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Title: The Principles of Leather Manufacture
Author: Procter, H. R.
Language: English
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Transcriber’s Notes
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subscript D. Small capitals have been transcribed as ALL CAPITALS.
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THE PRINCIPLES
OF
LEATHER MANUFACTURE
[Illustration: _Frontispiece._
PLATE I.
SECTION OF CALF-SKIN. (For key, see Fig. 9.)]
THE PRINCIPLES
OF
LEATHER MANUFACTURE
BY
H. R. PROCTER, F.I.C. F.C.S.
PROFESSOR OF LEATHER INDUSTRIES AT THE YORKSHIRE COLLEGE, LEEDS;
PAST PRESIDENT OF THE INTERNATIONAL ASSOCIATION
OF LEATHER TRADES CHEMISTS
[Illustration]
=London:=
E. & F. N. SPON, LIMITED, 125 STRAND
=New York:=
SPON & CHAMBERLAIN, 123 LIBERTY STREET
1903
=Dedicated to=
PROFESSOR F. L. KNAPP
GEHEIMEN HOFRATH, DR. PHIL. AND DR. ING.
THE PIONEER OF SCIENTIFIC RESEARCH
IN LEATHER MANUFACTURE
PREFACE.
The origin of the present work was an attempt to prepare a second
edition of the little Text-Book of Tanning which the Author published in
1885, and which has been long out of print. Though persevered in for
years, the work was never brought to completion, partly owing to the
constant pressure of other duties, but still more to the rapid advances
which have been made in our knowledge of the subject, and in the
scientific thought which has been devoted to it. For his share in the
initiation of this work, much credit is due to Wilhelm Eitner, Director
of the Imperial Royal Research Institute for Leather Industries in
Vienna, but the advance he began has been energetically carried forward
not only in Vienna, but in the Tanning Schools and Research Institutes
of Freiberg, Leeds, London, Liège, Copenhagen, Berlin and elsewhere, and
to a less extent in private laboratories.
Under the pressure of this rapid growth, as it was impossible to
complete the work as a whole, the Author published an instalment dealing
with the purely chemical side of the subject in 1898, under the title of
the ‘Leather Industries Laboratory Book’; which has been translated into
German, French and Italian, and of which the English edition is rapidly
approaching exhaustion.
The present work, which should by right have preceded the Laboratory
Book (and which frequently refers to it as “L.I.L.B.”), attempts to deal
with the general scientific principles of the industry, without
describing in detail its practical methods (though incidentally many
practical points are discussed). To complete the subject, a third volume
ought to be written, giving working details of the various methods of
manufacture; but apart from the difficulty of the subject, and the
weariness of “making many books,” the methods of trade are so
fluctuating, and dependent on temporary conditions that they have not
the same permanent value as the record of scientific advance.
As the present volume is intended to appeal both to the chemist and to
the practical tanner, it must to a certain extent fail in both, since
many matters are included which are already familiar to the former, and
it is to be feared, some, which may prove difficult to the latter. For
these and other imperfections the Author claims the indulgence of his
Readers.
The Author must here acknowledge his indebtedness to Dr. TOM GUTHRIE and
to Mr. A. B. SEARLE for assistance in writing several of the chapters;
to Dr. A. TURNBULL and Mr. F. A. BLOCKEY for much help in reading proofs
and preparing the MS. for the press; and to the many gentlemen who have
furnished or allowed him to use their blocks and drawings in
illustration.
THE YORKSHIRE COLLEGE,
LEEDS.
CONTENTS.
CHAPTER I.
_INTRODUCTORY AND HISTORICAL._
Primitive methods of leather manufacture -- Use of leather by the
ancients -- Progress of leather manufacture in England -- Methods of
production of leather -- Vegetable tannages -- Combination tannages --
Use of aluminium, iron and chromium -- Oil- and fat-leathers --
Difficulties of scientific treatment PAGE 1
CHAPTER II.
_INTRODUCTORY SKETCH OF LEATHER MANUFACTURE._
The object of tanning -- Washing and soaking -- Removal of hair by
liming -- Unhairing by putrefaction -- Unhairing and fleshing --
Deliming -- Bating, puering and drenching -- The vegetable tanning
process -- Currying -- Alum, chrome and chamois leathers PAGE 7
CHAPTER III.
_THE LIVING CELL._
The structure of cells -- White blood-corpuscles -- The yeast-cell --
Epidermis cells -- The building up of plants PAGE 10
CHAPTER IV.
_PUTREFACTION AND FERMENTATION._
The nature of ferments -- Organised and unorganised ferments --
Classification of organised ferments -- General properties of ferments
-- The alcoholic fermentation -- The action of enzymes or unorganised
ferments -- The destruction of ferments by heat and antiseptics -- The
products of fermentation -- The fermentations of the tannery --
Fermentation in bating and puering -- Fermentation in the tanning
liquors -- Moulds and mildews -- Control of fermentation PAGE 15
CHAPTER V.
_ANTISEPTICS AND DISINFECTANTS._
Distinction of antiseptics and disinfectants -- Lime -- Sulphur
dioxide -- Manufacture of sulphuric acid -- Bisulphites and
metabisulphites -- Boric acid and borates -- Mercuric chloride --
Mercuric iodide -- Copper sulphate -- Zinc salts -- Arsenic --
Fluorides -- Phenol -- Use of carbolic acid -- Eudermin -- Creasote --
Creolin -- Salicylic acid -- Benzoic acid -- Cresotinic acid --
Anticalcium -- “C.T.” bate -- Naphthalene sulphonic acid -- Naphthols
-- Hydronaphthol -- Oxynaphthoic acid -- Carbon disulphide --
Formaldehyde -- Triformol -- Camphor and essential oils PAGE 21
CHAPTER VI.
_THE ORIGIN AND CURING OF HIDES AND SKINS._
Marking of hides -- Fellmongering of sheep-skins -- The use of salt --
Salting of packer hides -- Brining -- Dry-salting -- Indian plaster
cures -- Analysis of salt-earths -- Salt- and iron-stains -- Drying of
hides and skins -- Damage by insects -- The warble-fly -- Damage by
branding -- Cockle PAGE 33
CHAPTER VII.
_STRUCTURE AND GROWTH OF SKIN._
Similarity of Mammalian skins -- Development of skin -- Structure of
calf-skin -- The epidermis -- The structure of hair -- The sebaceous
glands -- The development of hair -- The hair-sheath -- The
hair-muscle -- The hyaline layer -- The corium -- Connective tissue --
Fat cells -- Striped muscle -- Elastic fibres -- The unhairing process
-- The sweating process PAGE 46
CHAPTER VIII.
_THE CHEMICAL CONSTITUENTS OF SKIN._
The keratin tissues -- Production of gelatine from connective tissue
-- Analyses of hide and gelatine -- Constitution of gelatine --
Analysis and Reactions of gelatine -- Decomposition of gelatine --
Reactions of gelatine -- Chondrin -- Coriin -- Hide-albumin -- “Acid”
and “alkali” albumins -- Egg-albumin -- Vitellin -- Casein -- Keratins
-- Elastic fibres -- Analytical methods -- Kjeldahl process PAGE 56
CHAPTER IX.
_THE PHYSICAL CHEMISTRY OF THE HIDE-FIBRE._
Causes of swelling and contraction -- The essentials of the tanning
process -- The constitution of matter -- The nature of molecules --
Vapour-pressure -- Surface-tension -- Solution-pressures -- Jellies --
Crystals -- Osmotic pressure -- Electrolytic dissociation --
Electrolysis -- Reactions of ions -- Absorption of water by gelatine
-- Dehydration by alcohol -- Action of acids, alkalies and salts on
gelatinous fibre -- Physical explanation of swelling -- Action of
acids on gelatine -- Action of alkalies on gelatine -- Effect of salt
-- The pickling process PAGE 73
CHAPTER X.
_WATER AS USED IN THE TANNERY._
Impurities of natural water -- Hardness -- Soap test -- Temporary
hardness -- Clark’s softening process -- Archbutt and Deeley’s
softening apparatus -- Other appliances -- Effect of temporary
hardness in tanning and dyeing -- Permanent hardness -- Boiler scale
-- Mud -- Iron -- Alumina -- Soda -- Copper, lead, etc. -- Sulphuric
acid -- Nitrates and Nitrites -- Chlorine -- Carbonic acid -- Silicic
acid -- Effect of hardness on plumping -- Peaty waters PAGE 93
CHAPTER XI.
_SOAKING AND SOFTENING OF HIDES AND SKINS._
Washing of fresh hides -- Danger of putrefaction -- Soaking of salted
hides and skins -- Soaking and softening of dry and dry-salted hides
-- American wash-wheel -- Chemical methods -- Difficulty of softening
hides dried at high temperature PAGE 108
CHAPTER XII.
_DEPILATION._
Methods of depilation -- Sweating process -- Liming -- Sources of lime
-- Quicklime -- Slaking of lime -- Solubility of lime in water --
Analysis of lime -- “Available” lime -- Action of lime on hide --
Liming in pits -- Suspension limes -- Effect of warming limes --
Quantity of lime required -- The Buffalo method -- Action of old limes
-- Solution of hide substance by limes -- Sodium and potassium
hydrates -- Payne and Pullman’s process -- Alkaline carbonates --
Alkaline sulphides -- Sodium sulphide -- Calcium Sulphydrate --
Gas-lime -- Tank-waste -- Lufkin’s liming preparation -- Barium
sulphydrate -- Realgar, or red sulphide of arsenic -- “Inoffensive”
unhairing solution -- Earp’s patent -- Unhairing on the beam --
Unhairing machines -- Vaughn machine -- Leidgen machine -- Unhairing
in stocks and wash-wheel -- Jones machine -- Fleshing -- Vaughn
fleshing machine -- Rounding PAGE 119
CHAPTER XIII.
_DELIMING, BATING, PUERING AND DRENCHING._
Methods of removing lime and reducing swelling -- Use of acids --
Lactic, acetic and formic acids -- Boral -- Sodium bisulphate --
Boric (boracic) acid -- Borax -- “Pulling down” process -- Use of
ammonium chloride and sulphate -- Pickling solutions -- Drenching with
lactic acid -- Metabisulphite of soda -- Washing out lime, French
process -- Nesbitt’s process -- Use of carbonic acid -- Carbolic acid
-- Cresotinic acid -- Oxynaphthoic acid -- “Anticalcium” -- “Acrilene
bating acid” -- “C.T. Bate” -- Use of sulphides and polysulphides --
Babool pods -- Bran-drenching -- Bating and puering -- Causes of
bating effect -- Pepsin -- Trypsin, or Pancreatin -- Wood’s researches
-- Erodin -- Palmer’s experiments -- Other artificial bates --
Relative effect of dog- and pigeon-dung bates -- Analysis of dungs --
“Scudding,” or “fine hairing” -- Preservation and use of dung PAGE 152
CHAPTER XIV.
_ALUM TANNAGE, OR TAWING._
Nature of leather -- Mineral tanning substances -- Salts of aluminium
-- Alums -- Aluminium sulphate -- Effect of salt in tawing -- Basic
alumina solutions -- Tawing of skins for rugs -- Calf-kid manufacture
-- Glove-kid -- Green leather and other combination tannages PAGE 184
CHAPTER XV.
_IRON AND CHROME TANNAGES._
Iron tannages -- Chrome tannages -- Chemistry of chromium compounds --
Knapp’s method of chrome tannage -- Cavallin -- Swan -- Heinzerling --
Hummel’s improvement -- Schultz’s method -- Theory of the two-bath
process -- Practical management of the two-bath process -- Dennis’s
chrome tanning liquor -- Procter’s liquors -- Theory of basic process
-- Practical use of basic liquors -- Washing and neutralisation --
Effect of sulphur on chrome leather -- Bluebacking -- Fat-liquoring --
Dyeing of chrome leather -- Glazing and finishing PAGE 198
CHAPTER XVI.
_PRINCIPLES OF THE VEGETABLE TANNING PROCESSES._
Methods of sole-leather tanning -- Finishing of sole-leather -- Theory
of vegetable tannage -- Deliming of sole-leather -- “Mellowness” of
liquors -- Penetration of tannage -- Drying of sole-leather -- Tanning
of dressing leathers -- Preparation for tannage -- Avoidance of
“bloom” -- Tannage of moroccos and other skins PAGE 220
CHAPTER XVII.
_COMBINATION OF VEGETABLE AND MINERAL TANNAGES._
Early combination tannages -- Respective effect of mineral and
vegetable tannages -- Use of fat-liquor -- Action of mineral and
vegetable tanning materials on each other -- Danish and Swedish glove
leathers -- Green leathers -- Making of fat-liquors -- Chrome
combinations PAGE 236
CHAPTER XVIII.
_VEGETABLE TANNING MATERIALS._
Distribution of tannin in plants -- Structure of barks -- Botanical
list of important tanning materials PAGE 242
CHAPTER XIX.
_THE CHEMISTRY OF THE TANNINS._
Sources of tannins -- General qualities of tannins -- Chemical
constitution -- Catechol- and pyrogallol tannins -- Catechins --
Tendency of Catechol tannins to darken with light -- “Physiological”
and “pathological” tannins -- Presence of mordant colouring matters
PAGE 294
CHAPTER XX.
_THE SAMPLING AND ANALYSIS OF TANNING MATERIALS._
The International Association of Leather Trades Chemists -- The
American Official Association of Agricultural Chemists -- The sampling
of material -- Preparation of solution for analysis -- Extraction of
solid materials -- Total soluble matter -- Evaporations of solutions
-- The weighing of residues -- The determination of non-tannins -- The
hide-powder filter method -- The hide-powder shake method --
Determination of moisture -- Colour-measurement PAGE 300
CHAPTER XXI.
_THE GRINDING OF TANNING MATERIALS._
Primitive methods of grinding -- The bell mill or coffee mill -- Disc
mills -- Disintegrators -- Carr’s disintegrator -- Carter’s
disintegrator -- Adjustment of disintegrators -- The Williams
pulveriser -- Myrobalans and Valonia crushers -- Sawing mills --
Shaving mills -- Dyewood cutting machines -- Screening of ground
materials -- Hatching of bark -- Disintegrators and fire insurance --
Dust from disintegrators -- Chain conveyors -- Belt conveyors --
Vibrating conveyors PAGE 316
CHAPTER XXII.
_THE EXTRACTION OF TANNING MATERIALS, AND THE MAKING OF EXTRACTS._
Leaching -- Early forms of leaches -- The press-leach system --
Handling of liquors -- Distributing troughs and valves -- Construction
of leaches -- Influence of temperature -- Use of silent boiling jet --
Closed extractors -- Sprinkling leaches -- Manufacture of extracts --
Decolorisation of extracts -- Soluble extracts -- Concentration of
extracts -- Yaryan evaporator -- Multiple effects -- The use of
extracts in the tannery -- Effect of temperature on extraction and
colour PAGE 328
CHAPTER XXIII.
_FATS, SOAPS, OILS AND WAXES._
Characteristics of fats and oils -- Chemical constitution -- Nature
and production of soaps -- Insoluble soaps -- Distillation of fats --
Solvents of oils -- Drying oils -- Saturated fatty acids -- Non-drying
liquid fatty acids -- Less-saturated liquid fatty acids -- Castor oil
-- Tallow -- Neatsfoot oil -- Wool fat -- Holden fat -- Distilled wool
grease -- Distilled stearine -- Olive oil -- Castor oil -- Turkey-red
oil -- Linseed oil -- Boiled oils -- Japan for leather -- Cottonseed
oil -- Sesame oil -- Cod oil -- Shark liver oil -- Whale oil -- Seal
oil -- Menhaden oil -- Fish oils -- Fish tallow -- Dégras and Sod oil
-- Waxes -- Sperm oil -- Beeswax -- Carnauba wax -- Japan wax --
Volatile or essential oils -- Birch oil -- Wintergreen oil -- Mineral
oils and waxes -- Vaseline and vaseline oil -- Paraffin wax --
Ozokerit -- Resin oils -- Resin PAGE 350
CHAPTER XXIV.
_OIL TANNAGES, AND THE USE OF OILS AND FATS IN CURRYING._
Primitive use of oil in leather manufacture -- Chamoising and the
production of washleather -- Manufacture of Moellon, or Dégras -- Sod
oil -- Formaldehyde leathers -- “Crown” and “Helvetia” leathers --
Theory of oil leathers -- Processes of currying -- Theory of the
stuffing process -- Hand-stuffing -- Drum-stuffing -- Stuffing of dry
leather -- “Spueing” and its causes -- Fat-liquoring PAGE 378
CHAPTER XXV.
_DYES AND DYEING._
Coal-tar colours -- Acid and basic colours -- Theories of dyeing --
Fixation of colours on leather -- Mordants and mordant colours --
Curriers’ inks -- Glazes and finishes -- “Assistants” in dyeing --
Bronzing -- Fading of colours -- Practical methods of leather dyeing
-- Use of dyewoods -- Iron “strikers” -- Tannin blacks -- Staining --
Theory of colour-mixtures -- Finishing dyed leathers -- Testing of
dyes -- Injurious effects of metals in dyeing PAGE 394
CHAPTER XXVI.
_EVAPORATION, HEATING AND DRYING._
Theory of evaporation -- Boiling point and vapour-pressure --
Consumption of heat in evaporation -- Heat-units -- Mechanical energy
of heat -- Evaporation by “multiple effect” -- Vapour-pressure of
atmospheric moisture -- Wet and dry bulb thermometers -- Heat and air
required in leather-drying -- Loss of heat by buildings -- Quantity of
heat given by steam and hot-water pipes -- Screw-fans for drying --
Centrifugal fans -- “Turret” dryer -- Downward ventilation --
Arrangement of steam-pipes -- Hot water pipes PAGE 420
CHAPTER XXVII.
_CONSTRUCTION AND MAINTENANCE OF TANNERIES._
Selection of site -- Arrangement of buildings -- Fire insurance --
Automatic sprinklers -- Possibility of extension -- Production and
distribution of power -- Electric motors -- Shafts, pulleys and
belting -- Balancing of machinery -- Fire-risk from bark mills --
Chain-conveyors -- Lubricating oils -- Construction of pits --
Underground pipes and overhead troughs -- Pumps and pumping appliances
PAGE 444
CHAPTER XXVIII.
_WASTE PRODUCTS AND THEIR DISPOSAL._
Hair -- Fleshings and glue-stuff -- Fat -- Bate-shavings -- Horns --
Spent tan -- Tan-furnaces -- Sewage and other waste liquids --
Chemical purification of sewage -- Settling tanks -- Filter-presses --
Bacterial purification of sewage -- Tannery waste-liquors PAGE 460
APPENDIX A.
_METHOD OF THE INTERNATIONAL ASSOCIATION OF LEATHER-TRADES CHEMISTS
FOR THE ANALYSIS OF TANNING MATERIALS: Corrected to 1901._
Sampling from bulk -- Preparation for analysis -- Preparation of
infusion -- Determination of tanning matters and non-tannins --
Colour-measurement -- Analysis of used liquors PAGE 475
APPENDIX B.
_THE DECIMAL SYSTEM._
Metrical weights and measures -- Centigrade thermometer PAGE 481
APPENDIX C.
_METHOD OF ANALYSIS OF TANNING MATERIALS OF THE AMERICAN ASSOCIATION
OF OFFICIAL AGRICULTURAL CHEMISTS: Corrected to 1901._
Preparation of sample -- Quantity of material -- Moisture -- Total
solids -- Soluble solids -- Non-tannins -- Tannins -- Testing of
hide-powder -- Testing non-tannin filtrate PAGE 482
APPENDIX D.
_LISTS OF COAL-TAR DYES SUITABLE FOR DYEING AND STAINING LEATHER,
furnished by Mr. M. C. LAMB._
Colours for staining leather -- Colours for dyeing vegetable-tanned
leather -- Dyeing and finishing chrome-leather -- List of colours
suitable for chrome-leather PAGE 485
INDEX PAGE 499
PRINCIPLES
OF
LEATHER MANUFACTURE.
[Illustration]
CHAPTER I.
_INTRODUCTORY AND HISTORICAL._
The origin of leather manufacture dates far back in the prehistoric
ages, and was probably one of the earliest arts practised by mankind.
The relics which have come down to us from palæolithic times, and the
experience of the modern explorer, alike tell us that agriculture is a
later and a higher stage of development than the life of the hunter; and
since, in the colder regions, clothing of some kind must always have
been a necessity, we may conclude that it was first furnished by the
skins of animals.[1]
[1] See also Gen. iii. 21.
While wet skins putrefy and decay, dry ones are hard and horny; and
nothing could be more natural to the hunter than to try to remedy this
by rubbing the drying skin with the fat of the animal, of which he must
have noticed the softening effect on his own skin. By this means a soft
and durable leather may be produced, and this process of rubbing and
kneading with greasy and albuminous matters, such as fat, brains, milk,
butter and egg-yolks, is in use to this day, alike by the Tartars on
Asiatic steppes and the Indians on American prairies; and not only so,
but we ourselves still use the same principle in the dressing of our
finest furs, and in the manufacture of chamois, and many sorts of lace-
and belt-leathers.
Such a process is described in the _Iliad_ (xvii. 389-393) in the
account of the struggle over the body of Patroclus:
“As when a man
A huge ox-hide drunken with slippery lard
Gives to be stretched, his servants all around
Disposed, just intervals between, the task
Ply strenuous, and while many straining hard
Extend it equal on all sides, it sweats
The moisture out and drinks the unction in.”
It must also have been early noticed that wood smoke, which in those
days was inseparable from the use of fire, had an antiseptic and
preservative effect on skins which were dried in it, and smoked leathers
are still made in America, both by the Indians and by more civilised
leather manufacturers. To this method the Psalmist refers[2] when he
says, “I am become like a bottle in the smoke;” and such bottles, made
of the entire skin of the goat, are still familiar to travellers in the
East.
[2] Ps. cxix. 83.
The use of vegetable tanning materials, though prehistoric, is probably
less ancient than the methods I have described, and may possibly have
been discovered in early attempts at dyeing; an art which perhaps had
its origin even before the use of clothing! The tannins are very widely
distributed in the vegetable kingdom, and most barks, and many fruits,
are capable of making leather.
The employment of alum and salt in tanning was probably of still later
introduction, and must have originated in countries where alum is found
as a natural product. The art was lost or unknown in Europe till
introduced into Spain by the Moors.
Leather manufacture reached considerable perfection in ancient Egypt. A
granite carving, probably at least 4000 years old, is preserved in the
Berlin Museum, in which leather-dressers are represented. One is taking
a tiger-skin from a tub or pit, a second is employed at another tub,
while a third is working a skin upon a table. Embossed and gilt leather
straps have been found on a mummy of the ninth century B.C., and an
Egyptian boat-cover of embossed goat leather, as well as shoes of dyed
and painted morocco, are still in comparatively good preservation. The
art is of very early date in China, and was well understood by the
Greeks and Romans. In the Grosvenor Museum at Chester is the sole of a
Roman _caliga_, studded with bronze nails, which is yet pretty flexible.
After the fall of the Roman empire many arts were lost to Europe, and
it was not until the Moorish invasion of Spain that the art of dyeing
and finishing the finer kinds of leather was reintroduced.
England was very backward in this manufacture up to the end of the last
century, owing to the fossilising influence of much paternal
legislation, and of certain excise-duties, which were only repealed in
1830. Since this time the art has made rapid strides, especially in the
use of labour-saving machinery, and England may at the present moment be
considered fairly abreast of any other country as a whole; though in
some special manufactures we are surpassed by the Continent and by
America. In making comparisons of this kind, it must, however, be
remembered that, especially in sole-leather tannage, the most rapid
progress has been made during the last few years in those countries
which were more backward, and that therefore our superiority is much
less pronounced than formerly, and in a few years will probably cease to
exist unless marked improvements are introduced in the methods of
production.
In the sketch of the development of leather manufacture which has just
been given, it has been implied that its object is to convert the
putrescible animal skin into a material which is permanent, and not
readily subject to decay, while retaining sufficient softness or
flexibility for the purposes for which it is intended. As these range
from boot-soles to kid-gloves, there are wide divergences, not only in
the processes employed, but also in the materials used and in the
principles of their application.
The most important method of producing leather is by the use of
vegetable tanning materials, and this is perhaps the only one which is
really entitled to be called “tanning,” though the distinction is not
very strictly adhered to. It includes the whole range--from sole
leather, through strap, harness and dressing leather, to calf and goat
skins, and the various sumach tannages which yield morocco and its
imitations. All of these products but the first and the last undergo,
after tanning, the further processes of “currying,” of which the most
important operation consists in “stuffing” with oily and fatty matters,
both to increase the flexibility and to confer a certain amount of
resistance to water. Sumach-tanned skins are not strictly “curried” but
usually receive a certain amount of oil in the process of “finishing.”
Next in importance to the vegetable tannages are the “tawed” leathers
produced by the agency of alum and salt, including the “white leathers”
for belt laces and aprons, and calf- and glove-kid. A connecting link
between tanning and tawing is found in the “green leather,” “Dongola,”
and “combination” tannages, in which alum and salt are employed in
conjunction with vegetable tanning materials, and especially with
gambier.
Salts of several of the metals, and particularly those of aluminium,
iron, and chromium, have the power of converting skin into leather; and
processes in which salts of chromium are used have recently attained
very considerable commercial importance.
In the production of calf- and glove-kid, in addition to alum and salt,
albuminous and fatty matters, such as egg-yolk, olive oil and the gluten
of flour, play a considerable part, and are thus linked both to the
primitive methods in use by the Indians and Kalmucks, and to those by
which “crown” and “Helvetia” leather, and many other forms of belt- and
lace-leathers are now produced by treatment with fats and albumens.
From these again the step is a short one to the “chamois” and “buff”
leathers, and the German “_fettgar_” leathers, in which oils and fats
only are used; and these are probably again related chemically to
leather produced by the aid of formaldehyde and other aldehydes.
In an attempt to view all these complex processes from the scientific
standpoint, the reader should constantly realise that the present
methods of leather manufacture are the results of tens of centuries of
experience, and of innumerable forgotten failures, and must not
therefore expect that they can be easily superseded. Science must follow
before it can lead, and its first duty is to try to understand the
reasons and principles of our present practice, for we can only build
the new on the foundation of what has been already learned. Another
fact, which is scarcely understood by the practical man in his demands
on science, is that in leather manufacture every question which is
raised seems to rest on the most recondite problems of chemistry and
physics; the chemistry of some of the most complex of organic compounds,
and the physics of solution, of osmose, and of the structure of colloid
bodies--problems which are yet far from completely conquered by the
highest science of the day.
It may seem bold to attempt the scientific treatment of such a subject
at all; and, indeed, it must be admitted that our knowledge is still far
from adequate for its complete accomplishment, but enough has been done
to lay a foundation for future work, and this can at least be summarised
and arranged in an available form. The subject falls naturally into two
sections, in the first of which the processes of manufacture would only
be described in general terms, and with sufficient fulness to enable the
reader to understand the scientific considerations on which they are
based, and the methods of investigation which can be applied to them;
while in the second an effort should be made to give working details of
the various processes sufficient to enable those with a general
knowledge of the trade to experiment successfully in its various
branches. It was at first intended that these two sections should be
published in one book as a second edition to the Author’s ‘Text-book of
Tanning,’ but owing to the long delay in its publication, it was decided
to publish the first section under the present title ‘Principles of
Leather Manufacture,’ leaving the latter section ‘Processes of Leather
Manufacture’ to a later, and I fear, somewhat uncertain date; while the
strictly chemical portion has already appeared in the ‘Leather
Industries Laboratory Book,’ frequently referred to in the following
pages under the abbreviation “L.I.L.B.” Where quantities and details are
given, they must not be taken as recipes to be blindly followed; or
even, in every case, as the best known methods; but rather as mere
guides to experiment, which must be modified to suit varying conditions
and requirements. It is the special virtue of the scientific, as opposed
to the merely traditional way of looking at such questions, that knowing
the cause and effect of each part of the process, it can so adjust them
as to get over difficulties, and to suit novel conditions. It is
needless to add that many methods are jealously preserved as trade
secrets, and full details are frequently unattainable.
After what has just been said, it may be well to emphasise the great
importance of practical knowledge and experience to the leather
manufacturer. Even in trades which have reached the highest scientific
development, such, for instance, as the manufacture of the coal-tar
colours, the small experiments of the laboratory are not transformed
into manufacturing operations without experience and sometimes even
failure; and this must still more often be the case in a trade like
that of leather-making, where our knowledge of the actual changes
involved is still so incomplete. On the other hand, the cost of
experiments on a manufacturing scale is usually so heavy that the least
scientific must admit the advantage of learning all which the laboratory
can teach before venturing on anything more; while even our present
imperfect knowledge of the chemical changes involved will often warn us
off hopeless experiments, and give us hints of the directions in which
success may be attained. A knowledge of chemistry will probably prove at
least as important to the future of our trade as that of mechanics has
been in the past.
CHAPTER II.
_INTRODUCTORY SKETCH OF LEATHER MANUFACTURE._
The object of tanning has been stated to be the rendering of animal skin
imputrescible and pliable, but as we now rarely require leather with the
hair on, preliminary processes are needed to remove it, and to fit the
skin for tanning, and the nature of these processes has great influence
on the subsequent character of the leather produced.
The first step is usually a washing of the skin to remove blood and
dirt; while, where it has been salted or dried, a more thorough soaking
is needed to remove the salt, and to restore the skin to its original
soft and permeable condition.
The hair is then loosened by softening and partial solution of the
epidermis structures (see p. 47) in which it is rooted. This is most
generally accomplished by soaking for some days in milk of lime, which
is occasionally assisted by the addition of caustic alkalies or of
sulphides. When the latter are used in concentrated solution, the hair
itself, as well as the epidermis tissues, is softened and destroyed in
the course of a few hours. The lime not only serves to loosen the hair,
but swells and splits up the fibre-bundles of which the hide tissue is
composed, and so fits it to receive the tannage (cp. p. 125).
For some purposes a regulated putrefactive process is substituted for
the liming; the hides or skins being hung in a moist and warm chamber
(see p. 119), when the soft mucous layer which forms the inner part of
the epidermis is disintegrated, partly by direct putrefaction, partly by
the action of the ammonia evolved, so that the hair can be scraped off.
In this case the hide-fibre is not swollen, and the necessary swelling
has to be obtained by subsequent processes.
In whatever way the hair has been loosened, it is scraped off with a
blunt and somewhat curved two-handled knife on a sloping rounded “beam”
of wood or metal; this operation being termed “unhairing” (see p. 144).
This is generally followed by “fleshing,” which is performed on the same
beam with a somewhat similar knife, which, however, is two-edged and
sharp. In this operation, portions of flesh, and the fat and loose
tissue which underlie the true skin (see p. 147) are removed by scraping
and cutting. Machines for fleshing are also largely in use for certain
purposes (see p. 148).
For sole leather, the hide, after some washing in soft water to cleanse
from lime, is then ready for the actual tanning process; but for the
softer leathers more thorough treatment is needed to remove the lime,
and to still further soften the skin by solution and removal of a
portion of the cementing substance of the fibres.
This treatment is generally of a fermentive or putrefactive nature, and
the most common form is that known as “bating,” which consists in
steeping in a fermenting infusion of pigeon- or hen-dung. The theory of
its action is not yet thoroughly understood, but the effect is largely
due to the unorganised hydrolysing ferments produced by the _bacteria_
present; while at the same time the lime is neutralised and removed by
the weak organic acids and salts of ammonia which are produced; and the
fibre which had been plump and swollen with lime, becomes extremely
relaxed and flaccid.
In the lightest leathers, such as kid- and lamb-skins for gloves, and
goat and sheep for moroccos and the like, dog-dung is substituted for
that of fowls, and the process is then called “puering” (see p. 170).
These processes are often followed by “drenching,” which sometimes
indeed takes their place, the skins being soaked in a fermenting bran
infusion. In this, the small quantities of acetic and lactic acid formed
by fermentation are the active agents, neutralising and dissolving the
lime, and cleansing and slightly plumping the pelt (see p. 166).
The tanning process which follows consists in soaking the pelt in
infusions of various vegetable products containing bodies of the class
known as “tannins,” which have the power of combining with skin-fibre
and converting it into leather.
If at first strong infusions were used, they would act too violently on
the surface of the skin, hardening and contracting it so that the
subsequent tannage of the interior would be impeded, and the “grain” or
outer surface would be “drawn” and wrinkled. This is avoided by the use
at first of very weak infusions which have already been used on goods in
a more advanced stage. In the later part of the process much stronger
solutions are employed, and the hides are frequently “dusted” in them
with ground tanning material.
In the case of sole leather, these processes may require from two to
twelve months for completion; after which the leather is dried,
smoothed, and compressed by mechanical means, and is then ready for use.
Dressing-leathers, ranging from calf-skins to harness-hides, receive a
much shorter tannage, and the subsequent treatment with fats and oils,
which, together with mechanical manipulations, constitute “currying.”
The thin film of grease distributed over the surface of the fibres
renders them supple, and to some extent waterproof.
The lighter fancy leathers, such as morocco, are dyed, and undergo many
complex processes to fit them for their required purposes and improve
their appearance.
Many skins such as calf, glove, and glacé kid, are not tanned, but
“tawed” by a solution of alum and salt, which is often supplemented with
mixtures of flour and egg-yolk to fill and soften the leather.
Salts of chromium are also employed in place of alum and salt, and
produce an equally soft, but more permanent and enduring leather.
Lastly, wash-leather, or so-called “chamois,” and buff-leather are
produced by fulling the prepared pelt with fish or whale oil, which
converts the skin into leather by subsequent oxidation, during which
aldehydes are evolved.
CHAPTER III.
_THE LIVING CELL._
The larger part of the materials employed in leather manufacture are
organic in their origin, and the skin itself is an organised structure,
while the life-processes of putrefaction and fermentation play a large
part in the tannery. Some knowledge, therefore, of biological structures
and processes is necessary to a full understanding of much which
follows, and a few words are not out of place with regard to the
foundations of life itself.
The bricks of which all living structures are built are the living
“cells” and their products, and these first elements differ little, if
at all, whether the life is animal or vegetable, the distinction being
produced rather by the way in which they are put together, than by
differences in the cells themselves. This is so much the case that it is
often difficult to decide in which of the two classes to place the
simplest organisms, since most of these forms are capable of active
movement, and their modes of nutrition and reproduction are common to
both kingdoms.
In its simplest form, the cell, whether animal or vegetable, is strictly
speaking not a cell at all, but consists merely of a minute mass of
living jelly or protoplasm. Such is the amœba found in water and damp
soil, such are the lymph-cells and white blood-corpuscles of our bodies,
and such also some stages at least of the lowest forms of fungi, like
the _Æthalium septicum_ which is sometimes found on old tan-heaps as a
crawling mass of yellow slime. If a drop of saliva be examined with the
microscope under a cover-glass, with one-sixth objective and small
opening of diaphragm,[3] a few scattered semi-transparent objects will
be found, of the apparent size of a lentil or small pea, and of rounded
form. These are lymph-corpuscles (Fig. 1). Their contents are full of
small granules, and if they be observed quickly, or if the slide be kept
at about the warmth of the body, it will be noticed that these are in
constant streaming motion. If the warmth can be kept constant, which is
difficult without special apparatus, and the cells can be observed from
time to time, it may be seen that they lose their circular form, and put
out protuberances (pseudopodia, “false feet”) one of which will
gradually increase in bulk, till it absorbs the whole cell, which thus
crawls about. It will now readily be understood how these cells wander
through all the tissues of the body, passing through the smallest pores
like the fairy who put her finger through a keyhole, and grew on the
other side till she was all through! This independent vitality, in a
warm and suitable nutrient liquid, may continue for more than a week,
and, in the case of amœba, quite indefinitely.
[3] For details of microscopic manipulation in this and the following
chapter see L.I.L.B., p. 234 _et seq._
[Illustration: FIG. 1.--Lymph-corpuscle of frog, showing gradual change
of form.
(Ranvier.)]
It is possible that by close attention, a rounded or elongated body,
somewhat like an oil-globule, may be seen within the cell, though it is
generally more obvious when the latter has been killed and stained with
a weak solution of iodine. This is the nucleus, and within it is a still
smaller speck called the nucleolus, which bears an important, and as
yet little understood, part in the life-history of the cell. After a
period, it undergoes certain somewhat complicated changes, and divides
into two, the nucleus elongates, and also divides, each half carrying
with it a portion of the living protoplasmic jelly, and thus forming two
complete and independent cells. This is the life-history, not only of
the lymph-cell, but with more or less modification, of every living cell
or tissue.
[Illustration: FIG. 2.--Yeast-cells, much magnified.]
These cells, like all living things, feed on the nutriment which
surrounds them, and even enclose small particles of solid food, which
are gradually dissolved and disappear. In this way the white
blood-corpuscles are said to feed upon and destroy the still smaller
organisms which gain access to the blood, and which might otherwise
cause disease. The matter which cells consume is not, of course,
destroyed, but simply converted into other forms, some of which are
useless, or even poisonous to the cells, and which, like the secretions
of higher animals, are discharged into the surrounding fluids; while
others are retained, and contribute to the growth of the cell. Thus most
vegetable cells secrete cellulose, or plant-tissue, which forms a wall
enclosing the protoplasm, and so justifies the name of cell. If to warm
water and a little sugar we add enough yeast to render it slightly
milky, and examine it like the saliva, we shall have before us typical
vegetable cells of the simplest form (Fig. 2). There is the same
granular protoplasm, and there is the nucleus, though it cannot be seen
without special preparation, the rounded spaces which look like one,
being simply filled with transparent fluid, and called vacuoles. There
is, however, no motion, as in the case of amœba, for the cells are
enclosed in a tough skin of cellulose, which will be evident if they are
crushed by putting some folds of blotting paper on the cover-glass, and
pressing it with the handle of a needle or a rounded glass rod, when the
protoplasm will be forced out and the skin remain like a burst bladder.
This will be more obvious if the cells are previously stained with
iodine or magenta, which will stain the protoplasm, but not the
membrane. It is easy to observe the multiplication of the yeast-cells,
which is somewhat different to that of the corpuscles. Instead of
enlarging as a whole, and dividing into two equal cells, a small bud
appears on the side of the parent-cell, and enlarges till it becomes
itself a parent-cell with buds of its own. These do not break away at
once, and hence chains and groups of attached cells are formed which are
easily noticed in growing yeast if a microscope be employed. The
principal nutriment of yeast is grape-sugar or glucose; and much more of
this is consumed than is needed to produce the cellulose wall and the
substance of new cells; just as in the animal, sugar, starch and fat are
consumed to give heat and energy. In the yeast, this extra sugar is
split up into carbon dioxide, which escapes as gas, and to which yeast
owes its power of raising bread; and into alcohol, which in too large
proportion is poisonous to the yeast itself.
[Illustration: FIG. 3.--Epithelium-cells. Ranvier. _p_, pressure-marks;
_g_, granular protoplasm.]
In examining the saliva for lymph-cells, it is probable that some much
larger objects may have been noticed of irregular polygonal outline and
with a well-marked nucleus. These are cells from the lining _epithelium_
of the mouth, and only differ from those of the _epidermis_ of skin in
their form and size (Fig. 3). Note the markings caused by the pressure
of overlapping cells. In these cells the wall is formed of keratin or
horny tissue, which takes the place of the cellulose of the yeast.
[Illustration: FIG. 4.--_Penicillium glaucum_, a common green mould.]
Other simple forms of cell are those of _Saccharomyces mycoderma_ or
_torula_ which forms a skin on the surface of old liquors, and which
much resembles a small yeast; and of the various ferments which are
found in liquors, bates and drenches, which will be more fully described
in the chapter following.
Many of these, such as the acetic and lactic ferments, which, like all
other _bacteria_, multiply by division, do not separate, but remain
connected in chains or chaplets, like a string of beads. From these, the
step is not a long one to the _hyphæ_ or stems of the higher moulds,
which are too frequently found on leather which has been slowly dried,
and which consist simply of tubular cells which elongate and divide by
the formation of _septa_ or cross-partitions, and thus build up a
complicated plant-structure (Fig. 4). As we proceed higher in the scale
of plant and animal life, the forms and products of the cells become
more varied, and instead of one single cell, fulfilling all the
functions of the plant or animal, each class of cell has its own
peculiar duties and properties, while all work together for the
maintenance of the complex structure of which they form a part.
CHAPTER IV.
_PUTREFACTION AND FERMENTATION._
The chemical changes produced by the unicellular plants, such as yeasts
and bacteria, to which allusion has been made in the last chapter, are
known as fermentation and putrefaction, and are of such importance to
the tanner, both for good and evil, that the subject must be treated in
some detail. No scientific distinction exists between fermentation and
putrefaction, though it is customary to restrict the latter term to
those decompositions of nitrogenous animal matter which yield products
of disagreeable smell and taste.
The organisms which are the cause of both fermentation and putrefaction
are known by the general term of “ferments.” This term has also been
extended in recent years so as to include the so-called “unorganised
ferments” (enzymes, zymases) which are active products secreted by the
“organised ferments” or living organisms.
These latter are again divided into three classes:--
1. Moulds.
2. Yeasts (Saccharomycetes).
3. Bacteria.
The members of one class are distinguished from those of another by
their form, and, more especially, by the substances they produce during
their life-history. All three classes are now considered to be fungi.
All ferments possess the following three properties:--
1. They are nitrogenous bodies.
2. They are unstable, i.e. they are destroyed by heat, chemicals,
etc.
3. A relatively small quantity of the ferment is capable of producing
great changes in the substances upon which it acts, especially if the
products of the change can be removed as they are formed.
The general character of fermentation will be best understood by a
closer study of the yeast cell, which has already been described (p.
12), and its life-history briefly sketched. It has been shown that it is
a growing plant of a very simple type, belonging to the fungi. These are
devoid of the green colouring matter which enables the higher plants to
utilise the energy of sunlight to assimilate the carbonic acid of the
atmosphere, exhaling its oxygen, and employing its carbon for the
building up of tissue; and they must therefore, like animals, have their
nutriment ready formed, and capable of supplying energy by its
oxidation. For yeast, as has been stated, the appropriate nourishment is
glucose, or “grape-sugar.” This is broken down, in the main, into the
simpler compounds, alcohol and carbonic acid, while a small portion is
utilised for the building up of the cell and the formation of secondary
products. The main reaction is represented by the following equation:
C₆H₁₂O₆ = 2C₂H₆O + 2CO₂
_Glucose_ _Alcohol_ _Carbon dioxide_
Yeast cannot directly ferment ordinary cane-sugar (C₁₂H₂₂O₁₁), but
secretes a substance called invertase, which so acts on the sugar as to
break it up, with absorption of one molecule of water, into two
molecules of fermentable glucose (dextrose and levulose) which serve as
nourishment for the yeast.[4] This invertase is the type of the series
of bodies which are known as “unorganised ferments,” enzymes, or
zymases, differing from the organised ferments in being simply chemical
products without life or power of reproduction, but capable of breaking
up an unlimited quantity of the bodies on which they act, without
themselves suffering change. The way in which this is done is not
clearly understood, but some parallel may be found to it in the action
of sulphuric acid on alcohol, of which it will convert an unlimited
quantity into ether, without itself suffering any permanent change. The
action of enzymes is limited to breaking down complex bodies into
simpler forms, often with absorption of water, as in the case of sugar,
while some of the products of living ferments are often complex, a part
of their nutriment being broken down into simple products such as
carbonic acid, marsh gas and ammonia, to supply the necessary energy to
elaborate the remainder.
[4] Compare O’Sullivan and Thompson, Jour. Chem. Soc., 1890, p. 834;
1891, p. 46.
Very many different unorganised ferments are known to exist, as they are
not only produced by yeasts and bacteria, but are formed by the cells of
higher plants and animals; thus the digestive principles, pepsin,
trypsin, ptyalin, are of this character--ptyalin, like diastase,
converting starch into sugar; and such bodies fulfil many functions both
in animal and vegetable economy. In fermentation, as in disease, it is
often difficult to distinguish what is due to the direct action of
bacteria, and what to the unorganised ferments which they produce, and
the question is further complicated by the fact that in most natural
fermentations more than one ferment-organism is present. Sometimes the
action of the unorganised ferments may be distinguished by the fact that
the addition of chloroform has little effect on their activity while it
paralyses that of the living organism. By exposure to high temperature
both are destroyed, the bacteria, yeasts and moulds being killed and the
unorganised ferments coagulated like white of egg, and so rendered
inoperative. Many antiseptics also destroy the activity of both
organisms and enzymes; but others, like chloroform, have no action on
the latter. In some cases, as in that of invertase, the actual zymase
can be precipitated by alcohol from its aqueous solution, filtered off,
and restored to activity by transference into water. Since both
classes of ferments are destroyed by high temperatures, all
fermentation-processes are completely and permanently arrested by
exposure to sufficient heat, and subsequent preservation in vessels so
closed that no new ferment-germs can gain access. A familiar instance is
that of tinned meats. All fully developed bacteria are destroyed by a
very short exposure to a boiling temperature, and most by 60° to 70° C.,
but many species produce spores which are extremely difficult to
destroy. The thermophilic bacteria discovered by Globig and further
investigated by Rabinowitsch,[5] thrive at a temperature of 60° C. About
eight species are known, and they take part in the heating of hay and
similar fermentations where high temperatures are involved, and are
therefore presumably present in spent tan.
[5] Centr. Blatt für Bakt., II. Abth. vol. i. p. 585.
For absolute sterilisation it is therefore necessary either to boil
under pressure so as to raise the temperature to, say 110° C., or to
heat repeatedly for a short time to temperatures of 80°-100° C. at
successive intervals of 24 hours, in order to allow the spores to
develop. This process is frequently performed for bacteriological
observation in flasks or test-tubes merely stopped with a plug of
sterilised cotton-wool, which has been found to efficiently filter the
germs from the air which enters through it (see L.I.L.B., p. 270).
The ferment-organisms cannot thrive and multiply unless they have proper
nourishment and conditions of growth, the amount of moisture and the
temperature being two of the most important of the latter. Use is made
of this in the preservation of many articles of food, etc., since by
ensuring that at least one of the conditions necessary for growth shall
be absent, these substances are prevented from decomposing. For
instance, hides are preserved by drying them; the absence of sufficient
moisture hindering the growth of any organisms in them so long as they
are dry, but as soon as they become somewhat damp, putrefaction
commences at once.
The waste products of organisms are often poisonous to themselves, and
for this reason fermentations frequently come to an end before the whole
of the substance is fermented. Thus neither beer nor vinegar can be
obtained of more than a certain strength by direct fermentation, the
alcohol or acetic acid checking the growth of their respective ferments.
A solution of glucose “set” with the lactic ferment of sour milk will
only produce lactic acid to the extent of about half a per cent.; but if
chalk be added, the lactic acid will be neutralised as produced, and the
fermentation will go on till the whole of the glucose is converted into
insoluble calcium lactate.[6] When this is accomplished the lactic
ferment dies from want of nutriment, and its place is taken by another
organism, of which some germs are sure to be present, which ferments the
calcium lactate into calcium butyrate. If the nourishment fails, or the
conditions become less favourable for one ferment than for some other
which exists even in small quantity in a liquid, the former is quickly
overgrown and killed, and the latter takes its place. Thus the ordinary
ferment of the bran drench will die out rapidly unless constantly
transferred to fresh bran infusions.
[6] For the practical preparation of lactic acid, the solution may
contain 7¹⁄₂-11 per cent. of glucose, and some nitrogenous
nourishment. The solution should be slightly acid. See Journ. Soc. Ch.
Ind., 1897, p. 516.
Many of the products of bacteria (like those of some of the higher
plants) are intensely poisonous both to animals and man. Many of the
severe symptoms of disease are caused by these poisons produced in the
body. Thus the tetanus-bacteria produce a poison similar in its effects
to strychnine, and quite as virulent. Not only are such poisons produced
by disease-bacteria in the body, but frequently also in the earlier
stages of putrefactive fermentation. The latter are known as
_ptomaines_, and when present in cheese and preserved foods are liable
to cause poisoning. Such putrefactions are often unaccompanied by any
disagreeable odour or flavour.
The fermentations which are most important in the tannery are, firstly,
the ordinary putrefaction which attacks hides as well as other animal
matter, and which is usually a complicated process carried on by many
sorts of bacteria and other micro-organisms. This may be regarded as
generally injurious to the tanner; but it is utilised in the “sweating”
process for depilation and in the “staling” of sheepskins, in both of
which advantage is taken of the fact that the soft mucous layer of the
epidermis, which contains the hair-roots, putrefies more rapidly than
the fibrous structure of the hide itself. In soaking also, use is made
of the power of putrefactive ferments to dissolve the cementing
substance of the hide, though in this case with doubtful advantage to
the tanner. In the liming process putrefaction makes itself felt when
the limes are allowed to become stale and charged with animal matter,
softening the hide and finally rendering the leather loose, empty and
inclined to “pipe.” Here the effect is in many cases useful if not
carried too far.
In bating and puering, the action is almost entirely due to the enzymes
and other products of bacterial activity, the original chemical
constituents of the dung being apparently of minor importance. Naturally
the liquid is adapted to the growth of many other organisms beside those
acting most advantageously on the hide, and injury in the bates from
wrong forms of putrefaction is very common, if indeed it is not always
present in greater or less degree.
In drenching, the effect is, at first, entirely due to the weak acids
produced by bacterial fermentation of the bran, but becomes complicated
in its later stages by putrefactive and other fermentations which may be
desirable or otherwise.
In the tanning liquors, fermentation is not so marked, but is of great
importance owing to the production of acids by bacterial action from the
sugars present in the material. The acids themselves are apt to be
fermented and destroyed, principally by the oxidising action of
_Saccharomyces mycoderma_ and the higher moulds (see p. 14), which also
act destructively on the tannins.
The effect of these acids on the hides is to swell them and to
neutralise any lime they may contain. They also give to the liquors a
characteristic sour taste, as a consequence of which, liquors containing
acetic and lactic acids are usually known in the tannery as “sour
liquors.”
It is doubtful whether the action of fungi is completely stayed even by
the drying process. The heating of leather in the sheds is due to
bacteria and the higher moulds, and Eitner considers their growth one of
the causes of the “spueing” or “gumming” of curried leathers.
From what has been said, it is obvious that, with regard to
fermentations, a double problem is presented to the leather
manufacturer, since he desires to utilise those which make for his
advantage, while controlling or destroying those which are injurious.
The first step to a solution of these problems is a more complete
knowledge of the organisms which serve or injure us, that we may, as it
were, discriminate friends and enemies. We may then approach the
question in two ways. Taking the drenching process as an example, we may
on the one hand introduce a “pure cultivation” of the right ferment into
a sterilised bran infusion, and so induce only the one fermentation
which we require; or, on the other hand, as different ferments are
affected in varying degrees by antiseptics, we may perhaps choose such
as permit the growth of the organism we want, while killing or
discouraging the rest. We may also arrange the nutriment, temperature,
degree of acidity and other conditions, so as to favour one organism
rather than another. All three methods have been applied in brewing with
good results.
CHAPTER V.
_ANTISEPTICS AND DISINFECTANTS._
“Antiseptics” are often defined as substances which check putrefaction
without necessarily destroying bacteria and their spores, while
“disinfectants” are poisonous to ferment-organisms, and actually destroy
them; great differences exist in the extent of their sterilising power,
and the whole distinction is one rather of degree than of kind, and has
little practical value. Thus common salt is incapable of _killing_ most
bacteria, even in concentrated solution, though it holds putrefaction in
check both by withdrawing water from the hide and by directly preventing
the multiplication of bacteria. If the salt be washed out of the hide,
putrefaction is at once resumed by the organisms present. Hides, on the
other hand, which have once been sterilised by powerful disinfectants,
such as phenol (“carbolic acid”) or mercuric chloride, do not again
putrefy till the organisms which are killed are replaced by fresh ones
from outside. The action of sodium sulphate, and many other salts, is
similar to common salt in this respect, while a large proportion of the
aromatic compounds are permanently disinfectant, though their efficiency
varies with the species of bacteria involved.
Biernacki and others have shown that some disinfectants when extremely
diluted actually stimulate alcoholic fermentation, and probably the
growth of other ferments, e.g. mercuric chloride 1 in 300,000, salicylic
acid 1 in 6000, and boric acid 1 in 8000, and in many cases organisms
become habituated to antiseptics in doses which would at first have
proved fatal.
The number of antiseptics available is now so great that it is
impossible to give a detailed account of all, but the following are
among those which are best known and have been practically employed.
_Lime_ possesses some antiseptic properties, and is largely used in the
preservation of fleshings before they are sent off to the glue factory.
They are most conveniently stored in a large vat filled with a strong
milk of lime. Dilute solutions of caustic alkalies have an effect
similar to that of lime.
_Common salt_, sodium chloride, NaCl, acts to a certain extent by its
solubility and a dehydrating effect on animal tissues common to
chlorides, which removes water from hides and other materials which it
is used to preserve. Probably the latter characteristic has a good deal
to do with its effect in checking the development of bacteria, since
many species thrive quite well in weak salt solutions, and some even in
brine, and the dehydrating effect of the salt enables it to harden many
animal tissues if used in sufficient quantity, the water they contain
running away in the form of brine.
Ordinary rock salt frequently contains ferric chloride, and this, either
originally present in the salt, or in some cases derived from the action
of the latter upon the iron contained in the blood, is the cause of what
is known as “salt-stains.” These show but little during the liming of
the hides, unless sulphides are used, when stains appear of a greenish
black, from the formation of sulphide of iron; when, however, the hides
come into the tanning liquors, black or blue stains are produced by the
action of the tannin, which are partially removed by the acids of the
liquors during the tanning process, but generally show to some extent in
the finished hide. There is another species of salt-stain, not
apparently due to iron, but to the colouring matter produced by some
fungoid or bacterial growth, which it is practically impossible to
remove, and which is stated to be sometimes caused by the use of old
salt with which hides have been previously salted. Iron stains are most
readily recognised by the use of a solution of potassium ferrocyanide or
thiocyanate slightly acidified by hydrochloric acid. If this be applied
to the leather, the stains will be changed from a blackish to a blue, if
the former, or a red colour if the latter salt has been used. A more
absolutely conclusive proof is to lay a piece of filter paper soaked in
dilute hydrochloric acid upon the stain, and then to test for iron upon
the paper with ferrocyanide or thiocyanate. The freedom of the paper
itself from iron must be ascertained before use. Iron-stains produced in
the salted state are more difficult to discharge than those which are
caused later in the tanning process, since iron salts have distinct
tanning power, and attach themselves firmly to the untanned fibre. On
the Continent, where common salt is heavily taxed, alum, carbolic acid,
naphthalene and other materials are frequently added to it to
“denaturise,” or render it incapable of being used as food, and these
additions are often the cause of trouble to the tanner.
_Sodium sulphate_, Na₂SO₄, has little if any disinfectant power in
dilute solution, but if used in the calcined form (anhydrous sodium
sulphate) as proposed by Eitner[7] as a substitute for common salt in
preserving hides, it withdraws water from the hide and crystallises with
10 Aq (about 56 per cent.). This does not run away like brine, but
remains in the hide, which retains its weight, and remains plump and
swells well in the limes and liquors, which chlorides have a great
tendency to prevent; 10-15 per cent. on the weight of the hide is
sufficient, while salt must be used in nearly double this quantity. Care
must be taken that the sulphate used is free from bisulphate, NaHSO₄,
which has a powerful swelling effect upon the hide-fibre, like sulphuric
acid. The neutral sulphate does not redden methyl orange or litmus.
Pickled skivers may be in part preserved by the sodium sulphate formed
by the action of sulphuric acid upon the salt employed in the pickling
bath (see p. 90).
[7] Gerber, 1880, p. 185.
_The stronger mineral acids_ have considerable antiseptic power, and are
of course especially fatal to such ferments as thrive best in alkaline
solutions. The use of sulphuric acid in pickling skivers has already
been alluded to, and a very dilute solution applied without salt to raw
hides prevents putrefaction, though the principal object in using it is
to plump the hides and produce a fictitious weight and substance which
disappear on tanning. Such hides of course have a powerful acid reaction
to litmus. Sulphuric acid in small quantities has been used with
advantage in soaking E.I. kips. A very small excess of hydrochloric acid
will sterilise putrid effluents, and no doubt nitric or sulphuric acid
would have the same effect. The powerful effect of mineral acids on
animal fibre, and their solvent action on cements and iron, preclude
however, their general use as antiseptics.
More important is the use of sulphurous acid and sulphur dioxide, which,
from their mild acidity and great antiseptic powers, are capable of a
variety of useful applications. Considerable doubt has been raised as to
the germicide power of sulphur dioxide, and it is certain that the dry
gas is less effective on dry objects than when applied in solution, or
to moist materials, as is almost invariably the case in the tannery. It
may possibly be more efficient in its action on some moulds and
putrefaction-ferments than on the pathogenic bacteria which have been
most frequently used to test the power of disinfectants; but in practice
it is found extremely useful in the brewery and in gelatine manufacture,
and there is no reason that it should be less so in the tannery.
The gas is most conveniently produced by burning sulphur, which produces
double its weight of sulphur dioxide. If used for “stoving” drying rooms
and other places infested with moulds, care must be taken to avoid risk
of fire. A shallow cast-iron pot set on bricks or sand is generally the
most suitable vessel, and the sulphur may be ignited by a piece of
red-hot iron or a rag which has been previously dipped in melted
sulphur. It is corrosive to metalwork, and bleaches many colours, but
does not produce any marked injurious effect on leather, though the
sulphuric acid formed by oxidation may, if not removed, ultimately make
it tender.
For many purposes a solution of the gas is required, and this is most
easily made by burning the sulphur in a small metal or firebrick stove
from which the fumes are sucked through a “scrubber,” which, on a small
scale, is conveniently made of large glazed sanitary pipes, packed with
coke or broken earthenware, over which water is allowed to trickle. The
lowest pipe has an opening for a branch pipe, which is connected with
the stove and rests on three bricks in a tub, which collects the acid
solution and forms a water-seal to prevent the escape of gas. Above the
inlet for the gases is fixed a wooden grating on which the coke rests.
The scrubber may be 10-15 feet in height and connected at the top with a
chimney or steam ejector to produce the draught. The arrangement is
illustrated in Fig. 5. Another method is to burn the sulphur in a closed
cylinder and to force the products through water with an air-compressor
or steam-jet injector.
In place of using a scrubber, the fumes may be blown by a steam ejector
direct into a tank. This is a very good arrangement for washing and
bleaching hair, etc., but where large quantities of solution are
required is inferior to the scrubber. Ejectors of hard lead or regulus
metal should be used, and are less acted on by the dry gases than by the
very dilute moist exhaust from the scrubber (see p. 335).
[Illustration: FIG. 5--Sulphurous acid apparatus.]
_Bisulphites_ have also strong antiseptic properties. “Bisulphite of
soda” (hydric sodic sulphite) solution may be made by supplying the
scrubber with solution of soda-ash or washing soda; bisulphite of lime,
by using milk of lime or packing the scrubber with chalk or limestone
(free from much iron) in place of the coke. In either case a much
stronger solution is obtained than with water alone.
Boakes’ “metabisulphite of soda”[8] is a very convenient source of
sulphurous acid when the latter is wanted in small quantities. It is an
anhydrosulphite, Na₂O.2(SO₂), and contains 67·4 per cent. of its weight
of SO₂. One molecule of the salt (= 190) requires one molecule of H₂SO₄
(= 98) to set free the whole of the sulphurous acid. For many purposes
the sulphate of soda formed may be neglected and the acidified solution
used direct.
[8] Patented by Boakes, Ltd., Stratford, London, E.
For analysis of sulphites and sulphurous acid solution, see L.I.L.B.,
pp. 16 and 37.
_Boric acid_, _borax_ and other _borates_ are not very powerful
disinfectants. They have no injurious action upon the skin, but to be
effective require to be employed in pretty strong solutions, say 1 per
cent., and their comparatively high cost unfits them for general use as
antiseptics in the tannery, though boric (boracic) acid is very useful
as a drenching and deliming agent (see pp. 156, 229, and L.I.L.B., p.
37).
_Mercuric chloride_, corrosive sublimate, HgCl₂, is an extremely
powerful antiseptic, preventing the growth of some species of bacteria
in solutions so dilute as 1 in 300,000 (Koch). 1 in 14,000 is
disinfectant (Miquel), but its power varies very much upon different
organisms (Jörgensen states that 1 in 400 is required to kill
_Penicillium glaucum_), and it is unsuited for most purposes in leather
manufacture, both from its extremely poisonous character, and because it
is rendered inactive by various substances present in the materials
used.
_Mercuric iodide_ dissolved in iodide of potassium solution was patented
by Messrs. Collin and Benoist as an antiseptic in tanning, but it is
ineffective for the same reasons as mercuric chloride; although under
favourable circumstances it is even more powerful than the latter.
_Copper sulphate_, _zinc chloride_ and _sulphate_, and many other
metallic salts are powerful antiseptics, but have only a limited
application in leather industries, and do not usually actually
sterilise. _Arsenic_ (arsenious acid), which has been used in curing
hides, is an excellent insecticide, but not particularly effective as an
antiseptic; and sulphide of arsenic (realgar) when used in limes (see p.
139) seems to have but little antiseptic effect. Arsenious acid is
easily soluble in alkaline solutions.
_Fluorides_ have been suggested as antiseptics in the tannery, but do
not seem of much practical value.
The most important antiseptics at present are those derived from coal
tar, and belonging to the aromatic series. Of these, the phenols
(carbolic acid, cresol, etc.) are the most used.
_Pure phenol_, “pure crystallised carbolic acid,” is hydroxybenzene
C₆H₅(OH), but the crude forms which are generally employed contain
cresols and higher members of the series in which one or more of the
atoms of hydrogen are substituted by CH₃ groups. These are oily bodies
scarcely soluble in water, and even pure phenol is only soluble in cold
water to the extent of some 7 per cent. Crude carbolic acid should not
be employed in the tannery, since the insoluble oily particles stain the
hide, and render it unsusceptible of tanning. Suitable carbolic acid
should be of a pale yellow colour when fresh (though it will darken on
exposure to air and light), and it should be wholly soluble in a
sufficient quantity of water. Its specific gravity should be 1·050 to
1·065. For methods of chemical examination, see L.I.L.B., p. 40. A
saturated solution of carbolic acid sterilises hide completely against
most putrefactive organisms, but has a sort of tanning effect, adhering
obstinately to the fibre so that it cannot be removed by washing; and
hides which have been cured with it cannot be unhaired by sweating,
though they may be limed in the usual manner, if somewhat more slowly.
Care should be taken in mixing with water or liquor, as undissolved
drops will produce the same effects as those of the crude acid. Hides
are occasionally stained, as has just been described, by salt which has
been denaturised with common sorts of carbolic acid. Eitner recommends
the use of a solution of carbolic acid in an equal weight of crude
glycerine, which readily dissolves in water, and seems to prevent any
injurious effect on the hide.
An aqueous solution containing 1 per cent. of carbolic acid is
sufficient for mere sterilising of hides, but if it be desired to
preserve them for a long period, stronger solutions (up to 4 per cent.)
may be employed.[9]
[9] Gerber, 1889, p. 98.
Quantities so small as 1 part per 1000 control the fermentation of
liquors, and prevent the formation of moulds on the surface, economising
tannin, and preserving vegetable acids already present, but at the same
time lessening their production by fermentation, and therefore sometimes
leading to difficulties in the early stages of tanning. Carbolic acid is
not, strictly speaking, an acid, but rather of the nature of an alcohol,
although it forms weak combinations with bases. It is a powerful
narcotic poison, and if dropped on the skin in a concentrated form it
produces severe burns; these are best treated with oil, while in cases
of poisoning, oil and chalk must be administered internally, but if the
quantity of carbolic acid taken has been large, are not likely to be
effective. From its cheapness and efficiency, carbolic acid is likely to
be increasingly used, although for special uses some of the newer
antiseptics have great advantages.
_Eudermin_ is a tar-oil manufactured by Speyer and Grund, of
Frankfort-on-Main, which is intended as an antiseptic addition to
stuffing greases to prevent mould and spueing. It is recommended for the
purpose by Eitner[10] and can be used in proportions such as 10 per
cent. of the grease. Creasotes and cresols can be dissolved in oils and
stuffing greases, and act as antiseptics, though less powerfully than
in aqueous solution. Rosin oils and turpentine have also antiseptic
properties.
[10] Ibid., 1893, p. 41.
_Creasote_, “heavy coal oil,” or “dead oil,” is a complex mixture of
hydrocarbons, phenols and cresols, obtained by distillation of coal tar,
heavier than water, and almost insoluble in it. It is largely used as a
preservative for timber. Carbolineum is an oil of this class, boiling at
over 300° C., and intended for application to wood. One or more coats
are applied to the dry wood at a temperature of 80° C. The workman’s
hands must be protected by gloves, as the hot creasote raises painful
blisters. Eitner[11] recommends its use for preserving pits, posts and
other woodwork in tanneries. Wood-creasote is a somewhat similar product
obtained from wood-tar.
[11] Gerber, 1889, p. 183.
The heavier cresols are so little soluble in water as to be valueless in
their ordinary form as antiseptics, but several preparations are made
under the names of “Creolin,” “Jeye’s fluid,” “Lysol,” “Izal,” “Soluble
phenyl,” etc., in which they are treated with additions of soap or
alkalies, which cause them to emulsify or dissolve in water, generally
as milky liquids. These are powerful germicides and have the advantage
over phenol of being non-poisonous. 0·1 to 0·5 per cent. solution of
creolin will sterilise hides after bating so that no putrefaction takes
place in the liquors. Mr. J. T. Wood specially recommends creolin for
the general purposes of the tannery, disinfecting pits and tubs, and for
checking the action of puers and drenches on goods which have gone a
little too far, by throwing them into a 0·2 per cent. solution.
_Salicylic acid_, orthohydroxybenzoic acid, C₆H₄OH(COOH), is now
artificially prepared from phenol. It is much less poisonous than the
latter and has no smell, which makes it valuable for certain purposes,
but is too dear for most technical applications. Many bacteria appear to
become gradually habituated to its action, and the same is true of
phenol to a less degree.
Salicylic acid is closely related to protocatechuic and gallic acids,
and, like these, gives a blackish colour with iron salts. It is freely
soluble in hot water, but very sparingly in cold. The addition of 1-2¹⁄₂
parts of sodium phosphate, sulphate, or potassium nitrate to each part
of salicylic acid greatly increases its solubility. It seems much more
powerful in preventing the development of bacteria than carbolic acid;
a solution of 1 part of salicylic acid in 666 of water is said to be
equal in this respect to 1 part of carbolic in 200.
_Benzoic acid_, C₆H₅COOH, though not much employed, except in medicine,
is a still more powerful disinfectant, and has the advantage of being
non-poisonous to human beings.
“_Cresotinic acid_,” which is derived from the cresols as salicylic acid
is derived from phenol, is more soluble than salicylic acid. It is not
very poisonous, and a powerful disinfectant. In a crude form it has been
introduced by Hauff, of Feuerbach, for bating or removing lime from
hides. This it does very well, though without the softening action of a
true bate. It has a tendency to produce a pinkish stain, and in some
degree a sort of tanning of the fibre. Its price, moreover, is rather
high for extensive technical use. (See also p. 162.)
“_Anticalcium_” is a more recent preparation introduced as a bate by the
same firm.[12] It is a solution of mixed sulphonic acids derived from
cresols, and has considerable disinfectant powers. It removes lime very
effectively, but from its acid character somewhat swells the skin. It is
used very successfully as a drench for thin skins (p. 163).
[12] Gerber, 1895, p. 133.
“C.T.” (coal-tar) bate is a grey crystalline pasty mass, with a tarry
smell, and is chemically very similar to anticalcium if not identical
with it.
_Naphthalene sulphonic acid_ has strong antiseptic properties. Its use
in bating has been patented by Burns and Cross. (See p. 163.)
_Naphthols_, C₁₀H₇(OH).--These bodies, which have the same relation to
naphthalene as the phenols to benzene, are powerful antiseptics; and
naphthalene itself appears to have antiseptic power, and is occasionally
used for denaturising salt. There are two naphthols, varying in the
position of the OH group in the molecule, and denominated α and β, of
which α naphthol is the more powerful antiseptic and the less poisonous,
though β, being cheaper, is the common commercial article. It is said
that quantities so small as 0·1-0·4 grams of α naphthol per liter are
sufficient to prevent the development of microbes, while of β naphthol
about ten times that quantity is required.
Naphthols are not very expensive, but their value is diminished by the
fact that they are insoluble in water. They are soluble in alkaline
solutions, but their compounds with bases are of much lower antiseptic
value, and the same is true of their alcoholic solutions; when an
alcoholic solution is added to water the naphthol is precipitated, but
if an addition of soap or camphor be made to the alcoholic solution, the
naphthol remains in a very finely divided condition, if not dissolved.
Adopting Eitner’s suggestion with regard to oxynaphthoic acid (see
below), hides may no doubt be sterilised by treatment first with an
alkaline naphthol solution, and then with a very dilute acid to set the
naphthol free.
“_Hydronaphthol_,” β tetra-hydro-naphthol, C₁₀H₁₂O, is obtained by the
reduction of β naphthol by sodium (Rideal). It seems to be an excellent
disinfectant.
_Oxynaphthoic acid_, α hydroxynaphthoic acid, C₁₀H₆(OH)COOH, which bears
the same relation to naphthol as salicylic acid does to phenol, is
cheaper than salicylic acid, and said to be a more powerful antiseptic.
Its salts have no antiseptic power. In its commercial form it is a
reddish crystalline powder, inodorous, but with a burning taste, and its
dust causes violent sneezing. It is scarcely soluble in water, and is
said to undergo some change on keeping which lessens its germicide
power; it is readily soluble in alcohol, and the solution produces a
milky fluid on mixture with water. Such a solution containing 15 grams
of the acid in 4 liters of water, will sterilise a hide. Eitner
recommends[13] that it should be dissolved in dilute soda solution, and
the hides, after soaking in it, passed through water slightly acidified
with hydrochloric acid, as has been suggested in the case of naphthol;
the method is also applicable to creosotinic acid, the hides being
permanently sterilised so that they cannot be unhaired by sweating,
though they will lime in the usual manner.
[13] Gerber, 1888, p. 101; 1889, pp. 99 _et seq._ See also p. 163.
_Carbon disulphide._--Moret has suggested an aqueous solution of this
compound as an antiseptic, and it seems to have considerable sterilising
powers, but from its inflammability, poisonous character, and unpleasant
smell, it is not likely to come largely into use.
_Formaldehyde_, COH₂, has recently been introduced as an antiseptic in
aqueous solution containing 40 per cent. of formaldehyde together with
a little formic acid, under the names of “formalin,” “formol,” etc. It
seems to have great disinfectant powers, and may possibly be valuable in
various processes of leather manufacture as it becomes cheaper, but has
a curious hardening tanning effect on hide fibre and gelatinous matters,
so that in very dilute solution it will produce leather.[14] The vapour
of formaldehyde, or of its condensation-product paraform, may be
employed to harden microscopic preparations. 1 part of formaldehyde, and
consequently 2¹⁄₂ parts of “formalin” in 12,000 parts of water, is said
to sterilise, and this proportion would form a good disinfectant
solution. Even in considerably larger proportion than the above, it does
not appear to be poisonous, and thus possesses the bactericidal power of
sublimate without the latter’s poisonous properties. Formaldehyde has
another advantage over most, if not all other antiseptics, in that it
may be used as well in the gaseous as in the liquid state, and on that
account it is largely employed in the disinfection of rooms or of
articles which would be spoiled if they were to be wetted, as the
gaseous formaldehyde, though thoroughly disinfecting them, will not
injure the colours of materials of the most delicate fabrics.
[14] Gerber, 1897, p. 67; ibid., 1899, pp. 101, 205, 218.
On account of its capability of rendering gelatinous matters hard and
insoluble in water, formaldehyde requires to be employed with great
care, but 0·2-0·3 per cent. may be successfully used in admixture with
egg-albumen in the preparation of “seasoning” in the finishing of
morocco leather. It is also used commercially to produce different
varieties of white leather for soldiers’ accoutrements and similar
purposes (p. 380).
_Triformol_ (tri-oxymethylene, “paraform”) is a product of the
polymerisation of formaldehyde, and is prepared by evaporating a
solution of the latter to dryness on the water-bath. It is said to be
more powerful than formalin in its antiseptic properties, but has not
entered very largely into use as a disinfectant, though considerable use
is made of it to “fix” bacteria in gelatin for bacteriological purposes.
_Camphor and essential oils_, as well as oil of turpentine, have
considerable antiseptic powers, and the cheaper essential oils such as
those of winter-green, black birch, sassafras and aniseed are frequently
employed, especially in America, in preserving pastes, finishes and
seasonings, and at the same time covering offensive odours. The odour of
essential oils becomes much more powerful as they are diluted, and very
small quantities suffice for the purposes mentioned. Birch-tar oil, such
as is used to give the scent to Russian leather (p. 372), has
considerable antiseptic effect.
CHAPTER VI.
_THE ORIGIN AND CURING OF HIDES AND SKINS._
A considerable proportion of the hides and skins used in leather
manufacture are those of animals killed by the butcher for food, and
these are frequently employed by the tanner without any preliminary
curing. Domestic hides and skins are now generally sold by auction in
weekly markets in the principal towns, after sorting and classification
in weight and quality.[15] This is in many respects an improvement on
the old method of purchase direct from the butcher, but it often leads
to delay in delivery, and in hot weather hides suffer from putrefaction.
In most cases, the damage is not sufficient seriously to affect the
durability of the leather, but the delicate membrane of the “grain” is
injured, and the hide or skin unfitted for coloured leather, or any
purpose where small damages to appearance are important. Butchers are
adverse to the use of salt, because it withdraws water from the hide in
the form of brine, and so causes it to lose weight; but much injury
would be saved by a light salting, and all hides or skins on which the
hair is “slipping” should be regarded as damaged for fine leather
manufacture.
[15] The weight of English market-hides as credited to the butcher is
usually marked on the edge of the butt near the tail, by cuts with a
knife, the mode of numeration being sufficiently explained by Fig. 6,
in which cuts crossing the horizontal line each represent 20 lb., that
above it 10 lb., while less amounts are expressed in Roman figures.
On the Continent weights are usually given in pounds of half a
kilogramme (50 kilos = 110 lb. English). In Paris the marking is on
the tail, and is also shown on Fig. 6.
[Illustration: FIG. 6.--Method of marking weight on hides; 97 lb.]
Sheep-skins are not usually bought direct by the tanner, but by the
fellmonger, who removes the wool; and as this is usually of much greater
value than the skin, the latter is frequently handled very carelessly,
and its quality sacrificed for the sake of real or fancied improvement
to the wool. In very many cases the skin is “sweated” or “staled” by
hanging in a warm and moist chamber, heavily charged with ammonia
derived from the putrefaction of the skin, until the wool is
sufficiently loosened to be “pulled.” If this treatment is conducted
with extreme care the skin may escape serious injury, but in most cases
the grain is weakened, and the foundation is laid of damage, which makes
itself felt throughout the tanning process. For the purposes of the
tanner, a much better way is to lime the skins by painting with thick
limewash on the flesh-side, and after folding the skins down the back,
flesh-side in, to prevent as much as possible the access of the lime to
the wool, to place them in a pit, and cover them with water, till the
wool is loosened by the penetration of the lime through the skin. A
still more satisfactory method, and one which is in general use in the
American stockyards, and to some extent also in Europe, is to wash the
skins in water to free them from blood and dirt, and then, laying them
in a wet condition, flesh side up, to paint them with a solution
containing about 25 per cent. of sulphide of sodium, thickened with
lime. The skins, as they are painted, are doubled down the back,
flesh-side in, and laid on a floor, overlapping each other like tiles on
a roof, for some hours, or overnight, till the wool is sufficiently
loosened to pull, after which the pelts are limed and treated in the
ordinary way. As a general rule the English fellmonger keeps his skins
in lime till they are sold to the tanner, and as in small yards some
time is taken to accumulate a parcel, the earlier skins may suffer great
injury from overliming. Even sweet fresh limes dissolve the cementing
substance of the fibre, and increase the naturally loose texture of the
sheep-skin, but the injury is much more considerable when old and stale
limes, charged with ammonia and bacterial products, are employed, as is
frequently the case. In the American stockyards the skins are generally
limed only for the necessary time to act upon the grease, and to swell
and differentiate the fibres, and are then at once puered, drenched and
preserved by “pickling.” For details of “pickling” see p. 89. It is very
probable that the Pullman process of liming (p. 137) would answer well
for fellmongered skins, as goods will keep for a considerable length of
time uninjured after treatment with calcium chloride.
Where hides or skins cannot be used at once in the fresh state, there is
probably no better method of preserving them than the use of salt.
Although salt is not fatal to bacteria, it so slows bacterial growth,
partly by its direct antiseptic effect on many organisms, and partly by
withdrawing water from the skin, that well-salted skins can be kept in
good condition for almost an unlimited time. Where it is only required
to preserve goods for a week or two, a moderate sprinkling on the flesh
side is efficient, but if they are to be preserved for any length of
time, more thorough treatment is necessary. It is said that however
carefully hides are salted they deteriorate if kept in this condition
above twelve months.
The method of salting employed in the Chicago stockyards for “packer”
hides may be taken as a good type of a thorough salting. The hides are
first trimmed from useless “switches,” and any large portions of
adhering fat are removed. The curing takes place in large and cool
cellars, with concrete floors. The detail is well given in the following
extract from the ‘Shoe and Leather Reporter’:--
“Great care is taken to make the sides of a pack higher than the middle,
so that the brine which is made by the juices of the hide coming in
contact with the salt will be retained. The brine can only escape by
percolation and hence the fibre of the hides is thoroughly cured. The
floor of a hide cellar is usually of concrete, and a pack is from 15 to
20 feet long and as wide as the space between the posts which support
the floor above. The sides of a pack are built first to a height of from
4 to 6 inches; the cross layers are then put on, generally three on each
side, two being inside and one having the butts drawn out to the edge.
In a pack 20 feet long, the side layers will contain about 25
medium-sized hides each, and a cross-layer 12 or 14. To begin a pack a
truck-load of hides is run along to the front of the place selected,
one spreader grasps the butt and his partner the head of a hide, and
together they carry it to what is to be the rear of the bed. The hide is
then dropped, so that the folded back is parallel to and from 15 to 20
inches from the inside line of the posts, the head a trifle closer than
the butt. The front man takes the dewlap and front shank in his left
hand, and extends his right along the belly of the hide as far as is
necessary to raise the edge, the rear man holding the flank with one
hand and the hind shank with the other. They keep their legs well out of
the way of the salt thrower, who with a single throw covers the whole
hide, being particular that enough salt strikes against the edges held
by the men to make a pronounced ridge when they are lapped down. A
little salt is thrown on the hair surface and the butt folded over about
a foot. The folded edge is then drawn out even with the outer line of
the pack. More hides are placed the same way until the corner is high
enough. After this, each hide is put further forward to make a level
surface from rear to front, the heads at the front corner being folded
back as the butts were at the starting place. The other side is built
the same way, and then the cross layers are put on alternately until the
pack is level, when sides are again built as before. In putting on the
first hides of the cross layers, they are thrown over the edge, to lap
back again when the salt is thrown on; the layer is then continued on to
the front. The spreader who holds the butt does the guiding in every
case. He drops the butt down at exactly the proper place, takes the
upper flank and shank in each hand, sets one foot on the lower shank to
keep it firm, and throws the one in his hands from him with considerable
force. The man at the head watches his partner, keeps the folded hide
taut, and drops it at the same time as the latter. He takes the
fore-shank at the knee in the one hand and the upper head-piece in the
other, and setting his foot on the lower side, throws the upper side
forward simultaneously with the rear man. Two expert spreaders,
accustomed to working together, spread a hide at a single throw, but
some little straightening has to be done by hand before the hide is
ready for the salt. A gang composed of two spreaders, one salt thrower
and a salt trucker put down forty hides an hour. When gangs are doubled,
two men do all the spreading; the other two place the hides where they
can be got at conveniently. A double gang put down eighty hides an
hour. The salt trucker brings the salt to the pack in box-trucks open at
one end to permit the entrance of a shovel. The salt thrower keeps the
edges and corners of the pack full of salt. He must see that every part
of the flesh-side is well covered. Each hide takes two scoop shovelfuls
of ground rock or coarse white salt, mixed with an equal quantity of old
or second salt. The salt thrower throws the shovel forward and to one
side and back again with a peculiar swinging jerk, causing the salt to
fall regularly over the entire surface of the hide. The ease and
rapidity with which a gang operates depends greatly upon the efficiency
of the salt thrower. When the pack gets too high to be comfortable for
the men, it is brought to a dead level and covered over with clean salt.
It then presents a very neat and workmanlike appearance. Spreaders and
salt throwers receive 20 cents an hour, and truckers get 17¹⁄₂ cents.
When the temperature is kept at an even average, two weeks is ample time
to cure the hides.
“In ‘taking up,’ two men strip the hides from the pack. As they were put
down from the rear to the front, they are taken up in the reverse
direction. No matter how much loose salt is lying on the top, the man
knows exactly where to place his hand on a shank; as the hides are moved
forward, the loose salt is thrown off toward the front. One man takes
away the salt as it accumulates and trucks it to the salt bins, where it
is mixed with new, to be used again. A ‘horse’ made of a network of
scantling about 3¹⁄₂ feet wide by 6 feet long, and standing 2¹⁄₂ feet
from the floor, is placed in front of the pack, on this the hides, flesh
side down, are shaken to remove the salt that is clinging to them. This
process requires four men, one at each corner. The hide is brought down
heavily on the horse twice, and then spread on the floor flesh side up
for examination by the inspectors, of which there are two, one
representing the house and the other the buyer of the hides. They sweep
off any salt that may be left, and examine for cuts, sores, brands,
manure and grubs. They also see that the hide is properly weighed and
classified. If the contract calls for a special trim it is now done. Two
men then roll the hide, beginning by lapping over the shanks, head and
neck. Then the sides are folded over and lapped again, leaving the roll
15 to 18 inches wide. The ends are thrown inward, slightly overlapping
each other; a final fold is then given, and the hide is ready to be
tied. Rope the size of clothesline is used for tying, and is cut into
lengths of about seven feet. It takes three men to tie for a gang such
as we have described. After tying, the neat bundles are weighed and
loaded on the cars for shipment. A small tare is allowed the buyer.
Ordinary workmen in hide cellars get 17¹⁄₂ cents an hour, and inspectors
25 cents an hour.”
About 25 per cent. of salt on the green weight of the hide is required
for thorough curing. Rock salt merely crushed is frequently employed,
but this is very liable to contain iron in the form of oxide and
chloride, which causes the peculiar marbled markings known as
“salt-stains.” It is therefore much better to use a white crystallised
salt, though it is possible even in this case that stains may arise from
the iron present in the blood. Some salt-stains appear also to be due to
the action of pigment-bacteria, and not to contain iron. A reddening of
the flesh side is often noticed in hides which have been kept in salt
long or under unsatisfactory conditions, and is very frequent in
wet-salted South American hides. Such hides are said never to produce so
firm a leather as those which are sound.
Hides are not unfrequently cured by steeping in salt brine, instead of
strewing with dry salt. This method is principally resorted to in order
to give fictitious weight. Brined hides do not plump well in tanning,
the leather is not so good in quality as from those salted with dry
salt, and the cure is much less efficient.
Many hides are not only salted but also dried in order to preserve them.
Not much detail has been published with regard to the methods used,
which no doubt vary much in different places, but probably in some cases
the hides are salted in pile and in others by brining, and then hung up
to dry. The principal object of this drying is to economise weight and
cost of transport, but it makes the hides much more difficult to wash
and soften for tanning, and probably the crystallisation of the salt has
a weakening effect on the fibre. Hides cured in this way are styled “dry
salted.”
A large number of the hides of the small native cattle of India are
imported into this country in a dry-salted condition. The following
particulars of their cure are taken from a paper by the Author and Mr.
W. Towse.[16]
[16] Journ. Soc. Ch. Ind., 1895, p. 1025.
Dry-salted, or, as they are commonly called “plaster cures,” such as
those of Dacca and Mehapore, are thickly coated with a white material,
which in the first instance is merely the insoluble portion of a saline
earth used in the cure; though in many cases it is applied in larger
quantities than necessary, with the simple object of giving weight. The
salting is thus described by Mr. W. G. Evans, who some years since had
considerable experience as a tanner at Cawnpore:--
“The salt used by the natives is a salt-earth; and is so called by them.
It is found extensively in the districts of Cawnpore, Agra, Delhi,
Lucknow, Patna, etc., and has no doubt something to do with the
localisation of the hide-curing and kindred industries in these places.
The mode of procedure used is pretty much as follows:--the salt-earth is
mixed into a very thin paste, and this is lightly brushed on to the
flesh side one day, and the hide allowed to remain over night under
cover. Next day, for best hides, the same solution is again spread on
the flesh side of the outstretched hide and rubbed into it with a porous
brick, and then for legitimate salting, the hide is allowed to dry under
cover. If for export, the saltings may be three or four, and the hides
are treated out in the open, subject to the intense heat of the sun;
which accounts for the number of hides which go back in the soaks in
England and elsewhere.”
“We had a clause in our agreement with hide-factors, that any hides
which did not come down to natural suppleness in two days in clean water
were to be returned. Of arsenic curing I know nothing, and it is not so
much in vogue as formerly. There is quite a trade in Cawnpore, Lucknow,
Allahabad, etc., in treating old and inferior hides with new for export,
and great efforts are made by native holders to get their stocks down
before the rains commence, as they say, and rightly I think, that hides
are not worth so much after the rains by 30 per cent. The peculiar
latent moisture of the rains affects them very detrimentally.”
Under certain circumstances this mode of cure gives rise to extensive
iron-staining of the skins, and analyses of the material scraped off
Dacca and Mehapore kips were undertaken with a view to elucidating the
causes of this injury. The following are the results of the analyses
referred to, which were made upon the residue after the rather
considerable quantity of fibrous organic matter, which had been scraped
off with the cure, had been destroyed by ignition, together no doubt
with traces of ammoniacal salts:--
------+---------------+------------+------------+------
| ---- | Dacca. | Mehapore. |
| |Entire Cure.|Entire Cure.|
|---------------+------------+------------+------
| | | |
|Sand and silica| 20·55 | 27·38 |
|Fe₂O₃ | 2·77 | 1·86 |
|Al₂O₃ | 2·48 | 2·74 |
|Mn₃O₄ | 0·60 | 0·40 |
|CaO | 2·60 | 3·70 |
|MgO | 3·38 | 3·69 |
|Na₂O | 28·97 | 26·80 |
|SO₃ | 38·90 | 33·75 |
|Cl | 0·22 | 0·18 |
|H₃PO₄ and CO₂ | Traces | Traces |
| +------------+------------+
| | 100·47 | 100·50 |
------+---------------+------------+------------+------
The soluble salts of the Dacca cure were also analysed separately with
the following result:--
CaO 0·70
MgO 0·60
Na₂O 29·00
SO₃ 37·90
Cl ·22
Insoluble 32·12
------
100·54
It thus consisted exclusively of sulphates, with the exception of a
trace of chloride. The cures, after ignition, were both neutral to
phenolphthalein, but before ignition the Dacca was distinctly alkaline,
in consequence probably of the presence of ammonium salts, and both
showed considerably larger traces of carbonates before than after.
The most striking feature of these analyses is the absence of more than
the smallest traces of chlorides. The cures are thus practically free
from common salt, and owe their antiseptic power to the sodium sulphate
which they contain, and which indeed forms their principal constituent.
Nitrates appear to be entirely absent. Sodium sulphate sometimes forms
large crystals in pits used for soaking these kips.
The iron-staining of hides which has been mentioned appears to result
only when the hides after cure are exposed for a lengthened period to a
moist atmosphere, in which the carbonic acid present probably also plays
its part, the iron passing into solution as hydric carbonate.
The analyses show a striking resemblance to those of the soda deposits
of Wyoming, given by Dr. Attfield,[17] except that their percentage of
sodium carbonate is smaller, which is quite intelligible in the light of
Mr. Brunner’s abstract on the ‘Probable origin of natural deposits of
sodium carbonate,’[18] which supports the view that the sodium carbonate
is derived from sodium sulphate by the reducing and carbonating action
of low organisms.
[17] Journ. Soc. Ch. Ind., 1895, p. 4.
[18] Ibid., 1893, p. 116.
It may be noted here that the preservative properties of sodium sulphate
are well known, and the anhydrous sulphate has been recommended as a
substitute for common salt (see p. 23).
Drying is a very common method of preserving hides as well as other
putrescible matters. It has no effect in killing bacteria, but
putrefaction can only go on in presence of a considerable amount of
moisture. As applied to hides, it is, to the tanner, one of the least
satisfactory modes of cure, involving very considerable difficulties in
bringing hides back to the moist and swollen condition which is
necessary at the outset of his operations, but it is the only practical
method in districts far from the coast and with primitive modes of
transit, both on account of the cost of salt, and the lessened weight of
the dried hide. Great differences are found in the ease with which dried
hides soften, according to the way in which the drying has been
accomplished, the difficulty being greater the higher the temperature
which has been used (see p. 111). The best mode of drying is to hang in
the shade in a good draught of cool air, with the flesh side out. Hides
or skins dried in a tropical sun are not only difficult to soften, but
are liable to damaged portions, which either refuse to soften, or
blister and go to pieces in liming, owing to the structure of the hide
being destroyed by heat, the outer surface drying first and forming an
impervious layer which hinders evaporation from the inside, so that the
moist interior becomes melted, while the outside appears quite sound.
Such injuries are often only to be discovered by soaking and liming.
Very similar damage may occur from putrefaction of the interior after
the outside has become dry, and to get good results, the drying must be
gradual, but rapid, especially in hot climates. South American hides are
mostly dried in the sun, suspended by head and tail from stakes, with
the hair side out.
The risk of injury by putrefaction during drying is diminished by the
use of antiseptics. Solutions of arsenic have been frequently used for
this purpose, and many of the dried Indian kips are of what are known as
“arsenic cures,” although the writer has never been able to detect
arsenic in any which he has examined, and its use seems by no means
general. The arsenious acid is usually dissolved in soda solutions.
Unless used pretty freely it has little antiseptic effect, but is useful
in preventing the attacks of insects, which are often very destructive.
The larva of a small beetle, _Dermestes vulpinus_, frequently devours
the whole tissue of patches of the hide, leaving only the epidermis.
It may be well here to say a few words about the injuries and defects to
which hides and skins are liable, although some of them are not strictly
due to the cure. The most serious, and yet preventable injury is that
due to butchers’ cuts. As the value of the hide bears only a small
proportion to that of the meat, many butchers do their work extremely
carelessly, and this is encouraged by the loose classification of
“damaged hides” in some markets. There is also an idea that the
appearance of the meat is improved by a thin layer of the white
skin-tissue being left on it, and for this reason as well as mere
carelessness, butchers frequently score the flanks of the hide with
shallow cuts which greatly diminish its value. The “packer hides” of the
United States, and the products of the large saladeros or slaughtering
(“salting”) establishments of South America, such as Liebig’s, show what
can be done by skilled work in this respect. In the United States, much
of the flaying is done by means of a wooden cleaver, instead of a sharp
knife. Another method to some extent in use, and which may be
recommended for calf and sheep skins, is to inflate the carcase before
skinning, with air from a compressing syringe, which tears the
connecting tissue between the skin and the body, and renders flaying
much easier.
Brands are a great source of damage to hides, but where cattle roam at
large on unfenced plains, as on the prairies of Texas and the Pampas of
South America, it seems indispensable for the recognition of ownership;
no other mode of marking being sufficiently permanent and conspicuous.
It is unfortunate, that as the animals crowd together, and cannot be
closely approached, it is necessary that the brands should not only be
large, but placed on the most valuable part of the hide. Generally on
the Pampas an effort is made to keep them on one side only, so that in
South American hides it is possible to select clear and branded sides.
In the United States much land is now fenced with barbed wire, which
while it obviates the necessity of branding, introduces another evil in
the form of “barbed wire scratches,” which are frequently troublesome in
“packer hides.”
[Illustration: FIG. 7.--_Hypoderma bovis._ 1, egg; 2, maggot; 4,
chrysalis case; 6, fly, magnified (Brauer); 3, 5, chrysalis and fly,
natural size (B. Clark).]
[Illustration: FIG. 8.--Sac of warble, showing growth of epidermis round
aperture.]
In countries where cattle are used for draught purposes, goadmarks are a
frequent source of injury, and some of the large cattle-ticks do
considerable damage to the hides of Spain and South America. From the
tanners’ point of view, however, the most injurious insects are the
“bot-flies” or “warble-flies” (_Hypoderma bovis_ and allied species,
Fig. 7). There is still some controversy as to how the eggs of these
insects are deposited. In the horse-bot fly it is known that the eggs,
first deposited on the skin, are licked off and swallowed by the animal,
and develop in the stomach, where they pass their larval and pupal life
hanging on to its interior coats, and only drop off and are passed out
with the dung before their final change to the complete fly. Fortified
by this, and by some direct observation, some American naturalists are
of opinion that the American species at least, hatches in the stomach,
and as a minute larva wanders through all the intervening tissues till
it reaches the skin, where it undergoes its further development. The
late Miss Ormerod, who has made a careful study of the English
species,[19] states that the egg hatches on the hair, and that the
_larva_ simply eats its way below the skin, leaving a minute red
puncture which it subsequently enlarges to obtain air for its spiracles,
which are in the tail. As it grows it continues to irritate the lower
part of the cavity with hooked mandibles, and lives on the pus and
matter so produced. It grows to a length of fully ³⁄₄ inch, and the
cavity, Fig. 8, situated between the skin and the subcutaneous tissue is
often as large as half a walnut. It remains in the sac not only during
its larval, but its pupal stage, which do not differ much in appearance,
and falls out on the ground before complete development. In small
numbers, the warble seems to do little injury to the general health of
the animal, but cases have been known where animals have actually died
of the inflammation produced. Some idea of the extent of the plague may
be realised from the statement that an Indian kip in the possession of
the writer has not less than 680 warble holes, and that almost equal
numbers have been counted in English hides. Preventive measures are the
sheltering of the cattle during the summer months when the fly is most
prevalent; the application of mixtures of oil or grease with tar-oil and
sulphur to the hair, to prevent egg-laying; and the destruction of the
_larva_ in its early stages, in autumn and winter by smearing the
breathing aperture with grease, or better, with mercurial ointment.
When this is done sufficiently early, the hole heals up without
permanent injury, but when it is allowed to remain open during the
period of growth, its sides become partially coated by the growth of
epidermis, and this permanently prevents their proper union by
skin-tissue. It is believed that if the _larvæ_ were systematically
destroyed in a district, they would soon become extinct, as they are not
supposed to travel far.
[19] ‘Some Observations on the Œstridæ,’ E. A. Ormerod, Simpkin and
Marshall, London, 1884, price 4_d._
A very troublesome injury to the skins of lambs and sheep is the disease
known as “cockle,” in which the skin becomes thickly dotted with spots
of thickened tissue, which bear some fanciful resemblance in form to a
cockleshell. The affection is prevalent during the spring while the wool
is thick, and disappears almost immediately on shearing, but little is
known of its causes or mode of prevention.
Climate and breed have a considerable effect on the quality of hides and
skins. As a rule the less highly bred races, and those which are most
exposed to the extremes of weather, have the thickest hides, and in most
cases highly bred animals have had their meat-producing, or in the case
of sheep, their wool-bearing qualities developed at the expense of the
characteristics most valued by the tanner.
CHAPTER VII.
_STRUCTURE AND GROWTH OF SKIN._
Although, at first sight, the skins of different animals appear to have
little in common, a closer examination shows that all the Mammalia
possess skins which have the same general structure, and thus an
anatomical description of the skin of an ox applies almost equally to
that of a sheep, goat, or calf, though on account of the difference in
texture and thickness the practical uses of these various materials may
differ widely. The skins of lizards, alligators, fishes and serpents
differ from those of the higher animals, chiefly in having considerable
modifications in the epidermis, so that it becomes harder and forms
“scales,” and the arrangement of the fibres presents considerable
difference. In many fish-skins for instance, the fibres are in
successive layers, at right angles to each other and diagonal to the
skin, but not interlaced.
In its natural condition, the skin is not merely a covering for the
animal, but at the same time an organ of sense and of secretion, and
hence its structure is somewhat complicated. It consists of two
principal layers, the _epidermis_ (_epithelium_, cuticle) and the
_corium_ (_derma_, _cutis_ or true skin). These are totally distinct,
not only in structure and functions, but in their origin. In the egg of
a bird and the _ovum_ of a higher animal, the living germ consists of a
single cell, which, as soon as fertilised, begins to multiply by
repeated division. The mass of cells thus formed early differentiates
into three distinct layers, from the upper of which the epithelium
arises, while the true skin, together with the bones and cartilages, is
derived from the middle one.
This distinction of origin corresponds with a wide difference of both
anatomical and chemical characteristics. A diagrammatic section of
calf-skin is shown in Fig. 9, and a more correct representation of its
actual appearance is given in Plate I. (Frontispiece). The _epidermis_
is very thin as compared with the true skin which it covers, and is
entirely removed preparatory to tanning; it nevertheless possesses
important functions. It is shown in Fig. 10 at _a_ and _b_, more highly
magnified. Its inner mucous layer _b_, the _rete malpighi_, which rests
upon the true skin _c_, is soft, and composed of living nucleated cells,
which multiply by division and form cell-walls of keratin. These are
elongated in the deeper layers, and gradually become flattened as they
approach the surface, where they dry up, and form the horny layer _a_.
This last is being constantly worn away, thrown off as dead scales of
skin, and as constantly renewed from below, by the multiplication of the
cells. It is from the epithelial layer that the hair, as well as the
sweat and fat-glands, are developed.
[Illustration: FIG. 9.--Vertical section of calf-skin, magnified about
50 diameters. _a_, epidermis; _b_, grain or papillary layer; _c_,
fibrous layer of skin; _d_, hairs; _e_, fat-glands; _f_, sweat-glands;
_g_, opening of ducts of sweat-glands; _h_, hair-muscles.]
Each hair is surrounded by a sheath which is continuous with the
epidermis, and into which the young hair usually grows as the old one
falls out. The hair itself is covered with a layer of overlapping
scales, like the slates on a roof, but of irregular form. These give it
a serrated outline at the sides, and when strongly developed as in wool
and some furs, confer the property of felting. Within these scales,
which are called the “hair cuticle,” is a fibrous substance which forms
the body of the hair; and sometimes but not always, there is also a
central and cellular pith, which under the microscope frequently appears
black and opaque, from the optical effect of imprisoned air. On boiling
or long soaking in water, alcohol, or turpentine, the air-spaces become
saturated with the liquid, and then appear transparent.
[Illustration: FIG. 10.--Epidermis layer.]
The fibrous part of the hair is made up of long spindle-shaped cells,
and contains the pigment which gives the hair its colour. The hair of
the deer differs from that of most other animals in being almost wholly
formed of polygonal cells, which, in white hairs, are usually filled
with air. In dark hairs, both the hair and sheath are strongly
pigmented, but the hair is much the most so, and hence the bulb has
usually a distinct dark form. The dark-haired portions of a hide from
which the hair has been removed by liming still remain coloured by the
pigmented cells of the hair-sheaths, which can only be completely
removed by “bating and scudding.”
[Illustration: FIG. 11.--_a_, sebaceous gland; _b_, hair; _c_, erector
muscle. Mag. 200.]
[Illustration: FIG. 12.--_a_, hair; _b_, hair cuticle; _c_, inner
root-sheath; _d_, outer root-sheath; _e_, dermic coat of hair-sheath;
_f_, origin of inner sheath; _g_, bulb; _h_, hair-papilla.]
Near the opening of the hair-sheath to the surface of the skin the ducts
of the sebaceous or fat-glands pass into the sheath and secrete a sort
of oil to lubricate the hair. The glands themselves are formed of large
nucleated cells arranged somewhat like a bunch of grapes; the upper and
more central ones being highly charged with fatty matter. Their
appearance is shown in Fig. 11. The base of the hair is a bulb,
enclosing the hair papilla _h_ (Fig. 12), which is a projecting knob of
the true skin and which by means of the blood-vessels contained in it
supplies nourishment to the hair. The hair-bulb is composed of round
soft cells, which multiply rapidly, and pressing upwards through the
hair-sheath, become hardened, thus increasing the length of the hair.
The cells outside the bulb, shown at _f_ in Fig. 12, pass upwards as
they grow, and form a coating around the hair, known as the “inner
root-sheath.”
In embryonic development, a small knob of cells forms on the under side
of the _epidermis_, over a knot of capillary blood-vessels in the
_corium_, and enlarges and sinks deeper into the latter, while the
root-bulb of the young hair is formed within it, surrounding the
capillaries from which it derives nourishment, and which form the
hair-papilla, Fig. 13. In the renewal of hair in the adult animal the
process is very similar. The bulb of the old hair withers, and the hair
falls out, and in the meantime a thickening takes place in the epidermal
coating of the bottom of the sheath, and the young hair is formed below,
and usually to one side of the old one, growing into the sheath, and
taking the place of the old hair. This is one cause of the difficulty of
removing ground-hairs in the process of unhairing, since they are not
only short, but deeper seated than the old ones.
The process of development of the sudoriferous or sweat-glands is very
similar to that of the hairs. They consist of more or less convoluted
tubes with walls formed of longitudinal fibres of connective tissue of
the _corium_, lined with a single layer of large nucleated cells, which
secrete the perspiration. The ducts, which are exceedingly narrow, and
with walls of nucleated cells like those of the outer hair-sheaths,
sometimes open directly through the epidermis, but more frequently into
the orifice of a hair-sheath, just at the surface of the skin. Each hair
is provided with a slanting muscle called the _arrector_ or _erector
pili_ (see Fig. 11), which is contracted by cold or fear, and causes the
hair to “bristle,” or stand on end; by forcing up the attached skin, it
produces the effect known as “goose-skin.” The muscle, which is of the
unstriped or involuntary kind, passes from near the hair-bulb to the
epidermis, and just under the sebaceous glands, which it compresses when
it contracts.
Beside the hair, and hair-sheaths, and the sebaceous and sudoriferous
glands, the _epidermis_ layer produces other structures of a horny
character, including horns, hoofs, claws and finger-nails; which both
chemically and anatomically are analogous to exaggerated hairs, such as
the quills of the porcupine.
The whole of the epidermis, together with the hairs, is separated from
the _corium_ by an exceedingly fine membrane, called the hyaline or
glassy layer. This forms the very thin buff-coloured “grain-” surface of
tanned leather, which is evidently of different structure from the rest
of the _corium_, since, if it gets scraped off before tanning, the
exposed portion of the underlying skin remains nearly white, instead of
colouring. The whole of the hair-sheath is enclosed in a coating of
elastic and connective-tissue fibres, which are supplied with nerves and
blood-vessels, and form part of the _corium_.
[Illustration: FIG. 13.--Development of young hair.]
[Illustration: FIG. 14.--Connective-tissue fibres. (Ranvier.)]
The structure of the corium or true skin is quite different from that of
the epidermis which has just been described, as it is principally
composed of interlacing bundles of white fibres, of the kind known as
“connective tissue” (see Fig. 14); these are composed of fibrils of
extreme fineness, cemented together by a substance somewhat more soluble
than the fibres themselves. The fibres are not themselves living cells,
but are apparently produced by narrow spindle-shaped cells lying against
them. The felted fibre-bundles are more loosely interwoven in the middle
portion of the skin, but become compacter again near the flesh. In the
case of sheep-skins this is especially marked, the middle part being
full of fat-cells and very loose. Any ill treatment of the pelt during
the wet-work is liable to still further loosen this middle layer so that
grain and flesh may sometimes be torn apart. The flesh-splits of
sheep-skins must have this loose fatty layer frized off before
chamoising, and American “waxed fleshes” from ox-hides are levelled by
splitting away this portion, and finished on the flesh. The outermost
layer, just beneath the epidermis, is exceedingly close and compact, the
fibre-bundles that run into it being separated into their elementary
fibrils, which are so interlaced that they can scarcely be recognised.
This is the _pars papillaris_, and forms the lighter-coloured layer,
called (together with its very fine outer coating) the “grain” of
leather. It is in this part that the fat-glands are embedded, while the
hair-roots and sweat-glands pass through it into the looser tissue
beneath. It receives its name from the small projections or _papillæ_,
with which its outer surface is studded, and which form the
characteristic grain of the various kinds of skin.[20] (See Fig. 9 and
Plate I.)
[20] It will be noted that the word “grain” is used by the tanner in
at least three different senses, which are productive of much
confusion. The extremely thin hyaline layer forms a natural glaze to
the skin, and might well be spoken of as such; the form and
arrangement of the _papillæ_ and hair-pores might be called the
“pattern” of the grain, leaving the use of the word “grain” itself
restricted to the _pars papillaris_.
[Illustration: FIG. 15.--Fat-cells in connective tissue. _a_,
fat-globule; _p_, protoplasm; _n_, nucleus; _m_, cell-wall. (Ranvier.)]
The study of the structure of the grain, and especially of the
arrangement of the hair-pores is very important, as it is usually the
readiest means of identifying the kind of skin of which a leather is
made, which in finished skins with artificially printed grain is often
very difficult. (Plate II.) The examination is facilitated by wetting
and stretching the skin, and by the use of a good lens, or a low power
of the microscope.[21]
[21] Under the microscope, the skin is of course lighted from above by
direct light from a window, or by that of a lamp concentrated by a
“bullseye” condenser. The reversal of the image in the microscope
often causes a pseudoscopic effect very puzzling to the beginner,
prominences appearing as hollows, and _vice versa_ till the real
direction of the lighting is considered.
[Illustration: PLATE II.
PHOTO-MICROGRAPHS OF GRAIN OF VARIOUS SKINS (_A. Seymour-Jones_).
1. Cow-hide; 2. Calf-skin; 3. East India Goat; 4. Pig-skin; 5. East
India Sheep; 6. Welsh Sheep.
[_Face p. 52._]
[Illustration: FIG. 16.--Striped, or voluntary muscular fibre.
(Ranvier.)]
As stated above, the surface of skin which is next to the flesh is
firmer than that in the centre, and as the fibres run nearly parallel
with the surface it has a more or less membranous character. The skin is
united to the body of the animal by a network of connective tissue
(_panniculus adiposus_), which is frequently full of fat-cells and is
then called adipose tissue. This constitutes the whitish layer which is
removed, together with portions of actual flesh, in the operation of
“fleshing.” If a minute portion of adipose tissue be examined
microscopically, it will appear to consist of a mere mass of
fat-globules entangled in connective tissue. If, however, it be stained
with carmine or logwood it may be at once observed that each globule is
contained in a cell, of which the nucleated protoplasm, by which the fat
was secreted, is pressed closely against the wall (Fig. 15). Similar
cells are contained in considerable quantities throughout the hide, and
especially in the loose tissue of the central part; hence in leather
manufacture it is impossible to expel or wash out the fat until the
cells have been broken down by “liming” or in some other way.
Many animals (ox, horse, etc.) possess a thin layer of voluntary muscle
(red flesh) spread over the inner side of the skin, and used for
twitching to drive off flies. In rough fleshing this is sometimes left
on and may be a cause of dark flesh in sole leather. Even in the
finished leather its striped structure may be detected microscopically
(Fig. 16).
Besides the connective-tissue fibres, the skin contains a small
proportion of fine yellow “elastic” fibres. If a thin section of hide be
soaked for a few minutes in a mixture of equal parts of water,
glycerine, and strong acetic acid, and then examined under the
microscope, the white connective-tissue fibres become swollen and
transparent, and the yellow “elastic” fibres may be seen, as they are
scarcely affected by the acid. The hair-bulbs and sweat- and fat-glands
are also rendered distinctly visible by this treatment. On the other
hand, the white gelatinous fibres are most easily seen by examining the
section in a strong solution of common salt, or in one of ammonium
sulphate; or by staining with some aniline dyes such as safranine.
Sections are most readily cut for these purposes by the use of the
freezing microtome, or after previous hardening in alcohol. For further
details see L.I.L.B., p. 254.
Ordinarily in the production of leather only the corium, or true skin is
used, and in order to obtain it in a suitable condition for the various
tanning processes, the hair or wool, together with the epithelium, must
be completely removed without damaging the skin itself; and especial
care must be taken that the grain, or portion next to the epidermis,
does not suffer any injury during the treatment. All the methods
employed depend upon the fact that the epidermis cells, especially the
soft growing ones next to the corium, and those of the epidermis layer
which surround the hair-roots, are more easily destroyed than the corium
itself owing to their different chemical character. The “unhairing”
process consists essentially in breaking down these cells by chemical or
putrefactive agents, and removing the hair together with the rest of the
epidermis by mechanical means. Of the various substances which may be
used for this purpose, lime is one of the most convenient, as its
solubility in water is so slight, that a solution of such a strength as
to injure the hide cannot be easily made. Caustic alkalies, on the other
hand, are much more soluble, and unless care be taken to use only the
proper quantity, a dangerously strong solution may be made with
consequent damage to the skin. The addition of small amounts of
sulphides to the lime-solution accelerates the unhairing owing to their
special solvent action on the epidermis-structures, and also in the case
of alkaline sulphides, by the caustic alkali which is produced by their
reaction with the lime. Even if used alone, strong solutions of alkaline
sulphides rapidly destroy both hair and epidermis, converting them into
a mass which may be swept off the skin like wet pulp, and yet they have
practically no injurious action on the true skin.
In the “sweating” process the epidermic cells are broken down by
putrefactive organisms and their products, so that the hair becomes
loose and may then be either rubbed or scraped off. Ammonia, which is
produced during the putrefaction, has also an important solvent action,
and its presence doubtless tends to quicken the processes both of
unhairing and of destruction.
To obtain useful knowledge of the structure of any particular skin, it
is not necessary to have a very elaborate or expensive microscope, and
it is quite possible to obtain useful information merely by the use of a
good pocket lens, as for instance, in the examination of various forms
of “grain,” and the embossing of one skin to imitate another.
For further details of the manipulation and selection of the microscope,
the reader must consult L.I.L.B., pp. 234 _et seq._
CHAPTER VIII.
_THE CHEMICAL CONSTITUENTS OF SKIN._
The chemistry of the various constituents of skin is still very
imperfectly understood, but Beilstein, in his great handbook of organic
chemistry, places gelatin, albumens and keratins in the “aromatic”
series, and implies therefore that they contain the “benzene” ring. It
is at least certain that all are very complex.
The epidermis structures belong to the class of keratins, which are
closely related to coagulated albumin; while the white fibres of the
corium (or true skin) are either identical with gelatin, or only differ
from it in their molecular condition or degree of hydration. This
gelatinous tissue constitutes the bulk of the corium, but it also
contains albumen as a constituent of the lymph and blood which supply
its nourishment, keratins in the epithelial structures of the blood and
lymph vessels, and “yellow fibres,” which are perhaps allied to the
keratins, but which cannot well be isolated for analysis.
The white connective tissue of the corium is converted into gelatin
(glutin) by boiling with water. Owing to the impossibility of obtaining
unaltered hide-fibre free from the other constituents, and still more to
that of deciding to what point it should be dried to remove uncombined
water, it is impossible to prove by analysis whether its composition is
identical with that of glutin; but as the white fibre constitutes by far
the largest part of the corium, and the other constituents do not differ
largely from it in their percentage composition, an analysis of
carefully purified corium is practically identical with that of the
actual fibre. The following analyses of hide and gelatin are therefore
of interest.
The analyses of Von Schroeder and Paessler[22] are of special importance
as being the average of a large number of separate determinations.
Their nitrogen determinations are by Kjeldahl’s method. Small amounts of
ash and traces of sulphur are neglected, and probably included in the O,
which is obtained by difference.
[22] Ding. Polyt. Journ., 1893, cclxxxvii. pp. 258, 283, 300.
ANALYSES OF PURIFIED CORIUM.
--------------------------+------------------+----+----+----+----+----
Analyst. | Material. | C | H | N | O | S
--------------------------+------------------+----+----+----+----+----
Stohmann and Langbein | .. |49·9| 5·8|18·0|26·0| 0·3
| | | | | |
Müntz |Ox-hide |51·8| 6·7|18·3|23·2| ..
| | | | | |
|{Ox, calf, horse,}| | | | |
Von Schroeder and Paessler|{camel, pig, }|50·2| 6·4|17·8|25·4| ..
|{rhinoceros }| | | | |
„ |Goat and deer |50·3| 6·4|17·4|25·9| ..
„ |Sheep and dog |50·2| 6·5|17·0|26·3| ..
„ |Cat .. .. |51·1| 6·5|17·1|25·3| ..
--------------------------+------------------+----+----+----+----+----
ANALYSES OF GELATIN (free from Ash).
--------------------------+----+----+----+----
Analyst. | C | H | N | O
--------------------------+----+----+----+----
Von Schroeder and Paessler|51·2| 6·5|18·1|24·2
Mulder |50·1| 6·6|18·3|25·0
Fremy |50·0| 6·5|17·5|26·0
Schützenberger |50·0| 6·7|18·3|25·0
Chittenden and Solly[23] |49·4| 6·8|18·0|25·1
--------------------------+----+----+----+----
[23] Contained also 0·7 sulphur. Journ. Physiol., xii. p. 23.
It will be noted that the above analyses of skin differ more widely
among themselves than their average does from that of the gelatin
analyses, though on the whole the nitrogen is somewhat higher in the
latter. The molecular weight of gelatin must be very high,[24] and any
empirical formula founded on ultimate analysis therefore quite
hypothetical. Bleunard,[25] Schützenberger and Bourgois,[26] and
Hofmeister agree on the formula C₇₆H₁₂₄N₂₄O₂₉, which leads to the
following percentage composition:--
per cent.
C₇₆ = 912 = 49·7
H₁₂₄ = 124 = 6·8
N₂₄ = 336 = 18·3
O₂₉ = 464 = 25·2
---- -----
1836 100·0
[24] Paal (Berichte D. Ch. Ges., xxv. (1892) pp. 1202-36, and Ch. Soc.
Abst., 1892, pp. 895-7) calculates a molecular weight of about 900
from physical (freezing, boiling point) methods.
[25] Annales de Chimie [5] xxvi. p. 18.
[26] Compt. Rend., lxxxii. pp. 262-4.
The addition of a molecule of water would make a difference in the
percentage composition indicated by these formulæ which would be less
than their probable experimental error, and the change may therefore be
one of hydration.
Gelatin certainly contains both carboxyl and amido-groups, and is
capable of combining with both acids and alkalies (see p. 84).
Reimer[27] obtained what he supposed to be pure unaltered
fibre-substance by digestion of purified hide with ¹⁄₂ per cent. acetic
acid for many days and subsequent neutralisation. His analysis showed C
= 48·45 per cent., H = 6·66 per cent., N = 18·45 per cent., O = 26·44
per cent., thus deviating considerably from direct analysis of unaltered
skin. It is obvious that little weight can be placed on this result,
Reimer’s precipitate being probably a mere decomposition product.
[27] Ding. Polyt., ccv. p. 164.
Hofmeister[28] notes that on heating gelatin it loses water and forms an
anhydride which he considers identical with collagen or hide-fibre. When
gelatin is dried at a temperature of 130° C. it becomes incapable of
solution in water, even at boiling temperature, and can only be
dissolved by heating under pressure. It is certain that collagen
(hide-fibre, ossein) is less easily soluble in hot water than ordinary
gelatin.
[28] Bied. Centr., 1880, p. 772.
So far as our present knowledge goes, we may regard hide-fibre as merely
an organised and perhaps dehydrated gelatin.
_Gelatin_ or _glutin_ (not to be confounded with the gluten of cereals),
when pure and dry is a colourless, transparent solid of horny toughness
and of sp. gr. 1·3. It begins to melt about 140° C., at the same time
undergoing decomposition. It is insoluble in hydrocarbons, in ether, or
in strong alcohol. In cold water it swells to a transparent jelly,
absorbing several times its weight of water, but does not dissolve. In
hot water it is soluble, but a solution containing even 1 per cent. of
good gelatin sets to a weak jelly on cooling. Gelatin jellies melt at
temperatures which vary considerably with the quality or freedom from
degradation products, but which within pretty wide limits (5-10 per
cent.) are little affected by the concentration. A 10 per cent. solution
of best hard gelatin melts about 38° C., while low glue may fail to set
at 15° C. A useful technical test for the setting power of gelatin,
based on this fact, consists in placing an angular fragment of the jelly
in a small tube attached to a thermometer, and stirring in a beaker of
water, which is slowly heated till the jelly melts, when the temperature
is noted. The exact point is perhaps more easily seen if the tube is
drawn to a conical point. The jelly may also be allowed to set in
capillary tubes open at the bottom, and the moment noted when water
rises into the tube. The temperature of fusion is raised by the addition
of formaldehyde, salts of chromium, alumina and ferric salts, which
produce a tanning effect, and in a less degree by sulphates, tartrates,
acetates, some other salts, and diminished by iodides, bromides,
chlorides and nitrates.[29] Solutions of gelatin too weak or too warm to
gelatinise possess considerable viscosity. Gelatin may therefore be
estimated, in the absence of other viscous matters, by the viscosimeter,
an instrument which measures the time taken by a liquid in flowing
through a capillary tube.[30] The firmness of a jelly, which is often
important for commercial purposes, is frequently measured by Lipowitz’s
method, in which a slightly convex disc, conveniently of exactly 1 cm.
diameter, and cemented to the bottom of a thistle-head funnel tube, is
loaded gradually with mercury till it sinks in the jelly. The jelly (5
or 10 per cent.) should be allowed to set some hours before the test is
made.
[29] See Pascheles, ‘Versuche über Quellung,’ Archiv für ges. Path.,
Bd. 71.
[30] See Prollius, Ding. Polyt. Journ., ccxlix. p. 425, who employs a
1 per cent. solution; also Stützer, Zeit. Ann. Ch., xxxi. pp. 501-15.
Solutions of gelatin from skin and bone are powerfully lævorotatory to
polarised light. At 30° C. (A)_{D} = -130°, but temperature and the
reaction of the solution have much influence on the value found.
Gelatin is precipitated from aqueous solution by the addition of strong
alcohol and concentrated solutions of ammonium sulphate and some other
salts. Many other colloid bodies such as dextrin and gums behave
similarly. In the absence of these substances, precipitation by alcohol
may be utilised for the technical analysis of gelatins and glues,
printers’ roller compositions and gelatin confectionery. 25 c.c. of the
gelatinous solution, which is preferably of about 10 per cent., is
placed in a small beaker tared together with a glass stirring rod, and
thrice its volume of absolute alcohol added. On stirring, the gelatin
sets firmly on the rod and sides of the beaker, and may be washed with
dilute alcohol or even with cold water, dried and weighed. A very pure
French gelatin gave 98·6 per cent., while a common bone-glue only
yielded about 60 per cent. precipitate. Absolute alcohol withdraws water
from gelatin-jelly, leaving a horny mass. Gelatin may also be
precipitated completely by saturating its solution with sodium chloride,
and then acidifying slightly with sulphuric or hydrochloric acid; and
masses of jelly become hardened in acidified salt solution as in
alcohol, though a neutral solution has little effect. The cause of this
is difficult of explanation, but its bearing on the pickling of
sheep-skins (p. 89) and the production of white leather (p. 186) is
obvious.
_Decompositions._--When aqueous solutions of gelatin are heated under
pressure, or in presence of glycerin and other bodies which raise the
boiling-point, or more slowly at lower temperatures, they gradually lose
the power of gelatinising on cooling, the gelatin being converted into
modifications soluble in cold water, but still capable of being
precipitated by tannin. Hofmeister[31] states that the gelatin takes up
3 molecules water and is split up into _hemicollin_, soluble in alcohol
and not precipitated by platinic chloride solution; and _semiglutin_,
insoluble in alcohol and precipitated by platinic chloride solution.
Both are precipitated by mercuric chloride. Dry gelatin is soluble in
glycerin at high temperatures, but probably suffers a similar change.
Hence high temperatures and long-continued heating must be avoided in
gelatin manufacture; and in making printers’ roller compositions, which
are mixtures of gelatin and glycerin, the gelatin must be swollen with
water and melted at a low temperature with the glycerin.
[31] Bied. Centr., 1880, p. 772, and Ch. Soc. Abs., 1881, p. 294.
Gelatin is also converted into soluble forms (peptones), perhaps
identical with the above, by the action of heat in presence of dilute
acids and alkalies. These, like gelatin, are precipitated by tannin and
by metaphosphoric acid.[32] Heated for longer periods or to higher
temperatures with aqueous solutions of the caustic alkalies, baryta, or
lime, gelatin is gradually broken down into simpler and simpler
products, ending in nitrogen or ammonia, water and carbonic acid. Among
the intermediate products may be mentioned various acids of the
amido-acetic series, as amido-acetic (glycocine, glycocoll),
amido-propionic (alanine), and amido-caproic (leucine); and of the
amido-succinic series (amido-succinic = aspartic acid).[33]
[32] Lorenz, Pflüger’s Arch., xlvii. pp. 189-95; Journ. Chem. Soc.,
1891, A. p. 477.
[33] Compare Schützenberger, Comptes Rend., cii. pp. 1296-9; Journ.
Chem. Soc., 1886, A. p. 818.
Treatment with acids produces very similar effects. The first products
are soluble peptones. Paal[34] on treating 100 parts of gelatin on the
water-bath with 160 parts water and 40 parts concentrated HCl till the
product was soluble in absolute alcohol, obtained, on purification, a
white hygroscopic mass of peptone salts containing 10-12 per cent. of
hydrochloric acid.[35]
[34] Berichte, xxv. pp. 1202-36; Journ. Chem. Soc., 1892, A. p. 895.
[35] See also Buchner and Curtius, Ber., xix. pp. 850-9; Journ. Chem.
Soc., 1886, A. p. 635.
The products of digestion of gelatin with gastric and pancreatic juice
are peptones which do not differ materially from gelatin in ultimate
composition, and the action is probably mainly hydrolytic.[36]
[36] Chittenden and Solly, Journ. Chem. Soc., 1891, A. p. 849.
The earlier products of putrefaction are very similar. Many bacteria
have the power of liquefying gelatin-jelly. This has been shown by
Brunton and McFadyen[37] to be due not to the direct action of the
bacteria, but to a soluble zymase secreted by them which peptonises the
gelatin. Its action is favoured by an alkaline condition, and destroyed
by a temperature of 100° C.[38] As putrefaction progresses, the solution
becomes very acid from the formation of butyric acid, and later on
ammonia and amido-acids are formed.
[37] R. S. Proc., xlvi. pp. 542-53.
[38] Compare pp. 17, 171; also Ch. Zeit., 1895, p. 1487.
Fahrion,[39] starting with the idea that albuminoids and gelatin were
condensation products of a lactone character (L.I.L.B. p. 185), and
that they might, like lactones, be depolymerised by saponification,
digested these bodies with alcoholic soda till they were dissolved, and
on neutralising the solution with hydrochloric acid, of which the excess
was driven off by repeated evaporation, and removing the sodium chloride
by treatment with alcohol, obtained in each case bodies of acid
reaction, which from their composition he supposed to be identical with
Schützenberger’s proteic acid, C₈H₁₄N₂O₄, which is soluble in water and
alcohol, insoluble in ether and petroleum, uncrystallisable, and forming
uncrystallisable salts. Fahrion suggested that the nitrogenous character
which Eitner attributed to his “dégras-former” (p. 370) was probably due
to contamination by this body; and that its formation might be utilised
in the analysis of leather and other proteid bodies. These products have
since been further investigated by Prof. Paal and Dr. Schilling,[40] who
show that they contain hydrochloric acid, to which their acid reaction
is due, and that they are identical with the peptone salts previously
obtained by Prof. Paal (v. s.) by digestion of proteids with
hydrochloric acid. The free peptones are strongly basic.
[39] Ch. Zeit., 1895, p. 1000.
[40] Ch. Zeit., 1895, p. 1487.
By dry distillation of gelatin a mixture of pyrrol and pyridin bases are
produced. This is commercially obtained by the distillation of bones,
and is known as “bone oil,” or “Dippel’s animal oil.” Pyrrol, C₄H₅N,
resembles phloroglucol in giving a purple-red colour to fir wood
moistened with hydrochloric acid (p. 299).
_Reactions of Gelatin._--Gelatin is precipitated by mercuric chloride,
in this respect resembling peptones, but not by potassium ferrocyanide,
by which it is distinguished from albuminoids, and it differs from
albumin in not being coagulated by heat. Solution of gelatin dissolves
considerable quantities of calcium phosphate; hence this is always
present in bone-glues. Gelatin and some of its decomposition products
are precipitated by metaphosphoric acid.[41] The precipitate contains
about 7 per cent. P₂O₅, but gradually loses it on washing. Various salts
diminish the solubility of gelatin in hot water, and especially those of
the alum type. Chrome alum and basic chrome salts are especially
powerful, rendering it practically insoluble. The addition of about 3
per cent. ammonium or potassium dichromate causes glue or gelatin to
become insoluble by the action of light with the formation of basic
salts of chromium, and has been utilised in photography and as a
waterproof cement. Other colloids besides gelatin are similarly
affected.
[41] Lorenz, Pflüger’s Archiv, xlvii. pp. 189-195.
Gelatin is precipitated by all tannins, even from very dilute solutions;
one containing only 0·2 grm. per liter is rendered distinctly turbid by
gallotannic acid or infusion of gall-nuts; but some other tannins give a
less sensitive reaction. The precipitate is soluble to a considerable
extent in excess of gelatin, so that in using the latter as a test for
traces of tannin care must be taken to add a very small quantity only.
The addition of a little alum renders the reaction more delicate.
Whether the precipitate is a definite chemical compound has been
disputed, as its composition varies according to whether gelatin or
tannin is in excess. Böttinger[42] states that the precipitate produced
by adding gelatin to excess of gallotannic acid contains 10·7 per cent.
of nitrogen, indicating the presence of 66 per cent. of gelatin on the
assumption that gelatin contains 16·5 per cent. N (see p. 57). Digested
with water at 130° C., the precipitate is decomposed, yielding a
solution which precipitates tannin, and probably indicating the
formation of a more acid compound. Gelatin with excess of oak-bark
tannin gives a precipitate containing 9·5 per cent. of nitrogen,
corresponding to 57·5 per cent. of gelatin. Treated with water at 150°
C., this precipitate yielded three products: one soluble in cold water,
another in hot only and one insoluble. On addition of a solution of
formaldehyde (formalin) to one of gelatin no visible action takes place
in the cold, unless the solution of gelatin be very concentrated and
alkaline, but on heating, the gelatin is rendered insoluble owing to the
formation of a compound with the formaldehyde. From the very small
amount of formalin which is required to produce formo-gelatin it is very
doubtful if this is a definite compound.
[42] Liebig’s Ann. der Ch., ccxliv. pp. 227-32.
Weiske[43] states that bone-gelatin, carefully freed from all mineral
matter, is not precipitated by tannin till a trace of a salt (e. g.
sodium chloride) is added. So far as is known, bone-gelatin is identical
with that of skin.
[43] Bied. Centr., 1883, p. 673.
_Chondrin_ is the gelatinous body produced by the digestion of
cartilage with water at 120° C. for three hours. In most of its physical
properties it is identical with gelatin, but differs from the latter in
being precipitated from its solution in water by acetic acid, lead
acetate, alum, and the mineral acids when the latter are not present in
excess. Chondrin also differs from gelatin in producing a substance
capable of easily reducing cupric oxide when it is boiled for some time
with dilute mineral acids. It is extremely probable that chondrin is
merely an impure gelatine.[44]
[44] Cp. Petri, Berichte, xii. p. 267; Mörner, Skand. Archiv f.
Physiol., i. pp. 210-243; and Journ. Chem. Soc., 1889, A. p. 736 and
Zeit. Physiol. Chem., 1895, xx. pp. 357-364; and Journ. Chem. Soc.,
1895, A. i. p. 254. See also Richter, Org. Chem., i. p. 559.
_Coriin._--Rollet[45] has shown that when hide and other forms of
connective tissue are soaked in lime- or baryta-water, the fibres become
split up into finer fibrils, and as the action proceeds, these again
separate into still finer ones, till the ultimate fibrils are so fine as
to be only distinguished under a powerful microscope. At the same time,
the alkaline solution dissolves the substance which cemented the fibres
together, and this may be recovered by neutralising the solution with
acetic acid, when the substance is thrown down as a flocculent
precipitate. This was considered by Rollet to be an albuminoid
substance; but Reimer[46] has shown that it is much more closely allied
to the gelatinous fibres, and, indeed, is probably produced from them by
the action of the alkaline solution. Reimer used limed calf-skin for his
experiments, and subjected it to prolonged cleansing with distilled
water, so that all soluble parts must have been pretty thoroughly
removed beforehand. He then digested it in closed glasses with
lime-water for 7-8 days, and precipitated the clear solution with dilute
acetic acid. He found that the same portion of hide might be used again
and again, without becoming exhausted, which strongly supports the
supposition that the substance is merely a product of a partial
decomposition of the hide-fibre, and indeed that there is no distinct
“cementing substance,” but merely a difference in the hydration or
physical condition of the fibre substance which causes it to split more
readily in certain directions. The dissolved substance, which he called
“coriin,” was purified by repeated solution in lime-water and
reprecipitation by acetic acid. It was readily soluble in alkaline
solutions but not in dilute acids, though in some cases it became so
swollen and finely divided as to appear almost as if dissolved. It was,
however, very soluble in common salt solution of about 10 per cent.,
from which it was precipitated both by the addition of much water and by
saturating the solution with salt. Reimer found that a 10 per cent. salt
solution was equally effective with lime-water in extracting coriin from
the hide, and that it was partially precipitated on the addition of
acid, and completely so on saturating the acidified solution with salt.
Other salts of the alkalies and alkaline earths acted in a similar
manner, so that Reimer was at first deceived when experimenting with
baryta-water, because, being more concentrated than lime-water, the
coriin remained dissolved in the barium salt formed on neutralising with
acid, and it was necessary to dilute before a precipitate could be
obtained. The slightly acid solution of coriin gave no precipitate
either in the cold or on boiling with potassium ferrocyanide, being thus
distinguished from albuminoids. The neutral or alkaline solution showed
no precipitate with iron or mercuric chloride, copper sulphate, or with
neutral lead acetate; but with basic lead acetate, basic iron sulphate,
or an excess of tannin a precipitate was produced. Reimer’s analysis
showed: Carbon, 45·91; hydrogen, 6·57; nitrogen, 17·82; oxygen, 29·60;
and he gives a formula showing its relation to the original fibre, which
does not seem supported by sufficient evidence. In all probability
coriin is merely an impure degradation-product of hide-fibre or gelatin.
[45] Sitz. Wiener Akad., xxxix. p. 305.
[46] Ding. Polyt. Journ., ccv. p. 153.
_Hide Albumin._--The fresh hide contains a portion of actual albumin,
viz. that of the blood-serum and of the lymph, which is not only
contained in the abundant blood-vessels, but saturates the fibrous
connective tissue, of which it forms the nourishment. This albumin is
mostly removed from the skin by the liming and working on the beam,
which is preparatory to tanning. Probably for sole-leather, the albumin
itself would be rather advantageous if left in the hide, as it combines
with tannin, and would assist in giving firmness and weight to the
leather. It is, however, for reasons which will be seen hereafter,
absolutely necessary to get rid of any lime which may be in combination
with it. The blood must also be thoroughly cleansed from the hide before
tanning, as its colouring matter contains iron, which, by combination
with the tannin, produces a bad colour.
The albumins form a class of closely allied bodies of which white of egg
may be taken as a type. They are also related to the casein of milk, to
fibrin, and more distantly to gelatin. A good deal of information on the
class may be found in Watt’s Dict. of Chem., 2nd ed., article
‘Proteids,’ and Beilstein’s article ‘Albuminaten,’ and in Allen’s
‘Commercial Organic Analysis,’ vol. iv.
The most characteristic property of albumins is that of coagulation by
heat. The temperature at which this takes place differs somewhat in
different members of the group, egg and serum albumin coagulating at
72-73° C. Dry albumins become insoluble if heated to 110° C. for some
time. Traces of acid tend slightly to lower, and traces of alkali to
raise the temperature of coagulation. Sodium chloride and some other
neutral salts favour coagulation. Solutions of albumin become opalescent
at a temperature slightly below that at which flakes form.
Albumins are also coagulated by alcohol and by strong mineral acids.
Coagulated albumin is only soluble in strong acids and alkalies by aid
of heat, and strongly resembles keratin (pp. 56, 68).
Solutions of albumin are lævorotatory to polarised light.
“_Acid_” and “_Alkali_” _Albumins_ are formed by the action, in the
cold, of dilute acids (such as acetic, hydrochloric) and alkalies on
albumin solution. They are uncoagulable by heat, and are precipitated by
careful neutralisation, but are soluble in excess of either acid or
alkali, or alkaline carbonates. They are thrown out of solution by
saturation with sodium chloride or magnesium sulphate. It is doubtful
whether albumins combine with either acids or bases, and it is probable
that the “acid” or “alkali” albumins are identical with the parapeptones
formed in the first stage of peptic digestion.
On putrefaction, or on more severe treatment with acids and alkalies,
albumins break down in a way similar to gelatin, and yield almost
identical products (see p. 57); amido-acids of the acetic series, and
tyrosin (para-oxy-α-amido-phenyl-propionic acid) and aspartic
(amido-succinic) acid, being the most important.
Treatment with alcoholic soda (see p. 62) yields peptones similar to
those of gelatin.[47]
[47] Paal, Ch. Zeit., 1895, p. 1487.
Heated for some days with dilute nitric acid (1 : 2) all proteids,
including albumins, gelatin and keratins, yield yellow flocks of
“xantho-proteic acid,” a substance of somewhat indefinite composition,
soluble in ammonia and in fixed caustic alkalies with production of an
orange-red or brownish-red colour.
Millon’s reagent gives an intense red coloration when heated with
albumins, keratins, or gelatin. The reagent is made by dissolving 2·5
grm. of mercury in 20 c.c. of concentrated nitric acid, adding 50 c.c.
of water, allowing to settle and then decanting the clear liquid.
Albumins, previously purified by boiling with alcohol and washing with
ether, when dissolved in concentrated hydrochloric acid (sp. gr. 1·196)
by aid of heat, give a violet-blue coloration, but the reaction is often
somewhat indefinite. Gelatin, chondrin and keratins do not give this
reaction.
Treated with a trace of cupric sulphate and excess of caustic potash
solution, albumins give a violet, and gelatin and peptones a pink
solution (biuret reaction).
Dissolved in glacial acetic acid and treated with concentrated sulphuric
acid, albumins and peptones give a violet and feebly fluorescent
solution. A somewhat similar reaction is obtained if sugar solution be
substituted for acetic acid.
A solution of albumin rendered strongly acid with acetic acid is
precipitated by potassium ferrocyanide, salt, sodium sulphate, lead
acetate, mercuric chloride, tannin and picric and tungstic acids.
_Egg-Albumin_ is contained in the whites of eggs in membranes which are
broken up by beating with water and can then be removed by filtration.
When fresh its reaction is slightly alkaline, and it is lævorotatory.
According to Lehmann, white of egg contains 87 per cent. of water, and
13 per cent. of solid matter, the latter being almost entirely composed
of egg-albumin. This latter coagulates and becomes insoluble in water on
heating to 60° C.
_Vitellin_ (the albumin or globulin[48] of the yolk) is insoluble in
water, and is obtained as a white granular residue on extracting undried
egg-yolk with large quantities of ether. It closely resembles myosin,
the chief globulin of muscle, but differs from other globulins in being
soluble in a saturated solution of common salt. A neutral solution of
vitellin in very dilute brine coagulates at 70-75° C.
[48] Globulin is an albumin soluble in dilute salt solutions, but
insoluble in water.
Yolks of eggs, preserved by the addition of salt, borax, or formalin,
are used for dressing skins in the process of “tawing” (see p. 191). For
the analysis of such yolks, see L.I.L.B., p. 159. Their most important
constituent for the leather-dresser is egg-oil of which they contain
about 30 per cent.
_Casein_, the principal proteid of milk, may be mentioned here in
connection with the albumins to which it is closely related, since,
though in no way connected with the animal skin, since it is used to
some extent as a “seasoning” or glaze for leather, for which it is well
adapted, and it is now to a considerable extent a waste product of
butter manufacture. It differs from albumins in being very incompletely
if at all coagulated by boiling, but separates at once in curdy flakes
on the addition of acids (hydrochloric, acetic, butyric), and by the
action of rennet. The curd is easily soluble in small quantities of
dilute alkalies, lime-water, and salts of alkaline reaction, such as
sodium carbonate and borax. If no more than the necessary quantity of
alkali is employed for solution, the compound has an acid reaction to
phenolphthalein, and like the original milk, is curdled by rennet and
dilute acids. Casein may also be dissolved by digestion with diluted
mineral or organic acids.
_Hair, Epidermis and Glands._--These are all derived from the epithelial
layer, and hence, as might be inferred, have much in common in their
chemical constitution. They are all classed by chemists under one name,
“keratin,” or horny tissue, and their ultimate analysis shows that in
elementary composition they closely resemble the albumins. It is
evident, however, that the horny tissues are a class rather than a
single compound.
The keratins are gradually loosened by prolonged soaking in water, and,
by continued boiling in a Papin’s digester at 160° C., evolve
sulphuretted hydrogen, at the same time dissolving to a turbid solution
which does not gelatinise on cooling. Keratin is dissolved by caustic
alkalies; the epidermis and the softer horny tissues are easily
attacked, while hair and horn require strong solutions and the aid of
heat to effect complete solution. The caustic alkaline earths act in the
same manner as dilute alkaline solutions; hence lime easily attacks the
epidermis, and loosens the hair, but does not readily destroy the
latter. Alkaline sulphides, on the other hand, seem to attack the
harder tissues with at least the same facility as the soft ones, the
hair being often completely disintegrated, while the epidermis is still
almost intact; hence their applicability to unhairing by destruction of
the hair. Keratins give the xanthoproteic reaction with nitric acid, and
a red coloration with Millon’s reagent, and also resemble albumin, in
the fact that they are precipitated from their solution in sulphuric
acid by potassium ferrocyanide. By fusion with potash, or prolonged
boiling with dilute sulphuric acid, keratin is decomposed, yielding
leucin, tyrosin, ammonia, etc. The precipitate produced by the addition
of acids to alkaline solution of keratin (hair, horns, etc.), mixed with
oil and barium sulphate, has been employed by Dr. Putz as a filling
material for leather, for which purpose it acts in the same way as the
egg-yolks and flour used in kid-leather manufacture. Eitner attempted to
use it for the same purpose with bark-tanned leather, but without much
success. Putz has also proposed to precipitate the material after first
working its solution into the pores of the leather.
_Elastic Fibres._--The elastic or yellow fibres of the hide are of a
very stable character. They are not completely dissolved even by
prolonged boiling, and acetic acid and hot solutions of caustic alkalies
scarcely attack them. They do not appear to combine with tannin, and are
very little changed in the tanning process. They are present in hide and
skin to the extent of less than one per cent.
_Analytical Methods._--The reactions distinguishing the principal skin
constituents are summarised in the following table:--
+------------------+--------------+---------------+------------------+
| Reagent. | Gelatin. | Albumins. | Keratins. |
+------------------+--------------+---------------+------------------+
|Cold water |Swells only |Soluble |Insoluble. |
| | | | |
|Heated in water |Soluble |Coagulate at |Soluble only at |
| | |72°to 75° C. |temp. over 100° C.|
| | | | |
|Acetic acid and }| | | |
|potassium ferro- }|No precipitate|Precipitate |Precipitate |
|cyanide to aque- }| | | |
|ous solution }| | | |
| | | | |
|Millon’s reagent |No reaction |Red coloration |Red coloration. |
| | | | |
|Hot concentrated }|No coloration |Violet blue |No coloration. |
|hydrochloric acid}| | | |
+------------------+--------------+---------------+------------------+
There is no simple method for the quantitative separation of the
different constituents of skin. It is, therefore, customary to simply
determine the amount of nitrogen which any particular portion of the
material may contain, and, as gelatinous fibre, which constitutes by far
the greater portion of the true skin, contains 17·8 per cent. of
nitrogen, to base the estimation of the amount of skin present upon this
figure (see p. 57).
The most convenient process for the determination of the nitrogen is
that devised by Kjeldahl, which is most easily carried out as follows:--
A known weight of the substance which contains about 0·1 gram of
nitrogen (0·5 gram of skin, or a corresponding quantity of liquor) is
placed in a flask of Jena glass, capable of holding 500-700 c.c.
together with 15 c.c. of concentrated sulphuric acid. The contents of
the flask are then boiled over a small Bunsen flame for 15 minutes, or
more, until all the water has been driven off and the material is quite
disintegrated; and are then allowed to cool below 100°. 10 grams of dry
powdered potassium persulphate is now added, and the boiling continued
till the liquid has become colourless. The operation of boiling should
be conducted in a good draught, or in the open air. Before the substance
has begun to char it is advisable to place a small funnel in the neck of
the flask to prevent, as far as possible, spirting and loss of sulphuric
acid.
[Illustration: FIG. 18.--Kjeldahl Apparatus.]
The colourless liquid is allowed to cool thoroughly, and the flask is
then fitted with a tapped funnel and tube, as shown in Fig. 18. This
tube must not be less than 4 mm. in diameter, and with the end in the
flask cut diagonally to facilitate drops of liquid falling back again
into the flask. It rises obliquely for a height of 12 to 15 inches, is
then bent over as shown in the figure and connected by a rubber tube[49]
to a 100 c.c. pipette, or similarly shaped tube, the other end of which
dips just below the surface of a volume of exactly 50 c.c. of “normal”
hydrochloric acid contained in a second flask. About 50 c.c. of
distilled water is introduced into the flask containing the treated
sample, and after this 100 c.c. of a solution of 50 grams of caustic
soda in 100 c.c. of water is carefully and slowly run into the flask by
means of the tapped funnel with which it is provided. The contents of
the flask are now boiled for about half an hour,[50] the normal acid in
the receiving flask being kept cool by immersing the latter in cold
water. The liquid in this second flask is then titrated with normal
sodium carbonate, using methyl orange as indicator. The difference in
c.c. between 50 c.c., the volume of acid used, and the quantity of
normal sodium carbonate required to neutralise it, when multiplied by
0·014 represents the amount of nitrogen (in grams) in the weight of the
substance used for the determination; or if multiplied by 0·0786 shows
the weight of hide-fibre in the same quantity of material. Some
chemists add copper sulphate, or a drop of mercury before boiling up the
substance with the strong sulphuric acid, but the use of such substances
introduces complications in the process without, in the case of
gelatinous matter, securing more accurate results. It is absolutely
necessary that the acids and alkali used should be free from ammonia,
and a blank experiment should be made using pure sugar which contains no
nitrogen, and a correction applied if necessary for the ammonia they
contain.[51]
[49] The ends of the glass tubes should fit closely together, so as to
expose the rubber as little as possible to the action of ammoniacal
vapour.
[50] “Bumping” is often very troublesome at this stage, and may be
prevented by passing a current of steam from another flask, or
ammonia-free air through a tube with a capillary opening into the
boiling liquid; fragments of pure zinc, of platinum, or broken
tobacco-pipe are much less efficient. It is an additional safeguard
against the escape of ammonia to fix a small absorption-tube
containing fragments of glass to the absorption-flask. The normal acid
is run through this tube into the flask, so as to wet the broken
glass, and is finally rinsed into the absorption flask before
titrating its contents.
[51] Cp. Procter and Turnbull, Jour. Soc. Chem. Ind., 1900, p. 130;
also Nihoul, Composition des Cuirs Belges, p. 14 (Bourse aux Cuirs de
Liège, Sept. 1901), who advocates the use of potassium permanganate in
the oxidation; and Law (Jour. Soc. Ch. Ind., 1902, p. 847).
In place of using 10 grm. of potassium persulphate as described, 10
grm. of ordinary potassium sulphate may be used, and potassium
persulphate added in small quantities towards the end of the operation
till a perfectly colourless solution is obtained.
CHAPTER IX.
_THE PHYSICAL CHEMISTRY OF THE HIDE-FIBRE._
The nature of the changes which take place in the conversion of raw hide
into leather, and the causes of swelling and “falling” in the various
stages of the wet-work and tannage are among the most difficult problems
with which we have to deal, and no intelligible explanation can be given
without taking into account facts which are among the most recent
discoveries of physical chemistry; and of which even yet our knowledge
is by no means complete.
We know from our study of the structure of hide, that it consists in its
natural state of gelatinous fibres which are soft and swollen with
water, and easy putrescible. When these are dried, they contract and
adhere to each other, forming a hard and almost homogeneous mass,
resembling in degree, a sheet of glue or gelatine. After the tanning
process, the fibres are changed in character, though not in form; they
no longer absorb water so freely, and in drying they do not adhere
together, but remain detached and capable of independent movement. The
leather is therefore porous, flexible, and opaque on account of the
scattering of light from the surfaces of the fibres, although the
individual fibres are translucent. At the same time, chemical changes
have taken place which render the fibres incapable of ordinary
putrefaction. Our first necessity, therefore, in the conversion of skin
into leather is to dry the fibres without allowing them to adhere. This
is accomplished in the most primitive mode of leather dressing, by
mechanically working fatty substances into the skin as it slowly dries,
so as to coat and isolate the fibres, which are loosened by kneading and
stretching; while at the same time the fat forms a waterproof coating
which prevents them from again absorbing the water which is necessary to
putrefaction. Similar results may be produced by causing chemical
changes in the fibres themselves, which render them insoluble in water,
and consequently non-adhesive; and a sort of leather may even be made by
merely replacing the water between the fibres with strong alcohol, in
which they are insoluble, and which absorbs and withdraws the water from
them, allowing them to shrink and harden, while preventing their
adhesion. The merit of having first clearly seen and expressed these
cardinal principles in leather production belongs to the now venerable
Professor Knapp, who published in 1858 a short paper (_Natur und Wesen
der Gerberei und des Leders_) which is a model of clear explanation and
practical experiment. Knapp, however, deals mainly with the changes in
the condition of the fibre which are necessary to convert it into
leather, and not with their physical causes; and before we can explain
the means by which these changes are brought about, we must be
acquainted with certain facts and theories about solutions which have
become much clearer since he wrote.
The particles (molecules) of all substances are drawn together by
attractive forces somewhat of the same character as the attraction of
gravitation which holds together the solar system, and which is the
cause of weight. It is indeed even possible that these forces are
identical. Like gravitation, these molecular attractions increase
rapidly in intensity as the distance of the attracting bodies
diminishes, so that in solids and liquids, where the molecules are near
together, they are immensely powerful, while in gases and vapours they
are barely perceptible. These attractions are opposed by the motion of
heat, which takes the same part in molecular physics which the energy of
planetary motion does in the solar system. In solids, the attractive
forces hold the molecules rigidly in position, the motion of heat being
limited to short vibrations round a fixed point, the effects of which
are visible in the expansion caused by rising temperature. If the
temperature is increased, most substances become liquid, a condition in
which the particles can roll round each other, but are still held
together by their mutual attractions, as the sun holds the earth from
flying off into space. If the temperature goes on rising, the orbits of
the molecules become greater, the liquid expands, and finally molecules
fly off at a tangent out of reach of the attractions of the mass of
liquid, and are only diverted from their course by colliding with solids
or with other flying molecules, from which they rebound. This
constitutes the state of vapour or gas.
The molecules usually consist of _groups_ of atoms. Thus in the vapour
of water, each molecule contains one atom of oxygen combined with two of
hydrogen, and it is only at immense temperatures that this inner
grouping is broken up. Naturally, the more complicated and heavier the
molecular group, the more easily it is broken up by outside causes into
simpler groupings, and molecules may exist in liquids or solids, which
break up before they reach the gaseous form. Of such substances the
chemist says that they “cannot be volatilised without decomposition.” In
very rare instances does the gaseous molecule consist of a single atom;
even those of the most perfect gases, such as hydrogen, oxygen and
nitrogen consist of pairs which are not broken up at any known
temperature. The pressure of a gas, and its tendency to expand is due
simply to the motion and impact of its flying molecules, and it may be
noted that at the same temperature and pressure equal volumes of all
gases have the same number of molecules, the lighter molecules making up
for their want of weight by their greater velocity. The average velocity
of a molecule of oxygen (O₂) at freezing point is 461 meters per second
or about that of a rifle-bullet. It must not be taken however, that in
any given solid, liquid, or gas, all the molecules at any temperature
move at a uniform velocity, but that each individual molecule may vary
from moment to moment from rest up to a very high speed, while the
temperature of the mass only represents the average. Thus it happens
that in all liquids, and even in solids, a certain proportion of the
molecules at any temperature will have a speed sufficient to enable them
to leave the surface, and take the form of vapour, while a certain
proportion will fall back and be caught and retained. Thus every liquid,
and theoretically every solid, has a “vapour-pressure,” rising with the
temperature, and depending on it only, and at the boiling temperature of
the liquid equal to that of the atmosphere, or about 15 lb. per square
inch, and therefore able to form bubbles in the interior of the liquid.
If a little of a liquid is confined in a flask, the flask will become
filled with its vapour, and so long as any of the liquid is present, the
pressure of the vapour will depend only on the temperature and not at
all on the respective quantities of liquid or vapour. Neither will it
be affected by the pressure of other vapours or gases present in the
flask, the total pressure in which will be the sum of the “partial”
pressures of all the gases and vapours present.[52]
[52] Cp. p. 421.
The behaviour of gases and vapours has been described in some detail
because it possesses very close analogy to that of substances in
solution. The molecules of liquids are held together by attractions
which are very powerful over the short distances which separate them,
amounting in most cases to many tons per square centimeter of sectional
area, but the range over which they act is very small. In the interior
of the liquid the attractions on one side of a molecule are of course
exactly balanced by those on the opposite side, so that it is free to
move within the liquid without hindrance, but at the surface a very
small part of the force due to the attractions of the surface-layer is
unbalanced and acts as a sort of elastic skin holding the liquid
together, and is called “surface-tension.” Familiar examples of this are
found in the force which supports a drop on the end of a tube, the
possibility of laying a slightly oily needle on the surface of water
without sinking, and the ability of some flies to walk on water as if it
were covered with a sheet of india-rubber. Many liquids will mix or
dissolve in each other in any proportions, e.g. water and alcohol; the
attraction of the alcohol for the water-molecule being as great or
greater than that of alcohol for alcohol, or water for water. In other
cases, such as water and oil, or water and petroleum spirit, practically
no mixture takes place, their mutual attraction being small; and each
retains a considerable surface-tension at the points of contact, though
less than that of the free surfaces, since each exerts an attraction on
the other. There are also many intermediate cases, such as water with
chloroform, carbolic acid, or ether, in which each solvent dissolves a
portion of the other, but the two solutions do not mix, but form
separate layers. In these cases an equilibrium is attained, in which
there is just as much tendency for either of the liquids to pass into as
out of the other layer. In this there is an extraordinary resemblance to
what has been said of vapour-pressures; and the tendency to pass into
solution is often called solution-pressure; and it may be noted that
when equilibrium has been reached, not only is the solution-pressure,
but the vapour-pressure of each constituent equal in both solutions.
Like vapour-pressures, the solution-pressures usually increase with rise
of temperature, more of each constituent passing into the other, till at
last the composition of the two layers becomes identical, their
surface-tensions disappear, and complete mixture takes place. With
phenol (carbolic acid) and water this takes place at about 70° C.
Most of what has been said of the mutual solution of liquids is also
true of the solution of solids, but the latter may be divided into two
very distinct classes, colloids and crystalloids (which, however, shade
off into each other). The colloid or gluey bodies are mostly miscible in
any proportion with liquids in which they dissolve, and there is no such
thing as a definite point of saturation. There are however some which
form _jellies_ which have great analogy to the partially miscible
liquids; there is a mutual solubility, a portion of the solid dissolving
to a liquid solution, while the remainder of the liquid dissolves in the
solid, increasing its volume, but still retaining the characteristics of
the solid state. As the temperature is raised, this mutual solubility
generally increases, till at a given point the jelly melts, and complete
solution takes place, as in the case of partially miscible liquids.
These phenomena are of prime importance in the theory of tanning, but
their further consideration must be deferred till a few words have been
said about the crystalloids. These are characterised by regular
crystalline form, indicating that the attractive forces of their
molecules are exerted in definite directions, giving them a tendency to
attach themselves together in definite geometrical arrangements. They
dissolve in themselves no part of the solvent, but are dissolved by it
till an equilibrium is reached in which the tendency of further
particles of the solid to pass into the solvent is balanced by that of
those already dissolved to attach themselves to the remaining solid, or
“crystallise out.” Such a solution is “saturated” with respect to the
solid residue, but the word has no meaning unless solid crystals are
present, and where a body has, as sometimes happens, more than one
crystalline form, a solution may be saturated with regard to one of
them, and more or less than saturated with regard to another. In
“supersaturated” solutions, crystallisation is at once started by the
addition of a “seed” crystal of the proper form.
If a crystalloid substance, such, for instance, as copper sulphate, be
placed in a solvent (e. g. water), the dissolved salt will gradually
spread itself through the whole body of the solvent, though in the
complete absence of currents in the liquid, the motion is extremely
slow, and years may be taken for the diffusion to rise through a few
feet. In many cases salts diffuse through aqueous jellies at the same
speed as they would through still water. Colloid substances on the other
hand have little or no power of diffusion and mostly cannot pass through
jellies at all. This is the reason why tannage with mineral salts is so
much more rapid than with vegetable tannins which are of colloidal
character, and diffuse through the gelatinous fibres of the hide with
extreme slowness.
All dissolved crystalloids do not pass through gelatinous membranes with
equal ease, and substances are known, mostly gelatinous precipitates,
which do not permit the diffusion of dissolved salts, though they allow
water to pass freely. Thin layers of such precipitates form what are
called “semipermeable membranes.” The existence of such membranes
affords us the possibility of direct measurement of the tendency to
diffusion, or as it is generally called the “osmotic”[53] pressure of
dissolved bodies. Thus a porous earthenware battery-cell may be immersed
in a solution of copper sulphate, and filled with one of potassium
ferrocyanide. In this way its pores will be filled with a gelatinous
precipitate of copper ferrocyanide, which is pervious to water, but
impervious to most dissolved substances. If now the cell be filled with
a dilute solution of some crystalloid, say sugar, and its top closed by
a perforated cork fitted with a vertical tube, and the cell be plunged
in water, the latter will pass into the cell, and the dilute solution
will rise in the tube to a height of many feet above the water outside.
By substituting a mercury pressure gauge for the vertical tube, exact
measures of the pressure in the cell can be made, which is the osmotic
pressure of the dissolved substance. At first sight it is paradoxical
that the water should flow into the solution, apparently against a heavy
pressure, but the explanation is simple. Mention has already been made
of the enormous internal pressures of liquids produced by the
attractions of their molecules. In the solution a portion of this is
borne by the dissolved substance, and the water flows in from the
outside till an internal mechanical pressure is produced, equal in
amount to the osmotic pressure of the dissolved substance. The
resemblance of the phenomena of solution to those of vapour-pressure has
already been mentioned, and it is found to be even quantitative, since
the measured osmotic pressures are exactly equal in amount to those
which the dissolved body would produce if it were in the state of vapour
at the same temperature and occupying the same volume as the solution.
It acts, in fact, precisely as the “partial pressure” of a vapour. There
are several indirect ways of measuring the osmotic pressure of dissolved
bodies, as for instance, from the lowering of the freezing point, or the
raising of the boiling point of the solution as compared to those of the
pure solvent, all of which confirm the direct measurements, and show
that in a given volume at the same temperature, the same number of
molecules will produce the same osmotic pressure whatever their nature,
or conversely, that at the same osmotic pressure and temperature equal
volumes of any solution must contain the same number of molecules. The
use of these facts in determining molecular weight is obvious.
[53] Solution-pressure and osmotic pressure are really two names for
the same force; the former being employed to signify the tendency of a
solid to dissolve, and the latter the pressure produced by the
dissolved body which tends to prevent further solution. Thus, in a
saturated solution in contact with its solid, the two pressures are
always equal, but exerted in opposite directions.
A curious apparent deviation from this law is however noticed in
solutions of salts, acids, and alkalies, and indeed of electrolytes
generally; thus a dilute solution of sodium chloride produces an osmotic
pressure nearly double that corresponding to the number of molecules of
NaCl present; and in fact behaves as if it were a solution of Na and Cl
existing separately. Such a solution conducts a current of electricity
very readily, while at the same time the chlorine is carried to the
positive, and the sodium to the negative pole, where they separate as
Na₂ and Cl₂ (the Na decomposing the water present and forming NaOH). In
fact, the modern theory of electrolysis asserts that these dissociated
atoms are not separated from each other by electricity, but that they
exist already separated in the solution of the electrolyte, and merely
act as carriers for the electricity, and that the work done by the
latter is not that of breaking up the salt-molecule, but of giving its
dissociated atoms fresh charges of electricity which enable them to
combine as new molecules, and escape from the electrolyte. Complex
salts do not always break up into single atoms, thus calcium sulphate
dissociates into Ca and SO₄, hydrogen sulphate (sulphuric acid) into 2H
and SO₄, and so on. These dissociated atoms and atom-groups are called
“ions,” and may be monovalent, divalent, and so on; the divalent ion
carrying double the electrical quantity or charge of the monovalent.
Without discussing the ultimate nature of electricity itself, the matter
is most easily pictured by assuming that the molecule of the undissolved
salt is made up of an ion with a + charge (“kation,” e.g. Na), and an
ion with a - charge (“anion,” e.g. Cl), by the electrical attraction of
which charges they are held together. In the solution these attractions
are balanced by those of other ions, so that they can wander freely
within the liquid, but in order to take the molecular form of free
elements and escape, say as Na₂ and Cl₂, the pair of kations must go to
the - pole and give up one + charge, and at the same time a pair of
anions must go to the + pole and receive a + charge. Thus the Na and all
other kations separate at the - pole, and the Cl and all other anions at
the + pole.
From what has been said, it will be obvious that free ions can only
exist in solution, and can neither evaporate, nor separate as solids;
but that in the liquid they act much like other dissolved molecules,
exerting their own osmotic pressure independently of each other or of
the dissolved salt, but with the limitation that a solution must always
contain at the same time equal numbers of + and - ions. As a solution is
diluted, more ions are liberated; as it is concentrated, more recombine
to form undissociated salt. This will be made clearer by an example. In
a saturated solution of sodium chloride with solid salt present, we have
dissolved salt at the solution-pressure of the crystallised salt, and Na
and Cl ions at the dissociation-pressure of the saturated salt solution,
and neither affect the others. If we now add hydrochloric acid, it has
no effect directly on the solubility of the _salt_, but as HCl
dissociates largely into H and Cl, it increases the pressure of the Cl
ions, and so compels the salt to recombine till the Cl pressure is
reduced to its normal amount. This increases the concentration of the
undissociated salt-solution, and thus salt is precipitated or
crystallises out till the solution is no longer super-saturated with
respect to the salt-crystals.
Most chemical reactions, and especially those between acids and bases,
are really reactions of the ions. Thus NaOH in dilute solution is mostly
ionised into Na and OH, while HCl is similarly ionised into H and Cl. On
the other hand, water ionises only very slightly. Hence, on mixture, the
H and OH combine and form water, with evolution of heat, while no actual
combination occurs between the Na and Cl, so long as they remain in
_dilute_ solution. For this reason, the heat of neutralisation of all
strong acids and bases is the same, independent of their nature, since
strong acids, bases and salts are almost completely ionised. The
rapidity of action, and consequently what we call the “strength” or
“avidity” of an acid or base depends on the number of its free ions in
solution; very weak acids and bases are very little ionised, though
their salts ionise almost completely in dilute solution. On this depends
the explanation of a fact of great practical importance. Hydrochloric
acid, a strong acid, is almost completely ionised in solution; acetic, a
weak one, very little; while sodium acetate and sodium chloride as salts
are both almost completely ionised. If we add hydrochloric acid to a
solution of sodium acetate, we shall have sodium-ions, acet-ions,
chlorine-ions and hydrogen-ions in the solution. As the pressure of the
acet-ions and the hydrogen-ions will be greater than the
dissociation-pressure of acetic acid, they will combine to form it, till
the pressure is equalised, and we shall have in the solution, free
acetic acid slightly ionised, the sodium- and chlorine-ions of sodium
chloride, and the sodium- and acet-ions of any excess of sodium acetate
left. If the hydrochloric acid were just sufficient to combine with the
whole of the sodium, we should have an equilibrium containing much
(ionised) sodium chloride and little sodium acetate, together with much
free acetic acid, and little hydrochloric. Thus the “strong” acid would
displace the weak one.
Taking another example, we add sodium acetate to a solution of acetic
acid. As the ionisation-pressure of the acetic acid is much less than
that of sodium acetate, and both have a common acet-ion, the ionisation
of the acetic acid will diminish, and more undissociated acetic acid
will form, till by its concentration the two pressures are equalised.
The total quantity of free acetic acid will be unchanged, but a less
proportion of it will be ionised, and it will act like a weaker acid.
This reduction of the activity of a weak acid by the addition of its
neutral salt is often made use of by chemists. Instances in tanning
practice are the use of excess of potassium dichromate with chromic acid
in the chrome tanning process, the effect of neutral salts in
“mellowing” the action of tanning liquors, and the use of salt in
“pickling.”
Let us now try to apply these facts to the physics of tanning, taking
first the simplest cases, where electrolytic dissociation does not take
place. We may consider the wet hide as made up of a mass of fibres of
gelatine-jelly, with interspaces which are filled with water. In fact,
for many purposes of experiment we may substitute for hide, mere sheets
of swollen gelatine, so as to avoid the complications introduced by the
water or solution mechanically retained between the fibres.
If we place a sheet of dry gelatine in water, it swells, absorbing
perhaps seven or eight times its weight of water, but does not
appreciably dissolve. A condition of equilibrium is reached when the
attraction of the water-molecules for the gelatine is equal to the sum
of the cohesive attraction of the gelatine for itself and the internal
attraction of the water outside. An increase of the cohesion of the
gelatine would tend to make it contract and expel part of the water, and
this contraction would tend further to increase both the cohesion of the
gelatine, and its attraction on the diminished number of water molecules
it contained, and clearly these causes would act in opposing directions.
The equilibrium is therefore a very unstable one, and slight causes
might be expected to produce great changes in the degree of swelling,
which is indeed the case. If we increase the temperature we diminish the
cohesion of the gelatine, till at a point it becomes less than its
attraction for the water, and the jelly suddenly loses its solid
condition and dissolves.
The absorption of water by colloids (including gelatine) is accompanied
by contraction of volume (compression) of the water absorbed, and by
evolution of heat, and, as has been pointed out by Koerner,[54] it is
opposed (and swelling decreased) by increase of temperature. Solution,
on the other hand, absorbs heat, and is therefore favoured by rise of
temperature.
[54] Beiträge zur wissenschaftlichen Grundlage der Gerberei, Freiberg,
1899.
If we place the swollen jelly in alcohol, it parts with water and
contracts. The gelatine and alcohol are not mutually soluble, the sum of
the attraction of water for alcohol, and the cohesive attraction of the
gelatine is greater than the attraction of the latter for water, and as
the alcohol cannot pass into the gelatine, the water passes out, and the
jelly contracts. The greater the concentration of the alcohol, the more
completely is the jelly dehydrated, and in strong alcohol it may become
quite hard and solid. If we like to express the same facts in language
more familiar to the modern chemist, but perhaps less clear to the
non-chemical reader, we may say that the alcohol exerts an osmotic
pressure outside the gelatine, but little or none inside it, and
therefore the water is squeezed out. It would be equally true to say
that the water passes out of the jelly till its osmotic pressure is
equal in both the jelly and the alcohol. The jelly is a true “solid
solution” of water in gelatine, and in a solution we may regard either
of the two constituents as the solvent. Exact parallels may be found in
the distribution of a third substance between two immiscible solvents
(see p. 76), say alcohol between water and ether.
The osmotic pressure of water into alcohol may be demonstrated in a very
simple way, taking advantage of the fact that a film of jelly is
permeable for water but not for alcohol. If the experiment described on
p. 78 be made by placing alcohol in a cell previously washed out with a
gelatine solution, and the cell be placed in water, the water will pass
into the cell, and the alcoholic solution will rise many feet in the
vertical tube. The insolubility of gelatine in alcohol may be made use
of for its estimation. If three times its volume of absolute alcohol be
added to a solution containing gelatine, the latter will separate as a
solid mass on a stirring rod, or on the sides of the beaker, and may be
washed with further portions of alcohol. The method is useful in the
analysis of gelatine lozenges and “jelly squares,” roller compositions,
hectograph masses, and the like; and for the determination of true
unaltered gelatine in glues, and commercial gelatines (see page 60).
Many other colloids are however also precipitated by alcohol.
If hide be treated with alcohol, as in Knapp’s experiment (p. 74), the
action is precisely the same as has been described with gelatine-jelly.
The water is withdrawn, first from the spaces between the fibres, and
then from the fibres themselves, and the skin dries with the fibres
isolated and non-adherent, and is in fact converted into a sort of
leather, which, however, returns to raw pelt on soaking in water.
The action of solutions of sugars, glycerine, and the like is in
principle similar to that of alcohol, but more complex, since in general
these bodies are soluble not only in the water, but in the gelatine or
hide-fibre, so that their effect cannot be foretold, though usually it
tends towards contraction rather than swelling. In general terms the
equilibrium is a balance of the attraction of the water and the sugar
for the gelatine, against the sum of their mutual attraction in the
solution outside and the resisting cohesive force of the gelatine; and
will depend not only on the nature of the substances, but on temperature
and concentration.
The action of acids, alkalies and salts on gelatinous fibre is yet more
complex, since not only electrolytic dissociation, but most probably
actual chemical combination comes into the question. The chemical
constitution of gelatine is as yet quite uncertain, but it is known that
the molecule contains both amido-groups capable of linking with acids,
and carboxyls which will combine with bases (see p. 58). Hence
hide-fibre absorbs both acids and bases with great avidity, so much so
that the sulphuric acid of a decinormal solution may be completely
removed by hide, leaving only water without a trace of acid recognisable
by litmus. Alkalies are absorbed in a similar way, and in both cases the
gelatine or gelatinous fibre acquires a greatly increased power of
absorbing water, and consequently of swelling. Familiar cases of this
are the swelling of hide by acid, and by lime, and in neither case can
the added substance be removed in any reasonable time by mere washing
with water. Hence to free hides from lime or acids it is necessary to
neutralise the alkali with acids (see p. 153) or the acid with chalk or
alkalies (p. 91). No accurate determination has yet been made of the
amount of acid or alkali with which gelatine or hide-fibre will combine,
since the matter is complicated by the volume of acid or alkaline
solution which is absorbed mechanically, and by the tendency of the
compound to partially decompose on washing with water. Experiments made
by the author lead to the conclusion that 1 grm. of air-dried gelatine
will combine with about 0·025 grm. of actual hydrochloric acid (HCl)
when placed in a very dilute solution of the latter, and this compound
will absorb 40 or 45 grm. of water while still retaining the jelly
state. The maximum swelling, with both acids and alkalies, is obtained
with dilute solutions; and with the stronger acids, the outside solution
must be almost neutral when equilibrium is attained, increasing
quantities of acid diminishing the amount of water absorbed. The same
statement is true of the strong alkalies. Thus in both cases, where
swelling is desired, the object is defeated by the use of too strong
solutions, and the quantity of acid or alkali should be rigidly adjusted
to the weight of pelt, and not to the volume of solution.
As regards a physical explanation of the effect of acid and alkaline
solutions upon gelatine, anything which can yet be said must be regarded
rather as speculation than as actual scientific knowledge. It must also
be admitted that while the view that actual _chemical_ combination takes
place between the gelatine and acids (or alkalies) seems much the most
probable, difficulties arise from the fact that different acids
apparently do not always combine in proportion to their equivalents,
though it is probable that these will prove only apparent anomalies when
more accurate means are known of determining how much acid is really
combined, and how much merely mechanically absorbed.
Leaving out of account for the moment the question of swelling, a few
words must be said about a property of these acid- (and alkali-)
gelatine compounds, a knowledge of which is essential to understanding
the swelling process. If a mass of acid-gelatine be suspended in pure
water, a certain portion of it will be decomposed into neutral gelatine
and free acid, and the latter will diffuse into the water. Thus
acid-gelatine can only exist in presence of a certain amount of free
acid. This dissociation by water is a common property of all salts, and
necessarily follows from what has been said of ionisation; but it is
only where the combining affinity of the constituents is weak, that it
becomes practically perceptible. Water to a _very_ small extent ionises
to H and OH. If we imagine a salt dissolved in it, such as NaCl, which
ionises almost completely to Na and Cl, we see that a certain proportion
of NaOH and HCl must be formed by combination with the water-ions. In
the case named the quantity is absolutely negligible, since both sodium
hydrate and hydrochloric acid are almost completely ionised themselves,
but if either the acid or the base is weak (that is little ionised), the
process of combination must go on till the acid or basic solution is
strong enough to have an ionisation-pressure equal to that of the salt.
As this acid or base is no longer in an ionised condition, it may be
removed from the solution by volatilisation or diffusion. For instance,
if a solution of ferric chloride be confined in a tray of parchment
paper, through which it has little power of diffusion, and this tray
floated upon water which is frequently changed, the dissociated acid
will diffuse through the membrane into the water, and in this way the
whole of it may be ultimately removed, leaving nothing but a colloid
solution of hydrated ferric oxide in the tray. Actions of this sort, in
which the gelatinous fibre of the hide plays the same part as the
parchment-paper membrane, have an important share in many of the
phenomena of tanning.[55] Thus, in the case of hide swollen with acid,
the acid compound with the fibre is somewhat dissociated, and if the
hide be hung in water which is constantly changed, the acid diffuses
into it, and the whole may be ultimately, though slowly removed. A
similar effect is produced in the familiar operation of removing acid
from pelt or chromed leathers by paddling with “whitening” (calcium
carbonate). The latter is insoluble in water, and therefore cannot
penetrate into the hide, but as it instantly combines with any acid
which diffuses out, the acid-gelatine compound is rapidly decomposed,
since it is only permanent in a solution containing enough free acid to
have an ionisation-pressure equal to that of the compound. Similar
statements are true of the alkali-gelatine and lime-gelatine compounds.
[55] Probably chrome, aluminium and iron salts are decomposed in this
way in mineral tanning, and thus fixed in the hide as insoluble basic
salts. Cp. pp. 186, 215.
It will be easier to follow the results of what has been said if we take
a concrete case which has been carefully investigated by the author and
others; that of the action of hydrochloric acid solutions on gelatine.
If a weighed sheet of gelatine be placed in a very dilute solution of
the acid, it swells much more considerably than it does in water, a
maximum swelling being attained with a concentration of the outer
solution of 0·1 to 0·2 grm. of HCl per litre. The swollen jelly has then
a volume of about 45 c.c. per gram of the air-dried gelatine, and a
concentration equal to about 0·75 grm. of HCl per litre of swollen
jelly, or at least about five times that of the outer solution. As the
concentration of the latter is increased, the concentration in the jelly
also increases, but in a much smaller ratio, while the volume of the
jelly diminishes, till, with a concentration of 5 grm. of HCl per litre
in the outer solution, the volume of the jelly is only about 18·5 c.c.,
and its concentration not quite 6 grm. per litre. These facts cannot be
accounted for by any theory of simple solution of the HCl in the jelly,
since the law of such solutions is that the concentration in each
maintains a constant ratio, unless chemical change takes place. It is
possible that they might be explained by adsorption (surface
attraction), but as it is known that gelatine contains both amido-groups
capable of combining with acids, and carboxyl-groups which can combine
with bases, it is much more likely that actual chemical combination
takes place, and that the apparent irregularities in the amount of acid
fixed are due to partial hydrolysis of the compound.[56]
[56] Cp., however, Walker and Appleyard on the ‘Absorption of Acids by
Silk,’ Chem. Soc. Trans. 1896, p. 1334.
The following may be suggested as a working hypothesis. As both water
and hydrochloric acid can pass freely in and out of the jelly, it must
be in osmotic equilibrium with the outer solution in every respect, and
neither the un-ionised hydrochloric acid of the solution, nor the small
amount which may be formed by hydrolysis of the gelatine compound can
have any effect on the swelling. So long as the outer solution is very
dilute, by far the greater part of the acid present is absorbed and
fixed by the gelatine, and almost the whole of the outer acid will be
ionised, as well as a portion of that in combination with the gelatine.
In the latter case, however, the ions will be unable to pass out of the
jelly, and will therefore cause an internal osmotic pressure, and the
gelatine will swell till the Cl-ions are in osmotic equilibrium with
those of the outer solution. At the same time, this internal pressure of
Cl-ions will oppose the entry of the Cl-ions (and therefore also of
their associated H-ions) from the outer solution, and the acid solution
absorbed mechanically will be somewhat less concentrated than that
outside. As the concentration of the outer solution is increased, the
pressure of the outer Cl-ions will repress the ionisation of the
gelatine-chloride, and at the same time its tendency to hydrolyse. Thus
the acid actually combined with the gelatine should somewhat increase,
but the swelling should diminish, as is actually the case.[57] It is
impossible to carry the concentration of the hydrochloric acid much
above 5 grm. per liter without causing solution of the gelatine, but the
addition of common salt to the outer solution should equally increase
the pressure of its Cl-ions, and cause further diminution of swelling,
the Na-ions in this case increasing the outside pressure in the same way
as the hydrogen ions. In fact the addition of salt in sufficient
quantity will reduce the swelling till the gelatine becomes quite solid,
and retains only about its own weight of water, while at the same time
the apparently combined acid largely increases. This cannot be
attributed to any direct dehydrating action of the salt, since
concentrated sodium chloride solutions have no dehydrating, but rather a
swelling effect on gelatine in the absence of acid, and the
concentration of the salt in the outer solution and in the jelly proves
precisely the same within the limits of experimental error. Several
other facts may be noted, tending to support the explanation which has
been given. The tendency to swell gelatine is common to all acids of
appreciable strength, and in all cases where the concentration of the
acid could be increased to a moderate extent without causing solution of
the jelly, the effect of a maximum swelling, diminishing as the
concentration of the acid increased, has been observed. Other salts also
produce similar effects to sodium chloride; thus the swelling caused by
sulphuric acid is repressed by sodium sulphate. Sodium chloride seems to
diminish the swelling caused by all acids, but in presence of large
excess of sodium chloride, most of the acid in combination with the
gelatine will probably be hydrochloric, whatever the acid used to
originally produce the swelling. A curious fact observed by the author,
is that absolute alcohol, which so effectually dehydrates neutral
gelatine, is almost powerless to remove either water or acid from
gelatine swollen by hydrochloric acid. HCl is freely soluble even in
absolute alcohol, but H- and Cl-ions can only exist in it to a very
small extent, so that we may conclude that the acid which causes the
swelling and retains the water of the jelly exists either in actual
combination with the gelatine, or in an ionised condition.
[57] The acid retained by the gelatine, as measured by deducting from
the total contained in the jelly, a quantity equivalent to the volume
of solution absorbed, at first rises rapidly to a maximum, then
slightly diminishes and remains practically constant. On the theory
suggested, it is evident, however, that the absorbed solution must be
more dilute than that outside, and the actual combined acid greater
than that shown by the above calculation. The “combined” acid, as
determined by indicators, shows slight but continuous increase. It is
acid to phenolphthalein, but neutral to methyl orange.
Solutions of caustic alkalies are in most respects analogous in their
swelling action to those of strong acids. A portion of the alkali is in
some way fixed by the gelatine, while another portion is simply absorbed
as solution. A maximum swelling effect is also noticed with dilute
solutions, which is diminished as the concentration increases. Swelling
by alkalies is not diminished by chlorides so far as has been observed,
and especially it may be noted that the swelling produced by caustic
soda is not diminished by sodium chloride. On the theory which has been
suggested there is no reason why alkaline swelling should be reduced by
chlorides, since the swelling agent has no Cl-ion, but it is somewhat
singular that the sodium salt, having a common Na-ion should produce no
repression of the swelling by caustic soda. In the present state of our
knowledge no definite explanation can be given, but it is quite possible
that the swelling in this case is not produced by the sodium-ion but by
some more complex one, or even by the hydroxyl-ion, like most of the
characteristic reactions of alkalies. Apparently the gelatin-alkali
compound is still strongly alkaline, affecting phenolphthalein indicator
like uncombined alkali--an effect which is known to be due to the
presence of free HO-ions.
The effect of acids and alkalies has been studied by Procter and others
on actual pelt as well as on gelatine, and has been found to be
qualitatively, if not quantitatively quite similar to that on gelatine,
though from the acid retained mechanically in the interfibrous spaces,
exact quantitative determination is more difficult. The amount of
swelling produced is not proportional to the strength of the acid, some
weak and little ionised acids such as lactic producing larger swelling
than stronger acids such as hydrochloric and sulphuric, of which the
ionic pressure in the external solution is greater. Dilute solutions
generally produce greater swelling than more concentrated, so that where
swelling is required without destructive effect on the fibre, dilute
solutions of such weak acids are to be preferred, and the presence of
neutral salts is to be avoided. On the other hand, where it is desired
to remove lime, or to bring the pelt into an acid condition without
swelling, the addition of neutral salts, and especially of chlorides is
advantageous. A very important application of this principle is the
“pickling” of sheep-skins, and especially of sheep-grains, in order to
preserve them for export. The principle of this operation is that the
skins are first swollen slightly with sulphuric acid, and the swelling
is then reduced by salt, either added, or used in a subsequent bath. In
practice, salt is now generally also added to the first bath to moderate
the swelling. A suitable strength for the “rising solution” is about 80
grm. common salt, and 7·5 grm. sulphuric acid per litre. 100 c.c. of
this solution will therefore require about 15 c.c. of N/1 alkali to
neutralise it, and it should be tested after each lot of skins, and
maintained at the same strength by suitable additions of acid. The acid
absorbed by the skins is mainly hydrochloric, sodium sulphate
accumulating in the bath. The salt is not absorbed by the skins in the
same way as the acid, but will be continually diluted by the water they
bring in, and occasional additions of salt must therefore be made, the
density being maintained at about 65° Bkr. (1·065 sp. gr.) After
paddling or being stirred in this bath for about ¹⁄₂ or ³⁄₄ hour the
skins are transferred to saturated brine, and stirred in it till fully
fallen in thickness, the density of the liquid being maintained by
excess of salt. They may be allowed to remain some hours in the
saturated brine with advantage.
Within moderate limits, the strength of the rising liquor is not of
great importance, since the skins will only absorb a certain amount of
acid (increasing with the concentration of salt). In the second or
falling liquor the large excess of salt forces all the acid present into
the skins, none diffusing into the bath. Skins may be effectively
pickled with very much smaller quantities of acid than those prescribed
above, or ordinarily used, and are much easier to tan satisfactorily;
but it is said that they are more liable to suffer from mildew. Pickling
may also be done by placing the skins in a concentrated brine-bath, and
adding a calculated quantity of acid, not exceeding 0·1 grm.-molecule of
sulphuric acid per kilo. of dry hide substance, but the method is not
economical in practice from the dilution of the bath produced by the
water brought in by the skins and the necessity of constant large
additions of salt.
Pickled skins must not be brought in contact with water, which by
diluting the brine they contain, allows the excess of acid to act upon
and destroy the fibre. Even drops of water, accidentally sprinkled on
the skins produce this effect, and it is said that it spreads to parts
which have not been wet. For similar reasons, it is necessary in
tanning pickled skins, at least to begin the process in liquors to which
salt has been added, the quantity required being dependent on the amount
of acid used in pickling the skins, and where this is reduced to a
minimum, it is even possible to tan without further addition of salt
than that contained in the skins.[58] The pickling process converts the
skins into a species of white leather, and skins tanned in salted
liquors after pickling, or by addition of both acid and salt to sumach
liquors give good colour, and tough leather with a much diminished
consumption of sumach. The permanency of such leather is somewhat
doubtful, but the writer was unable to detect free sulphuric acid in a
sample which he examined, and it may be that when no acid is added to
the later liquors, that derived from the pickling is expelled by the
tannin; but this is very doubtful.
[58] Instead of using salted liquors, the skins maybe “depickled” by a
bath of whitening and water, borax, or some other mildly alkaline
solution before tanning.
The facts which have been discussed in the preceding pages offer a
sufficient explanation of the causes which operate in those deliming
processes which depend on the simple neutralisation of the alkaline
matters present in the hide, and of the swelling by means of acid which
forms a step in the manufacture of many sorts of sole-leather, but they
by no means fully elucidate the causes of the much more complete
depletion of the pelt brought about by the bacterial products of bates
and puers. It has been pointed out (p. 82) that gelatine and hide-fibre
in a neutral condition are swollen by water, but that the equilibrium so
reached is an unstable one, easily influenced by slight causes. Among
these, as has been pointed out by Koerner,[59] the surface-tension
between the water and the swollen fibre holds a place; and
surface-tensions of this sort are greatly influenced by many substances
of the class to which bacterial ferments belong. Many salts also alter
the water-absorption of gelatinous fibres, sometimes causing swelling,
and sometimes contraction, according to temperature, concentration, and
the nature of the salt. Though most salts do not seem to be absorbed by
hide-fibre, it is possible, as suggested by Koerner (_loc. cit._), that
in some instances the base may combine with the acid-groups, and the
acid of the salt with the basic groups of the gelatine-molecule, while
other cases are known in which salts are actually dissociated, and their
acid fixed by the affinities of the hide-fibre. An interesting case of
this sort was recently proved by Paessler and Appelius,[60] who showed
that sulphuric acid was absorbed from a solution of hydric sodic
sulphate, and the neutral sulphate left in the solution. Similar
reactions undoubtedly occur with some salts of strong acids and weak
bases, but this point must be more fully discussed in connection with
the theory of mineral tannages.
[59] Beiträge zur wissenschaftlichen Grundlage der Gerberei,
Jahresberichte der deutschen Gerberschule zu Freiberg, 1898-9 and
1899-1900.
[60] Wissenschaftliche Beilage des Ledermarkt, 1901, ii. p. 106.
CHAPTER X.
_WATER AS USED IN THE TANNERY._
Of all the materials employed in tanning, none is of more indispensable
importance than water, and its quality has undoubtedly great influence
on tanning, though it is constantly blamed for faults and troubles which
are really due to the mistakes of the tanner.
Water is chiefly used in tanneries for soaking and washing hides and
skins, for making the limes, the bates, and the tanning liquors, for
steam boilers, and in dyeing. For all these purposes it should be as
free as possible from impurities, but since water is the most universal
solvent in Nature, it is never found pure, but always contains mineral
matter derived from the rocks and soil through which it has flowed, as
well as organic impurities from decaying animal and vegetable matter.
Associated with the latter are usually living organisms of putrefaction
(_bacteria_) which may affect the quality of the water for tanning even
more seriously than the mineral impurities. The purest natural waters
are those which have flowed only over hard sandstones and volcanic
rocks. Water sufficiently pure for laboratory use can only be obtained
by distillation. The steam-water from heating pipes usually contains
large quantities of dissolved iron, and often also volatile organic
matters from the oil, etc., which finds its way into the boiler. It may
sometimes be made fit for use by boiling (which precipitates the ferrous
carbonate present), and subsequent settling or filtration. The use of
steam-water containing iron is a frequent source of stains and
discolorations in the tannery which more than counterbalances the
advantage of its softness.
The “hardness” of natural waters is mostly due to the salts of lime and
magnesia which they contain, which precipitate soap in the form of
insoluble stearates and oleates, which are useless for washing. It is
commonly estimated by determining the amount of a standard alcoholic
soap solution which must be added in order to produce a permanent froth
on shaking. Theoretically about 12 parts of soap (sodium stearate or
oleate) are destroyed by 1 part of calcium carbonate or an equivalent
quantity of other lime salts, with formation of insoluble lime soaps
(calcium stearate or oleate). Really, the reaction is much more
complicated, owing to the dissociation of the soap into free alkali and
acid-salts on solution in water. Teed[61] estimates that ¹⁄₃ to ¹⁄₂ more
is required than the theoretical quantity, and more in hot water than
cold. This uncertainty is partially overcome by testing the soap
solution against a known solution of calcium chloride. The presence of
magnesia also complicates the test and leads to discrepant results.
[61] Journ. Soc. Chem. Ind., 1889, p. 256. Cp. also Allen, ibid. 1888,
p. 795.
The methods of determining hardness originated by Hehner (see L.I.L.B.,
p. 19) are simpler and more accurate than the soap-test, and are to be
preferred, except for direct determination of the suitability of a water
for scouring with soap. “Degrees” of hardness in England are calculated
as parts of CaCO₃ per 100,000, or sometimes grains per gallon (70,000
grains).
Hardness is of two kinds, “temporary” and “permanent”; the former being
removed by boiling, while the latter is not so removed.
Temporary hardness consists of the carbonates of alkaline earths held in
solution by an excess of carbonic acid. Lime combines with 1 molecule of
carbon dioxide to form the ordinary normal carbonate (chalk), which is
practically insoluble in water. When, however, excess of carbonic acid
is present, hydric calcic carbonate (bicarbonate) which is fairly
soluble is produced. This is easily demonstrated by passing carbon
dioxide into somewhat diluted lime-water, which at first becomes turbid
from precipitated chalk, but soon clears by formation of soluble hydric
carbonate. If the solution be now boiled, the hydric carbonate is
decomposed, and the excess of carbonic acid is driven off as CO₂, and
the chalk again precipitated. The reactions are represented by the
following equations:--
Ca(OH)₂ + CO₂ = CaCO₃ + OH₂. (1)
{ CaCO₃
CaCO₃ + CO₂ + OH₂ = { (2)
{ H₂CO₃
Magnesia forms soluble double carbonates in a similar manner, but on
continued boiling gradually loses the whole of its carbonic acid, and is
precipitated as magnesium hydrate, Mg(OH)₂.
One of the most important reactions in connection with temporary
hardness is that caused by the addition of calcium hydrate (slaked
lime), which forms the basis of Clark’s softening process. When an
equivalent amount of lime is added to a solution of hydric calcic
carbonate, it displaces the water of the “half-bound” carbonic acid,
forming a second molecule of calcium carbonate, which is precipitated
together with that originally present, as is represented in the
following equation:--
CaCO₃ }
} + Ca(OH)₂ = 2CaCO₃ + 2OH₂. (3)
H₂CO₃ }
Hydric magnesium carbonate is also precipitated by lime, but the
reaction is somewhat different, the magnesia being removed as hydrate as
follows:--
{ MgCO₃
{ + 2Ca(OH)₂ = 2CaCO₃ + 2OH₂ + Mg(OH)₂. (4)
{ H₂CO₃
It will be noted that 2 equivalents of lime are required to precipitate
1 of magnesia. Two molecules of sodium hydrate (NaOH) or potassium
hydrate (KOH) may be substituted for 1 of Ca(OH)₂ with similar results,
and in some cases it is practically advantageous to use the former, as
the sodium carbonate formed in precipitating the temporary hardness
reacts again on the permanent, throwing down the lime and magnesia as
carbonates. (See p. 101.)
[Illustration: FIG. 19.--Plan of Archbutt and Deeley’s Apparatus.]
The use of lime for softening temporary hard waters was originally
proposed by Thomas Henry, F.R.S., of Manchester, but was first applied
as a practical process by Clark, who, after adding the requisite
quantity of lime to the water in a mixing vat, allowed it to stand in a
large tank to clear by subsidence, the precipitated carbonate of lime
taking from 6 to 12 hours to settle. The process in its original form is
a perfectly satisfactory one, except for the capacious settling tanks
which are required, which in some cases are inconvenient and expensive.
Messrs. Archbutt and Deeley[62] have patented a modification of the
Clark process, by which the time of subsidence is much shortened, and
according to which the precipitated carbonate of lime of previous
operations is allowed to remain in the tank, and the fresh charge of
water and lime is mixed up with it by means of steam-injectors, which
blow in a current of air through perforated pipes at the bottom of the
tank, and at the same time very slightly warm the water. The action goes
on much more rapidly at a slightly raised temperature than in the cold;
and rather curiously, the stirred up precipitate, instead of increasing
the time of clearing, settles rapidly and carries down with it that
formed in the new operation. It is particularly suitable for treating
waters containing magnesia, from which a compound of lime and magnesia
is apt to be precipitated in a colloid form which chokes filter-cloths
and will not readily settle. After softening, the water is usually
“carbonated” by passing the gases produced by burning coke into the
floating exit-pipe through which it falls, in order to retain any
remaining traces of carbonates of lime and magnesia in a soluble form,
and prevent their subsequent precipitation in the pipes. The apparatus
is made by Messrs. Mather and Platt, of Manchester, and its arrangement
is shown in Figs. 19 and 20.
[62] Journ. Soc. Chem. Ind., 1891, p. 511.
[Illustration: FIG. 20.]
Several modifications of the Clark process have been introduced, in
which the precipitation is carried on continuously instead of
intermittently. The most important of these is the Porter-Clark, in
which one portion of the water to be softened flows through an agitator
containing excess of lime, with which it forms saturated lime-water,
which is passed slowly up a cylinder where it deposits the excess of
suspended lime. The clear lime-water so produced is mixed with a fresh
portion of the water to be softened in a second cylinder also provided
with an agitator, the proportion of the two liquids being regulated by
cocks. The carbonate of lime is at once precipitated, and is removed by
passage through a filter press. This process is in successful operation
on a considerable scale at Messrs. Hodgsons’ tannery at Beverley.
Several other forms of filter have also been employed with success, and
also methods in which the treated water traverses tanks with sloping
partitions on which the carbonate of lime is deposited. The latter plan
was originally patented in France by Gaillet-Huet, and has been
introduced into England by Stanhope.
So far as is yet known, from the tanner’s point of view, it is hardly
necessary to make any distinction between lime and magnesia, either or
both of which may be considered simply as “hardness.” A hard water
probably softens dried hides more slowly than a purer water, though it
is possible that the observed difference in the time required may be due
in many cases to the lower temperature of wells from which hard water is
generally derived. In the actual “limes” the hardness of the water can
have no appreciable influence, though if sodium sulphide be used alone
for unhairing, a certain waste occurs from temporary hardness which may
render it advisable to add a little lime. It is in washing the hides
free from lime that the influence of hard water is first distinctly
felt. If limy goods, after unhairing, are placed in a water with much
temporary hardness, the same action occurs as in Clark’s water-softening
process, and chalk is deposited in the surface of the hides, making them
harsh and apt to “frize” or roughen the grain in “scudding.” The common,
but not wholly satisfactory expedient is to add a little lime, or
better, a few pailfuls of lime liquor to the water before putting in the
hides. The best plan is to use a properly softened water. Permanent
hardness is not injurious in this way.
Unfortunately it is not the grain alone which is injured by the use of
hard water for washing the hides, but on coming into the liquors the
precipitated bases combine with the acids and tannins, forming compounds
which oxidise and darken when exposed to the air, and which are the
commonest causes of stains and markings on all descriptions of leather.
Even when goods are drenched or bated before tanning the injury is not
prevented, since the weak organic acids which are capable of removing
the lime (as such) from the hide have little effect on the precipitated
carbonate, which can only be dissolved by the use of stronger acids. It
must be noted that the same injurious effect on limed goods is produced
by free carbonic acid, which may be present even in soft waters.
When temporarily hard waters are employed for leaching tanning
materials, the carbonic acid is displaced by the tannins, which form
compounds similar to those just mentioned, which are incapable of
tanning, and darken and discolour when exposed to the air. Though the
amount of lime present in a liter of even the hardest water is very
small, yet in the aggregate of thousands of gallons used weekly in a
good-sized yard it amounts to something very considerable, and as the
molecular weight of tannins is very high, the quantity destroyed is many
times that of the lime present. This loss can be prevented (_a_) by the
addition of sufficient mineral acid to convert the temporary into
permanent hardness, (_b_) by the use of oxalic acid, which precipitates
the whole of the lime as oxalate, or, (_c_) best of all, by softening
the water by suitable treatment before use. Each part of temporary
hardness reckoned as CaCO₃ (L.I.L.B., p. 19), requires 1·26 parts of
crystallised oxalic acid or 0·98 parts of H₂SO₄, or say one part of
ordinary oil of vitriol of sp. gr. 1·840 per 100,000 parts of water.
As the lime and magnesia of temporarily-hard water is thrown down by
boiling, it is deposited in steam boilers as a soft precipitate, much of
which can be blown out by suitable sludging; but if oils or fats obtain
access to the boiler, a soft, bulky, adherent deposit is formed, keeping
the water from the plates, which may become red hot, and lead to
collapse or explosion. This effect is not produced by mineral oils,
which, on the contrary, tend to prevent adherence of scale to the
plates, and as suitable mineral oils are not only cheaper, but much less
injurious to the working parts of steam engines than animal or
vegetable oils or tallow, they should always be used in preference for
cylinder purposes.
Water which is temporarily hard owing to calcium and magnesium
carbonates, is unsuitable for _dyeing_, as the carbonates react with
basic dyes, precipitating the colour-base, and so rendering a part of
the dye useless. Further, as this precipitate is deposited on the skins
it causes uneven dyeing and gives rise to spots and streaks. In dyeing
with basic dyes, therefore, it is advisable to add sufficient acetic
acid to the water before use to exactly neutralise the carbonates
present. Of course this treatment is quite unnecessary when acid dyes
are employed, as acid is usually added with the dye, and with dyewoods
the presence of a little calcium salt is advantageous.
As each “degree” of total hardness represents a soap-destroying power of
at least 2 oz. of soap per 100 gallons of water, allowance must be made
in making up “fat-liquors” with soap and oil for the loss of soap due to
its precipitation by the mineral matter in the water. The sticky
lime-soaps are apt to adhere to the leather and interfere with glazing;
so that it is much better to employ a soft water.
_Permanent hardness_ of water is generally caused by sulphates of lime
and magnesia, and more rarely by chlorides and nitrates. As none of
these can be precipitated by lime, permanent hardness cannot be removed
by Clark’s process, nor can it produce the injurious effect on limed
hides which have been attributed to temporary hardness. Neither can the
lime and magnesia present combine with the tannins if used for leaching,
since they are already fixed by stronger acids, and at most can only act
injuriously by slightly lessening the solubility of the tannins. Even
this effect cannot be regarded as proved, though it deserves further
investigation.[63] Permanent hardness is therefore of little moment as
regards the ordinary uses of the tannery, though it has considerable
influence in some of the processes of dyeing, and acts very injuriously
where soap is used for scouring, as in the washing of sheep-skins for
wool mats, since each part of lime reckoned as carbonate destroys at
least twelve parts of pure soap (sodium stearate or oleate), producing
a sticky and insoluble lime-soap which adheres to the fibre. In
sole-leather tanning, permanent hardness is sometimes advantageous,
especially if it be due to calcium and magnesium sulphates, and Vignon
recommended that sulphuric acid should be added to the water before use
in quantity sufficient to exactly neutralise the carbonates which cause
temporary hardness, as magnesium and calcium sulphates are not
injurious, but tend to plump the hides. It must be remembered, however,
that the carbonic acid liberated may still have prejudicial effects on
limed hides.
[63] Recent investigations by Nihoul (‘Influence de la nature de l’eau
sur l’extraction des matières tannantes,’ Bulletin de la Bourse aux
Cuirs de Liège, Sept. 1901) on the tanning waters of Belgium seem to
show that permanent hardness is more injurious in the extraction of
tannin than has generally been supposed.
Permanent hardness is most objectionable in waters employed for
boiler-feeding, and calcium sulphate is especially so, as it becomes
nearly insoluble in water at 150° C. or 55 lb. steam-pressure, and is
deposited on the plates as a hard crystalline scale which has to be
chipped off with a hammer. Where many boilers have to be worked with a
hard water, it is much the most satisfactory to soften the water with
caustic soda, or with lime and soda together before it comes into the
boiler, but in cases where the plant required would be too costly,
boiler-compositions are sometimes used with good effect, though
considerable caution is advisable, since some of them affect the plates
injuriously. The active constituent of many boiler-compositions is
soda-ash or sodium carbonate, which acts by double decomposition with
the calcium sulphate, forming sodium sulphate, and precipitating calcium
carbonate as a sediment which is easily washed out. Most tanning
materials, and even spent tan liquors, will prevent or lessen
incrustation if mixed with the feed water, but sometimes corrode the
plates if used too freely. This danger is lessened if they are used in
conjunction with soda. Heavy mineral oils, either introduced in small
quantity with the feed water, or painted on the sides of the boiler when
cleaned, are useful in preventing the formation of a coherent scale.
The removal of permanent hardness from water is easily effected in most
of the forms of apparatus employed for the softening of water by lime,
by using a calculated quantity of sodium carbonate in addition. The
reaction is represented in the case of calcium sulphate by the following
equation--
CaSO₄ + Na₂CO₃ = CaCO₃ + Na₂SO₄.
The conversion of magnesium sulphate into carbonate may be similarly
effected, but as the latter is somewhat soluble, an additional
equivalent of lime must be used to precipitate it as hydrate. Magnesium
salts, from their solubility, do not cause scale on boilers (though the
chloride is apt to produce corrosion), but they are equally destructive
of soap with the calcium salts. Caustic soda will remove temporary
hardness, and after becoming converted into carbonate will further react
on any permanent hardness present; and its use is therefore sometimes
convenient in small softening plants, but it is not more effective, and
considerably more costly than a suitable mixture of lime and sodium
carbonate. Even with these, Archbutt states that the cost of softening
permanent hardness is about ten times as great as that of removing
temporary hardness with lime only.[64]
[64] Proceedings of Inst. of Mech. Engineers, 1898, pp. 404-54, in
which much valuable information on water-softening is given.
As regards the influence of other impurities, our knowledge is far from
complete, but the following are the most important matters likely to be
present.
_Mud_ under any circumstances is objectionable. It frequently contains
organic slime and organisms which encourage the putrefaction of hides
placed in it to wash or soften. It also almost invariably contains iron
as one of its constituents, and hence stains leather and gives dark
coloured liquors. It is not easily removed by filtration, as large
filter-beds are expensive and difficult to keep in order, and much space
is required to clear water by subsidence. Some mechanical filter which
can be easily cleaned, and used under pressure, offers the best chance
of success. The Pulsometer Company make one consisting of sponge tightly
packed below a perforated piston. To cleanse the filter a stream of
water is passed the reverse way, and the piston raised and worked up and
down, either by hand or power, so as to loosen and knead the sponge.
Filter-presses, in which cloths, or in some cases sand, are used as the
filtering medium, are also well adapted for the purpose. If a water be
softened by Clark’s or other process the precipitated chalk carries down
the mud with it, together with most of the organisms.
_Iron_ is always an objectionable impurity in the tannery, though it is
less injurious to the quality than the appearance of the leather
produced, and indeed German sole-leather tanners frequently put old
iron in the handlers to darken the colour of the leather, and
apparently, if not really, to quicken the tannage. It must not be
present in waters used for dyeing. Iron oxide is frequently present as a
mud merely, and in this case can be removed by filtration. It is rarely
in solution in any other form than that of acid carbonate, since
sulphate or chloride could not exist in presence of bicarbonate of lime.
In this form, iron is precipitated at once by boiling or on the addition
of lime, like the temporary hardness due to other bases, in the form of
ferric hydrate, and more slowly by oxidation on exposure to the air. The
mud produced by softening waters which contain iron must be completely
removed by filtration, or subsidence, before the water is used for
leaching, or the iron will redissolve in the acids of the liquors. Iron
is not perceptibly injurious in the limes, but in the bates and
wash-pits sometimes causes stains, which are scarcely visible till
blackened by the tanning liquors. In presence of sulphur (from sulphide
of sodium or the decomposition of sulphates by the sulphur-bacteria
nearly always present in bates and soaks), the stains become bluish or
greenish black, and a black deposit is frequently produced on the sides
of the pit, in which the threads of sulphur-bacteria (_Thiothrix_) can
often be recognised by the microscope. As ferric salts not only combine
with the tannins, but are themselves tanning agents (see p. 198), they
are rapidly absorbed by leather, and iron is always present in leather
ash. (For detection and estimation see L.I.L.B., p. 218.)
_Alumina_, except as clay, is rarely present in waters, and probably
harmless in any water likely to be used in tanning.
_Soda_ is sometimes present in considerable amount, as sulphate,
chloride, or carbonate. The sulphate is probably inoperative. The
chloride, if present in material quantities, prevents plumping, and may
be the cause of thin and soft leather, and in large amounts will greatly
impede the proper exhaustion of many tanning materials. Sodium carbonate
is sometimes present in considerable quantities, as in some of the
waters of the Leeds district. It may coexist with temporary hardness,
and produces similar injurious effects. Waters in which it is present
cannot have any real permanent hardness. It may be neutralised by the
very cautious addition of an acid; or by admixture of a permanent-hard
water. It tends to increased plumping in the limes, but neutralises the
free acids of the tan-liquors which are necessary in sole-leather
tanning.
_Copper_, _lead_, and other metallic bases are not likely to be present
in any waters used for tanning in quantities sufficient to be injurious.
_Sulphuric acid_ rarely occurs free in water, and then only in such
traces as would be harmless for tanning, though possibly injurious to
steam boilers. As sulphates it is most common. Alkaline sulphates are
not known to have any deleterious action. The sulphates of lime and
magnesia are the principal cause of permanent hardness, q.v. Iron
sulphate is sometimes found in colliery waters.
_Nitrates and nitrites_ in water are usually the result of “previous”
sewage contamination, and are only important as an indication of the
possible presence of the putrefactive ferments, and are of little moment
in waters only used for manufacturing purposes, while they seem to be
even useful in promoting the “working” of bran drenches, by supplying
the nitrogen required by the ferment.
_Chlorine_ is seldom or never present in water in the free state, but
only in the form of chlorides, most frequently of sodium chloride
(common salt), the effect of which has been referred to above, and also
at p. 88. The action of other chlorides is probably similar as regards
the swelling of hide. _Magnesium chloride_ is very objectionable as a
constituent of boiler-waters, as it liberates hydrochloric acid at high
temperatures, and corrodes the plates at the surface of the water. This
injury can be prevented by addition of soda.
_Carbonic acid_ has been referred to under temporary hardness. Its
presence in the free state is a matter of some importance to the tanner
(see p. 99).
_Silicic acid_ in a soluble form is present in some waters in
considerable quantity. Such waters are said to harden leather, but of
this the writer has no personal experience.
Few accurate researches have been made on the effect of the impurities
of water on tanning,[65] and though, from what has already been said, it
will be seen that they are not without effect it is probable that in
many cases the water is blamed for troubles which are simply the result
of mismanagement, and credited with virtues which are really due to
careful and skilful manufacture.
[65] See Nihoul, ‘Influence de l’eau sur l’extraction des matières
tannantes,’ Bulletin de la Bourse aux Cuirs de Liège, Sept. 1901.
The hardness of water, and the dissolved carbonic acid which it
contains, are, together with its temperature, the principal factors
which determine whether a hide will plump or fall in it. Almost the only
accurate investigation of this point has been made by W. Eitner.[66] He
placed pieces of hide, unhaired by sweating, and quite flat and fallen,
in water for four days at a temperature of 46° F. (8° C.), with the
following results:--
1. In distilled water Scarcely at all
plumped.
2. „ water saturated with CO₂ Well plumped.
3. „ „ with lime bicarbonate, 20° German} Tolerably plump.
scale of hardness }
4. „ „ „ magnesia bicarbonate, 20° do. „ „
5. „ „ „ lime sulphate 20° do. Well plumped.
6. „ „ „ magnesia sulphate, 20° do Best plumped.
7. „ „ „ magnesium chloride, 20° do. Not at all plumped.
8. „ „ „ common salt, 20° do. „ „
(1 German degree of hardness corresponds to 1 of CaO in 100,000.)
[66] Gerber, iii. (1877) p. 183.
The peculiarities which were shown by the hide pieces on removal from
the water were maintained throughout the tanning, which was conducted in
imitation of the German method, the hide being swollen and coloured
through in weak birch-bark liquors, made with distilled water and
acidified in each case with equal quantities of lactic acid, and finally
laid away, till tanned, in a mixture of oak bark and valonia. No. 6,
from magnesium sulphate, was the best; then No. 2; No. 3 was less good,
but all the pieces from 1 to 6 were firm, close and of good substance
and texture, No. 1 having swelled well in the sour liquor. On the other
hand, 7 and 8 scarcely swelled in liquor, but remained flat throughout,
and were looser, thinner and of finer fibre. From this experiment it is
clear that while sulphates and carbonates exert a favourable influence
on plumping, chlorides do the reverse, as they themselves not only do
not plump, but they place the hides in an unfavourable condition for the
plumping action of acids in the liquors. These experiments are quite
borne out by the writer’s experience in practice. The water at the
Lowlights Tannery, which in dry weather was mostly obtained from beds of
what was originally sea-sand, and which consequently contained a very
abnormal proportion of chlorides (up to 68 pts. NaCl per 100,000),
required special and very careful management to make thick leather,
notwithstanding the fact that it contained a considerable quantity of
calcium and magnesium sulphates. These facts also indicate the
importance of the thorough removal of salt from hides intended for
sole-leather. Plumping is not a desirable thing in leather intended for
dressing purposes, and it is possible that the use of a small percentage
of salt in the liquors or wash waters might in some cases enable bating
to be dispensed with. Like a bate, salt would dissolve a small
proportion of hide substance (see p. 65). There is no practicable means
of removing chlorides from water, but Eitner suggests the addition of a
small quantity of sulphuric acid to water containing much temporary
hardness (bicarbonates), in order to convert it into permanent hardness
(sulphates), which, as stated above, plumps better. The amount required
may be calculated from an acidimetric determination of temporary
hardness (see L.I.L.B., p. 19). A simple but not very accurate guide, is
to add enough acid to purple, but not to redden litmus paper even after
moving the latter about in the water for some minutes. In practice the
acid must of course be very thoroughly mixed with the water by stirring
and plunging. It must be borne in mind that Eitner’s experiment was on
sweated hides, and that with limed hide, which is kept plump by the
dissolved lime retained in the hide, different results as regards
carbonic acid and bicarbonates would be obtained. Both these would
convert the lime in the hide into chalk, which is insoluble and inert,
and the hide would fall, at any rate when the lime was completely
carbonated, while hides would remain plumpest in waters most free from
substances capable of neutralising lime. From this we may conclude, what
may be _a priori_ expected, that the purer the water, the plumper limed
hides remain in it. In soft but peaty waters, hides fall rapidly, from
the neutralisation of the lime by the weak organic acids of the peat.
Such waters are dangerous for domestic use from their solvent action on
lead, but this danger can be entirely removed by storing the water in
limestone reservoirs, or allowing it to flow slowly through a limestone
culvert before use. In some towns in the north of England a small
quantity of lime is added so as to neutralise the water as it leaves the
reservoir and before it enters the mains.
Wherever the conditions of putrefaction or decaying organic matter are
present, as in a bate, hides fall rapidly, and in extreme cases even the
presence of the stronger acids will not maintain plumpness. Eitner
mentions the case of a stream at Vissoko in Bosnia, which was in special
repute among the tanners from its power of pulling down hides rapidly,
and which took its rise in a common on which the pigs of the town were
pastured. The causes of this action are no doubt due to the products of
putrefaction, but are somewhat obscure. Bacteria present in water are a
frequent source of injury in the soaks, and probably in other stages of
the tanning process.
Rain water and the water of streams in mountain districts of hard
igneous rock are generally nearly free from mineral constituents. This
is the case with the Glasgow water from Loch Katrine, and the Thirlmere
water which supplies Manchester. Such water, if cold enough, and free
from mud and organic impurity, is the best for almost every purpose in
the tannery. Most river-water contains material quantities of mineral
matter, though it is usually softer than that of springs or wells.
For further details as to the chemical examination of water, and the
methods of determining the amounts of its different constituents, see
L.I.L.B., pp. 18 _et seq._
CHAPTER XI.
_SOAKING AND SOFTENING OF HIDES AND SKINS._
As has been explained in the last chapter, hides and skins come into the
hands of the tanner either uncured (“green”), as they are taken off the
animal, preserved with salt or some other antiseptic, dried, or
“drysalted” in which both methods are combined. His object in each case
is to remove blood and dirt, and to restore the hide to its soft and
natural condition; but the treatment required varies much with the state
of the hides.
_Fresh hides_ merely require cleansing from blood and dirt. This is
necessary because the blood causes bad colour, and both blood, lymph and
adhering dung are sources of putrefaction, which ultimately attacks the
grain and fibrous structure of the hide. Hence washed hides keep better
than unwashed. Cold water is most desirable, as checking putrefaction.
If the water is much over 10° C., or if it is charged with organic
matter and ferment-germs; or if, as is too generally the case, the hides
are in a partially putrid state when received, the time of soaking must
be reduced as much as possible, and it may be necessary to sterilise the
water with carbolic acid or creolin (pp. 26, 28). In such cases the use
of a wash-wheel, or tumbler, is very desirable, rapidly cleansing the
hides and removing adhering dung, which interferes with the liming, and
is a serious cause of damaged grain. The American pattern of wash-wheel
shown in Fig. 21 is very suitable for the purpose. In no case is it
desirable to allow green hides to lie for more than a few hours in
water; and unwise treatment at this time is the cause of many troubles,
which are only detected at later stages, and which are very difficult to
trace to their source. “Weak grain,” in which the hyaline layer (p. 50)
is destroyed, and which tans a whitish colour; “pricking,” or
perforation of the grain with small pinholes, which may go on to
“pitting” with larger holes, and a general weakening of fibre, with
softening and needless loss of weight, are among these results. An
instructive instance may be quoted. A large tanner found that his
curried leather was affected with small spots and rings of darker
colour, which rendered it quite unfit for staining, and which reappeared
even when the leather was buffed. When finished as black grain, these
spots had a tendency to “spue,” or rise as little pimples of resinous
matter. Before the leather was stuffed no defect was noticeable to the
eye, but either then, or on stripping the grease by a solvent, they
could be seen under the microscope as lighter patches of open and porous
grain which absorbed more than their share of fat. During the tanning
process they could hardly be detected, but in the first colouring they
appeared for a few hours as blackish specks almost exactly like those
caused by particles of iron or iron-rust. By careful observation they
were traced back to the limes; specimens of the limed hide were
submitted to Director Eitner, who identified the defect as “_Stippen_,”
caused by a species of bacteria, which cannot subsist in limes, and
which therefore must have been in the soaks. These, which had been
somewhat neglected from pressure of work, were cleaned out and
sterilised with creolin solution, and the mischief ceased. It is worth
noting that the tanner dated the beginning of the trouble from the
soaking of some “Spanish” horse-hides, which may have introduced the
infection. Several very similar cases have come under the writer’s
notice.
It is not absolutely necessary to soak fresh hides or skins at all
before liming, and where the water is scarce or unfavourable, or the
skins tainted or “slipping” hair, it is best to pass straight into a
weak lime. In this case the limes must be worked in shifts (see p. 131)
and the whole of the oldest liquor run away and the hides rapidly
changed into a fresh lime, or the limes will become so charged with
organic matter and bacteria that the hides will cease to plump, and may
even putrefy.
_Salted hides and skins_ require more soaking and more thorough washing
than fresh ones, as it is not only necessary to remove the salt, but to
soften and plump the fibre which has been dehydrated and contracted by
salting. If goods with salt in them are taken into limes, they will not
plump properly,[67] and creases and wrinkles (drawn grain) are formed
which no after-treatment will remove. This is especially important in
sole leather. In deciding on a method, we must bear in mind that salt is
easily soluble, and diffuses rapidly into water or weaker solutions, and
that weak salt solutions tend to prevent the plumping of the fibre,
while those of about 10 per cent. have considerable power of dissolving
the cementing matter of the fibres (p. 65) and so lessening weight and
firmness. It may also be noted that though salt is not a true
disinfectant (p. 22), salted hides are much less prone to putrefaction
than fresh ones, and therefore a longer soaking may be safely given.
[67] Experiments mentioned on p. 89 throw some doubt on the power of
salt to prevent plumping in the limes, though the opinion in the text
is generally held by tanners.
These conditions point to the desirability of free exposure to water,
attained by suspending, handling frequently, or tumbling, and repeated
changes to remove the salt. The degree of removal of salt is easily
determined by the estimation of Cl in the last wash-water (L.I.L.B., p.
18). American tanners universally soak wet-salted hides three or four
days with as many changes of water, and frequently finish by a few
minutes in a wash-wheel. Any washing tumbler may be used; but the cheap
and simple construction of the American wash-wheel will be easily
understood from Fig. 21. The sides are open, so that hides can be put in
or removed between the spokes. The rim of the wheel is generally
perforated, for the escape of water which is supplied by a pipe passing
through the axis; and the wheel is often driven by a chain or rope round
its circumference. No severe mechanical treatment, such as “stocking,”
is necessary or desirable for green or salted hides.
_Dry and dry-salted hides_ require much longer soaking than wet-salted,
the amount naturally depending on the thickness of the hide and the
character of drying. Even thin skins when strongly dried require
considerable time to soften and swell the fibres, although they soon
become wet-through and flexible. Many different methods of soaking have
been employed. Sometimes hides are suspended in running water; sometimes
laid in soaks which may be either renewed, or allowed to putrefy;
sometimes in water to which salt, borax or carbolic acid has been
added, to prevent putrefaction; and more recently weak solutions of
caustic soda, sulphide of sodium or sulphurous acid have been used with
much success.
[Illustration: FIG. 21.--American Wash-wheel.]
The first of these methods, were it desirable, is rarely possible in
these days of River Pollution Acts; of the others, it is difficult to
say which is better, since the treatment desirable varies with the
hardness of the hide and the temperature at which it has been dried. The
great object is to thoroughly soften the hide without allowing
putrefaction to injure it. As dried hides are often damaged already from
this cause, either before drying, or from becoming moist and heated on
shipboard, it is frequently no easy matter to accomplish this. The fresh
hide, as has been seen, contains considerable portions of albumin, and
if the hide is dried at a high temperature, this may become wholly or
partially coagulated and insoluble. The gelatinous fibre and the coriin
(if indeed the latter exists ready formed in the fresh hide) do not
coagulate by heat, but also become less readily soluble. Gelatin dried
at 130° C. can only be redissolved by acids, or water at 120° C.
Eitner[68] experimented with pieces of green calf-skin of equal
thickness, which were dried at different temperatures, with results
given in the following table:--
-------+-----------+---------+---------+-------------+--------------
Sample.|Temperature|Remarks. |Time of | Remarks. | Coriin
|of Drying. | |Softening| |Dissolved by
| | |in Water.| |Salt Solution.
-------+-----------+---------+---------+-------------+--------------
I. | 15° C. |In vacuo |24 hours |{Without } |1·68 per cent.
| | |{mechanical} |
II. | 22° C. |In sun |2 days |{ work } |1·62 „
| | | | |
III. | 35° C. |In drying|5 „ |Twice worked |0·15 „
| | closet | | |
| | | | |
| | | {Refused to soften} |
IV. | 60° C. | „ | {sufficiently for } |traces
| | | {tanning } |
-------+-----------+---------+---------+-------------+--------------
[68] Gerber, 1880, p. 112.
Hence it is evident that, for hides dried at low temperatures, short
soaking in fresh and cold water is sufficient, and, except in warm
weather, there would be little danger of putrefaction. With harder
drying, longer time is required, and more vigorous measures may be
necessary. A well-known tanner recommended a brine of 30°-35° barkometer
(sp. gr. 1·035, or about 5 per cent. of NaCl). This has a double action,
not only preserving from putrefaction, but dissolving a portion of the
hide-substance in the form of coriin, which is undoubtedly a loss to the
tanner, though it is questionable if there is any process which will
soften overdried hides without loss of weight; since even prolonged
soaking in cold water at a temperature which is too low to allow of
putrefaction taking place will dissolve a serious amount of
hide-substance. Chlorides, however, do not seem well adapted for the
purpose in view, from their weak antiseptic power and tendency to
prevent swelling. To prevent this Jackson Schulz advised the use of
water at 80° F. for soaking during the winter months. Water containing a
small quantity (0·1 per cent.) of carbolic acid has been recommended for
the purpose, and will prevent putrefaction, while it has no solvent
power on the hide, but, on the contrary, tends to coagulate and render
insoluble albuminous matters. Borax has been proposed for the same
purpose, and, in 1 per cent. solution, certainly prevents putrefaction,
and has considerable softening power, but is far too costly. Other
methods of chemical softening are described on p. 115.
For some descriptions of hides, and notably for India kips, putrid soaks
were formerly much employed, the putrefactive action softening and
rendering soluble the hardened tissue. In India the native tanners
soften their hides in very few hours by plunging them in putrid pools,
into which every description of tannery refuse is allowed to run.
Putrefactive processes, however, are always dangerous, as the action,
through changes of temperature, or variation in the previous state of
the liquor, is apt to be irregular, and either to attack one portion of
the hide before another, or to proceed faster than was expected. Hides
are also frequently more or less damaged by putrefaction and heating
during the process of cure, and these damages are accentuated in a
putrid soak. Hence hides in the soaks require constant and careful
watching, and the goods must be withdrawn as soon as they are thoroughly
softened, for the putrefaction is constantly destroying as well as
softening the hides. It is possible that putrefactive softening is less
injurious to kips, and such goods as are intended for upper-leather,
than to those for sole purposes, as it is generally considered necessary
in the former case that a good deal of the albumen and interfibrillary
matter be removed, and that the fibre be well divided into its
constituent fibrils for the sake of softness and pliability; and thus
the putrid soak, if acting rightly, accomplishes part of the work which
would afterwards have to be done by the lime and the bate, as the actual
fibre of the hide seems less readily putrescible than the softer
cementing substance.
Putrefaction is caused, as we have seen, by a great variety of living
organisms, each of which has its own special products and modes of
action. It is quite possible that, if we knew what precise form of
putrefaction was most advantageous, we might by appropriate conditions
be able to encourage it, to the exclusion of others, and obtain better
results than at present. Putrid soaks (in the old sense) are, however,
disused in the present day by all enlightened tanners, as it is
recognised that the risks outbalance the advantages, and when drysalted
hides are worked, the soluble salts of the cure accumulate to an
injurious extent. The modern method, where no chemicals are used, is to
give one fresh water at least to each pack of hides or skins. Even in
this case considerable putrefaction takes place where the soaking
occupies 7 to 14 days, as is the case with kips and hides, and it is
probable that the use of chemical and antiseptic methods of soaking will
ultimately be generally adopted, both on technical and sanitary grounds.
The use of dilute acids for softening has much to recommend it, their
power of causing the fibre to swell and absorb water being quite equal
to that of the alkalies, while few, if any, putrefactive bacteria can
thrive in an acid liquid. Very dilute sulphuric acid has been used with
success to dissolve the alkaline “plaster” of East India kips (p. 39).
It has considerable disinfectant power (p. 23), but its action on the
hide-fibre is undesirably strong.
Sulphurous acid is much more suitable. Its use for this purpose was
patented by Maynard, along with a number of other possible uses, but the
patent has now lapsed, and he does not seem to have succeeded in
introducing it into practice. Experiments at the Yorkshire College, and
also at a tannery on a manufacturing scale, have shown that the method
is capable of excellent results. The hides are soaked for 24-48 hours in
a solution of sulphurous acid containing about 2 per cent. of SO₂ (for
manufacture, compare p. 24; for testing, L.I.L.B., pp. 16, 37), and are
then transferred to water, where they swell freely to their full
thickness. They may be either limed at once, or first neutralised with
dilute caustic soda, ammonia, or sulphide of sodium, which, for dressing
leather, is perhaps desirable. No putrefaction takes place, even if they
are retained for a considerable time in water, and the acid has little
or no solvent effect on the hide-fibre, the strength of which is well
preserved. The liming, however, must either be conducted with the aid of
sodium sulphide or in old limes, since the sterile condition of the
hides renders liming in fresh lime very slow (cp. p. 137). For
experimental purposes a ¹⁄₂ per cent. solution of Boakes’
“metabisulphite of soda” may be used, to which ¹⁄₄ per cent. of
concentrated sulphuric acid previously diluted with water is gradually
added during the soaking, the hides being first withdrawn. For permanent
work it will be found much cheaper to manufacture the acid on the spot
by burning sulphur.
The use of solutions of caustic soda (1 part per 1000), or of sodium
sulphide (1¹⁄₂-3 parts per 1000) as suggested by Eitner, seems at
present likely to supersede all other methods of softening from their
simplicity and safety. Twenty-four to forty-eight hours in either of
these solutions, which may if necessary be followed by a short soak in
plain water, seem sufficient to soften either kips or hides. Experiments
at the Yorkshire College have shown that solutions of this strength have
little or no solvent action on the hide-fibre, but promote its swelling
in water so effectively that no mechanical softening is needed (though a
slight drumming is advantageous), while putrefaction is almost entirely
prevented, so that the solution may be repeatedly used if kept up to its
original strength, which is easily determined with standard acid and
phenolphthalein (see L.I.L.B., p. 17). Neither caustic soda nor sodium
sulphide have any injurious effect on liming, though it may prove
somewhat slower than with the older methods, where the epidermis was
partially destroyed by the action of putrid ferments. The dilute
solutions used are not only less injurious to the hide than those of
greater strength, but they are also more effective in softening. Eitner
(Gerber, 1899, p. 584) states that when using a solution of caustic soda
of 1 part in 1000 strength, the time required to soften some hides was
only two days, as against three days for a sodium sulphide liquor, and
four days for pure water, and that with the soda solution only about 0·6
per cent of the hide-substance of the skin was dissolved out, whilst
when sodium sulphide was used it was 0·7 per cent., and with pure water
alone no less than 1·9 per cent. was lost by solution.
The use of moderately warm water (40° C.) in a drum is quite successful
in rapidly softening sound hides after they have previously been soaked
for some days in cold water; but if they are tainted in the cure, it is
very apt to intensify the mischief. Hides which have partially putrefied
internally, or which have been exposed to a hot sun while the interior
is still moist, are very apt to appear sound while dry, but to blister
or go to pieces from the destruction of the fibres as soon as they are
limed, and this in spite of even the most careful treatment. For tainted
hides, caustic soda is probably preferable to sodium sulphide.
Many chemicals have been patented for softening hides. Sulphide of
arsenic is said to be in use, and if dissolved in caustic soda solution
would differ little in its effect from ordinary sulphide of sodium.
Saltpetre has also been employed, but its effect, if any, was probably
merely antiseptic. Ordinary sodium carbonate has been used, but is less
effective than caustic soda. Gas liquor and mixtures of this with tar
and water were patented by Barron, and probably the first would soften
by virtue of its ammonia and sulphides, while tar contains carbolic
acid. Probably the most absurd mixture of all was patented by Berry,
which consisted of ¹⁄₂ bucket of slaked lime, ¹⁄₂ bucket of wood-ashes,
12 lbs. of potash, 5 lbs. of oil of vitriol, and 4 lbs. of spirit of
salt!
[Illustration: FIG. 22.--Faller Stocks.]
Beside merely soaking the hides, it is sometimes necessary to work them
mechanically, to promote their softening; this was formerly accomplished
by “breaking over” the hides on the beam with a blunt knife. This
process is still in use for skins of many sorts, but for the heavier
classes of leather is now usually superseded or supplemented by the use
of “stocks,” or drums. The former consist of a wooden or metallic box,
of peculiar shape, wherein work two very heavy hammers, raised
alternately by pins or cams on a wheel, and let fall upon the hides,
which they force up against the curved end of the box with a sort of
kneading action. The ordinary form of this machine is shown in Fig. 22.
A more modern form, which seems to possess some advantages, is the
American “double-shover,” or “hide-mill,” seen in Fig. 23. “Crank
stocks,” similar in form to the faller stocks, but driven by cranks,
are sometimes used for softening, but are better adapted to lighter
uses.
[Illustration: FIG. 23.--American Hide-Mill.]
The number of hides which can be stocked at once naturally varies with
the size of both hides and stocks, but should be such that the hides
work regularly and steadily over and over. The whole number should not
be put in at once, but should be added one after another, as they get
into regular work. The duration of stocking is 10-30 min., according to
the condition and character of the hides. Hides should not be stocked
until they are so far softened that they can be doubled sharply, without
breaking or straining the fibre. After stocking, they must be soaked
again for a short time, and then be brought into an old lime. A small
quantity of sodium sulphide added to the soaks or in the stocks has been
recommended as of great value in softening obstinate hides, and probably
with justice, from its well-known softening action upon cellular and
horny tissues.
Tumbler drums of various forms may also be used with good effect for
softening purposes, especially for skins, and are much less detrimental
than stocking, both as regards the weight and quality of the goods.
For sole leather, and even for kips, the use of stocks has in recent
years been entirely discarded by many of the more advanced tanners. If
mechanical work is required at all, the drum is preferred, and is
sometimes employed after a few days’ liming, the goods being first
merely softened in fresh water. The use of caustic soda, sodium
sulphide, or sulphurous acid renders mechanical softening almost
unnecessary.
[Illustration: FIG. 24.--Drum for Washing or Tanning.]
The drums employed are in principle like a barrel-churn, and are large
cylindrical wooden chambers 6 to 12 feet in diameter, and fitted inside
either with shelves like the floats of a water-wheel, or with rounded
pegs on which the hides fall. The American wash-wheel figured on p. 111
is a machine of this kind, and one of a more elaborate description is
shown in Fig. 24. Drums are not only used for softening, but for
tanning, dyeing, and many other purposes in leather manufacture. It is
advantageous to be able to reverse the direction of their rotation to
prevent the rolling up of the hides.
CHAPTER XII.
_DEPILATION._
After the softening and cleansing of the hide or skin is completed, and
before proceeding to tan it, it is usually necessary to remove the hair
or wool. The earliest method of accomplishing this was by means of
incipient putrefaction, which attacks in the first instance the soft
mucous matter of the epidermis, and thus loosens the hair without
materially injuring the true skin. This loosening of the hair often
takes place accidentally in hides which have been kept too long without
salting, and is known as “slipping,” and is apt to be accompanied by
some degree of injury to the grain. The old method of loosening the hair
by putrefaction, or, as it is generally called, “sweating,” was to lay
the hides in piles, usually in some warm and damp place. Occasionally a
slight preliminary salting was given to prevent too much putrefaction of
the hide. The action in this case, however, was very irregular, and it
has been quite abandoned in all civilised countries.
[Illustration: FIG. 25.--Sweat-Pit.]
The method which is now used is to hang the hides in a closed chamber,
generally called a “sweat-pit,” Fig. 25, but usually constructed above
the ground-level and protected from sudden changes of temperature by
double walls, or by mounds of earth. The hides are hung in the
sweat-pit, in small chambers each capable of holding 50 or 100 hides.
The temperature is kept at about 15° to 20° C., the air being warmed, if
necessary, by the admission of steam below a perforated floor, or cooled
by a shower of water from sprinklers, so arranged as not to play
directly on the skins, and is thus always kept saturated with moisture.
Little if any ventilation is allowed, and a large quantity of ammonia is
given off from the decomposition of the organic matter, and no doubt
contributes to the solution of the epidermis and the loosening of the
hair, as the writer has found that ammoniacal vapours alone very
speedily produce this effect.
After 4-6 days of this treatment, the hair is sufficiently loosened to
be removed by working the skin over the beam with a blunt knife, or by
means of the stocks or hide-mill (see p. 116). Great care and
watchfulness are required to avoid injury to the grain by putrefaction.
The hide is in a slimy and completely flaccid and “fallen” condition,
and some trouble is occasioned by the hair being worked into the flesh
by the hide-mill, to obviate which, a slight liming is frequently given
after the sweating. Hides which have been unhaired in this way require
to be swollen by acid in the liquors in order to produce a satisfactory
sole-leather, as the sweating process does not swell or split up the
fibres.
In some European tanneries a similar process, but at a higher
temperature, is employed, and it is also largely used for sheep-skins
under the name of “staling,” but in this case is sometimes conducted in
a very rude and primitive manner, and frequently with the result of
considerable injury to the pelt.
The great objection to the sweating process, however carefully
conducted, is the liability of putrefaction to attack the skin itself,
causing “weak grain.” Its most advantageous use is for sole leather, as,
although the solution of the hide-substance may not be very much less
than in the case of liming, the dissolved matter remains in the hide
instead of being washed out, and being fixed by the tannin, contributes
to the solidity of the leather.
In England, lime is the agent almost universally employed for unhairing,
though every tanner admits its deficiencies and disadvantages. It is
hard, however, to recommend a substitute which is free from the same or
greater evils, and lime has one or two valuable qualities which will
make it very difficult to supersede. One of these is that, though it
inevitably causes loss of substance and weight, it is also impossible,
with any reasonable care, totally to destroy a pack of hides by its use;
which is by no means the case with some of its rivals. Another advantage
is that, owing to the very limited solubility of lime in water, it is of
comparatively small consequence whether much or little is used; and even
if the hides are left in a few days longer than necessary, the mischief,
though certain, is only to be detected by careful and accurate
observation. With all other methods, exact time and quantity are of
primary importance, and it is not easy to get ordinary workmen to pay
the necessary attention to such details. Again, the qualities of lime,
its virtues and failings, have been matter of experience for hundreds of
years, and so far as such experience can teach, we know exactly how to
deal with it. A new method, on the other hand, brings new and
unlooked-for difficulties, and often requires changes in other parts of
the process, as well as in the mere unhairing, to make it successful. As
our knowledge of the chemical and physical changes involved becomes
greater, we may look to overcoming these obstacles more readily.
The universal source of lime is chalk or limestone, which consists of
calcium carbonate, and from which the carbon dioxide is driven off by
burning in a kiln. Many limestones, however, are far from being pure
calcium carbonate, but contain large proportions of magnesia, iron and
alumina, the latter perhaps originally deposited in the form of clay
with the sediment from which the stone was formed. Such clay limestones
when burnt yield natural cements, like oolite and other “hydraulic”
limes, which are capable of setting even under water. The presence of
magnesia and clay is injurious not only by diminishing the amount of
lime present, but by making the lime much more difficult to slake; and
iron oxide, though quite insoluble, may become mechanically fixed in the
grain of the hide, and may be the cause of subsequent stains. The
burning of lime in the kiln is probably not quite so simple an operation
as the equations of the text-books would suggest. By mere heating, the
carbonate can, it is true, be decomposed, but to do this completely a
good white heat is required, which is rarely attained in practical
burning, and it is probable that at least a part of the carbon dioxide
present is reduced to carbon monoxide by the combustible fuel-gases, and
so separated from the lime, for which it has no affinity. Carbon
monoxide is the cause of the intensely poisonous character of limekiln
gases, the pure dioxide being irrespirable, but not strictly poisonous.
Quicklime, CaO, on coming in contact with water, combines with it with
the evolution of considerable heat, becoming slaked or converted into
hydrate, Ca(OH)₂. This change takes place rapidly and easily when the
lime is light and porous, such as is obtained by the burning of chalk or
good limestone at a low temperature; but if it has been too intensely
heated or “over-burnt,” or contains silicates or other salts which fuse
at the temperature of the kiln, a compact lime is formed which slakes
with difficulty and extreme slowness, thus being lost to the tanner, or
leading to the still more serious result of burning holes in the hides
by the heat produced by slaking in contact with them. It is stated by Le
Chatelier[69] that for dense limes 24-48 hours is frequently required
for complete slaking in the cold, while magnesia is still more
obstinate, months being sometimes necessary for the complete hydration
of hard-burnt samples; and mixtures of lime and magnesia are
intermediate in their character. Slaking is greatly assisted by heat,
even heavily burnt magnesia being hydrated in about six hours at 100° C.
Slaking is also much more rapid in a dilute solution (2 per cent.) of
calcium or magnesium chloride. From these facts it is easy to deduce the
reason why a suitable quantity of water, neither too much nor too
little, is desirable for the rapid and effectual slaking of lime. If too
little is used, the lime is only partially slaked, and it is not easy
for further portions of water to gain access to the interior of the
powdery mass. On the other hand, if it is “drowned” by excess, the
temperature is lowered, the process goes on slowly, and the mass does
not readily fall into powder, and so fails to be utilised in the liming
process. Of all methods of slaking lime, the ordinary one of tipping it
direct into the lime-pits is perhaps the most irrational, leading to the
formation of unslaked lumps which may burn the hides, and which,
together with stones and dirt, rapidly choke the pits with useless
matter. The best process is that adopted by builders and in many
Continental yards, in which a large quantity of lime is slaked in a
shallow tank by throwing on it sufficient water to thoroughly wet it,
and after allowing it to heat and fall for 24 hours, adding enough water
to convert it into a stiff paste. In this form it may be kept for months
without material deterioration. When required for use, a suitable
quantity of the paste is dug out, and well stirred with water in a tub
or tank before running into the pit when the stones and sand remain in
the tank. In this way all nuisance from dust is also avoided. If lime is
stored unslaked, it gradually absorbs moisture from the air, falling,
and soon becoming dusty and difficult to slake completely, while the
traces of carbon dioxide in the air gradually convert it into useless
carbonate.
[69] Bull. de la Soc. d’Encouragement, 1895, x. pp. 52-62; Journ. Soc.
Chem. Ind., 1895, p. 575.
The solubility of lime in water is very limited, and the figures
determined by different chemists do not agree very satisfactorily. The
following table gives the result of determinations made by Mr. A.
Guthrie in the Author’s laboratory, and is probably one of the most
accurate[70]:--
100 c.c. of saturated lime water at 5° C. contain 0·1350 grm. of CaO.
„ „ 10° „ 0·1342 „
„ „ 15° „ 0·1320 „
„ „ 20° „ 0·1293 „
„ „ 25° „ 0·1254 „
„ „ 30° „ 0·1219 „
„ „ 35° „ 0·1161 „
„ „ 40° „ 0·1119 „
„ „ 50° „ 0·0981 „
„ „ 60° „ 0·0879 „
„ „ 70° „ 0·0781 „
„ „ 80° „ 0·0740 „
„ „ 90° „ 0·0696 „
„ „ 100° „ 0·0597 „
[70] Journ. Soc. Chem. Ind., 1901, p. 224.
It will be noticed that unlike that of most substances, the solubility
of lime in water diminishes as the temperature is raised. It is
therefore necessary in employing lime-water as a standard solution to
take care that it is saturated at a constant temperature. The results
given in the above table are those from pure marble lime. Where the
ordinary impure limes from limestone are employed, a somewhat stronger
lime-water is often obtained. This is difficult to explain, but possibly
some double hydrate of lime and magnesia is formed which is more
soluble than either hydrate alone. The results harmonise with the old
belief of tanners that chalk-lime is milder in its action on skin than
that made from less pure limestones. The solubility of any given lime is
easily determined by adding it in excess to water in a stoppered flask,
and shaking frequently until a solution of constant strength is
obtained. A known volume of this solution (which must be clear and free
from undissolved lime) is then titrated with N/10 hydrochloric acid,
using phenolphthalein as the indicator.
Saturated lime-water may be conveniently used as an alkaline standard
solution for many purposes, and if kept on excess of lime is always
caustic, and varies very little in strength at ordinary laboratory
temperatures. The solution is nearly ¹⁄₂₀ normal, but for accurate work
its strength should be exactly determined with N/10 acid. 1 liter of
pure lime-water at 15° C. should require 471·4 c.c. of N/10 acid for
neutralisation.
Lime is much more soluble in sugar solutions than in water. Such
solutions have been used as standard solutions, and sugar has been added
to limes to increase the action on the hides.
The following is the analysis of a lime used in a Leeds tannery, which
was made by Mr. G. W. Flower, B.Sc., in the Leather Industries
Laboratory of the Yorkshire College[71]:--
Per cent.
SiO₂ and insoluble matter 17·70
Fe₂O₃ 6·42
CaO 49·86
CaCO₃ 14·21
CaSO₄ 3·01
CaCl₂ 0·33
MgO 2·09
Organic matter 0·80
Moisture by difference 5·58
------
100·00
[71] Journ. Soc. Chem. Ind., 1901, p. 224.
The sample only contained 31·02 per cent. of available lime, the
remainder being probably combined with the silica. It also contained an
appreciable quantity of iron oxide, which might lodge mechanically in
the pores of the skin and become dissolved in later processes, darkening
the colour of the leather. The lime was also under-burnt, judging from
the amount of carbonate it contained.
For comparison with this, the analysis of a good specimen of
carboniferous-limestone lime from Buxton may be given:--
Per cent.
CaO 91·95
MgO 1·30
CO₂ and moisture 6·75
------
100·00
_Determination of “Available” Lime._--The practical value of lime for
the tanner is easily determined by drawing a sample by breaking off
small pieces from a number of lumps of the bulk, coarsely pulverising
them in a mortar, and then rapidly grinding a portion as fine as
possible, and transferring it at once to a stoppered bottle for
weighing. A portion of this, not exceeding 1 grm., is shaken into a
stoppered liter flask, which is filled up roughly to the mark with hot
and well-boiled distilled water, and allowed to stand for some hours
with occasional shaking. When cold it is filled exactly to the mark with
cold distilled water, well shaken again and allowed to settle, or
rapidly filtered, and 25 or 50 c.c. of the clear liquid withdrawn with a
pipette and titrated with N/10 hydrochloric or sulphuric acid and
phenolphthalein. Each cubic centimeter of N/10 acid equals ·0028 grm.
CaO. It is generally a very mistaken economy to make use of an inferior
lime for tanning purposes, as any saving in cost is discounted by the
larger quantity required, the more frequent cleaning of the pits, and
the danger of stains and of burns from imperfect slaking.
The action of lime on the hide has already been spoken of to some
extent. It is throughout a solvent one. The hardened cells of the
epidermis swell up and soften, the mucous or growing layer and the
hair-sheaths are loosened and dissolved, so that, on scraping with a
blunt knife, both come away more or less completely with the hair
(constituting “scud” or “scurf,” Ger. _Gneist_ or _Grund_). The hair
itself is very slightly altered, except at its soft and growing
root-bulb, but the true skin is vigorously acted on. The fibres swell
and absorb water, so that the hides become plump and swollen, and, at
the same time, the “cement-substance” of the fibres is dissolved, and
they become split-up into finer fibrils: the fibrils themselves become
first swollen and transparent, and finally corroded, and even dissolved.
A similar swelling of the fibres is produced by both alkalies and acids,
and is probably due to weak combinations formed with the
fibre-substance, which have greater affinities for water than the
unaltered hide.[72] This swelling is useful to the tanner, since it
renders the hide easier to “flesh” (i.e. to free from the adhering
flesh) on account of the greater firmness which it gives to the true
skin. It also assists the tanning, by splitting up the fibre into its
individual fibrils, and so exposing a greater surface to the action of
the liquors. This is advantageous in dressing-leather which is
afterwards tanned in sweet liquors, and which must have the
cement-substance of the fibres dissolved and removed for the sake of
flexibility; and, in the case of sole-leather, it is necessary for sake
of weight and firmness that the hide be plumped at some stage of the
process; but it is probable that this effect is produced with less loss
of substance and solidity by suitable acidity of the tanning-liquors.
Another advantage of lime is that it acts on the fat of the hide,
converting it more or less completely into an insoluble soap,[73] and so
hindering its injurious effects on the after tanning process, and on the
finished leather. If strong acids whether mineral or organic are used
later on, this lime soap is decomposed, and the grease is again set
free. In sweated or very low-limed hides this grease is a formidable
evil, causing darkening or grease spots on the finished leather.
[72] Cp. p. 84.
[73] This has been questioned, but I have satisfied myself it is
correct.
The customary method of liming is simply to lay the hides horizontally
one at once in milk of lime in large pits, taking care that each hide is
completely immersed before the next is put into the pit, so as to ensure
a sufficiency of liquor between them. Every day, or even twice a day,
the hides are drawn out (“hauled”), and the pit is well plunged up, to
distribute the undissolved lime through the liquor. The hides are then
drawn in again (“set”), care being taken that they are fully spread out.
How much lime is required is doubtful, but owing to its limited
solubility, an excess, if well slaked, is rather wasteful than
injurious. Great differences exist in the quantity of the lime used, the
time given, and the method of working, not only for various classes of
leather, but for the same kinds in different yards. Lime, as we have
seen, is only soluble to the extent of about 1·25 grm. per litre, or (as
1 cub. foot of water weighs about 1000 oz.) say 1¹⁄₄ oz. per cub. foot,
or, in an ordinary lime-pit, not more than ¹⁄₄ lb. per hide. Only the
lime in solution acts on the hide, but it is necessary to provide a
surplus of solid lime which dissolves as that in the liquor is consumed
or absorbed by the hide; and this is especially the case where, as is
generally customary, the hides are laid flat in pits, so that no
circulation of liquor is possible. Where hides are suspended in
lime-water, which is constantly circulated and kept up to its full
strength by agitation in another vessel with solid lime, they unhair as
quickly as with milk of lime, but the method seems, in the case of lime,
to present no special advantage over the ordinary one, if in the latter
the hides are hauled sufficiently often to keep the lime uniformly
distributed. The case is otherwise in dealing with more soluble
depilatories. Various patents have been taken for methods of liming by
suspending in liquors, but the idea is now public property, and is
largely used on the Continent. It is necessary that the lime which
settles to the bottom of the pit should be agitated and kept in
suspension, which may be effected either by moving the hides on a frame
as in “suspenders” (p. 221), or by agitators acting on the principle of
pumps, and raising the liquor and sludge from the bottom. Such agitators
have been patented in Germany, but had been in use much earlier in the
Author’s tanyard. An agitator on the principle of the screw-propeller of
a steamship, placed near the bottom of the pit, and protected by a
lattice, may also be usefully employed (Fig. 26). Skins are frequently
limed in paddles, or stirred up by blowing air into the pit. The latter
method is neither effective nor economical in power.
[Illustration: FIG. 26.--Suspension Lime-Pit.]
As has been noted, the solubility of lime, and consequently the strength
of the lime-liquor, is diminished by rise of temperature, but its
solvent action on hide-substance is much increased. As a consequence,
the loosening of the hair proceeds much more rapidly in warm limes, but
the hides do not plump well, and become loose, hollow and inclined to
“pipe” in the grain, and to weigh out badly, and for sole leather the
method is therefore in every way disastrous. In the few cases among the
lighter leathers where a decided softening and loosening of the texture
of the skin is required, it is possible that useful advantage may be
taken of this effect; but it would be exceedingly difficult to regulate
the temperature of an ordinary lime-pit with accuracy, and better
results could probably be obtained with suspenders in which the liquor
could be constantly circulated. When limes are very cold, in spite of
the greater strength of solution, the action is very much checked, and
where goods are frozen into pits in severe weather, there is but little
danger of overliming, although the usual time may be much exceeded. It
is generally best to work limes at about the ordinary summer
temperature, and this is better done in winter by warming the limeyard
than by any direct heating of the limes. If lime which has cooled after
slaking is used, the water with which limes are made may safely be
warmed in midwinter to a temperature not exceeding 20° C.
The quantity of lime used by different tanners, and for different sorts
of hides and skins, is very variable, not only according to the effect
which it is desired to produce, and the way in which it is used, but
from the arbitrary fancy of the user, since its limited solubility
renders an excess comparatively innocuous. For sole-leather, the amount
recommended varies from under 1 per cent. to 10 or 12 per cent. on the
green weight of the hide; but probably 2-3 per cent. is all that can be
really utilised, the remainder being wasted. In order, however, to
utilise the whole of the lime, very frequent handling or agitation is
required to ensure its uniform distribution. It must also be borne in
mind that the strength of commercial limes varies from above 80 down to
30 per cent. of available calcium oxide.
Von Schroeder has found that a strength of 6 grams of calcium oxide
(CaO) per liter was sufficient, but, in practice, much more is generally
added. It is also noteworthy that a perfectly fresh milk of lime must be
made much stronger than one which has been used. This is partially due
to the fact that some bacterial action takes place in an old lime and
that ammonia is formed which assists unhairing, in addition to the
effect of the lime itself, and partially because the lime in old liquors
remains in suspension for a much longer time, and is thus more evenly
distributed.
A method of liming, sometimes known as the “Buffalo method,” has been
largely adopted for sole-leather in America, and is now used in many
Continental yards. It consists in a very short liming and the subsequent
use of warm water. The limes are also often sharpened by the addition of
a little sodium sulphide or of some other sulphide. Thus, in one large
yard in the States, the hides for sole-leather (salted “packers”) are
limed for 10 hours only with 2 lb. lime and 2¹⁄₂ oz. of sulphide of
sodium per side, and after lying overnight in water of a temperature of
35-45° C., are easily unhaired. A Continental firm lime two days in weak
fresh limes with a little tank-waste, and then treat with water at 32°
C. for 6-8 hours, when the hides are unhaired and returned to the warm
water for two hours before scudding. All sorts of combinations between
liming and hot water treatment can be employed. The longer and stronger
the liming, the lower temperature or shorter time in the water will
suffice. The method is much to be recommended for firm sole-leather, but
it does not saponify grease or swell the fibres thoroughly, and usually
vitriol is used for the latter purpose in a later stage. The hide goes
into the liquors practically free of lime, and the loss of
hide-substance is much less than in the ordinary method of liming.
A point of probably much greater importance than the quantity of lime
used is the length of time during which a lime is worked without change
of liquor. An old lime becomes charged with ammonia and other products
of the action of lime upon the skin, such as tyrosin, leucin
(amidocaproic acid), and some caproic acid, the disagreeable goaty odour
of which is very obvious on acidifying an old lime-liquor with sulphuric
acid, by which considerable quantities of partially altered gelatin are
at the same time precipitated (compare p. 64). Lime has considerable
antiseptic power, and a new lime is practically sterile, but very old
limes, especially in hot weather, often contain large numbers of active
bacteria, which may be seen in the microscope under a good ¹⁄₆-inch
objective. Their presence is always an indication that putrefaction is
going forward, and if their number be very excessive, the leather out of
such limes will generally prove loose, hollow and dull-grained, and in
extreme cases hides may be totally destroyed. Spherical concretions of
calcium carbonate may also be seen under the microscope, resembling on a
smaller scale those found in Permian limestone, and caused perhaps in
both cases by crystallisation from a liquid containing much organic
matter. It is hardly probable that in many tanneries the ammonia would
pay for recovery from the lime-liquors, though it could be easily done
by steaming the old limes in suitable vessels, and condensing the
ammoniacal vapours in dilute sulphuric acid. Its quantity rarely exceeds
0·1 per cent. of NH₃. For methods of estimation of ammonia, see
L.I.L.B., p. 30.
Up to a certain point, it is found that old limes unhair much more
readily, and have a greater softening effect than new ones, which is
often advantageous for dressing goods; though for sole leather, where
weight and firmness are of primary importance, the use of stale limes
must be kept within the narrowest limits. In the finer leathers also,
such as kid and moroccos and coloured calf, where a sound and glossy
grain is desired, the effects mentioned are generally better obtained in
other ways, such as by the use of sulphides. On East India kips and
other dried hides, which are difficult to soften, and which have great
power of resistance to the action of lime, old limes are distinctly
useful, but, even there, there are limits which should not be passed.
Probably no lime ought to be allowed to go for more than three months at
the outside limit without at least a partial change of liquor, and the
system of allowing all the limes in a yard to run for twelve months, and
then cleaning them all together, is almost the worst which can be
planned. A very much better way is to clean the limes in regular
rotation, using, if desired, a portion of the old liquor in making the
new lime, so as to avoid a too sudden transition. The old liquor is
valuable, if at all, for the ammonia and organic matter which it
contains, as the amount of lime in solution is not worth considering.
The ammonia considerably increases the solvent and unhairing power,
while swelling the hide less than an equivalent amount of lime. In some
cases it may be desirable to add ammonia artificially for this purpose.
In this case it will be cheaper and more convenient to add it in the
form of ammonium sulphate than as liquid ammonia. If it be desired to
retain ammonia, the lime should be kept covered. Very old limes
containing excess of ammonia and lime, sometimes in hot weather cause a
transparent swelling of the goods, with destruction of the fibrous
texture.[74] The writer has observed a similar phenomenon in very weak
and old limes strengthened with sulphides, in which hide was left
experimentally for several weeks. The principal effect of the dissolved
animal matter is to enable bacteria to thrive in it, which they will not
do in a fresh lime, but putrid limes probably also contain liquefying
ferments produced by the bacteria present (p. 17), and which dissolve
hide. Eitner has published researches on the amount of hide-substance
dissolved by limes,[75] in which he shows that the loss of substance in
liming sufficiently to unhair is materially greater in old limes than in
fresh ones, although during the first two days of liming the new limes
are decidedly the most active. As he remarks, this justifies the wisdom
of the method, now largely adopted, of working limes in shifts, and
beginning the operation in old limes and completing it in fresh ones.
(See also p. 131.)
[74] Gerber, 1884, pp. 150, 184.
[75] Gerber, 1895, pp. 157-9, 169-72.
For details of the analytical methods employed, Eitner’s original paper
must be consulted, but the annexed table (see next page) summarises his
results. The letters heading the columns have the following meanings.
A. Hide substance precipitated by neutralisation of the lime with
carbonic acid.
B. A further precipitate obtained by slight acidification with
hydrochloric acid.
C. Soluble peptones precipitated by hypochlorous acid or mercuric
nitrate.
It is obvious that none of these figures represent the _total_ dissolved
organic matter, and it is to be regretted that this was not determined.
It is, however, fairly safe to assume that the table correctly
represents the relative solubility in the different liquors. In each
case 2 liters of liquor were used for each kilo of green hide. When old
liquors were employed, the hide-substance they originally contained was
determined, and deducted from the final result.
-------+-----------------------+-------+------------------------+-----
Hide | Description of | Days | Hide-substances in |Loss
Used. | Lime Liquor. |Liming.| Grams per Liter. | per
| | | |cent.
| | | | on
| | +-----+-----+-----+------+ Dry
| | | A. | B. | C. |Total.|Pelt.
--+----+-----------------------+-------+-----+-----+-----+------+-----
1|Ox- |Fresh lime 30 grm. per}| 6[76] |1·068|0·324|2·370| 3·762| 2·35
|hide|liter }| | | | | |
| | | | | | | |
2| „ |Ditto | 9 |2·764|0·540|3·624| 6·928| 4·14
| | | | | | | |
3| „ |Fresh lime 30 grm., }| | | | | |
| |¹⁄₂ grm. sulphide of }| 5[76] |0·852|0·172|1·816| 2·840| 1·75
| |sodium per liter }| | | | | |
| | | | | | | |
4| „ |Ditto | 8 |1·240|0·514|3·846| 5·600| 3·36
| | | | | | | |
5| „ |5 weeks old lime, }| | | | | |
| |through which four }| 2 |0·180|0·212|0·988| 1·380| 0·87
| |packs had passed }| | | | | |
| | | | | | | |
6| „ |Ditto | 5[76] |0·868|1·318|3·356| 5·542| 3·46
| | | | | | | |
7| „ |5 months old lime, }| 2 |0·196|0·188|0·864| 1·248| 0·77
| |with sodium sulphide }| | | | | |
| | | | | | | |
8| „ |Ditto | 5[76] |0·928|1·198|3·004| 5·130| 3·06
| | | | | | | |
9|Cow-|Fresh lime as above | 5[76] |1·982|0·413|4·501| 6·896| 4·30
|hide| | | | | | |
| | | | | | | |
10| „ |Ditto | 8 |3·132|0·672|5·741| 9·545| 5·94
| | | | | | | |
11| „ |Fresh lime as above, }| | | | | |
| |and ¹⁄₂ grm. sodium }| 5[76] |1·012|0·403|2·315| 4·730| 2·96
| |sulphide per liter }| | | | | |
| | | | | | | |
12| „ |Ditto | 8 |2·521|0·653|5·026| 8·200| 4·87
| | | | | | | |
13| „ |Old disused lime | 5 |0·344|0·291|2·341| 2·976| 1·84
| | | | | | | |
14| „ |Ditto | 8[76] |2·119|1·697|6·952|10·768| 6·45
| | | | | | | |
15| „ |Used sulphide of }| | | | | |
| |sodium lime 4 weeks }| 5[76] | .. |1·600|1·047| 2·527| 1·58
| |old }| | | | | |
| | | | | | | |
16| „ |Ditto | 8 |0·791|0·519|4·592| 5·892| 3·43
--+----+-----------------------+-------+-----+-----+-----+------+-----
[76] Hides unhaired.
Taking into account the liming necessary for unhairing only, as shown in
the table, it will be noted that the percentage of loss is invariably
greater in old limes than in new ones, and less in limes sharpened with
sulphide of sodium than where lime alone is used. The only exception to
this rule is in No. 15, where a sulphide lime 4 weeks old shows the
least loss of any in the time required for unhairing; and indeed
sulphide limes if kept strengthened with the requisite addition of
sulphide, seem to deteriorate very slowly, No. 8, with a lime 5 months
old, showing a result which may still be considered good. Another point
especially noted by Eitner is the slight action of old limes during the
first stages of liming, as compared with their rapid solvent effect as
the hair becomes loosened. The loss in any case does not appear to be so
great as the advocates of other unhairing processes have often claimed.
If we assume that all the dissolved hide-substance might have made
leather, the worst loss on oxhide only limed to the point of unhairing
amounts to less than 3¹⁄₂ per cent. on the possible total; and it must
be remembered that at least a part of this consists of dissolved
epidermis matter, which could not by any possible method have been
converted into leather. It will be noted in Nos. 2, 4, 10, 12 and 16,
what considerable losses are produced by plumping limes after unhairing,
but it must be borne in mind that, in the case of dressing-leather,
solution of at least a part of the cementing matter is essential to
produce the necessary softness and flexibility. Eitner calculates the
dry pelt-weight from that of the green hide on the assumption, based on
experiment, that 100 parts of the original skin corresponds to 32 parts
of dry pure pelt in green oxhide, 25 parts in green calf-skins, and 56
parts in dried calf-skins. In some of the smaller skins, such as kid
worked for glove leather, where great softness and stretch is required,
the loss is necessarily much greater than in ordinary dressing-leathers,
amounting, in the case of kid, to from 20 to 27 per cent.
The parts taken by the purely chemical activity of the lime, and by the
action of bacteria and bacterial ferments in the unhairing process must
still be regarded as uncertain. The late Professor von Schroeder[77]
carried out a series of experiments on liming and sweating which were
characterised by his usual care and thoroughness, and which tend to
prove that the chemical action is far more important than the bacterial.
He had fresh hides well washed in a tannery immediately after slaughter,
and fleshed. The butts were then cut into pieces of about 10 cm. (4
inches) square, and salted in brine repeatedly changed, and finally
preserved for use in glass jars in saturated salt solution. He found
that when washed free from salt, and placed in a moist chamber at a
temperature of 16° C., the hair was sufficiently loosened by bacterial
action in four to five days. Pieces placed in the moist chamber without
previous removal of the salt only showed signs of sweating after about
ten weeks’ exposure. Liming experiments were made with similar pieces of
salted hide, both after three days’ washing to free them from salt, and
unwashed, and in both cases the pieces unhaired freely in three to four
days. These experiments were varied by using 6, 18 and 30 grms. of lime
per liter of water in which about 200 grms. of hide were placed, but
neither in the washed, nor unwashed portions was there any material
difference in the time required to loosen the hair. Addition of 1 vol.
of used lime-liquor to 3 vols. of water in making up the limes was
equally without perceptible influence, and careful bacteriological
examination of hide and liquors showed that the former was almost
sterilised by the intense salting, and that the lime-liquors were
practically free from bacteria.
[77] Gerberei-Chemie, Berlin, 1898. p. 646.
Von Schroeder’s conclusion that no gain arises from the use of
excessive quantities of lime, so long as the solution is kept saturated,
is fully justified both by experience and scientific reasoning, but his
results with regard to the effect of old liquors and bacteria contradict
the conclusions both of practical tanners and of other scientific
experimenters.
The different effects of old and new limes are too well known to
practical tanners to be discounted by laboratory experiments, even if
they were not confirmed not only by Eitner’s results, but by a
considerable amount of work done in the Author’s laboratory and
elsewhere; while the necessity of bacterial action is at least rendered
probable by the fact that soda solutions, which are completely sterile
to bacteria, fail to unhair hides which have not previously undergone
some putrefaction (see p. 137). In some experiments undertaken at the
suggestion of the Author it was found that a perfectly fresh and
sterilised calf-skin which was not unhaired after ten days’ liming in
sterilised lime-liquor unhaired rapidly on the addition of a bacterial
culture to the lime. It is extremely difficult to exclude bacteria, and
even where perfectly fresh skins treated with chloroform or carbon
disulphide were employed, bacteria were always to be recognised when the
skin was ready for unhairing. Von Schroeder’s work, is, however, so
painstaking and reliable, that these divergent results must be explained
as other than experimental errors. With regard to old liquors, it is
known that ammonia is a powerful aid to the unhairing process, and it is
not certain to what extent the liquors he used were charged with it. It
is also certain that old limes containing much organic matter, support
bacterial life freely, while 25 per cent. of a possibly not very old
liquor would probably be sterilised by the addition of lime and 75 per
cent. water. In order to test the matter fairly under exact tannery
conditions, the lime should have been made up entirely with old
lime-liquor well charged with ammonia and organic matters, instead of
with water. It is also probable that the hides had undergone a
sufficient amount of bacterial change in the tannery before they came
into Von Schroeder’s salt solutions, and it is not at all unlikely that
the salt solution itself exercised some specific effect on the
unhairing. It is also possible that his bacterial cultures were made on
gelatine media unsuitable for the growth of alkaline bacteria, and
therefore gave blank results. Under these circumstances it is scarcely
possible to arrive at any very definite conclusions, and it is obvious
that further experiments on these points are extremely desirable.
_Sodium and Potassium Hydrates._--From the earliest antiquity,
wood-ashes, consisting mainly of potassium carbonate, have been used for
unhairing, either alone or in conjunction with lime, and indeed the
German name of the process (_Aeschern_) is derived from the fact. In
more recent times, caustic soda, either ready formed, or causticised on
the spot by the addition of lime, has often been recommended as a
substitute for lime. Its action is very similar to lime, but, from its
greater solubility, is far more powerful, and probably this has hitherto
formed one of the greatest obstacles to its use, since a solution of the
strength of lime-water is almost immediately exhausted, while a much
stronger one is too violent in its action on the hides. Experiments made
in the Author’s laboratory show that caustic soda, in solutions of the
same strength as lime-water, dissolve considerably less hide substance
than the latter, but it is more antiseptic than lime, and does not
unhair readily without the aid of bacterial action (cp. p. 137). It also
swells more violently, and it is difficult to keep the grain smooth and
unwrinkled.
Caustic soda has the great advantage that from its solubility, and that
of its carbonates in water, it is much more easily and completely
removed by washing than is the case with lime. It has been successfully
applied in some instances to soften skins of which the texture is
naturally too compact for moroccos and the softer leathers; and is
usefully employed in softening dried goods (p. 115). Where caustic soda
is required merely to “sharpen” limes, it is best added in the form of
sodium carbonate (soda-ash or crystals), which are causticised by the
lime in the pits. One-quarter or one-half per cent. on the weight of
hides added in this way decidedly increases the plumping power of the
lime. It may be noted that in the use of sodium sulphide in conjunction
with lime, caustic soda is one of the products of its decomposition,[78]
and is probably one great cause of the difference of effect of this
material for sharpening limes as compared with red arsenic.
[78] This has been denied, but is probably correct, though the actual
reaction is not easy to prove analytically; but the effect on the hide
is practically what is stated.
An indirect method of liming has recently been patented by Messrs.
Payne and Pullman of Godalming,[79] which is of both scientific and
practical interest. From the difficult solubility of lime, and the
consequently weak solutions which must be employed, the ordinary process
of liming is a slow one. Caustic soda, however, can be used in much
stronger solutions without producing injury to the hide, or larger
solution of hide substance, and from its great diffusibility, it
penetrates very rapidly. Used alone, however, the hide becomes too much
swollen for most purposes, and for certain classes of leather at least
(e.g. buff and chamois leather) the presence of a portion of lime in the
hide appears to be necessary for successful work. If a hide which has
been swollen with caustic soda be afterwards treated with a solution of
calcium chloride, double decomposition takes place, and caustic lime is
formed actually in the interior of the fibre of the hide, while the
sodium unites with the chlorine to form common salt. Both solutions may
be used in any convenient way, and by the employment of drums, the whole
liming process may be accomplished in five or six hours. It is found,
however, that perfectly fresh hides treated in this way cannot be
unhaired, and the explanation appears to be that in the ordinary liming
process, the epidermis is made soluble by the joint action of bacterial
ferments and of the alkaline solutions. If sodium sulphide be added to
the caustic soda used for unhairing, the goods will unhair without the
use of putrefactive means, but the process is difficult to manage
without destruction of the hair, and Messrs. Pullman now recommend that
all hides or skins for unhairing by their process should be soaked for
forty-eight hours in winter, and twenty-four hours in summer in a really
putrid stale soak. This necessity constitutes for very many purposes a
serious weakness in the method, as putrid soaking is always extremely
dangerous to the grain of the hide, and especially so in hot weather.
For certain purposes, however, advantage may be taken of the fact that
the hide or skin can be fully limed by Pullman’s process and the fibres
swollen so as to be prepared for tanning without any loosening of the
hair, and the Author has seen deerskins which have been treated in this
way, on which the hair was perfectly firm, while they possessed a
softness and fulness which could not be attained without liming.
[79] Eng. Pat. 2873, 1898.
Messrs. Pullman now recommend that the treatment with their solutions
should take place in pits, in preference to drums or paddles, and that
the caustic soda should not exceed a strength of one pound in ten
gallons (1 per cent.). The hides or calf-skins remain in this for about
forty-eight hours, during which they are once drawn and returned, by
which time, if the putrid soaking has been properly done, the hair
should be fully loosened. The hides are then drained for two hours, and
passed into another pit containing a solution of calcium chloride, which
should be slightly stronger than the caustic soda, say of about one and
a half pounds per ten gallons. The goods remain in this for about
forty-eight hours, during which they are drawn once, and are then well
washed in soft water (free from temporary hardness) in which they may be
kept for some time without injury. As both the caustic soda and the
calcium chloride solutions are quite sterile to ordinary putrefactive
bacteria, both can be used for an almost unlimited time, and they are
conveniently kept up to strength by the addition of strong
stock-solutions. These may be made of a sp. gr. of 1·4 (80 deg. Tw.)
which gives a strength of about 5¹⁄₂ lb. of caustic soda and 5³⁄₄ lb. of
calcium chloride per gallon.
In addition to the advantage of considerable saving of time, the effects
can be much more easily regulated than in ordinary liming, and the
amount of soda (and subsequently of lime) absorbed by the hide can be
exactly determined by titration of the liquors. Grease is better removed
than by ordinary liming, as soda-soaps are soluble in water, but if this
result is to be obtained, the soap must be worked out before passing
into the calcium chloride solution, which would otherwise convert it
into an insoluble lime-soap. A great gain in many districts is that the
process yields practically no effluents and no lime slab, both of which
are frequently very difficult to dispose of. The serious disadvantages
of the stale soaking, however, have already been mentioned.
In place of applying the caustic soda first, and the calcium chloride
subsequently, hides may be first treated with calcium chloride solution,
and then with caustic soda, or the caustic soda may be applied to the
flesh side of the hide by painting. These modifications are covered by
Messrs. Pullman’s patent, but they are willing to grant licences for
experiments at a nominal fee.
_Alkaline carbonates_ are much milder in their action on hide than the
corresponding hydrates, and although they will unhair hides, in absence
of lime, their action is somewhat uncertain and slow. “Polysulphin”
(Polysulphin Co., Keynsham) owes its unhairing power principally to the
sodium carbonate, and not to the small traces of sulphur compounds which
it contains.
Sodium carbonate occurs in commerce in three forms: “soda ash,” a more
or less pure dry sodium carbonate; “soda crystals,” or washing soda,
Na₂CO₃·10Aq, containing 62·95 per cent. of water of crystallisation, and
efflorescing in the air; and Gaskell and Deacon’s “crystal soda,”
Na₂CO₃·1Aq, containing only 14·5 per cent. of water of crystallisation.
It must be remembered that where carbonate is used in conjunction with
lime it becomes causticised and converted into NaOH.
_Sulphides._--The practice of using realgar, or red sulphide of arsenic
(Ger. _Rusma_) as an addition to limes for fine leathers is one of
considerable antiquity. It has the property of loosening the hair and
epidermis structures with less solution of cement-substance than lime
alone, and hence produces a leather of fuller and closer texture. It
will, however, be convenient to defer the consideration of this agent
till after that of some of the more modern and simpler substitutes, such
as the sulphides of sodium and calcium. Sulphides of the alkalies and
alkaline earths, if used in strong solution, say 5 per cent. or upwards,
have the effect of very rapidly reducing the harder keratin-structures,
such as hair and wool, to a pulp, attacking first the interior cells, so
that the hair crumples up like a string of sausages, and in a few hours,
or even, with very strong solution, in a few minutes, the whole mass
becomes so completely disintegrated that it can be swept off the hide
with a broom, or washed off in a tumbler. At the same time, the action
on the substance of the hide, and especially on the cementing substance,
is very slight, though the grain is swollen and temporarily rendered
somewhat tender. On the other hand, when used in weak solutions, say ¹⁄₄
per cent. and under, in conjunction with lime, the hair is but little
injured, while the hair-roots and dirt are rapidly loosened, and results
are obtained very similar to those with arsenic.
_Sodium Sulphide_ (Na₂S·9OH₂).[80]--For the methods of valuation and
determination of sodium sulphide, see L.I.L.B., p. 28.
[80] In the Laboratory Book the water of crystallisation is given as
10 Aq. Later researches show that pure crystals of the commercial
sulphide only contain 9 Aq., or 67·5 per cent. of water.
Hides suspended in solutions of sulphide of sodium of 2 to 3 per cent.
strength unhair rapidly.
For the commoner classes of sole-leather, hair is frequently removed by
painting on the hair side with a 15°-28° Tw. (30-40 per cent.) solution
of (crystallised) sulphide of sodium thickened with lime, applied with a
fibre-brush, and folding the hide in cushions in a damp place, or
packing in a tub. The hair is reduced to paste in a few hours. The same
effect is produced by drawing the hides through a similar solution
without lime, of which sufficient is retained by the hair to destroy it.
The workmen must be provided with indiarubber gloves to prevent the
caustic effect of the solution on the skin and nails. Skins and lighter
hides are conveniently unhaired by painting the mixture on the flesh
side, when it will loosen the hair or wool in a few hours without
destroying it.
For dressing-leathers and the finer sorts of sole it is best employed as
an addition to ordinary limes to the extent of ¹⁄₄-¹⁄₂ per cent. on the
weight of the hides or skins, when the hair is loosened more rapidly
than with lime alone, and with less loss of hide substance.
Good samples of sulphide of sodium consist of pale-brown, almost
colourless crystals, containing 28 to 32 per cent. of dry sodium
sulphide, which readily deliquesce on exposure to air. Fused sodium
sulphide can now be obtained, which contains nearly twice as much actual
sulphide as the crystalline form. The dark green colour possessed by
many samples of sodium sulphide is due to the presence of iron sulphide.
If carefully used no serious harm can accrue from its presence. If
allowed to stand a short time in solution the iron sulphide will settle
out.
_Calcium sulphydrate_, Ca(SH)₂, sometimes called Böttger’s _Grünkalk_,
is a powerful depilatory, while it has probably less destructive action
on the hide-fibre than even the sulphide of sodium, and would no doubt
be largely used but for its unstable character. It is probably the
principal active product produced by the use of sulphide of arsenic in
conjunction with lime, though it is possible that a sulpharsenite may be
formed. It may be produced by passing hydrogen sulphide (SH₂), into milk
of lime. According to von Schroeder, it is _not_ formed by the reaction
of sodium sulphide on lime solutions (see note, p. 136). It may be
obtained crystallised, and is soluble in water, but is decomposed on
boiling. The sulphide, CaS, is insoluble in water, but by the action of
steam under pressure it is said to be converted into a mixture of
equivalent parts of hydrate and sulphydrate. It may also be dissolved in
a solution of hydrogen sulphide, forming a solution of sulphydrate. In
this way it might be formed on a large scale from the “tank waste” of
the Leblanc soda process.
Gas-lime is principally active on account of the calcium sulphide which
it contains, but is very variable in its strength, as both sulphydrate
and sulphide are decomposed by the carbon dioxide always present in the
gas, forming carbonates. Lime has nearly gone out of use for purifying
gas, its place being now taken by iron oxide, but formerly gas-lime was
a good deal used for unwooling the small lambskins used for the commoner
sort of glove-kid, usually by painting a cream of it on the flesh side,
but sometimes by immersing in a strong solution, which of course
destroyed the wool. Its place is now taken by a solution of sodium
sulphide of 15°-18° Tw. (approximately 30-35 per cent. crystals),
thickened with lime to a soupy consistence, the use of which is much to
be recommended for unwooling sheep-skins.
The tank-waste from the Leblanc process, consisting principally of
calcium sulphide, is, when fresh, quite insoluble, and has no depilatory
powers; but when exposed to air and moisture, decompositions take place,
resulting in the formation of sulphydrates and polysulphides, which form
a solution which has been the subject of several patents for
unhairing.[81] Polysulphides alone have probably no unhairing effect,
but in conjunction with lime, sulphydrates are formed which rapidly
loosen the hair. This fact was the basis of an ingenious and effective
unhairing process used very many years ago by Mr. John Muir, of Beith,
who, after liming for 24 hours in the usual way, submitted the hides to
a pretty strong solution of weathered tank waste for 24 hours, and
finally to water for 24 hours, to remove the surplus lime and sulphides.
The sulphydrates formed in the hide attacked the hair-roots with little
injury to the hair itself, and the hides contained so little lime that
they could be tanned for dressing without bating, and made about 10 per
cent. more weight than those treated in the ordinary way. Some trouble
was occasioned by stains caused by impurities in the tank-waste.
[81] Squire, E. P., 756, 1855; Claus, E. P., 1906, 1855.
A somewhat similar unhairing mixture to that obtained from tank-waste,
which is now seldom to be got, was patented by Prof. Lufkin,[82] who
mixed equal parts of sulphur and soda-ash with a little water till
combined, and then added 8 to 10 parts of lime, slaked and still hot.
Schultz[83] states that such a mixture containing 10 lb. of sulphur,
will unhair fifty hides in the same way, and in about the same time as
an ordinary lime, the pelt being little plumped and easily reduced
without bating by a few minutes’ wheeling in warm water. By boiling lime
and sulphur with water a yellow solution is obtained which can be used
in the same way as that from the tank-waste. A further quantity of water
can be boiled on the same materials, more lime and sulphur being added
as required. Polysulphides appear to have a marked effect in preventing
plumping.
[82] Eng. Pat. 2053, 1860.
[83] ‘Leather Manufacture,’ p. 35.
_Barium sulphydrate_ has been put on the market experimentally as an
unhairing agent, in the form of a strong solution containing yellow
polysulphides, and which deposits crystals of sulphydrate in cold
weather. It is more stable than calcium sulphydrate, but, on the whole,
does not seem to present any advantages over sodium sulphide.
_Realgar or red sulphide of arsenic_, As₂S₂, is made by fusing arsenious
acid and sulphur. (Orpiment is As₂S₃, but its action is different from
that of realgar.) Mixed with lime it produces calcium sulphydrate and
possibly hyposulpharsenite. To produce a rapid and complete reaction it
must be mixed with hot lime, and the hotter the mixture is made the more
powerful is its unhairing action. Milder forms may be made by mixing
cold, or with the aid of hot water only. It is used with great advantage
in conjunction with lime in varying proportions for unhairing lamb- and
kid-skins for glove-kid and other fine leathers, to which it gives the
necessary stretch and softness and cleanness of grain, without the
loosening of texture and loss of hide-substance which would be caused by
an equivalent amount of ordinary liming. For glove-kid about 0·1-0·3 per
cent. of realgar and 5 per cent. of lime is used, reckoned on the green
weight of the skin.
For painting the flesh side of calf- and lamb-skins 1 part of realgar
is mixed with 10 parts of hot lime, made into a paste with water. Calf
will unhair in 8 or 10 hours.
“Inoffensive” unhairing solution contains a large quantity of arsenic
sulphide apparently dissolved in caustic soda, although Moret’s original
patent claimed the use of wool-sweat potash only!
W. R. Earp[84] has suggested the use of compounds of sulphur and arsenic
(thio-arsenates, thio-arsenites, etc.), in 5 per cent. alkaline
solution. He prefers to add the compounds to the ordinary lime-liquors,
or to manufacture them _in situ_ by adding the proper quantities of
arsenious or arsenic acid mixed with one-third of its weight of sulphur
to a solution of an alkaline sulphide in lime-liquor. The pelt is not
bated or drenched in the ordinary way, but, after unhairing, is passed
directly into the tanning liquor to which sulphurous acid has been
previously added.
[84] Eng Pat., No. 2052, Feb. 12, 1886.
There is more danger of injury to the hide from the very prolonged
action of weak solutions of sulphides, which tend ultimately to destroy
the structure and reduce the fibre to a gelatinous condition, than there
is from too concentrated solutions. No danger need, however, be
apprehended in the course of any ordinary liming. Arsenical limes are
not suited for tainted skins, and they should not be made so strong as
to destroy the hair or wool.
For methods of analysis of both old and new lime-liquors, see L.I.L.B.,
pp. 27 to 34.
Whichever method of loosening the hair be adopted, the actual removal
must be effected by placing the hide on a sloping beam with a convex
surface, and then scraping it with a blunt two-handled knife (Fig. 27),
the workman pushing the hair downward and away from himself. The beam
may be either of cast iron or of wood, usually covered with zinc to
increase its wearing capacity. The hides after being removed from the
lime-pits, are allowed to drain for half an hour or so before the hair
is removed, and immediately this operation has been completed, they
should be placed in soft water. It is of great importance that the limed
hides should not be exposed to the air longer than is absolutely
necessary for the removal of the hair, as the carbonic acid present in
the atmosphere quickly carbonates any lime contained in the surface of
the skin, forming chalk, and leading to uneven tanning at a later stage.
When hide has been insufficiently limed it is often easy to remove the
longer hair but excessively difficult to get rid of the short
under-growth of the young hairs, which even in properly limed skins can
often only be removed by shaving them with a sharp handknife. This
difficulty is caused partly by the small resistance which the short
hairs offer to the unhairing-knife, and partly by their being more
deeply rooted in the skin than the older hairs (see p. 49).
[Illustration: FIG. 27.--Unhairing (Penketh Tannery).]
Various machines have been devised to accomplish the removal of the
hair, but owing to the rapidity with which it may be worked off by hand,
and the fact that the work is not difficult, no machine has as yet come
into general use. Hand-work has the further advantage that in those
portions of skin where the hair is tighter than usual it may be removed
by greater pressure of the knife or by hand-shaving, whereas after goods
have been unhaired by machine they must always be examined and any
patches of hair removed by hand on the beam. The edges invariably
require to be gone over by hand.
Several machines with spiral knives have been introduced for the
purpose. That made by the Vaughn Company (Peabody, Mass.) for fleshing
is one of the most satisfactory for unhairing, though any other machine
of a similar type, and provided with spiral knife-blades, purposely kept
blunt, may be used. The Leidgen unhairing machine, shown in Figs. 28 and
29, is one of the latest and most ingenious.[85]
[85] E. H. Munkwitz, Milwaukee.
[Illustration: FIG. 28.--Leidgen Unhairing Machine.]
Occasionally goods are unhaired by fulling in the “stocks”; but it is
very doubtful whether the saving in labour is not more than counteracted
by the loss of weight caused by submitting the hide, while its gelatin
is in a partially dissolved condition, to such rough usage.
The use of the wash-wheel (see pp. 111, 118) for the same purpose is
much more satisfactory, and may be profitably employed for common
goods, especially when the hair has been loosened by painting with a
sulphide mixture.
After being unhaired, the hides are “fleshed” on the beam. This work,
which consists in removing any small pieces of flesh and fat left by the
butcher on the inner side of the skin, should be carefully and
thoroughly done; but the closeness of the fleshing required is dependent
on the purpose to which the hides or skins are to be applied.
[Illustration: FIG. 29.--Leidgen Unhairing Machine.]
It is necessary not only to remove those portions of fat which are
easily visible, but also to force out that contained in the loose
areolar tissue. The form of knife used in England in fleshing is shown
in Fig. 30. It differs from the one used for unhairing in being
somewhat broader and heavier, and both its edges are sharp, so that
where the flesh is too tight to remove by mere friction of the knife, it
may be actually cut away by holding the knife almost flat on the beam,
and using the convex sharp edge. The strokes in cutting must not be too
broad, or, from the convexity of the beam, the substance of the hide
will be cut into in the middle, or flesh will be left at the edges of
the stroke. This difficulty is avoided by the flexible knife commonly
used in Germany, but in other ways its work is less rapid and effective.
[Illustration: FIG. 30.--Fleshing.]
Machines have long been used for fleshing and scudding light goods, such
as lamb-, kid-, and goat-skins, and their use for fleshing dressing
hides has now become very general in the United States, and is gradually
gaining ground in England. The type of machine used for these heavier
leathers, varies considerably from that used for light skins, but the
general principle is the same. In most cases the working tool of the
machine is a cylinder with spiral blades, which are generally arranged
right-handed on one half, and left handed on the other, so as not only
to scrape the hide in the direction in which the cylinder works, but
also to extend it sideways. Much of the efficiency of these machines
depends on the exact adjustment of the pitch of the spiral, and in the
Vaughn machine, which is probably most in practical use, the blades are
so arranged as to form two intersecting spirals, one of steeper pitch
than the other. The great difference in the machines for skins and for
heavy work, consists in the means adopted to support the skin, and to
carry it under the spiral blades.
[Illustration: FIG. 31.--Jones Fleshing Machine.]
In the machine invented by the late J. Meredith Jones, the skins are
supported upon an india-rubber blanket stretched over two rollers, so
that the knife-cylinder works on that part of the blanket which is
between them, by which great elasticity is obtained, and this machine
has proved most successful in treating delicate skins. In some other
forms of machine, cylinders thickly covered with rubber have been
substituted for this arrangement. The Jones machine is shown in Fig. 31.
For heavy hides the Vaughn machine is most generally used, and may be
taken as the type of the rest, as the Vaughn Company certainly
originated the semi-cylindrical “beam,” which forms a very important
feature. Its construction will be seen from Fig. 32.
[Illustration: FIG. 32.--Vaughn Fleshing Machine, front view.]
It will be easily noticed that if a hide be thrown over the
half-cylinder so that one half hangs outside it, and the other half
falls in its hollow, and it be then rotated, the hide is first caught
firmly by a spring-clamp, which has been supported above the edge of the
half-cylinder by blocks attached to the frame. As the edge rises, it
lifts this clamp off the blocks, and thus carries the hide under the
spiral knife-cylinder. The blades of this spiral knife-cylinder are
ground to a sharp rectangular edge, and partly scrape and partly cut the
loose tissue of the flesh. When the half-cylinder has made a
semi-revolution, it returns to its original position, and the sizes of
the driving pulleys are so arranged that the cylinder travels downwards
more rapidly than it rises, in order to economise time, though in both
cases the hide is worked upon by the knife-spiral which is rotated at a
still higher speed. The hide is of course turned on the beam-cylinder
and the other half is similarly fleshed. The beam-cylinder reverses
automatically, or may be reversed by hand, and its nearness to the
spiral knife is also under control. It is usually covered with a thick
sheet of rubber.
It is obvious that machines of this type can not only be used for
fleshing, but for unhairing and scudding, by the substitution of
suitable knife-cylinders, and in the case of light skins, cylinders
fitted with slates are frequently employed for the latter operation. The
slate for the purpose must be of a peculiarly fine and even grain, and
is mostly obtained from a single quarry in Wales. The Vaughn machine is
frequently used in America for fleshing hides after soaking but before
they go into the limes, and much is to be said in favour of this method,
as the removal of the flesh permits even and uniform action of the lime.
It is, however, a distinct disadvantage to the method that the flesh
appears rough-looking after tanning, and the method is most suitable in
conjunction with the American system of splitting the tanned leather.
In the production of sole-leather, fleshing machines have not as yet
come into very general use. This may be accounted for by the fact that
if used before liming a rough flesh is produced, which is unsightly on
sole-leather, and which cannot well be afterwards improved, while
something of the same objection attaches to fleshing after liming, with
the added disadvantage that the hide is too much pressed, and is not
easy to plump again, so as to make a satisfactory sole-leather.
In America, both sole- and dressing-leathers are usually tanned in
sides, the hide being cut down the centre of the back. In England, the
hide is usually “rounded” for sole-leather into “butts” or “bends” and
“offal,” as shown in Fig. 33. The rounding is done by hand with a sharp
knife on a table, and in some of the best tanneries frames made of wood
or metal are employed, to mark the sizes required. The chief advantage
of rounding before tanning is that the different parts of the hide can
be differently tanned, and appropriated to the purposes for which they
are most suitable. The offal is now frequently split and worked up for
light leather, or in other cases is tanned with a cheaper and more rapid
tannage than the butts.
[Illustration: FIG. 33.--Diagram of Hide.]
Dressing leather is more frequently rounded after tanning, according to
the purposes for which it may be required.
CHAPTER XIII.
_DELIMING, BATING, PUERING AND DRENCHING._
Although lime is in many respects the most useful and satisfactory means
of loosening hair from hides and skins, it is of the greatest importance
that it should be completely removed when it has done its work, since
its action on tannins is most injurious, and it is often harmful in
tawing. For soft leathers it is also necessary that the skin should be
brought from a swollen to a soft and flaccid condition.
In practice this is mainly accomplished for dressing leathers by bating,
puering and drenching; while sole-leather and strap-butts are only too
frequently left to chance, and to the natural acidity of the tanning
liquors.
_Bating_ consists in handling, or steeping the goods in a weak,
fermenting infusion of pigeon- or hen-dung for a time usually extending
over some days, and is applied to the heavier classes of dressing
leather, such as “common” and shaved hides, kips and calf-skins.
_Puering_ is a very similar process, applied to the finer and lighter
skins, such as glove- and glacé-kids and moroccos, in which dog-dung is
substituted for that of birds, and, as the mixture is used warm and the
skins are thin, the process is generally complete in a few hours at
most. Neither bating nor puering are very effective in removing lime,
and seem to act principally by some direct effect of the bacterial
products on the swelling of the pelt.
_Drenching_ is occasionally used (e.g. on calf-kid) as a substitute for
bating or puering, but more frequently follows the latter, and serves to
cleanse and slightly plump the skins before tanning, and complete the
removal of lime. The drench-liquor is an infusion of bran made with hot
water, and allowed to ferment under the influence of special bacteria,
which are always present in vats used for the purpose, and which
develop lactic and acetic acids.
It will be noted that all these methods are fermentative, and their
effect is not simply the chemical one of removing the lime, but the
bacterial action leads also to solution of the cementing substance of
the hide-fibres, and produces a marked softening effect on the leather,
together with considerable loss of hide-substance. In the manufacture of
the softer leathers this effect is generally desired, and no process
would be satisfactory which did not produce it; but in other cases, such
as harness- and strap-butts, firmer and heavier weighing leathers would
be preferred, if it were known how to make them. The putrefactive
processes would be gladly relinquished, if satisfactory substitutes
could be found, not only on account of their offensive character, but
because of their uncertainty and danger to the goods; and even if lime
only were removed, the necessary softness could often be obtained by
appropriate liming and tanning.
It will be best, therefore, to deal first with the purely chemical
methods which aim only at removal of lime, before considering those
involving bacterial action. Unfortunately, the chemical problem is not
so simple as it might at first sight appear. The alkaline lime clings
obstinately to the hide-fibre, and can only be removed very slowly, if
at all, by mere washing. On the other hand, the use of any excess of
strong acid is absolutely precluded, because of its powerful swelling
effect on the pelt, in the tanning of which it would prove even more
injurious than the lime, making dark-coloured and brittle, or tender,
leather. This effect is not to be avoided by the use of even very dilute
solutions of strong acids, since the affinity of hide-fibre for them is
so strong that it will abstract practically all the acid from even a
decinormal solution, leaving it quite neutral. What is required is an
acid of extremely weak affinities, forming soluble lime salts, and
obtainable at a low cost; or, on the other hand, a salt of some weak
base which could be displaced by lime, and which would not act
injuriously on the pelt. With certain precautions, and in special cases,
however, the stronger acids may be used successfully.
In the cases of sole- and belting-leather no softening is desired, and
formerly tanners usually contented themselves with a very perfunctory
washing in water, trusting to the acids present in the liquors to
complete the removal of the lime. Even pure distilled water effects this
removal very slowly and imperfectly, owing to the strong attraction of
the lime for the fibre; and if “temporary hard” water is used, the lime
present in the hide combines with that present in the water and is
precipitated as chalk in the surface of the hide. This may be prevented
by previously adding a small quantity of lime or lime-liquor to the
water before use to soften it (see p. 95); but unless this is very
carefully done, the free lime present in the water prevents it from
removing any from the hide. The safest way is not to add lime direct to
the water, but to change the latter gradually, so as to allow the lime
already present to soften the new portion of water.
A much more efficient method is to suspend the butts in water to which
small portions of diluted acid are successively added till the lime is
nearly, but not quite, neutralised. If carefully used, sulphuric
acid[86] is perhaps as good as any, but, of course, any excess will
spoil the colour or “buff” of the leather.
[86] The use of sulphuric acid for this purpose was patented by H.
Belcher of Wantage (No. 14,943), but was used some years previously in
several tanneries known by the author.
Acetic, formic, and lactic acids are safer than sulphuric, but are
somewhat costly, and must not be used in appreciable excess. Crude
pyroligneous acid may be used, and it has a considerable antiseptic
effect owing to the phenols, etc., which it contains. Hydrochloric acid
is not suitable for sole-leather, on account of the bad effect of
chlorides on plumping. Sulphurous acid[87] is perhaps the best, and its
acid properties are so weak that slight excess does little harm, but the
neutral calcium sulphite is insoluble, and to actually dissolve the lime
the hydric sulphite must be formed, which can only occur in presence of
excess of the acid. Unless such excess is used, the colour of the pelt
in the early liquors is apt to be somewhat greyish. Probably a very good
method would be to suspend the butts in a solution of sulphurous or some
other acid of about N/20 strength, sufficiently long to remove all lime
from the surface and slightly to plump it but not to penetrate to the
centre of the hide, which should then be suspended in water until any
excess of acid had been taken up by the unneutralised lime still present
in the middle of the butt, which at the end of the operation should be
rather alkaline than acid. The course of this, or any other bating
operation can be followed by cutting the hide, and moistening the cut
surface with alcoholic solution of phenolphthalein, which is turned red,
or pink, by the least trace of free lime.
[87] Manufacture of sulphurous acid, see p. 24; testing, see L.I.L.B.,
p. 37.
In using mineral acids it is of great importance that they should be
perfectly free from iron, and that the vat employed should contain no
iron which could become dissolved, since, if present in the bating
liquid, it is sure to be fixed by the hide, especially if the quantity
of acid used is insufficient to neutralise the whole of the lime.
Besides the direct use of mineral acid which has been described,
sulphuric, or still better, oxalic acid may be very advantageously
employed in precipitating lime from used bating liquids containing weak
organic acids, or other lime solvents, so as to restore their original
activity. Not only is the bate economised by being used repeatedly, but
some of the organic products dissolved from the hide have themselves
considerable power of removing lime. Putrefaction should not be allowed
to take place; but many of the organic acids which have been proposed
for bating belong to the aromatic series, and have considerable
antiseptic power. Where organic acids are employed, the presence of
their neutral lime-salts in the liquor, resulting from previous
operations, will reduce the swelling action of the acid on the skin,
without diminishing its power of removing lime (cp. p. 81).
In place of sulphuric acid, some tanners have employed a material
advertised under the name of “boral.” This substance consists simply of
sodium anhydrosulphate melted up with about one-seventh of its weight of
boric acid, the quantity of which is, however, too small to have
appreciable influence as an antiseptic, while it is said to form
insoluble borates with the lime present, which are sometimes a source of
subsequent trouble.
There is no reason why ordinary sodium bisulphate should not be used for
the purpose, and its action is more mild than that of sulphuric acid
itself, but great care must be taken that no nitric acid is present, as
is frequently the case in the crude product obtained in the manufacture
of nitric acid from sodium nitrate, and known in commerce as
“nitre-cake.” The presence of a trace of sodium chloride would not be
disadvantageous for dressing leather, but would tend to prevent
plumpness in sole. Paessler and Appelius[88] have recently shown that
raw hide absorbs sulphuric acid from sodium bisulphate, leaving the
neutral sulphate in solution.
[88] ‘Wissenschaftlich-Technische Beilage des Ledermarkt,’ 1901, p.
107.
Boric (boracic) acid, though used to a slight extent for a number of
years past, has recently come much into favour as a deliming agent, for
which purpose it is in many respects particularly suitable. Sole-leather
may be improved in colour by giving a short bath in 1¹⁄₂-2 per cent.
boric acid solution to remove surface-lime. In this case the acid is
best applied just before the hide enters the suspenders. Boric acid may
also be suitably employed on hides which have been bated. It then acts
as a drench and removes traces of lime still left in the hides, so that
the liquors have a more even effect on them. Experience has shown that
the skins should never be allowed to lie for any length of time in the
boric acid solution in a motionless condition, as this tends to produce
patches of partially delimed skin, which cause irregular colour. It is
best to keep the skins in fairly constant motion in a paddle or by
frequent handling. Boric acid has considerable influence in preventing
drawn grain in the early liquors, but if it gets into the forward
liquors it renders the leather loose and light (cp. p. 229, and L.I.L.B.
p. 37).
Borax has also been suggested as a deliming agent, and as it is
chemically an acid salt, it has naturally some deliming effect, but it
cannot compare with boric acid in either price or efficiency.
Both boric acid and borax are antiseptics (see p. 25).
In the employment of either sulphuric, boric, or any other acid forming
calcium salts of limited solubility, it must be borne in mind that if
the solution is repeatedly re-strengthened, it will become saturated
with the lime-salt, and although the acid will still combine with the
lime and render it neutral, it will no longer remove it from the hide.
Under these conditions, sulphuric acid may cause the deposition of
crystalline calcium sulphate in minute nodules between the fibres.
Calcium borate may be similarly deposited, and has the further
disadvantage of becoming decomposed by the tanning liquors, which form
dark compounds with the lime. In using sulphuric acid alone it is
therefore best to renew the water each time. When it is used in
conjunction with some other acid, forming very soluble lime salts, this
danger is not to be apprehended, while oxalic acid precipitates the lime
almost completely from the solution.
It is to be borne in mind that in all cases of using acids, any
carbonate of lime present on the pit sides or elsewhere will be
decomposed, and the carbonic acid will become dissolved in the liquor,
and unless acid is used in sufficient quantity to remove the whole of
the lime, may tend to fix the remainder as carbonate. In the case of
dressing leather there is less danger of this, as warm water is
generally used, in which little carbonic acid dissolves. It is probable
that some of the coal-tar acids which have been advertised for bating
dressing leather might be advantageously employed for sole. Hauff’s
“anticalcium” (see pp. 29, 163), would appear to be very suitable for
this purpose, and if the liquor were regenerated by the addition of
sufficient sulphuric acid to neutralise the lime dissolved from the
hide, might be used repeatedly, and would not then prove expensive;
while its sterilising power would be very advantageous to the proper
swelling of the butts in the handlers, since nothing tends to check
plumping so much as putrefactive action.
Turning from sole to dressing leather, mineral acids are very
successfully employed for “pulling down,” the goods being thrown into a
paddle containing warm water of about 30°-35° C., and the calculated
quantity of sulphuric or hydrochloric acid, previously largely diluted
with water, is then added in two or three successive portions at
intervals of perhaps ten minutes. The acid must in no case be sufficient
to neutralise quite the whole of the lime. Goods treated in this way can
be further bated, puered, or drenched as required by the ordinary
methods, if they are not sufficiently soft. If too much acid has been
used, and the skins show signs of swelling, they may be brought down by
the addition of a little ammonia, borax, or even soda.
In many cases the addition of salt in small quantity to the acid liquor
will tend to deplete the hides, and at the same time prevent any
injurious action of the acid. Ammonium chloride may also be used with
advantage (see p. 159). A solution containing about 15 per cent. of salt
and 0·3 per cent. of sulphuric acid, with some molasses, has been a good
deal used in the States as a bate, and seems to answer well on some
classes of goods, but the acid and salt are apt, ultimately, to find
their way into the liquors and destroy tannin. The process is well
suited for chrome-leather, and may also be usefully applied in cases
where goods have become “wind-blasted” or otherwise impregnated with
carbonate of lime, since in presence of salt the acid can be used in
sufficient excess to dissolve the carbonate. Vegetable acids may, of
course, be used in conjunction with salt in the same way. The salt does
not neutralise the acid, but simply controls the swelling of the skin,
and if acid has been used in any material excess, the first part of the
tanning must be done in salted liquors, or the acid neutralised with
ammonia, sodium carbonate, or chalk, previous to tanning, as, otherwise,
the goods will plump up in the liquors, and be tender when tanned (cp.
p. 91).
Lactic acid has recently come largely into use as a deliming agent. It
is best known as the acid which gives a characteristic taste to sour
milk, and is the chief product of the lactic ferment. It may be very
successfully used for neutralising the lime left in the skins after the
depilation, but, if used in excess, it tends to plump or swell the
leather very strongly, being one of the best plumping agents known. When
used for deliming, a solution of 2 lbs. in 100 gallons is very suitable.
It may, in many cases, be substituted for the bran-drench with
advantage, and is much more rapid and less dangerous in hot weather, but
the effect is not in all respects identical.[89]
[89] On the manufacture of lactic acid by fermentation, see Claflin,
Journ. Soc. Chem. Ind., 1897, p. 516. Campbell states that practically
pure cultures of the lactic bacteria are obtained by continued culture
in milk. These cultures employed as a ferment for drenches have given
good results in the Yorkshire College Experimental Tannery.
When lactic acid is used for bating, or drenching, the operation should
always be conducted in a paddle, and the liquid works more
satisfactorily if it is at a temperature of 30-35° C. As regards cost,
it will be found that in practice it is not appreciably more expensive
than dung or bran. About an hour’s paddling will generally suffice, if
the right quantity of acid has been used, but in some cases it is best
to add the acid in several portions and take more time.
The estimation of the amount of lactic acid in the commercial article
may be carried out by diluting exactly 9 grms. with about ten times its
volume of water, and then titrating it with normal caustic soda as
described in L.I.L.B., p. 16, for acetic acid. As each c.c. of normal
alkali is equivalent to ·090 grm. of lactic acid it will represent one
per cent. of real lactic acid in the sample. If other acids are present,
they are of course included. Commercial lactic acid is usually of about
50 per cent.
It is important that the lactic acid should be free from iron, a dilute
solution should give no blue coloration on addition of either potassium
ferrocyanide or ferricyanide. Acid perfectly free from iron is now
easily obtained.
Formic acid in 60 per cent. solution, formed synthetically by the
combination of carbon monoxide with caustic soda and the subsequent
decomposition of the sodium formate so produced, has recently been
brought into commerce at a cheap rate, and will probably form a
satisfactory substitute for acetic acid in the deliming of hides and
many other technical operations.
Instead of acids, many neutral salts may be used to neutralise lime, and
in sole-leather, it is not generally disadvantageous to leave the lime
in the hide, so long as it is in an insoluble and fixed condition, and
combined with an acid which cannot be displaced by tannin. Thus
phosphates, or oxalates of sodium or ammonium will convert the lime into
insoluble phosphate, or oxalate, setting free sodium- or
ammonium-hydrate which form soluble tannates and other salts which are
easily washed out of the hide. Zinc sulphate will form sulphate of lime
and zinc oxide in the hide, and seems worth further experiment for
sole-leather, but must be free from iron. Alum, or sulphate of alumina,
would similarly form calcium sulphate and alumina, but the tanning
effect of alumina salts is too great to admit of their general use for
bating. Ammonium sulphate will form calcium sulphate with liberation of
ammonia.
For dressing leather, the use of ammonium chloride would be still more
advantageous, and it is a powerful bating material, converting the lime
into calcium chloride with the evolution of ammonia, which has but
little plumping power, and which is easily washed out. Ammonium chloride
has been very successfully used in calf-kid manufacture as a preparation
for drenching, instead of puering, which was formerly in vogue. As,
however, only about ³⁄₄ oz. per dozen skins was employed, the cleansing
must have mainly depended on the warm water with which it was used, and
the free ammonia evolved.
The use of ammonium chloride as a bate was patented by Zollickoffer in
1838.
A bating liquor which was proposed by the writer, and which has been
used with some success on harness-leather, is made up with a ¹⁄₄ lb. of
good white ammonium chloride (sal ammoniac) and a ¹⁄₄ lb. of Boakes’
“metabisulphite of soda” per hide, and for successive packs sufficient
sulphuric acid to neutralise the ammonia formed, together with a small
quantity of metabisulphite and ammonium chloride to restore that carried
out by the hides is added. It is probable that this would also answer
well for deliming sole-leather as it entirely removes lime without
pulling down the hides much, and they would remain still plumper if
ammonium sulphate were substituted for ammonium chloride, while the
sulphuric acid might be safely increased till the liquor was but
slightly alkaline when the bating was finished. About 2-4 oz. of good
white oil of vitriol is required per hide, but the exact quantity will
depend on the mode of liming, and the amount of washing the hides
receive before going into the bate, and can therefore be only
ascertained by experience. As no free sulphuric acid can exist in the
liquor so long as the quantity of metabisulphite is maintained, there is
no practical danger of spoiling the leather if the acid be in slight
excess. The quantities given may in most cases be advantageously
diminished, since it is not always advisable in practice to remove the
whole of the lime, which in small quantity renders tannage and
penetration of the liquor much more rapid, either by acting as a mordant
to the tannin, or by temporarily neutralising it and diminishing its
astringent action on the hide-fibre.
Turning to dressing leather, we find that the use of cold water alone
has been practically abandoned in this country, though the finest French
calf is produced by repeated soakings in cold water with alternate
workings over the beam, sometimes extending to nine or more. In this
case, from the lengthened exposure to waters which are only gradually
renewed it is probable that putrefactive action takes place, and that a
sort of bating is effected by the decomposing products of the hide
itself; in fact, in many French yards, bran-drenches have been
introduced to supplement the action of the water alone. Waters differ
greatly in their power of removing lime from skin. Slightly acid and
peaty waters, and those in general which contain much organic matter,
are much more powerful in reducing than those which are purer (cp. p.
107).
Warm water has much more effect in removing lime than cold, since the
heat lessens the risk of dissolved carbonic acid, and seems to have a
direct depleting effect on the pelt. A good tumbling in warm soft water
will remove a great deal of lime, and is an excellent preparation for
bating, but heat must be used cautiously, and should never exceed
30°-35° C.; some skins, such as seals, being very readily tendered by
its action, while others, especially sheep-skins, will stand a
comparatively high temperature.
The use of a solution of carbonic acid for removing lime has been
patented by Nesbitt,[90] who takes advantage of the fact that calcium
carbonate is soluble in excess of carbonic acid (p. 94). The gas, which
he generates, as for soda water, by the action of acids on chalk, or
limestone, is received in a gasholder, and forced by a compressing pump
into the vessel containing the hides, which is preferably a rotating
drum lined with copper, and capable of bearing a pressure of about three
atmospheres. The invention excited considerable interest on its
introduction, as the gas is, certainly, quite uninjurious to the hides,
and it was claimed that it enabled the grease and dirt to be better
removed than by the ordinary methods. Further experience has shown,
however, that the removal of the lime is far from complete, since, for
success, it is not only necessary to bring it into solution, but to wash
it out with carbonic acid solution under pressure, as on exposure to the
air, solutions of lime in excess of carbonic acid rapidly deposit
calcium carbonate. At the present time, the only tannery in which to my
knowledge the process is in use is that of Messrs. Mossop and Garland,
of Capetown, who state that it answers very well for harness-leather
when a pure lime made by calcining sea-shells is used for liming, but is
not satisfactory with ordinary stone lime. It is difficult to account
for this on chemical grounds. Gluestuff may be treated very
satisfactorily by simply blowing carbon dioxide, or washed and cooled
lime-kiln- or furnace-gases, into an open pit in which the material is
kept agitated. In this case, however, there is no need for the actual
removal of the lime, so long as it is carbonated and its caustic
character destroyed. Carbonic acid does not decompose lime-soap, and
hence sets free no fatty acids, which, together with grease, are the
main cause of the turbidity of glue, and the process therefore yields a
more brilliant though darker coloured glue than does treatment with
sulphurous acid.
[90] Eng. Pats. 7744 and 12,681, 1886.
Several acids of the aromatic series have been from time to time
recommended as deliming agents, and generally possess the merit of
acting at the same time as powerful antiseptics. In this connection it
may be well to mention the solution of 1 per cent. of phenol and 2 per
cent. of boric acid used by Dr. Parker and the writer for preparing and
preserving skins for colour tests (L.I.L.B., p. 133). This answers very
well as a bate even when much diluted, and may be rendered cheap enough
for use in practice by the employment of a good commercial carbolic acid
instead of pure phenol, and the use of sulphuric acid to remove lime
from the solution and render it capable of repeated employment. The
carbolic acid should not be too dark in colour, and should be carefully
dissolved, or “carbolic” stains will result.
“Cresotinic acid,” a mixture of impure acids obtained from cresols in
the same way as salicylic acid is manufactured from pure phenol, was
introduced as a bate and unhairing and deliming agent by J. Hauff, of
Feuerbach.[91] He also claims the use of hydrochloric acid to liberate
the acid after it has been combined with lime in the deliming process.
It is only soluble to the extent of about 1 in 800 of water, so that,
even if used in excess, no dangerously strong solution is formed, but it
has a tendency to slightly swell, and somewhat harden, the hides or
skins, so that it is perhaps more suitable for sole than dressing
leather. It has also powerful disinfectant properties (see p. 29).[92]
[91] Eng. Pat. 14,889, 1888.
[92] Compare also Journ. Soc. Chem. Ind., 1889, p. 954.
Hauff states that a solution of 18 lb. of cresotinic acid in 500 gallons
of water at 30° C. will bate one lot of 50 heavy hides, and that the
same liquor may be used continuously, by adding 4-5 lb. more cresotinic
acid for each successive 50 hides. For bating glove-leather, Hauff
recommends the use of 5 kilos. cresotinic acid dissolved in 1000 liters
of warm water for every 500 kilos. of wet skins, to which is added
ammonia nearly sufficient to neutralise the cresotinic acid, leaving the
solution still slightly acid to litmus paper; and he also advises the
addition of 5 kilos. of ammonium chloride or sulphate. The goods are
paddled in this solution for about half an hour.
“Oxynaphthoic acid,” the corresponding mixed acids of the naphthols (p.
30), has also been patented by Hauff as a bate, since cresotinic acid
sometimes acts too powerfully on light skins.[93] He mentions that
mixtures of this and cresotinic acid, or salicylic acid, may also be
used. Oxynaphthoic acid requires for its solution 20,000 to 30,000 parts
of water.
[93] Eng. Pats. 10,110 and 12,521. Journ. Soc. Chem. Ind., 1889, pp.
124, 809; 1890, p. 85.
A mixture of the α and β mono- and di-sulphonic acids of naphthalene has
also been patented for bating,[94] under the name of “Acrilene bating
and puering acid.” 150 calf-skins, weighing 880 lb., were pured in a 3
per cent. solution of the α acid, and gave 266 lb. of leather as against
255 lb. from a lot of similar weight treated with hen-dung, and this
gain was more than maintained on stuffing, while the shoulders were
plumper and fuller. This patent appears to anticipate a part of Hauff’s
claim mentioned in the next paragraph.
[94] Burns and Hull, Eng. Pat. 8096, 1891; Journ. Soc. Chem. Ind.,
1892, p. 48.
More recently Hauff has patented, under the name of “anticalcium,” a
mixture of impure sulphonic acids of various cresols and hydrocarbons.
This is cheaper than cresotinic acid, and like it, possesses
considerable antiseptic powers. One-half to one-quarter per cent.
solution will keep hides uninjured for a considerable time, but at this
strength it plumps considerably, and seems more suitable as a deliming
agent for sole-leather than as a bate for dressing-leather, though it
may replace drenching. No doubt, by the use of warm water, and avoidance
of excess of acid, skins could be pulled down satisfactorily, or the
plumping could be controlled by addition of salt, but the disinfectant
powers of the acid would render further treatment with an ordinary bate
or puer very difficult.[95]
[95] J. Hauff, Eng. Pat. 22,546, 1894; Journ. Soc. Chem. Ind., 1895,
p. 170, Gerber, 1895, p. 133.
The “C. T. Bate,” manufactured by the Martin Dennis Chrome Company, is
of a very similar character; and is in the form of a greyish crystalline
paste, consisting mainly of sulphonic acids of naphthalene and probably
other hydrocarbons. It is very possibly made by sulphonating
coal-creasote oils, which contain much naphthalene and phenanthrene. The
following directions are given by the company for its use.
“1. After unhairing and fleshing from the lime, the skins should be
thoroughly washed with water (preferably warm) so as to remove as much
lime as possible.
2. If, in the liming process, the sulphide of sodium is used in
combination with the lime, it will render the lime more soluble and
therefore more easily removed with water.
3. The more completely the skins are cleansed with warm water the less
will be the quantity of bate required.
4. After washing, the skins should be thoroughly worked on the beam,
especially on the grain.
5. A solution of C. T. Bate is now prepared in the proportion of from
one-half pound to one pound of bate in 100 gallons of warm water (90°
F.). In making the solution do not have the water over 140° F. Under no
circumstances boil it.
6. If the hides or skins have been treated as above indicated, one pound
of bate should be sufficient for 400 pounds wet hide, washed from the
limes. The hides or skins are placed in the bating solution and worked
for an hour. They are then allowed to rest in the solution with
occasional stirring for some hours or over night.
7. The length of time that the bating should continue will depend upon
the degree of softness and pliability required in the leather. For
instance, for sole-leather fifteen minutes is sufficient; for satin
leather thirty minutes; for glove-leathers four to six hours or even
longer.
8. On removing the skins from the bating solution it is sometimes
desirable, especially for the finer grades of leather, to wash them in
warm water and again work them over the beam. They are then ready to be
placed in the tanning liquors.
9. In preparing the bating solution for the second pack, draw down the
old solution one-third and replace with fresh water; then add in
solution just one-half the quantity of bate used at first, and so on
with each succeeding pack.
10. When fresh white limes are used toward the end of the liming
process, and a manure bate is deemed necessary to reduce the harshness
of grain caused by the fresh lime, it is very beneficial to give the
skins from the manure bate a drench of C. T. Bate, thereby arresting
the bacterial action of the manure bate, preserving the grain, besides
cleansing, bleaching and neutralising the skins preparatory to placing
them in the tanning liquors.
11. Again, when it is considered desirable to use a manure bate, it is
good practice to treat the skins as above indicated (down to item No.
7), and then place them in the manure bate. By this previous treatment
the antiseptic action of the C. T. Bate tends to arrest the destructive
bacterial action of the manure bate, thereby lessening the risk of
damage to the grain. In all cases where the value of the leather is
dependent on the quality and perfection of the grain, this is an
important advantage to gain.”
All these coal tar “bates” are rather suitable to replace drenching than
bating or puering, as their effect is mainly that of removing lime. From
their antiseptic character they are very useful in stopping the effects
of putrefaction, and preventing ferments being carried into the tanning
liquors, and skins may safely be kept at least for some days in weak
solutions, but any necessary fermentive puering or bating should usually
be done before and not after their use.
A writer in the ‘Gerber,’ 1875, p. 279, recommends the use of dilute
solution of sulphide of sodium as a bating agent. Possibly it removes
lime as sulphydrate, and the writer named seems to have obtained good
results with glove lamb-skins. In experiments made at the Yorkshire
College, a solution of 4 grm. per litre used on 40 grm. of pelt was
found to plump it considerably, but probably a much weaker solution
might be sufficient and more satisfactory. Polysulphides, such as “liver
of sulphur,” or the yellow solution obtained by boiling dilute sodium
sulphide or sodium hydrate solution with excess of sulphur, have great
power of “bringing down” the pelt, and seem well worthy of experiment as
bating agents.
In India, the pods of the babool (_Acacia arabica_) are much used as a
bate, the infusion being allowed to ferment. In their dry state they
contain about 12 per cent. of an easily changeable tannin, which does
not precipitate lime-water, and which by fermentation is very probably
converted into gallic acid. The use of gallic acid itself as a bate has
been patented by Albert Hull,[96] and would undoubtedly accomplish the
removal of the lime if used in sufficient quantity; but as he only uses
a solution of 25 mgr. per litre (one part in 40,000) any effect must be
mainly due to the washing with water. Gallic acid forms dark oxidation
products with lime.
[96] Eng. Pat. 14,595, 1889.
Of the fermentive methods of removing lime, “drenching” with fermenting
bran-infusions is the simplest in theory, and has been very carefully
investigated by Mr. J. T. Wood.[97] It will, therefore, be convenient to
consider this process first, although it is frequently employed as a
means of cleansing and slightly plumping the skin after the lime has
been removed by puering or bating. In calf-kid manufacture, however, it
is now used without previous puering, and in some other cases it is
substituted for the use of dung bates. The most important of the active
ferments are two species of bacteria, named by Wood _Bacterium furfuris_
α and β, which are very similar in their form and action (see L.I.L.B.,
p. 264), but produce a somewhat better fermentation together than
separately. They are shown in Figs. 34 and 35.
[97] Journ. Soc. Chem. Ind., 1890, p. 27; 1893, p. 422; 1897, p. 510;
Brit. Assoc. Rep., 1893, p. 723.
[Illustration: FIG. 34.--_Bacterium furfuris_ α.]
[Illustration: FIG. 35.--_Bacterium furfuris_ β.]
Neither species has any direct action on the hide substance, but
ferments the glucose produced by the action of the cerealin of the bran
on the starch which is present. A considerable quantity of hydrogen,
with carbon dioxide, nitrogen and small quantities of hydrogen sulphide,
are produced during the fermentation, together with lactic and acetic,
and traces of formic and butyric acids and amines. Active drenches
contain 1-3 grm. of mixed acids per liter, to which they owe their
action, a perfectly satisfactory drenching being produced by an
artificial drench containing 0·5 grm. of glacial acetic acid and 1 grm.
of lactic acid (sp. gr. 1·210) per liter in which the skins were worked
for 1¹⁄₂-2 hours, while 12-16 hours would have been required in the
ordinary drench. An experimental drench gave the following results on
analysis:--
Formic acid 0·0306 grm. per litre
Acetic acid 0·2402 „
Butyric acid 0·0134 „
Lactic acid 0·7907 „
------
Total 1·0749 „
It is probable that other organisms are capable of producing similar
fermentations, and it is not certain that in all tanneries the same
ferments are present. Mr. A. N. Palmer states that at the Cambrian
Leather Works at Wrexham, he has been unable to detect lactic acid in
the drenches, all the acids present being of the acetic series.
The drench-ferments investigated by Wood are incapable of attacking or
injuring the hide, and, in his opinion, when the skin is attacked, it is
generally due to putrefactive and gelatine-liquefying organisms
introduced from the bates, or from the air in hot sultry weather.
Drenching takes place most safely and satisfactorily at temperatures not
exceeding 30°-35° C., when the process is usually complete in 12-24
hours. In hot sultry weather a butyric fermentation of an active
character sometimes suddenly takes the place of the normal one (Ger.
_Umschlagen_), the skins swell rapidly, become translucent (_glasig_)
and finally dissolve to a jelly. If tanned in the swollen condition,
tender and useless leather results, and the injury, once begun, proceeds
with alarming rapidity, skins being sometimes completely ruined in a few
hours. Prompt action is therefore necessary, and the first step to take
is to add salt, which checks the fermentation, and acts in the same way
as in the pickling process, controlling the action of the acid, and
producing a sort of tawing. Such skins will yield sound leather, though
the grain is apt to be somewhat drawn. If the skins can be immediately
got out of the drench, the acid may be neutralised by the cautious
addition of ammonia, soda, or whitening to the water in which they are
placed, preferably in a paddle, and if they are insufficiently drenched
they may then be paddled in tepid water, though this is hardly likely to
be needed, as the effect of the acid is to remove the lime very
completely. The objection to the use of whitening, which otherwise is
the safest and best material to employ for removing acid from pelt, is
that it is apt to become mechanically fixed in the grain, and, thus, to
produce bad colour with vegetable tans. For white or chrome leather it
would do no harm. Precautions to prevent the recurrence of the injury
are to keep the temperature of the drench low, and to free the bran from
flour by washing in two or three cold waters, before adding to it the
hot water with which the actual drench-liquor is made, since the flour,
or at least its starch, is the source from which the butyric acid, as
well as the lactic, is formed. In cold weather, where drenching is
proceeding in a normal way, the flour is useful, since it is the natural
nutriment of the drench-ferment; and, in England, flour is frequently
added purposely to the bran to increase the activity of the drench. To
retain the flour, the bran may be washed first with boiling water, which
gelatinises the starch and makes it adhere to the bran, and, according
to Eitner, removes a sticky fatlike matter from it, and fits it better
to remove the fat of the skin. After soaking in hot water for two hours,
it is washed in several cold waters and infused at about 40° C. for
use.[98] Many tanners use the bran without previous washing, but if much
flour is present it rises to the top with the gas evolved by the
fermentation, and forms a pasty mass on the skins, which interferes with
even drenching.
[98] Gerber, 1882, p. 246.
The quantity of bran used in ordinary drenching is very variable, but
about 4 parts per 1000 of water used and from 5 to 10 per cent. on the
weight of pelt may be taken as an average quantity, more being
frequently employed. The temperature may vary from 10° up to about
30°-35° C., and the time inversely from days or weeks down to two or
three hours, according to the temperature of the drench, the amount of
ferment present, and the thickness and character of the skins. The skins
are usually thrown into the freshly prepared drench, to which a few
pailfuls of old drench-liquor is frequently added as a ferment.
Fermentation soon sets in, and the gas evolved causes the skins to float
to the surface; this is called the “working” of the drench. Thin skins
may be sufficiently drenched after once rising, while thick ones require
to be put down two or three times. A certain sign of sufficient
drenching is the appearance of small blisters on the grain, caused by
the evolution of gas in the substance of the skin. When these are seen
the drenching should be at once discontinued, as otherwise the blisters
will increase in number and burst through the grain, causing minute
holes or “pricks” (one of the many forms of the complaint called in
German _Pikiren_ or _Piquieren_). When a bubble of air is enclosed in a
fold of the sufficiently drenched skin and pressed, it raises the grain
without actually separating it from the substance of the skin. The
properly drenched skin also falls easily in folds when held between the
hands either lengthways or crossways, and if thin, the skin tightly
stretched over the hand shows grains of bran underneath it as little
lumps, round which the skin clings to the hand. The drenched skin should
not be transparent, but white and soft; and when pressed should retain
the mark of the finger. Some experience is required to determine
certainly the point of sufficient drenching, which, of course, varies
with the character of the skins, and the kind of leather which is to be
produced; and the feel of the skin to a practised hand is one of the
most important criteria.
A writer in the ‘Gerber’[99] divides drenching into three
classes--“sweet,” “alcoholic” and “sour.” Sweet drenching is done in a
bath of tepid bran-water, made by infusing in hot water and drawing the
clear liquor off the bran, which settles to the bottom. The skins are
only allowed to remain in 2-3 hours, or not long enough for fermentation
to set in. The process is only suited for very thin or soft skins, which
will not stand any further loosening. The use of bran-water has the
advantage of saving the labour of “branning,” or removing adhering bran
with the knife on the beam, but it is doubtful if unfermented bran has
much actual effect. Bran-water can, however, be used for drenching by
fermentation, and for small glove-lamb has largely superseded the older
method. The mechanical action of the bran in cleansing the pelt is
however often useful. In sour drenching the bran is allowed to steep and
soften in cold water for many hours, and boiling water is then added
till the temperature is raised to 75° C., and it is allowed to infuse
with frequent stirring for some hours, and after cooling to 45° a
considerable quantity of old drench-liquor is added as a ferment. If the
drench is used warm (30°-35°, or, in cold weather, even 40° C.), the
skins only remain in 1-3 hours, but if cold the drenching can be
extended over a period of 2-3 days, the skins being frequently handled.
This modification is suitable for glacé-kid and the harder sorts of
skins, but glove-lamb are always treated by the warm and rapid process.
What the writer in the ‘Gerber’ describes as the “alcoholic” bran-drench
is probably the method of fermentation investigated by Mr. Wood, in
which ordinary inflammable gases, but no alcohol, are produced.
[99] Gerber, 1888, p. 257.
A normal drench plumps the goods slightly, but if it contains much of
the putrid ferments carried in from the bate or puer the skins fall in
it as they would do in a bate. To increase this effect, putrid
soak-liquor is sometimes added to the drench, but with doubtful
advantage.
In drench-liquors the total acidity may be determined by titration with
lime-water or N/10 caustic soda, with phenolphthalein as indicator; and
the volatile acids may be distilled off as described under the analysis
of tanning liquors (L.I.L.B., p. 126). For more complete methods of
analysis the reader is referred to Messrs. Wood and Willcox’s paper on
the “Nature of Bran Fermentation.”[100]
[100] Journ. Soc. Chem. Ind., 1893, p. 422.
Drenches are said to “work” somewhat better if made with water
containing nitrates, and this is quite probable; but the necessary
nitrogen can easily be supplied if required by the addition of a very
small quantity of saltpetre.
Wood is of the opinion that the ferments found in bran do not originate
in the drench itself, but come from the bated skins, as the
drench-bacteria soon die out without finishing the fermentation, and
constant renewing of the nutrient material is necessary (cp. p. 18).
_Bating and puering_, though differing practically in many ways, are
identical in theory, and most of what follows applies to both of them.
The action is much more complex than that of the drench, involving both
chemical reactions and those of organised and unorganised ferments, and
it is a matter of no little difficulty to say what proportion of the
observed effect should be ascribed to each of these agencies.
Formerly, the principal effect was attributed to organic salts of
ammonia and its homologues, and to amido-acids which combine with lime.
Phosphoric acid is also present, and if any exists in the form of
soluble salts, it will combine with lime, and render it insoluble and
inactive. It is probable, however, that most if not all the phosphoric
acid is already in the form of tricalcium phosphate, and therefore
without effect.
It is now, however, recognised that the effects of these chemicals are
of no importance as compared with the products of bacterial action, and
the researches of J. T. Wood have cleared up much that was until
recently quite inexplicable.[101]
[101] Journ. Soc. Chem. Ind., 1894, p. 218; 1895, p. 449; 1898, pp.
856, 1010; 1899, pp. 117, 990.
Much effect has been ascribed to the digestive ferments, such as pepsin
and trypsin, which are present in fresh dung. It is known that the
animal organism secretes these in considerable excess of its
requirements, but it is doubtful whether any exist undecomposed, even in
fresh dung; though they are apparently more resistant to putrefaction
and decomposition than would _a priori_ have been expected of such
complex organic compounds, and there is therefore a possibility of their
existence in the dung, even as it comes to be used in the tannery. Both
pepsin and trypsin are enzymes (see p. 16), and belong to the great
class of albuminoids. They are soluble in water, but insoluble in
alcohol, and hence are precipitated by the addition of the latter to
their solution, but are not altered by it, and regain their activity on
solution in water. By heat they are coagulated and decomposed, and their
activity permanently destroyed.
_Pepsin_ is the active principle of the secretion of the glands of the
stomach, and large quantities are prepared for medical use as an aid to
digestion from the stomachs of pigs. Pepsin only acts in slightly acid
solution, and, though fresh bate liquor is slightly acid to litmus, it
speedily becomes alkaline from the lime of the skins and the ammonia
present, so that the action of pepsin in a bate can only be a very
limited one. Wood[102] compared the action of a 1 per cent. solution of
pepsin, acidified with 0·2 per cent. of hydrochloric acid, with that of
a dogs’ dung puer liquor, both at the temperature of 40° C. At the end
of one hour the skin in the pepsin-solution was considerably fallen, but
that in the puer-solution was almost dissolved. Since the solution here
employed was much stronger than is likely to occur in practice, and the
conditions much more favourable to its action, it may be assumed that
the practical effect of traces of pepsin in the bate may be neglected.
[102] Journ. Soc. Chem. Ind., 1894, p. 220.
_Trypsin_ or _pancreatin_[103] if present, is more likely to have an
effect, since it is active in neutral and in alkaline solutions. It is
the product of the pancreas, and is largely concerned in intestinal
digestion. Chemically it much resembles pepsin, but is more resistant to
heat, retaining its power of digestion after heating to a temperature of
160° C. in a dry condition. Its warmed solution dissolves fibrin almost
instantly, and in large quantity, and peptonises gelatin and hide-fibre,
so as to render them soluble in water. Wood found that a 1 per cent.
solution of pancreatin acted far more rapidly than a solution of pepsin
of equal strength. At 40° C. in neutral solution, the skin fell rapidly,
and the action continued even in the cold. In 15 hours the liquid was
swarming with minute bacteria. At the suggestion of the Author, the
experiment was therefore repeated, with the addition of 15 per cent. of
chloroform, which prevented the development of bacteria, while it did
not stop the action of the pancreatin. The skin fell as before, but in
neither case had it the peculiar touch of puered skin, nor were the
characteristics of the leather produced from it the same. We may
therefore conclude that, though trypsin may contribute to the action of
the bate or puer, it can only do so in a minor degree, and that the
principal effect of the bate or puer is due to other causes. It is
certain, however, that fresh bird-dung, and probably that of all
animals, contains ferments capable of liquefying gelatin. An instance of
this is found in the observation, common in glue manufacture, that if
the dropping of a sparrow falls on a cooler full of solidified gelatine
size, it will liquefy a track quite down to the bottom of the cooler.
Trypsin, or at least the secretion of the pancreas, as well as the gall
from the liver, have great power of wetting and emulsifying fats, and
this has possibly something to do with the action of the bate in
enabling the skins to be cleansed of fat.
[103] Loc. cit. and Beilstein, iii. p. 1308, 2nd ed.
Bacterial fermentation and its products are however the main factor in
the action of puers and bates, and on this subject we owe most of our
knowledge to the work of J. T. Wood, since, though Popp and Becker have
worked over much of the same ground, they have not nearly so freely
published their results.
Wood showed that a fresh puer liquor, even when boiled for half an hour
and so freed from living organisms and albuminoid ferments, has still
considerable action on a limed skin, though much less than the unboiled
puer. He found that this action was principally due to amines and their
compounds with organic acids, which removed lime, but did not remove the
interfibrillary substance or give the proper feel of puered skin. A very
similar result was obtained with aniline (phenyl-amine) hydrochloride in
1 per cent. solution.
A considerable variety of bacteria from dung and other sources were
cultivated in various media and their puering power tested, but though
greater than that of the unorganised chemical compounds such as amine
salts and organic acids, it was in no case equal to that of an ordinary
puer, or sufficient for practical use. When, however, a small quantity
of the amine salts obtained from the puer were added to a mixed
bacterial culture the effect on the skin was almost as rapid and
considerable as with an actual puer.
In order to determine whether the puering effect was due to the direct
action of the bacteria or to their enzyme-products, the latter were
separated from a filtered puer solution by adding it to a large volume
of 98 per cent. alcohol in which the enzymes are insoluble. When
redissolved in water, they had a decided puering effect, and a solution
of 0·5 grm. of the mixed enzymes and 0·5 grm. of the mixed amine
hydrochlorides in 100 c.c. of water at 350° C. brought down a piece of
limed sheep-skin in thirty minutes exactly like a puer. The action is
therefore dependent on the mutual action of the enzymes and amine salts,
but as the separation of these would be too costly for practical use,
and the puering proved more effectual when they were formed in contact
with the skin by active bacteria, Wood adopted the method of preparing a
suitable sterilised nutritive liquid, which was inoculated before use
with a mixed culture of suitable bacteria. For laboratory purposes a
suitable culture-medium was obtained by digesting 10 grm. of gelatine
with 5 grm. of lactic acid (reckoned water-free) and 100 c.c. of water
for three hours in a closed vessel on the water-bath. The resultant
solution was neutralised with sodium carbonate and diluted to 1 litre
with addition of a small quantity of potassium phosphate.
The bacteria of fresh dog-dung were not found to possess a satisfactory
puering effect, but those from dung which had been fermented a month (as
in practice) gave a result nearly equal to actual puer. A still better
result was obtained by a mixed culture from the roots of wool loosened
by sweating. The bacteria were principally of two species, of which
neither separately was capable of satisfactory puering; but which
together acted more rapidly than an actual puer. These bacteria do not
liquefy gelatine.
During the course of his experiments, Wood found that filtered puer
solutions were less active than turbid ones and that their activity was
increased even by the addition of inert substances, such as kaolin.
Wood attributes the differences in action between dog-dung and bird-dung
not only to different bacteria, but to the fact that in the latter case
the urinary products, and especially uric acid are contained in the
dung.
From the results of these and similar researches, Wood in England, and
Popp and Becker in Germany succeeded in producing a practical artificial
puer, which they now manufacture in conjunction under the name of
“Erodin.”
“Erodin” consists of a solid nutrient medium and a liquid “pure culture”
of the bacteria necessary to effect the required bating or puering.
The following are the directions for working with erodin bate, as
supplied by the manufacturers:--
“For 100 lb. of wet skin washed ready for bating, about 1 lb. of erodin
is required. Or in the metric system, 1 kilo. wet skin requires about 10
grm. erodin. The strength or concentration of the bate must not fall
below 3 grm. per litre of bate liquor, i.e. ¹⁄₂ oz. per gallon.
For preparing the bate a sufficiently large cask or tub carefully
_cleaned_ and steamed out is placed near the bating paddle. The cask
should be fitted with a steam pipe easily screwed on and off, and also
furnished with a _clean_ cover.
The requisite quantity of erodin is weighed out and put into the tub
with fifty times its weight of water, and the whole brought up to a
temperature reaching but not exceeding 40° C. (104° F.) by direct
admission of steam, thoroughly stirred, and the pure culture of
_Bacillus erodiens_ added to the mixture. The temperature must not be
allowed to fall below 25°C. (87° F.), and a little steam should be
admitted first thing in the morning, again at noon, and in the evening,
to bring the temperature up to 40° C. (104° F.).
A practical mode of procedure is as follows:--On Friday make up and
start fermenting twice as much erodin as will be required for a day’s
work. This is allowed to remain under the above-mentioned conditions
until Monday. On Monday half the amount will be used for bating; this is
replaced by an equivalent amount of fresh erodin powder, dissolved in
fifty times its weight of water, which is added to the already fermented
erodin in the tub. Proceed in this way each day until the following
Friday, when there will be left in the tub sufficient erodin for one
day. This is put into a smaller tub for use on Saturday, and the cycle
of operation begun again.
One pure culture of _Bacillus erodiens_ should be used for every 11 lb.
(5 kilos.) erodin powder or less quantity.
Suppose the amount of erodin required for a day’s work to be 11 lb. (5
kilos.), then on Friday 22 lb. (10 kilos.) erodin must be mashed as
above described in 110 galls. (500 litres) water, 2 pure cultures added,
and allowed to ferment until Monday.
On Monday half of this is used, and to the remainder 11 lb. (5 kilos.)
erodin and 55 galls. (250 litres) water is added. This is repeated on
Tuesday, Wednesday and Thursday; and on Friday half is used and the
remainder put into a separate cask for use on Saturday, and in the
mashing cask a fresh quantity of 22 lb. (10 kilos.) erodin with 110
galls. (500 litres) water is made up for use next week.[104]
[104] Mr. Wood has found that in many cases it is unnecessary to start
afresh at the end of each week, but that additional quantities of
erodin solution with the accompanying bacterial culture may be added
continuously to the stock-tub as required. In puering, the
concentrated solution from the tub may be diluted with 4 to 6 times
its volume of warm water. The diluted liquor should usually only be
used for one pack of skins.
On Saturday the remainder of the old mash is used up.
In case this mode of procedure is for any reason not suited to the
conditions of work, erodin may be used by making up every day a fresh
quantity with fifty times its weight of water, adding the pure culture,
and allowing it to ferment three days before use.” In some cases the
solution may be used for several consecutive packs, merely adding water
and a small quantity of erodin without a new culture.
Erodin is being used most successfully in several large works both in
England and abroad, and on calf-skins and sheep-skins has proved quite
as effective and much safer than dog-dung; the skins coming out clean
and free from stains. It has been a good deal used in the experimental
tannery of the Yorkshire College, and has proved a satisfactory
substitute for puer, but with the present bacterial cultures can only be
employed warm, and does not answer used cold like the ordinary
pigeon-dung bate. No doubt a suitable bacterial medium and culture can
be found for cold bating, which for thicker leathers is often preferable
to puering, and experiments in this direction are being undertaken.
From the multiplicity of germs present, and the adaptability of the dung
infusion as a nutrient medium for any putrefactive organisms which may
gain access to it, the bating and puering process is necessarily a
dangerous one for the goods, always leading to loss of weight, and, if
the process is carried on too long, to the more or less complete
destruction of the skins. Loss of weight, however, in greater or lesser
degree is inevitable, and indeed necessary where a soft leather is to be
produced. If the skins are allowed to lie in the bate or puer liquor,
mud, containing organisms, and zooglœa-forms of bacteria settle in the
folds, and produce marbled markings, streaks and lines by the
destruction of the grain surface (hyaline layer). Black or bluish stains
are also often produced, known as bate-stains, and either due to
bacterial pigments, or in some cases, to the action of evolved hydrogen
sulphide on iron present from salting or other sources. Frequent change
of position is therefore necessary, especially when the liquor is active
from being used at a high temperature, but it does not seem to be
desirable to keep the skins in constant motion, and if puering is done
in a paddle, it should only be run at intervals.
T. Palmer[105] determined in experiments on pigeon-dung bates that there
is considerable loss of nitrogen during the process, and recommended
bating in pits from which the air was excluded as much as possible, both
as effecting a considerable economy in the dung, and in excluding false
ferments, which, he concludes, are mostly aerobic. It is not improbable
that the method is advantageous, since it has been shown by Roscoe and
Scudder that liquefaction of gelatin only takes place in presence of
oxygen, and its partial exclusion would therefore lessen the risk of
overbating, and consequent damage and loss of weight.
[105] Leather Trade Circular, 22nd Sept., 1891; 1887, p. 667; and
Sanford, Journ. Soc. Chem. Ind., 1893, p. 530.
Starting from the presumption that bating and puering are, in the main,
bacterial processes, more or less successful attempts had been made
previous to those of Wood, Popp and Becker, to substitute other
fermenting substances for dung; and probably these efforts failed in
many cases, not so much because they were wrong in principle, as from
want of knowledge of the necessary details, such as the use of proper
ferments, and the provision of suitable culture-media. Guano, prepared
horse-flesh, urine, yeast, and fermenting vegetables have all been
tried. A solution of glucose or treacle of about 10 per cent., to which
3 per cent. of pasty dog-puer is added about a week before use, was
tried many years since in a morocco-factory, at the suggestion of the
writer, as at least a partial substitute for puer, and is still in use
there. The mixture keeps for some time in an active state, and is added
to the puer liquors in the same way and in approximately the same
proportions as the dung paste. Similar in principle is the solid bate
supplied by an American firm, in which glucose is mixed with a small
amount of nitrogenous matter and phosphates, together with a lactic
ferment, and which only requires dissolving in warm water some little
time before use. Its results are good for some purposes, but rather
resemble those of a drench than a bate. In a similar way, puer may be
added to bran-drench liquors, and induces in them a fermentation which
brings the skins down much lower than the ordinary drench. It is
probable that a weak glucose solution, with traces of mineral
constituents similar to Cohn’s solution (see L.I.L.B., p. 269) and “set”
with sour milk, or fermenting drench-liquor, might in some cases be used
with advantage for drenching, with a saving of cost. A writer in ‘Hide
and Leather’ describes a bate in which two parts by weight of glucose
are dissolved in about 25 parts of water, and fermented, for about three
days, till a foam gathers on the top, with about one part of old bran
drench-liquor, or 0·1 part of pressed yeast, and then made up with water
to 1000 parts. The goods are bated 24-36 hours at a temperature of about
35° C, and the bate is strengthened for a second pack with about
one-fifth of the original glucose, a new bate being made at the end of a
week, and set with one part per thousand of the old one. A short bating
of say 10 hours produced very nice harness-leather, but the general
tendency was to make the goods looser and more spongy than a dung-bate.
It is obviously not a matter of indifference whether old drench, or
yeast, is used to start the fermentation, since in the latter case only
alcohol could be produced directly by the ferment introduced, though
this might be fermented later, by other accidental organisms, into
acetic acid. These mixed bates, containing glucose, are however probably
wrong in principle, since the true puering and bating bacteria will not
thrive in presence of acids, and require nitrogenous nutriment.
As regards the relative effect of dog- and hen- or pigeon-dung bates,
the chief of the published experiments are those made by W. J. Salomon
at the Vienna Versuchsanstalt für Lederindustrie,[106] in which he
determined the relative solvent power of equal quantities as being, for
dog-dung 2¹⁄₂, for pigeon-dung 2, and for hen-dung 1. It is obvious that
these figures, though interesting, must be taken with some reserve, as
the composition even of pure dungs is by no means constant, depending on
the feeding of the animals, and adulteration is common. The writer has
heard stories of a certain dealer who used to fabricate his product from
clay by the aid of a popgun, though he does not vouch for the statement!
It is generally held that the action of bird-dung is more penetrating,
but less softening and loosening than that of dog-dung, which is thus
generally used for descriptions of leather where great softness and
stretch are required. It is to be remembered in this connection that
bird-dung bates are generally used cold, and hence are much slower in
their action, which allows them time to penetrate thicker hides more
uniformly. Few analyses of the dungs used in leather manufacture have
been published, and these mostly with a view to manurial value.
Schulze[107] gives the result of forty analyses of pigeon-dung as
follows:--
Min. Max. Mean.
per cent. per cent. per cent.
Water 3·80 40·00 21·00
Nitrogen 1·47 5·04 2·53
Phosphoric acid 1·00 2·77 1·79
Potash 0·71 2·57 1·46
One sample contained 43·3 per cent. of sand!
[106] Tech. Quart., 1892, v. p. 81.
[107] Der Landwirt, 1895, li. p. 301.
Wood[108] quotes the following:--
_Hen-Dung._
Per cent.
Water 60·88
Organic matter[109] 19·22
Phosphates 4·47
Calcium carbonate and sulphate 7·85
Alkaline salts 1·09
Silica and sand 6·69
_Dog-Dung._
Water 31·0
Ca 43·0
Na, K, Mg 0·8
PO₄ 3·4
CO₂ 7·5
Organic matter 14·2
Traces Fe, Cl, Si, loss 0·1
[108] Journ. Soc. Chem. Ind., 1894, p 220.
[109] Containing nitrogen equal to 0·74 per cent. of ammonia.
This was apparently a sample from a dog fed on bones; that from the
kennels, which is more commonly used in leather manufacture, contains
much less lime; a sample analysed by Wood gave 4·7 per cent. mineral
matter, 9·7 per cent. organic, and 85·6 per cent. of water, part of
which was no doubt added.
_Analysis._--Little or no attention has been paid to the analysis either
of dungs for bating purposes, or of the bating liquors, and although the
total cost of manure bates is a high one, it is evident that such low-
priced and irregular articles will not pay for elaborate analysis.
Probably in some cases it would be worth while to make a determination
of moisture and organic and mineral constituents by drying and ignition.
Where a further investigation is desired, the determination of the
soluble matter by filtering and evaporating a portion of the solution to
dryness, and that of the nitrogen by Kjeldahl’s method (see p. 70),
would be advisable, and of course in the future, when the subject is
better understood, a bacteriological examination may be useful. If it is
desired to estimate the solution of hide-substance in the use of bate or
drench liquors, the determination of the nitrogen in a measured quantity
by Kjeldahl’s method will afford the best basis of calculation,
allowance being made for the nitrogen present in the original bate
liquor. Hide-substance contains about 17·8 per cent. of nitrogen. In
many cases, simple weighing of the solid residue, left on evaporating
the liquor to dryness and drying for several hours at 100° C., with
subsequent ignition to determine lime and other mineral matters, will
suffice.
The quantity of hen- or pigeon-dung used in bating hides is very
variable, but may be stated at from 12 to 60 litres per 1000 kilos of
raw hide, in at least 2000 litres of water. The bate is generally used
cold, the hides remaining in it 4-8 days, with frequent handling; but
some tanners, especially in the United States, prefer bating in a paddle
or drum at a temperature of about 35° C., in which case the time must be
diminished to a few hours. The dung is best infused with warm water in a
separate vessel,[110] and allowed to ferment for at least a week without
use, when it will be found to swarm with micrococcus-chains. Only the
clear liquor should be run into the bate-pit, the sediment and dirt
being thrown away, or used as manure. In this way the danger of stains
and flaking is much reduced. Bates may be mended with fresh portions of
dung-infusion for several successive packs of hides, but should not be
used too long, as they gain in solvent power by the dissolved
hide-substance and the increased fermentation, and the method is not
without risk.
[110] This seems to have been first suggested by T. Palmer, Eng. Pat.
13,636, 1886.
After bating, the hides are usually “worked” (“scudded,” “fine-haired”)
on the beam, to remove dirt and grease, but in America a wash in the
wash-wheel is often considered sufficient. Goods are occasionally
“stocked” (p. 116) from the bates, but this is not to be recommended, as
it is likely to drive out much of the partially dissolved hide-substance
and produce undue looseness and loss of weight.
It is difficult to give any definite marks of sufficient bating other
than the soft and fallen feel of the hides, which is easily recognised
by a practised hand. One of the earliest signs of commencing overbating
is the occurrence of bluish patches, or a bluish tinge somewhat similar
to an iron-stain, which, if slight, generally disappears in a few days
after the hides are taken into the liquors. Hen- and pigeon-dung is
probably best kept air-dried, though, if very wet, or for convenience
for immediate use, it may be kept in paste like dog-dung.
Dog-dung should never be allowed to lie exposed to the air, or it
putrefies and turns black, the bating ingredients are destroyed, and it
will not puer the goods which turn black and putrid without softening.
Dung should, therefore, be mixed to a paste with water and kept in
tanks, so as to be but little exposed to the air, when it will retain
its puering properties for a long time unaffected. Fresh dung should be
allowed to ferment for at least a week before use. No accurate statement
can be made as to the quantities required. Eitner states that 1-1¹⁄₂
pails of dung-paste (say 14-20 litres) is sufficient for 200 medium to
large lamb-skins for glove-kid. It should be sufficient to make the
water quite turbid, but not thick or soupy. For lamb-skins a temperature
of 18°-20° C. is suitable, which may be raised in very cold weather to
25° C., to allow for cooling. The time required is from two hours for
the thinnest slink skins, to 12-14 hours for strong ones. It is well to
use wooden, and not iron, utensils for handling the dung, and it should
be strained through a coarse cloth after diluting with water. As has
been remarked, it is not desirable to keep the skins in constant motion
in the puer; they should be stirred or paddled for the first 20-30
minutes, and then for 10 minutes every hour for five or six hours, after
which they can be allowed to lie for a longer period without injury.
Puering is sufficient when the skins feel quite soft and flaccid,
hanging in folds in any direction and allowing the flesh to be scraped
off with the finger-nail.
Wood recommends that, for the puering of sheep-skins, dung should be
allowed to ferment one month before use, and states that it deteriorates
if kept over three months. The puering products are the result of the
successive action of many sorts of bacteria, and Wood is of opinion that
those actually concerned in puering originate from the air, or from the
vessels in which the dung is stored, and are not present in it when
excreted. Borgman[111] advises that the dung should be kept in a dry
condition, and only made into a paste between a fortnight and three
weeks before use, by covering in a clean cask with cold water, and on
the following day mixing to a smooth paste with a clean wooden
“poss-stick,” made from wood free from tannin. The cask should then be
covered up, and allowed to rest undisturbed till required. Clean
extract-casks are very suitable for the purpose, if carefully and
repeatedly steamed out, and Borgman advises that a regular series should
be arranged, so as to supply the dung required, the date of mixing being
carefully marked on each cask. Throughout the process the utmost
cleanliness should be observed, and the casks should be carefully
steamed out as soon as emptied. Immediately before use the dung-paste
should be heated by steaming nearly but not quite to boiling point, care
being taken to avoid the introduction of condensed water containing
iron, and the dung thoroughly mixed with a large quantity (say 100
gallons) of water at 45°-50° C., allowed to settle, and drawn off
through a basket, and strained into the puering paddle through a second
basket lined with coarse open canvas (such as is used by plasterers to
cover windows while the plaster is drying). A further quantity of warm
water should be poured on the residue in the mixing tub, and used for
diluting that in the paddle to the proper volume. The temperature of the
liquor may reach 42° C. before the skins are introduced. The liquor
should be of a light colour, greenish to brownish yellow; if darker, it
indicates decomposition of the dung by improper storing, or too long
fermentation, and will be liable to cause staining and injury to the
skins. About 33 liters of dry dung is required per 100 kilos. of wet
skin prepared for puering (33 gallons per 1000 lb.). Dry dung should be
of yellow to brown colour, dark brown or black dung is spoiled and
unsuitable for use. Wet dung is more difficult to judge, but very dark
brown or black should be rejected, as well as that with a very strong
smell, indicating that it has already fermented. Borgman’s directions
bear the stamp of experience and common sense, and the book as a whole
repays study.
[111] ‘Die Feinleder-Fabrikation,’ Berlin, 1901, p. 69.
Borgman recommends that the skins should be warmed by paddling for some
time in water of about 40° C. to which a couple of pails of puer-paste
have been added, before bringing them into the puer, the temperature of
which they should reduce to perhaps 38° C. The puered skins should feel
silky on the grain, and even somewhat slippery, and when pressed
between the finger and thumb a dark impress should be left, and the
flesh should be tender and easily scraped off. The requisite condition
will, however, vary somewhat with the kind of skins, and the purpose for
which they are intended. After puering, the skins may be paddled for
half an hour in water of about the same temperature as the puer.
CHAPTER XIV.
_ALUM TANNAGE OR TAWING._
We have now followed the raw material up to the final stage of
preparation for its actual conversion into leather, and it remains to
consider the means by which that important change is produced. Though as
yet the vegetable tanning process is most largely used, and possesses
the greatest commercial importance, the use of mineral salts has long
been known, and, through the advent of chrome tanning, has placed the
permanent supremacy of the vegetable tannins in considerable doubt. Not
only the importance of mineral tanning processes, but their greater
simplicity from the scientific side, justify their consideration before
those of vegetable origin.
In the previous chapters it has been shown that to produce a permanent
leather, it is not only necessary to dry the fibres in a separate and
non-adherent condition, but so to coat them or alter their chemical
character that they are no longer capable of being swelled and rendered
sticky by water. All salts which produce a contraction or dehydration of
the fibre similar to that caused by alcohol are capable of the first
effect in a greater or less degree. Many sulphates, and particularly
those of sodium and magnesium, though they will not alone produce
leather, will so far contract the fibres as to greatly hasten tanning by
vegetable tanning materials, and they are therefore capable of useful
application in quick tanning processes, especially where tough and
light-weighing leathers are aimed at, which may be subsequently weighted
and solidified by further treatment. Strong solutions of ammonium
sulphate are almost as strongly dehydrating as alcohol, and will produce
white leathers very similar to those formed by pickling, a fact which is
certainly of considerable commercial importance. None of these salts,
however, can form a complete leather in themselves, but require the
assistance of metallic salts which will permanently fix themselves in
the fibre, and diminish or destroy its attraction for water. Many
substances have this power in a greater or less degree, but all those of
commercial importance belong to the group of which aluminium, iron and
chromium are representative, and which are capable of producing
salt-forming oxides of the formula M₂O₃ (e.g. alumina, Al₂O₃).
Manganese, of which the salts of this type are very unstable, has very
slight tanning power, while titanium, which in many ways is allied to
the group, though it does not strictly belong to it, has recently been
patented as a tanning agent. For the present, however, we may limit our
attention to the three metals first named.
Alumina and its salts demand the first attention, not only as having
been used for leather manufacture in very early times, but as being
still important commercially. The metal aluminium is now well known, and
its oxide, alumina, Al₂O₃ is abundant in nature, combined with silica in
the form of clay and bauxite, as fluoride in combination with sodium
fluoride in cryolite, and in some cases as a native sulphate.
Alum-shale, which was formerly the principal source of alum, is a
bituminous clay containing much iron sulphide, and which when calcined
yields aluminium sulphate. As aluminium sulphate does not crystallise
readily, and was difficult to free from iron, potassium sulphate was
added to the liquor obtained by leaching the calcined shale, from which,
after concentration by boiling, potash-alum, a double sulphate of
potassium and aluminium, Al₂(SO₄)₃,K₂SO₄,24Aq, was easily crystallised
out. Alum is now usually made by decomposing clay or bauxite with
sulphuric acid, and ammonium sulphate is generally substituted for the
potassium salt, yielding ammonia-alum, a double sulphate of aluminium
and ammonium of similar constitution to potash-alum. Ammonium alum is
easily distinguished from the potassium salt, by the strong smell of
ammonia which it evolves on the addition of caustic soda or lime. So far
as is known, there is no practical difference in tanning effect between
the two salts, and ammonium alum is cheaper, and slightly stronger, its
molecular weight being 906, as against 948 for the potassium salt.
Either alum dissolves readily in cold water to the extent of about nine
parts in 100 of water, and more easily, and to a much larger extent in
hot water, from which the excess crystallises on cooling. It is said
that for purposes of leather manufacture, alum solutions should not be
boiled, and, though it is improbable that this produces any change, it
must be remembered that chrome alum on boiling really does undergo
decomposition to free acid and a more basic salt, indicated by change of
colour from violet to green, from which it slowly returns to the violet
form on cooling.
Alums are only valuable in leather manufacture in proportion to the
aluminium sulphate which they contain, the potassium or ammonium
sulphate taking no part in the reaction, and since improved methods have
rendered possible the production of aluminium sulphate practically free
from iron, it has largely taken the place of alum, than which it is both
cheaper and stronger. Crystallised aluminium sulphate, Al₂(SO₄)₃, 18Aq,
has a molecular weight of 666, which is of equal value to 906 of
ammonia-alum, and 948 of potash-alum. Iron is the most objectionable
impurity in both alums and aluminium sulphate, and may be detected by
the addition of potassium thiocyanate, which will produce a red colour,
or potassium ferrocyanide (yellow prussiate of potash), which will
produce a blue. As the iron may be present in the ferrous condition, it
is safer first to boil the alum solution with a few drops of nitric acid
or bromine water. For more accurate determination of iron see L.I.L.B.,
pp. 20, 136.
No satisfactory leather can be produced with a solution of alum or
aluminium sulphate alone, the skin drying horny, and incapable of
softening by stretching. In practice, salt is always used in addition,
the proportion being very variable, but averaging about half the weight
of alum, or two-thirds the weight of sulphate of alumina employed. The
mode of action of the salt has long puzzled chemists, and it has been
supposed that its use was to convert the aluminium sulphate into
chloride, a reaction which takes place to some extent, but which fails
to explain the production of a soft leather, since aluminium chloride,
though freely taken up by the skin, produces no more satisfactory
leather than aluminium sulphate. The real explanation is found in
Chapter IX. Alumina is a weak base, which readily gives up its acid to
the pelt, becoming converted into a basic salt (see p. 187). The acid
not only swells the pelt, and renders it incapable of producing a soft
leather, but the swollen pelt is less ready to absorb the alumina salt,
and so remains undertanned. The addition of salt prevents the swelling
effect of the acid, and produces a partial pickling of the skin (p.
89), which, in conjunction with the tanning effect of the basic alumina
salt formed, yields a satisfactory leather, though one which is readily
affected by washing. If instead of adding common salt to the alum
solution, an alkali such as soda is added, it combines with a portion of
the acid, forming sodium sulphate, while the alumina remains in solution
as a “basic salt.” As the term “basic salt” must be frequently employed
in connection with mineral tannage, it may here be explained. Basic
salts are compounds intermediate between the normal salt, in which the
whole of the base is combined with acid, and the hydrated oxide in which
the whole is combined with OH groups. Thus aluminium chloride, Al₂Cl₆,
is a normal salt, in which the whole of the combining powers of the
aluminium are saturated with chlorine: aluminium hydrate, Al₂(OH)₆, is
the hydrated oxide, and Al₂Cl₅OH, Al₂Cl₄(OH)₂, and so on are basic salts
in which successively more of the Cl is substituted by OH. Generally, as
a salt becomes more basic, its solution in water becomes more unstable,
and very basic salts are either insoluble, or are precipitated from
their solutions by very trifling causes, such as boiling, dilution, or
the attraction of animal or vegetable fibres; separating into free acid
and either hydrate or a still more basic and insoluble salt. On this
property depends their importance in tanning and dyeing, many of the
metallic mordants being solutions of basic salts. Basic salt solutions
are formed in various ways, the most common being the direct solution of
a hydrated oxide in a solution of the normal salt, or the neutralisation
of a part of the acid of the normal salt by the addition of a stronger
base. This is what takes place on the addition of soda to an alum
solution. If the soda is added in excess, the whole of the alumina is
precipitated as hydrate, or as an insoluble basic salt, but if a
proportion not exceeding about four parts of crystallised sodium
carbonate be dissolved separately, and added _slowly with constant
stirring_ to the ten parts of alum dissolved in water, no precipitation
will take place. In this solution leather can be tanned, either with or
without addition of salt, the alumina is taken up more freely than from
the normal alum, and the leather is more easily softened, and more
resistant to water. In fact such leather bears a strong resemblance to
the chrome tannages, standing a great deal of washing, and considerable
temperatures without returning to a pelty condition. The more basic the
solution that is used, the fuller and softer is the leather produced.
The alumina-salt taken up by the skin from such basic solutions is
always basic, while that absorbed from alum or alumina sulphate is
_apparently_ the normal aluminium sulphate. It is probable however that
the actual tanning salt is in both cases basic, and that the acid is
fixed as free acid, as in the pickling process, as the proportions of
acid and base found in the residual liquor are somewhat variable.
Basic alumina solutions have hardly taken the place in practice which
they deserve, though they were described by Knapp in 1858[112] and have
since been patented by Hunt, but the patent (probably invalid) has been
allowed to lapse. A good stock solution for practical use is made by
dissolving 10 lb. of sulphate of alumina in 10 gallons of water, and 4
lb. of washing soda in 4 gallons, and gradually mixing the latter with
the former. Salt can be used in addition if desired, and flour and
egg-yolk may also be added.
[112] ‘Natur und Wesen der Gerberei,’ Braunschweig, 1858.
In curing small skins, where it is not desirable for the fur to come in
contact with the liquid, or in the tawing of wool rugs, it is often
convenient, after freeing the skin as much as possible from blood and
dirt, and adhering flesh, to stretch it on a frame, or nail it out on a
board, and apply a strong alum-and-salt solution, as hot as the hand
will bear, with a sponge, repeating the operation till the skin is
struck through. About 1 lb. of alum and ¹⁄₂ lb. of salt per gallon is a
suitable strength. In place of applying the solution, powdered alum and
salt is sometimes rubbed into the wet skin. Alumed goods should
generally be dried out rapidly, and finally at a good temperature, as
this tends to fix the tannage, which is also made more permanent and
resistant to water by keeping the skins for a month or more in the
alumed condition, an operation known as “ageing.” When first dried,
alumed goods are invariably stiff and horny, and, to give them softness,
must first be damped back to a half-dry condition, and then gradually
softened by mechanical means. “Staking,” and “perching” are the usual
methods, the first consisting in drawing the goods vigorously over a
bluntish blade fixed on the top of a post, and the second in fixing the
skins on a horizontal pole (the “perch”), and working them with the
“crutch stake,” a tool formed somewhat like a small shovel with a
semicircular blade, in place of which a “moon-knife” (a round blade
somewhat like a broad thin quoit) is often fixed in a wooden crutch. The
tools, and mode of using them are shown in Figs. 36 and 37.[113]
Machines, described on p. 192, are now generally used for these
operations. After the first staking or softening, the skins are allowed
to become nearly dry, and are then staked a second time. Some judgment
is required as to the precise degree of moisture in each case: in the
first instance the skins must be sufficiently damp to yield without
injury to the mechanical stretching, but in this state they retain
sufficient moisture to enable the fibres again to adhere on drying; and
at the second staking or perching, they must be damp enough to allow
these fibres to be again loosened without violence, and dry enough to
prevent their again adhering.
[113] The process shown in Fig. 37 is not actually “perching,” but
“grounding,” in which a moon-knife with a sharp turned edge is used to
reduce the thickness of the skin on the perch, at the same time as it
stretches and softens it.
[Illustration: FIG. 36.--Staking White Leather.]
[Illustration: FIG. 37.--Grounding with the Moon-knife.]
The following slight sketch of the manufacture of calf-kid will serve to
illustrate the practical manufacture of the finer alumed or “white”
leathers. The raw material is in England mostly large market-calf,
though salted and dried skins are sometimes employed. After sufficient
soaking or washing in water, they are limed without arsenic or other
sulphides, in limes which must not be allowed to grow stale or putrid,
until the hair can be easily removed. After unhairing and fleshing in
the usual way, they receive a few days in a pretty fresh lime, in order
to plump them, and are then freed from lime gradually but as completely
as possible, by successive steepings and washings in water softened by a
mixture of that already used on other goods and by working on the beam.
This acts as a partial substitute for puering with dung, which is now no
longer used on calf-kid. The goods are next drenched in the ordinary
way, 3-4 % of bran being used, and the goods allowed to rise two or
three times in the drench, which should be conducted with the usual
precautions (p. 167) to avoid the danger of butyric fermentation in hot
weather. The goods should come out of the drench free from lime, and
unswollen by acid, but full, white, and soft. The tanning (or “tawing”
as it is usually called in the case of alumed goods) is done in a
rotating drum with a mixture of alum or sulphate of alumina, salt,
flour, egg-yolk, and olive oil. About 5 per cent. of flour, 2·5 per
cent. of alum, 1 per cent. of salt, the yolks of 25 eggs, or 1¹⁄₂ lb. of
preserved egg-yolk, 2 oz. of olive oil, and 1¹⁄₄-1¹⁄₂ gallon (12-15 lb.)
of water are required per 100 lb. of wet pelt. The flour is first made
into a smooth paste with a little water, the egg-yolk, somewhat diluted
with warm water and strained, is mixed in together with the oil, and
finally the alum and salt solution is added at such a temperature as to
bring the whole mixture to blood-heat (38° C.). The length of drumming
depends on the thickness of the skins, several hours being required for
very thick ones, but care must be taken to stop and ventilate the drum
at frequent intervals, so as to prevent the skins becoming hot by
friction. This part of the process was formerly accomplished by treading
with bare feet in a tub. After tawing, the goods are allowed to lie in
piles over-night, or are sometimes laid in tanks for a day or so with
any that remains of the tawing paste, to complete the absorption of the
salt and alum, and are then frequently split with the band-knife
machine, though it would be better, as is often done on the Continent,
to split them before tawing, the materials of which are not only costly,
but unfit the splits for many purposes for which they might be employed.
The drying should be rapid, but is best done first at a moderate
temperature, or in the open air, and then in a rather hot stove. They
may now be allowed to “age” from one to three months, but it is usually
better before ageing to do the first part of the finishing process,
consisting of damping back, staking, and if necessary, shaving. Machines
are now almost invariably used for the staking, the principle of which
may be described as that of a pair of tongs, carrying one or generally
two staking blades on one limb, and a roller on the other which closes
on the skin, and presses it against and between the blades, while the
tongs are drawn backwards, allowing it to slip through. Fig. 38
illustrates the Slocomb, one of the most popular machines of this type.
After staking and ageing, the skin is soaked in water till thoroughly
wet in all parts. This not only softens the skin, and prepares it for
dyeing, but takes out the superfluous alum and salt, and at the same
time a good deal of flour and egg. To replace these, “re-egging” is
necessary, and while some manufacturers give egg-yolk, or egg-yolk and
flour only, many add a proportion of salt, and sometimes also of alum.
This is done before dyeing, if the skins are to be blacked on the table,
but as tray-dyeing (see p. 406) would again wash out the egg, the
re-egging is deferred till after dyeing if this process is resorted to.
Before dyeing, the skins receive an alkaline mordant to overcome
greasiness, and enable them better to take the colour. In former times
this was usually stale urine, but this has mostly been superseded by
solutions of “hydroleine” (a washing powder), or of soap rendered more
or less alkaline with ammonia. Eitner gives the following recipe, viz.
¹⁄₂ lb. Marseilles soap dissolved in boiling water, 5 or 6 egg-yolks
added, and the whole made up to 4 gallons with water and ¹⁄₄ lb. potash
bichromate. The colour used is infusion of logwood or its extract, or
two-thirds logwood and one-third fustic, which is best extracted without
alkali, a small quantity of soda or ammonia being afterwards added. It
is fixed and darkened by a wash of iron-liquor or a solution of 1 of
ferrous sulphate in 75 of cold water. After being again dried, the skins
are sometimes grounded with the moon-knife, softened again by staking or
perching, for which a machine with inclined or spiral blades attached to
a drum and working on a sort of leather apron is often preferred to
machines of the Slocomb type, and rubbed over on the grain with a
composition containing oil, wax, etc., and are finally ironed with a
heavy flat-iron, to give them a fine and smooth surface. Eitner gives a
recipe for the gloss:--1 kilo gum arabic, ¹⁄₂ kilo yellow wax, ¹⁄₂ kilo
beef-tallow, ³⁄₄ kilo Marseilles soap, 1 liter strong logwood infusion,
and 5 liters water. The water is brought to a boil in an earthen pot,
and then the soap, wax, gum, and tallow are added successively, each
being stirred till dissolved before adding the next, and lastly the
logwood. After boiling for an hour, it is allowed to completely cool,
being incessantly stirred during the whole process. After ironing the
goods are rubbed over with a final gloss, for which Eitner gives the
following recipe:--8 liters olive oil, 500 grm. tallow, 500 grm. yellow
wax, 500 grm. rosin, 500 grm. gum arabic. (No water is given in the
recipe, but the gum arabic is presumably softened in water.) The
mixture is cooked for two hours in an earthen pot till the water is
evaporated, and allowed to cool with constant stirring. The skins are
then rubbed with a flannel with a very small sprinkling of French chalk,
and are ready for sale.
[Illustration: FIG. 38.--The Slocomb Staking Machine.]
The manufacture of glove-kid is quite similar in principle to that just
described, but varied in detail to suit the softer and more delicate
skins employed, to give greater softness, and especially the quality of
stretching in any direction without springing back, which is so
characteristic of the leather. Lamb-skins are the principal raw
material, though genuine kid is also employed for the best qualities.
The manufacture varies much with the quality and character of the goods.
The skins, which are mostly dried, are soaked in clean and cool water
for three to four days, according to age and thickness. Common qualities
(small imported slink lambs) are often unhaired by dipping in or
painting with a paste of gas-lime, lime and sulphide of sodium, or lime
and red arsenic, so as to destroy the wool. Better skins are sometimes
unhaired by painting on the flesh with lime alone or in mixture, and in
other cases ordinary lime-pits are used, with limes, which are most
usually strengthened with red arsenic, which is added to the lime while
hot from slaking (cp. p. 142).
The calcic sulphydrate (and perhaps sulpharsenite) thus formed hastens
the unhairing, and preserves the gloss of the grain. Well conducted
glove-kid establishments avoid as much as possible the use of old limes,
which produce a loose, porous leather, with a rough, dull grain. The
liming lasts on the average ten days, and is of the greatest importance.
It is essential that the inter-fibrillary substance should be dissolved,
that the leather may have the quality known in Germany as _Stand_, that
is to say, may be strongly stretched in either length or breadth without
springing back. It also depends upon the liming (and this is of special
importance in the case of lamb-skins), whether the tissue of the
fat-glands is well loosened, so that the fat, either as such, or as
lime- or ammonia-soap, may be readily and completely worked out. Skins
in which this is neglected can never be properly dyed.
When the hair (or wool) is well loosened, the skins are rinsed in water,
and then unhaired on the beam with a blunt knife. The water employed in
washing should not be much colder than the limes, or it will prevent
the hair from coming away readily. The wool or hair is washed and dried
for sale. The skins are thrown into water, to which a little lime-liquor
has been added, to prevent precipitation of the lime in the skins by the
free carbonic acid of the water, which would have the effect of making
them rough-grained.
Next comes the first fleshing (_Vergleichen_) or “levelling.” By this,
the loose cellular tissue on the flesh-side is removed, together with
the head, ears, and shanks; and the flanks are trimmed. The skins are
then again thrown into water softened with lime-liquor as above
described, and then into a puer of dogs’-dung. This is prepared by
stirring up white and fermented dogs’-dung with boiling water, and
straining it through a sieve or wicker basket. The puer must be used
tepid, and not too strong. The skins “fall” (lose their plumpness) in it
rapidly, and become extremely soft and fine to the touch; and the
fat-glands, remaining hairs, and other dirt, can now be very readily
scudded out.
Too strong puers, or too long continuance in them, produce evident
putrefactive effects on the skins. (See also p. 181.)
When the skins come out of the puer, they are stretched and worked on
the flesh with a sharp knife, and any remaining subcutaneous tissue is
removed. This constitutes the second fleshing. They are then rinsed in
warm water, and beaten with clubs in a tub, or worked in a tumbler-drum,
in either case with a very little water only; and finally brought into a
tank of water, not too cold, and kept in constant motion with a
paddle-wheel.
The skins are next cleansed on the grain-side by working on the beam
with plates of vulcanite set in wooden handles, so as to remove fat,
lime- and ammonia-soaps, and other lime compounds, together with all
remaining hair or wool. The skins are now a second time washed in the
“paddle-tumbler,” first in cold, and then in tepid water; and after
allowing the water to drain from them, they are transferred to the
bran-drench.
This is prepared by soaking wheaten bran in water at about 50° C., and
diluting with warm water. Sometimes the mixture is strained, and the
bran-water only used, to save the trouble and cost of removing adhering
particles of bran from the delicate skins. Sufficient of the liquid must
be employed to well cover the skins, and the temperature may range from
50° F. (10° C.) to 68° F. (20°C.). These conditions are favourable to
bacterial activity, which comes into play, and, on the one hand, evolves
acetic and lactic acids, which dissolve any remaining traces of lime,
and on the other, loosens and differentiates the hide tissue, so as to
fit it to absorb the tawing solution. Much care is required in the
management of the bran-drench, especially in summer, since the lactic
readily passes into the butyric fermentation (see also p. 167). The
tawing mixture is composed (like that employed in the fabrication of
calf-kid, q.v.) of alum, salt, flour and egg-yolks, in a quite thin
paste. A small quantity of olive oil is also generally used. The skins
are either trodden in it with the feet, or more generally put into a
tumbler-drum with it. Kathreiner pointed out, some years ago,[114] that
a mixture of olive-oil and glycerine might be partially substituted for
the egg-yolks, in both the tanning and dyeing of glove-kid leather.
[114] Gerber, i. (1875) p. 170; ii. (1876) p. 664.
The tawed skins are now dried by hanging on poles, grain inwards. Rapid
drying in well-ventilated, but only moderately heated rooms is essential
to the manufacture of a satisfactory product.
The dry leather is rapidly passed through tepid water, and after being
hung for a very short time, to allow the water to drain off, is trodden
tightly into chests, and allowed to remain in them for about 12 hours,
so that the moisture may be uniformly distributed. It is then trodden on
hurdles (German _Horden_) composed of square bars of wood, joined corner
to corner, so as to make a floor of sharply angular ridges. The next
operation is stretching with the “moon-knife”; after which the leather
is dried nearly completely, and staked again.
This completes the tawing process. The goods are now “aged” as in
calf-kid manufacture. Before dyeing they are washed with tepid water to
remove part of the tawing mixture, and especially, superfluous alum and
salt, and are re-egged much like calf-kid, before dyeing if the latter
is done by brushing, and after if in the dye-tray or paddle. Aniline
colours are more used than formerly, especially for topping and
brightening the natural colours, but the dyewoods and other mordant
colours are still largely employed. The leather is first prepared with
an alkaline mordant (stale urine, ammonia, etc.) (cp. p. 413), then
repeatedly brushed with or dipped in the dyewood liquor, and a wash
(“striker,” German _Ueberstrich_) containing some metallic salt is
generally applied, with the object either of bringing out the special
tone required, or of making the colour more lively and permanent. The
striker is usually a solution of one of the so-called “vitriols”: “white
vitriol” (zinc sulphate), “blue vitriol” (copper sulphate), “green
vitriol” (iron sulphate), or occasionally other salts.
After the dyeing, the skins, if dipped, are wrung out and re-egged; if
brush-dyed, sleeked out with a brass or ebonite sleeker to get rid of
superfluous water. They are then dried in an airy room. Before staking
(stretching), the skins are laid or hung in a damp cellar, or in moist
saw-dust. They are staked twice: once damp, and once nearly dry; and are
finished by glassing or ironing.
Skins which are much damaged on the grain, or otherwise faulty, are
smoothed with lump pumice on the flesh side, or fluffed with fine emery
on the fluffing wheel. They are then dyed on the flesh side, mostly by
dipping, but occasionally with the brush, in which case, the method
described is slightly modified.
Tawing with alum and salt is frequently employed for commoner and
stronger leathers, such as aprons (of sheep-skin), leather for
whip-lashes, laces for belts, and “skivers” for capping chemists’
bottles. The process is practically the same as for calf-kid, except
that no egg, and little flour is used. Often flour is entirely omitted,
and the goods may then be alumed in tubs, in which they are merely
handled, as the alum solution penetrates quickly. Goods which are
required white are frequently handled or tumbled with a milk of
“whitening,” both to improve the colour, and to neutralise any acid
present, and fix the alum by rendering it more basic. Alumed goods can
be stuffed with greases, either by hand or in the drum, after thorough
softening by staking.
Alum, and other salts of alumina are frequently used in
combination-tanning with vegetable materials (see Chap. XVII.). “Green”
leather for laces, “dongola,” and “dog-skin” glove-leathers are made in
this way. Glazed kid for ladies’ shoes must be slightly vegetable-tanned
on the surface, or it will not glaze, but this is frequently
accomplished by the use of materials in the dye-liquor containing
tannins.
CHAPTER XV.
_IRON AND CHROME TANNAGES._
_Iron tannages_ may be very shortly dismissed, as their practical
interest is at present either historical or prospective, but iron salts
enter in so many ways into the chemistry of leather manufacture, that
their properties must be briefly considered. Iron exists in salts in two
states, the ferrous, and the ferric, in the first of which it is
divalent, and in the second trivalent. Thus ferrous chloride is FeCl₂;
ferrous oxide, FeO; ferrous sulphate, FeSO₄; ferrous hydrate, Fe(OH)₂.
The compounds of ferrous iron are mostly green, like ferrous sulphate
(“green vitriol,” “copperas”): exposed to air and moisture, they easily
absorb oxygen, and pass into the ferric form. Ferric chloride is FeCl₃
(or, as it is sometimes written without much reason, Fe₂Cl₆), ferric
hydrate Fe(OH)₃, ferric oxide Fe₂O₃, ferric sulphate Fe₂ (SO₄)₃, and so
on. The atomic weight of iron is 56. Ferric salts are mostly yellow or
orange, ferric hydrate is yellow-brown, and on ignition is converted
into deep red ferric oxide, which is very difficultly soluble in acids.
Ferric salts in contact with more easily oxidisable matters, readily
give up oxygen, and pass into the ferrous state; and especially does
this happen in the presence of organic matter, under the influence of
sunlight. Thus iron-salts often act as carriers of oxygen, and oxidisers
of organic matter, absorbing oxygen from the air, and giving it up again
under the influence of light or heat. There are several other oxides of
iron which do not form salts, and there is a ferric acid, apparently
corresponding to chromic acid, which is so unstable that it has been
very imperfectly investigated.
Ferric salts correspond in structure to those of alumina, and like these
are powerful tanning agents, and readily form basic salts, while the
ferrous salts have no tanning effect until they become oxidised, when
they form basic ferric salts. Ferric salts are characterised by giving
blue-black or green-black compounds with tannins, and with many other
allied bodies, while the corresponding ferrous compounds are mostly
colourless, though they rapidly oxidise and darken.
Ferric iron, like alumina, forms an “alum,” a double sulphate of iron
and potassium, Fe₂(SO₄)₃K₂SO₄, 24Aq, forming fine pale-violet crystals,
but dissolving to a yellow-brown solution. (It must be distinctly
understood that iron-alum and chrome-alum contain no alumina, but are
simply called alums because of their similarity of constitution, iron or
chrome taking the place of the aluminium. Iron-alum, in conjunction with
salt, can be used for tanning, giving a pale buff-coloured leather very
similar to an ordinary alum leather. Thus the presence of a small
quantity of iron in an alum used for tawing is of no consequence, except
as affecting the colour of the leather. In impure sulphate of alumina
such as “alumino-ferric,” it, however, generally exists in the green
ferrous state, and only acquires tanning properties on oxidation.
Without common salt iron-salts are still less satisfactory tanning
agents than those of alumina under the same conditions, as the acid is
yet more loosely held, and though basic ferric salts are taken up in
considerable quantities by hide, the leather produced is thin, and
usually brittle. Professor Knapp devoted much study to the production of
a commercial sole-leather by basic iron-salts; and took several patents,
which did not prove practically successful, though the brittleness was
to some extent overcome by the incorporation of compounds of iron with
organic materials such as blood and urine, of iron-soaps, and of rosin
and paraffin in the leather. Like most mineral tannages, the process was
far more rapid than that with vegetable materials. Knapp’s basic tanning
liquor was made by the oxidation of ferrous sulphate with a small
quantity of nitric acid. Patents have also been taken for the oxidation
of ferrous sulphate by peroxide of manganese in presence of sulphuric
acid, which produces basic ferric sulphate in mixture with manganese
sulphate, which has also some tanning properties. Attempts have also
been made to tan by treatment of the hide with solutions of ferrous
sulphate, and subsequent exposure to the air, in order to oxidise the
iron on the fibre and convert it into a basic ferric salt, but have not
proved of any commercial value.
The principal use of iron at present in leather manufacture is in dyeing
blacks (see p. 413), but in this case, its feeble hold upon acids in the
ferric state, and its tendency to act as an oxidising agent, or oxygen
carrier, renders the blacks somewhat unstable, and is frequently
injurious to the leather. There is also little doubt that the presence
of ferric salts in leather blacks has a great tendency to cause the
resinification of the oil, known as “spueing,” by promoting its
oxidation.
_Chrome tannages_, from a practical point of view, stand on a very
different footing to those which have just been mentioned; having
established their position in the manufacture of almost all sorts of
light leathers, in competition with all the older methods, and making a
serious claim to a share in the production of belting and even of sole
leathers.
Chromium is a grey, and very infusible metal, which chemically much
resembles iron in its compounds, and has an atomic weight of 52, or a
little over. Like iron, it possesses a divalent and a trivalent form,
but the divalent has so strong an affinity for oxygen, and passes so
readily into the trivalent form, that until easier means are found for
its preparation, it is of little practical interest. Its salts are blue.
On the other hand, salts of the trivalent form, corresponding to the
ferric salts of iron, are very stable, and powerful tanning agents. They
are mostly green, but violet modifications are known, corresponding to
the violet crystals of iron-alum, but of a much deeper tint. There is
also a hexavalent form, probably corresponding to that of iron in the
unstable ferrates, but in the case of chromium, of considerable
stability. Its oxide is chromic anhydride, CrO₃, commonly called chromic
acid, which combines with bases, and especially with the alkalies to
form yellow or orange-red salts, and the anhydride itself is almost
crimson in the solid form, though dissolving to orange or yellow
solutions. Chromic acid though it hardens and preserves animal tissues,
has no tanning properties till it becomes reduced to chromic oxide.
There is also a higher, but very unstable oxide, perchromic acid,
possibly corresponding to persulphuric acid, which is soluble in ether
to an intensely blue solution. The name chromium is derived from the
intense colour of many of its compounds.
Our supplies of chromium are derived from chrome-iron-ore, a mineral
which contains oxides both of chrome and iron. This is furnaced with a
mixture of lime, and soda or potash, when it absorbs oxygen from the
air, the chromium becoming converted into chromic acid which combines
with the alkali present, while the iron remains undissolved as ferric
oxide. Lixiviating the mass, and evaporating the solution, lime and
potassium or sodium chromates are obtained, according to the alkali
used, and on adding sufficient sulphuric acid to combine with half the
base, potassium or sodium dichromate (or as it is commonly called
“bichromate”) can be crystallised out. Potassium dichromate is most
commonly made, because it crystallises well, and is not deliquescent,
but sodium dichromate is somewhat cheaper, though less convenient.
Dichromates, at least in the crystallised state, are not hydric salts
like bisulphates, but anhydrochromates corresponding to the potassium
anhydrosulphate obtained by fusing ordinary bisulphate, and to fuming
sulphuric acid. Thus the formula of potassium dichromate is
{ CrO₂OK
{ O ,
{ CrO₂OK
or Cr₂K₂O₇ and its molecular weight is 294, while that of sodium
dichromate, which is similar in constitution, but crystallises with 2Aq,
is 298. The molecular weight of CrO₃ is 100. Chromic acid, and acidified
potassium dichromate are powerful oxidising agents, and are used as such
in many processes, and especially in the manufacture of alizarine. If
sulphuric acid be used in molecular proportions, the product of the
reaction is chrome-alum: 4H₂SO₄ + Cr₂K₂O₇ = 3O + 4OH₂ + K₂Cr₂(SO₄)₄.
This, like ordinary alum, crystallises with 24Aq, and hence has a
molecular weight of 998. It forms dark purple, almost black crystals,
which are a fine garnet-red by transmitted light. In cold water it
dissolves to a violet solution, which becomes green on boiling, but very
slowly resumes the violet condition when cold. This change, which is not
uncommon in chrome solutions, is probably due to a partial decomposition
into free acid and a basic salt, the basic salts of chromium being
generally green. It has been noticed that raw pelt swells much more in
the green, than in the violet solution. Being derived from waste
products, chrome-alum is often a cheap and valuable source of chromium
for chrome tanning.
For the analysis of chrome compounds see L.I.L.B., p. 141 _et seq._
Chrome oxide, and basic chrome salts, when strongly ignited, become
insoluble even in concentrated acids, and their analysis is therefore
attended with some difficulty. If, however, the ignited residue (for
instance a leather-ash) be finely powdered, and intimately mixed with a
fusion-mixture consisting of equal parts of pure calcined magnesia and
pure dry sodium carbonate, and ignited (preferably over a Teclu burner),
in a platinum crucible, in which it is occasionally stirred with a
platinum wire, it will be quantitatively converted into chromate, which
may be dissolved in acid, and estimated with potassium iodide and
thiosulphate in the usual way. If it is desired at the same time to
estimate sulphuric acid, it is sometimes preferable to substitute lime
or calcium carbonate for the magnesia, which is apt to be contaminated
with sulphates.
Chrome is not only of importance in tanning, but in dyeing; on account
of its power of forming insoluble colour-lakes with many mordant
colouring matters. For this purpose normal or basic chromic salts are
sometimes used, sometimes chromic acid or dichromates, the latter acting
not only by yielding chrome-oxide on reduction, but as oxidising agents
to the colouring matters. Most of the colours produced with chrome
mordants are of dark shades, that with logwood being deep violet or
black. The mordanting power of chromium is important in the dyeing of
chrome leather. Bichromate of potash is often used in dilute solution
for darkening the shade of leather dyed with other materials, but is not
to be recommended on account of its destructive action on the leather.
Numerous patents have been taken for processes of chrome tannage. The
first practical method was described by Professor Knapp in 1858 (see p.
210), though he did not recognise its value. Some of the patents have a
historical interest, though of no importance. Among these may be
mentioned that of Cavallin, a Swedish apothecary, whose object was
dyeing rather than tanning, but who treated raw hide with a solution of
bichromate, which was afterwards reduced on the fibre by one of ferrous
sulphate. The leather produced is dark reddish brown, and tender from
the amount of basic ferric salt formed at the same time. Mr. J. W. Swan,
well known in connection with photographic processes, and electric
lighting, also patented a process of chrome tannage (as an addendum to a
patent on carbon printing), in which the chromic acid first fixed in the
pelt was reduced by “oxalic, or other suitable acid.” Although it is
possible to produce leather within the lines of the patent, the
strongly acid reaction of the reducing agent renders it unsuitable for
practical use. The first chrome tanning process which made any show of
practical success, was that patented in 1879 by Heinzerling, which was
acquired in this country by the Eglinton Tanning Company, and also
worked under their license for a short time by the Yorkshire Tanning
Company at Leeds. Though the process was not commercially successful on
any considerable scale, it possesses points of interest which make a
brief description desirable. The hides or skins, after preparation in
the usual way, were treated in a mixed solution of salt, alum (or
aluminium sulphate), and potassium bichromate, but no systematic attempt
was made to reduce the chromic acid to a tanning form, the product
being, at first at least, merely an alum tannage, coloured, and perhaps
somewhat hardened with chromic acid, though on keeping for a length of
time, reduction gradually took place at the expense of the hide-fibre,
and of the fats employed in currying, so that the leather internally
became greyish-green, and really chrome-tanned. Specimens of the early
products of the process, preserved in the museum of the Leather
Industries Department at Leeds, have now all undergone this change, but
are still tough and flexible, showing that the rapid tendering of the
Heinzerling leather, which was one of the causes of its failure, must
have been due to some error in manufacture, and was not inherent in the
process. Interesting, historically, is the fact, that at an early stage
in the life of the patent, a specimen of the leather was submitted to
the late Professor Hummel, in order that he should suggest some means of
overcoming the disagreeable yellow colour of the product. He reduced it
with a bisulphite, and coloured it with an aniline dye, and a piece is
still in the possession of the Yorkshire College, and in perfectly sound
condition. If legal publication of this experiment could have been
proved, it would have invalidated the important Schultz patents under
which most of the chrome-kid of the United States has been manufactured.
As bearing on modern chrome-tanning, the most important reaction in the
process is that of the alum with the bichromate. It has been shown by
Heal and Procter[115] that pelt absorbs practically no chromic acid from
bichromate, unless it has been previously set free by acidification.
When however alum, or sulphate of alumina is added, its sulphuric acid
liberates the chromic acid, leaving a basic alumina salt in solution,
and this fact has been utilised in some modern tanning processes.
[115] Journ. Soc. Chem. Ind., p. 251, 1895.
The first really important advance in practical chrome tanning was made
by Augustus Schultz, in 1884. Schultz was not a tanner, but a chemist,
employed by a New York firm of aniline colour merchants, and his
attention was accidentally drawn to leather by a friend who asked him if
it were possible to produce a leather for covering corset steels, which
would not rust them as ordinary alumed leathers do. The process which he
adopted was probably suggested by a method then recently patented for
the mordanting of wool by chrome oxide, and depended on the power of the
pelt to absorb free chromic acid (as it does all other free acids), and
the subsequent reduction of the latter on the fibre to a basic chrome
salt, which produced the tannage. The reducing substance employed was
the free sulphurous or thiosulphuric acid of an acidified solution of
sodium thiosulphate (hyposulphite), and as it was not certain which of
the two acids was the really active agent, Schultz duplicated his
patent, so as to cover both. Though he made no claim in his patent to
having discovered the best proportions of his ingredients, those which
he specified have proved practically useful after allowing for the
modifications required by different skins, and slightly different
methods of working. His first bath consisted of a solution of 5 per
cent. of bichromate of potash, and 2¹⁄₂ per cent. of concentrated
hydrochloric acid (or 1·25 per cent. of concentrated sulphuric acid),
reckoned on the wet weight of the prepared pelt, and dissolved in
sufficient water for convenient use in the paddle or drum which was to
be used in the process. In this bath the skins were worked till they
took a uniform yellow colour throughout, but without any tanning effect
being produced. They were now freed from superfluous chrome liquor by
draining or “putting out,” and transferred to the second bath, which
consisted of 10 per cent. of “hypo” and 5 per cent. of hydrochloric acid
similarly dissolved. In this, they rapidly took a duck-egg green colour
from the reduction of the chromic acid; and when this was uniform
throughout the skin, the tannage was complete. The exact quantity of
water is not of great importance, and good results can be obtained with
anything varying from 20 to 50 gallons per 100 lb. of pelt (200 to 500
per cent.) if time be allowed for the weaker solution to act. The
quantities of “hypo” and hydrochloric acid given for the second bath are
often somewhat insufficient, and have to be slightly increased to
complete the reduction. The reactions which take place are represented
by the following formulæ, in which the weights of the materials taking
part in the reaction are also given below the symbols. In the first
bath--
Potassium Hydrochloric Potassium Chromic Water.
dichromate. acid. chloride. acid.
K₂Cr₂O₇ + 2HCl = 2KCl + 2CrO₃ + OH₂
294 + 73 = 149 + 200 + 18
As ordinary concentrated hydrochloric acid does not contain more than
about 30 per cent. of actual HCl,[116] about 2·5 parts would be required
to completely decompose 2·94 parts of dichromate, while in Schultz’s
formula 2·5 parts of hydrochloric acid are used to 5 parts of
dichromate. This excess has been found useful in the production of a
good leather, both to prevent accidents from an overdose of hydrochloric
acid, and because of the modifying effect of an excess of neutral salt
on the action of the chromic acid (cp. p. 82).
[116] Acid of S.G. 1·16 (32° Tw.) contains 31·5 per cent. of HCl by
weight or 36·6 grm. per liter, and therefore is practically 10 ×
normal strength. Acid of S.G. 1·2 (40° Tw.) contains 39·1 per cent. or
469 grm. per liter.
The reactions which take place in the second bath are somewhat
complicated. Eitner, in a valuable series of articles on chrome tannage,
which have been appearing in the ‘Gerber’ since January 1900, states
that even better results are obtained by using the hydrochloric acid in
slight excess, as the action of chromic acid (in the presence of the
potassium chloride of the chrome-bath) is not swelling but hardening to
the skin, and the slight swelling action of the hydrochloric acid tends
to counteract this, and also to facilitate the subsequent reduction. The
two views are not contradictory, as the excess of bichromate behaves to
the hide as an alkaline salt, which also produces a slight swelling
effect, and it is quite probable that better results are attained when
the solution is either alkaline or acid, than when the potassium
chromate is exactly decomposed. Eitner recommends the use of four parts
by weight of bichromate, and four parts of the strongest hydrochloric
acid, dissolved in 400 parts of water, for each 100 parts of wet pelt,
which should yield about 40 parts of dry leather. He states that if such
a bath be used, it may be safely and economically exhausted by a second
pack of skins, which is impossible in a bath containing excess of
unacidified bichromate. He gives[117] the following explanation of the
successive changes which take place when acid is gradually added during
the reduction, but points out that in practice the reactions always to
some extent go on simultaneously.
[117] Gerber, p. 297, 1900.
In the first stage, very slight acidification is required, and if the
skins have been chromed with excess of hydrochloric acid, may be
altogether dispensed with. The skins become brownish from the conversion
of the chromic acid into so-called “chromium dioxide” (probably really a
basic chromic chromate, Cr₂CrO₄(OH)₄, which on ignition leaves Cr₃O₆);
no sulphurous acid is liberated, or sulphur deposited, but sodium
tetrathionate is formed in the bath, and the reaction may be represented
as follows:
(1) 3CrO₃ + 6HCl + 6Na₂S₂O₃ = 3Na₂S₄O₆ + 6NaCl + 3OH₂ + Cr₃O₆.
Further addition of hydrochloric acid brightens the colour of the skins,
while the liquid still remains clear, and chromium chloride is formed
instead of chromic chromate, the main reaction being:
(2) 2CrO₃ + 12HCl + 6Na₂S₂O₃ = 3Na₂S₄O₆ + 2CrCl₃ + 6NaCl + 6OH₂.
On still further addition of hydrochloric acid, sulphur is separated
according to the following equation, and is deposited partly in the
skins, and partly in the bath:
(3) 2CrO₃ + 6HCl + 3Na₂S₂O₃ = 3Na₂SO₄ + 3S + 2CrCl₃ + 3OH₂.
After complete reduction, and consumption of the free hydrochloric acid,
further reactions take place at the expense of the excess of
thiosulphate which should be present, resulting in the production of
basic chromic salts, and the further deposition of sulphur, mostly
within the skin, as shown in the following equations:
(4) Cr₂(SO₄)₃ + Na₂S₂O₃ + OH₂ = 2CrOH.SO₄ + SO₂ + S + Na₂SO₄.
(5) 2CrCl₃ + Na₂S₂O₃ + OH₂ = 2CrOH.Cl₂ + SO₂ + S + 2NaCl.
The thiosulphate bath therefore not only reduces, but precipitates
sulphur in the skin, and reduces the chromic salt to a basic state. In
boiling solution, thiosulphate precipitates the whole of the chromium as
chromic oxide, but in the cold, and in presence of free sulphurous acid,
it only reduces to a basic salt. Eitner does not consider the
possibility, which certainly requires investigation, that instead of
basic salts, sulphite-sulphates are formed at least in the first
instance. Such salts of one base and two acids are quite possible, and
it is very probable that in the use of chroming baths containing organic
acids, they have considerable influence on the tannage.
The free sulphur which is liberated is partially deposited on and among
the fibres of the leather, and adds to its softness, and also acts
chemically on the oils used in “fatliquoring,” so that it is probably
one of the main causes of difference between the products of the Schultz
or “two-bath” method, and the “one-bath” processes subsequently to be
described.
It does not fall within the scope of this book to describe in detail the
working methods for the production of the different kinds of chrome
leather, but a few precautions common to all forms of the process may be
named. It is not absolutely important in all cases that goods should be
completely freed from lime before chrome-tannage, but in this case a
sufficiency of acid must be allowed in the first bath to neutralise the
lime introduced. Pretty thorough liming is generally advisable, to plump
and separate the fibres, but as a rule the bating or puering of goods
for chroming should not be excessive,[118] but should be planned not to
remove more than is absolutely necessary of the hide-substance, as the
chrome tannage is in its nature soft and light, and does not lend itself
to artificial fillings, such as the flour and egg-yolk of the calf-kid
process. Skins are sometimes freed from lime by “pickling” (p. 89), and
pickled skins may be chromed without depickling, which will be done by
the dichromate, but in this case the acid contained in the skins must be
considered in the composition of the chroming bath. Skins, indeed, which
are pickled with a sufficiency of acid may be chromed in a neutral
dichromate bath, and this is sometimes a convenient mode of procedure.
To prevent drawing of the grain during tanning, skins not unfrequently
receive a preliminary tannage with alum, or sulphate of alumina, and
these materials, together with salt, may be introduced into the chroming
bath, in which case they will liberate a portion of the chromic acid, as
has been mentioned in connection with the Heinzerling process. Alum,
chrome-alum, and acid salts, such as sodium bisulphate, may be
substituted for the acid in the chrome bath, but organic acids must not
be used, as they would reduce the chromic acid. The quantity of free
chromic acid in the chrome bath is of the most vital importance to
success, as it, and not the dichromate (which may be present in
considerable excess), regulates the amount of chrome taken up by the
skin, and the subsequent degree of tannage. It is very possible to
injure leather by overchroming, rendering it rough, harsh and even
tender. If a bath containing excess of bichromate is to be
re-strengthened, it may be assumed as a rule that all the free chromic
acid has been absorbed by the skins, and while it is merely necessary to
restore the strength of the dichromate to its original amount, the full
quantity of acid must be used which would be required in preparing a new
bath. Where, as in Eitner’s acid chrome bath, the whole of the chromic
acid is liberated, the bath may be exhausted by a second pack of skins.
Many tanners, in order to avoid the complications of remaking a bath,
run away their chrome liquors after once using, containing all the
excess of dichromate which has been used. With proper chemical control,
this should not be necessary, and is objectionable, not only from its
wastefulness, but on account of the very poisonous character of the
unreduced bichromate. Even weak dichromate solutions, especially if
warm, are liable to cause painful and obstinate eruptions on the hands,
but this rarely occurs to tanners, as the poisonous action of the
solution is removed on reduction. It is well, however, to arrange that
men who handle skins in the chrome bath, should subsequently also work
in the reducing bath. Methods of analysis of used chrome liquors are
given, L.I.L.B., pp. 142 _et seq._ Those for the determination of
acidity are not however easily applicable in the presence of alum and
salts of chromic oxide.
[118] Goat-skins for glacé kid need thorough puering to produce a
smooth grain.
The skins, on coming from the chroming bath may be allowed to lie for
some time without serious injury, but should be carefully protected from
the action of light, which reduces the chrome at the expense of the
skin, and renders the subsequent tannage irregular. It is found that
skins, if brought into a weak or neutral reducing bath, are apt to
“bleed” or lose chromic acid, which is reduced wastefully in the bath.
On the other hand a strong “hypo” bath is apt to draw the grain and
contract the skins, owing to the tannage taking place too suddenly. A
somewhat strong “hypo” bath is therefore often employed as a preparatory
“dip,” the skins being simply drawn through it to fix the chrome on the
surface, piled on a “horse” and subsequently reduced in a bath of
ordinary strength. The tendency to bleed is lessened, but at the expense
of the pelt, by the reduction which takes place if the skins are allowed
to lie overnight in the chromed state. Eitner states that skins chromed
in an acid bath (i.e. where the whole of the chromic acid is in a free
state) show little tendency to bleed. After reduction, the skins are
well washed with warm water, and their subsequent treatment is the same
as that of skins tanned by the one-bath process, which is subsequently
described (see p. 211).
Naturally in practical work, the reduction cannot be made to proceed
rigidly in the definite steps described by Eitner on p. 206, but all go
on in different proportions together, though by supplying the acid in
proper quantities, and at proper intervals, they may be made in the main
to follow in the given order. Both on this account, and because neither
the exact amount of chromic acid in the skins, nor the sulphurous acid
lost by escape into the air can be exactly determined, the reduction
cannot be conducted on theoretical principles, but the best conditions
must be empirically determined. Eitner states that 12 parts of
thiosulphate dissolved in 400 parts of water, and 6 parts of (40 per
cent.) hydrochloric acid are sufficient for 4 parts of bichromate per
100 of wet pelt employed in the chrome bath, of which not more than
one-half to two-thirds is absorbed; and that if equal parts of
bichromate and acid are employed in chroming, the acid used in reducing
may be lessened to 5 parts. In this case it must not be forgotten, that
if the partially exhausted chrome-bath is used for a second pack of
skins, which are afterwards finished in a bath of full strength, nearly
the whole quantity of bichromate used in making up one bath will be
absorbed by the skins. The amount of acid consumed in reduction will be
greater, the more rapidly it is added, owing to increased escape of
sulphurous acid. It is better to add the acid, previously diluted with
water, in 8 or 10 successive portions, more rapidly at first, and more
slowly during the latter half of the operation, each portion of acid
being added as soon as no further change of colour appears to be caused
by that already given. These changes are the more rapid the lighter the
goods. The colour darkens at first to olive-brown, then gradually
becomes green, and finally blue, and when this colour is uniform
throughout the thickness of the goods, no further acid need be added.
For goods which have been chromed in an acid bath, Eitner states that no
acid will be needed for the first twenty to thirty minutes. It is
important to have a sufficient excess of thiosulphate in the bath when
reduction is complete, in which case the goods may be left for some
hours or overnight in the bath, to complete “neutralisation,” but Eitner
prefers to use a fresh bath of 1¹⁄₂ parts of thiosulphate in 400 parts
of water for this purpose, the bath being used, after settling, for
making up the reduction bath for the next lot of goods, for which 1¹⁄₂
parts less thiosulphate is used. The goods must be kept in motion during
reduction, either in a drum or a covered paddle.
In a paper on “_Die Natur und Wesen der Gerberei_” published by
Professor Knapp, in 1858, he describes clearly a chrome tanning process
with basic chromic chloride, formed by the addition of sodium carbonate
to a solution of the normal salt, but he expressly states that the
product was not more resistant to water than the ordinary alum tannages.
How he fell into this error is hard to explain, for leathers produced
according to his directions, resist not merely washing in cold but
boiling water. As soon as the Schultz process proved successful, many
attempts were made to evade the patent by the use of other reducing
agents than the “hypo,” and other salts of sulphurous acid which it
covered. Among these, the use of hydrogen sulphide, and acidified
solutions of alkaline sulphides, and especially of polysulphides,[119]
proved capable of practical use, though less convenient than
thiosulphate, but were soon acquired by a combination, the Patent
Tanning Company, together with Schultz’s original patents.
[119] “Liver of sulphur” or solutions, made by boiling sodium sulphide
or soda with excess of sulphur.
Under these circumstances, Martin Dennis, either by fresh discovery, or
otherwise, revived the original process of Knapp, which he patented[120]
almost word for word, and offered a basic chrome tanning liquor for
sale, without further restrictions on its use. This liquor was made by
dissolving precipitated and washed chromic hydrate (easily prepared by
precipitating chrome-alum solution with excess of alkali) in
hydrochloric acid to saturation, and adding washing soda until the
solution was rendered sufficiently basic. Such a solution may be used on
skins prepared in the ordinary way, by diluting with water, and
strengthening as the tannage proceeds, like a vegetable tan-liquor. It
is doubtful if the patent is a valid one, as it was known that the use
of such a solution was not new, and it was only granted in America on
the representation, which has since been found to be mistaken, that
chlorides alone were applicable for tanning, while Knapp had not
restricted his statement to these salts. In reality chlorides and
sulphates seem equally suitable, but to produce similar results the
former must be made more basic than the latter. In any case the patent
cannot cover the general principle of basic tanning, but only the
particular liquor and mode of preparation specified. It was soon
afterwards shown by the writer,[121] that a good chrome tanning liquor
might be prepared by direct reduction of dichromate with sugar in
presence of such a limited quantity of hydrochloric acid as to produce a
basic salt. Suitable proportions are 5 mol. HCl to 1 mol. potassium
dichromate, which produces a salt approximately Cr₂Cl₃(OH)₃. The
solution is easily made by dissolving three parts of dichromate in a
convenient quantity of water, adding six parts by weight of concentrated
hydrochloric acid, and then cane-sugar gradually till a green solution
is obtained, when the whole may be made up to one hundred parts, and
will be approximately of the same strength as a 10 per cent. solution
of chrome-alum. A little heat may be needed to start the reaction, but
too much should be avoided, as considerable heat is evolved by the
oxidation; and as much carbonic anhydride is produced, which causes the
solution to effervesce briskly, the vessel used should be of ample size.
In place of cane-sugar, a good quality of glucose may be used, but some
samples contain some impurity which produces a violet solution which
will not tan satisfactorily. This liquor is in regular use in many
tanneries, producing a good quality of chrome calf, but is somewhat
variable in its effects according to the temperature employed in its
preparation, and it appears to have no real advantage over a simple
solution of chrome-alum, rendered basic by soda and with some addition
of salt. A somewhat similar preparation is Eberle’s “chromalin,”[122] in
which some organic substance, probably crude glycerine, is used to
reduce the bichromate. The organic matters, and especially the organic
acids which result from the oxidation of the sugar or glycerine, are not
without influence on the tanning properties of the liquor. Of course
these solutions may be rendered still more basic by the addition of
sodium carbonate. A good stock-liquor, of approximately the same
strength as that above described, is made by dissolving 10 parts of
chrome-alum in 80 parts of tepid, but not hot water,[123] and adding
with constant stirring a solution of 2¹⁄₂ to 3¹⁄₂ parts of washing soda
in 10 parts of water. The chrome alum dissolves somewhat slowly without
the aid of heat, and the solution is best made either in a small drum
driven by power, or by suspending the crystals in a basket near the
surface of the liquor, so that the saturated solution can descend.
[120] Martin Dennis, U.S.A. Pat. 495028, 1893; and 511411, 1893, 7732,
1893. E. Pat. Gallagher.
[121] Leather Trades Review, Jan. 12, 1897.
[122] Compare Eberle’s German patents, 119042, 1898, and 130678, 1899.
The last of these appears to be anticipated, at least as regards the
use of glucose, sugar and starch, by the writer’s publication in 1897
above cited.
[123] Leather Trades Review. Later investigations have shown that the
temperature of the water is unimportant if alkali be added, but
chrome-alum dissociates to some extent in hot water, and comparative
experiments have shown that solutions of the normal salt made with the
aid of heat act on skin as if more acid than those made in the cold.
Eitner[124] has pointed out the important effect that differences of
basicity have on the tanning properties of chrome solutions. Normal
chrome sulphate or chrome-alum colours the leather quickly and equally
throughout, and swells the pelt on account of its practically acid
character, but gives a thin and lightly tanned leather, from which much
of the chrome washes out, unless it is at once “neutralised” in alkaline
solutions. As the chrome solution is made more basic, the tannage
penetrates more slowly, but is heavier and more thorough, the colour is
darker and bluer, and much less of the chromic salt is removed by
washing with water. When the basicity becomes excessive, the solution
becomes unstable, and decomposes on dilution with water into a very
basic salt which is precipitated, and a more acid solution than that
given by a moderately basic salt. The effect of such solutions on the
leather is very unsatisfactory, producing the bad effects both of too
acid and too basic salts. The pelt is apt to be swollen and lightly
coloured by the more acid salt, but at the same time the actual tannage
proceeds very slowly, and in extreme cases it is difficult to tan
through, while the surface becomes over-tanned, and the grain often
tender and even brittle from the incrustation of precipitated basic
salt. Eitner likens the effect of the more acid liquors to the quickly
penetrating and lightly tanning vegetable tans, such as gambier, and
that of the more basic to the heavier tannages, such as valonia; and
within limits, advantage may be taken of these facts in adjusting the
liquors to the character of the leather it is desired to produce. In
sulphate liquors, he considers the salt CrOH.SO₄ as most suited to
general use, and in the case of chrome-alum, this is produced by the use
of 286 parts of soda-crystals, or 106 parts of dry sodium carbonate (1
molecule) to 998 (or practically 1000) parts by weight (1 mol.) of
chrome-alum. (In using washing soda, care must be taken to employ
_clear_ crystals of the salt, and not those which have become white by
loss of water.) In place of soda, Eitner makes a similar basic liquor by
boiling 1000 parts of chrome-alum with 248 parts (1 mol.) of sodium
hyposulphite until the whole of the liberated sulphurous acid is driven
off, and the sulphur deposited. In comparative experiments by the
Author, no difference could be detected between the tanning effects of
the two solutions, and that with soda is both cheaper and more easily
made. If the solution with hyposulphite is not boiled, a more acid
liquor results, in which part of the chromium is combined with
sulphurous acid, forming an unstable compound which may prove useful in
certain cases.
[124] Gerber, 1901, pp. 3 _et seq._
Eitner states that he has made chrome-solutions of various types,
containing organic compounds in combination with the chrome-salt, which
combine with the leather, producing a fuller and softer tannage, but he
gives no details as to their preparation, as they are made commercially
by the “Erste Oesterreichische Soda-Fabrik” at Hruschau. The writer has
found that in some cases by the addition of say 3 parts of sugar, or
still better of glucose, to 10 parts of the chrome-alum in making up the
basic liquor, a much fuller and plumper leather is produced, which dries
perfectly soft, even without staking or fat-liquoring; and it is
probable that many other organic compounds may be found which produce
similar effects. The addition of very small quantities of even neutral
tartrates or lactates, and probably of many other organic salts or
acids, have a remarkable effect in lowering the apparent basicity of the
solution, and it is possible that these may also be usefully employed in
combination with very basic liquors. It is highly probable that the
unsatisfactory tanning liquors produced by direct reduction with some
samples of glucose are due to the presence of small quantities of some
organic acid produced during the oxidation. It has been found that these
solutions may be made to tan by the liberal addition of soda. It is
probable that more satisfactory results in chrome-tanning will be
attained by the direct addition of known organic substances to basic
liquors of definite constitution, than by the somewhat uncertain
products of organic oxidations.
The quantity of salt to be added depends on the qualities desired in the
leather, and upon whether chloride or sulphate liquors are employed;
salt in chloride-liquors increasing the softness of the leather, but in
excess tending to flatness, while in sulphate-liquors it practically
diminishes their basicity by converting the chromium sulphate into the
equivalent chloride, which, as Eitner points out, behaves as a less
basic salt, and hence but little advantage is to be gained from its use.
It is best to begin with a very weak liquor, to avoid drawn grain, and
for the same purpose a preparatory tannage with alumina salts, or an
addition of alum or sulphate of alumina and salt may be made to the
first liquor, as the attraction of the chrome salt for the fibre is
sufficient to produce a chrome tannage, even in presence of excess of
alumina salts. 10 lb. of chrome alum will tan about 100 lb. of wet pelt,
but more must be used for the first parcel; as to avoid loss of time,
the skins may be tanned out in a pretty strong liquor. The bath has a
tendency to become acid by use, and before strengthening, it may be
necessary to add some more soda solution. Very little additional salt is
required, as it is only absorbed by the skins to a small extent,
probably as chromic chloride. As the liquors gradually become charged
with sulphates, it is best to work them out like bark liquors, and not
to go on strengthening the same liquor indefinitely. If old liquors are
used for green goods, it is not necessary to neutralise them with soda
before use, as Eitner has shown that less basic liquors colour more
evenly and with less tendency to produce drawn grain.
Basic chrome liquors, such as have been described, may also be used in
chrome combination tannage. It is generally best to let the light
vegetable tannage precede the chrome, and lightly tanned skins, such as
“Persians” and East India kips, acquire many of the qualities of
chrome-tanned leather by the treatment. The effect is still further
increased by a previous detannisation of the leather with alkaline
solutions (see p. 241). Several firms beside Dennis now supply basic
chrome liquors ready prepared for use.
The time of tannage will of course vary with the thickness of the goods,
and for calf-skins will usually extend over some days, though it can be
much quickened by drumming. The tannage is generally best accomplished
in the paddle, but can be carried out by frequent handling in pits or
tubs, or, where very smooth grain is important, by suspension. When the
goods come out of the final liquor, they may be allowed to lie in pile
for twenty-four hours, or even for some days, with advantage, as the
surplus chrome liquor is pressed out, and the tannage becomes more
complete. They are then washed with plenty of warm water, till it ceases
to be coloured with chrome. They may be kept for an almost unlimited
time in a wet condition, as they do not bleed, and have little tendency
to heat even in pile. They have now reached the stage at which we left
the “two-bath” leather, and the subsequent treatment may be the same in
both cases.
Although by both processes, the chrome-salt fixed in the fibre is of a
decidedly basic character, it still contains enough acid to act
injuriously on the leather in course of time, and to lead to serious
inconveniences in its subsequent treatment. Before proceeding further,
this access of acid must be removed or neutralised, and it is not too
much to say that most of the troubles experienced in the fat-liquoring
arise from neglect or mistake in the washing and neutralisation. The
difficulty in the process arises from the fact that while the acid
should be reduced to a mere trace, it must not be entirely removed,[125]
as chromic oxide itself does not seem capable of tanning, and at any
rate the effect of excess of strong alkalies is at once to render the
leather hard and pelty. Borax is one of the safest neutralising
materials, about 3 per cent. on the wet weight of the pelt being
required, in not more than ¹⁄₂ per cent. solution. Eitner recommends the
use of silicate of soda, which, sold as a solution of S.G. 1·5, is
somewhat stronger and much cheaper than borax. Hyposulphite of soda and
whitening together neutralise more rapidly and completely than either
alone. Other salts of weak acids may also be used, the acids exercising
a regulating influence which prevents neutralisation going too far.
Sodium carbonate or bicarbonate, or ammonia may also be used, but with
these it is difficult to get even “neutralisation,” or to avoid the risk
of carrying the process too far. Even a thorough drumming with a milk of
“whitening” (calcium carbonate) is effective. With the latter there is
no danger of overdoing the process, but in some cases the adhering
whitening and precipitated calcium sulphate are troublesome in later
operations. In any case the neutralising should only be carried so far
that the skins show no acid reaction to litmus paper.
[125] Procter and Griffith, Journ. Soc. Chem. Ind., 1900, p. 223.
It is probable that one of the great causes of difference between
“one-bath” and “two-bath” leathers is the presence of free sulphur in
the latter. This may also be introduced into “one-bath” leather, by
treating it in the wet chromed state, without washing out the chrome
liquor, with excess of a solution of hyposulphite, or of an alkaline
polysulphide, which at the same time will neutralise the skin. The more
acid the chrome liquor, the greater the quantity of sulphur which will
be introduced. The simplest means of distinguishing “two-bath” from
“one-bath” tannages is to test for the presence of sulphur, by wrapping
up a silver coin, with a piece of the leather in paper, and leaving the
parcel for an hour in the water-oven, or some other warm place, when
the presence of sulphur will be shown by the blackening of the coin. Of
course a sulphurised “one-bath” leather will give the same reaction.
The leather must now be dyed and fat-liquored. Which of these two
operations should be first undertaken will depend on circumstances. Most
leathers dye more easily before fat-liquoring, but as many dyes are
soluble in the alkaline fat-liquor, a good deal of colour is often lost.
This may be compensated by dissolving a suitable aniline (acid) colour
in the fat-liquor. “Bluebacking” is generally done before fat-liquoring
by drumming with methyl-violet, or some other aniline colour (with or
without logwood, which gives alone a very dark violet). Any shaving or
splitting required must of course be done before bluebacking.
The fat-liquor is an emulsion of soap and oil, which for chrome leather
should be as neutral as possible, if the neutralising has been thorough;
but if any acid be left on the skins, a neutral fat-liquor will be
precipitated as a greasy mass. This can sometimes be remedied by the
addition of a little ammonia or borax, or by re-fat-liquoring with soap
solution only, but if the washing of the skins has been incomplete, and
soluble chrome-salts remain, the mischief is almost irretrievable, as
sticky chrome-soaps are formed, often coloured with the aniline violet,
which adhere to the skins, and which can scarcely be removed by any
solvent which does not injure the leather. As regards the soaps and oils
used, there is considerable latitude: 1¹⁄₂ per cent. of castor-oil soap,
and ³⁄₄ per cent. of castor or olive oil on the wet weight of the pelt
has done good service in my hands, but many manufacturers employ soft
soaps, curd soaps, etc., with castor, olive, cod or neatsfoot oil, and
sometimes sod-oil or degras. Eitner considers olive-oil and olive-oil
potash soap the most suitable, and particularly warns against the use
either of drying oils or of oils containing tallow (such as neatsfoot),
which are not only apt to cause a white efflorescence, but to give the
leather a disagreeable rancid smell. Fish-oils are unsuitable, but
mineral oils are often useful constituents of fat-liquors. Wool-fat also
makes a good fat-liquor, but is unsuitable for goods which are to be
glazed. “Turkey-red oil” (which is sulphated castor) may be used as a
fat-liquor, simply mixed with warm water, without soap, and has been
recommended where delicate colours are to be dyed after fat-liquoring;
but it is said to have an unsatisfactory after-effect, hardening and
tendering the leather. Some soaps made from the saponifiable part of
wool grease, such as “Lanosoap,” also act well in conjunction with
olive, castor, or other oils. Where leather is to be glazed, the amount
of fat-liquoring must be kept very moderate. Fat-liquors should be
thoroughly emulsified, and are generally used warm. They penetrate
better if the leather is partially dried by sleeking out, or pressing,
or cautious “samming,” but the leather must not be completely dried out
before fat-liquoring and dyeing, unless it has been previously treated
with glycerine, glucose, treacle or some deliquescent salt, which will
enable it to be wet back. Chrome leathers are not “waterproof,” as has
often been stated, unless rendered so by treatment with soaps and
greases, and are apparently easily wetted, but the fibre will no longer
absorb water after thorough drying, and consequently will neither dye
nor stuff satisfactorily. In order to enable chrome leather to be kept
in an undyed condition, glycerine or syrup is sometimes mixed with the
fat-liquor, but as the watery portion of this is not generally
completely absorbed, the process is somewhat wasteful. Mr. M. C. Lamb
avoids this difficulty by applying a solution of glycerine to the
grain-side with a sponge after fat-liquoring. In this case the leather
may be dried sufficiently for staking or shaving without risk.
Chrome leather can be dyed by many of the acid aniline colours without a
mordant. Basic colours are only fixed when the leather has been first
prepared with a vegetable tannin, gambier, or a mixture of gambier and
sumach being the most suitable. Considerable care must be employed in
the application of tannins to chrome leather, as they have a tendency to
harden it and diminish its stretch, or even to render it tender, but
traces of tannin in the dye probably facilitate glazing. Before dyeing,
it is advantageous to fix the tannin with tartar emetic, or for browns
and yellows, with titanium potassium oxalate solution, which itself
gives a good yellow-brown with tannin. In place of employing the tannin
and titanium salt in two separate baths, they may be combined, using a
weight of the gambier or tanning extract (oakwood, chestnut, etc.) about
equal to that of the titanium salt, or titanium tanno-oxalate solution
may be used. Chrome leather may be dyed with the various dye-woods,
which are mordanted by the chromium present, but the colours are mostly
dull, that of logwood being nearly black. A good black of a very
permanent character is obtained by dyeing with logwood, and saddening
with a hot solution of titanium oxalate in the drum. A little iron-alum
added to the chrome liquor in tanning will facilitate dyeing the skins
black with logwood and help it to penetrate through the leather, which
is sometimes desired. Several aniline blacks, and notably the
“corvolines” of the Badische Anilin und Soda Fabrik, Casella’s “leather
black C,” and Claus & Rée’s chrome-black give very satisfactory blacks
by brushing or dyeing.
Chrome skins may be glazed in the ordinary way with blood or albumen
mixtures under glass or agate, but require good pressure and repeated
seasonings and glazings, and much care is required in fat-liquoring. The
glazing is often assisted by the previous application of barberry juice
(épine vinette) or of lactic or tartaric acid solution with a trace of
sugar. Much of the difficulty which has been experienced in glazing
chrome leathers is due either to the natural fat of the skin, or to oils
used in fat-liquoring in excessive quantity or of unsuitable character.
CHAPTER XVI.
_PRINCIPLES OF THE VEGETABLE TANNING PROCESSES._
The processes employed in the production of leather with the vegetable
tanning materials vary extremely according to the class of leather which
is being produced, both in the materials selected and in the time
required. In sole-leather tanning, where thick hides are used, and where
diffusion is the only force acting to carry the tannin into the hide,
many months are frequently needed, while with thin skins, and with the
aid of mechanical motion, which circulates the tanning liquid between
the fibres, the process is often complete within a few hours.
Differences in the strength of the liquors according to whether hard or
soft leathers are to be produced, and the mutual action of the acids
naturally present in the liquors, and of the tan, have also a
determining effect upon the quality of the product.
The simplest form of tanning in principle is probably the old-fashioned
method of sole-leather manufacture. For this purpose, the hides are
usually “rounded” or trimmed after liming, unhairing and fleshing, so
that the most valuable part, the “butt,” can be tanned separately from
the “offal.” The butts are usually washed in water to remove a portion
of the lime, considerable care being required at this stage to avoid
carbonation and fixation of chalk by means of free carbonic acid, or
hydric calcium carbonate (temporary hardness) in the water employed, or
by the free carbonic acid of the air. This somewhat primitive process
can at best only remove a small portion of the lime, since so long as
the lime remains in the caustic condition, it is very obstinately held
by the hide-fibre. Advanced tanners now frequently employ weakly acid
baths, in addition to washing, in order to produce more complete
deliming, and this effects a very considerable improvement of colour in
the early liquors. The use of lactic acid (free from iron) or boric
(boracic) acid in solutions of about 4 lb. per 100 gallons, in which the
butts are kept in motion, are among the safest and most satisfactory
ways of removing surface-lime and improving the colour, but even the
stronger mineral acids may be used successfully with caution (see Chap.
XIII.).
Whether acid be used or not, the butts are now usually suspended in deep
pits containing old and nearly exhausted tan liquors. These liquors
contain a certain amount of lactic and acetic acids, derived by
fermentation from the sugary matters of the tanning materials, and also
in some cases, weak acids originally present in the materials
themselves. These acids are most important to successful tannage, and
their effect is twofold; in the first place, they neutralise and remove
any lime which still remains in the butts; and, secondly, they bring the
butt into a slightly acid condition, in which it remains plump and
swollen in the liquors, while the tannin gradually penetrates and tans
the fibre. If, as frequently happens, especially in modern yards where
extracts are very largely used, the natural acid of the liquors is not
sufficient for this purpose, the lime combines with the tanning matters,
and the butts either become discoloured at once, or darken by exposure
and oxidation, when they come to be dried, while the pelt remains flat
and insufficiently swollen. To avoid this trouble, resort is sometimes
had to artificial acidification of the liquors. As a general rule, it
may be stated that it does not answer to mix the stronger mineral acids
directly with the liquors, but lactic and acetic acids may be used, or
even oxalic acid may be added to the suspenders in such quantities as to
precipitate and remove the lime which they contain, setting free the
organic acids with which it had been combined. The use of oxalic acid
should never be pushed further than this, as it has a most powerful
swelling action on the hide; and goods which are too much swollen by
acids tan dark and brittle.
After the hides have remained from ten days to a fortnight in the
suspenders, they are usually laid in pits called “handlers” which are
worked in series of 6, 8, or 10 pits, containing the same number of
packs of goods. The weakest liquor from the youngest pack is run to the
suspenders daily, a new and stronger liquor is run to the pit, which now
becomes the head of the series, into which the oldest and most tanned
pack of butts is moved; and the next takes its place and liquor, and so
on down the series, the youngest pack finally occupying the place which
had previously been taken by the last but one. In this way each pack
receives a change of liquor of regularly graduated strength; and during
the time which it remains in the handlers, passes from a strength of
perhaps 20° Bkr. (sp. gr. 1·020) to one of about 40° Bkr. (sp. gr.
1·040). During this part of the process the butt is completely or nearly
coloured through, and is then ready for the “layers.”
In the forward handlers, dustings of ground bark or other tanning
material are very frequently given, and the layers only differ from
these in having much heavier dusting, stronger liquors, and being
allowed to remain undisturbed for greater lengths of time, ranging from
a week up to a month or even six weeks, as the tannage progresses. The
handler-liquors are principally from the old layers, though they are
frequently made up with weak liquors from the leaches, and strengthened
with extracts or gambier.
Very varied materials are used in the manufacture of sole-leather.
Oak-bark is one of the oldest, and as regards quality one of the most
satisfactory, but it is costly, not only on account of its weakness in
tannin but from the light weight of leather which it gives. Valonia is
one of the favourite materials, giving heavy weight and a solid leather,
in which it deposits a great deal of bloom. Oakwood, chestnut-wood, and
hemlock-bark extracts are now very largely consumed, principally in
strengthening the layer-liquors; the great object being not only to
lessen the cost in material, but to save time, and produce greater
weight and firmness. The layer-liquors in some yards where extract is
used, reach strengths of even 120° to 150° Bkr. (sp. gr. 1·12 to 1·15),
while in pure oak-bark yards it is difficult to get above 30° or 35°
Bkr.; and even these figures are only reached by repeatedly
strengthening the same liquor, in which large quantities of non-tanning
substances accumulate. The opinion of the most intelligent tanners is,
however, that better results are attained by a regular change of liquor,
even if the apparent strength is less.
When the leather has remained a sufficient time in the layers to have
attained all the weight and solidity of which it is capable, it is
washed up in a clear and somewhat weaker liquor or even in warm water,
and taken into the shed to be dried and finished. As this finishing is
almost purely mechanical, and scarcely comes within the scope of the
present volume, a very brief sketch must suffice.
[Illustration: FIG. 39.--Wilson’s Striking Machine.]
The mode of finishing which was formerly, at least, in vogue in
Lancashire and Cheshire may be taken as a type of the best work. (In the
present day, the various methods are so widely known that they have
ceased to be local, and are varied according to the quality and tannage
of the goods.) The butts, which in earlier times were largely
bark-tanned, are taken wet from the pits, and scoured on a rounded beam
or “horse” with stone and brush, till the bloom is completely removed,
and are then lightly oiled on the grain, half dried (“sammed”), laid in
pile to temper, and “struck out” with the “pin,” a two-handled tool of
triangular section shown in Fig. 29. The use of this tool has now been
largely superseded by Wilson’s striking machine Fig. 39, in which knives
or sleekers (or stones and brushes), held in jointed arms, are made to
work on the butt, which is extended over a slowly rotating cylinder. The
object of the pinning is not so much to remove bloom or dirt, which has
been previously effected by the scouring, as to smooth and flatten the
grain. After further drying, a second pinning is generally given, and
the goods are then twice rolled, first with a light weight, and somewhat
moist grain, and then more heavily with the grain nearly dry. This was
formerly accomplished by a sort of box or car, heavily loaded with
weights, supported on a smooth brass roller of about 5 inches diameter
and 9 inches long, and manipulated with a long wooden handle on a floor
of hard wood, or zinc plates. One type of the machines which have now
almost entirely replaced this primitive contrivance is shown in Fig. 40,
but is principally used for offal and common classes of goods. For
better work, traversing rollers, such as Wilson’s ingenious double bed
roller shown in Fig. 41, are to be preferred. After rolling, the goods
are dried pretty rapidly by the aid of moderate heat, and, after
polishing with a brush (hand, or machine, Fig. 42), are ready for sale.
It may be pointed out that although the tools are different, the process
is almost the same as that used for “_vache lissée_” in France and
Belgium, and closely resembles that of currying harness leather except
that the “stuffing” with fats and oil is omitted.
[Illustration: FIG. 40.--Offal Roller.]
In contrast with the rather elaborate method just described, we may
place the American finish of red hemlock sides, which are tanned
throughout with a material which yields no bloom. On these, the
scouring and “striking” is altogether omitted: the goods are completely
dried out from the pits, which is found to fix the dark-coloured liquor,
and result in better colour; they are then damped back, and tempered,
and heavily rolled under a rapidly moving pendulum roller, which
polishes at the same time that it smooths the leather. The saving of
cost by so simple a process is not inconsiderable.
[Illustration: FIG. 41.--Wilson’s Double-bed Butt Roller.]
In the West of England, much heavy leather is still manufactured from
South American hides, which are tanned with a large proportion of
valonia; and which consequently are heavily bloomed. No attempt is made
to remove this bloom, which would too much lessen the weight and
firmness, but the goods, after a light oiling to preserve the colour,
are hung up and partially dried, and are then laid in pile to temper.
The grain side is now wet with soap and water, with which a little oil
is often mixed, and the bloom is “struck in” with the pin or machine; a
somewhat blunt pin being used, or a blunt tool in the striking machine;
which is held at such an angle as to smooth and compress the grain
without taking too much hold on it. After a little further drying, the
striking is generally repeated, the goods are washed over with water,
and rolled “on.” They are now coloured with a mixture of pigment
colour, generally containing a large proportion of whitening, or
sometimes of French chalk coloured with ochres, chrome-yellow and
orange, or whatever may suit the tint preferred by the tanner, or best
imitate the colour of a clean-scoured tannage, and usually mixed with
size and oil, or sometimes with oil and tan liquor. This mixture is well
rubbed in, and smoothed over with a cloth, and then polished by
brushing, when the goods are “rolled off,” rapidly dried, and again
brushed. If the work has been well done, it is not easy to distinguish
from clean scouring, and is much cheaper.
[Illustration: FIG. 42.--Brushing Machine.]
A method intermediate between this and the first described, and which
was formerly much used in London, was to proceed as above, but using
more water and holding the pin in the first striking so as to scour out
as much bloom as possible, and assisting this by the free use of water
and the brush. Instead of using an opaque pigment-colour, the goods were
generally coloured either between striking and the first rolling, or
between the two rollings, with a transparent colour, such as dissolved
annatto, or a mixture of aniline dyes, so as to conceal the traces of
bloom, and to render slight damages to the grain less conspicuous.
The principles of the manufacture have been fully explained in previous
sections, up to the time when the goods are taken into the tanning
liquors. At this stage complicated reactions take place between the lime
in the butts, the free vegetable acids in the liquors, and the tannins;
and on the right adjustment of these three factors much of the success
of the operation, and indeed of the whole manufacture depends. If the
lime is in excess of the acids present, it forms insoluble compounds
with the tannins in the surface of the hide. If these are protected from
the air, they are generally redissolved as they advance into more acid
liquors, but they readily become oxidised into dark-coloured matters,
which can no longer be removed. Their presence in the finished leather
is one of the great causes of darkening in drying. If the hide in the
limy condition has been exposed either to the carbonic acid of the air,
or to free carbonic acid, or acid calcium carbonate dissolved in water
(“temporary” hardness, p. 94), a precipitate of calcium carbonate will
be formed in the surface, which is much more difficult to remove than
free lime, and which is perhaps the most common cause of the stains and
discolorations which are so serious a source of loss to the sole-leather
tanner. These stains may, if not too much oxidised, be removed by
treatment of the tanned leather with weak warm sulphuric acid, but this
remedy brings other evils in its train, and should not be required. The
great remedy is to keep the goods from the time of unhairing till they
go into the liquors, under water in which there is always a trace of
caustic lime, or which at any rate are free from carbonic acid. In
deliming sole-leather with acids, it is best to give the full dose of
acid required, at once, and not gradually, so that it may act most
powerfully on the exterior, and remove any carbonates present, before it
penetrates to and becomes neutralised by the excess of lime in the
interior. This is exactly the reverse of what is advisable with dressing
leather, where the object of the tanner is to remove lime as uniformly
and completely as possible, without excessive acidity of any part. Of
course hides should not, even in the case of sole-leather, be allowed to
go into the liquors while any acid swelling of the surface remains, but
this will soon disappear if the goods are suspended for a time in cold
water after deliming, unless excess of acid has been used (cp. p. 153
_et seq._).
If the proportion of free acid in the suspender liquors is as it ought
to be, it is probably rather advantageous than otherwise for a little
lime to remain in the interior of the hide, as it keeps the pelt in a
plump condition during the first stages of colouring, quickens the
penetration of the tannin, and lessens the tendency to “drawn” or
wrinkled grain, which arises when the goods go into the liquors in a
flat or fallen condition. The causes of drawn grain are often a little
obscure. Of course that case needs no elucidation in which the hides are
submitted to the tanning liquor in a creased or wrinkled condition,
which is simply fixed and made permanent. This may arise, either from
carelessness in handling the goods before taking into the suspenders, or
from the way in which they are slung to the sticks, which often draws
them into long wrinkles, afterwards difficult to remove. Drawn grain in
general, however, arises from the grain-surface becoming tanned and
fixed in area, while the substance of the hide is in a more extended
condition than that which it assumes as tannage proceeds. Hides in a
flat and unswollen condition are thinner, the fibres are slenderer and
looser than when swollen, and consequently the hide has a larger area.
If, after the grain is tanned, the substance of the hide becomes
contracted in the liquor, either by swelling with acids, or by the
direct action of the tannin on the interior fibres, the grain is certain
to be shrivelled, like the skin of a dried apple. A similar effect,
produced in a mechanical way, may always be noted where a hide has been
coloured hanging grain-side out over a pole, so that the surface is
extended at the bend, on which long wrinkles are formed as soon as it is
straightened.
A hide in a slightly alkaline condition colours, and even tans more
quickly than one which is acid. In presence of a trace of lime, and
deficiency of free acid in the suspender liquor, tannages of valonia and
bark give the butt a sort of lemon-yellow colour, which is not in itself
injurious, and which disappears as the hides advance into more acid
liquors, but which is a sign of danger, as showing that no excess of
acid exists in the suspender-liquors. Gambier gives pelt perfectly free
from lime a pale buff colour, but where lime is present, the colour is
always reddish, and much darker, and this coloration does not disappear
so readily as that with valonia, so that if gambier is to be used in the
first liquors, care should be taken to remove all lime from the surface.
The only known tannin which gives no insoluble compound with lime is
that of the babool pod (sometimes called “gambia-pod”), which is
frequently used in India as a bate, and which would probably prove very
useful in colouring liquors (pp. 165, 288).
When sole-leather first goes into liquors, it is generally swollen with
lime to some extent. If the liquors contain, as they usually do,
sufficient free acid (acetic, lactic) in addition to the tannins, these
combine with and neutralise the lime, and the pelt, without absolutely
becoming flat and thin, loses its firmness, and becomes soft and spongy.
This is a favourable condition for the absorption of tannin, but care
should be taken not to allow the pelt to be squeezed or pressed, or
water will be squeezed out, and the pelt will not easily resume its
plumpness. As the tannage proceeds, both the tannin and the acid of the
liquors penetrate deeper into the pelt, the former tending to contract
and the latter to swell the fibres. Thus a given quantity of acid will
cause the greater swelling, the less tannin is present; and therefore in
strong tanning liquors more acid is required. The presence of certain
products of bacterial putrefaction has a great but unexplained effect in
preventing hide from swelling with acids; and in hot weather, much
better swelling is obtained by sterilising and deliming the hides with
one of the coal-tar products mentioned on pp. 30, 162. Boric acid may
also be satisfactorily used for this purpose, but should not be allowed
to get into sole-leather liquors, as it tends to produce a soft and
loose tannage, and from its inorganic and indestructible character, is
apt to accumulate in a yard in which it is used. The same reasons render
unadvisable its introduction into any liquors which are to be returned
to the leaches even in the tannage of dressing leather, though its
presence in the colouring liquors is otherwise very useful in lessening
the astringency of the tannins (“mellowing the liquors”), and making a
fine grain. Its mode of action is by no means clearly explained, but is
in some way connected with its tendency to produce “conjugated acids”
(L.I.L.B., pp. 37, 46).
The so-called “mellowness” of old liquors requires a word of comment. It
is well known to practical tanners, that old liquors are much less
liable to produce drawn grain, and a harsh surface, when used to colour
green goods, than liquors, even equally weak, which have been made from
fresh materials. This is probably due, in part at least, to more than
one cause. Most natural tanning materials contain tanning matters of
varied degrees of astringency and power of attaching themselves to the
leather-fibre. It is obvious that if a tanning liquor is used, the most
astringent and energetic tannins will be first removed from it, leaving
those of a milder character. It is also known that the presence of
neutral alkaline salts of weak acids has considerable influence in
producing mellowness; the addition, for instance, of sodium acetate has
a marked effect. This effect is probably due in the first place to the
action of neutral salts in diminishing the energy of weak acids (see p.
81), and secondly to the fact, that their bases combine to some extent
with the tannins; and that, as was perhaps first pointed out by the
writer, such tannins are, as it were, partially paralysed in their
action on hide (p. 339). Sodium sulphite acts powerfully in this way,
and may perhaps prove of technical value in temporarily diminishing the
astringency of liquors in quick tannage. Borax has a similar effect, but
is too alkaline, and, unless used with extreme caution, spoils the
colour of the liquors by causing oxidation. It is probable that similar
causes explain the mellowness of palmetto extract, which contains large
quantities of alkaline salts, and of some extracts which have been
treated with sulphites, when used undiluted in drum tannage. The
addition of free acid will generally restore these tannins to an active
condition.
As the tannage proceeds and penetrates further into the hide, the
liquors are used stronger, as the outside, once tanned, is to a large
extent protected from their action, and it is only by continuously
increasing the strength of the liquors that more tannin can diffuse into
the interior, since diffusion only takes place from a stronger into a
weaker liquor. The liquor in the interior of the butts is always
exhausted of tannin so long as any part of the hide-fibre remains
untanned, but as the layer of tanned fibre between this and the outside
gets thicker, a greater difference is required to maintain a reasonable
rate of exchange, just as a greater head of liquor is required to
maintain a flow of liquor through an increased number of
percolation-leaches. If the strength of the liquor outside be allowed to
fall off, this graduation of strength from the outside to the inside of
the butt is disturbed, and takes some time to re-establish. As the
liquors become stronger in tannin, they may also become somewhat
stronger in acid, since, as has been stated, the two act to some extent
in opposition to each other. The acid-swollen fibre absorbs the tannin
more slowly than if it were in more neutral condition, but it absorbs it
apparently in larger quantity, and at any rate, makes a firmer, solider,
and less flexible leather.
It has been mentioned that in the latter stages of the process, solid
tanning materials are generally strewed between the butts in the tanning
liquor. It may be pointed out that many materials vary in their tanning
effect, according to whether they are used in solid form or merely in
liquors. It has been shown by Youl and Griffith[126] that such materials
as valonia, oakwood and chestnut extracts, and myrobalans, which contain
both gallotannic and ellagitannic acids, lose strength rapidly when kept
in the form of liquor, the ellagitannic acid becoming decomposed with
separation of insoluble ellagic acid. Now it is just this ellagic acid,
which deposited _in_ or _on_ the leather, gives weight, solidity and
bloom, and the investigation points out not only an important source of
loss in the tanning industry, but also, why valonia, which in
sole-leather tannage is known to give hard and heavy leather, can be
used in large quantities on dressing leathers in Yorkshire, with
gambier, in the form of liquor, giving a soft and mellow leather almost
destitute of bloom. If weight and solidity are required from the use of
such materials, it is obvious that they must be brought into immediate
contact with the leather to be tanned, so that as large a part of the
bloom as possible is deposited in, and not outside the leather. With
many other materials, such as hemlock, quebracho, and mimosa, which
yield no bloom, but “difficultly soluble” tannins (reds or
phlobaphenes), the same rule holds, since in contact with the hides, the
small proportion of these materials which is soluble in the liquors, is
replaced from the materials as rapidly as it is absorbed by the leather,
while, when liquors or extracts only are used, the greater part of these
solidifying and weight-giving constituents remain unutilised in the
spent tanning materials. At the same time, the long “layers” afford an
opportunity for the acetic and lactic fermentations to go on which are
the principal source of the natural acidity of liquors. It must be
understood that what are called layers in England, are not to be
identified with the _Sätze_, but rather with the _Versenke_ of the
German tanner, the former being layers given in much the same manner as
was current in England 150 years ago; in which the leather, with thick
layers of tanning material between it, is laid into the empty pit, which
is afterwards filled up with liquor, often of a comparatively weak
character. In such layers, the acidification, and the solidification of
the leather both go on to a still greater degree; the acid formed,
apparently gradually penetrating to the heart of the leather-fibres, and
producing a solidity, and cheesy texture which can hardly be obtained by
layers of the English kind; which nevertheless have the advantage in
rapidity and cheapness.
[126] Journ. Soc. Chem. Ind., 1901, p. 428.
In drying sole-leather, one of the great objects which must be aimed at
is to remove the dark coloured liquor, with which the goods are
saturated, from the surface, and to prevent further portions of it from
finding their way there from the interior. If a strip of filter-paper be
allowed to rest with one end in a basin containing a little liquor, and
be placed in a draught of air, the exposed end of the paper will rapidly
become dark brown or black, the liquor which evaporates there being
constantly replaced by fresh portions sucked up by capillary attraction
from the basin. A similar action is constantly seen, when filtering
liquors through paper, if the latter be allowed to project above the
edge of the funnel. Precisely the same effect occurs, perhaps increased
by the oxidation of the tannins, on the edges and other parts of a butt
which are most exposed to draughts of air. The use of oiling the grain
is not only, to a certain extent to protect it from oxidation, but also
to check evaporation, and the consequent accumulation there of the
dark-coloured solids contained in the liquor. A very similar result is
attained by wetting the grain-side, and allowing as much of the
evaporation as possible to take place from the flesh.
The process of sole leather tanning has been discussed in considerable
detail on account of its simplicity and importance. It is now time to
point out in what respects the tannage of the lighter leathers differs
from it in principle. Taking the case of ordinary dressing leathers,
such as kips and shaved hides, the first point to remember is that these
goods come into the liquors not merely almost entirely deprived of lime
by bating, but in a very flat and fallen condition from the action of
the bacterial ferments of the bate. As a general rule in this country
the colouring is done in paddles, but where a very smooth grain is
required, the use of suspenders is to be recommended, and in America is
largely adopted. Indeed in the States the entire tannage of much of the
cheaper leather is done in suspension, and the sides are only removed
from the laths to which they have been nailed, when they are required
for splitting. It is obvious, from what has been said of sole leather,
that as the hides are brought into liquors in a very fallen and extended
condition, the grain will be likely to be wrinkled; and indeed this is
sure to be the case unless, by suspension, the hide is more or less kept
in tension till its fibres are fixed by tanning. The free motion in the
paddle favours the formation of a “pebbled” grain, since the hide is
bent now this way, now that, and minute wrinkles and creases are formed
in all directions. For many purposes, and especially if a grain is
afterwards to be raised by “boarding” the curried leather, this graining
in the paddle is not disadvantageous, so long as it is not excessive. In
some other cases it causes much trouble and labour to the currier before
it is removed, and if the English tanner and currier are ever to compete
with the American in smooth grain finishes, it will be necessary for
them to obviate this source of wasted labour. The graining is the less
considerable, and the easier to remove, the weaker and more mellow are
the liquors employed in colouring and the more gradually their strength
is increased.
The production of a soft leather depends on the fibre being tanned in a
fallen and unswelled condition. It is for this reason that bating is in
many cases essential, though where somewhat firmer leathers are
required, mere reduction of the swelling by removal of the lime is
sufficient. For the same reason, no acid-swelling is permissible either
before tanning, or in the liquors, and though liquors for soft leathers
must be rather acid than alkaline, they are incapable of removing any
large quantity of lime, and for the best results, the deliming must be
complete before tanning. As mere bating or puering is mainly designed to
reduce swelling by the action of bacterial products (p. 172), and is not
a very efficient means of removing lime, it is desirable where it is
employed, to supplement it by some more active deliming process. In the
lighter leathers, drenching (p. 166) generally fulfils this purpose and
many of the more intelligent tanners now give bated hides a bath in
boric acid before tanning, which not only removes the last traces of
lime without acid-swelling, but checks the bacterial fermentation, and
prevents its introduction into the liquors. In gambier tannages, a
decidedly better colour is obtained by this treatment (p. 228).
In most cases the production of bloom is not desired in dressing leather
tannage, and is prevented by relying chiefly on liquors, and avoiding
the use of bloom-giving solid materials, which include most pyrogallol
tannins. Dressing leather tannages can frequently be advantageously
hastened by drumming: which by continuously bending the leather in all
directions, constantly widens and contracts the spaces between the
different fibres, and, as it were, pumps the liquor through the skin.
The softness of dressing leathers is increased, and the hardening action
of acids present in the liquors is prevented by the addition of salt, or
of some sulphates (sodium, magnesium, ammonium) which exercise a sort of
pickling action on the fibre, and prevent its swelling, but at the same
time tend to light weight and a somewhat empty tannage. It by no means
follows that a hide or skin which is thoroughly coloured through, is
really fully tanned; as, though the fibres may be actually tanned or
coated on the surface, time is required for the tannins to penetrate
them to the centre. This incompleteness of saturation is often found in
drum tannages. Such leathers are generally tough, and gain weight and
softness in currying. In order to “carry grease” well, that is, to
absorb a large quantity without appearing greasy, it is essential that
the fibre-bundles should be thoroughly split up or differentiated; and
the degree to which this is attained largely depends on the extent of
liming. There is also considerable difference in different tannages, as
to the amount of grease which they will carry.
It is now not uncommon to combine a degree of alum or chrome tannage
with vegetable tannage in the finer dressing leathers. For further
information on this the reader must be referred to the next chapter.
The finest sorts of leather such as goat, calf, sheep and seal for
bookbinding, upholstery and the like, are mostly tanned with sumach;
paddles and drums being largely used to quicken the operation. Leather
tanned with sumach has been proved by the researches of the committee of
the Society of Arts on the decay of bookbinding leathers[127] to be the
most durable leather for this purpose, some other tanning materials of
the pyrogallol class coming near it in this respect, while all catechol
tannages are found peculiarly liable to destruction by the action of
sunlight, dry heat, gas fumes, and traces of sulphuric acid from other
sources, although in many cases more durable than the pyrogallol
tannages when exposed to mechanical wear and moisture, as is the case
with shoe-leather (p. 298). East India sheep- and goat-skins (so-called
“Persians”) are tanned with the catechol tannin of turwar or cassia
bark.
[127] Soc. of Arts Journ., 1901, p. 14.
[Illustration: FIG. 42_a_.--Interior of Light Leather Tannery.]
The finer leathers of which we are now speaking are almost invariably
prepared for tanning by puering with dog-dung, and drenching with bran,
as colour and softness are the special characteristics aimed at. A
somewhat interesting style of tannage is occasionally used for
sheep-skins (roans), and calf-skins, in which the skin is sewn into a
bag, flesh side out, with only a small aperture left for filling at one
of the shanks. It is then turned grain-side out, and filled with strong
sumach liquor, and a little leaf sumach, and floated in a bath of warm
sumach liquor. After a short immersion, the skins are piled on a stage,
so that the liquor is pressed through them by their weight; and when
partially empty, they are refilled and the process repeated. The time of
tannage is very short, not exceeding about twenty-four hours, and the
leather produced is very soft.
CHAPTER XVII.
_COMBINATION OF VEGETABLE AND MINERAL TANNAGE._
In very early times leathers were produced, which were partly tanned
with alum, and partly with vegetable materials. One of the earliest of
these was probably the Swedish or Danish glove-leather. The principle
has long been applied to the production of certain very tough and
flexible leathers known as “green leather,” and used for “picker-bands”
for looms, laces for belting, “combing-leathers” and some other purposes
where softness and toughness are of principal importance. About
twenty-five years since, it was applied in America by Mr. Kent to the
manufacture of an imitation of glazed kid, which he named Dongola
leather; and since that time, the method in various modifications, has
taken a considerable place in the manufacture of the finer leathers for
shoe purposes, especially in the United States.
Alum-tanned leathers, as has been already stated, are remarkable for
softness and toughness, and the mineral (crystalloid) tannages have the
power of penetrating and isolating the individual fibrils of the skin in
a much greater degree than the vegetable tannins, and hence are less
dependent than the latter on a previous isolation produced by liming. On
the other hand, they give much less plumpness and solidity, and more
liability to stretch, and are less resistant to the action of water; and
are, as a general rule (to which some chrome-tannages are an exception),
incapable of producing a soft leather without mechanical softening
(staking) after the tannage is completed. Purely mineral tannages have
always a woolly fibrous structure, and never the firm and compact flesh
which is required in leathers which are to be “waxed,” or finished on
the flesh side to a smooth surface, and as they communicate more or less
of these peculiarities to combination-tannages, the latter are mostly
used, either for grain-finish, or for uses where a soft and velvety
flesh-side is required, as in the case of “ooze-” or “velvet-” calf. On
the other hand, the partial use of vegetable tannage communicates to
them a degree of plumpness, fulness and resistance to water which is not
possible to alum-tannages pure and simple, and a softness which is not
easily obtained in vegetable tannage without the use of large quantities
of fats or oils. A preliminary mineral tannage also greatly increases
the rapidity of the penetration of the vegetable tans, by isolating the
fibres, and rendering them less gelatinous. Once a leather is
_thoroughly_ tanned by vegetable materials, it is little affected by
subsequent treatment with alumina, or even with chrome; and on the other
hand, though chrome and alumina leathers are still capable of absorbing
considerable quantities of vegetable tannins, they always retain, in a
degree, the qualities which the mineral tannage has communicated to
them. The resulting leathers are thus not only modified by the different
proportion of vegetable and mineral tannages which have been given, and
by the properties of the particular vegetable tannage used; but by the
order in which the several treatments have been given, and always
retain, to a considerable extent, the characteristics of that which has
been first applied. We have thus in our hands a powerful means of
modifying the character of our leather to suit the special requirements
which it is to fulfil.
So long as tanners were restricted on the one hand, to the ordinary
methods of stuffing tanned leathers with oils and fats, and on the other
to the use of egg-yolk, which had long been common in alum-tannages,
combination-tannage remained of but secondary importance, and it was the
application of the method of “fat-liquoring” by James Kent to his
Dongola leather, which gave them the place they now possess, by
providing a cheap substitute for egg-yolk, and enabling the tanner to
obtain softness and resistance to water, without producing the greasy
feel which is common to curried leathers. The process of fat-liquoring
has already been mentioned in connection with chrome leathers, to which
it was subsequently applied, and we shall return to it, after having
given some further details of the methods of tannage.
In the first place we must consider briefly the mutual action of the
mineral and vegetable tannages on each other. It has been pointed out
by Eitner, and also mentioned (p. 339) in connection with the
decolorisation of extracts, that the addition of say ¹⁄₂ per cent. of
alum, or aluminium sulphate to tanning liquors, lightened their colour,
not only by giving a degree of acidity to the solution, but by
precipitating a portion of the less soluble and more darkly coloured
tannins. Chrome-alum, and basic chrome salts produce a similar effect,
though from their marked colour, the lightening of the solution is not
so easily observed. It is therefore advisable if these salts are to be
used in actual mixture with the vegetable tans, to allow the solution
time to subside, or to filter off the dark-coloured precipitated
matters. Larger quantities than ¹⁄₂ per cent. of the alum do not appear
materially to increase the effect just described.
A second effect produced by these mineral salts on vegetable tannins, is
in many cases to develop mordant colouring matters which are present;
and thus, since most of these colouring matters are yellow, to produce a
yellower leather than would be obtained with the vegetable material
alone. This effect is very marked in the cases of sumach, gambier and
quebracho. The compounds which these colouring matters form with chrome
are mostly of a darker shade than those with alumina, tending to olive,
and therefore chrome-combination leathers are generally dull in colour.
Potassium dichromate, especially if acidified, generally oxidises and
precipitates tannins, and darkens their colours, so that it is not
practical to follow a vegetable tannage by the two-bath chrome process;
and though the reverse order may be pursued, the single-bath chrome
process, and that following and not preceding the vegetable tannage,
generally gives the best results. If lightly tanned leathers, such for
instance as the imported East Indian tannages, with babool or turwar
barks, be treated with a basic chrome tanning liquor, such as is
described on p. 215, so large a proportion of chrome will be absorbed,
that the leather will possess most of the characteristics of a genuine
chrome leather.
Combination-tannages for glove-leathers, such as the Danish and Swedish
leathers already alluded to, are generally first tawed with alum and
salt, with or without addition of flour and egg-yolk, and are then
coloured, and more or less tanned with vegetable materials. That
employed on the original Danish leather was willow bark (of _Salix
arenaria_). In France, where this willow is not found, the bark of the
commoner _Salix caprea_ was substituted; and as it is much weaker in
tannin, additions of oak-bark or sumach to supply the deficiency, and of
madder to give a redder colour were made to it. The dyeing of these
leathers is frequently combined with the tannage, dyewoods or dyewood
liquors being mixed with the tanning liquors. In the manufacture of
glazed French kid, indeed, the process is so arranged, by brushing on
dye-liquors mixed with tannins, as merely to tan the grain-surface,
which is necessary to enable it to be glazed by friction, leaving the
substance of the leather of purely alum tannage.
On the other hand, in the “green leathers” (so-called from their
greenish-yellow colour, and largely made in the West Riding of
Yorkshire), the hides usually receive a light gambier tannage, extending
over a week or so in weak gambier liquors in handlers, and are then
“cured” by handling in hot and strong solution of salt and alum, in
which they are finally left all night, and then dried rapidly without
washing out the alum, much of which consequently crystallises on the
surface. This is slicked off, and the leather damped back, and heavily
stuffed with sod-oil. If, however, the combination-tannage is properly
carried out, it will stand liberal washing without losing the necessary
alum, and of course a tougher and more satisfactory, though somewhat
lighter weighing leather results. It is in many cases a better plan to
combine the two tannages in one bath, mixing the alum and salt with the
gambier, and handling or paddling the goods in the mixture. This is the
plan usually adopted for Dongola leather, in the United States. For
skins which are to be glazed, it is important that the _surface_ should
be tanned with the vegetable material, and the goods are therefore
worked into gambier liquors, to which the salt and alum are only added
after the tannage has made some little progress; while for dull Dongola,
intended rather to imitate calf-kid, it is best for the alum and salt
tannage to begin first. For goat-skins for glazed Dongola kid, about 4
lb. of block gambier, ¹⁄₂ lb. of alum, and ¹⁄₄ lb. of salt are used per
dozen, and the tannage occupies in all about twenty-four hours.
After the skins are tanned, they are thoroughly washed out with tepid
water, to remove loose alum and gambier, and are then ready for
fat-liquoring. As in the case of chrome leather, it is of great
importance that this washing should be done thoroughly, as any remaining
alum which diffuses into the fat-liquor, will cause it to curdle. If the
washing is thorough, the more neutral the fat-liquor and the better; but
a somewhat alkaline soap-solution is less liable to curdle. The original
fat-liquor used by Mr. Kent was the alkaline liquor which had been used
in washing the surplus oil from chamois leather (see p. 380), but now
soap- and oil-solutions are generally made specially for the purpose.
Most of the remarks in the chapter on chrome tannages are applicable in
this case, but probably fat-liquoring is somewhat easier than in the
case of chrome. Mixtures of either soft soap or curd soap with cod, sod,
and olive oil are frequently employed. Sesame oil also seems well
adapted for the purpose. The better these are emulsified, and the more
satisfactory is the result; a cylinder of zinc or copper fitted with a
plunger, something like that of a “Lightning Egg Beater,” but covered
with perforated zinc, or wire gauze, does very good service as an
emulsifier on a small scale.
Another method is to melt the soap with just sufficient water to make it
pasty, and to incorporate the oil thoroughly with the mass, which is
afterwards dissolved in hot water. Oils are the most easily emulsified
when they are somewhat acid. For this reason rancid olive oils are often
used for the Turkey-red process, but a similar effect can be obtained by
adding a small quantity of candlemaker’s oleic acid to the oil before
mixing. The addition of sulphated castor oil (Turkey-red oil) also helps
emulsification, and is in itself a very good softening agent. One of the
commonest mistakes in fat-liquoring is the employment of too strong an
emulsion--even so small a quantity as ¹⁄₂ per cent. of soap, and half
that quantity of oil, reckoned on the wet weight of the well-drained
leather, will produce a very notable softening effect. Of course, for
dull finishes, much larger proportions may be used. Not only
combination-tannages, but those entirely vegetable, can be fat-liquored
with excellent effect, and the process is now largely used for coloured
calf, and other leathers, which are required soft and nourished, but
without any appearance of greasiness. Leathers absorb the fat-liquor
most readily if put into it in a sammed or partially dried condition,
but even if quite wet, they soon take up the whole of the oil and soap
on drumming, leaving only a little clear water in the drum. Goods may
be blacked while still wet with fat-liquor, but should generally (except
in the case of chrome leathers) be dried out before dyeing, as this
fixes the oil and soap in the fibres.
Many coloured leathers are now made by a process which may be considered
a combination of the Dongola process itself with the ordinary process of
vegetable tanning, the goods being coloured and partially tanned as if
for a vegetable tannage, and then finished in Dongola liquors with alum,
salt and gambier. Very good leathers are made in this way in the United
States, with a tannage begun in suspension in hemlock bark liquors.
Imitations of Dongola leather are made by treating East India sheep or
goat with alum liquors, and afterwards fat-liquoring (if necessary)[128]
and finishing like genuine Dongola leather. The treatment is most
effective, if a portion of the original tan be removed by washing with
warm water, with a little borax, ammonia, or even soda, and the goods
then alumed with a “neutralised” or basic alum solution such as that
described on p. 187. Goods treated with a basic chrome-liquor, like that
used for the one-bath chrome process, p. 212, are almost converted into
chrome-tanned goods, and will even stand some degree of boiling. The use
of a liquor made like the Martin-Dennis liquor, by dissolving chromic
oxide in hydrochloric acid, was the subject of an American patent[129]
which in this country is owned by Wichellow and Tebbutt, but which
expires in 1903.
[128] East India sheep and goat are generally so heavily oiled with
sesame oil (up to 30 per cent. of their weight), that it is desirable
in many cases rather to diminish than increase the oil, which may be
done by washing with soap solutions, preferably before aluming.
[129] Eng. Pat. Jensen 13126, 1889.
Chrome-combinations may also be made by retanning goods tanned by either
of the chrome processes with vegetable materials, of which gambier seems
the most suitable. The use, even of very weak liquors of sumach and most
other tanning materials, deprives chrome leather of its stretch, and if
carried to excess, readily makes it hard and tender.
CHAPTER XVIII.
_VEGETABLE TANNING MATERIALS._
As has been stated in the previous chapter, our knowledge of the
chemistry of tannins is not sufficiently advanced to render possible any
strictly chemical classification, while an additional complication
arises from the fact that very different tannins may coexist in the
wood, bark, fruit, galls, etc. of the same plant. It therefore seems
best to follow the example of Prof. Bernardin in his ‘Classification de
350 matières tannantes,’[130] and arrange the plants under the orders of
the natural system of botany, as has already been done by von
Höhnel[131] and A. de Lof.[132] In the following pages, only those
materials which from their high percentage of tannin, or from some other
cause, are of commercial interest or value, are included, as the tannins
are so widely distributed in the vegetable kingdom, that any exhaustive
list would be quite out of the question.
[130] Gand, 1880.
[131] ‘Die Gerberinden,’ Berlin, 1880.
[132] ‘Matières tannantes,’ Halle aux Cuirs, Paris, 1890. See also
‘Agricultural Ledger,’ 1902, No. 1 (Government Printing Office,
Calcutta, 6_d._), by Mr. D. Hooper, which contains much valuable
information.
Tannins are not confined to any particular part of the plant, though
they are usually most abundant in barks and fruits. Insect-galls are
often very rich in tannin, usually gallotannic acid; while in several
cases woods are of commercial importance from their cheapness, though
the percentage of tannin they contain is not generally high. The
function of tannins in the vegetable economy is not well understood. In
some cases they are probably a waste product of plant-life, and may help
to ward off attacks of insects. They usually exist as cell-contents, and
as vegetable cells have frequently thick and impermeable walls, and the
diffusive power of tannins is low, much time is required for extraction,
unless the cells have been previously crushed or broken.
It would be beyond the scope of this text-book to describe in detail the
structures of the tannin-yielding parts of plants; but barks are of such
general importance, that some particulars seem desirable.
The detailed structure of bark varies greatly in different trees, though
its general principles remain unaltered. One of the best short accounts
of these is given by Prof. H. Marshall Ward on page 199 of his little
book on ‘Timber and some of its Diseases,’[133] and further information
may be found in Van Tieghem’s ‘Traité de Botanique’ and other works on
structural botany.
[133] Macmillan & Co.
With regard to detailed structure of various tanning barks, ‘Die
Gerberinden’ of von Höhnel[134] is one of the best authorities.
[134] ‘Die Gerberinden,’ Berlin, 1880.
The inner surface bark of a young tree, or twig, consists of a layer of
soft and living cells resting on the outer surface of the wood, and
called the _cambium_. These cells multiply by division (cp. p. 12) and
produce from their inner surface the successive annual layers of wood,
and on their outer a fibrous tissue called the bast (_phloem_),
consisting of lengthened cells, and tubes with perforated divisions
(sieve-tubes) which convey sap, and mostly run in the direction of the
branch, but are crossed transversely by cells in a line with the
medullary rays of the wood. All these cells when first produced in the
cambium-layer have thin and soft cellulose walls, but the inner layer
forming the wood becomes lignified, or hardened, by deposits of lignine
on the interior of the cell-walls, while their contents of living
protoplasm disappear. The outer layer forming bast remains much softer
and more fibrous, and retains its vitality longer. The outer surface of
the young branch is covered by a thin layer of flat cork-like cells
forming the epidermis, developed from the growing tissue of the bud,
beneath which is a layer of growing cells frequently called the
_cork-cambium_. This produces, on its inner side, a layer of soft,
juicy, thin-walled cells (_parenchym_), which are living and capable of
growth, and contain protoplasm and often chlorophyll, to which the green
colour of young twigs is due. This layer at first rests on the bast. On
the outer side, the cork-cambium produces corky layers beneath the
epidermis. The section of an oak-twig is shown in Fig. 43.
[Illustration: FIG. 43.--Section of Oak Twig, drawn by Prof. Bastin:
_c_, corky layer; _t_, tannin-cells; _St_, stone-cells; _Ca_, cambium;
_Mr_, medullary ray; _P_, pith.]
As the tree grows, it is obvious that the corky epidermis which grows in
thickness, but not in breadth, must become distended and finally
ruptured. In some cases the surface is renewed by fresh corky layers
constantly developed below it, and then the bark remains smooth and
unfurrowed, as in the beech and young oak, or in the birch, from which
thin corky layers are continually peeling; or it may produce a thick
layer of cork, as in the cork-oak. In many cases, and especially in
older trees, the outer or primary layer of cork-cambium ultimately dies
for want of nourishment, and a fresh cork-producing layer is developed
in the still living parenchym. As cork is practically air- and
water-proof, the new layer cuts off from its source of nourishment and
kills all the parenchym exterior to it. In some cases this peels off, as
in the Oriental plane (_Platanus_), but usually it forms a constantly
increasing coat of dead tissue forming the “ross” or “crap” (Ger.
_Borke_), which as it cannot increase in breadth, becomes deeply
fissured as the tree becomes old. In some cases the new growing layer or
secondary cork-cambium forms a complete coating parallel with the first,
but more often it consists of a series of arcs convex towards the tree
and cutting the primary cork-cambium at various places, so as to divide
the tissue outside itself into scales. Later on the process repeats
itself, new arcs forming inside the first, and cutting off further
portions of the parenchym. In this way the cork-forming layer gradually
sinks deeper and deeper into the bark, till it frequently passes even
into the bast-layer, and very complicated arrangements of tissue result,
in which corky layers from the secondary cork-cambium are interspersed
with bast-cells and sieve-tubes.
As a rule the outer and dead part of the bark contains but little
tannin, though to this there are exceptions, as, for instance, in the
hemlock and Aleppo pines. It always contains a large proportion of dark
colouring matters (reds, phlobaphenes, p. 297).
Cork consists of thin, and often roughly cubical cells, which are filled
with air, while tannin is usually contained in somewhat similar cells
with thicker walls. The walls of many vegetable cells are perforated
with fine holes, and become thickened by internal deposits of hard
ligneous matter which sometimes almost fill the entire cell
(“stone-cells”). Bark-cells often contain starch-granules, frequently of
peculiar and characteristic forms (which are easily recognised by the
blue colour produced on treating the preparation under the microscope
with a drop of a solution of iodine in potassium iodide), as well as
crystals of oxalate of lime and other matters. These, and the form and
arrangement of the cells as seen in sections under the microscope, form
useful marks of recognition of the various barks. Tannin is most easily
detected by staining, before cutting sections, with a solution of ferric
chloride in absolute alcohol.
Apart from microscopic characteristics, the external appearance of
barks, both to the naked eye and by the aid of a lens, forms a valuable
means of recognition. The arrangement of the bast and corky layers, the
remains of epidermis, or the form and character of the fissures, and of
the lenticels or small corky protuberances which take the place of
stomata in the epidermis, should be observed.
Space does not permit of any detailed account of the structure of
fruits, wood and leaves, which are also cellular structures in many
respects resembling the bark. The cuticles of leaves, and especially the
stomata or breathing pores, and the hairs are often very characteristic.
(Cp. Plates III. and IV., and p. 272.)
Valuable hints may also be obtained from the chemical reactions which
are described on p. 70 _et seq._, L.I.L.B.
BOTANICAL LIST OF TANNING MATERIALS.[135]
[135] The percentage of tannin given where the source of information
is not stated must in many cases be regarded as uncertain, many
analyses having been made before the introduction of modern methods,
but those quoted as having been done in the author’s laboratory are of
recent date and have been made by the latest methods.
CONIFERÆ, Pines, Cypresses, mostly containing Catechol tans, yielding
reds.
_Abies excelsa_, Lam. (_Pinus Abies_, _Pinus Picea_, _Picea vulgaris_,
Link.), Norway Spruce. Fr. _Faux sapin_; Ger. _Fichte_, _Rottanne_. The
source of the so-called larch-extract, and a principal tanning material
of Austria. Contains 7-13 per cent. of a catechol-tannin and much
fermentable sugar, and on this account is useful for swelling and
colouring, but does not tan heavily. English and Scandinavian bark does
not seem much utilised. Best bark 2-8 mm. thick; smooth, yellow inside,
with reddish-brown ross outside. For detailed description of structure
see von Höhnel, ‘Die Gerberinden,’ p. 35.
_Abies pectinata_, Silver Fir. Fr. _Sapin_; Ger. _Edeltanne_,
_Silbertanne_, _Weisstanne_. Used to a limited extent, but apt to be
confused with spruce. Contains 6-15 per cent. iron-blueing tannin. Used
in Styria, Austria, Russia. Without “ross,” but silver-grey and smooth
outside. (Von Höhnel, ‘Die Gerberinden,’ p. 40; ‘Gerber,’ 1875, p. 375.)
_Abies (Pinus, Tsuga) canadensis_, Hemlock Fir (Fig. 44). The principal
American tanning material, and source of hemlock extract; averages 8-10
per cent. of a catechol-tannin, but variable, 18 per cent. reported,
possibly from a different species. Abundant in Canada and the Northern
and North-western States of America. The bark of old trees, which is
principally used for tanning and extract-making, is 2-4 cm. thick,
smooth and yellow within, greyish and deeply fissured without. The
ross, which is red and thick, contains a considerable quantity of
tannin, with much dark-red phlobaphen. It does not differ in structure
from the inner living and yellow “flesh.” The bark is easily
recognisable by its well-marked concave lamellæ of cork, cutting off
successive layers of “ross” of several millimetres in thickness. (Von
Höhnel, _ibid._, p. 42.)
[Illustration: FIG. 44.[136]--Hemlock Fir (_Tsuga canadense_).]
[136] Bastin and Trimble’s American Coniferæ, American Journal of
Pharmacy.
_Abies alba_ (_Picea alba_), White Spruce, North America. In character
of tree and bark very similar to Norway spruce.
_Larix europæa_ D.C. (_Abies_ or _Pinus Larix_), Larch. Fr. _Mélèze_;
Ger. _Lärche_. Contains 9-10 per cent. pale catechol-tannin, mild and
suitable for light leathers. Used, especially in Scotland, for basil
tannage.
_Pinus halepensis_, Aleppo Pine. An important tanning material of the
Mediterranean coasts. The outer bark, stripped like cork from the living
tree (_Scorza_ or _Cortegia rossa_), is a deep red tannage, and contains
about 15 per cent. of tannin very similar to hemlock. It is largely used
in the island of Syra. The inner and fleshy part of the bark, only
obtained when the tree is cut, is _Snoubar_ or _Snobar_ bark, containing
up to 25 per cent. of lighter coloured tannin. This bark is reddish
brown, and pretty smooth on both sides, except for shell-like
depressions on the outer surface. The “scorza rossa” is dark red-brown
internally, grey and irregular outside, frequently very thick, and
divided into successive layers of 1-2 mm. thickness by cork lamellæ.
(Von Höhnel, _ibid._, p. 44.) In appearance the tree resembles the
Scotch fir.
_Pinus tæda_, America; _P. Laricio_, Austrian Pine; _P. maritima_,
Mediterranean; _P. Cembra_, Alps, Tyrol, 3-5 per cent.; _P. sylvestris_,
Scotch Fir. Ger. _Kiefer_; Fr. _Pin sauvage_, 4-5 per cent. _P.
longifolia_ Roxb., India, 11-14 per cent.
_Juniperus communis_, Juniper. Bark used in Russia.
_Podocarpus elongata_ and _Thunbergii_, Cape of Good Hope; _Geelhout_,
Yellow woods.
_Phyllocladus trichomanoides_, New Zealand; _Tanekahi_, _Tarsekahi_,
_Kiri-toa-toa_, “Golden Tan.” Used in dyeing glove-leather. Tannin, 30
per cent., gives green blacks with iron.
_P. asplenifolia_, Tasmania, Celery-topped Pine; 23 per cent.
_Phyllocladus_ belongs to Yew family.
LILIACEÆ.
_Scilla maritima_, Squill. Tannin stated from 2-24 per cent. More
valuable for pharmacy.
PALMÆ.
_Areca catechu_, Betel-Nut Palm of India. Yields a species of cutch of
no importance for tanning.
_Sabal serrulata_, Saw Palmetto of Florida (Trimble). (“Dwarf” palmetto
is _S. Adansonia_.) Palmetto root has been much talked of as a tanning
material; and makes a light-coloured leather.
An extract is now made from the roots of the Saw Palmetto, which grows
freely in the Southern States of America, and is especially abundant on
the east coast of Florida. The plant is an evergreen, the stem growing
flat along the ground, being held in place by numerous roots each the
size of a pipe-stem. The leaves are fan-shaped and ribbed, and two to
three feet in diameter. In its hardihood the palmetto resembles a weed,
as the leaves may be cut off quite close to the stem without damaging
the plant, which will grow freely on poor sandy land which is worthless
for other purposes. The average yield is stated to be about 10 cwt. to
the acre, but in good seasons and with rich land, over a ton per acre
has been obtained.
The air-dried leaves contain about 13 per cent. of tannin, but the
results obtained by different chemists vary from 5 to 20 per cent.
Possibly these variations are caused by the different amounts of
moisture in the various samples.
The leaves must be treated with a solution of caustic soda, to remove
the glossy siliceous shield which covers them and prevents their being
easily extracted. After the tanning matter has been extracted the
remaining fibre can be profitably disposed of to paper and rope
manufacturers.
As the supply of palmetto is very large it is likely that it will, to a
considerable extent, substitute the employment of gambier, and in the
United States the extract has already met with a considerable sale.
Samples of the extract examined by the Author analysed from 16-22 per
cent. of tanning matter, and several per cent. of mineral matter, and
produced a very soft and mellow leather of good colour. The extract
contains noticeable quantities of common salt, and organic salts of soda
which leave sodium carbonate on ignition.
_Cocos nucifera_, the Cocoa-nut Palm, also contains tannin in roots.
CASUARINÆ.
_Casuarina equisetifolia_ L. (_laterifolia_ Lam.); _Filao_ bark,
Reunion; _Tjamara laut_, Java; Casagha or Tinian Pine, Ceylon. Widely
distributed in Southern Asia, bark used for tanning and dyeing. Tannin
gives blue-blacks with iron. Several other species very similar in
structure and properties. (Von Höhnel.) Hooper found 11-18 per cent. of
tannin.
MYRICACEÆ.
_Myrica Gale_, Sweet Gale, or bog-myrtle.
_Myrica (Comptonia) asplenifolia_, U.S.A.; “Sweet Fern.” Covers millions
of acres in Michigan. Yields 40 per cent. of “extract.” Leaves 4-5 per
cent., roots 4-6 per cent. tannin, according to season (Trimble). Has
been much talked of, but in Prof. Trimble’s opinion is not likely to
prove of much importance.
_Myrica nagi_ (Hind. _Kaiphal_), India, contains 13-27 per cent. of
tannin in the bark, and a colouring matter, myricetin, identical with
that of sumach.[137] Leather tanned with it is of a somewhat reddish
colour which is much brightened by sumaching, and converted into a pale
yellow by treatment with alum. It promises to be a valuable tanning
material.
[137] Perkin and Hummel, Trans. Chem. Soc., 1896, p. 1287.
BETULACEÆ.
_Alnus glutinosa_, Common Alder. Fr. _Aulne_; Ger. _Erle_. Contains
16-20 per cent. iron-green tannin, with much red colouring matter; old
barks as low as 10 per cent. Colour develops during and after tannage.
Used alone it gives a red, hard and brittle leather, but with galls,
valonia, etc. it produces a satisfactory tannage. Its principal use is
to furnish gunpowder-charcoal, and it is possible the bark might be
obtained from powder-factories, if the use of gunpowder is not
superseded by nitro-compounds. (Von Höhnel.)
_Alnus maritima_, _Hannoki_, Japan; and _A. firma_, _Minibari_. Fruits
(_yashi_) contain 25 per cent. tanning matter (iron-blueing), and little
colouring matter. Used in Japan for dyeing and tanning. _A. nepalensis_
and _A. nitida_ used in India. Several other species of _Alnus_ contain
tannin.
_Betula alba_, White or Common Birch. Fr. _Bouleau blanc_; Ger. _Birke_.
Inner bark used in Scotland (in conjunction with larch for tanning
sheep-skins), Norway, Russia, etc. It contains only 2-5 per cent. of
iron-greening tannin, and much fermentable sugar. By far the most
important use of birch bark in tanning is to produce the birch-bark tar
used to give scent and insect-resisting power to “Russia” leather
(_Youft_; Ger. _Juchten_). The outside bark consists of thin layers of
cork, often white with a crystalline deposit of betulin, which when
distilled yields the odorous oil. The distillation is a dry one, and
tarry products accompany the true oil, and at first give a strong
empyreumatic smell to the leather, which it loses by keeping, while the
true “Russia” odour remains. This “ageing” may be hastened by hanging
the leather in a hot stove. If the oil is distilled in a current of
steam, or with petroleum ether, the tarry matter passes over, while the
matter giving the true odour remains in the retort (p. 372).
_Betula lenta_, American Black Birch. The bark and twigs distilled with
water yield an essential oil, which is almost pure salicylate of methyl,
and largely substituted for oil of wintergreen (_Gaultheria
procumbens_), with which it is chemically identical. Used for perfumery,
and as a rheumatism remedy. Often erroneously spoken of as the source of
“Russia” oil. A mixture of a trace of wintergreen oil with sandal-wood
oil considerably resembles the “Russia” scent (p. 373).
CUPULIFERÆ.
_Castanea vesca_, True or Spanish Chestnut. Fr. _Châtaignier_; Ger.
_Kastanie_. Abundant in Italy, South of France and Corsica, where it
forms great forests. Bark said to be nearly as strong in tannin as oak
(up to 17 per cent., de Lof), but not much used in tanning.
Wood only contains 3-6 per cent. tannin, but is the source of the
valuable chestnut extract, first employed for dyeing, and introduced as
a tanning agent by Aimé Koch. The strength of extract is of course very
variable, even for the same density (see p. 339), but it usually
contains from 28 to 32 per cent. of tannin.
The tannin gives blue-black with iron, but is not identical with either
oak-bark or gall tannins, but apparently a mixture, or possibly a
methylated derivative of the latter, and identical with oakwood tannin,
or so nearly so as to be indistinguishable; it may also be identical
with divi tannin. Decolorised chestnut extracts, sometimes mixed with
quebracho and other materials, are often sold as “oakwood” or
“oak-bark” extracts. The extract gives a firm leather, with a good deal
of bloom if used strong, and a more reddish tint than valonia. The
extract often contains dark colouring matters, and the colour of leather
tanned with it is readily darkened by traces of lime derived from
calcareous waters or imperfectly delimed hides. Like all wood-extracts
it tans rapidly, the colour penetrating first, and the tan following,
but, according to Eitner, it does not, alone, make full or solid
tannage, perhaps from want of acid-forming matters, but answers
particularly well in combination with spruce-bark. It is largely used in
England for sole-leather in combination with valonia, myrobalans and
other materials.
The higher the temperature of extraction, the more colouring matter is
contained in the extract in proportion to tannin matter and the greater
is its viscosity. Much colouring matter remains undissolved if the
extract is dissolved in cold water, but there is, in addition, a loss of
tanning power, the colouring matter being also capable of combining with
hide. It has in fact been used for tanning by dissolving it in solutions
of borax or alkaline salts. By improved methods of manufacture the
colouring matter has been much reduced.
The chestnut is an important food tree, the nuts forming a considerable
part of the food of the inhabitants of Corsica and Sardinia, and even of
Italy.
Oaks.
Almost all species of oak contain useful quantities of tannin in the
bark, and probably in the wood. Most if not all oaks yield
catechol-tannins with, probably, some mixture of ellagitannic acid.
_Quercus robur_, Common Oak. Fr _Chêne_; Ger. _Eiche_. It is frequently
separated into the two subspecies:--
_Quercus pedunculata._ Commonest oak of lowlands, England, Ireland and
Scotland. Acorns in bunches or spikes on a stalk ¹⁄₆ inch long, hence
Ger. name, _Stiel-Eiche_. Leaves sessile or short-stalked. In favourable
situations, _said_ to yield about 2 per cent. more tannin than _Q.
sessiliflora_, but this is doubtful. It is the commonest oak in
Slavonia, and the source of commercial oakwood-extract.
_Q. sessiliflora_, Ger. _Traubeneiche_. Common in hilly districts, and
scattered throughout the country. Acorns in bunch on the branch, or with
very short stalk; leaves on stalk ¹⁄₂-1 inch long.
Of English barks, Sussex and Hampshire are considered the best, and
contain up to 12-14 per cent. of tanning matter; a coppice bark from
Wastdale, Cumberland, is however, recorded to have yielded 19 per cent.
tanning matter (Hellon).
Probably each of the two varieties of oak gives best bark where it
thrives best (v. Höhnel).
Belgian bark is sometimes equal to English, and contains 10-12 per cent.
tanning matter. Dutch bark as exported is generally inferior and not
cleaned; Swedish is bright, but very poor.
Oak-bark contains a tanning matter, quercitannic acid, giving
green-blacks with iron salts, and possibly containing both catechol and
pyrogallol groups, but its constitution is not fully understood. It
yields both red anhydrides and ellagic acid; and gallic acid has been
obtained by the action of hydrochloric acid, though not by fermentation
in the tannery. The tannin is not a glucoside, but the fact that a
sugar, lævulose, is also present in the bark has led some observers to
erroneous conclusions regarding the constitution of the tannin. The
unpurified infusion of the bark of _Q. robur_ gives a blue-black with
iron-salts, from the presence of a colouring matter; but those of most
other oaks give green-blacks.
Most tannin is contained in the living part of the bark. The yield
diminishes in trees over twenty-five years, and coppice barks, from
absence of ross, are often strong, and also contain less colouring
matter and more fermentable sugar.
Warm and rich soils seem to yield the best barks.
The brighter the colour of the fresh cut “flesh,” the better the bark.
Dark brown inner side shows that bark has been exposed to rain, which
deteriorates strength and colour; but a very light colour is thought by
some to indicate poorness in tannin. White lichen is said to be a mark
of poor bark, and probably indicates a damp and unfavourable situation.
Oaks are generally cut when the sap is rising (15th April to 15th June),
and the buds open and new soft cells begin to grow, for the bark is then
more easily detached.
Experiments in France have shown that the bark of timber felled at other
seasons may be loosened by steaming, and it is said there is no
practical loss of tannin. Superheated steam, produced in a small boiler
in the woods, is used.
The bark is peeled with tools of various forms, the branch and knotted
places being loosened by beating with a mallet. The bark must be peeled
immediately the tree has been felled.
The peeled bark, in pieces up to three feet long, is laid on hurdles
sloped in such a way that the rain runs off as much as possible, and in
this way it is dried, but in wet seasons is much damaged. Bark so dried
in the woods often retains 40-50 per cent. water, and must be stacked or
stored so as to allow of further drying.
English bark is sometimes sold in “long rind,” and sometimes “hatched”
or chopped in pieces about four inches long. Belgian and Dutch barks are
generally hatched. Belgian tree bark is “cleaned” (and cleanings often
mixed back with bulk), Dutch bark is not cleaned. Much sand and dirt is
contained in most Continental bark: screenings of Belgian bark yielded a
black liquor, and contained so much sand that they would not even burn!
_Oak-bark extract_ is occasionally offered for sale, but is not usually
genuine or of good quality, except that of the American chestnut-oak,
_Q. prinus_, from which an excellent extract has been manufactured in
the Alleghanies. Factitious extracts often contain myrobalans and
quebracho.
[Illustration: FIG. 45.--Turkey Oak (_Quercus cerris_).]
_Oakwood_ contains only a very small percentage (from 2-4 per cent.) of
a tannin, practically identical with that of chestnut, but different to
that of oak-bark. It is stated by de Lof to reach 9-14 per cent. in old
heart-wood; but this is doubtful. The wood retains the tannin in its
interior for a long time. Wood of a Roman bridge built at Mainz 55 B.C.
is stated by de Lof to have still contained 2·14 per cent. tannin in
1881 A.D. A good deal of imitated oakwood extract is undoubtedly made
from chestnut wood, and unfortunately no very satisfactory way of
distinguishing it is known, though oak-bark extract can be distinguished
from oakwood by giving a precipitate at once, even in dilute solution,
with bromine-water, while the wood gives a brown precipitate only after
long standing. Precipitation by bromine-water is a general
characteristic of catechol tannins, and hence a mixture of quebracho (a
cheap catechol tan) with chestnut would simulate oak-bark in this
respect. If a few drops of the non-tannin solution or an alcoholic
extract from the “total soluble” of extracts containing quebracho or
other catechol tannins be treated with concentrated sulphuric acid in a
test-tube, a deep crimson will be produced, especially at the surface of
the acid, which remains pink on dilution with water. With pyrogallol
derivatives, such as genuine oakwood, a yellow or brown only is produced
(J. Hughes). The test is very delicate. Another distinction is that bark
extracts contain perceptible traces of manganese, but this cannot be
relied on as many wood extracts also contain some, probably derived from
the twig and branch bark which is used along with the wood. Oakwood
extract is now manufactured on an enormous scale in Slavonia, and is
used both by sole- and dressing-leather tanners, chiefly to increase the
strength of the layer liquors. The extract is also used to increase the
weight of leather after tannage by mopping on the flesh-side. All the
best oakwood extract manufacturers contract to sell on analysis and
colour estimation, and good Slavonian oakwood extract generally contains
26-28 per cent. of tanning matter, giving a tintometer measurement of
4-5° red, and 20-25° yellow, when a solution containing ¹⁄₂ per cent. of
tanning matter is measured in a 1 cm. cell. For particulars of the
manufacture of concentrated extracts see p. 337.
[Illustration: FIG. 46.--Cork Oak (_Quercus suber_).]
_Q. cerris_, Turkey Oak. Ger. _Zerreiche_. Common in southern Europe, a
fine tree, but bark inferior to _Q. robur_. Fig. 45.
_Q. pubescens._ Fr. _Chêne velu_; Ger. _Weiss-_ or _Schwarzeiche_. In
mountain districts and scattered in Southern Europe, about equal to _Q.
robur_.
_Q. ilex_, Evergreen Oak. Fr. _Chêne vert_, _Chêne yeuse_; Ger.
_Grüneiche_, _Steineiche_; Span. and Ital. _Encina_. South Europe,
Algeria. Said to be somewhat stronger in tannin than common oak,
yielding 5-11 per cent. of a rather darker coloured tannin, but well
adapted to sole-leather. Good bark is smooth outside, without fissures,
short in fracture.
_Q. Suber_, Cork Oak. F. _Chêne liège_; Ital. _Sughero_, _Suvero_.
(Figs. 46, 47.) The outer bark is cork; the interior bark contains 12-15
per cent. of tannin which is redder than that of ordinary oak. Trees at
first produce an irregular cork, sold as “virgin cork” for ferneries,
etc. After this is stripped, later growths are more uniform, and fit for
use; tanning bark is only obtained when the tree is cut down. Bark is
rough but pale-coloured on both sides and about 1 cm. thick; interior
like ordinary oak, but more strongly furrowed. Produced chiefly on
Mediterranean coasts, and formerly largely used in Ireland.
[Illustration: FIG. 47.--Section of Cork Oak, showing cork, inner bark
and wood.]
_Q. pseudosuber_, African Oak. Fr. _Chêne faux liège_. Algeria. Not
stronger than English oak, but with more colouring matter, hence strikes
quickly through leather. Bark very thick.
_Q. Mirbecki._ Fr. _Chêne Zeen_. Algeria. Rapid growth. Bark contains 8
per cent. of tannin.
_Q. Tozæ._ Fr. _Chêne tauzin_. Pyrenees and S. France. Bark contains 14
per cent. of tannin.
_Q. coccifera_, Kermes Oak. Fr. _Kermes_, _Garouille_ (Fig. 48). South
Europe and Algeria. Root bark is called “rusque” or “garouille”;
averages 10-18 per cent. tannin, but trunk bark does not exceed 11 per
cent. This tree is the food of the kermes insect, used for dyeing
scarlet before the introduction of cochineal. Garouille is principally
used in the south of France, giving a firm sole-leather of a
disagreeable odour and dark brown colour.
[Illustration: FIG. 48.--Kermes Oak (_Quercus coccifera_).]
_Q. Ægilops_ (and probably other species--_Q. macrolepis_, _græca_,
_Ungeri_, _coccifera_), Valonia. Fr. _Valonée_; Ger. _Valonea_,
_Ackerdoppen_, _Orientalische Knoppern_. Best Smyrna contains up to 40
per cent., Greek 19-30 per cent., Candia valonias up to 41 per cent.,
and Caramanian (probably not _Q. Ægilops_) 17-22 per cent. of tannins
which are at least principally pyrogallol derivatives and which give
blue-blacks with iron, no precipitate with bromine-water, and which
deposit a great deal of bloom consisting of ellagic acid.
[Illustration: FIG. 49.--Valonia Oak (_Q. Ægilops_).]
_Q. Ægilops_ (Fig. 49) is said to be most abundant in the highlands of
Morea, Roumelia, Greek Archipelago, Asia Minor and Palestine, while
_macrolepis_ forms large forests in many parts of Greece, and especially
on the lower slopes of Mount Taygetos. In Asia Minor the fruit ripens in
July-August, when the trees are beaten and acorns left on the ground to
dry. They are afterwards gathered, and carried on camels to stores in
the towns, and thence by camel and rail to Smyrna, where they are placed
in heaps 5-6 feet deep in large airy stores, and allowed to ferment and
heat for some weeks, when the acorn, which contains but little tannin,
contracts and falls from the cup, and is used for feeding pigs. This
fermentation is risky, and if carried too far the cups become
dark-coloured and damaged. The acorn contains a considerable amount of
fermentable sugar.
When ready for shipment, the valonia is hand-picked, the largest and
finest cups (prima) going to Trieste, the second selection to England
(Inglese), and the remainder, known as “natural,” also coming largely to
England. The “Inglese,” although inferior in appearance to the very
large selected cups, is, of course, less costly, and gives an equal
yield of tannin.
In 1887, Smyrna exported about 23,000 tons to England, and 16,000 tons
elsewhere, principally to Austria, Germany and Italy. The largest known
crop is stated at 70,000 tons in Asia Minor, and 14,000 in Greece, but
the average yield is considerably less than this.
The beard contains considerably more tannin than the cups, sometimes
over 40 per cent. It is often sold separately at the same or a lower
price, and in Smyrna is known by the Turkish name _tirnac_ (Ital.
_trillo_).
In Greece the best valonia is collected (in April?) before the cup is
matured and while it still encloses the acorn, and is known as _chamada_
(It. _camata_ and _camatina_). The colour of these kinds is excellent
and the percentage of tannin high. Mainly used by dyers, but often worth
attention for tanning where colour is important. In camatina the acorn
is completely covered in the cup, while in camata it is partly exposed.
The next quality, _rhabdisto_, is beaten down by sticks in
September-October (hence name), while after the first rains the fruit
falls and turns black, and is called _charcala_. It contains but little
tannin, and is not generally collected.
Sometimes valonia is attacked by a sort of honeydew, probably caused by
an aphis, which renders it very sticky, and perhaps more liable to heat,
but does not in itself damage its tanning properties.
The lighter the colour, the heavier the weight, and the thicker the
scales of the beard, the better the quality usually proves, but analysis
is the best guide. Caramanian valonia is very inferior.
The tannin contained in valonia is especially suitable in the
manufacture of sole-leather. It deposits much bloom, and if used as a
dusting material, has the characteristic of making the leather solid and
compact, but leaves the grain somewhat rough and hard to work. In
mixture with gambier and other materials, it is an excellent tannage for
dressing leather, and with proper management deposits little or no bloom
(cp. p. 231).
_Q. infectoria_ (Fig. 50) is the source of the “Turkish” or Aleppo
galls. Galls are caused by insects, principally of the genus Cynips, or
gall-wasps, which lay their eggs in different parts of plants, and in
some way cause an abnormal growth of the bud, leaf, or other part.
[Illustration: FIG. 50.--Gall Oak (_Q. infectoria_).]
_Aleppo galls_ are developed from the young shoot of the oak, are best
before the insect has escaped, and contain in this stage up to 50 or 60
per cent. of gallotannic acid. When the insect has developed and
escaped, the galls are of course perforated, much lighter, and more
porous. These galls and those of _Rhus semialata_ are the principal
sources of the tannin of commerce.
The _Q. infectoria_ also bears a large gall like an apple, “Apples of
Sodom,” or “rove,” caused by a different insect. This, in a crushed
condition, has been somewhat largely used as a tanning material, and
contains 24-34 per cent. gallotannic acid.
English oaks have several species of galls and oak-apples, but they do
not seem to be of much value for tanning.
[Illustration: FIG. 51.--Chestnut Oak (_Q. prinus_).]
_Knoppern_ are galls produced on the immature acorns of various species
of oaks, principally _Q. Cerris_ in Hungary, and were formerly largely
used there for tanning, as they contain up to 35 per cent. gallotannic
acid. They are now less abundant, and have been largely replaced by
valonia, sometimes called _orientalische Knoppern_. Like all purely
gallotannic materials, they naturally give a soft and porous tannage,
ill-adapted for sole-leather purposes, which has led to the Austrian
practice of drying, or rather stewing, the leather in very hot and damp
stoves, which make it hard and brittle.
_Chinese_ and _Japanese galls_ are the product of the action of an
aphis on a species of sumach, and will be mentioned again under sumachs
(_Rhus_).
_Djaft_, _dchift_, _jift_, or _jaft_ is a material apparently of Eastern
origin, and said to be derived from an oak of Kurdistan. Dark red scales
or fragments, origin uncertain, very astringent and darkish tannage,
liquor when spilt dries whitish, apparently from crystallisation of some
sort. It contains a large amount of tannin. It appears very irregularly
in commerce and the writer would be glad to obtain further samples and
details of origin. He once used 6 or 7 tons successfully in sole-leather
tannage. It has also been attributed to a shrub allied to the
_Cæsalpinias_ (p. 286).
The most important American oaks are--_Q. prinus_ (_castanea_,
_monticola_), the Chestnut or Rock Oak (Fig. 51). About equal to our oak
in strength, bark very thick, and infusion strongly fluorescent,
especially in presence of ammonia. Source of chestnut-oak extract. The
most important tanning oak-bark of the United States.
_Q. alba_, or “white oak,” is perhaps the most widely distributed and
abundant of any of the American oaks, and very closely resembles the
European _Q. robur_.
_Q. tinctoria_ or _nigra_, Black or Quercitron Oak. Poor as a tanning
material, but used for dyeing yellow, and for modifying the colour of
hemlock tannages. The dyestuff, _quercetin_, is closely allied to that
of fustic, and gives yellows with alum and tin mordants.
A good deal of information is given by Trimble[138] on American oaks and
other tanning materials.
[138] ‘The Tannins,’ vol. ii., Lippincott, Philadelphia, 1894.
Important Indian oaks are _Q. glauca_, _Q. lamellosa_ and _Q. incana_;
bark of last said to yield 22 per cent. of tannin.
SALICACEÆ, Willows.
The bark of various willows, especially _Salix arenaria_ and
_Russeliana_, is used for tanning in Russia, and for Danish
glove-leather. Some contain up to 12-14 per cent. of an iron-blueing
tannin. They impart a strong odour to leather, but different to that of
birch-tar oil, and the scent of genuine Russia leather is due to a
combination of both. In many cases the bark peeled off osiers for
basket-making is employed. A Russian willow (species unknown), in the
form of thin bark of osiers or small branches, gave 9·5 per cent. tannin
when examined in the Leather Industries Laboratory of the Yorkshire
College; and willow barks certainly demand more attention than they have
received in England as tanning materials for fine leathers. _Salix
caprea_ has been used in France for glove leathers, but is weaker than
_S. arenaria_.
Poplars belong to the same natural order, and have been used for
tanning, but their barks at the most contain 2-3 per cent.
POLYGONACEÆ, Docks.
Most members of this family contain tannin.
_Rumex hymenosepalum_, Canaigre, Gonagra (Cana agria), Red Dock, wild
pie-plant (Fig. 52). Common in sandy alluvial plains of Mexico and
Texas, and considerably resembling rhubarb. Its tuberous roots resemble
those of the dahlia, and contain, when air-dried, 25-30 per cent. of a
catechol tannin, probably allied to that of mimosa. Undried, the roots
contain about 68 per cent. of water and only 8 per cent. of tannin. When
well harvested by slicing thin and rapidly drying, it gives leather a
bright orange colour, and, it is said, considerable weight and firmness,
and is thus specially suitable for use in retanning and finishing light
goods and harness leather. Besides tannin, the root contains a yellow
colouring matter, and about 8 per cent. of starch, of which the granules
are very variable in form and size, but mostly oval or elongated. They
do not stain readily with iodine till they have been well washed, or
treated with dilute sulphuric acid. Both the starch and tannin are
contained in large and somewhat thin-walled cells, and the sliced
material is easily extracted at low temperatures. Greater heat
gelatinises the starch, and extracts a darker colour. The best
temperature for extraction is between 30° and 50° C. (see p. 348).
[Illustration: FIG. 52.--Canaigre (_Rumex hymenosepalum_). ‘New
Commercial Drugs and Plants,’ T. Christy.]
The root is most readily grown from tubers or portions including the
crown, as the plant seeds sparingly. Sandy soils, subject to inundation
or irrigation, seem best suited to its culture. In California and
Arizona the growth begins in October or November with the winter rains,
blooming about the end of January, while the leaves die down in May and
the roots remain dormant during the summer. It is not important at what
time the roots are harvested, and they seem to improve in percentage of
tannin up to the second year, after which they become darker and
deteriorate.
The harvested crop should be sliced into thin pieces and rapidly dried
at a low temperature, or still better, converted at once into extract.
This is already done on a considerable scale at Deming, New Mexico. The
residue after extraction is used in America as cattle-food; and might no
doubt be also applied to the production of alcohol.
Planting takes place in autumn, in rows, say 30 inches apart, with 10
inches between each root. Roots for “seed” should be kept in the ground
or stored in dry sand. This should yield a crop of 10 tons per acre in
an average season.
_References._--Report U.S. Commissioner of Agriculture, 1878, pp. 119
_et seq._; Trimble, Am. Jour. of Pharmacy, p. 395, 1889; Canaigre, Bull.
No. 7, Arizona Agr. Expt. Station, 1893; ‘Canaigre or Tanner’s Dock,’
Bull. No. 105, University of California, Berkeley, Cal.; ‘Canaigre
Tannin,’ Trimble and Peacock, Philadelphia, 1893; ‘Report to the German
Leather Trades Association,’ by V. Schroeder, 1894; ‘Il Canaigre,’ E.
Andrieis, Turin, 1899.
_Rumex maritima_, or _maritimus_. Central Europe, England, Ireland. It
is said by de Lof to be found in California, where it is used by the
Indians for tanning; but he probably confounds it with canaigre. De Lof
found its roots, wet, to contain 6 per cent. and after drying, 22 per
cent. of tannin, together with starch and an acid allied to malic.
Several English docks contain tannin; the writer had a sample of leather
tanned with dock-root (very possibly _R. aquaticus_), many years old,
but still soft and close in texture, and of excellent quality.
_Polygonum amphibium._ Said to grow on thousands of acres (?) on the
lower Missouri. Roots contain 22 per cent., branches 17 per cent. of
tannin. _P. amphibium_ is a common English and European plant, with
spikes of pink flowers, growing in marshes and ponds. Probably this is
the _Polygonum_ analysed by Fraas, who found 20-26 per cent. tannin.
_Polygonum Bistorta._ Common in damp places in England. Bistort,
Snakeweed, called “Eastermer giants” in Cumberland, where the young
leaves are used for making herb-puddings. Fraas found 16-21 per cent.
tannin in the roots.
Other species are known to contain much tannin. Perkin found a red
colouring matter in _P. cuspidatum_, a native of India and China,
commonly grown in gardens as a foliage plant (Journ. Chem. Soc., 1895,
p. 1084). _P. tinctorium_, used as a source of indigo in China and
Japan.
_Coccoloba uvifera_, Seaside Grape of West Indies; source of West Indian
kino. Whole plant rich in tannin.
LAURACEÆ, Bay Family.
_Persea_, or _Laurus lingue_. Bark used in Chili for tanning Valdivia
leather. (According to Arata, _Laurus caustica_.) A tree 25-30 feet high
and 2 feet in circumference. Bark rough outside, and whitish, with an
aromatic smell and taste, brittle and easily ground, contains 17-19 per
cent. of a catechol-phloroglucol tannin, greening iron salts (Journ.
Chem. Soc., 1881, p. 600). About 60,000 heavy hides are tanned yearly
with this bark in Valdivia and district, and mostly sent to Hamburg. The
hides are thick and scarcely tanned through, colour fair, leather soft
and porous.
_Persea Meyerina_ N. and _Laurus Pneumo_. Said to be also used in Chili.
SANTALACEÆ.
_Osyris compressa_ (_Fusanus compressus_, _Colpoon compressum_, _Thesium
Colpoon_), “Cape Sumach,” “_Pruim Bast_,” leaves and bark, Cape of Good
Hope. Leaves contain about 23 per cent. of tannin and form a useful
substitute for sumach; but the tannin is not identical, and is of the
catechol class, resembling gambier.
_O. arborea._ Northern India. Leaves rich in tannin.
_Fusanus acuminatus_ (_Santalum acuminatum_), “_Quandony_.” Australia.
18-19 per cent. tannin, dark coloured.
_Exocarpus cupressiformis._ Australia. Bark contains 15 per cent.
tannin.
DAPHNOIDÆ, Spurge Laurels.
_Daphne Cnidium_ L., “_Garou_.” Algeria. Used for dyeing and tanning.
PROTEACEÆ.
_Banksia serrata_, Heath Honeysuckle. Australia. Specimen examined
contained 11 per cent. tannin; according to Maiden it reaches 23 per
cent.
_Banksia integrifolia._ Queensland. Bark contains 11 per cent. tannin.
_Grevillia striata._ Australia. Bark contains 18 per cent. tannin.
_Leucospermum conocarpum. Kruppelboom._ Knotted Tree. Cape of Good Hope.
Said by de Lof to contain 22 per cent. of tannin; but a specimen
examined by the Author yielded 10·9 per cent. on analysis.
_Protea mellifera._ Sugarbush. _Suikerbosch._ Cape of Good Hope.
Contains 25 per cent. tannin, according to de Lof; but Palmer found 18·8
per cent.
_Protea grandiflora. Waagenboom._ Cape. Contains 25 per cent. tannin (de
Lof); 15·9 per cent. (Palmer); 15·6 per cent. (Procter).
_Protea speciosa._ Cape of Good Hope.
_Leucadendron argenteum_, Silver Tree, _Silverboom_, _Witteboom_ Cape of
Good Hope. Bark said to contain 16 per cent. tannin (de Lof); a specimen
examined by the Author yielded 9·2 per cent.
_Brabium stellatifolium_, _Wilde Amandelboom_, Wild Almond.
PLUMBAGINÆ.
_Plumbago Europea_, Leadwort. Fr. _Dentelaire_. A garden plant in
England, native in France; contains much tannin, especially in the
root-bark.
_Statice coriaria_, Marsh Rosemary. South of Russia. Roots up to 3
metres long and 2-12 cm. thick; used by Kalmucks for tanning
sheep-skins; contain 22 per cent. of tannin (de Lof).
_Statice limonum_, Sea Lavender. Coasts and salt marshes of Europe and
America. Richer in tannin than _S. coriaria_; used in France, Spain and
Portugal.
Several other species contain tannin. These plants are allied to
“Thrift” (_Armeria_).
MALPIGHIACEÆ.
_Byrsonima spicata_, Antilles, “Tamwood.”
_Byrsonima coriacea_, Jamaica, “Golden Spoon.”
_Byrsonima chrysophylla_, etc.
_Malpighia punicifolia_, Nicaragua, “Nancite”; “Mangrutta.” Bark
contains 20-30 per cent. of light-coloured tannin.
POLYGALACEÆ, Milkwort Family.
_Krameria triandria_, Rhatany, Peru.
The root is used in medicine, and is stated to contain 40 per cent. of
tannin.
Wittstein found only 20 per cent. of an iron-greening
catechol-phloroglucol tannin allied to tormentil tannin, in the
root-bark, the only active part of root.
ANACARDIACEÆ.
_Loxopteryngium Lorenzii._ Span. _Quebracho colorado._ South America,
especially Argentine Republic; the highest proportion of tannin
occurring in the wood from Gran Chaco district. Wood contains on an
average about 20 per cent. of a red, difficultly-soluble tannin,
yielding “reds,” and containing catechol and phloroglucol. The tannin is
not very soluble in water, and hence can only be used in weak liquors,
but is very astringent, and gives a firm, reddish leather. The wood also
contains a catechin[139] and a colouring matter, fustin, identical with
that of “young fustic.” It is imported into England, and more largely to
Havre and Hamburg, in logs, which are there chipped like logwood, and
either used direct for tanning, or made into extract. A very cheap tan.
With alum it gives a yellow colour. The extract usually dissolves to a
fawn-coloured turbid solution. Many quebracho extracts are now made
completely soluble by treatment with alkalies or sulphites (cp. p. 338).
[139] See P. Arata, Journ. Chem. Soc., 1878, A, p. 986; 1881, A, p.
1152; and Perkin and Gunnell, Trans. Chem. Soc., 1896, 1303.
“Quebracho” means “axebreaker,” and is consequently applied to a variety
of hard woods. Its specific gravity is 1·27-1·38, and it therefore sinks
in water.
_Pistacia lentiscus_, Ital. _Pistacio_, Fr. _Lentisque_. Sicily,
Cyprus, Algeria. Small myrtle-like leaves contain from 12-19 per cent.
of a catechol-tannin, and are very largely used in the adulteration of
sumach. Leather tanned with sumach adulterated with this material
darkens and reddens on exposure to light and air, and for this reason
its use in many cases is decidedly injurious. In Cyprus and the East it
is known as “_Skens_,” Ital. _Schinia_, Fr. _Poudre de Lentisque_, in
England, often called Cyprus sumach. (Cp. p. 272.)
_P. orientalis_, _terebinthus_, _vera_, etc., India, Mediterranean.
Various aphis galls, 30-40 per cent. tannin. A sample of galls of
_Pistacia vera_, “_Gool-i-pista_,” India, recently examined in the
Author’s laboratory, contained 30 per cent. of a light-coloured tannin.
_Schinus molle_, “Molle,” Buenos Ayres. Leaves only used; said to
contain 19 per cent. tannin.
_S. Aroeira_, Brazil. Said to contain 14 per cent. tannin.
_Rhus coriaria_, Sicilian sumach. Ital. _Somacco_. (Fig. 53.) A shrubby
bush, of which leaves and small twigs are used.
[Illustration: FIG. 53.--Sicilian Sumach (_Rhus coriaria_).]
Mostly propagated by suckers from older plants, which are planted in
rows about two feet apart in early spring, and pruned to 6-8 inches.
Bushes begin to bear the year after planting, though the strength is not
so good as from more mature plants. Cropping is either by pruning off
shoots, or gathering leaves by hand; in the latter case shrubs are
pruned in winter. The leaves are dried either in the fields, or on
covered threshing floors, where they are afterwards separated from the
stems by beating. Some is exported in this state, as “leaf” or “baling”
sumach, but most is ground to fine powder under edge-runners.
“Ventilated” sumach is winnowed to remove dust and sand, which often
contains iron. “_Mascolino_” is the best sumach from Palermo and
district; “_feminella_” consists of weaker sorts from other parts, and
is generally used for mixing.
The different varieties of sumach are classed as follows:--
Relative
Market Value.
Sumach for baling 2·5
„ „ grinding 2·3
„ from yearling plants 1·5
„ „ ends of branches collected in autumn 1·0
To prepare these different grades for ultimate consumption, they are
ground in mills similar to those employed for crushing olives, that is,
in which two large stone wheels follow each other, revolving upon a
circular bed, the whole construction being about the same as the Spanish
or Mexican _arrastre_. The sumach thus pulverised is passed through
bolting-screens to separate the finer from the coarser particles.
After the sumach leaf has been subjected to the first process of
trituration, the coarse remaining portions are re-ground and the product
added to that which has been already obtained. The still unpulverised
residue known as _peduzzo_ is sifted, and the coarser and ungrindable
parts are used as fuel, while the finer are mixed with the
partially-ground, small, leaf-bearing branches (_gambuzza_, _gammuzza_),
and ground again.
Palermo is the principal seat of the sumach trade. The material is
generally bought from the small growers by middlemen, who hold it till
market conditions are favourable. The quotations are always in _tarì_ of
42·5 centimes per _cantar_ of 79·342 kilos, which are obsolete even in
Sicily, and have to be reckoned into _lire_ (francs) and kilos.
Consequently 1 tarì per cantar equals 0·53565 lira per 100 kilos.
In 1894, the prices delivered at the mills were about 41-42 tarì for
mascolino, 37-38 tarì for femminello, 14-18 tarì for brusca, and 10 tarì
for stinco, per cantar; the lira being worth about 9_d._[140]
[140] Cf. ‘Kew Bulletin,’ No. 107, pp. 293-6.
Sumach has been introduced into Australia, and is said to thrive well in
the dry plains of the Wimmera district.
Sumach often contains much sand, and sometimes particles of magnetic
iron ore, which cause black stains, and may be collected by a magnet,
and which dissolve in dilute hydrochloric acid without evolution of
hydrogen, to a yellow solution. Metallic iron, which is also attracted
by the magnet, dissolves in hydrochloric acid with effervescence to a
colourless or green solution.
Good sumach contains at least 25-27 per cent. of tannin. The Author has
analysed samples of undoubted genuineness containing as much as 32 per
cent. of a tannin, principally gallotannic, with some ellagitannic acid,
and a colouring matter (myricetin) identical with that of _Myrica nagi_
(p. 250), which gives yellows with alumina and tin mordants, and is
fugitive to light.
Sumach is the best tanning material known for pale colour and soft
tannage, and is hence used for moroccos, roans, skivers, etc., and also
for brightening leathers of darker tannages, such as mimosa, gambier,
the colouring matters of which warm sumach liquors seem able to
dissolve.
In the report of the Society of Arts Committee on bookbinding
leathers,[141] it is stated on abundant evidence, that sumach-tanned
leathers are less affected by light and gas-fumes, and less liable to
decay than those of any other known tannage.
[141] Soc. Arts. Journ., 1901, p. 14.
Sumach is frequently adulterated with the ground leaves and twigs of
_Pistacia lentiscus_ (“schinia” or “skens”), _Coriaria myrtifolia_
(“stinco”), _Tamarix africana_ (“brusca”), _Ailantus glandulosa_, _Vitis
vinifera_ (leaf of the common grape vine) and some other species of the
Rhus family, but _Pistacia lentiscus_ is used to a much larger extent
than any of the others. _Pistacia_, _coriaria_, and _tamarix_ all
contain considerable quantities of tannin, though less than genuine
sumach, and of a different chemical constitution.
The most satisfactory method of detecting these adulterants is by
microscopic examination, none of the chemical methods proposed being
very satisfactory; though, as many of the added matters contain catechol
tannins, while those of sumach are purely pyrogallol derivatives, the
method proposed by Hughes for the detection of quebracho in oakwood by
the reaction of concentrated sulphuric acid (p. 296) might render good
service, and any sumach infusion which was rendered turbid by
bromine-water would at least be open to grave suspicion.
[Illustration: PLATE III.
_Ailantus glandulosa._
_Coriaria myrtifolia._
_Colpoon compressa._
_Rhus cotinus._]
[Illustration: PLATE IV.
_Pistacia lentiscus._
_Rhus metopium._
_Rhus coriaria._
_Tamarix Africana._]
The most important work on the microscopic structure of the tissues of
sumach and its adulterants was done by Andreasch, when during the later
stages of his last illness he was obliged to winter in Sicily.[142] His
work will well repay study, but unfortunately does not admit of useful
abstraction here. A very useful investigation was also made in the
Author’s laboratory by Messrs. M. C. Lamb and W. H. Harrison,[143] as
regards the treatment and examination of the leaf-cuticles, which
renders the detection of mixture comparatively easy. For details, the
original memoir must be consulted, but if the suspected sumach be gently
warmed for a few minutes with strong nitric acid, its more delicate leaf
structure is entirely destroyed, and after washing and neutralising with
sodium carbonate the strong cuticles of the leaves of the more common
adulterants, “schinia” (_Pistacia lentiscus_), “stinco” (_Coriaria
myrtifolia_), “brusca” (_Tamarix africana_), and _Ailantus glandulosa_
are uninjured, and easily recognised. Examination is rendered easier by
dyeing the cuticles; safranine, acid green, Bismarck brown, and naphthol
yellow being suitable for the purpose. Mr. Lamb’s photographs of the
cuticles are reproduced on Plates III. and IV., but if possible, it is
most satisfactory to compare the suspected sample direct with known
specimens of the adulterants.
[142] ‘Sicilianischer Sumach und seine Verfalschung,’ Wien, 1898.
[143] ‘Sumach and the Microscopic Detection of its Adulterants,’
Journ. Soc. Dyers and Colorists, March 1899.
[Illustration: FIG. 54.--American Sumach (_Rhus glabra_).]
_R. glabra_, Southern States, U.S.A. (Fig. 54). Very largely used in the
States to take the place of Sicilian sumach. A sample collected by the
late Professor Trimble, and analysed in the Leather Industries
Laboratory, contained 25 per cent. of tannin and produced a leather of
very much darker colour than Sicilian.
_R. typhina_, “staghorn” or Virginian sumach, contains 10-18 per cent.
of tannin. A sample from same source as above contained 13 per cent.
_R. cotonoides_, U.S.A. The analysis of a sample of this material gave
21 per cent. of tanning matter, and leather tanned with it was almost
equal in colour to that from _R. glabra_.
Other sorts found in States: _R. semialata_ (5 per cent. tannin); _R.
aromatica_ (13 per cent. tannin); _R. metopium_ (8 per cent.); _R.
copallina_, _R. pumila_, _R. canadensis_; _R. toxicodendron_ is the
well-known “poison ivy,” a climbing plant which causes a severe and
irritating eruption if touched.
_R. glabra_ and _R. copallina_ are chiefly recommended for extended
cultivation in the United States.
In Virginia, the leaves are collected and cured by the country people,
and sold and delivered to owners of mills for grinding. Their particular
object being to secure the largest possible quantity of product at the
lowest cost, little attention is given to the quality obtained, or the
manner of collecting. The most intelligent dealers in the raw material
urge upon collectors to observe the following particulars:--To ensure a
maximum value for tanning purposes, the leaf should be taken when full
of sap, before it has turned red, has begun to wither, or has been
affected by frost. Either the leaf-bearing stems may be stripped off, or
the entire stalk may be cut away, and the leaves upon it allowed to
wither before being carried to the drying shed; but care must be
observed that they are neither scorched nor bleached by the sun. When
wilted, they are carried to a covered place, and spread upon open
shelving or racks to dry, avoiding the deposit in any one place of a
quantity so great as to endanger the quality of the product by
overheating and fermentation. Sumach should be allowed to remain in the
drying-house for at least one month before sending to the market; in
case of bad weather, a longer period may be required. When ready for
packing for shipment, it should be perfectly dry and very brittle,
otherwise it is likely to suffer injury in warehouses from heating and
fermentation.
Buyers of sumach leaves for grinding depend largely upon colour for the
determination of the value; the leaves should, therefore, when ready for
market, present a bright-green colour, which is evidence that they have
suffered neither from rain after being gathered, nor from heating during
the process of drying. Leaves having a mouldy odour or appearance are
rejected. The Virginian crop reaches 7000-8000 tons, and is collected
at any time between July 1st and the appearance of frost.
There is an important difference in the value of the European and
American products. The proportion of tannic acid in the latter is
generally lower than that found in the former, which is much preferred
by tanners and dyers. By using Sicilian sumach it is possible to make
the finest white leathers, while by the employment of the American
product, the leather has a disagreeable yellow or dark colour,
apparently due to a colouring matter, which exists in larger quantity in
the American variety than in the Sicilian.
Experiments upon the presence of colouring matters made by treating an
infusion of sumach with a solution of gelatine, gave the following
results:--
Virginia, mixed, collected in June, gave A nearly
white
precipitate.
„ „ „ July, „ A decidedly
yellowish-
white
precipitate.
„ _R. copallina_ „ August, „ A dirty-
yellow
precipitate.
„ _R. glabra_ „ „ „ A very dirty-
white
precipitate.
Fredericksburg, mixed „ „ „ A dirty-
yellow
precipitate.
Sicilian „ „ „ A slightly
yellowish-
white
precipitate.
For the purpose of tanning white and delicately coloured leathers,
therefore, the collection should be made in June; while for tanning
dark-coloured leathers, and for dyeing and calico-printing in dark
colours, where the slightly yellow shade will have no injurious effect,
the collection may be made in July. It appears that for all purposes,
the sumach collected after the 1st of August is inferior in quality.
Experimental results as regards percentage of tannin obtained by
collecting sumach at different seasons showed:--
Per Cent. of
Tannic Acid.
Virginia, mixed, collected in June, gave 22·75
„ „ „ July, „ 27·38
„ _R. glabra_ „ August, „ 23·56
„ _R. copallina_ „ „ „ 16·99
Sicilian, _R. coriaria_ „ „ „ 24·27
It is evident, therefore, that in order to secure the maximum amount of
tannic acid, the sumach should be collected in July, but the colouring
matter of the leaves has an important influence upon the value of the
product. The leaves of the upper extremities of the stalks are always
richer in tannic acid than those of the base; and the increase of age of
the plant is accompanied by a general diminution of this acid.
The mill used for grinding sumach leaves consists of a heavy, solid,
circular, wooden bed, 15 feet diameter, with a depression around the
edge a few inches deep and 1 foot wide, for the reception of the ground
sumach from the bed, and two edge-rollers, weighing about 2500 lb. each,
5-6 feet diameter, and provided with numerous teeth of iron or wood,
thickly inserted. In Europe and in some parts of the Southern States,
sumach is still ground by stones revolving on a stone bed, and the
sifting is often done by hand.
[Illustration: FIG. 55.--Venetian Sumach (_Rhus cotinus_).]
_R. cotinus_, Venetian sumach. Fr. _Arbre à perruques_; Ger.
_Perrukenstrauch_ (Fig. 55). More important as a dyeing than as a
tanning material, its twigs and wood, “young fustic,” containing a large
proportion of a colouring matter (fisetin), which with tin and alumina
mordants dyes bright yellows; and much resembles, but is not identical
with the myricetin present in _R. coriaria_.[144] Its leaves, known as
Turkish or Venetian sumach, contain about 17 per cent. of tannin, and
are used for tanning.
[144] Perkin and Allen, Trans. Chem. Soc. 1896, 1299.
_R. pentaphylla_, “Tezera,” Algeria, is used by the Arabs for tanning
goat-skins.
_R. Thunbergii_, _Kliphout_, Cape of Good Hope. A sample of the bark
analysed in the Author’s laboratory contained 28 per cent. of tanning
matter. A valuable tanning material, of reddish colour. The tannin is of
the catechol class.
Several other species of Rhus are used in tanning. _R. semialata_
yields Chinese and Japanese galls, containing up to 70 per cent.
gallotannic acid. They are caused, not by a fly, but by the attack of an
aphis, as are those of the allied Pistacia.[145] The aphides pass their
asexual stage inside the gall, which is large and thin-walled. A similar
aphis-gall is found on the American sumach. A specimen of the leaves
examined at the Yorkshire College yielded only 5 per cent. of tannin.
[145] See Flückiger and Hanbury, ‘Pharmacographia.’
_Mangifera indica_, Mango, widely distributed in the Tropics. Bark and
leaves rich in tannin, which gives green-blacks with iron.
CORIARIACEÆ.
_Coriaria myrtifolia_, French sumach (of which there are four
kinds--_fauvis_, _douzère_, _redoul_ or _redon_, and _pudis_). A
poisonous shrub of South of France; leaves used for tanning, and as a
sumach adulterant under the name of “stinco”; contain about 15 per cent.
tannin. (Cp. p. 272.)
_Coriaria ruscifolia_ bark, the _tutu_ of New Zealand, contains 16-17
per cent. of tannin.
Other Coriarias merit examination, and are known to contain much tannin.
RUBIACEÆ.
_Rubia_, Madder, allied to Galiums, which are almost the only English
representatives of the family. The coffee- and cinchona-plants are
foreign representatives.
_Nauclea_, or _Uncaria gambir_. East Indies. (Fig. 56.) A climbing
shrub, source of “gambier,” or “Terra Japonica”; also called “Catechu,”
in common with several other solid extracts. Gambier is first described
by the Dutch trader Couperus, in 1780; plant introduced in Malacca,
1758; plantations established in Singapore in 1819.
Culture is mainly by Chinamen, and is very rude; it yields rapid return,
but under the treatment to which it is subjected a plantation is worn
out in ten to fifteen years. Cropping commences three years after
planting, and is continued two to four times annually, with little
regard to fitness of shrubs, the plant being cropped till it has barely
leaves left to support existence. It is found advantageous to combine
pepper-culture with that of gambier, the spent leaves form a good
protection for the pepper-plant roots, but they have little actual
manurial value.
[Illustration: FIG. 56.--Gambier Shrub (_Nauclea gambir_).]
Cropping is done with a knife called a _parang_, while a larger knife is
used for chopping the leaves and twigs before they are put in a boiler,
in which they are heated with water till the liquid, which is constantly
stirred during the operation with a wooden five-pronged stirrer, becomes
syrupy. The leaves are then brought out with a wooden fork, and allowed
to drain on a tray, so that the liquor runs back into the boiler. The
coarser matter still remaining in the boiler is removed with a strainer
like a racquet, and the finer by straining the liquor through a
perforated cocoanut shell into small shallow tubs, where it is allowed
to cool with constant stirring with a cylindrical wooden bar, which is
worked up and down with a rotary motion until the catechin
crystallises. When quite cool the pasty mass is turned out of the tub,
cut into cubes with sides 1 inch long with a hoop-iron knife, and dried
on bamboo trays in racks under sheds, or sometimes smoke-dried with wood
fires.
Good cube gambier is an earthy-looking substance and is dark outside,
but pale within from crystallisation of catechin. Catechin is not itself
a tanning material, but is apparently converted into a tannin by drying
at 110°-126° C., when it parts with a molecule of water. It is very
probable that a similar change occurs in the tannery. The tannin is a
catechol-phloroglucol derivative, less astringent than most of this
series, and of pale colour. (See p. 297.)
A commoner quality, called “block-gambier,” instead of being cut into
cubes, is run into large oblong blocks of about 250 lb. weight, which
are wrapped in matting and exported in a pasty condition. These contain
35-40 per cent. of tannin, as estimated by the hide-powder method, while
the best cubes reach 50-65 per cent. Besides the forms named, various
others are made, principally for native use in chewing with betel-nut in
the form of small biscuits, or in thin discs (“wafer gambier”) by
running the pasty mass into bamboos and cutting the cylinder so formed
into thin slices. These forms are usually light in colour, and very rich
in catechin.
For details of the chemistry and employment of gambier, see pp. 228,
231, 239, etc.
APOCYNACEÆ.
_Aspidospermum quebracho_. Sp. _Quebracho blanco_. Brazil. Bark contains
aspidospermin, an alkaloid used in medicine, but both bark and wood are
poor in tannin.
_Quebracho colorado_, see ANACARDIACEÆ, p. 269.
ERICACEÆ, Heath Family.
_Arctostaphylos_ (or _Arbutus_) _uva-ursi_, Bearberry. Used in Russia,
Finland; twigs and leaves said to contain 14 per cent. tannin. Often
adulterated with leaves of _Vaccinium vitis-idæ_ or Cowberry.
_Arbutus unedo_, Common Arbutus. Leaves, fruit and bark used on
Mediterranean coasts.
VACCINIÆ.
_Vaccinium Myrtillus_, Bilberry. Used in Piedmont.
SAXIFRAGEÆ.
_Weimannia glabra_ L., “Curtidor” bark. Venezuela.
_Weimannia macrostachys_ D.C. Reunion.
_Weimannia racemosa_, New Zealand Towai or Tawheri bark.
These species contain 10-13 per cent. of iron-blueing tannin, and have
been practically used, but are not of much importance.
TAMARISCINIÆ.
Most of the members of this group are poor in tannin, but several
species have galls which are rich.
_Tamarix africana_; Egypt, Algeria. Galls containing 26-56 per cent.
tannin. The small twigs are collected in Tunis, and when dried and
ground are imported into Sicily to be used for the adulteration of
sumach under the name of “Brusca,” and contain about 9 per cent. of
tannin. (Cp. p. 272.)
_T. articulata_, Morocco, yields galls produced by aphides, called in
Arabia _Takout_, and stated by Vogel to contain 43 per cent. of tannin.
_Tamarix gallica_, used in Spain and Italy.
OXALIDEÆ.
_Oxalis gigantea_, source of _churco_ bark, Chili. A thin, brittle, dark
red bark, mostly about 2 mm. thick, cork and ross entirely absent. The
bark is brittle, and the cells thin. It contains about 25 per cent. of
an easily extracted, dark red tannin, giving green-blacks with iron. The
bark has been incorrectly attributed to _Fuchsia macrostemma_. (Cp. Von
Höhnel, ‘Die Gerberinden,’ p. 125, and this book, p. 284.)
COMBRETACEÆ.
Several families of this genus contain trees rich in tannin, but most
important are the Myrobalans (often, but incorrectly, written Myrabolams
or Myrabolans), the unripe fruit of various species of Indian
_Terminalia_.
[Illustration: FIG. 57.--Myrobalan Tree (_Terminalia Chebula_).]
_T. Chebula_ (Fig. 57), a tree 40-50 feet high, and yielding good
timber, is the source of all the ordinary varieties, which differ only
in the district from which they are obtained, and the state of maturity
of the fruit. The nuts contain from 30 to 40 per cent. of tannin. Of the
various sorts, probably those known as Bombays are least unripe, while
“lean greens” are the most so. The unripe fruit is the richest in
tannin. “Bombays” have a smooth skin in coarse wrinkles, and when cut
are porous and light coloured. “J’s” (Jubbalpores) and “V’s”
(Vingorlas), have finer and shallower wrinkles, and are harder, solider
and consequently darker looking, but do not give a darker liquor, while
“lean greens” are greener, have less yellow colouring matter, and
consequently more nearly approach in character to sumach, which the
tannin in many respects resembles, though probably containing more
ellagitannic acid in proportion to gallotannic acid than the latter.
The “nuts” should be bright in colour, not worm-eaten, nor “waxy” or
soft. If kept in a damp place they rapidly absorb moisture, and fall
into the “waxy” condition, in which they are very difficult to grind,
sticking to and choking the cutters or beaters of the mill.
Neither the large hard stones nor their kernels contain tannin, but the
latter have an oil which gives a peculiar odour to leather. The tannin
exists in large and rather thickly-walled cells, and is not very easily
extracted; the skin is wrinkled, but the uncrushed nuts swell up to
their original plum-like form when placed in water for some time. The
bark is almost as rich as the fruit, and the tree also yields galls.
_T. Belerica_ yields Beleric or “Bedda nuts,” which are downy, rounder
and larger than ordinary myrobalans, and contain about 12 per cent. of
tannin, used as adulterant of ground myrobalans. A sample of solid
extract made from the bark of T. Belerica contained 70 per cent. of
tannin.
_T. tomentosa_ has downy nuts, containing about 10 per cent. of tannin,
bark stated by de Lof to contain 36 per cent. of tannin. A sample of
solid extract contained 56 per cent. of tannin. The bark contains about
11 per cent.
There are several other Indian species.
_T. Catappa_, “Badamier bark” of Mauritius, contains 12 per cent. of
tannin.
_T. mauritiana_, “Jamrosa bark,” said to contain 30 per cent. of tannin.
_T. Oliveri_, Malay Archipelago, yields “Thann leaves,” from which an
extract is made as a cutch substitute. A sample of the extract from
Burmah examined recently in the Author’s laboratory, contained 62 per
cent. of tannin. The tannin is a catechol derivative, differing from
that of _Acacia catechu_ in containing no phloroglucol (p. 297).
A sample of bark from Mandalay contained 31 per cent. of tannin, while
the leaves from the same tree contained 14 per cent.
Emblic myrobalans, see p. 293.
RHIZOPHORACEÆ, Mangles or Mangroves.
_Rhizophora Mangle_, and other allied species, Mangrove or Mangle,
_Manglier_, _Paletuvier_, _Cascalote_, grows on tropical coasts all
round the world. The barks vary much in strength, from 15 up to 40 per
cent. in different species (see _Ceriops_). Leaves, used in Havana, are
said to contain 22 per cent. tannin. According to Eitner, the younger
plants contain the highest proportion of tannin. _R. Mangle_ seems to
yield a bark inferior to several other species.
All trees growing in swamp, and of the same character of growth as
mangrove, are called “Bakau” in the East Indies (Anglice, mangrove) and
various species of _Ceriops_ yield the best tanning bark. A tidal
mangrove swamp at low water is a tangle of arched roots like inverted
branches on which the trees are supported.
The catechol-tannin, which is easily extracted, is of deep red colour
and allied to that of the mimosas. In admixture with other materials the
red colour has a much smaller effect, and mangrove bark is now largely
used in combination with pine, oak and mimosa.
Several other species are also rich in tannin, and used in different
parts of the world under the name of mangle, as are also several species
of _Conocarpus_ belonging to the _Combretaceæ_.
_Rhizophora mucronata_. India and Burmah. Bark varies considerably;
David Hooper, Indian Museum, Calcutta, gives 26·9 per cent. of tannin.
Dr. Koerner (Deutschen Gerberschule, Freiberg) analysed two samples in
1900, one containing 48 per cent. and the other 21 per cent. of tannin;
two samples from the British Imperial Institute recently examined by the
Author showed only 4·5 and 6·1 per cent. of tannin respectively.
_Ceriops Candolleana_, _Bakau_ or _Tengah_ Bark, East Indies. _Goran_,
Bengal. Contains up to 27 per cent. of tannin and yields an extract
which promises well as a substitute for cutch, to which, for dyeing
purposes it is nearly or quite equal. The solid extract contains up to
65 per cent. tannin, making a good but dark red leather.
_Ceriops Roxburghiana_, a somewhat larger tree, also growing in the
Sunderbans, bark very similar in strength and character to the above.
ONAGRACEÆ, the Œnothera Family.
_Fuchsia excorticata_, the only deciduous tree of New Zealand. Contains
5 per cent. tannin.
_Fuchsia macrostemma_, Chili. Yields Tilco or Chilco bark. Churco bark
has been incorrectly attributed to this plant, but it is certainly
derived from an _oxalis_, as stated by the Kew authorities. (Cp. von
Höhnel, ‘Die Gerberinde,’ p. 125.)
GUNNERACEÆ.
_Gunnera scabra_ (_Pangue?_), _Pauke_, Chili. Used occasionally in the
tanning of goat-skins.
MYRTACEÆ.
_Eucalyptus globulus_, and other species of _E._ common in Australia,
and introduced into Algeria and Southern Europe (gum-trees), are more or
less rich in catechol-tannins, their sap being the source of Botany Bay
or Australian kinos, which contain up to 79 per cent. tannin. Several
species of _Eucalyptus_ afford astringent extracts; those from the
“red,” “white,” or “flooded” gum (_E. rostrata_), the “blood-wood” (_E.
corymbosa_), and _E. citriodora_, being quite suitable for replacing the
officinal kind. The gum is chiefly obtained by woodcutters, being found
in a viscid state in flattened cavities in the wood, and soon becoming
inspissated, hard and brittle. Minor quantities are procured by incising
the bark of living trees; a treacly fluid yielding 35 per cent. of solid
kino on evaporation is thus obtained. The gum is imported from
Australia, but there are no statistics to show in what quantity.[146]
[146] Compare Journ. Soc. Chem. Ind., 1902, p. 159.
_Eucalyptus longifolia_ bark, the “woolly-butt” of Australia, contains
8·3 per cent. of tannic acid, and 2·8 of gallic. The “peppermint” tree
contains 20 per cent. of tannic acid in its bark. The “stringy-bark”
(_E. obliqua_) gives 13¹⁄₂ per cent. of kinotannic acid. The Victorian
“iron-bark” (_E. leucoxylon_) contains 22 per cent. of kinotannic acid,
but is available only for inferior leather.
_Myrtus communis_, and several other myrtle species, contain a
considerable amount of tannin in the bark and leaves.
GRANATACEÆ.
_Punica Granatum_, Pomegranate. Peel of fruit employed in Spain and the
East as substitute for sumach, containing up to 25 per cent. of tannin.
Bark said to contain 22 per cent. tannin. _Balaustines_, wild
pomegranates, East Indies. Fruit, said to contain 46 per cent. tannin.
ROSACEÆ.
_Tormentilla erecta_, _Potentilla tormentilla_. Root variously stated to
contain 20-46 per cent. tannin. Red coloured leather, formerly used in
Orkneys, Shetland, and Faroe Islands, and in some parts of Germany.
_Sorbus_ or _Pyrus Aucuparia_, Mountain Ash. Bark said to be stronger
than oak.
Many other plants of the family contain tannin, among others the
strawberry.
PAPILIONACEÆ.
_Butea frondosa_.[147] This (with _Pterocarpus marsupium_)[148]
furnishes East Indian kino. The flowers are used in India as a dye,
under the name of Tesu. Bark fairly rich in tannin.
[147] ‘Dictionary of Economic Products,’ I.B., p. 944; Hummel and
Cavallo, Proc. Chem. Soc. 1894, p. ii.
[148] Agricult. Ledger, 1901, No. 11, Gov. Printing Office, Calcutta.
_Pterocarpus_ or _Drepanocarpus senegalensis_ is the source of African
kino, which contains up to 75 per cent. of tannin.
_Cæsalpinia coriaria_, Divi-divi. A tree of 20-30 feet, native in
Central America, introduced successfully in India, but principally
imported from Maracaibo, Paraiba and Rio Hache. The dried pods contain
40-45 per cent. of a pyrogallol-tannin, mainly ellagitannic acid, and
would be a most valuable tanning material, but for a liability to
fermentation and sudden development of a deep red colouring matter. The
causes are not well understood, but apparently the risk can be
materially lessened by use of antiseptics. If used in strong liquors it
gives a heavy and firm leather, but is principally employed as a partial
substitute for gambier on dressing leather. Used in rapid drum-tannage
for light leathers, an excellent colour may be obtained. It is said to
give an especially firm and glossy flesh. Leather tanned with it, even
when of outwardly good colour, has often a blueish-violet shade within,
perhaps due to the development of a colouring matter allied to that of
logwood. The seeds do not contain tannin, which lies almost free in the
husk of the pod. The pods are about 3-4 cm. long, dark outside, and curl
up in drying to an S-shape.
_C. digyna_, _Tari_ or _teri_ pods. Occurs in Prome, Toungoo, Bassin,
Mynang and other parts of India and Burmah, where it is used as a drug.
The pod-case is said to yield over 50 per cent. of tanning matter. A
sample from Burmah, kindly sent by the Imperial Institute, examined by
the Author in 1900, contained 24 per cent. of tannin, but after removing
the seeds the remaining pod-cases yielded 44 per cent. of tannin on
analysis. _C. digyna_ promises to become a valuable tanning material if
it proves free from the tendency to ferment which is so troublesome in
divi-divi. It has been introduced into England under the name of “white
tan,” which yields a leather quite as white as sumach; but the supply
seems at present uncertain.
_C. cacolaco_, Cascalote, Mexico. Pods rich in tannin (up to 55 per
cent., Eitner). Pods larger and fleshier than divi, seeds smaller,
tannin similar.
The pods of several other Cæsalpinias are used in tanning, sometimes
under the name of “Algarobilla,” which is simply a diminutive of
Algaroba, the carob, or locust-bean, derived from Arabic _al Kharroba_,
and applied to several small pods. (See _Balsamocarpon_ and _Prosopis_.)
_C._ (or _Balsamocarpon_) _brevifolia_, Chili, ordinary Algarobilla.
Fig. 58. One of the strongest tanning materials known, containing an
average of 45 per cent. of a tannin very like that of divi, but less
prone to discoloration. The tannin lies loose in a very open skeleton of
fibre, and is easily soluble in cold water; the seeds contain no tannin.
If not allowed to ferment it produces a very bright-coloured leather.
Algarobilla has been attributed to _Prosopis pallida_, but this appears
incorrect. Several species of _P._ are known to yield tanning pods;
those of _P. Stephaniana_ of the desert of Kaschan, in Persia, are
_dschigh dschighe_, perhaps identical with _dchift_ or _jaft_. (See p.
263.) Bark of _P. spicigera_ used in Punjab.
_C._ (or _Hæmatoxylon_) _campechianum_, Logwood, Central America. In
addition to colouring matter, and a glucoside which it yields on
oxidation, this wood contains about 3 per cent. tannin. Its principal
use is in dyeing blacks with iron or chrome mordants. (See p. 413.)
[Illustration: FIG. 58.[149]--Algarobilla (_Cæsalpinia brevifolia_).]
[149] ‘New Commercial Drugs and Plants,’ No. 5, T. Christy.
_C. echinata_ yields “Brazil-wood.” (See p. 413.)
_C. Sappan_, Sappan-wood, India.
_Cassia auriculata_, _Turwar_ or _Tanghadi_ bark, Southern India. Used
for tanning so-called “Persian” sheep- and goat-skins, contains about
17 per cent. of a catechol tannin. Leather tanned with it is of a pale
yellow colour, but rapidly reddens in sunlight. Cp. p. 235.
_C. fistula_, India. Husk of pod, 17 per cent. tannin. The pulp of pod
is used as an aperient.
_C. elongata_ and _lanceolata_. Senna leaves. Upper Egypt.
_C. Sophora_, “_Bali-babilan_.”
[Illustration: FIG. 59.--Babool (_Acacia arabica_).]
MIMOSEÆ, a Tribe of Leguminosæ.
_Acacia arabica_, “Babool,” “Babul,” India, Egypt. Fig. 59. Bark
contains about 12-20 per cent. of catechol tannin; one of the principal
Indian tanning materials, used for kips and heavier leathers. Pods, used
in India for bating, contain about same amount of tannin as bark, but of
a different kind, that of the bark being a catechol-tannin, with a good
deal of red colouring matter, while the pods contain a paler tannin
allied to divi, which is not precipitated by lime-water. In Egypt the
pods are called _bablah_, a name which is also applied to pods of _A.
cineraria_ and _A. vera_, and others. They are used for dyeing
glove-leathers.
_A. nilotica_, Egypt. Pods called _neb-neb_ or _bablah_.
[Illustration: FIG. 60.--Cutch Tree (_Acacia catechu_).]
_A. catechu_, India. The wood yields cutch or “dark catechu.” A lighter
coloured variety called _kath_, containing much crystallised catechin,
is also made in India, and principally used for chewing with betel. _A.
catechu_ is a tree 30-40 feet high, common in India and Burma, and also
in tropical East Africa, where, however, it is not utilised. In Southern
India, _A. suma_ is also used for the same purpose.
Trees of about 1 foot diameter are cut down, and the wood (some state
the heart-wood only) is reduced to chips, and boiled with water in
earthen jars over a mud-fireplace. As the liquor becomes thick and
strong, it is decanted into another vessel, and the evaporation
continued till the extract will set on cooling, when it is poured into
moulds made of leaves or clay, the drying being completed by exposure to
the sun and air. “Kath,” or pale cutch, is made in Northern India, by
stopping the evaporation at an earlier point, and allowing the liquor to
cool, and crystallise over twigs and leaves thrown into pots for the
purpose. It contains a large proportion of catechin, apparently
identical with that of gambier, but its tannin is much redder. Good
cutch contains about 60 per cent. tanning matter, but is principally
used for dyeing browns and blacks with chrome and iron mordants. It
contains quercetin, a yellow colouring matter (p. 263).
_A. leucophlea_, India and Java “_Pilang_.” Pods and bark equal to _A.
arabica_.
Australia abounds in acacias (mimosas), many of which are used in
tanning, but vary greatly in strength, not only according to species,
but probably also by situation and growth. Probably the best information
is to be found in a pamphlet on ‘Wattles and Wattle-Bark,’ by J. H.
Maiden, F.L.S., published by the Department of Public Instruction at
Sydney, 1890. His analyses were made by the Löwenthal process, and can
only be roughly compared with those by the hide-powder method. The
analyses given are by the I.A.L.T.C. method, and mostly on samples
furnished by Mr. Maiden.
A peculiarity largely developed in the mimosa family is the tendency for
the true leaves to be suppressed, and their place taken by the flattened
and expanded midrib (phyllode). Thus leaves of two very distinct forms
are common in the genus, and some acacias, as _A. heterophylla_, may
have both forms on the same branch. Compare _A. pycnantha_ and _A.
decurrens_.
The Australian mimosas have been naturalised in India, and grow freely
in the Nilgiri Hills, but the bark does not appear to be utilised.
The most important species are as follows:--
_A. pycnantha._ (Fig. 61.) “Broad-leaved” or “Golden Wattle,” South
Australia. One of the strongest tanning barks known. A sample marked
“special,” analysed in the Yorkshire College, contained 50 per cent. of
tannin; another sample marked “ordinary” contained 40 per cent.
[Illustration: FIG. 61.--Broad-leaved Wattle (_Acacia pycnantha_).]
[Illustration: FIG. 62.--Green Wattle (_Acacia decurrens_).]
_A. longifolia_, the Golden Wattle of New South Wales, only contains
half as much tannin as _A. pycnantha_.
_A. mollissima_, with its two varieties _A. decurrens_ (Fig. 62) and _A.
dealbata_, are among the most important of the Wattle family
commercially. Two samples of the former marked “Green Wattle” showed
36-39 per cent. of tanning matter; another sample marked “Sydney Green
Wattle,” contained 41 per cent. A sample of _A. decurrens_, the second
variety, was much weaker, showing only 12 per cent. on analysis.
_A. penninervis_ (Hickory bark) is said to be particularly hardy, but
its strength seems to vary. A sample from Bateman’s Bay contained 38 per
cent. of tanning matter.
_A. binervata_, another “Black Wattle” contains up to 30 per cent.
tanning matter, as does also the “Weeping Willow,” _A. saligna_. The
latter is poisonous, and is said to be used for killing fish.
_A. prominens_, the bark of which resembles that of the Golden Wattle,
_A. longifolia_, in appearance contains only 14 per cent. tannin.
The cultivation of wattles in Australia has been somewhat neglected, but
would render possible the utilisation of many acres of land lying waste,
or which have already been exhausted and rendered unfit for the growth
of cereals. It requires so little attention as to make it very
profitable, and wattle-growing and sheep-grazing can be combined
satisfactorily after the first year, when the young trees in the
plantation have reached the height of 3-4 feet. In Natal the Australian
wattles, especially _A. mollissima_, have been acclimatised and
cultivated with success, and large quantities of excellent bark are now
exported to England. African wattle-barks usually contain about 30 per
cent. of tannin.
Wattles grow in almost any soil, even the poorest, but their growth is
most rapid on loose, sandy patches, or where the surface has been broken
for agricultural purposes. When the soil is hard and firm,
plough-furrows should be made at a regular distance of 6-8 feet apart,
and the seeds dropped into these. The seed should be sown in May, having
been previously soaked in hot water, a little below boiling temperature,
in which they may be allowed to remain for a few hours. It should be
dropped at an average distance of 1 foot apart along the furrow, in
which case, about 7200 seeds would suffice for one acre of land. The
seed should not be covered with more than about ¹⁄₄ inch of soil.
On loose sandy soil, it might even be unnecessary to break up the ground
in any way; the furrows may be dispensed with, and the seed sown
broadcast after the land has been harrowed. After the plants have come
up, they should be thinned so that they stand 6-8 feet apart. When the
young trees have attained the height of 3-4 feet, the lower branches
should be pruned off, and every effort afterwards made to keep the stem
straight and clear, in order to facilitate the stripping, and induce an
increased yield of bark. It is advisable that the black and broad-leaved
kinds should be grown separately, as the black wattle, being of much
larger and quicker growth, would oppress the slower-growing broad-leaved
one. Care should be taken to replace every tree stripped by re-sowing,
in order that there should be as little variation in the yield as
possible. In Victoria, the months of September-December are those in
which the sap rises without intermission, and the bark is charged with
tannin. Analysis proves that the bark from trees growing on limestone is
greatly inferior in tannin to that obtained from other formations,
differing 10-25 per cent.
The following are South American mimosas:--
_A. cavenia_, Espinillo. Bark, contains 6 per cent., pods, 18-21 per
cent., or more of tannin.
_A. cebil_, Red Cebil. Bark, contains 10-15 per cent.; leaves, 6-7 per
cent. tannin. Argentine Republic.
_A. Guarensis_, Algarobilla of Argentine Republic. Bark, pods and
flowers said to be used for tanning.
_A. timbo_, Buenos Ayres.
_A. curupi_, Curupy bark.
_A. angico_, or _Piptadenia macrocarpa_, Brazil, yields “angica bark,” a
sample of which contained 20 per cent. of tanning matter when analysed
recently in the Author’s laboratory.
“White Bark,” South America, probably an acacia, bark internally very
similar to angica, if not identical.
_A. horrida_, “_Doornbosch_,” Cape of Good Hope, contains 8 per cent. of
tannin.
_Inga feuillei_, “_Paypay_,” Peru. Pods said to contain 12-15 per cent.
of tannin (doubtful). Several other species of _Inga_ known to contain
tannin.
_Elephantorrhiza Burchellii_, _Elandsboschjes_, _Tugwar_, or _Tulwah_,
South Africa; a papilionaceous plant. The air-dry root contains 12 per
cent. of tannin, and a great deal of red colouring matter. The roots are
several feet long, and about 2 inches in diameter, growing by the sides
of rivers.
The following additions may be made to the above list:--
EUPHORBIACEÆ.
_Cleistanthus collinus_, “_Kodarsi_,” Deccan. Bark stated to contain 33
per cent. of tannin.
_Phyllanthus emblica_, India, yields emblic myrobalans, which in
immature condition contain considerable tannin. Leaves (18 per cent.)
and bark used for tanning.
_Phyllanthus distichus_ and _nepalensis_ both yield tanning barks.
COMBRETACEÆ.
_Anogeissus latifolia_, India. Bark and leaves rich in tannin.
GUTTIFERÆ.
_Garcinia mangostana_, India. The rind of the mangosteen fruit contains
much tannin.
CHAPTER XIX.
_THE CHEMISTRY OF THE TANNINS._
The essential constituents of tanning materials are members of a large
group of organic compounds known as “tannins” or “tannic acids,” which
are widely distributed throughout the vegetable kingdom, and said to
have one representative among animals, in the body of the corn-weevil.
Their use in vegetable physiology is as yet uncertain, and indeed they
appear in some cases to be waste products of organic change. The
tannins, though varying considerably in their chemical constitution, and
in many important characteristics, are all marked by the power of
precipitating gelatine and some allied bodies from their solutions, of
converting animal skin into the imputrescible material known as leather,
and of forming dark-coloured compounds with ferric salts which are often
utilised as inks. They are also precipitated by lead and copper
acetates, stannous chloride, and many other metallic salts, and form
insoluble compounds with many organic bases, such as quinine, and with
the basic aniline colours. They are possessed of feeble acid character.
All tannins are soluble in water to a greater or less degree; they are
also soluble in alcohol, in mixtures of alcohol and ether, in ethyl
acetate, acetone, and a few similar solvents, but are not dissolved by
dry ether alone, nor by chloroform, petroleum spirit, carbon disulphide,
nor benzine.
As the tannins are uncrystallisable, and incapable of being distilled
without decomposition, they are exceedingly difficult to obtain in a
state of purity, and, owing to the considerable differences in their
character, no one method is equally applicable to the whole group. As
their successful separation requires considerable chemical training, and
experience, detailed description is outside the scope of the present
work, but some particulars of the more important methods employed are
given on p. 43, L.I.L.B.
Their chemical constitution is complex and in most cases imperfectly
understood, but all the natural tannins which have been investigated
prove to be derivatives of the trihydric phenol, pyrogallol, or of the
dihydric phenol, catechol, the latter of which is often accompanied by a
trihydric phenol, phloroglucol, which is isomeric with pyrogallol. The
phenols are, themselves, a class of derivatives of benzene, C₆H₆, in
which one or more of the hydrogen atoms are replaced by OH groups.
Common phenol or “carbolic acid” is their simplest representative. Many
of them, including pyrogallol and catechol, are used as photographic
“developers.” The phenols on replacing another hydrogen by carboxyl
(CO.OH) form true acids, of which salicylic corresponds to common
phenol, protocatechuic to catechol, and gallic to pyrogallol; and the
tannins are apparently complicated acids, in which one of the two latter
acids is linked to a second molecule of the same or another acid as an
anhydride, in some cases possibly with the addition of phenols or other
organic groups. For more detailed information, see L.I.L.B., p. 45.
Gallotannic acid is apparently digallic acid, in which two molecules of
gallic acid are linked together after giving up the elements of a
molecule of water. Natural gallotannic acid and many other tannins are
glucosides, or at least contain glucose, which in many cases can be
removed by purification.
From what has just been said, it is obvious that a classification of the
tannins according to constitution, is at present impracticable, not only
from our imperfect knowledge, but from the difficulty of separating and
determining the products of their decomposition. It is not, however,
difficult to distinguish the catechol- from the pyrogallol-tannins by
their chemical characteristics, apart from actual separation of the
phenols, and the division is important as it is marked by certain broad
differences in their properties which affect their use in tanning.
The catechol-tannins, dissolved in water, yield a precipitate when
bromine-water is added till the solution smells strongly of it. The
precipitate is occasionally crystalline, but generally amorphous, and of
yellowish or brownish colour. When the infusion of tannin is very weak,
the precipitate is sometimes only slight, or forms slowly.
Pyrogallol-tannins give no precipitate with bromine-water. Another
reaction, which is generally characteristic of catechol-tannins, is that
if concentrated sulphuric acid is added to a single drop of the infusion
in a test-tube, a dark red, or crimson ring is formed at the junction of
the two liquids, and on dilution with water, the solution is generally
pink. Pyrogallol-tannins on the other hand give a yellow, or at most a
dark brown ring, which dilutes to a yellowish solution. This reaction is
of great delicacy, which may be further increased by the use of an
alcoholic instead of an aqueous extract. It is often given also by the
non-tannin residue of catechol-tannins which is left after treatment
with hide-powder, in which case it is probably due to the presence of
catechins allied to the tannins. With ferric salts (preferably a
solution of iron-alum), pyrogallol-tannins give blue-blacks, while
catechol-tannins generally give greenish blacks, though the reaction is
apt to be rendered uncertain by the presence of colouring matters, or
perhaps in some cases by the constitution of the tannin. Thus aqueous
infusions of common oak-bark (_Quercus robur_) give a decidedly blue
black with iron, though the tannin is a catechol one, and the purified
tannin gives a green-black. Most of the barks of American oaks, such as
_Q. prinus_, give green-blacks without purification. The Australian
mimosas generally give dull purple-blacks with iron-salts, though they
all contain catechol-tannins. The iron test was first proposed by
Stenhouse as a means of classification. Trimble has shown that while the
purified pyrogallol-tannins only contain about 52 per cent. of carbon,
the catechol-tannins have about 60 per cent.[150]
[150] ‘The Tannins,’ ii. p. 131.
Only two tannins of the pyrogallol group have been definitely
distinguished, though it is very possible that more exist. These are
ordinary tannic acid of gall-nuts (probably digallic acid), which yields
gallic acid when heated with dilute acids, or by the action of certain
unorganised ferments or zymases (p. 16) which are generally present in
tanning materials; and ellagitannic acid (usually present in greater or
less proportion in mixture with the gallotannic acid), which, under the
same conditions yields “bloom” (an insoluble deposit of ellagic acid),
as one of its products. Hence it happens that most pyrogallol tannins
deposit “bloom” on leather, though in very different proportions,
gall-nuts and sumach giving very little, and myrobalans, valonia and
divi-divi a great deal. English oak-bark deposits a good deal of “bloom”
on leather, though it is certain that its principal tannin is a catechol
one, but it is possible that the blue-black which it gives with iron
salts may be due to the presence of ellagitannic acid, though
gallotannic acid is known to be absent. The tannins of oakwood, chestnut
and valonia are principally if not entirely pyrogallol derivatives,
closely allied to, if not identical with the two just named, but, if so,
very difficult to obtain in a pure condition. It is noteworthy that so
wide a difference exists between the various products of the oak; galls,
bark, fruit and wood yielding tannins of very varied properties. The
tannin of other galls, such as those of the sumach and pistacio,
generally contain gallotannic acid, even when, as in the last case, the
remainder of the plant yields catechol-tannins.
The tannins of the catechol group appear to present much more variety
than the pyrogallol-tannins, though it is possible that many apparent
differences may be due to the presence of impurities. It is, however, at
least certain that the tannins of gambier and cutch contain phloroglucol
as one of their constituents, while it is absent from most other tanning
materials. Its presence is easily detected by moistening pine-wood (a
deal shaving) with an infusion of the tannin in question, and applying a
little concentrated hydrochloric acid, when after a few minutes, a
bright red or purple stain is produced. The catechol-tannins, on boiling
with acids, yield no gallic acid, or bloom, but generally a deposit of
“reds,” insoluble in water but soluble in alkaline liquids, and in
alcohol, and which are closely allied to resins, and especially to the
red resin known as “dragon’s blood.” These reds are anhydrides of the
tannins, that is, are produced from them by the abstraction of water;
and are consequently formed by any agency which tends to remove water,
such as long boiling or high temperature. The lower anhydrides (that is,
those from which least water has been abstracted) are not wholly
insoluble, but form the “difficultly soluble” tannins which are
naturally present in many materials. They are much more readily soluble
in hot than in cold water, which is one of the causes why liquors made
by the aid of heat generally give darker colour to leather than those
extracted cold. They exist in large quantity in hemlock extract and
quebracho. Attempts have been made to utilise their alkaline solutions
for tanning, but without much success; though alkalies or alkaline
sulphites are frequently used to obtain “soluble” quebracho extracts (p.
338).
Many catechol tanning materials, and especially gambier, cutch and
quebracho, contain in addition to the tannin, considerable portions of
colourless bodies called catechins, which are only slightly soluble in
cold water, but readily in hot, and which crystallise out on cooling.
These bodies do not tan, but are in a sense the source of the tannins,
which appear to be their first anhydrides, the reds being formed by the
successive loss of further molecules of water. These bodies very
probably ultimately become converted into tannins by changes in the
tanyard. The change may be brought about very rapidly by heating to a
temperature of 100 to 120° C.[151] The catechin of gambier, by
crystallising on and in the leather, is the cause of a trouble known as
“whites,” which is common where gambier is largely used.
[151] Some doubt exists as to the exact temperature at which catechins
become converted into anhydrides, and Perkin puts it higher than that
stated.
An unfortunate peculiarity, apparently common to all catechol-tannins,
is that, however light-coloured the leather produced by them, it darkens
and reddens rapidly by exposure to strong light, and ultimately becomes
quite friable and rotten.[152] Cp. pp. 234, 272.
[152] Cp. Report of Committee on Leathers for Bookbinding, Journ. Soc.
of Arts, 1901, p. 14.
Wagner, a German chemist, attempted to classify the tannins into
“physiological” tannins, which were produced in the natural growth of
the plant, and “pathological” which were caused by the attack of insects
such as the gall-wasps, and he further ventured the assertion that only
the former class were capable of producing leather. It has since been
shown that the tannins produced in galls are identical with some of
those found in healthy plants, and galls themselves have been used in
tanning from very ancient times. It is only necessary to remind the
reader of the use of Turkish gall-nuts, in place of sumach, which was
common in the East in the tannage of moroccos, and of the “Knoppern,” or
oak-galls formerly so largely used in Austria as a tanning material for
sole leather. It is true that the tannin of galls is not very suitable
for the latter purpose, consisting as it does mostly of gallotannic
acid, which, giving no solid deposit of bloom or reds, is incapable of
making a heavy or solid leather. Pure gallotannic acid itself produces a
very white and soft leather.
The class to which the tannins of the different tanning materials belong
is mostly mentioned in the Botanical List (Chap. XVIII.), but it may be
well here to specify a few of the most common. Galls and sumach contain
gallotannic acid with a little ellagitannic; myrobalans, valonia,
divi-divi, algarobilla, oakwood and chestnut are all pyrogallol-tannins
giving ellagic and gallic acid among their decomposition products. All
the pine barks, including the American hemlock, and the larch, all the
acacias and mimosas, including the Indian Babul (_Acacia arabica_), the
oak barks (though not the oak wood, fruits, or galls), quebracho wood,
cassia[153] and mangrove barks, canaigre, cutch and gambier are
catechol-tannins, and the two last contain phloroglucol, of which minute
traces are also present in many other catechol-tannins (p. 297).
[153] _Cassia auriculata_, or “turwar” bark, is the ordinary tannage
of the East Indian or “Persian” sheep- and goat-skins, largely used in
bookbinding, but which redden and decay very rapidly.
Gallotannic acid, and several artificial tannins with the characteristic
reactions of the class have been produced in the laboratory, but there
is no present prospect of their manufacture at prices which can in the
faintest way compete with those of natural production.
Tanning materials frequently contain mordant colouring matters, often
derived from the same phenols as the accompanying tannins. They also
usually contain gums, starch and glucose. Oak bark contains lævulose
which is not combined with the tannin. Many tannins, however, exist in
nature in combination with the sugars as glucosides, which are easily
decomposed by the action of acids or by fermentation. These sugary
matters are important as furnishing by fermentation the acetic and
lactic acids of tanning liquors.
CHAPTER XX.
_THE SAMPLING AND ANALYSIS OF TANNING MATERIALS._
Although the analysis of tanning materials falls more properly within
the scope of a book for chemists than one intended primarily for
tanners, and though it has been treated at considerable length in the
‘Leather Industries Laboratory Book,’ a slight sketch must now be given
of the methods in general use, since it is of great importance that at
least the principles on which they are based should be understood by all
to whom they are of practical interest, and also because an approximate
analysis of a tanning material by the hide-powder method is within the
scope of any intelligent tanner who will provide himself with the
necessary implements. Much attention has been paid to the subject area
by the International Association of Leather Trades Chemists, and also by
the American Official Association of Agricultural Chemists, and as the
methods prescribed by one or other of these are with very little
exception employed by all qualified chemists throughout the world, their
directions, corrected up to date, are given in Appendices A and C. As,
however, these directions are addressed to chemists already familiar
with the usual course of analysis, a somewhat fuller explanation must
here be given.
It must specially be insisted on, that absolute adherence to the methods
given is essential to obtaining concordant results, and little points of
manipulation which appear in themselves unimportant, are frequently the
result of long experience and careful discussion. The members of the
International Association, especially, are bound by their rules to make
note in their analytical reports of any deviation, however small, from
the prescribed process.
The first step in the analysis of any material is to draw a sample
truly representing the bulk, which is often by no means easy, while
failure to accomplish it is probably the cause of more errors and
disputes than any inaccuracy of the method of analysis itself. In very
many cases, chemists are blamed for discrepancies which really exist in
the samples supplied to them. The chemists of the International
Association only hold themselves responsible for the accuracy of their
analyses when the sampling has been done strictly according to the rules
prescribed by their Association. On this account, all important samples
should be drawn in the presence of a principal, or some other
responsible person.
_In liquid extracts_, the thorough mixing of the liquid is of the
greatest importance. Most extracts contain a portion of “difficultly
soluble” tannins (see p. 297), which slowly settle to the bottom, or
adhere to the sides of the cask; from which such expedients as merely
rolling a full cask are quite inefficient to dislodge them. In fact
nothing but taking the heads out of a sufficient number of casks, and
actually stirring them with a suitable plunger, which should be
specially applied to the sides and bottom, or emptying the entire
contents of the casks into a tank in which the whole can be adequately
mixed, is really thoroughly reliable, though at times it is necessary to
be content with less satisfactory methods. In any case, when it is
probable that samples must be submitted to more than one chemist, the
whole should be drawn at once, thoroughly mixed and divided, and sealed
in separate bottles, and in dividing a sample the same care must be
taken to ensure complete mixture, as in drawing the original sample.
_Solid and pasty extracts_, such as quebracho, cutch and gambier, are
still more difficult to sample fairly, as the outside is almost
invariably much drier than the interior. Generally the only way is to
select such portions as are thought fairly to represent the bulk, to
chop them into moderately small pieces, mix and seal in an air-tight
tin, leaving it to the chemist to draw from these the smaller sample
required for analysis. Gambier is best sampled with a tubular tool like
a cork-borer, designed by Mr. Kathreiner, Fig. 63, which should be
passed completely through the bale, or the cylindrical sample of gambier
cannot be withdrawn. The same tool may also be used for sampling sumach
in bags, if the damage to the bag is not objected to. If such a tool is
not available, the only fair way to sample gambier is to cut slices
completely through the bale with a clean fleshing knife. In any case it
is of the utmost importance that the sample once drawn, should be mixed
as rapidly as possible, and at once enclosed in an _air-tight_ box or
jar, sealed and labelled.
[Illustration: FIG. 63.--Kathreiner’s Sampling Tools. A, strong
cross-handle; B, guard-disc; C C´, brass tube sharpened at C´; D, brass
or wooden plunger.]
_Dry tanning materials_, such as bark and valonia, require judgment in
selecting samples which fairly represent bulk. If they are of a nature
which do not readily separate into dust and fibre, a good method is to
grind a sufficient quantity in an ordinary bark-mill, and after well
mixing, to draw the sample from the ground portion. In other cases it is
best to empty a sufficient number of bags one upon another in layers on
a smooth floor, and to take out a section down to the floor. In such
materials as valonia and divi-divi, the dust or beard is usually much
stronger than the average of the pods or cups.
The same sort of precautions are required in drawing the still smaller
sample required for analysis from the larger original sample, but these
are sufficiently detailed in the directions of the I.A.L.T.C. given in
the Appendix. As materials usually require finer grinding than can be
managed with the mills employed in the tannery, a suitable mill must be
provided, and one of the simplest, at a moderate price, is a No. 4
drug-mill made by A. Kenrick and Sons, Limited, West Bromwich, Fig. 64.
Coffee mills are seldom strong enough for the purpose, but if nothing
better is available, the sample must be _thoroughly_ dried before
grinding, and its loss of weight noted, and taken into account in
calculating the analysis, care being taken that the sample after
grinding is so preserved that it cannot re-absorb moisture. Valonia,
myrobalans and even barks, may before grinding be broken with a
flat-faced hammer, on a thick cast-iron plate, with raised edges to
prevent loss from flying fragments.
[Illustration: FIG. 64.--Kenrick’s Drug-Mill.]
_Preparation of solution for analysis._--As the method of analysis only
gives satisfactory results when the quantity of tanning matter in the
solution is within certain limits, the International Association
prescribes that it must be such as to contain between 3·5 and 4·5 grms.
of tanning matter per liter, or as near as possible, on the average, to
4 grms. If, as will rarely happen, the strength of a material is quite
unknown, it may be necessary to make a trial test to ascertain the
quantity of substance to be used, but the following table gives the
quantity with sufficient accuracy for most ordinary materials.
TABLE SHOWING THE AMOUNT OF DIFFERENT MATERIALS TO BE WEIGHED OUT FOR
ANALYSIS TO MAKE UP ONE LITER OF SOLUTION.
_Barks, etc._
Grams.
Algarobilla 9
Canaigre 15
Divi-divi 9
Hemlock bark 16
Mimosa bark 11
Myrobalans 15
Oak-bark 30
Oak-wood 100
Quebracho wood 20
Sumach 15
Valonia 15
Valonia beard 11
_Extracts._
Oak-wood, sp. gr. 1·2 or over 15
Chestnut ditto 14
Quebracho (solid) 6
„ (liquid) 9 to 13
Gambier (block) 10
„ (cube) 7
The best method of weighing out exact quantities may be here described
for those to whom it is not already known, as much time may be wasted by
attempting it unsystematically. The material is of course weighed in a
basin, which together with the weight which is desired of the material,
is exactly counterpoised by weights in the other pan. Where many
weighings of the sort have to be made, it saves time to keep one
particular basin for the purpose, which should be properly marked[154];
and to make a counterpoise of lead or brass exactly equal to it in
weight, so that it is only necessary to add weights corresponding to the
quantity required to be weighed out. Supposing now, that it is a liquid
extract which is to be weighed, a sufficient quantity is introduced into
the basin with a pipette, to slightly exceed the required weight. The
pipette is now emptied, and a small quantity is withdrawn with it from
the basin. If the basin is still too heavy, the pipette is emptied, and
the process repeated until the basin is too light. The true weight now
lies between that in the basin and the small quantity retained in the
pipette, from which extract is added till the basin is again
over-weighed, and the same process is repeated, each time reducing the
margin, till a sufficient approximation is obtained. It is not necessary
in weighing out the sample, to be accurate to a single milligram; but
with practice, this amount of accuracy is easily attained. If the
material is solid, a spatula is substituted for the pipette. The
weighing of liquid or pasty extracts should be as rapid as possible, as
they lose weight on the balance by evaporation.
[154] Porcelain basins may be indelibly marked by writing on them with
an ordinary iron ink, and heating strongly with a blowpipe.
Liquid extracts are most easily dissolved by placing a large funnel in
the neck of a liter flask, and after pouring a little boiling water into
the flask, holding the basin inclined in the funnel, and washing out its
contents with boiling distilled water from a glass wash-bottle, or a
perfectly clean copper kettle, till the flask is filled to the mark. The
flask is now covered with a small beaker, which must hang loose on its
neck, without resting on its shoulders, and is rapidly cooled by placing
it under a cold water tap, to a temperature as little above 15° C. as
possible, and is then filled up to the mark on the neck with cold water,
and well mixed by shaking very thoroughly.
Solid or pasty extracts are dissolved in a beaker by stirring with
successive quantities of boiling water, which are poured off into the
flask, leaving the undissolved matter in the beaker. When the flask is
nearly full, if any small portions remain undissolved or insoluble, they
may be rinsed into it with the last portions of hot water, and the flask
is now cooled and mixed as already described.
[Illustration: FIG. 65.--Procter’s Extractor.]
_Extraction of solid materials_, such as barks, or valonia, is more
difficult, but the following is a convenient method, which has been
recognised as official by the International Association. An ordinary
beaker, of about 200 c.c. capacity, but which may be varied in size
according to the weight of the material which it is necessary to treat,
is placed in a water-bath, as shown in Fig. 65. A thistle-headed funnel,
the stem of which is bent twice at right angles, and of which the head
is covered with a piece of fine silk gauze (such as is used by millers)
to act as a strainer, is placed in the beaker and held in position by a
clamp as shown in the figure. To the free end of its stem a piece of
glass tube, six or eight inches long, is attached by indiarubber tube,
which is provided with a pinchcock to regulate the flow of liquid. Fine
silver-sand, freed from iron and soluble matters by washing first with
hydrochloric acid, and then very thoroughly with water, is now poured
into the beaker, so as to surround the head of the funnel to about half
an inch in depth; and the weighed quantity of tanning material is next
introduced. It is best to cover the material with cold water, and allow
it to stand all night, but in case of haste, water of 30° to 50° may be
used, and the extraction proceeded with after the material is thoroughly
soaked. Percolation is started by sucking the syphon, and allowing the
liquid to drop slowly into a liter flask, the temperature of the
water-bath being maintained by a Bunsen burner, and the beaker being
refilled as it requires it with water at the desired temperature.[155]
At least 500 c.c. must be percolated before the temperature is allowed
to exceed 50°, after which, except in the case of sumach and canaigre,
which should be begun about 30°, and at no time allowed to rise above
50°, the temperature may be raised to boiling point. At least 1¹⁄₂ hour
should be employed in percolating 800 to 900 c.c. and if the material is
not then practically exhausted, the liter flask must be withdrawn, and
an ordinary ungauged flask substituted, into which the percolation is
continued till the material is exhausted. The very dilute liquor in the
second flask is now boiled down till its volume is sufficiently small to
be added to that in the liter flask, a small funnel being placed in its
neck during ebullition, to prevent spirting and the access of air. Under
no conditions must the stronger liquor of the first part of the
percolation be boiled down, as this would involve destruction of tannin.
The solution is now cooled, and made up to the mark as has been before
described. Most ordinary materials may be practically exhausted by the
liter of water if percolation is slow, and the trouble of evaporation
may thus be avoided.
[155] The material should be kept in an even layer, and if necessary
the surface may be stirred at intervals with the thermometer or a
glass rod.
_Total soluble matter._--The solution of which the preparation has been
described, must now be filtered, the size and kind of paper, and exact
method of filtration prescribed by the International Association being
strictly adhered to. All papers and methods of filtration absorb traces
of tanning matters, and but few will give a clear filtrate with such
solutions as those of quebracho and hemlock extracts; and to obtain
uniform results exact uniformity of method is essential.[156] Deviations
from the exact method, in the case of quebracho, easily cause
discrepancies of several units per cent. in the result. The object of
rejecting the first portions of the filtered solution is to prevent, as
far as possible, errors which would arise from the absorption of tannin
by the paper, and to insure a clear filtrate. 50 c.c. of the clear
filtrate is now measured by an accurate pipette, and evaporated to
dryness in a weighed porcelain basin, on a steam-bath, in order to
determine the “total soluble.” This and succeeding operations should be
done in duplicate, even if this has not been the case in making up the
original solution, which is certainly desirable.
[156] Methods of correction for absorption of filter-papers have been
worked out in the Author’s laboratory, and adopted by the last
conference of the International Association. Cp. Collegium, pp.
145-158, 1902, and App. A, p. 477.
Ordinary light porcelain basins, generally of about three inches
diameter, are employed for evaporation, which takes place somewhat more
rapidly if they are flat-bottomed (saucer-shaped). In place of
porcelain, thin glass basins of hemispherical form may be used, and, but
for the cost, platinum would be better than anything else. Aluminium and
nickel basins have been tried, but are slightly attacked by some
liquors, and hence are more liable to vary in weight, though they have
the advantage in rapidity of evaporation. Evaporation takes place most
quickly if the steam-bath can be placed in a draught of air, so as to
rapidly carry away the vapour formed, but the basins must be protected
from dust. Under favourable circumstances, evaporation of 50 c.c. in
porcelain basins occupies one to one and a half hour. An ordinary pan
fitted with a lid of thin copper perforated with holes of two and
three-quarter inches in diameter, makes a useful water-bath; but where
much work is done, it is desirable to have a rectangular bath of thin
sheet copper, taking a single, or at most a double row of basins, and
fitted with the usual appliance for keeping the water at constant level;
or with a supply of steam from a boiler, and an overflow for condensed
water.
As soon as the contents of the basins appear completely dry, they may be
transferred to the drying oven. The most satisfactory form is one in
which the basins are placed in a closed chamber, surrounded by steam at
the atmospheric pressure, and at the same time subjected to a vacuum
maintained by a water-jet air-pump; but as this apparatus is somewhat
costly, it will probably only be provided in laboratories which make a
speciality of such work. Next to the vacuum-oven, an air-oven, heated by
a gas-burner, and with its temperature controlled by a mercurial
regulator to 100-105° C., gives the best results, and it is also the
cheapest; but considerable care and some scientific knowledge are
required to work it satisfactorily. In intelligent hands good results
may be got from the small “breakfast cooker” gas ovens made by Fletcher
of Warrington, which are placed on an iron plate heated by a gas burner,
the supply of gas to which is regulated by a thermostat, or mercurial
gas-regulator, inserted, together with a thermometer, through holes
drilled in the top. The basins must not be placed too near the bottom of
the oven, which must be protected by a perforated metal plate supported
perhaps one inch above it, to prevent radiation and to distribute the
hot air. Any cold air required for ventilation should be admitted below
this plate, and care should be taken to exclude the products of the
burning gas. Contact of the basins with any heated part of the
metal-work should be carefully avoided, and they are best supported on
grid-shelves covered with wire gauze or perforated metal, so as to allow
of free circulation of air. If perforated zinc is used, it must be well
supported, as it is much softened at the temperature used. The least
satisfactory appliance in skilled hands, but probably the most easy to
work by the inexperienced, is the ordinary water- or steam-oven. It is
impossible, in this apparatus, to raise the temperature of the interior
fully to boiling point, and below this gambier, quebracho, and other
solutions containing catechins (p. 298), dry very slowly. On the other
hand, so long as it is kept boiling and supplied with water, the
temperature is necessarily constant, and there is no danger of
overheating, which easily occurs in ovens heated directly by gas. Such
ovens are often fitted with openings at the top for use as a steam-bath.
To get the best results, the basins must be as freely exposed as
possible to the air in the interior of the oven (in no case must basins
be set one inside another, except in the exsiccator for cooling), and
little or no ventilation from the outside is required, as only traces of
moisture remain after evaporation on the steam-bath; so that, after an
hour’s drying, any ventilators may safely be closed. As a good deal of
cooling takes place through the door, it is best to protect it with some
non-conducting material, such as asbestos millboard, which may be
attached with rivets, or even with ordinary paper-fasteners. One to one
and a half hours will be required to dry to constance in the
vacuum-oven; two to three in the air-oven at 105°; and probably about
four hours in the water-oven, except in the case of gambiers, which may
require somewhat longer. Too long heating is disadvantageous, as the
residues begin to oxidise and gain in weight. As soon as it is judged
that the basins will be constant in weight, they are withdrawn from the
drying oven, and at once placed in an exsiccator (a glass vessel with an
accurately fitted lid, which should be slightly greased, in the bottom
part of which is placed either dry calcium chloride or concentrated
sulphuric acid, to absorb the moisture of the air it contains). In this
they are left till thoroughly cold, which if several basins are put in
together, may require half an hour. They are then weighed accurately,
but as rapidly as possible; returned to the drying oven for half an
hour; and replaced in the exsiccator. The exact weight of each basin, as
it comes in turn to be weighed, is now placed on the balance before
removing the basin from the exsiccator, so that it can be seen instantly
if there is loss or gain of weight, before it has time to absorb any
moisture from the air. The weight should not be more than a milligram or
so less than at the first weighing; if weight has been gained, it is
caused by oxidation, and the first weight should be taken as correct if
it is certain that the basin was then perfectly cold; a very slight
amount of warmth easily reducing the apparent weight by several
milligrams. If material loss has occurred, the basin must of course be
returned to the oven, and re-weighed in another half hour; but with
experience, this should rarely be needed.
It is necessary that the balance used should weigh accurately to
milligrams; and it must carry at least 50 grm. on each pan; while it is
more convenient that it should carry 100 or more, it is always possible
with a little ingenuity, to manage within 50 grm.; and if a cheap
balance must be used, the smaller size will probably be more accurate.
Balances of this sort can now be got for two or three pounds, though it
is in all respects better to obtain one of first rate quality, which
should cost about ten pounds. The balances of Verbeek and Peckholdt, of
Dresden, from their simplicity and rapidity of weighing, have given
great satisfaction in technical work in the Yorkshire College. Whatever
economy be exercised in the choice of the balance, it is essential that
the set of weights should be of the greatest accuracy, and especially
that all the weights of one denomination (10 grm., 1 grm., etc.), should
accurately balance each other. Even after all precautions are taken, it
is desirable that those weights which are in duplicate should receive
distinguishing marks (e.g. with a centre-punch), and should always be
placed on the scale in the same order; and, not only on account of
possible inaccuracy, but to save time, it is desirable to reject basins
which are so nearly of an even weight (20, 25, 30 grm.) that when
weighed with the residue (0·3-0·4 grm.), a change of the larger weights
may be required, since it is to be remembered that any error of the
weights employed is concentrated on the small weight of the residue.
After deducting the weight of the empty basins, the weight _in
milligrams_ of the two residues of 50 c.c., which should be practically
alike, are added together, and the sum divided by the weight _in grams_
of tanning material used; which gives the percentage of “total soluble
matter.”
[Illustration: FIG. 66.
Hide-powder Filter.]
_Non-tannins._--It is now necessary to determine the proportion of the
“total soluble” which consists of “non-tanning matters,” that is, of
substances not removed from the solution by treatment with hide-powder.
The so-called “tanning matters” removed, include colouring matters and
some other substances, which though absorbed by hide, are certainly not
tannins in a strictly chemical sense. (See note, p. 480.)
According to the method of the International Association, the apparatus
shown in Fig. 66 is employed for this purpose. The glass bell is
carefully and uniformly stuffed with hide-powder, care being taken that
no channels are left, especially at the sides, through which the liquor
can reach the syphon without traversing the hide-powder. Before filling
the bell, the short leg of the syphon-tube should be loosely plugged
with cotton-wool (of which a little is allowed to project from the end),
in order to prevent the powder from gaining access to the tube. The
powder is retained in its position in the bell by a piece of muslin held
by an indiarubber band, and the bell is then placed in a beaker or
tumbler as shown in the figure; and filtered liquor is gradually added,
as it is absorbed by the powder, till the whole is uniformly wetted. The
liquor which was first filtered through the paper, and rejected for
“total solubles,” may be used for this purpose, and it is not necessary
that it should be absolutely clear. The syphon is now gently sucked, and
the filtrate is allowed to fall, drop by drop, into a gauged cylinder.
The first 30 c.c. which collects is rejected, since it contains traces
of dissolved hide-substance even from the purest hide-powder; and the
next 50 c.c. should give no turbidity if a few drops are mixed either
with clear tannin solution (absence of dissolved hide-substance), or
with the first 30 c.c. (absence of tannin). This 50 c.c. is used for
determination of non-tannins, by evaporation and drying precisely as has
been described in the case of “total soluble.” Some chemists, with very
accurate balances, prefer to evaporate only 25 c.c., which effects a
little saving of time in evaporation; but in any case the whole of the
50 c.c. must be allowed to run through the filter before it is measured,
as the filtrate varies somewhat in solid contents as the filtration
proceeds. The filtration and evaporation should be done in duplicate.
The weight of the residue is calculated into percentage as “soluble
non-tanning matters” precisely as has been described for the “total
soluble”; and when subtracted from the latter, the remainder is the
percentage of “tanning matters.” If the hide-powder now employed by the
English members of the International Association (manufactured by
Messrs. Mehner and Stransky in Freiberg in Sachsen), be employed, no
difficulty will be found in the filtration. This powder is quite
neutral, and contains between 10 and 20 per cent. of cellulose to render
it more absorbent. It does not swell in the filter, and hence should be
stuffed into the bell almost as tightly as possible, about 10 grm. being
required. If the bell is properly filled, the filtration should
altogether take about one hour, but if the liquid runs too fast, it must
be regulated by a pinchcock on the indiarubber tube of the syphon. If
other powders are used, which often contain acid, and swell very much in
the bell, the filling is much more difficult, and while the sides of the
bell must be closely packed, great care is requisite to keep the powder
loose in the centre, or the filter will not run. One point requires
mention with regard to neutral hide-powders. If an extract which has
been rendered soluble by the addition of alkalis or sulphites (p. 388)
be analysed with a perfectly neutral powder, it has been shown by
Paessler and Appelius[157] that a part of the tannin combined with the
alkali will not be absorbed, while with acid powders, the whole will be
estimated.
[157] Wissenschaftliche Beilage des ‘Ledermarkt,’ 1901, p. 107.
[Illustration: FIG. 67.
American Milk-shaker.]
_The “shake-method” adopted by the American Association of Official
Agricultural Chemists_, possesses some advantages, especially in the
analysis of used liquors which, from the acids they contain, are apt to
give somewhat too high results by the filter method (see App. B, page
480). It has the further advantage of being much less dependent than the
filter-method on the quality of the hide-powder employed. It has
therefore been accepted by the International Association as permissive
for all tanning materials, and as compulsory for used liquors (see App.
A), and must therefore be briefly explained. It can be carried out
successfully with somewhat inferior hide-powders to those required for
the filter, but generally gives results 1 or 2 per cent. lower in
tannins than the latter. A special shaking machine must be employed,
capable of thoroughly agitating a mixture of hide-powder and the liquor
to be analysed; and if many analyses have to be done, it is convenient
that it should be driven by power, as otherwise the work becomes
somewhat laborious. A machine called a “milk-shaker,” Fig. 67, employed
in the mixing of summer drinks, is generally used. The quantity of
powder required for the analyses to be made (about 8 grm. of ordinary
air-dried powder for each determination, with say 5 grm. added), is
stirred in a large beaker with 25 times its weight of distilled water,
and allowed to soak for 24 hours, 1·5 per cent. of chrome-alum
previously dissolved in water being added at the beginning of the
operation, and 1·5 per cent. more not less than 6 hours before its end.
The powder is then washed by squeezing through linen, and the washing is
continued till the wash-water no longer gives a precipitate with barium
chloride; and is then well squeezed out in linen, preferably with the
aid of a press. The damp squeezed powder is now roughly weighed, to
determine approximately what quantity it is necessary to take, to give
7·5 grm. of the original dry powder to each estimation (air-dried powder
contains about 15 per cent. of moisture), and a portion is accurately
weighed in a basin, and dried, first on the water-bath, and then in the
drying oven, to determine its moisture by loss. The approximate amount
of powder required for each determination--if possible a round number of
grams--is now weighed into as many bottles of about 300 c.c. capacity as
determinations are to be made, 100 c.c. of the filtered liquors,
prepared as before described, are introduced into each bottle, and the
bottles are then each shaken for 10 minutes (Mr. Alsop states that in
his experience 5 minutes is sufficient). The contents of the bottles are
now filtered through funnels, the stems of which are plugged with pure
cotton-wool, and the liquor is returned till a clear filtrate is
obtained, of which 50 c.c. is evaporated as in the International method.
It is now necessary to accurately correct the residue obtained, for the
amount of water carried in by the wet powder. The loss of weight of the
powder which has been dried, divided by its wet weight, gives the water
contained in each gram of wet powder, and this multiplied by the weight
of wet powder added to the liquor, gives the weight in grams (or volume
in c.c.) of water which has been added to each 100 c.c. of liquor.
Consequently, if the residues found be multiplied by this weight plus
100, and the product divided by 100, the weight will be obtained which
should have been given by 50 c.c. of undiluted but detannised liquor;
and from this the non-tannins are calculated exactly as in the case of
the residues from the filter process. Of course, in practice, a factor
is found, by which it is simply necessary to multiply all the residues,
to correct them to undiluted weight. The process sounds somewhat
complicated, but in reality, where a large number of determinations have
to be made, is quite as quick, if not quicker than the filter method;
which it is quite possible it may ultimately supersede, as much
attention is being devoted to its improvement.
Having determined the tanning, and soluble non-tanning matters of the
materials, it remains to determine the moisture, and the insoluble which
make up the whole. To determine moisture, a quantity, not exceeding two
or three grams of dry solid materials, or half a gram of moist or liquid
extracts, is weighed into a basin, and dried in the same way as has been
described for the residues, only that a considerably longer time will be
required before constancy is attained. The object of employing so small
a quantity of liquid extracts is to abridge this time, and the
consequent oxidation, as much as possible, as the extract soon forms a
hard skin on the exterior, which renders further drying very tedious. It
is advantageous to add a little alcohol to liquid and semi-liquid
extracts, and so dilute them that by inclining the basin they can be
distributed in a thin layer over its sides, while at the same time the
alcohol facilitates the evaporation of the water. The weight of the
dried residue in the basin is the “total solids,” while the loss is the
“water”; and these can be converted into percentages by multiplying by
100 and dividing by the weight of substance originally taken. An
alternate method, which is frequently convenient with extracts, is to
pipette off 50 c.c. (in duplicate) of the dissolved and well-mixed
extract-solution _before filtration_, and dry exactly in the same way as
for “total soluble.” The sum of the two residues in milligrams, divided
by the weight of extract taken for analysis, gives the “total solids”;
subtracting this from 100 leaves the “water,” while the difference
between the “total solids” and the “total soluble” is the percentage of
insoluble matter. Two further points must be noted. If the total solids
are determined by the first method, and the total soluble in the
ordinary way, in an extract which contains no insoluble matter, it
frequently happens that they differ by 0·1 or 0·2 per cent., owing
either to the difficulty of driving off the whole of the water, or to
slight oxidation of the total soluble residue. On the other hand, if the
second method is adopted, a small amount of “insoluble” is invariably
found, even in perfectly soluble extracts, which is due to the
absorption of tannin or colouring matter by the filter paper. On the
correction of this error, see Collegium, 1902, pp. 145-158, and App. A,
p. 477.
As the value of a tanning material often depends very much on the
paleness of its colour, it has become customary to specify in contracts
the intensity of colour of a solution of it containing one-half per
cent. of tanning matter (as measured by the I.A.L.T.C. method of
analysis), in a glass cell of one centimeter thick, by comparison with
standard coloured glasses in the tintometer. On the method of making the
measurement see L.I.L.B., p. 131.
NOTE.--All the apparatus named in this chapter can be obtained of
Messrs. Reynolds and Branson, Commercial Street, Leeds; or of Messrs.
Portway, Jamaica Road, S.E.; and of most other dealers in chemical
apparatus.
CHAPTER XXI.
_GRINDING OF TANNING MATERIALS._
Before the tannin they contain can be extracted, most materials require
to be ground, almost the only exceptions to this rule being divi-divi
and algarobilla, in which the tannin is very loosely contained.
Extracts, whether solid or liquid, merely require to be dissolved in
water or liquor, in which they are, for all practical purposes,
perfectly soluble. With the less soluble extracts it is generally
preferable to dissolve at a temperature of 50° to 60° C. with vigorous
stirring.
The actual method of grinding, and consequently the machinery employed
for the purpose, vary not only with the material to be ground, but with
the method of leaching adopted, as it is essential that the mass of
ground material should be completely permeated by the liquor employed in
leaching; and if it be ground too finely, or subjected to too much
pressure on account of the height to which it is piled in the leaches,
it is apt to form a compact and clay-like mass, the interior of which
remains unextracted.
[Illustration: FIG. 68.--Cone-Mill.]
In the laboratory, where thorough extraction must be completed in a few
hours, the material can hardly be too fine; but on the larger scale a
much coarser product must be used, and leaching requires days, or
sometimes even weeks, and is then seldom successful in removing all the
tannin. It is probable, however, that in the future these mechanical
difficulties of extraction will be overcome; and the material will then
be as finely divided, and as completely extracted on the large scale, as
it is in the laboratory at the present time.
One of the earliest methods of grinding oak-bark, and which is still
used for sumach (p. 271) consists in crushing it under large circular
edge-stones, frequently turned by a horse. This process was very slow
and inefficient for barks, and both it and horizontal millstones
similar to those used for wheat were long ago superseded by iron or
steel mills on the same principle as the ordinary coffee-mill.
These mills, Fig. 68, consist of a “bell” or inner cone, covered with
blades or teeth arranged at a slight angle to the vertical section of
the cone, and which are made finer and increased in number towards its
lower and wider part. This cone rotates within an outer hollow cone or
casing, also provided with blades or teeth which are sloped slightly in
the opposite direction to those of the inner cone, so as to meet them at
an angle, like the cutting-blades of a pair of scissors, and the angles
of the cone are so chosen that the blades approach each other more
closely towards their base. The outer cone is fixed, and is provided
with a hopper like a coffee-mill, while the inner cone is so rotated on
its axis that bark placed in the hopper is screwed down between the two,
and cut finer and finer till it reaches the lower edge, when it drops
out. The blades or teeth are usually cast in one piece with the metal
cones, and sharpened when required by chipping with cold chisels. This
operation should not be conducted in the mill-house, or small chippings
of iron may get mixed with the bark, and cause stains on the leather.
This form of mill, which is run in England at about 30 revolutions per
minute, and at nearly three times this speed in America, works very well
with dry material, but clogs badly if it be appreciably damp. On this
account it is always well to run the mill with a fairly slack belt which
will slip before exerting sufficient pressure to break the machine, as
in such operations as grinding, safety clutches are of but little use.
A type of mill varying somewhat from the above, consists of a pair of
discs or very obtuse cones, the inner one of which runs on a horizontal
axis. The teeth are generally arranged in concentric rings and interlock
with each other. The material to be ground is fed at or near the centre
of the fixed disc, and escapes at the edges. The construction of this
class of mill will be easily understood from Fig. 69. Very small pieces
of iron or steel which get caught between the teeth will often result in
the breaking of the latter, and the formation of iron dust, which is a
serious objection to the employment of this type of mill (to which the
Schmeija “Excelsior,” the Glaeser “Favorita,” and the “Devil
Disintegrator” of the Hardy Patent Pick Co. belong) for grinding barks.
[Illustration: FIG. 69.--“Excelsior” Mill.]
Myrobalans and mimosa-barks have proved especially troublesome to grind,
the former from the hardness of the stones of the fruit, and a tendency
to clog the mill, and the latter from their combined hardness and
toughness. “Disintegrators” of various patterns are now made, which are
capable of grinding both these materials satisfactorily, and but for
their liability to cause fire, and the large proportion of fine dust
which they make, are usually to be preferred to toothed mills. In spite
of their disadvantages, however, they have come very largely into use,
on account of their efficiency in grinding obstinate materials.
Disintegrators work on the principle of knocking or beating the material
to powder, by means of very rapidly revolving beaters, which, in the
smaller machines, are driven at 2500 to 3000 revolutions per minute.
The first disintegrator was made by Carr and consisted of two concentric
cylinders or baskets of steel bars, rotating in opposite directions at a
very high speed. The material was fed between these and was dashed to
pieces by being thrown against the bars and the outer casing.
[Illustration: FIG. 70.--Disintegrator.]
A simpler form was soon introduced by Carter, in which only one axis was
employed, carrying radial beaters which dashed the material against the
serrated outer casing, a portion of the circumference of which was
fitted with gratings, through which the ground material was thrown as
soon as it was sufficiently reduced in size, the fineness of the
grinding being regulated by changing the grates as required. This type
of disintegrator is, with slight variations, made by all the leading
makers of tanners’ machinery; and one form is shown in Fig. 70, and a
similar but smaller machine, opened to show construction, in Fig. 71.
In the more modern machines the sides as well as the circumference of
the casing are frequently corrugated in order to increase the action on
the material.
Mills running at such high rates of speed as 3000 revolutions per minute
will grind most hard substances, such as stone or brick, without
injury, but pieces of iron among the tanning material are apt to cause
damage, and various magnetic devices have been employed for separating
this metal, but with only partial success. In the best mills, therefore,
the beaters and inner casings are constructed so that they can be easily
replaced, and the damage is then rarely serious.
[Illustration: FIG. 71.--Disintegrator opened, showing construction.]
In order to avoid vibration, the discs and beaters of all these
high-speed mills must be balanced with great accuracy. This is best
accomplished by removing the spindle from the mill, and allowing it to
roll on two levelled straight-edges, and then filing or chipping the
beaters on the heavy side until it will remain indifferently in any
position.
A new form of disintegrator has been recently brought out in America by
the Williams’ Patent Crusher and Pulveriser Company, in which a series
of discs are keyed to the main shaft, to the circumference of which a
number of sets of “hammers” are suspended by means of hinge-bolts. Each
of these steel bars, or hammers, has a free arc movement of 120°, and
when the machine is in motion take a position divergent from the centre
on account of the centrifugal force. After striking a blow against any
material fed on to a plate serving as an “anvil,” the hammers recoil,
and, after passing any material which is not shattered by the blow,
again resume their normal position, leaving the next set of hammers to
beat against the unground material. The hinged suspension of the hammers
imparts a degree of flexibility to the mill which is not found in any
other machine of this character, and lessens the risk of serious damage
to the machine by the introduction of pieces of metal along with the
bark. The makers claim that this machine can be repaired more rapidly
and with less expense than any other disintegrator of equal power on the
market. Considerable improvements have recently been made in the details
of its construction. Fig. 72 shows a section of this mill. Of course
only the end hammers of each set can be seen in the figure.
[Illustration: FIG. 72.--Section of Williams’ Crusher.]
When myrobalans or valonia is to be used for leaching, it is generally
better to crush it between toothed or fluted rollers, rather than to
grind it finely, as the cellular structure is just as completely broken
up, and the flakes formed by crushing allow of much freer percolation
than when the material is powdered by the disintegrator, while the
consumption of power is also less. The general construction of the
machine will be easily understood from Fig. 73, and it is only necessary
to point out that the small upper roller acts mainly as a “feed” to the
larger crushing rolls.
In the best mills, the rollers are made up of a series of toothed steel
discs on a square axis, and are on this account easily replaced or
sharpened when they have become broken or worn.
[Illustration: FIG. 73.--Myrobalans Crusher.]
Several mills have been introduced in America in which the bark is sawn
or rasped by toothed discs like circular saws, but these are only
capable of dealing with barks of a brittle nature, and are immediately
choked by tough materials like the bark of the mimosa or oak. A better
form of mill, but one which is, to some extent, subject to the same
disadvantage, is the “shaving-mill,” in which blades are fixed like
plane-irons upon a disc, cones or cylinder, and are rotated at a high
speed against the material which is fed against them by toothed rollers
at such an angle that the shavings are cut diagonally to the grain.
These shaving-mills are largely in use in America for hemlock-bark, with
which they are particularly successful. The principle of the machine is
exactly the same as that of the machines used in cutting oakwood,
quebracho, and the different dye woods. One type of shaving-mill is
illustrated in Fig. 74.
[Illustration: FIG. 74.--Shaving Mill.]
It frequently happens that the material is delivered from the mill in a
very unequal state of division, and it is sometimes necessary to screen
it and thus separate the coarser portion either for use in the leaches
or for re-grinding, while the finer portion is more suitable for
“dusting.” With disintegrators, which deliver the bark with considerable
impetus, the screening can be accomplished by placing a screen
diagonally below the mill, through which the finer parts are projected.
It is, however, essential that this screen should be quite smooth on its
upper surface and very strong, as ordinary wire gauze is immediately cut
through by the impact of the material. What are called “locked wire
screens” in which the wires are supported by being actually twisted
round the transverse bars are very suitable. Where the circumstances
will not permit of screening in this way, cylindrical rotating screens,
or nearly horizontal screens vibrated by an eccentric may be used. The
latter are cheaper to erect and have the advantage that they take up
less room, and by having lengths of wirework or perforated steel of
different coarseness, the material may be separated into more than one
degree of fineness.
[Illustration: FIG. 75.--Bark-Breaker.]
Oak-bark as it is taken off the trees is usually in lengths of perhaps
three feet, and it is necessary to cut or break it into smaller
fragments before it can be ground in most of the machines just
described. This is frequently done by hand by chopping the bark into
pieces about four inches long, and the operation is known as “hatching.”
Machines on the principle of the chaff-cutter, consisting of a fly-wheel
with curved blades radially attached to it, are sometimes used. Instead
of “hatching” it, the bark is frequently broken by passing through
toothed rollers fitting into each other, and often attached to the mill;
the construction of this machine will be readily understood from Fig.
75.
In Belgium, and some other bark-producing districts, the adhering moss
and dead outside bark are usually removed before hatching, but
apparently these impurities are frequently re-mixed with the bark after
the hatching is completed! As such barks often also contain much clay
and dirt, it is generally expedient to pass the hatched bark over a
coarse screen before letting it enter the mill, so as to remove the
greater part of such rubbish, since, if left in the bark, it produces
black and unsatisfactory liquors.
In drawing up policies for fire insurance, it is usual to charge a
higher rate where disintegrators are used to grind the tanning material,
as owing to the amount of dust and the production of sparks by the
striking of the steel parts of the machine on any chance piece of flint
or metal which may get into it, there is a greater liability to fire
than with toothed mills, although with proper precautions the risk is
really small. (Cp. p. 446.)
All disintegrators act like ventilating fans, and suck in air with the
material, blowing it out again with great force at the periphery. This
air is heavily laden with dust from the tanning material which is
extremely irritating to the lungs. The difficulty is to some extent
remedied by an air-channel or flue (generally cast in the casing of the
machine) connecting the discharge with the feed-opening so as to convey
the air back to the disintegrator. The air is thus circulated through
the arrangement, but some is always drawn in from the external
atmosphere and driven out with the ground material, and it is advisable
that the chamber into which it is discharged should be provided with
some means of filtering the air before it escapes. One convenient method
is to have a large flannel bag which is blown out by the air like a
balloon and out of which the dust can be shaken when the machinery has
stopped. Another efficient method is to have one of the walls or the
ceiling of the chamber made of canvas or of sacking; but in any case the
air should be allowed an escape where a little dust will not cause
annoyance.
_Chain-Conveyors._--While, in England, the ground material is usually
carried from the mill to the leaches in barrows or baskets, in America
the use of conveyors is practically universal, and there is no doubt
that they effect a great saving of labour at a comparatively small cost.
The most practical conveyor for tanning materials consists of a trough
through which an endless chain passes, carrying scrapers. The chain
generally used for this purpose is one consisting of square links
fitting into each other and capable of running over toothed wheels.
These chains are made by several firms in America, and in England by the
Ewart Chain Conveyor Co., of Derby, who supply not only plain links but
also those having projections to which buckets, scrapers and a variety
of attachments may be fixed.
[Illustration: FIG. 76.--Chain-Conveyor.]
In many cases the trough is V-shaped with the chain running in the
angle; in others flat-bottomed as in the illustration, or rectangular.
The scrapers may consist either of metal or of wood; and where materials
have to be carried up a steep incline buckets instead of scrapers should
be employed. The arrangement of such a conveyor is illustrated by Fig.
76.
A useful form of conveyor for dry materials consists in a woven cotton
belt running in a smooth trough and with laths riveted across it at
intervals. These laths should project slightly beyond the edges of the
belt so as to prevent wear. Care must be taken with belts of this sort
that the material does not get between the belt and the pulley.
Chain-carriers are often used for conveying the spent tan to the
furnaces from the leaches, and occasionally for carrying skins.
Several other kinds of conveyor are in use in corn-mills, spiral or worm
conveyors which work on the screw principle being very largely used for
carrying corn. They are not very suitable for tanning materials on
account of the coarseness of the latter, by which the friction is
greatly increased; they are however occasionally used. Those built up of
separate blades are specially to be avoided.
An ingenious form of conveyor has been recently introduced from Germany,
and consists of a light trough supported on steel springs and vibrated
longitudinally by means of an eccentric in such a way as to shake the
material from one end of the carrier to the other; the velocity of
motion of the trough being less in the outward than the return stroke,
so that the material is carried with it as it moves forward and slides
over it in its return. It is obvious that the principle may also be
applied to screening or sifting.
CHAPTER XXII.
_THE EXTRACTION OF TANNING MATERIALS, AND THE MAKING OF EXTRACTS._
_Leaching._--The material, having been reduced to a suitable state of
fineness, is ready for extraction. This requires a considerable amount
of time, as the tannin is contained in cells whose walls are of a
wood-like substance (cellulose and lignine), through which the water
diffuses but slowly. Hence, unless the material be very finely ground, a
long soaking will be necessary before it becomes “spent.” It should be
the aim of the tanner to have his barks, etc. ground so finely that they
may be extracted as rapidly as possible, and yet not be so fine that
they settle to a compact mass in the leaches and so prevent circulation.
Using the present methods of extraction on the large scale it is
necessary to have the material only somewhat coarsely ground or crushed,
so as to render its percolation practicable; but it is quite possible
that in the near future some better mechanical means will be found of
treating the dust and other excessively finely ground matter so as to
bring about a very rapid extraction.
Up to perhaps 150 years ago, no attempt was made to leach the tanning
material, which was simply strewed in layers between the hides, and
moistened with water. Leaching originated in England, and was first
applied merely to complete the exhaustion of the material which had been
already used for layers; but the use of even weak liquors instead of
water in the layers was found so advantageous, that new material was
soon applied to make stronger infusions. The earliest form of leach was
simply a pit with a perforated wooden “eye” or shaft down one corner, in
which a pump could be placed to remove the liquor without being choked
with solid matter. This was considerably improved by the addition of a
perforated “false bottom” to the pit, with which the eye communicated.
The perforations of the latter were found unnecessary, and it now serves
simply for pumping through, or for the manipulation of a plug in a hole
communicating with an underground “trunk” leading into a pump-well. The
false-bottom is best made of laths about 1 inch thick and 2 inches wide,
cut slanting so as to be wider on the upper than the lower surface,
which makes the spaces between them less liable to choke. The laths are
nailed on cross-battens with copper nails, which should be long enough
to clinch, ¹⁄₄-inch to ¹⁄₂-inch spaces being allowed between the laths
according to the fineness of the ground material. The lattice-bottom
should be in at least two sections, so as to allow of its easy removal
for cleaning, and should rest on detached blocks, which are best nailed
to the underside of the battens. A space of 2 inches to 3 inches below
the false bottom will prove sufficient if it is cleared every time the
pit is emptied, but not otherwise. Clearness from obstruction both below
the bottom and between the laths themselves is very important in
securing free running in the “press leach” system about to be described.
A section of the latticed bottom is shown in Fig. 77. The laths are
easily cut by employing a circular saw with a tilted table, and turning
the board at each cut. No advantage is gained by planing them.
[Illustration: FIG. 77.--Section of Leach-Bottom.]
As a strong liquor cannot be made by the use of a single leaching-pit, a
series of pits are now always employed, and it is the leaching,
systematic or otherwise, which determines how much of the total tannin
will be thrown away and lost in the “spent tan.” In the case of properly
extracted materials the “spent tan” will not contain more than one per
cent. of tanning matter, but the degree of extraction which is
profitable is dependent on the tanning material employed and the class
of leather to be produced.
The system of leaches now considered to be the best is based on the
“continuous” process of extraction. Of its different forms, the
“press-leach” is the simplest and in most cases is all that is required.
[Illustration: FIG. 78.--Plan and Section of Battery of Press-Leaches.]
A plan and vertical section of the leaches is shown in Fig. 78. Assuming
that the leaches have been working for some time and that the liquor in
the strongest leach has been run off to the tan-pits, or in the case of
manufacturing extracts to the decolorising tanks or evaporator, the last
vat in the series is now filled with water or spent liquor, which may be
heated by steam if desired, and this water, which completes the
exhaustion of the material in this vat, forces the liquor forward in the
whole series, so that it gets stronger and stronger as it passes from
vat to vat. The very weak liquor remaining in the last vat is now pumped
into a spare pit, or on to the next stronger vat, pressing the liquor
forward as before; the vat is emptied of the spent material and refilled
with new, and now becomes the head leach; and the strongest liquor is
pressed on to it by running water or weak liquor on the weakest vat.
As regards the construction of such a “battery” of leaches, details will
differ according to whether the usual English square sunk pits, or the
American form of circular tub-leaches is employed. In the former case
the vertical spouts connected with the space under the false bottoms are
usually made of wood, like the old fashioned “eye,” and placed at one
side or corner of each pit, and connected with the top of the next pit
by a short trough which may be open above or covered as preferred. Both
eyes and cross troughs must be of ample size, so as not to check the
running of the liquor, and for a set of six or eight leaches, the bottom
of the cross trough should be at least 10 or 12 inches below the actual
top of the leach, which should not be filled with material above that
level. The object of this is to allow of a sufficient fall from the
first to the last leach. Means must be provided for the temporary
closing of the cross-trough between the vats which form the first and
last leach. On a very small scale, this may be done with a plug; sliding
wooden doors are convenient, but difficult to keep tight. A hinged or
sliding door held against an indiarubber facing by a wedge or
toggle-joint would seem a practicable device.
If round tub-leaches are employed, the vertical connection may be
similarly made with a wooden trough, but copper tubes are almost
essential for the cross connections. If a vertical copper eye in the
centre of the leach be provided for boiling, or for emptying the leach
(p. 334), it may be utilised for the upflow by connecting it with the
cross pipe with a thin copper pipe of large diameter, which must be
movable for the purpose of casting the leach. A joint like that of a
stove-pipe will probably prove sufficiently tight, but if necessary may
be made tighter by rolling an indiarubber ring over it.
Six to eight leaches is generally a sufficient number to form a
press-leach “battery.” If more are connected in one series it will
usually be necessary to assist the circulation, either by pumping an
intermediate leach, or by one or more pumps on the Holbrook system, in
which a power-driven pump of simple construction is fitted in the eye of
the leach. It is hardly necessary to note that the liquor must run
_downward_ through the leaches, and _up_ through the vertical pipes, in
order to prevent mixture of the weaker with the stronger liquor.
Several additions and modifications to the system have been made with a
view of obviating the so-called “channel difficulty.” There is always a
fear on the part of some tanners that the liquid in the leaches may push
the material aside and form channels through it, thus preventing proper
extraction of the tanning matter. In the author’s opinion this evil has
been greatly exaggerated, as, unless the liquid be pumped from the
leaches at a very rapid rate while they are in circulation, it is not at
all easy for the formation of such channels to take place. In any case
it can be entirely avoided by turning over the material in the leaches
occasionally, so as to lighten it somewhat and rearrange it a little.
It may also be pointed out that the provision of a proper system for
pressing or circulating leaches does not prevent their being pumped off
as frequently as desired, though this is generally to be avoided, since
when the leach is emptied of liquor, the material tends to settle into a
compact mass, which is not easy to percolate, and which is liable to
shrink from the sides of the pit, thus causing the very trouble which it
is desired to avoid. There are some advantages in taking the first and
strongest liquors off the material in a separate tank, and then
finishing the exhaustion in the press leaches, since many materials
swell, and pack tightly when they are first wetted, but on the whole the
method hardly pays for its added cost.
The press-leach system as above described is well adapted for the
requirements of tanners, as its first cost is very small in addition to
that of the construction of the leaches themselves; it extracts the bark
well, and saves much labour in pumping, and greatly lessens the tendency
of the pumper to miss pits in the series, to save time, when the
master’s eye is not on him. Another advantage which is often important,
is that when the leaches are full, much more than a single liquor can be
run from the head-leach without pumping on; and similarly when they are
run down to their lowest level, much more than a single liquor can be
pumped on to the worst leach before it overflows. As the leaches flow
slowly in comparison to the rate at which liquors can be pumped by a
good steam pump, it is very advantageous to allow the pump to discharge
into a liquor-tank raised to such a height that the liquor can be run
from it into any leach at a suitable rate for the circulation, and it
also enables liquors to be pumped without waiting till room has been
found for them in the leaches. Similar tanks are very useful in running
liquors for the yard, and especially for the suspenders in a
sole-leather yard, enabling circulation to be kept up during the night,
and at other times when the pumps are not running. They may also be used
as filters for the suspender liquors by fitting them with false bottoms
covered with a layer of nearly spent tan. The liquors may be distributed
to the different pits and leaches by means of canvas hose-pipes, or,
what is often more convenient, by overhead troughs, carefully levelled,
and fitted with discharge valves where required. The latter are
conveniently made of lead in a hemispherical form, resting on an
indiarubber washer supported by a light brass casting, or a suitably
turned rebate in a block of wood. (Cp. p. 457 and Fig. 79.) Such valves
if good indiarubber is used, wear well, and are absolutely tight.
[Illustration: FIG. 79.--Valve for Liquor-Troughs.]
In England, leaches are usually sunk in the ground, and are frequently
made of brick and cement, or of large Yorkshire flagstones. Such leaches
are somewhat costly but very durable. Square wooden pits, puddled
outside with clay, are also used, and last well with cold, or even warm
liquors, but will not stand direct steaming, the wood gradually bending,
and allowing the clay to leak into the liquor, causing black stains. The
large round vats of thick pine, and often holding 10 or 12 tons, which
are generally used in the United States, stand boiling much better, and
are frequently supported above a tramway or conveyor, into which the
spent bark can be discharged through a manhole in the bottom. If this
method is adopted, it must be remembered that bark, and indeed most
other tanning materials, will not run through a hole like corn, but must
be cast into it, so that unless the vat is of great depth, it is simpler
and almost as easy to cast over the top. If the manhole is used, a
central hole must be made in the false bottom, and this must be
surmounted by a copper pipe made in sections of two or three feet, and
reaching to the top of the leach. When the pit is to be emptied, the top
length is removed, and the tan shovelled down the hole until the second
length is reached, and the process repeated. The central pipe serves
also for the circulation of the liquor when the pits are boiled, and may
be used as the ascending pipe for circulating on the press-leach system.
The question of the influence of temperature on extraction is discussed
on p. 344, but except where a pale colour is all important, it is
generally profitable to use a moderate degree of heat in extraction. In
the opinion of the writer (which is supported by a vast amount of
careful experiment) only the nearly exhausted leaches should be heated,
not merely to avoid discoloration, but to extract the maximum amount of
tannin. In American tanneries the boiling is frequently done by copper
coils fixed below the false bottoms of the vats, but such coils are very
costly, and, where weak liquors only are to be heated, seem to present
no advantage over a well-arranged system of heating by direct steam in
which care is taken that dry steam only is used, and that all water
condensed in steam pipes, and usually containing iron, is removed by
effective steam-traps. If steam is blown into cold liquor through an
open pipe, a very disagreeable rattling and vibration is produced, which
is not only annoying, but is very injurious to the leaches. This evil
may be avoided by the use of “silent boiling jets” on the principle of
the steam-jet water-raiser; and, following a suggestion of the writer,
these jets may be used at the same time to circulate the water through
the tanning material of the nearly exhausted vat, and so wash out the
last traces of tan. The simplest way to accomplish this is to lower the
boiling jet, directed upwards, and connected with a movable steam-pipe,
into the eye of the leach (which is preferably central) so that the
heated water flows over its top, and percolates downwards through the
material to be washed. Two forms of these boiling and mixing jets made
by Messrs. Körting are shown in Figs. 80 and 81.
[Illustration: FIGS. 80 and 81.--Boiling and Mixing Jets.]
Batteries of closed copper extractors, worked on the press system, and
similar to those used in extracting sugar from beetroot, have frequently
been advocated, but are very costly, and have no other advantage over
open vats than that the liquor can be forced through the series by
pressure, instead of circulating by gravity. No advantage is gained by
boiling under pressure, since even boiling in open vats has been shown
to destroy tannin, darken the colour of the liquor, and increase the
amount of insolubles, and higher temperatures are still more injurious.
Heating the weakest leach in the press-leach system promotes the even
circulation of the liquor, since the warm weak liquor is much lighter
than the colder and stronger liquors in the forward leaches, and so
floats on the top, and presses the stronger liquor uniformly downwards.
It also has the advantage that the liquors are cooled before they are
strong enough for the yard, while in tanneries where all the leaches are
heated, expensive tubular coolers are often employed. As the liquor
cools, much of the colouring matters and reds dissolved in the hot
liquor separate, and are filtered out by the tanning material, so that
much brighter and lighter coloured liquors are obtained.
[Illustration: FIG. 82.--Sprinkler-Leach.]
_Sprinkler-Leaches_, Fig. 82, were formerly used in many tanneries and
extract factories, especially in the United States. They were introduced
by Allen and Warren, and yield a liquor which is at first very strong,
but which becomes very rapidly weaker as the running is continued. These
leaches are similar in principle to the mashing-tub and sparger of the
brewer, but the process is not well adapted for tanners’ use, as the
material is left too much exposed to the air, which is apt to cause
oxidation and loss of tannin. It is also extremely difficult to
completely exhaust the material without using an impracticably large
volume of water. Sprinkler-leaches are arranged so as to spray the
liquor, or water, on to the top of the solid material which is to be
extracted at such a rate that it flows out just as rapidly as it flows
into the vat. Some idea of the great amount of oxidation and consequent
loss of tannin which takes place in this form of extractor may be
obtained when it is remembered that this same method is now used for the
destruction of sewage matter by spraying it on to beds of coke so that
it may be mixed with as much air as possible before it is attacked by
the bacteria of the coke-beds (see p. 473), and also to oxidise weak
alcohol to acetic acid in the “quick vinegar process.”
So far as extraction is concerned, there is no difference in principle
between the methods adopted by the tanner and the extract manufacturer,
though the latter usually works on a larger scale, and not unfrequently,
in order to increase his output, or the gravity of his extract, employs
a higher temperature. This is probably justified by practical
considerations in the manufacture of extracts from very low-grade
materials, such as oakwood, which only contains 2 to 3 per cent. of
tanning matter, or even of chestnut wood which is somewhat stronger, but
it is one of the causes why decoloration of the battery liquor is
generally necessary.
Dried blood is chiefly used as the decolorising agent, but a paste of
blood-albumen has been recently placed on the market, which is said to
be free from several of the disadvantages attending the use of the crude
material.
The liquor to be decolorised is run into a mixing vat fitted with a
steam coil capable of raising the temperature of the liquid to at least
80° C., and usually provided with a simple rotary stirring gear. The
liquor, as run into the mixing vat, must not have a temperature of more
than 48° C. (118° F.) nor a strength of more than about 20° Bkr. (sp.
gr. 1·020).
The blood or albumen dissolved in a little water, is added to the
contents of the vat, which are then well mixed, and the temperature is
raised to 70° C. when the albumen coagulates and carries down much of
the colouring matter. The solution is run into another tank where the
precipitate is allowed to settle, and the clear liquor is then drawn off
for the evaporation. The muddy portion, about 8 inches in depth, is
pumped through filter-presses (which can be cheaply constructed of
wood), the clear liquors going to the evaporators and the press-cakes
being dried for manure.
In addition to blood-albumen, several other substances, such as lead
acetate (sugar of lead), salts of alumina, casein and other albuminous
matters have been employed in the decoloration of extracts, but they are
by no means so efficient as albumen.
Decolorising always causes a loss of tanning matter, some of this being
carried down with the precipitated colouring matter; and is for this
reason to be dispensed with whenever its use is not really necessary. It
may often be avoided by careful extraction at moderate temperatures, and
this is especially to be aimed at in the case of strong tanning
materials, which easily yield battery liquors of much greater strength
than 20° Bkr., and which thus, if they can be sent direct to the
evaporator, save cost in evaporation, which is often an important
consideration.
Another method which is frequently used to brighten the colour of
extracts, is treatment with sulphurous acid. Dilute sulphurous acid
solution may be used for extraction, but a more common method is to pass
sulphur dioxide gas into the liquor before concentration. Sulphurous
acid acts partly as a weak acid, in decomposing compounds of the tannins
and colouring matters with bases, such as lime, iron, copper, but more
actively by reducing oxygen compounds and preventing oxidation.
Bleaching in this way does not actually destroy or remove the colouring
matters, which are apt to reappear on exposure to the air, either in the
liquor, or perhaps more often in the leather tanned with it, so that the
gain is frequently more apparent than real. If present in any
considerable quantities, sulphurous acid may also cause inconvenience by
its swelling action on the pelt, but is mostly expelled in
concentration.
Another process should perhaps also be mentioned here, though not
strictly a means of bleaching. Several tanning materials, and notably
quebracho and hemlock, contain large quantities of “difficultly soluble
tannins,” which render the liquors made from their extracts turbid on
cooling. These tannins form soluble compounds with alkalis and with
alkaline sulphites, in the latter case probably setting free the
sulphurous acid and combining with the base. This has been taken
advantage of in a recent patent[158] in which quebracho and other
extracts are rendered soluble by heating in closed vessels with
bisulphites, sulphites, sulphides, or even caustic alkalis; and many
“soluble quebracho extracts” made on this principle are now on the
market. In this case, even where bisulphites are used, the greater part
of the sulphurous acid, after serving its purpose in preventing
oxidation, escapes in course of manufacture, and the extracts remain
neutral or alkaline. There is no reason that such extracts should not
prove serviceable in tanning, but it has recently been shown by Paessler
that the alkaline tannin is not absorbed by neutral hide-powder, and it
therefore may lead, not only to discrepancies in analysis, but in case
of drum-tannage, where no acid is naturally present, to failure to
utilise the whole of the tannin, though, when added to ordinary liquors,
the acids contained in the latter will set free the tannins.
[158] Lepetit, Dollfus, and Gansser, Eng. Pat. 8582, 1896.
The use of ferrocyanides has been suggested as a means of precipitating
iron and copper present in extracts, and it may also be pointed out,
that with many red-coloured tanning materials, such as hemlock and
quebracho, the addition of small quantities of alum to the tanning
liquor effects considerable improvement in colour, not only by
precipitating a part of the difficultly soluble “reds,” but by
developing the yellow colour of certain colouring matters (quercetin,
myricetin, etc.) which may be present. Such an addition does no harm in
the case of soft leathers, but would probably be injurious in a
sole-leather tannage.
The liquors, whether direct from the leaches or from the decolorising
vats, must next be concentrated by evaporation (Chap. XXVI.), to sirupy
consistency for liquid extracts, or until they will become nearly solid
on cooling, if a solid extract is required. As has already been stated,
the action of heat tends to cause a loss of tannin and a darkening of
colour by decomposition and the formation of insoluble reds. To reduce
this loss to a minimum, the weak liquors are evaporated with as little
access of air and at as low a temperature as possible, and these
conditions are best obtained by the use of steam-heated vacuum pans.
[Illustration: FIG. 83.--Triple-effect Yaryan Evaporator.]
For concentration to gravities not exceeding 1·200, the Yaryan apparatus
made by Mirrlees, Watson and Yaryan, of Glasgow, is that most employed.
The general arrangement of a “triple effect” machine of this make is
shown in Fig. 83, and the internal construction in Fig. 84. Each body
consists of a strong casing into which steam is admitted, and which is
traversed by copper tubes which terminate in a separating chamber at the
further end, which is maintained at a low pressure by an air-pump. The
liquid to be evaporated is admitted into the tubes, and is immediately
converted into spray by the steam generated from it, and swept forward
into the separating chamber, from which it is withdrawn by a pump. The
steam before going to the air-pump (or, in the case of “multiple
effects,” to the next body), is passed through a “catch-all,” to
separate any spray still retained in the steam. Thus the liquor to be
evaporated will pass through the entire apparatus in four or five
minutes, and may be concentrated from a gravity of 1·02 or 1·03 to that
of 1·20 without ever having been heated above 70° C. (160° F.). Unless
fuel is very cheap, which is often the case where the spent tanning
material can be used to raise steam, it is advisable to use a double or
triple effect, in which the steam from the evaporation of the weakest
liquor in the first body is used to heat the second, which is maintained
at a lower vacuum, and so on. In this way the steam is made to do nearly
double or triple duty. As the steam from the extract-liquors contains
acids which corrode iron, it is necessary to have the casing as well as
the tubes made of copper in all bodies in which it is employed. Iron
must, in fact, be carefully avoided in every part of apparatus which
comes in contact with extract-liquor or its vapour. Besides the Yaryan,
there are several other evaporators in which the spray principle is
more or less completely employed. The simplest of these consists in
substituting for the heating coil of an ordinary vacuum-pan a copper
steam-box traversed by vertical tubes open at both top and bottom. This
is immersed in the liquid to be evaporated, which enters at the bottom
of the tubes and is sprayed out at the top. Paul Neubäcker, of Danzig,
constructs a pan on this principle with a very ingenious arrangement for
the destruction of foam, which seems worth attention.
[Illustration: FIG. 84.--Section of Yaryan Evaporator.]
It is unfortunately impossible to carry the evaporation of extracts much
further than sp. g. 1·2 with spray apparatus, as thicker liquors are apt
to clog the tubes, which are then difficult to clean, so that even
liquid extracts are usually finished in vacuum-pans of the ordinary
type, which may also be arranged in multiple effect.
In the case of a solid extract, the evaporation must be carried on until
it is as thick as can be run from the apparatus. To do this
satisfactorily, stirrers must be provided to keep the extract in motion
so long as it is in the pan. The thick, hot, liquid extract is then run
into boxes lined with paper, or other suitable material, where it is
allowed to cool and to solidify.
The pan for the final evaporation of solid extracts should be planned so
as to allow of easy cleaning and ready access to its interior, so that
if accidentally the evaporation is carried so far that the liquid will
not run out, the clearing of the pan may be a comparatively easy matter.
It is also important that the extract-exit should be of large size.
Probably a broad and somewhat shallow pan, heated merely by a steam
jacket, and fitted with rotating stirrers, is the most suitable.
_The Use of Extracts in the Tannery._--One of the great attractions of
extracts is that they save the trouble and cost of leaching, and as the
extract manufacturer makes this his specialty, he can often extract more
tanning matter from a material than the tanner who has no means of
concentrating his weak liquors. The extract manufacturer also can employ
methods of decoloration which would be impracticable to the tanner, and
so enable the latter to obtain better colour than if he employed the raw
material. By the use of extracts a tanner can strengthen weak liquors
without trouble, and with definite quantities of materials; and by
using extracts for this purpose the tanner is enabled to use up the
weaker liquors of his leaches and so employ more water and obtain better
extraction of his solid materials than if he used them alone. In the
case of very weak materials like oakwood, the difficulties of making
liquors of sufficient strength for tanning without evaporation are so
great as to render such materials useless to the tanner for his own
extraction, and their carriage even for short distances may amount to
more than their total value. Even with much richer materials, extraction
effects a saving if the carriage is a long one, as it rarely pays to
import any material containing less than about 25 per cent. of tanning
matter. Even when the strength of the natural material is considerable,
as in the case of quebracho, extraction may be profitable if from its
hardness, or other reasons, the material is difficult for the tanner to
handle. For long voyages, and especially from the tropics, solid
extracts are more suitable than liquid, as the expense of casks is
saved, and the danger of fermentation is lessened. As it is impossible
for the tanner to judge by appearance or consistency of the strength or
value of extracts, they should always be bought and sold on the analysis
of the particular shipment or parcel by a competent chemist. For
directions for sampling see pp. 301, 475.
Extracts simply require to be dissolved in a suitable quantity of water
or weak liquor at an appropriate temperature, to obtain a liquor of any
required strength. Some extracts are completely soluble in cold water or
liquor, but most dissolve better by the aid of heat. 40°-60° C.
(100°-140° F.) is generally sufficient, and probably no advantage can
arise from temperatures over 80° (180° F.). Boiling should be avoided,
as it facilitates the formation of insoluble “reds” with consequent loss
of tanning matter and darkening of colour. The extract should be run
into the vat in a thin stream, and continuously plunged up; where large
quantities of extract are to be dissolved, a mechanical agitator is
advantageous. A “silent boiling jet” (p. 335) may be used, fitted into a
small casing immersed in the liquor and open at both ends, and the
extract run into the current it produces.
Whether in the manufacture of extracts, or for direct use in the
tannery, the temperature at which tanning materials are extracted is of
prime importance. It is a common mistake to assume that the largest
amount of tannin is extracted by boiling. Mr. A. N. Palmer has pointed
out that this is by no means the case, but that each material has an
_optimum_ temperature of extraction, at which more tannin is extracted
than at any other; and the question has been carefully investigated by
J. G. Parker and the author,[159] with results which are given in the
following tables. For many purposes the colouring matter which
accompanies the tannin is a serious disadvantage, and it is usually most
extracted at the higher temperatures; and on this account it is
necessary for the tanner who will work his leaches economically to
ascertain at what temperature he can extract the largest amount of
tannin combined with no more colouring matter than he can permit to
enter his leather. Most materials are satisfactorily extracted at
50°-60° C., but as a general rule it is best to begin cold or nearly so,
and only raise the temperature as the extraction proceeds. The tables
show the percentages of tanning matter, and the amount of colour (as
measured by Lovibond’s tintometer), obtained by extracting materials in
a Procter’s extractor (p. 306 and L.I.L.B., p. 102) so long as any
colour or tannin could be obtained.
[159] Journ. Soc. Ch. Ind., 1895, 635.
BELGIAN OAK BARK.
-----------+---------+-----------+---------+---------------+---------
Temperature| Tanning | Soluble |Per cent.| Colour of ¹⁄₂ |Per cent.
of | Matters |Non-tanning|of Tannin| per cent. |of Colour
Extraction.|absorbed | Matters. | on | Solution in | on
| by Hide.| | Maximum |¹⁄₂ inch Cell. | Maximum
| | | Yield. +-------+-------+ Yield.
| | | | Red. |Yellow.|
-----------+---------+-----------+---------+-------+-------+---------
°C. |per cent.| per cent. | | deg. | deg. |
15 | 5·9 | 5·1 | 61·9 | 8·6 | 23·1 | 57·4
15-30 | 6·8 | 5·5 | 70·7 | 9·2 | 26·4 | 64·5
30-40 | 8·0 | 5·5 | 83·5 | 11·6 | 30·4 | 76·1
40-50 | 8·2 | 5·7 | 84·2 | 12·0 | 32·1 | 80·0
50-60 | 8·5 | 5·8 | 87·6 | 12·5 | 36·0 | 84·0
60-70 | 9·1 | 5·9 | 95·5 | 13·1 | 38·1 | 92·7
70-80 | 9·2 | 6·0 | 95·7 | 14·7 | 38·9 | 98·7
80-90 | 9·6 | 6·0 | 100·0 | 14·0 | 36·9 | 93·2
90-100 | 9·6 | 6·1 | 100·0 | 14·0 | 41·2 | 94·6
Boiled | | | | | |
¹⁄₂ hour | 9·1 | 6·6 | 93·7 | 15·0 | 42·6 | 100·0
-----------+---------+-----------+---------+-------+-------+---------
MYROBALANS.
-----------+---------+---------+---------+--------------+---------
Temperature| Tanning | Soluble |Per cent.| Colour of |Per cent.
of | Matters.| Non- | of | ¹⁄₂ per cent.| of
Extraction.| |Tannins. |Tannin on| solution in |Colour on
| | | Maximum |¹⁄₂ inch Cell.| Maximum
| | | Yield. +------+-------+ Yield.
| | | | Red. |Yellow.|
-----------+---------+---------+---------+------+-------+---------
°C. |per cent.|per cent.| | deg. | deg. |
15 | 28·5 | 12·8 | 79·2 | 1·09 | 4·9 | 97·4
15-30 | 30·1 | 13·6 | 83·6 | 1·00 | 4·1 | 82·5
30-40 | 32·3 | 14·3 | 89·8 | 1·03 | 4·1 | 82·7
40-50 | 33·5 | 13·6 | 93·0 | 1·03 | 4·2 | 84·4
50-60 | 34·7 | 14·4 | 96·4 | 1·03 | 4·4 | 87·6
60-70 | 34·8 | 14·4 | 96·6 | 1·03 | 4·5 | 89·3
70-80 | 34·9 | 14·9 | 96·8 | 1·10 | 4·7 | 94·1
80-90 | 35·1 | 15·0 | 97·4 | 1·16 | 4·8 | 96·7
90-100 | 36·0 | 14·9 | 100·0 | 1·12 | 4·9 | 97·0
Boiled | 35·4 | 15·5 | 98·1 | 1·26 | 4·9 | 100·0
-----------+---------+---------+---------+------+-------+---------
SMYRNA VALONEA.
-----------+---------+---------+---------+------------------+---------
Temperature| Tanning | Soluble |Per cent.| Colour of |Per cent.
of | Matters.| Non- | of | ¹⁄₂ per cent. | of
Extraction.| |Tannins. |Tannin on| Solution in |Colour on
| | | Maximum | ¹⁄₄ inch Cell. | Maximum.
| | | Yield. +----+-------+-----+
| | | |Red.|Yellow.|Blue.|
-----------+---------+---------+---------+----+-------+-----+---------
°C. |per cent.|per cent.| |deg.| deg. |deg. |
15 | 25·5 | 19·1 | 70·5 | 2·5| 6·0 | 0·3 | 74·6
15-30 | 29·1 | 18·3 | 74·5 | 2·5| 6·4 | 0·3 | 78·0
30-40 | 33·6 | 18·1 | 86·2 | 2·3| 6·4 | 0·3 | 76·2
40-50 | 35·5 | 18·1 | 86·2 | 2·3| 6·5 | 0·3 | 74·6
50-60 | 39·1 | 16·6 | 100·0 | 2·0| 6·0 | 0·3 | 76·2
60-70 | 38·6 | 17·0 | 99·0 | 2·0| 6·8 | 0·3 | 84·7
70-80 | 38·8 | 17·5 | 99·5 | 2·1| 7·4 | 0·4 | 84·7
80-90 | 36·9 | 17·2 | 95·0 | 2·2| 7·6 | 0·4 | 84·7
90-100 | 36·6 | 17·0 | 94·0 | 2·4| 7·8 | 0·5 | 90·6
Boiled | 35·4 | 17·6 | 90·6 | 3·0| 8·2 | 0·6 | 100·0
-----------+---------+---------+---------+----+-------+-----+---------
GREEK VALONEA.
-----------+---------+---------+---------+------------------+---------
Temperature| Tanning | Soluble |Per cent.| Colour of |Per cent.
of | Matters.| Non- | of | ¹⁄₂ per cent |of Colour
Extraction.| |Tannins. |Tannin on| Solution in | in ¹⁄₂
| | | Maximum | ¹⁄₂ inch Cell. |per cent.
| | | Yield. +----+-------+-----+of Tannin
| | | |Red.|Yellow.|Blue.|Solution.
-----------+---------+---------+---------+----+-------+-----+---------
°C. |per cent.|per cent.| |deg.| deg. | deg.|
15 | 16·0 | 13·0 | 64·0 | 2·9| 6·3 | 0·3 | 67·3
15-30 | 18·1 | 12·6 | 72·4 | 3·0| 6·6 | 0·3 | 70·0
30-40 | 21·1 | 12·0 | 84·4 | 2·8| 6·5 | 0·3 | 68·0
40-50 | 23·6 | 12·1 | 94·4 | 2·4| 6·6 | 0·3 | 65·9
50-60 | 24·8 | 12·4 | 99·2 | 2·7| 7·0 | 0·4 | 71·6
60-70 | 25·0 | 12·6 | 100·0 | 2·9| 7·3 | 0·5 | 75·8
70-80 | 24·6 | 12·5 | 98·4 | 3·1| 7·9 | 0·6 | 82·3
80-90 | 24·0 | 12·5 | 96·0 | 3·4| 8·1 | 0·6 | 85·8
90-100 | 23·6 | 12·6 | 94·4 | 3·5| 8·8 | 0·7 | 92·0
Boiled | 22·6 | 13·0 | 88·8 | 3·9| 9·4 | 0·8 | 100·0
-----------+---------+---------+---------+----+-------+-----+---------
NATAL MIMOSA.
-----------+---------+---------+---------+--------------+----------
Temperature|Tanning | Soluble |Per cent.| Colour of |Per cent.
of |Matters. | Non- | of | ¹⁄₂ per cent | of
Extraction.| |Tannins. |Tannin on| Solution in |Colour on
| | |Maximum. |¹⁄₂ inch Cell.|Maximum.
| | | +----+---------+
| | | |Red.| Yellow. |
-----------+---------+---------+---------+----+---------+----------
°C. |per cent.|per cent.| |deg.| deg. |
15 | 21·2 | 11·6 | 66·2 | 2·6| 4·1 | 51·1
15-30 | 29·0 | 9·8 | 90·6 | 3·0| 4·1 | 54·2
30-40 | 30·1 | 9·8 | 94·0 | 3·0| 4·4 | 56·5
40-50 | 30·2 | 9·8 | 94·4 | 3·1| 5·0 | 61·8
50-60 | 30·4 | 10·4 | 95·0 | 3·9| 6·5 | 79·9
60-70 | 31·5 | 10·6 | 98·4 | 4·2| 6·5 | 81·6
70-80 | 32·0 | 10·8 | 100·0 | 4·2| 7·0 | 85·5
80-90 | 30·8 | 11·2 | 96·2 | 4·9| 7·4 | 93·8
90-100 | 30·1 | 11·8 | 94·0 | 5·3| 7·8 | 100·0
Boiled | 29·4 | 12·0 | 91·8 | 5·7| 7·2 | 98·4
-----------+---------+---------+---------+----+---------+----------
SUMACH.
-----------+---------+---------+---------+--------------+---------
Temperature|Tanning | Soluble |Per cent.| Colour of |Per cent.
of |Matters. | Non- | of | ¹⁄₂ per cent | of
Extraction.| |Tannins. |Tannin on| Solution in |Colour on
| | |Maximum. |¹⁄₂ inch Cell.| Maximum.
| | | +----+---------+
| | | |Red.| Yellow. |
-----------+---------+---------+---------+----+---------+---------
°C. |per cent.|per cent.| |deg.| deg. |
15 | 14·2 | 17·8 | 70·0 | 1·6| 5·4 | 63·6
15-30 | 17·6 | 18·1 | 86·7 | 1·4| 4·3 | 51·8
30-40 | 18·5 | 18·1 | 91·1 | 1·3| 4·4 | 51·8
40-50 | 20·1 | 18·5 | 99·0 | 1·4| 4·4 | 52·9
50-60 | 20·3 | 19·1 | 100·0 | 1·5| 4·7 | 56·5
60-70 | 19·0 | 19·4 | 93·6 | 1·7| 5·6 | 66·6
70-80 | 18·0 | 19·9 | 89·1 | 1·9| 6·2 | 72·8
80-90 | 16·9 | 21·1 | 83·2 | 2·3| 6·8 | 82·7
90-100 | 16·6 | 22·3 | 81·7 | 2·6| 7·0 | 87·7
Boiled | 15·2 | 24·0 | 74·8 | 3·3| 7·7 | 100·0
-----------+---------+---------+---------+----+---------+---------
QUEBRACHO WOOD.
-----------+---------+---------+---------+--------------+---------
Temperature|Tanning | Soluble |Per cent.|Colour of |Per cent.
of |Matters. | Non- |of Tannin|¹⁄₂ per cent | of
Extraction.| |Tannins. | on |Solution in |Colour on
| | | Maximum.|¹⁄₂ inch Cell.|Maximum.
| | | +----+---------+
| | | |Red.| Yellow. |
-----------+---------+---------+---------+----+---------+---------
°C. |per cent.|per cent.| |deg.| deg. |
15 | 7·6 | 2·2 | 35·0 | 8·9| 14·1 | 71·3
15-30 | 10·1 | 2·4 | 46·5 | 6·4| 10·7 | 68·7
30-40 | 11·8 | 2·4 | 54·4 | 5·9| 9·6 | 65·2
40-50 | 15·1 | 2·4 | 69·5 | 5·3| 8·4 | 60·0
50-60 | 16·5 | 2·4 | 76·0 | 5·4| 8·5 | 60·4
60-70 | 17·4 | 2·4 | 80·0 | 5·6| 8·2 | 59·9
70-80 | 19·1 | 2·7 | 88·0 | 6·4| 8·6 | 67·4
80-90 | 21·7 | 3·0 | 100·0 | 6·4| 9·4 | 74·3
90-100 | 19·5 | 3·0 | 89·8 | 6·6| 9·8 | 100·0
-----------+---------+---------+---------+----+---------+---------
MANGROVE BARK (_Ceriops_).
-----------+---------+---------+---------+--------------+---------
Temperature| Tanning | Soluble |Per cent.| Colour of |Per cent.
of | Matters.| Non- |of Tannin|¹⁄₂ per cent. | of
Extraction.| |Tannins. | on | Solution in |Colour on
| | | Maximum.|¹⁄₂ inch Cell.| Maximum.
| | | +----+---------+
| | | |Red.| Yellow. |
-----------+---------+---------+---------+----+---------+---------
°C. |per cent.|per cent.| |deg.| deg. |
15 | 13·0 | 10·4 | 61·6 |14·2| 20·8 | 64·7
15-30 | 16·1 | 10·4 | 76·3 |16·1| 21·7 | 69·8
30-40 | 17·4 | 12·5 | 82·4 |15·8| 23·0 | 71·7
40-50 | 18·5 | 11·4 | 87·7 |16·5| 33·5 | 73·8
50-60 | 20·3 | 10·3 | 96·2 |16·0| 23·4 | 72·8
60-70 | 20·0 | 11·4 | 94·7 |17·5| 31·2 | 90·0
70-80 | 20·4 | 11·2 | 96·7 |16·5| 28·3 | 82·8
80-90 | 21·1 | 10·8 | 100·0 |15·4| 24·6 | 73·8
90-100 | 20·2 | 11·4 | 95·7 |23·0| 34·1 | 100·0
-----------+---------+---------+---------+----+---------+---------
CANAIGRE ROOT (three years old).
_Effect of Different Temperatures._
-----------+---------+---------+---------+-------------------+-------
Temperature| Tanning | Soluble |Per cent.| Colour of ¹⁄₂ per | Per
of | Matters | Non- |of Tannin| cent. Solution in | cent.
Extraction.| absorbed| Tanning | on | ¹⁄₂-inch Cell. | of
| by Hide.| Matters.| Maximum +----+-------+------+ Colour
| | | Yield. |Red.|Yellow.|Total.| on
| | | | | | |Maximum
| | | | | | | Yield.
-----------+---------+---------+---------+----+-------+------+-------
°C. |per cent.|per cent.| |deg.| deg. | deg. |
15 | 21·1 | 13·0 | 78·7 | 1·6| 4·1 | 5·9 | 41·5
15-30 | 26·2 | 12·5 | 85·6 | 1·6| 3·8 | 4·4 | 38·0
30-40 | 28·1 | 12·5 | 91·8 | 1·4| 3·7 | 5·1 | 35·9
40-50 | 30·5 | 13·1 | 99·6 | 2·1| 4·2 | 6·3 | 44·3
50-60 | 30·6 | 13·6 | 100·0 | 2·4| 4·8 | 7·2 | 50·7
60-70 | 27·2 | 14·1 | 88·8 | 2·5| 5·0 | 7·5 | 52·7
70-80 | 26·4 | 14·6 | 86·2 | 2·8| 6·1 | 8·9 | 62·6
80-90 | 23·2 | 14·8 | 75·8 | 3·1| 6·9 | 10·0 | 70·4
90-100 | 22·8 | 14·8 | 74·5 | 4·3| 7·4 | 11·7 | 82·4
Boiled | | | | | | |
¹⁄₂ hour | 19·2 | 12·3 | 62·7 | 5·6| 8·6 | 14·2 | 100·0
-----------+---------+---------+---------+----+-------+------+-------
CUBE GAMBIER.
_Effect of Different Temperatures._
-----------+---------+---------+---------+-------------------+-------
Temperature| Tanning | Soluble |Per cent.| Colour of ¹⁄₂ per | Per
of | Matters | Non- |of Tannin| cent. Solution in | cent.
Extraction.| absorbed| Tanning | on | ¹⁄₂-inch Cell. | of
| by Hide.| Matters.| Maximum +----+-------+------+ Colour
| | | Yield. |Red.|Yellow.|Total.| on
| | | | | | |Maximum
| | | | | | | Yield.
-----------+---------+---------+---------+----+-------+------+-------
°C. |per cent.|per cent.| |deg.| deg. | |
15 | 46·8 | 21·8 | 78·0 | 2·5| 7·8 | 10·3 | 57·2
15-30 | 48·8 | 21·0 | 81·3 | 1·7| 8·0 | 9·7 | 54·9
30-40 | 50·2 | 22·0 | 83·7 | 1·7| 8·6 | 10·3 | 57·2
40-50 | 51·9 | 23·0 | 86·5 | 1·7| 8·8 | 10·5 | 58·3
50-60 | 51·1 | 20·3 | 91·9 | 1·7| 8·9 | 10·6 | 58·8
60-70 | 55·6 | 20·3 | 92·7 | 1·9| 9·4 | 11·3 | 62·7
70-80 | 55·7 | 20·3 | 92·8 | 2·2| 10·1 | 12·3 | 68·3
80-90 | 55·8 | 21·2 | 93·1 | 2·3| 10·6 | 12·9 | 71·6
90-100 | 56·1 | 22·0 | 93·3 | 2·8| 11·6 | 14·4 | 80·0
Boiled | | | | | | |
¹⁄₂ hour | 60·0 | 20·0 | 100·0 | 3·2| 14·8 | 18·0 | 100·0
-----------+---------+---------+---------+----+-------+------+-------
BLOCK GAMBIER.
_Effect of Different Temperatures._
-----------+---------+---------+---------+-------------------+-------
Temperature| Tanning | Soluble |Per cent.| Colour of ¹⁄₂ per | Per
of | Matters | Non- |of Tannin| cent. Solution in | cent.
Extraction.| absorbed| Tanning | on | ¹⁄₂-inch Cell. | of
| by Hide.| Matters.| Maximum +----+-------+------+ Colour
| | | Yield. |Red.|Yellow.|Total.| on
| | | | | | |Maximum
| | | | | | | Yield.
-----------+---------+---------+---------+----+-------+------+-------
°C. |per cent.|per cent.| |deg.| deg. | |
15 | 30·1 | 27·4 | 50·1 | 2·6| 8·1 | 10·7 | 33·5
15-30 | 34·8 | 26·2 | 69·6 | 2·4| 8·0 | 10·4 | 34·0
30-40 | 40·8 | 27·2 | 81·6 | 2·0| 9·0 | 11·0 | 55·0
40-50 | 44·8 | 27·6 | 89·6 | 2·4| 9·8 | 12·2 | 61·0
50-60 | 46·8 | 27·8 | 93·6 | 2·4| 10·1 | 12·5 | 62·5
60-70 | 47·3 | 27·6 | 94·6 | 2·5| 10·6 | 13·2 | 66·0
70-80 | 47·4 | 27·6 | 94·7 | 2·8| 10·9 | 13·7 | 63·5
80-90 | 47·6 | 27·3 | 95·2 | 3·2| 11·6 | 14·8 | 74·0
90-100 | 48·2 | 27·1 | 96·4 | 3·8| 12·8 | 16·6 | 83·0
Boiled | | | | | | |
¹⁄₂ hour | 50·2 | 26·4 | 100·0 | 5·0| 15·0 | 20·0 | 100·0
-----------+---------+---------+---------+----+-------+------+-------
CHAPTER XXIII.
_FATS, SOAPS, OILS AND WAXES._
Fats and oils constitute a large class of substances, of animal or
vegetable origin, which may be solid, pasty or more or less viscous
liquids, but which in the latter case are commonly known as “fixed” or
fatty oils, to distinguish them from the volatile, or essential oils,
which may be distilled without decomposition, and which are the source
of most of the odours of plants, and of quite different chemical
constitution. The term “oil” is also applied to various products of
mineral origin, and especially to those derived from petroleum, on
account of their similarity in appearance and physical properties to the
fixed oils, though, chemically, they form a very distinct class. The
waxes are another group somewhat closely allied to the fats; and there
are certain fixed oils, such as sperm oil, which though very similar in
appearance and properties to the fatty oils, are chemically members of
the group of waxes.
As it is obvious that there is no chemical distinction between the fats
and fatty oils, except that of melting-point, it will be convenient to
treat them together; especially as what is a solid fat in one climate
may be an oil in another. Palm and cocoa-nut oils are cases in point, as
the first is buttery, and the second a hard fat in this country, though
they are both liquid in tropical climates.
For more detailed information on the chemistry of fats and oils, the
reader must be referred to the ‘Leather Industries Laboratory Book,’
sect. xviii., or to the larger manuals devoted specially to the subject
by Lewkowitsch, Jean, and others, or the very excellent section on oils
in Allen’s ‘Commercial Organic Analysis,’ vol. ii.; but a few general
facts must be recapitulated.
The true fats contain carbon, hydrogen and oxygen, but no nitrogen. They
are all compounds of glycerin with organic acids which are generally
termed “fatty acids,” and which resemble in many of their
characteristics the fats themselves. Glycerin is a very weak base, of
the nature of an alcohol, and consequently, when a fat is heated with a
solution of one of the caustic alkalis, the fatty acid combines with the
latter, and the glycerin is set free. The salts thus formed are
denominated “soaps.” The reaction with stearin (glycerin stearate), the
principal constituent of hard animal fats, is shown in the following
equation.
Stearin Sodium Sodium Glycerin
hydrate stearate
(C₁₇H₃₅CO.O)₃C₃H₅ + 3NaOH = 3C₁₇H₃₅CO.ONa + C₃H₅(OH)₃.
If a soap is treated with an acid stronger than its own, the latter is
set free, while the new acid combines with the base. The following
equation, for instance, shows the action of hydrochloric acid on the
stearic soap.
Sodium Hydro- Stearic acid Sodium
Stearate chloric chloride
acid
C₁₇H₃₅CO.ONa + HCl = C₁₇H₃₅CO.OH + NaCl.
If any soap be dissolved in hot water, and sufficient hydrochloric or
sulphuric acid added to render the solution acid, the latter will turn
first milky, and (if it be kept warm) the fatty acid will finally rise
in an oily layer to the surface, which in many cases will harden, as it
cools, to a solid mass. The amount of fatty acid in a soap may be
roughly determined by weighing 25 grm., dissolving in 50 c.c. of boiling
water, and adding excess of acid, and allowing the reaction to take
place in a graduated cylinder, or a flask with a graduated neck, in a
vessel of boiling water. When the fatty acid has risen to the top, its
volume may be noted, and each c.c. may be roughly reckoned as 0·9 grm.
(For more detailed methods cp. L.I.L.B., Sect. XVII.).
Soaps are insoluble in strong caustic alkaline solutions, and therefore
saponification (as the decomposition of fats by alkalis is called), does
not readily take place in them, and for this reason the soap-boiler
generally dilutes his caustic soda solutions to a strength not exceeding
18° Tw. (sp. gr. 1·090) in gravity, and separates the soap at the end
of the operation, by the addition of brine, in which it is insoluble. An
easier method, and one which is often useful for the preparation of
small quantities of special soaps for fat liquors and the like, is as
follows.[160] 10 lb. of a _good_ caustic soda, free from common salt, is
dissolved in 4 gallons of water, and 75 lb. of oil or fat is warmed to
about 25° C. or just sufficiently to render it liquid, and the soda
solution is added in a thin stream, with constant stirring, which must
be continued until the mass becomes too pasty. It is now set aside in a
warm place for at least twenty-four hours, during which saponification
gradually takes place. For leather purposes, a neutral soap, with a
slight excess of fat, is generally advantageous, so that the fat may be
increased to 80 lb.; or, in place of this, the operation will be
facilitated by the addition of 5 lb. of commercial oleic acid. If soft
soap is desired, 14 lb. of caustic potash may be used in place of the 10
lb. of caustic soda. The hardness or softness of soaps varies to some
extent with the fat used, but potash soaps are always much softer than
the corresponding soda soaps. It is obvious that with soaps made in this
way, all the glycerin remains mixed with the soap. If, on testing, the
soap does not prove to be free from caustic, it may be re-melted, which
will generally complete the reaction. Before attempting to work with
large quantities, a laboratory experiment is desirable, using 10 grm. of
soda in 40 c.c. of water, and 75 to 80 grm. of oil or fat. The
neutrality or freedom of the soap from caustic alkali may be tested by
touching a freshly cut surface with an alcoholic solution of
phenolphthalein, which the least trace of caustic soda or potash will
render pink.
[160] Carpenter, ‘Soap, Candles and Lubricants,’ p. 144.
If solutions of soaps are mixed with those of salts of the heavy metals
or of the alkaline earths, a mutual decomposition takes place, the acid
of the salt combining with the alkali of the soap; and the fatty acid
with the metallic base, to form a metallic soap. Most of these soaps are
sticky masses, insoluble in water, but not unfrequently soluble in
turpentine or petroleum spirit, if previously thoroughly dried, so that
some of them have been applied to the production of varnish. Alumina
soaps are occasionally used to thicken mineral oils, or render them more
viscous. The general reaction of the stearin soap with calcium sulphate
is shown in the following equation, though in practice it is sometimes
more complex:
Stearin soap Calcium Sodium Calcium
sulphate sulphate stearate
2C₁₇H₃₅CO.ONa + CaSO₄ = Na₂SO₄ + (C₁₇H₃₅CO.O)₂Ca
This is the reaction which causes the curdling of soap by hard water,
page 93.
True fats cannot be distilled alone without decomposition. When
distilled in a current of steam, some undecomposed fat passes over, but
the greater part is broken up into free fatty acid and glycerin; and
hydrocarbons practically identical with mineral oils are also formed.
Fats and oils are insoluble in water, and in most cases only sparingly
soluble in alcohol, but freely soluble in ether, petroleum spirit,
benzene, and most other hydrocarbons, as well as in chloroform, carbon
tetrachloride, and carbon disulphide. Petroleum spirit, often called
benzine, is largely used for their extraction, and for de-greasing
leather, and removing grease from clothes. In the laboratory, carbon
disulphide, or carbon tetrachloride is to be preferred. Castor oil is an
exception to the rule, owing to the large proportion of oxygen which it
contains, being readily soluble in alcohol, and very sparingly in
petroleum-spirit; and other oils, when oxidised, usually become more
soluble in alcohol, and less so in hydrocarbons.
Oils vary much in their tendency to “dry,” or become converted into
solid or sticky resin-like substances. This tendency is greatest in some
of the seed oils, and least in olive oil, and the oily part of animal
fats (tallow oil, neatsfoot oil). Sperm oil, a “liquid wax,” is also
very free from this tendency, but all fish oils possess it in a greater
or less degree. It is not due to evaporation, but to the absorption of
oxygen by the fatty acid. The tendency to oxygen-absorption, and
consequently to drying (and, in the case of leather-oils, to “spueing”),
is measured analytically by the “iodine-value,” the absorption of iodine
being proportional to that of oxygen, while it is much more easily
measured.
There are no simple tests by which the purity of oils can be determined,
though in a few cases the presence of particular oils can be detected.
The mixing and adulteration of oils is now a science, and those who
practise it are well acquainted with the customary tests, and take care
to adjust their mixtures so as to meet them. Taste and smell however,
with practice, often furnish useful indications.
Natural oils and fats are invariably mixtures of the glycerides of
several fatty acids, and their qualities depend simply on the character
of these glycerides and the proportions in which they are mixed. The
fatty acids form several groups, differing in their degree of
“saturation,”[161] or, inversely, in their power of taking up oxygen, on
which their tendency to drying depends. The members of any one of these
groups resemble each other strongly, differing principally in melting
points, density, and other physical characteristics.
[161] A “saturated” compound is one, the constituents of which are
present in such proportions that all the combining affinities of each
are satisfied by the others. Iodine value, see L.I.L.B., p. 176, and
Jour. Soc. Ch. Ind., 1902, p. 454.
_Saturated Fatty Acids._--Stearic acid, C₁₈H₃₅O.OH, and palmitic acid,
C₁₆H₃₁O.OH, are the most important. At ordinary temperatures they are
hard, white, crystalline bodies, and melt at 69° and 62° C.
respectively. They do not, under ordinary circumstances, absorb any
oxygen, nor iodine, and are very little liable to chemical change.
Together with oleic acid, they are the principal acids of tallow and
other animal fats, while palmitic acid and some lower members of the
same group are more common in vegetable oils. Free stearic acid is an
important constituent of the “distilled stearines” used in currying;
while “oleostearine” consists mainly of the neutral fats or glycerides
of stearic and palmitic acids.
_Liquid Fatty Acids, Non-drying._--Of these, oleic acid is much the most
common and important; its glyceride, olein, forming the liquid part of
animal fats, and being the principal constituent of vegetable non-drying
oils. Olive oil consists almost entirely of olein, with a little
palmitin. The formula of oleic acid is C₁₈H₃₃O.OH, thus differing from
stearic acid in having two less atoms of hydrogen. The “bonds” or
affinities corresponding to these two atoms are linked together, but can
separate, and attach two atoms of iodine, bromine, or chlorine, or one
of oxygen. The iodine-value of pure olein is 83·9 (that is, 100 grm.
absorb 83·9 grm. iodine); and that of olive oil about 83. Any oil with a
higher “iodine-value” than olein must contain drying oils, though a
lower value does not necessarily indicate their absence, if palmitin or
other saturated acids are also present.
_Unsaturated Liquid Fatty Acids._--Of these there are several groups,
differing in their degree of saturation, and also probably in their
structure. Their glycerides, together with olein, and sometimes
palmitin, are the constituents of the seed oils, the drying tendency of
which depends on their proportion of unsaturated acids, and the
particular group to which they belong. The fish oils contain a peculiar
group of unsaturated acids, together with olein, and usually stearin and
palmitin, like the other animal fats. Linolenic acid, C₁₈H₂₉O.OH, one of
the acids of linseed oil, has six hydrogen atoms less than stearic acid,
and therefore three double linkings, and will take up six atoms of
iodine. Its theoretical iodine-value is 274, while linseed oil itself
often has an iodine-value exceeding 180. The iodine-value of cod-liver
oil is sometimes nearly as high. Both oils therefore contain other acids
less unsaturated than linolenic.
The “spueing” of leather is due to the absorption of oxygen and
consequent resinification of the oils, and therefore all drying oils,
however pure, are capable of producing it, though some are more liable
to do so than others (cp. pp. 363, 365, 366, 368, 390).
Linolenic acid, and probably other allied acids, become converted by
absorption of oxygen into solid varnish-like substances, which are
important to the tanner, as furnishing the principal constituents of
japans for leather. The unsaturated acids of fish oils seldom give hard
varnishes, though menhaden oil (page 367) is sometimes used as paint-oil
for outside work.
Most fats are liable to become rancid by exposure to the air, acquiring
a disagreeable taste and smell, and an acid reaction from the liberation
of the fatty acids. The changes which take place are somewhat complex.
The fatty acid of castor oil is of peculiar constitution, being an oleic
acid in which one of the hydrogen-atoms is replaced by a “hydroxyl” or
OH group. The solubility of castor oil in alcohol has already been
alluded to. It does not dry, and is an excellent oil for lubricating
heavy machinery. It is sometimes adulterated with “blown” oils, which
are made from non-drying, or slightly drying seed oils, like cotton-seed
or rape, by blowing air through them in a warmed condition. Under this
treatment they increase greatly in viscosity and density and in their
solubility in alcohol, but do not acquire the other valuable properties
of genuine castor oil.
The “foots” or sediments which oils deposit on standing, sometimes
consist of animal or vegetable fibres, or mucilage combined with water,
but often are simply the harder fats, stearin, palmitin, etc., which
crystallise from the oil on cooling. In this case they are re-dissolved
on warming the oil. Such oils, which like neatsfoot and tallow oils
become turbid in cold weather, are styled “tender.”
NON-DRYING FATS AND OILS.
_Tallow_ (Fr. _Suif_; Ger. _Talg_) is the fat of various mammalia,
principally of the ox and sheep, but occasionally also of the goat. The
mixed fat obtained from all parts of the carcass is known as “rendered
tallow,” while that obtained from the region of the kidneys (suet) is
harder. A substance commonly referred to as “pressed tallow” or
“oleo-stearine” is obtained by pressing ordinary tallow, in cloths, in
the hydraulic press. The more liquid portion which is expressed is
tallow-oil, the finer qualities of which are used in making margarine.
Oleo-stearine must not be confounded with the “distilled stearine,”
obtained from Yorkshire grease by distillation and pressure (page 359),
nor with candlemakers’ “stearine,” which is a mixture of free stearic
and palmitic acids.
Pure tallow is white and tasteless, but much of that sold is yellowish
and of a disagreeable, slightly rancid flavour. Mutton tallow is usually
harder and whiter than that of beef. Goat tallow has a characteristic
odour, as have the recovered stearines and other waste greases from
glue-works. Buck tallow, which is particularly hard, has now been
largely replaced by oleo-stearine.
Beef tallow melts at about 40° C.; mutton tallow at 45°.
In chemical composition, tallow consists chiefly of a mixture of the
tri-glycerides of palmitic, stearic and oleic acids; its hardness
diminishing with the increase of the last.
Tallow should, when melted, be perfectly clear, turbidity indicating the
presence of water or other foreign matters, due either to carelessness
in the manufacture or, possibly, adulteration. Traces of phosphate of
lime, or fragments of animal tissue, may be present as accidental
impurities; lime, on the other hand, is sometimes added to thicken the
tallow and enable it to retain more water; starch, china clay, whiting,
heavy spar, etc., are also occasionally employed. Tallow has been not
infrequently adulterated with the distilled fatty acids from wool
grease. When this is the case, crystals of cholesterol (see L.I.L.B., p.
181) may be detected by examination of the unsaponifiable matter of the
mixture under a microscope. It would also give the tallow an unusually
high “acid-value.”
Methods for the proximate analysis of tallow are given in the
‘Laboratory Book,’ pp. 189 _et seq._
The fats produced by the boiling of fleshings for glue, and by the
pressing of sheep-skins, are of the nature of soft tallows. If the
fleshings are delimed with acid, and boiled fresh, the grease is
generally of good colour, and with little unpleasant odour, but contains
traces of free fatty acids derived from the decomposition of the
lime-soaps. If the fleshings have been dried and the lime carbonated,
the grease will generally be brown, and more or less rancid; but the
lime-soaps are not decomposed, unless the “scutch” or refuse be treated
with acid, when a further yield of grease is obtained. The grease from
sheep-skins is generally somewhat brown, and often smells of the
volatile acids and other constituents of the tan-liquors, especially if
larch bark has been used. These greases are usually much improved in
appearance and odour, if well washed by boiling or steaming on water, or
by blowing a mixture of air and steam through them, or sometimes even by
mere heating to a sufficient temperature to evaporate the water and
drive off the volatile matters. By allowing the grease to cool slowly,
so as to favour crystallisation, till it is of a soupy consistency, and
then pumping through a filter press with woollen cloths, the more liquid
is separated easily from a more solid portion, and both may in many
cases be used in leather manufacture, the tallow for currying, and the
oil in place of neatsfoot oil.
_Horse-fat_, and especially that from the fatty portions of the neck
(Ger. _Kammfett_), as well as various other animal greases, are used in
the manufacture of leather. They differ from tallow chiefly in that they
have a lower melting-point, and contain more olein in proportion to the
stearin and palmitin than true tallow, and are consequently somewhat
softer. Though often almost white, these greases are sometimes darkened
in colour by the products of putrefying animal matter, but this does
not, as a rule, interfere with the oil being used for leather dressing.
They are usually so cheap that they are but little adulterated; means of
determining their purity are, however, given in L.I.L.B., p. 191.
_Neatsfoot oil_ is a yellowish, nearly odourless oil, of bland taste,
which is largely employed in the dressing of calf-kid. It has a similar
composition to tallow oil and the other oils obtained by subjecting the
soft animal fats to great pressure at a low temperature. It is often
adulterated with bone oil, lard oil and cotton-seed oil, and
occasionally with mineral oil and recovered wool-grease.
As neatsfoot oil is somewhat costly, curriers may with advantage often
use ordinary animal greases (horse-fat, etc.) after they have had the
harder tallow extracted by cooling and pressure, the product thus
obtained being, chemically, the same as neatsfoot oil, and in every
respect as suitable, while it is much less liable to adulteration.
The true neatsfoot oil is prepared by boiling the feet of cattle, and
sometimes of sheep and horses, with water, and skimming off and
clarifying the oil which is thus obtained.
The physical and chemical characteristics of this oil are described in
L.I.L.B., p. 192.
_Wool-Fat_ (Fr. _Suint_, _oesype_; Ger. _Wollschweissfett_) is a grease
of high specific gravity, exsuded from the sebaceous glands of the
sheep, together with organic salts of potassium. It is obtained by
extracting wool with solvents; or by washing with alkaline solutions,
from which it is recovered by precipitation with acid, and subsequent
hot-pressing of the “magma,” or, more recently, by evaporating the
scouring liquor to small bulk, and centrifugating. Wool-fat is
characterised by its low percentage of glycerides, the fatty acids which
it contains being mainly combined with higher alcohols (bodies of
alcoholic structure, but of a waxlike consistency), and chemically it is
rather a wax than a true fat. Among the alcohols which it contains is
included a marked percentage of cholesterol and isocholesterol. It is
difficultly saponifiable, requiring to be heated to 105-110° C. with
alcoholic potash under pressure; and even then about 44 per cent. of
alcohols remain, which are incapable of further saponification. Care
must therefore be taken not to assume that unsaponifiable matter in
greases which may contain wool-fat is necessarily mineral oil. For
details of analysis see L.I.L.B., p. 194.
Pure wool-fat is nearly white, of salve-like consistency and very slight
smell, with a density of 0·973 at 15° C. Crude wool-fat is yellow or
brown, with an unpleasant and very persistent characteristic smell. Both
the pure and the crude wool-fat have an extraordinary power of
emulsifying with water, which makes them very valuable as substitutes
for dégras in stuffing greases. Lanoline (and several other preparations
under different names) are mixtures of purified wool-fat and water, of
which lanoline contains about 22 per cent.
“Yorkshire grease” differs from crude wool-fat, in being recovered from
the waters employed in scouring woollen cloths, as well as wool, and
hence contains the free fatty acids of soaps used in scouring, as well
as the “oleines,” etc., used in oiling the cloth, and although it often
contains much wool-fat, it is occasionally destitute of this substance.
_Holden Fat_ consists of ordinary wool-grease mixed with fish oil, and
is used either as a substitute for, or in admixture with dégras (q. v.).
_Distilled Wool Grease_ is produced by distilling crude Yorkshire grease
with steam. Most of the glycerides are broken up, but many of the free
fatty acids, alcohols and waxes distil over unchanged, though a
considerable part is decomposed into volatile hydrocarbons strongly
resembling mineral oils. The distillate is separated by cooling and
pressure into a liquid “oleine” and a solid “stearine.” The latter forms
a very valuable stuffing-grease which, in England, largely takes the
place of the “oleo-stearine” used in the United States--with which,
however, it must not be confounded.
_Distilled Stearine_, prepared as above described, is a pale
yellow-to-brown fat, which varies in hardness and in its melting point
according to the conditions of its preparation. It has a characteristic
odour which is very persistent, and it consists largely of free stearic
and palmitic acids; most of the liquid hydrocarbons formed by
distillation being removed with the “oleine.”
_Olive Oil_ (Fr. _Huile d’olive_; Ger. _Olivenoel_, _Baumoel_) finds
extensive use in leather dressing, and especially in the manufacture of
“fat-liquors” (pp. 217, 240). It is extracted from the fruit of the
olive tree by pressure, and of late years from the residues by
extraction with carbon disulphide. Although it chemically resembles
tallow and lard oils very strongly, its adulteration with these
substances may usually be detected, at any rate roughly, by the taste
and odour of the oil. It is principally characterised, from a chemical
point of view, by containing the glyceride of palmitic but not that of
stearic acid, and by having a much larger proportion of olein to solid
glycerides than most of the non-drying animal oils. At low temperatures,
olive oil solidifies to a product which can be separated by pressure
into a solid tallow-like fat, and a fluid oil consisting essentially of
tri-olein.
Olive oil is the type of a non-drying vegetable oil, but though it does
not thicken materially on exposure, it becomes rancid somewhat rapidly,
and is thus rendered unsuitable for lubrication. Unless the acidity is
excessive it does not appear to spoil the oil for leather manufacture,
and for some purposes is actually an advantage as aiding emulsification.
Free acids in oils may be removed by shaking with sodium carbonate
solution.
Olive oil always contains some free acid; which is of importance in the
preparation of fat-liquors, as it facilitates the production of an
emulsion. This quality may be increased by the addition, when necessary,
of a little oleic acid.
Olive oil is frequently adulterated with other vegetable oils. Probably
the most useful criterion is the iodine-value, which is raised by the
addition of any seed oil. Examination in the refractometer also affords
useful indications. Cotton-seed, sesame and arachis (earth-nut) oils are
the most frequent adulterants of the better qualities, and in many cases
may be recognised by special tests.
_Castor Oil_ (Fr. _Huile de ricin_; Ger. _Ricinusoel_) is the oil
expressed from the seeds of _Ricinus communis_, and is a transparent,
colourless or pale yellowish liquid, having a faint odour and a
disagreeable taste. At a low temperature it thickens and deposits
slightly, and at -18° C. it solidifies to a pale yellow mass.
Castor oil is distinguished from all other natural fixed oils by its
high density (0·960 to 0·964) and viscosity, and by its solubility in
alcohol and its insolubility in petroleum ether. Genuine castor oil is
completely soluble in an equal volume of absolute alcohol, or in four
times its volume of “rectified spirit” at the ordinary temperature. It
is practically insoluble in petroleum ether, but can dissolve an equal
measure of that liquid.
For the purpose of the leather manufacturer, the ordinary hot-pressed
oil, such as is used for lubricating machinery, is quite as good as the
more costly cold-pressed oil which is used for medicinal purposes. It is
generally imported in tins holding about 40 lb. of oil. Castor oil, and
castor-oil soap made as described on p. 352, are very good for
fat-liquors, seeming to interfere with dyeing and glazing less than most
other oils. Boots oiled with castor oil may be blacked at once, and will
take a good polish.
The only oils which are usually mixed with castor oil are “blown” or
oxidised seed oils, or resin oil. Any other oils would so seriously
lower the specific gravity as to render their use impracticable. For the
detection and estimation of these the ‘Laboratory Book’ should be
consulted, or if fuller details are required the reader is referred to
Benedikt and Lewkowitsch’s ‘Oils, Fats and Waxes,’ or to Allen’s
‘Commercial Organic Analysis,’ vol. ii.
Sulphonated castor oil or Turkey-red oil is now largely used for
“fat-liquoring,” for which it was probably first employed by the author,
about 1890. This material--which must be carefully distinguished from
the olive oil preparation which is also used for dyeing cotton a
Turkey-red colour--is made by treating castor oil with one-quarter of
its weight of strong sulphuric acid (specific gravity 1·8), adding the
latter in very small quantities at a time, and taking care that the
temperature of the mixture at no time exceeds 35° C. The mixture is then
allowed to stand for twenty-four hours, with occasional stirring, and is
washed with its own volume of water, allowed to stand until the water
has all separated, and the oil is then syphoned off. If desired, the oil
may be further washed once or twice with a solution of strong brine, but
this is of doubtful advantage, and should in no case be excessive. The
washed oil is finally neutralised by the cautious addition of
one-hundredth of its volume of strong ammonia solution (sp. gr. 0·880).
If properly prepared, Turkey-red oil (sulphonated castor oil) will, when
_largely_ diluted with water, bear the addition of ammonia to alkaline
reaction without showing any turbidity even on standing several hours.
If a turbidity is produced, it indicates that the castor oil used was
impure and contained some oil rich in stearin.
The alcohol test described on p. 360 may also be applied, as the oily
layer will be entirely soluble if castor oil alone was used in the
preparation of the red oil.
Turkey-red oil usually contains about 50 per cent. of fatty acids
(Allen).
_Linseed Oil_ (Fr. _Huile de lin_; Ger. _Leinoel_) is used by leather
manufacturers in the preparation of the japan for making “patent
leather,” and to some extent also in currying, for oiling off levants
and moroccos, though for these purposes it has been largely superseded
by mineral oils. It is obtained from the seeds of the flax plant, _Linum
usitatissimum_, chiefly grown in Russia and India. The Russian oil is
usually mixed with the oil from hemp to the extent of about 20 per
cent., while that from India, being grown as a mixed crop with mustard
and rape, is never perfectly pure. The Baltic oil is considered best for
japans, and is improved by storing for a considerable time in tanks in a
warm place.
When obtained by cold pressure of the seeds, linseed oil is of a bright
yellow colour; if a higher temperature be used in the extraction the oil
is more or less brown, and tastes much more acrid. On exposure to air,
linseed oil turns easily rancid, absorbs oxygen, and if spread out in a
sufficiently thin film it dries to a neutral substance (linoxyn), which
is insoluble in ether. This property is the one on which the chief value
of linseed and other “drying oils” depends.
Linseed oil is chiefly adulterated with other seed oils, cottonseed
being the most often used for this purpose, though menhaden and various
other fish oils are occasionally employed. As the density of raw linseed
oil varies between 0·932 and 0·936 at 15° C., the addition of other seed
oils or of mineral oil would cause an appreciable lowering of this
figure, whilst rosin or rosin oil would raise it. A judicious admixture
of both mineral and rosin oils would give a product of normal density.
Fish oils can be detected by their characteristic smell, especially on
warming.
Various methods have been proposed for judging the quality of linseed
oil, but none of them are perfectly satisfactory. The best oil is that
which dries the most perfectly; but the rapidity of the drying, and the
consistency of the dried product, are most important factors which must
also be taken into account. The iodine-valve, which is a measure of the
drying power, should not fall much below 180.
A satisfactory practical test, recommended by Allen,[162] consists in
mixing the oil with three times its weight of genuine white lead, and
covering a perfectly clean glass surface with the paint. An exactly
similar experiment is made simultaneously with a standard sample of
linseed oil, and the rates of drying and the characters of the coating
of paint compared.
[162] Commercial Organic Analysis, ii. p. 122.
J. Muter has simplified this test by merely flooding a plate of glass
with the oil and then exposing it to a temperature of 38° C. (100° F.)
in a good current of air. The time required for drying, to such an
extent that the coating will not come off when lightly touched, is
noted, and compared with standard samples of oil. By applying the finger
at intervals to different parts of the film surface the progress of the
drying can be readily observed.[163]
[163] Kathreiner states that this method is a useful test for fish and
liver oils, those which dry most rapidly being specially liable to
“spue.”
_Boiled Oils._--Its capacity for thus drying is much enhanced by
heating, with addition of “driers,” to a temperature of 130° C. and
upwards, while passing a current of air through the oil and then
increasing the temperature until the oil begins to effervesce (“boil”).
Large quantities of linseed oil are now treated in this way for use in
the arts. The driers used are metallic salts, principally those of lead
and manganese, which apparently act as oxygen-carriers. Litharge was
formerly most commonly used, but its place has been taken to a
considerable extent by acetate, borate and resinate of manganese. From 1
to 2 per cent. of either litharge or manganese borate may be used,
though less quantities produce a marked effect. Apparently litharge
gives the most rapid drying, and manganese a much paler colour.[164]
Linseed oil is usually darkened by boiling, and increases both in actual
weight and in specific gravity and viscosity. The chemical reactions
which take place in boiling are not well understood, but it is in the
main a process of oxidation and polymerisation, perhaps accompanied by
the formation of anhydrides of the fatty acids, and a portion of the
drier remains dissolved in the boiled oil. These driers may be detected
by boiling an ounce or so of the oil with dilute hydrochloric acid,
allowing the mixture to separate into two layers and then syphoning off
the lower into another vessel, and testing for metals (lead, manganese,
zinc) or acids (boric, oxalic, etc.).
[164] Cp. F. H. Thorpe, Abst. Jour. Soc. Chem. Ind., 1890, 628, from
Technology, Quart., iii. pp. 9-16.
Black japan for patent leathers is made by boiling linseed oil, without
blowing air through it, for at least seven or eight hours, with Prussian
blue, or with oxides of iron. The japan is brownish rather than blue in
colour, and it is probable that the Prussian blue serves merely as a
source of iron oxide, which acts both as a colouring matter and a drier.
Other driers, such as litharge, are sometimes added, and for coloured
enamels other pigments are substituted for the Prussian blue.
_Cotton-seed Oil_ (Fr. _Huile de coton_; Ger. _Cottonoel or
Baumwollensamenoel_) is now expressed in enormous quantities in the
United States, on the continent of Europe and in Great Britain. The
crude oil contains a very characteristic colouring matter which, though
naturally ruby red, is sometimes so intense as to make the oil appear to
be nearly black. This colouring matter causes the oil to produce stains,
and is therefore removed by a process of refining, and a product of a
straw- or golden-yellow colour is thus obtained. The refining is usually
effected by shaking the crude oil with a cold 5 per cent. solution of
caustic soda, using about ten times as much oil as soda solution.
Cotton-seed oil is, on account of its price, seldom or never
adulterated, but is itself frequently employed as an adulterant of olive
and neatsfoot oils. It is a semi-drying oil, and unsuitable for most
purposes in leather manufacture. For a description of its characteristic
properties, both chemical and physical, the reader is referred to
Lewkowitsch’s ‘Oils, Fats and Waxes,’ or to Allen’s ‘Commercial Organic
Analysis,’ vol. ii.
_Sesamé Oil_ (Fr. _Huile de sésamé_; Ger. _Sesamoel_; Teel oil, Gingeli
oil) is another seed oil, usually of paler colour than cotton-seed oil,
but resembling it in having scarcely any odour, and possessing a bland
and agreeable, though not very characteristic taste. It is often used as
an adulterant of olive oil.
Sesamé oil is a non-drying oil, which does not easily turn rancid. When
present in other oils, it may be detected by agitating 10 c.c. of the
sample with 5 c.c. of concentrated hydrochloric acid in which 0·1 grm.
of white sugar has previously been dissolved. After shaking together for
at least ten minutes, the oil and acid are allowed to separate, when,
if sesamé oil be present, the acid layer will have a marked rose colour,
the intensity of which increases with the amount of sesamé oil in the
sample (Baudouin’s test).
Sesamé oil is largely used in India for oiling tanned sheep- and
goat-skins (“Persians”), and has the characteristic property of being
assimilable in large quantities by leather without the latter appearing
oily. East India tanned skins often contain 25 and even 30 per cent. The
oil is applied to them in the wet condition before they are dried. It is
easily detected in the oils extracted from these skins by Baudouin’s
test. The oil seems well adapted for many purposes in leather
manufacture.
_Cod Oil_ (Fr. _Huile de morue_; Ger. _Leberthran_) is by far the most
important oil used by leather manufacturers, and is obtained from the
liver of the common cod-fish (_Gadus Morrhua_) and several other members
of the genus _Gadus_. The chief seats of the cod fishery are the coasts
and banks of Newfoundland, Nova Scotia, the Gulf of St. Lawrence, the
coasts of Norway, Denmark and Germany, the Dogger Bank in the North Sea,
and the shores of Alaska in the Pacific Ocean.
The oil was formerly obtained by keeping the livers of the fish in large
wooden vats, stirring constantly until so much decomposition has taken
place that the cells containing the oil burst, and the oil thus released
rises to the surface and is skimmed off with wooden ladles. The crude
oil is allowed to deposit any suspended matters by sedimentation in a
tank, and is then poured into casks ready for sale. The “brown oil” so
often used by tanners is obtained by boiling the solid matter left after
extracting the oil as above in iron tanks until all the water has
evaporated; the oil thus liberated is then strained off, clarified and
put into barrels.
The purer qualities of cod-liver oil are now obtained by boiling the
livers with water and skimming off the oil which rises to the surface.
Three grades are on the market at the present time: medicinal, or
ordinary bright; an inferior “light brown”; and “dark-brown,” or
“tanners’ oil.” It is probable that these steam-extracted oils are much
more liable to “spue” than those extracted by the old method at a higher
temperature, since Eitner[165] has shown that seal oils extracted at a
low temperature spue badly, but lose the tendency if heated for some
time to 250-300° C.
[165] Gerber, 1880, p. 244.
Genuine cod oil, as suitable for use in leather manufacture, is always
more or less brown in colour, of specific gravity about 0·928, and
refractive index 1·482. At present prices it can only be adulterated
with other fish oils, rosin, or mineral oil, or with water, gelatine or
mucilage. Of these, rosin oil and petroleum are the most frequently
employed in sophistication.
An inferior variety of oil, known as “coast cod,” made from the livers
of various fish, such as ling, haddock and hake, is also sold, but, as
it is frequently mixed with oils from other fish refuse, it has a very
poor reputation.
Cod oil, together with most of the other oils obtained from fish livers,
has the property of producing an intense reddish-violet colour when a
drop of strong sulphuric acid is dropped upon ten or fifteen drops of
the oil contained in a white porcelain tray or saucer. The reaction
succeeds still better, if, instead of the oil itself, its solution in
chloroform, carbon disulphide or tetrachloride is employed. This test,
although very useful for the detection of liver oils when they are
present in oils of a totally different character, such as rape or olive
oils, does not in any way indicate whether a sample of fish oil is pure
or otherwise. A very similar reaction is given by cholesterol which is
present in wool-fat.
_Shark-liver Oil_ (Fr. _Huile de requin_; Ger. _Haifischthran_) is
obtained from the liver of the “basking shark,” or “ice-shark,” chiefly
caught off the coast of Norway; but the livers of the dog-fish and
several allied fish also are sometimes substituted.
Shark oil has been employed in tanneries as a substitute for cod-liver
oil, but, according to Lewkowitsch, and to Allen, it is no longer
employed in England. From its pale colour it is probably principally
used to improve the appearance of darker oils. According to Eitner,[166]
its use causes leather to “spue” badly if not previously heated.
[166] Gerber, 1886, p. 266.
Shark oil is characterised by the very notable proportion of
unsaponifiable matter which it contains, which is of the same character
as that of sperm oil, and not easily removed from its soap solution by
petroleum ether. It gives a strong violet-blue coloration with
concentrated sulphuric acid, the reaction being even more marked than
with cod-liver oil itself, and of a bluer violet.
_Whale Oil_ (Fr. _Huile de baleine_; Ger. _Wallfischthran_) is extracted
from the blubber of various species of whale, and often contains traces
of spermaceti, the substance which characterises the oil from the sperm
whale. This yields on saponification higher alcohols, which are found in
the unsaponifiable matter; but in ordinary whale oil the total
unsaponifiable matter seldom exceeds 1¹⁄₂ to 2 per cent. Whale oil is
largely used on the Continent for “chamoising” (q.v.), and is
consequently a constituent of dégras. It is much less oxidisable than
cod.
_Seal oil_ (Fr. _Huile de phoque_; Ger. _Robbenthran_) is obtained from
the common rough-coated seal, abundant in the Arctic regions. It bears a
strong resemblance to both whale and fish oils, and cannot be detected
in mixtures of these. The Swedish “Dreikronenthran” (Three Crown Oil) is
a mixture of seal and fish oils. As genuine seal oil only contains about
¹⁄₂ per cent. of unsaponifiable matter, its adulteration by mineral or
resin oils may be detected by a determination of the matter extracted by
petroleum ether after saponification of the oil (see L.I.L.B., p. 178).
There is no simple test by which the purity or otherwise of a sample of
oil can be determined, as the dealers know all the best tests which the
users could try, and fake up their oils accordingly. For instance, if
petroleum is to be added surreptitiously to a cod oil, the decrease in
specific gravity of the oil caused by this addition would be corrected
by the addition of a suitable quantity of soap or rosin oil, which would
scarcely affect the colour, taste or odour of the sample. The only
satisfactory method of detecting adulteration is to submit the oil to a
complete chemical examination, and for this purpose L.I.L.B., pp. 156
_et seq._, or the larger text-books already named may be suitably
consulted.
_Menhaden Oil_ (Porgie oil, Straits oil) is largely used in certain
districts as an adulterant or substitute for cod oil. It is obtained
from the _Alosa Brevoordia_ or _menhaden_, a member of the herring
family, about a foot long. The fish is caught on the Atlantic coast of
America, and is so plentiful that it is very doubtful whether cod oil
can ever compete with it successfully in price. The fish are boiled in
steam kettles, the oil squeezed by hydraulic presses, clarified, and
bleached by exposing to the sun in shallow glass-covered tanks. An
inferior grade is known as “Bank oil.” Menhaden oil is chiefly
characterised by its very high “specific temperature reaction”
(L.I.L.B., p. 169) which is about 306. It is not a good leather-oil,
being very liable to “spue.”
Many other varieties of oil extracted from the bodies, and not from the
livers only of fishes, are classed as _fish oils_. Menhaden oil is the
principal of these; but Japanese oil, sardine and herring oils, and
those obtained from the refuse of other fish are scarcely less
important, though as they are derived from such different sources it is
not possible to quote any definite characteristics by which they may be
identified when mixed with more valuable oils. They are usually very
liable to “spue.”
_Fish Tallow_, which, according to Eitner, is a good and cheap
substitute for dégras, is the solid grease obtained from different kinds
of fish oil by subjecting them to a low temperature and separating the
matter which is thus precipitated, or (as in China and Japan) the solid
fat which is extracted at the same time as the oil from the body of the
fish. Formerly fish tallow was only obtained from and with Japanese
train oil, but it is now obtained from whale blubber. This latter yields
a very pure form of the tallow, which does not need any rectification;
but the Japanese variety, which is obtained from fish of the herring
family, contains a sort of fish glue, which greatly deteriorates the
quality of the product. By careful purification, however, this glutinous
matter may be removed, and the refined product has none of the
leather-staining properties so characteristic of the crude tallow. The
refined tallow is sold in square flat cakes, melts at 42° and is not
quite so stiff as ox tallow.
_Dégras_ and _Sod Oil_ are products of chamois-leather dressing (p. 378)
which are used in currying. Skins are treated with marine animal oils,
and submitted to oxidation, and the surplus and partially altered oil is
recovered. In the French method, whale and seal oils as well as liver
oils are used, and the oxidation is slow and gradual, and the residual
oil, being liquid, is recovered by pressure, and constitutes _moellon_,
of which the first pressing (_première torse_) is the best. This is
never sold for currying in its original purity; but, mixed with further
quantities of fish oils, tallows, and sometimes wool-fat, it constitutes
the ordinary dégras of commerce. The additions, though they lower the
value, are not to be considered as simple adulterations, since the
_moellon_ alone would be less suitable for the purpose. After removal of
as much oil as is possible by dipping in hot water and pressing, a
further quantity is recovered by washing with solutions of potash or
soda, from which it is separated by addition of acid, and constitutes a
lower quality of degras. The _moellon_ is of such value as a currying
material, that factories are run in which chamoising is carried on
solely for its production, the skins being oiled and oxidised
repeatedly, till reduced to rags.
In the English method of chamoising, liver oils are almost exclusively
used, and the oxidation is much more rapid and intense, the skins being
packed in boxes or piled, and allowed to heat. The product obtained in
this way is much more viscous, and can only be recovered by scouring
with alkalis; and the product, recovered with acid, constitutes sod oil.
In many English factories, a modified method is now adopted, and a
product recovered by pressure, which scarcely differs from _moellon_.
An important peculiarity of dégras and sod oil is its ready
emulsification with water, which from its mode of preparation, it always
naturally contains, and which should be present in a good dégras to the
extent of not less than 20 per cent. Such a mixture, containing water,
is a sort of natural fat-liquor and is absorbed much more perfectly by
the skins than an oil alone. Sod oils, however, are frequently
“evaporated,” or deprived of water by heating above 100°, with the
object not only of effecting a fancied improvement, but of getting rid
more completely of the sulphuric acid which the water is apt to contain.
This makes them more homogeneous, and consequently much darker in
colour. It is not easy to neutralise the acid in an aqueous sod oil by
direct addition of alkali; possibly ammonia is best adapted for the
purpose; or a suggestion, I think due to Eitner, may be adopted, of
incorporating a small quantity of a suitable soap. In any case, very
complete mixture is required. If the sulphuric acid used in recovery has
been insufficient for complete neutralisation of the alkali, the dégras
or sod oil will naturally contain soaps, and sometimes also free alkali.
Free acid and free alkali are both injurious to leather, the former if
anything the more so, darkening the colour, and even rendering the
leather tender. When dégras is used in mixture with other fats, care
should be taken not to raise the temperature of the mixture so high as
to drive off the water, to which a good deal of its special efficacy is
due.
The chemical changes which take place during the chamoising process are
as yet incompletely understood. A large proportion of the glycerine is
dehydrated during the “heating,” forming acrolein (acrylic aldehyde), to
the action of which it is very possible that the actual conversion of
the skin into leather is due, while the fatty acids also undergo
oxidation. Dégras therefore always contains considerable quantities of
oxidised fatty acids, which are sometimes associated with nitrogenous
products from the skins, and which are soluble in alcohol, but insoluble
in petroleum ether. To these products Simand gave the name of
_Degrasbildner_ (dégras-former, Fr. _dégragène_), and it has been
considered a measure of the quality of the degras, but its exact value
and function is rather doubtful. According to Simand, a genuine dégras
should contain not less than 15 to 20 per cent. of the dégras-former as
estimated by his method, calculated on the dry oil, and a smaller
percentage is also present in the original fish oils. (For method of
estimation see L.I.L.B., p. 182).
As the process of dégras manufacture is obviously mainly one of
oxidation, many attempts have been made to produce it by direct
oxidation of fish oils, without the agency of skins, both by blowing air
through the oil, and by addition of oxidising agents such as nitric
acid. Eitner states that such oxidised oils are more liable to “spue”
than the original oils, as they already contain large quantities of
resinised products; but this is certainly not true of all artificial
dégras, some of which answers its purpose perfectly as a currying
material, though it is very probably justified in other cases. Of course
the methods of successful manufacturers are kept as profound secrets.
Dégras and sod oil, when deprived of water, are dark and viscous oils,
of high specific gravity (0·945-0·955), and therefore heavier than the
oils which have been employed in their manufacture.
WAXES, as has already been stated, differ in their chemical character
from true fats, in that their fatty acids, which are mostly of high
molecular weight, are combined, not with glycerine, but with alcohols,
also of high molecular weight and of wax-like consistency. Most waxes
are solid bodies of high melting point, but some oils, especially sperm
and bottlenose oils, are chemically liquid waxes; woolfat contains a
considerable proportion of waxes; and many marine oils, such for
instance as shark-liver oil (p. 366), contain waxes in smaller quantity
in mixture with true fatty oils.
_Sperm Oil_ (Fr. _Huile de cachalot_; Ger. _Spermacetioel_, _Walratoel_)
is obtained from the sperm whale, an inhabitant of the Antarctic seas.
“Arctic sperm” (Ger. _Doeglingthran_) is a very similar oil obtained
from the “Bottlenose whale.” These oils are very fluid, do not dry, and
are excellent lubricating oils for light machinery, and also good lamp
oils. They contain little if any glycerides, and about 40 per cent. of
unsaponifiable solid alcohols, which are soluble in ethyl-alcohol, and
must not be confused with ordinary unsaponifiable mineral oils, which
are frequently used as adulterants in mixture with fatty oils to adjust
gravity and the “saponification value.” Mineral oils are liquid, and
insoluble in alcohol. Sperm oil is the lightest of ordinary oils, its
gravity being only about 0·880 at 15° C. From its price it is
particularly liable to sophistication. It is used in leather manufacture
in the finishing of some fine leathers, and sometimes as a constituent
of fat-liquors. Spermaceti, a wax also obtained from the sperm whale, is
an occasional constituent of leather polishes.
_Beeswax_ (Fr. _Cire des abeilles_; Ger. _Bienenwachs_) is one of the
most important waxes for the leather-dresser. As is well known, it is
obtained from the honeycomb of the ordinary bee. It is a yellowish solid
body, fairly plastic when fresh, and of “waxy” feel. At low temperatures
it is brittle and of fine granular texture, and when pure is almost
tasteless. It is often bleached by repeated melting and exposure to
sunlight. As wax always contains a considerable amount of pollen it may
be identified when in admixture with other substances by means of the
microscope.
Beeswax is almost insoluble in cold alcohol, but boiling alcohol
dissolves out the contained cerotic acid, which crystallises from it on
cooling. Wax is saponified by alcoholic potash, but the resulting
myricyl alcohol (about 54 per cent.) is not capable of further
saponification.
Beeswax is frequently adulterated. Water and mineral matters (ochre,
gypsum, etc.) also flour, starch, tallow, stearic acid, Japan wax,
carnaüba wax, resin and paraffin-wax are among the substances most
commonly used in its sophistication.
The detection of these, and especially of the other waxes, is so
difficult that it will not be described here. The reader is, however,
referred to Benedikt and Lewkowitsch’s ‘Oils, Fats and Waxes,’ for
further information.
_Carnaüba Wax_ (Fr. _Cire de carnauba_; Ger. _Cearenwachs_,
_Carnaubawachs_) has come largely into use recently owing to the advent
of the coloured leather shoe. As it is a very hard wax it has become
very popular with boot polish makers, its low price being also in its
favour. Carnaüba wax is an exudation from the leaves of _Copernica
cerifera_, a palm indigenous to Brazil, and is, on this account, often
known as Brazilian wax. It is difficult to saponify, and with different
experimenters has yielded very varied results on analysis; it is
generally agreed, however, that it is a complicated mixture of several
of the higher alcohols and acids.
_Japan Wax_ is not a true wax, but a fat consisting of glycerides. It is
a pale yellow, hard, waxy substance obtained from the berries of a
sumach (_Rhus succedanea_, etc.). At ordinary temperatures its specific
gravity is exactly that of water, and it melts at 56° C. Any admixture
with other fats would lower the melting point, but japan wax is often
adulterated with 15 to 30 per cent. of water. It is chiefly valuable to
leather dressers as a substitute for beeswax on account of its lower
price.
VOLATILE OR ESSENTIAL OILS.
These oils are distinguished from those described in the previous
section in that they are capable of distillation without undergoing any
serious amount of decomposition. They occur to some considerable extent
in nature, but those of most importance to the leather trade are
produced by the decomposition of more complicated materials.
_Birch Oil_ is by far the most important of this class of oils so far as
the leather-dresser is concerned, since it is the substance which gives
to “Russian leather” its characteristic odour.
The oil is obtained by destructive distillation, and the process by
which the peasants conduct this is one of the rudest that can be
imagined. A cauldron is filled with dry birch-bark, closed, and heated
over a fire. The vapours which are evolved are carried, by means of a
pipe, to another vessel which is buried in the ground, and are there
condensed. The dark-brown liquid (birch-tar) is allowed to cool, and the
liquor which rises to the surface skimmed off. The tar is sometimes
distilled, and an oil is thus obtained which does not give the true
birch-oil scent very strongly though occasionally sold as a refined oil.
The true odorous substance is evidently of very high boiling point and
remains mainly in the tar.
The birch tar is almost entirely used for giving leathers a “Russian”
odour, for although it smells somewhat strongly of tarry products, the
oils causing this smell are far more volatile than the birch scent
itself, and therefore disappear on storing the leather a short time. Tar
obtained from various species of pine is sometimes substituted for birch
tar, but it may readily be distinguished from the latter by the odour
and the difference in the specific gravity. Birch tar has a specific
gravity of 0·925 to 0·945, whilst fir tar has one of 1·02 to 1·05; thus
the former floats on water while the latter sinks if it be entirely free
from enclosed air. Fir tar, too, gives up a yellow colouring matter to
water shaken up with it, while birch tar leaves the water colourless.
Birch tar has a distinctly acid reaction, and must not be kept in iron
vessels. (See p. 251).
The leaves and twigs of American black birch when distilled with water
or steam, yield an oil which is practically identical with that of
_Gaultheria procumbens_ (wintergreen), and consists almost entirely of
methyl salicylate. It is clarified, and to some extent decolorised, by
filtration through woollen blankets and redistillation. A ton of
brushwood is said to yield about four pounds of oil. This oil has quite
a different odour to that of the real Russian oil, and cannot be used in
the scenting of “Russia” leather. Sandalwood oil with a little black
birch or wintergreen oil is sometimes employed for scenting small fancy
articles and bears considerable resemblance to the true “Russia” leather
odour. Black birch, aniseed, sassafras and various other essential oils
are occasionally used in small quantities as preservatives, and to cover
disagreeable odour in blood-seasonings, cements and other products used
in the leather trade. The methods employed for their detection and
estimation do not, however, come within the scope of a work such as the
present one. Most essential oils have considerable power as antiseptics,
and in preventing mildew and the attacks of insects.
MINERAL OILS AND WAXES.
This class of bodies is totally different in chemical constitution from
the true oils and waxes, containing neither glycerides, fatty acids nor
alcohols, but consisting of carbon and hydrogen only, approximately in
the proportion of one atom of the former to two of the latter. They
occur in underground lakes, from which they are obtained by springs or
borings; or in shales, from which they are separated by distillation. It
is commonly supposed that they have been formed, at some remote period
of the earth’s history, by the decomposition of animal and vegetable
matters, at a high temperature and under great pressure.[167]
[167] Oils from wells or springs are technically called “petroleum
oils,” those from shale, “paraffin” oils, but chemically, there is no
definite distinction.
The mineral oils and waxes are largely capable of being distilled
without decomposition, but if heated to high temperatures, are readily
“cracked” or broken up into simpler and generally more volatile
compounds--a fact which is employed in the production of gas, and the
utilisation of some of the heavier products.
They differ greatly in their gravity and boiling-point, but not much in
their ultimate composition, consisting largely of saturated or nearly
saturated hydrocarbons (cp. p. 354), and hence are little liable to
oxidation, and acted on by few chemical reagents. From their
constitution they are of course unsaponifiable, and in this way can be
separated from fats and oils with which they have been mixed. (For
particulars of the method see L.I.L.B., p. 178.)
The heavier mineral oils are a good deal used in mixture with other oils
and fats, for stuffing leathers, those of a specific gravity of
0·880-0·900 being usually most suitable. They are quite incapable of
“spueing,” and are useful in lessening that tendency in other oils with
which they are mixed. They have not, however, the same affinity for the
leather fibre as some of the true oils, and are to a certain slight
extent volatile, and should generally be used in mixture, rather than
alone.
Most mineral oils, when held so that a strong light (daylight or
electric light rich in ultra-violet rays) falls upon them, show a green
or violet fluorescence or “bloom.” This is very persistent, even when
the oil is mixed with a large volume of other oils, and is often relied
upon as a means of detecting them when used as adulterants. The test is,
however, not infallible, since the effect is due to impurities which may
be removed by purification, or masked by the addition of such substances
as nitrobenzene or nitronaphthalene, and it also occurs in the
hydrocarbon products produced in the distillation by steam of animal
oils, and is occasionally seen to some extent even in oils which have
not undergone distillation.
_Vaseline and Vaseline Oil_ are the most viscous and densest of the
petroleum oil products. They probably differ from the solid paraffins in
chemical constitution, though their ultimate composition is almost the
same. They are often useful constituents of stuffing greases.
_Paraffin Wax_ consists of a mixture of hydrocarbons similar in chemical
constitution to the paraffin and petroleum oils, but of higher boiling
point, and solid at ordinary temperatures. Its hydrocarbons are mostly
saturated, and hence very stable bodies, and little liable to oxidation.
They are completely unsaponifiable, and unaffected by boiling with
alcoholic potash, and in most cases by boiling with strong sulphuric
acid, by which they may be separated from animal and vegetable waxes or
fats with which they have been mixed. They are quite incapable of
resinising by oxidation, or of causing “spueing” in leather. They are
soluble in petroleum spirit, carbon disulphide and most of the ordinary
solvents of fats, but insoluble in alcohol.
Paraffin wax separates from the liquid oils by crystallisation on
cooling, and the remaining liquid which adheres is removed by hydraulic
pressing, as in the case of tallow. The hardness and melting point vary
according to the extent to which the pressing has been carried, and the
temperature at which it has been done. The paraffins of higher melting
point are as a rule the more costly.
Pure paraffin wax is a white, more or less hard and brittle substance
which does not melt so easily as ordinary fats, and is on this account
used in stuffing certain kinds of leather, hardening the stuffing
grease, and making the leather feel less oily. When melted, paraffin wax
forms a thin liquid, more resembling an ordinary petroleum lamp oil than
the viscous vaselines and leather oils. On ignition it burns with a
bright somewhat smoky flame, and leaves no ash behind. It is found on
analysis when mixed with other waxes or oils in the “unsaponifiable
matter” (see L.I.L.B., p. 178).
_Ozokerit_ is a natural paraffin material used for the manufacture of
cerasin candles, which sometimes occurs in the vicinity of petroleum
springs, especially in Galicia. It is of pale yellow colour when pure,
and has then a melting point of about 70° C. Its chief impurities are
petroleum oils, water and clay. These are removed by melting the
ozokerit, decanting off the clear oil, and filtering it through fine
animal charcoal. If liquid oils are present the material is treated with
alkali or with strong sulphuric acid, and is pressed before filtering
through charcoal. The refined product is termed “cerasin,” and is of a
more waxy and less crystalline texture than ordinary paraffin wax.
_The Resin Oils_ are derived from resins, and _mainly_ from colophony or
common pine rosin, by destructive distillation. Their specific gravity
ranges from 0·96 to 0·99, but their chemical composition is very
imperfectly understood, and appears to be by no means constant. Like the
mineral oils they are “unsaponifiable,” but often contain small amounts
of soap-forming material (resin acids).
The detection and estimation of resin oils is often a matter of
considerable difficulty, but further particulars on this point will be
found in L.I.L.B., p. 180. From their cheapness, they are considerably
employed as adulterants of other oils, and their high gravity makes them
convenient to adjust the gravity of mineral oils when used for this
purpose, as the latter are usually lighter than the fatty oils. As
currying oils, they are not particularly suitable, though often employed
in stuffing picker bands, and other heavily greased leathers. They have
considerable antiseptic powers, and for this reason are useful in
leather greases, preventing heating, and checking mildews.
Resin itself is occasionally used as an addition to stuffing greases,
and is said to increase the waterproofness of the leather, and to give
it a drier feel. In mixture with about half its weight of paraffin wax,
and with a little grease if necessary to soften the mixture, it is often
used in waterproofing mixtures, which can be made to melt at 50° to 60°
C. Leather will bear immersion in the melted mixture without scalding if
thoroughly dried in a hot stove at a temperature of not less than 50° C.
before dipping. Any great increase of the proportion of paraffin wax
causes the rosin to separate. Rosin consists mainly of free acids which
easily combine with alkalies and alkaline carbonates in boiling. It is
hence largely used in the manufacture of soaps on account of its
cheapness, and to render them more soluble in water. The rosin acids are
not so strong as many of the fatty acids, and rosin soaps are therefore
somewhat strongly alkaline. Rosin soap, precipitated among the ground
paper pulp in the rag engine, by addition of alum or sulphate of
alumina, is largely used as a sizing for common papers.
CHAPTER XXIV.
_OIL TANNAGES, AND THE USE OF OILS AND FATS IN CURRYING._
The conversion of skin into leather by the agency of oils and fats is
probably one of the most primitive methods, and is used in different
ways suited to the skins and fats which are available, by savage races
in all quarters of the globe. In its simplest form, it consists merely
in oiling or greasing the wet skin, and kneading and stretching it as it
slowly loses moisture and absorbs the fat. Under these conditions, the
fibres become coated with a greasy layer, which prevents their adherence
after they are once separated by the mechanical treatment. At the same
time some chemical change takes place in the fibre itself, which has a
part in its conversion into leather varying in importance according to
the method and fat employed, and of which the chemistry will be best
discussed after some slight sketch has been given of the methods
themselves.
The most complete sort of oil-leather is that produced by “chamoising,”
or oil-dressing with marine oils, a process applied to the ordinary
“chamois” or “wash-leathers” (now made from the flesh-split or “lining”
of the sheep-skin), and to the manufacture of “buff-leather” for
military purposes. The process varies somewhat according to the
character of the leather, but the manufacture of the common wash-leather
may be taken as a type. For this purpose the sheep-splits are freed from
the loose and fatty middle layer (p. 51) by “frizing” with a sharp knife
on a beam similar to that used for fleshing (Fig. 30, p. 147), but much
more steeply inclined. The process is rather one of scraping than
cutting, and was originally adopted to remove the grain from the
deer-skins which were largely used for glove-leathers, since
oil-dressing does not easily penetrate a skin with the grain surface
intact. The fleshes are usually delimed by drenching, but removal of
fat is unimportant. After being well drained, they are “stocked” for
some time with sawdust till they become partially dry and porous, the
common “faller” stocks shown in Fig. 22, p. 116, being generally
employed. During the stocking, care must be taken that the goods are not
overheated by the friction produced. When the skins have become opaque
from the inclusion of air between the fibres, they are, according to the
Continental method, shaken out and oiled on the table, and after folding
into bundles, are put back in the stocks. In England, the oil is usually
added to them during the stocking, in small quantities, which become
rapidly and evenly distributed by the motion of the skins. In England,
cod oil is almost exclusively employed, but on the Continent, a
considerable proportion of seal and whale oils is used. As the goods are
apt to heat, not only from friction, but from the oxidation of the oils
employed, they are removed from the stocks at intervals, and allowed to
cool, usually hung on hooks exposed to the air. In France this exposure
to the air is much more considerable than in England, the skins being
hung for eight or twelve hours after each stocking. The drying rooms are
kept moderately warm, and a good deal of oxidation of the oil takes
place in them, which materially affects the character of the product,
and especially of the residual oil or dégras, which is afterwards
squeezed out of the skins and used for currying (p. 368). Great care is
required to prevent any parts of the skins becoming dry before they are
completely saturated by the oil, which causes hard and transparent
patches which the oil will not afterwards penetrate. After each exposure
to the air, the skins are oiled on the table and returned to the stocks.
The stocking has to be continued for many hours, even for wash-leather;
and as it proceeds, the skins lose the smell of limed skin, and acquire
a peculiar mustard-like odour from the volatile products of oxidation of
the oils. When the skins are completely saturated, they are, according
to the English method, packed in boxes, and allowed to heat
spontaneously by oxidation of the oils, during which great care is
required, especially at the outset, that the heat does not rise so high
as to destroy the skins. To prevent this, they are removed at intervals
from the boxes and spread on the floor to cool, and then re-packed, and
this treatment is continued until the oxidation is complete, and the
skins cease to heat. During the heating, large quantities of volatile
and very pungent products are given off, and especially acrolein
(acrylic aldehyde, from the dehydration of the glycerine), which is
excessively irritating to the eyes. The German method is not unlike the
English, but in France, the packing in boxes is omitted, and the
oxidation is completely effected in warm stoves in which the goods are
hung on hooks. The heating in this case is much more moderate, and the
oil less thickened, a result which may be partly due to the different
oils employed, and which leads to differences in the subsequent
treatment of the leather.
In the French process, the oily skins are dipped in hot water and wrung
or hydraulic pressed, the expressed oil constituting _moellon_ or
_dégras_ (p. 368), and the skins are afterwards washed in a hot soda or
potash solution, from which a further portion of an inferior _dégras_ is
recovered. In the old-fashioned English method, the oil became so
thickened that it could not be pressed out, and the whole was removed by
washing with soda or potash solution, from which it was recovered by the
use of acid, constituting “sod oil” (p. 369.) Now many English
manufacturers adopt a modified method, and remove a good deal of their
oil by pressure.
Buff leather, much used for military accoutrements, is made in a similar
manner to chamois, from ox or cow hides, the grain of which is frized
off. The bleaching, both of buff and chamois, is done by exposing to the
sun in a damp condition, the skins being watered as required with water
or fat-liquor, or the alkaline emulsion of _dégras_ obtained in washing
the skins. It may also be bleached by oxidising agents, such as
permanganate of potash or acidified sodium peroxide. If permanganate is
used, the leather is treated in a solution of perhaps 5 grm. per liter
till of a deep brown colour, and then in a solution of sulphurous or
oxalic acid till the colour is removed.
Messrs. J. and E. Pullman, of Godalming, make a species of buff leather,
which they style “Kaspine” leather, by treating limed and drenched hides
or skins in a drum with a very dilute solution of formaldehyde
(“formalin”) rendered alkaline with sodium carbonate (Eng. Pat. 2872,
1898). The change to leather takes place very rapidly, and the leather
is afterwards treated with soap solutions of fat-liquors, to feed and
soften it. It is almost indistinguishable from genuine buff leather,
except from the fact that it is white throughout, and needs no
bleaching. It is finding considerable application for military purposes.
A type of leathers which bear a close chemical relation to oil-leathers,
is that including “Crown,” “Helvetia,” and fat-tanned leathers. The
first leather of the sort was invented by a German cabinet-maker named
Klemm, by whom the secret was sold to Preller, who manufactured it in
Southwark, under the name of “Crown” leather. Klemm used flour,
ox-brains, butter, milk, and soft fat, which was made into a paste with
water, and spread on the limed, drenched and partially dried skins,
which were rolled into bundles, and drummed in slightly warmed drums for
some hours; taken out, again dried slightly, and coated with the
mixture, and again drummed. For thick hides the process was repeated a
third time, drumming in each operation for about eight hours. The
leather was used for laces, picker-bands, light belts, and other
purposes where great toughness and flexibility were required. It was
found by further experience (if indeed, it was not known to Klemm
himself) that the only really essential ingredients of the mixture were
the soft fats and flour; and even the latter could, for some sorts of
leather, be dispensed with. It was further ascertained that only the
gluten or albuminous part of the flour was absorbed by the leather, the
starch serving mainly to facilitate the emulsification of the fats. The
proportions used in the paste are about seven parts of flour, seven
parts of soft fat such as horse grease, two parts of tallow, four parts
of water, and a little salt or nitre to act as an antiseptic. Other
greases, such as mixtures of tallow and oil, can be substituted for the
horse grease, and pipe-clay or ochre may to some extent take the place
of the flour, while soap may also be added. The similarity of the
mixtures used to the tawing paste in calf- and glove-kid dressing (pp.
191, 196) is obvious, and Klemm had an earlier process in which the
operation just described was preceded by a slight alum tannage, and
which was almost identical in its detail with the methods now in use for
the production of so-called “raw-hide.” On the other hand it is nearly
allied to the production of “Riems,” or raw-hide straps in South Africa,
for which a long thong is cut spirally from a hide, and wound into a
sort of skein which is suspended from a crossbar, with a heavy weight at
its lower end, and oiled and twisted, with frequent changes of position,
until the water is dried out, and the thong is saturated with fat,
forming a very tough and durable leather. A similar material can be made
by fulling or otherwise working grease into a raw hide prepared for
tanning. Eitner examined samples of “Crown-leather” chemically, by
removing the gluten of the flour with an alkaline solution, and found
that an imperfectly chamoised leather remained, which when restuffed
with fat, was much less full, and carried a much smaller quantity of
grease than before.[168]
[168] Gerber, 1878, p. 2.
Various theories have been proposed to explain the reaction which takes
place in the production of oil-leathers. Knapp supposed that it was
merely a case in which the smallest fibrils of the hide were coated with
the products of the oxidation of oils, and so prevented from adhering
together, and protected from the action of water by the sort of
waterproof coating which was formed. This explanation is scarcely
feasible in the face of the fact that chamois leather can be treated
even with hot dilute solutions of the caustic alkalies without
destruction, while cotton fibres waterproofed by treatment with drying
oils have their coating entirely removed by treatment with alkalies.
Lietzmann supposed that the whole of the gelatinous fibres were removed
in the liming and subsequent treatment, and that the finished leather
consisted only of the skeleton of yellow or elastic fibre which exists
in the skin, and which is remarkable for its resistance to heat, acids
and alkalis. Unfortunately the proportion of these fibres does not
exceed about 6 per cent. of the total, so that they are quite
insufficient to account for the production of the leather. We now know,
however, that aldehydes, including the acryl-aldehyde, which is evolved
in the oil oxidation of chamoising (and which is covered by Messrs.
Pullman’s patent) are capable in themselves of converting gelatinous
substances into a material identical in its properties, and especially
in its power of resisting hot water and alkaline solutions, with the
fibre of chamois leather. In all cases where perfect chamoising is
produced, intense oxidation takes place, and oxidisable oils are used
which will evolve acrylic and other aldehydes. Where oils of little
drying power are employed, as in the case of Crown- and other
fat-leathers, only an imperfect chamoising occurs, and we are therefore
justified in attributing the special qualities of chamois leather to a
natural aldehyde tannage. On the other hand, there is no doubt that the
coating of the fibres with oxidised oil-products really occurs, and is
probably a powerful factor in the leathering of Crown-leather, and other
similar products which are not washed out with alkaline solutions. Knapp
proved by treating raw pelt which had been dehydrated with alcohol (p.
74) with a very dilute alcoholic solution of stearic acid, that a thin
coating of stearic acid on the fibres would confer great softness and
considerable resistance to water. Even where no stearic or other fatty
acid is purposely added to alcohol used for dehydrating pelt, traces are
present from the decomposition of the natural fat of the skin, and there
is little doubt that this is the cause why such alcohol-leathers are
much more difficult to wet back again to the state of pelt than would _a
priori_ be expected; and why hide-powder dehydrated in this way is
unsuitable for use in the hide-powder filter (p. 311) from its
non-absorption of water.
[Illustration: FIG. 85.--Scouring large Seal-Skins by Hand.]
It is not within the scope of the present volume to describe in detail
the processes used in currying, many of which are purely mechanical, and
of no theoretical interest, whatever their practical importance; and
with which the writer hopes to deal fully in a future book. The leather
is usually scoured with stone, brush and sleeker to free it from “bloom”
and loose tan (Fig. 85); or by machines such as Fig. 86; and is often
reduced in thickness by shaving by hand (Fig. 87), or by machine (Fig.
88). In place of shaving, hides and skins are frequently split into two
or more thicknesses. This is done by various machines, of which the
“bandknife” shown in Fig. 89 is the most important; the cutting tool
being a thin steel belt stretched like a bandsaw and sharpened on one
edge by an emery-wheel.
[Illustration: FIG. 86.--Scouring Machine.]
[Illustration: CURRYING SHOP, LEATHER INDUSTRIES DEPARTMENT, YORKSHIRE
COLLEGE.]
Something must be said here about the function of the oils and fats used
in currying, and their general method of application. It is obvious that
the possibility of coating the finest fibrils of leather with a fatty
layer is not restricted to raw hide, but is present, sometimes even in a
higher degree, in tanned or tawed leathers, in which the fibres are
already so far isolated as to make the access of the fat easy. Even the
possibility of an aldehyde-tannage is not excluded, where the fibre is
not already completely saturated with other tanning agents or where
these agents, from their nature, have not so firm a hold on the fibre as
to be incapable of being displaced by the action of aldehydes. It is
therefore obvious that we may apply some of the ideas which we have
formed with regard to oil-tannages to the action of fats upon tanned
leather. In the first place, it must be remembered that gelatinous
matters are as a rule insoluble in fats; and _vice versa_, that fats are
incapable of penetrating dry and solid gelatinous fibres. If the skin
becomes dry in the chamoising process, that part remains raw. It may
therefore be concluded that fats and oils have little power in
themselves of isolating the fibrils, and that this must be accomplished
by other agencies, since if they are still adhering together, the fats
cannot penetrate them. Hence the necessity of moisture, which keeps the
fibres soft and divisible; and with raw hide, the importance of powerful
mechanical treatment, which will work the minute globules of fat between
the fibrils. In the case of tanned leathers, the last condition is less
important, since the fibres are already isolated by the tannage, and
capillarity assists the penetration. Even in this case the distribution
of the fat is much assisted if it is already in a state of fine division
(emulsification), and if the surface-tension (p. 76) between it and
water is low, as is the case with dégras and other partially oxidised
oils. On this rather than on any special chemical affinity probably
depends the importance of the “dégras-former” and other products of
oxidation which are present in dégras; and the difference in penetrating
power of different oils. So long as oil remains in an undivided
condition, so long can it be squeezed out, and the leather will feel
and appear greasy; while, when it is thoroughly emulsified, and adherent
to the fibre, it can no longer be expelled by mechanical means. No doubt
the different power of different tannages to “carry grease” without
appearing greasy, is also related to the degree of isolation of the
fibrils, and their surface tension with regard to fats. We may judge
that the more readily an oil can be emulsified, the more freely and
completely it is likely to fix itself on the leather fibre.
[Illustration: FIG. 87.--Hand Shaving.]
[Illustration: FIG. 88.--Shaving Machine.]
It is a practically invariable rule that the leather-fibre must be wet
when it is stuffed. The surface-tension between the water and the fats
is less than that of either with regard to air; and therefore, as the
water dries out of the small interstices of the leather, the fat follows
it in, and gradually takes its place. Generally speaking, the amount of
water should be such that some exsudes in minute drops when the leather
is pinched, that is, that not only the minutest spaces between the
fibrils are filled, but even the larger ones between the fibre-bundles
to a considerable extent.
[Illustration: FIG. 89.--Band-Knife Splitting Machine.]
In “_hand-stuffing_,” the leather is now coated on the flesh side, or
occasionally on both sides with “dubbing,” which is a pasty mixture of
fats usually mainly composed of cod-oil and tallow, which is applied
rather thickly with a brush and smoothed down with the fleshy part of
the forearm. When such constituents are melted together, the harder fats
dissolve in the oils, and as the mixture cools, much of the hard fats
again crystallise out. To make a good dubbing, the cooling fats must be
stirred continuously till this has taken place, as otherwise the mixture
separates into little globular masses of crystals with liquid oil
between them, instead of forming a uniform body of salve-like
consistency. The proportions of the hard and soft constituents of the
dubbing should be adjusted to the season, and to the temperature at
which the drying of the stuffed leather is to take place, so that on the
one hand, the dubbing will not melt and run off, and on the other, that
it should not solidify more than is necessary, as only the liquid
solution which remains entangled among the crystals can be absorbed by
the leather. The solid crystalline fats remain on the surface, and are
scraped off by the sleeker in finishing, as “table-grease,” which is
generally re-melted and used over again. It does not answer, in
hand-stuffing, to carry this re-use too far, as the table-grease
contains only the harder parts of the fat, with a continually increasing
proportion of stearic acid, so that if a dubbing be made continuously of
table-grease and oil, in the end little but the latter will be absorbed
by the leather; while where fresh tallow is used, a portion of its
softer constituents remains dissolved in the oil. The principal function
of the harder fats is the mechanical one of retaining the oil on the
surface of the leather; and to a certain extent they may be replaced by
other solids, such as steatite (“French chalk”), or perhaps other pulpy
materials. The use of a portion of soft fat, such as bone-fat, or the
better sorts of glue-grease, is quite practicable, especially if mixed
with the harder table-grease.
The drying of hand-stuffed leather should be slow, to allow time for the
absorption of the grease; and the temperature should be so regulated as
to keep the dubbing in a soft but not liquid condition. In winter, if
the temperature of the outer air be raised sufficiently for this, the
drying will be too keen (cp. p. 426) and the water will be dried out
before the grease is properly absorbed. It is therefore best, in cold
weather, to maintain the ventilation mainly by circulating the air in
the room, with little admission from the outside, and in extreme cases
even artificial damping of the air may be advantageous. Sometimes the
tendency to mildew during slow and warm drying is very troublesome. This
may be prevented by the addition of antiseptics to the stuffing grease.
Carbolic acid and creasote are effective, but generally objectionable
from their smell; rosin oil has considerable antiseptic power, and
mineral oils also in a less degree. Probably α-naphthol would prove an
efficient remedy, as it has little odour, and its antiseptic properties
are very strong, but it has not been tried by the writer. (Cp. Chapter
V.)
[Illustration: FIG. 90.--Haley’s Injector Stuffing Drum.]
In _drum-stuffing_ the conditions differ materially from those of
hand-stuffing. The goods, in a damp condition, are placed in a drum
(Fig. 90), which has been heated by steam to as high a temperature as
the leather will safely stand. Cold damp leather may be stuffed in a
drum heated to 60° C. and the grease may be run in at the same
temperature. The grease should generally be melted and mixed at a
somewhat higher temperature. Sometimes steam is merely blown into the
drum before introducing the leather, to heat it to the required
temperature; sometimes a steam-coil is placed in the drum itself. A more
modern method, which is now largely used in the United States, is to
heat by hot air, which is circulated by a fan over an external steam
heater and through the drum. The drum is set in rotation, and the
stuffing grease in a melted condition is run in through the hollow axle,
or if this is not provided, it is introduced through the door, and the
rotation is maintained for twenty to thirty minutes. During the last few
minutes the door is frequently replaced by an open grating or cold air
is drawn through the drum by means of the fan, in order to cool the
goods, which are set out with the sleeker on the table while yet
somewhat warm, and dried under much the same conditions as have been
described with regard to hand-stuffed goods.
In drum-stuffing, the hardness of the grease is limited by its
melting-point, which must not be so high as to damage the leather, but
it may be soft as is desired. As the grease is forced by mechanical
means into the interior of the leather, there is no danger of its
running off, but the drying must take place at such a temperature as to
keep it at least in a partially soft condition, as the drumming only
forces it into the coarser spaces of the leather, and does not complete
its distribution on the fibre. By the use of exceedingly hard greases,
such as “stearin” (p. 359) and oleo-stearin (p. 356), sometimes with
additions of paraffin wax, it is possible to introduce immense
quantities of grease, and yet to obtain a leather which will board up to
a good colour. In America, it is not unusual to reckon 100 or even 115
lb. of greases to 100 lb. of leather weighed dry after scouring, or
estimated from its wet weight; and the whole of this is absorbed,
scarcely anything coming off in “setting.” The leather, as it comes from
the drum, is dark brown, but when bent sharply in “boarding” to form the
grain, after cooling and drying, the very hard and crystalline fats
crumble into white powder, and the leather takes a light and pretty
colour. Such leather would of course darken at once if it were held to
the fire, but would again brighten on cooling and breaking up with the
“board.” Some portion of liquid fats, such as dégras or fish oil, should
be contained in the stuffing grease, as the solid fats alone will not
penetrate to the heart of the fibres, but will leave the leather dry and
harsh.
By drum-stuffing, it is possible to incorporate solid matter with the
leather, and barytes (ground heavy-spar or barium sulphate) was formerly
much used for this purpose, but has now been nearly abandoned. Glucose
is still used as an adulterant of leather, but is not introduced in the
drum, but by painting the goods with syrup before stuffing. It not only
adds weight, and gives the leather a lighter colour than an equivalent
quantity of grease, but at the same time lessens its toughness, and
ought to be prohibited in England, as it already is in Germany. On the
detection of adulteration of leather, see L.I.L.B., p. 212.
Drum-stuffing is in this country mainly applied to shoe-leathers, but in
America, with the hot-air drum, is coming into increasing use for
harness, and even belting.
A method of stuffing is used in Germany for heavy belting and the like,
which appears at first glance to contradict the axiom that leather must
be stuffed wet. It is called _Einbrennen_ (to burn in), and consists in
first drying at a high temperature (50° C.), to ensure the absence of
all moisture, and then either pouring hot melted tallow over the leather
on a table, and holding it over a brazier, to allow the grease to sink
in, or dipping it completely in a bath of melted tallow. The exception
is only apparent, because, though the leather is at this stage
completely saturated with tallow, it is only after wetting and drumming
that it attains the flexibility due to true stuffing. Similar methods
are applicable to alumed leathers, and even to chrome-leather; and
so-called “waterproof” or “anhydrous” leather is made by immersing
thoroughly dried leather in a bath of 2 parts of resin and 1 of
paraffin, or some similar mixture. If the leather is not first
thoroughly dried, it is scalded and destroyed by the hot grease.
The most troublesome defect to which stuffed leathers are liable, is
known as “spueing,” and is of two kinds, of which the first and less
serious (perhaps more properly distinguished as “striking out”) consists
of a white efflorescence rather like incipient mould, which is easily
wiped off, but generally reappears. This is due to the crystallisation
of the harder fats, and especially of the free fatty acids, on the
surface of the leather, and is almost sure to occur in greater or less
degree when the hard fats such as tallow or stearine are combined with a
non-drying oil such as neatsfoot, or when soft fats are present in the
leather. It is sometimes combined with actual mildew, from which it is
rather difficult to distinguish, even under the microscope, and may even
be caused by fungoid plants, which not only mechanically expel the fats
by their growth, but probably promote their rancidity and the separation
of the crystalline fatty acids. It is at most only a defect of
appearance, and does not in any way injure the leather. It is constantly
present in calf-kid, from the neatsfoot oil used in finishing, and is in
this case rather liked by the buyers, who for some reason regard it as a
proof of quality. A very similar appearance may be caused by the use of
solutions of barium chloride, alum or other mineral salts, for weighting
or other purposes; but is persistent when the leather is held to the
fire, while the crystallised fatty acids at once melt and disappear. The
fatty acids are at once removed by a drop of benzene or petroleum
spirit; but unaffected by water, while with water-soluble salts the
reverse occurs.
The second form of spueing is of a much more troublesome character, and
makes its first appearance as minute spots or pimples of resinous
matter, raised above the surface of the leather, which if removed,
generally reappear, and which may become so bad as to form a sticky
resinous coating over the whole surface. The exsuded matter consists of
the oxidised products of oxidisable oils, but the cause of its
appearance is not always easy to explain. The currier generally
attributes it to adulterated oils, and it must be admitted that some
oils almost invariably produce it, but it appears occasionally when only
the purest and absolutely genuine cod-oil has been used. It can only be
produced from drying or semi-drying oils, which include all the ordinary
fish oils and most of the vegetable seed oils, but can never arise from
tallow or stearine, from mineral oils or vaseline, or from genuine
non-drying oils, such as tallow, neatsfoot, sperm, or mineral oils, nor,
probably, from rosin oil. It is favoured by causes which promote the
oxidation of oils, such as moist heat with limited access of air, and by
the presence of oxygen-carriers, such as iron-salts in blacks, and
possibly also by the presence of free acids. A large amount of free
fatty acid in the oils themselves is suspicious, not only because the
free acids oxidise more freely than the neutral fats, but because their
presence is an evidence of the tendency to rancidity and change in the
oil. It is also said to be caused by previous mildewing of the leather,
and certainly often occurs where the grain has been rendered porous by
bacterial action in the soaks, limes, or bates, probably from the
greater quantity of oil absorbed by these parts. While it is easy to say
which oils may possibly spue, there is no known chemical test which will
foretell whether a given sample is likely to do so under ordinary
conditions. Eitner[169] states that seal oil extracted at a low
temperature is very liable to spue, but that when heated for a
considerable time to a temperature of 250°-290° C. it darkens in colour
and loses the tendency. This is probably true of many other marine oils;
and may be one cause of the frequent trouble with modern oils, many of
which, especially the lighter coloured kinds, are extracted by steam at
a temperature below boiling point. It is very probable that one effect
of heating to a considerable temperature is to dehydrate and separate
albuminous or gelatinous matters which are present in the fresh oils,
and which probably increase their tendency to decomposition. Many of
these substances separate as “foots” from oils during long storing, and
such old oils are said to be less liable to spue than those of recent
manufacture.
[169] Gerber, 1880, p. 243.
If oxidisable oils are used upon leather, they “dry” upon the fibre, and
if a sufficiency of non-drying constituents are not present at the same
time, the leather will ultimately become hard, and may even crack from
hardening of the fibre. Mineral oils are not liable in this way to form
a hard coating on the fibre, but as they are slightly volatile, though
of very high boiling point, they may ultimately evaporate, and leave the
leather insufficiently nourished. From their low surface-tension, they
have great powers of capillary penetration, as is witnessed by the way
that lamp oils “creep” over the surface of the lamp, but they have less
affinity for water than the more oxidisable oils, and probably do not
combine so intimately with the leather-fibre. They are probably better
used in combination with other greases than alone. The admixture of
solid paraffin with stuffing greases has the tendency to make the
leather feel less greasy and drier than it otherwise would; and crude
turpentine and rosin are said to have a still greater effect in this
direction.
The water which is required for satisfactory stuffing may in some cases
be introduced into the stuffing grease as well as into the leather. The
effect of dégras is largely due to the water with which it is intimately
mixed, and when dégras or sod-oil is deprived of that which it naturally
contains, by heating it to too high a temperature, either before or
after its mixture in a stuffing grease, its efficacy is greatly
lessened.
_Fat-liquoring_ (pp. 217, 239) may be considered a special case of
stuffing, in which the oil is very perfectly emulsified with a large
quantity of water. In this way, very considerable quantities of oil may
be introduced into leather without giving it the least greasy feel.
Egg-yolk contains about 30 per cent. of an oil chemically very like
olive, but with a larger proportion of palmitin, and may be considered
as a very perfect natural fat-liquor, containing also some albumen which
serves as “nourishment” for the leather. If a means of emulsifying
olive, lard, or tallow oil (with the addition of a little palm oil) with
albuminous matter as perfectly as in the egg could be discovered, the
problem of an egg-yolk substitute would in all probability be solved.
Milk and cream are also natural fat-liquors.
CHAPTER XXV.
_DYES AND DYEING._
Before the discovery of artificial organic dyestuffs, the only colouring
materials known to industry were those of mineral and direct organic
origin; and on this account the dyeing of leather was formerly subject
to great difficulties and limitations.
The discovery of the means of artificially preparing an organic dyestuff
(mauve) by Perkin some forty-five years since, opened up a new field for
research, and since that time, the list of commercial dyes has so
increased that there is now scarcely a tint or shade which cannot be
accurately matched and reproduced by the coal-tar colours. These colours
are often spoken of as “aniline dyes” owing to the fact that many of
them, and especially the earlier ones, have been derived from aniline,
one of the products of coal-tar; but more recently, a considerable
number of important colours have been prepared from other constituents
of the tar, and it is therefore more correct to term the whole of the
dyes obtained, either directly or indirectly, from coal-tar, the
“coal-tar colours.”
The coal-tar colours are generally soluble in water, or mixtures of
water and alcohol, and the majority of them combine with the fibre of
the leather without the use of any mordant, so that in most cases it is
only necessary to apply a solution of the dye direct to the leather,
though their suitability for the purpose varies considerably. A few
which are only soluble in oils or hydrocarbons, are not suitable for
leather-dyeing, though they may sometimes be utilised in conjunction
with fats in currying; and there are also certain colours which are not
applied to the fibre ready formed, but are developed on it by subsequent
chemical treatment, and which have only been applied to a limited extent
to leather.
A number of the coal-tar dyes, which are produced in the crystalline
form, have a totally different colour when solid to that of their
solutions, and to the colour they produce when dyed. A well-known
instance of this is magenta or fuchsine, which forms glistening green
crystals, while in solution it is a brilliant red dye. The colours of
the crystals are usually complementary to those of the solution, thus
several blues have the appearance of metallic copper, and violets, such
as methyl-violet, are greenish-yellow, generally with a pronounced
metallic lustre. This peculiarity is the cause of the defect in dyeing
known as “bronzing,” in which the dye, when applied in too concentrated
a form, takes a surface-shimmer of its complementary colour.
The coal-tar colours are mostly either “acid” or “basic.” The former are
the salts of organic colour-acids with inorganic bases (generally
sodium) and are usually readily soluble in water, but frequently do not
fix themselves on the fibre till the colour-acid is set free by the
addition of some stronger acid to the bath, and in many cases the free
colour-acid is of different colour to its salts. The “basic” colours are
salts of colour-bases (organic bases of the nature of very complicated
ammonia-derivatives) with acids (mostly hydrochloric, sulphuric or
acetic). Most of those in commercial use are soluble in water, though a
few require the addition of alcohol. The colour-bases themselves are
usually insoluble in water, and therefore precipitated by alkalies, and
in some cases they are also colourless. The basic dyes have generally
greater intensity of colour than the acid dyes, but large classes of
them are very fugitive when exposed to light, and in strong solution
many others are very liable to “bronze,” a defect which is generally
less marked with the acid colours.[170]
[170] It has recently been shown by Lamb (see App. D, p. 498) that
many basic colours are much faster to light on leather than on
textiles.
As it is not obvious at first sight whether a given dye is acid or
basic, a reagent to distinguish them is useful. For this purpose a
solution of 1 part of tannic acid and 1 part of sodium acetate in 10
parts (by weight) of water is conveniently employed, which gives
coloured precipitates with basic dyes, but is not affected by acid ones.
The fact that basic dyes are precipitated by tannins influences their
use in leather dyeing, not only as regards their fixation on the
leather-fibre by the tannin which it contains, but as the cause of their
precipitation in the dye-bath if great care is not taken to avoid the
presence of tannins in a soluble form. The use of the sodium acetate is
to combine with the mineral acid of the colour-salt, which if left free
would prevent complete precipitation, substituting for it acetic acid,
which is much weaker, especially in presence of excess of sodium acetate
(cp. p. 81).
In using the terms “acid” and “basic” with regard to dyes, it is not to
be understood that the dyestuffs as employed are acid or alkaline in the
sense that vinegar is acid, and soda basic, but merely that the actual
colour-constituent of the salt is in the one case of an acid nature, and
set free by stronger acids, and in the other case is basic, and
liberated (and often precipitated) by stronger alkalies.
There are several general theories with regard to the fixation of
colours in dyeing organic fibres, and it is probable that no one of them
affords a complete explanation in all cases. One holds that the action
of dyeing is mechanical rather than chemical, the colour adhering to the
fibre by surface-attraction; another, that an actual chemical compound
is formed between the dye and the dyed material or one of its
constituents; and a third, the “solid solution” theory of Witt, is in a
sense intermediate, holding that the colouring matter is actually
dissolved in the dyed fibre. The idea of a solid solution, strange at
first, offers little difficulty on consideration. The colouring metallic
salts in tinted glasses exist obviously in solution in the melted glass,
and can hardly be said to change their condition in this respect when
the glass becomes solid. Gelatine, indiarubber, and perhaps all other
colloid bodies, absorb water or other liquids without losing their solid
form, and these liquids may fairly be said to be dissolved in the solid.
All animal and vegetable fibres are in this respect like gelatine, and
during the process of dyeing are swollen with water. It is quite easy to
dye a mass of gelatine throughout with most water-soluble dyestuffs.
(Compare on these points what is said in Chapter IX. on the physical
chemistry of hide-fibre.) The distinctions between solution and
molecular surface-attraction on the one hand, and certain forms of
chemical combination on the other, are not wide ones, and probably all
three theories are true in different cases, and shade off into each
other by imperceptible gradations. The subject of leather-dyeing is, in
fact, a very complicated one, since we are not dealing with a fibre of
uniform composition, but with one which has had its structure (both
chemical and physical) altered by the processes to which it has been
subjected during its conversion into leather.
Although, strictly speaking, the constitution of the gelatinous fibre of
the skin is unknown, we are quite justified in stating[171] that, like
the amido-acids which are important proximate products of its
decomposition, it contains both acid and basic groups, and is therefore
capable of attracting both bases and acids. It is well known, for
instance, that the neutral fibre is capable of withdrawing sulphuric
acid from a decinormal solution with such vigour that the residual
liquid is neutral to litmus paper; and it will also absorb caustic
alkalies with perhaps equal avidity.[172]
[171] Procter, Jour. Soc. Chem. Industry, 1900, p. 23.
[172] Cp. Chap. IX.
It is thus readily dyed by colouring matter of either basic or acid
character, and in many cases will even dissociate their salts, dyeing
the characteristic colour of the free dyestuff, but possibly at the same
time fixing the liberated base or acid with which the colouring matter
has been combined. Many tanning processes consist in a somewhat
analogous fixation of weak bases and acids, and it is, therefore, to be
anticipated that they will profoundly modify the colour-fixing
properties of the original fibre, as indeed proves to be the case.
Exactly what the result of a particular tanning process in this respect
will be is less easy to foresee.
In the ordinary vegetable tanning process, the tannins, which are of
acid nature, are freely fixed by the fibre. It is, therefore, not
surprising that vegetable-tanned leather most readily fixes the basic
colours, especially as these form insoluble compounds with the tannic
acids, so that it is quite probable that the dyeing is mainly effected
by the formation of tannin-colour-lakes on the fibre, rather than by
actual fixation of the colour-base in combination with the original
matter of the skin. It is noteworthy, however, that even fully tanned
skin has by no means lost its attractions for acid colouring matters,
many of which will dye it even without the presence of free acid, though
it is possible that the tannic acid performs the function of saturating
the alkaline base with which the colour acid has been combined.
It should be pointed out that while the substance of animal skin
consists practically of gelatinous fibres, it is covered on the outer
surface with a thin membrane of extreme tenuity, called the hyaline or
glassy layer (p. 50) which, in the living animal, separates the true
skin from the epidermis. This layer, the chemistry of which is quite
unknown, reacts to colouring matters differently from the gelatinous
fibres, and probably is less absorbent for basic colours, and more so
for the coloured anhydrides of the tannins, and perhaps for acid colours
generally, than is the true skin. As a result, it colours more darkly in
tanning, and less so in dyeing with basic colours, and as it is
extremely liable to damage in the preliminary operations of removing
hair and lime by the tanner, this irregularity of colouring is a serious
disadvantage which is most marked with the basic colours. Small
quantities of lime left in the skin are also probably important causes
of irregular dyeing.
_Mordants_ are chemicals used to enable the fibre to fix dyes for which
it would not otherwise have sufficient attraction, and hence are
generally substances which have affinity both for the fibre and the dye.
Thus cotton, which does not itself attract the basic colours, is
mordanted for them by a solution of tannin, which it attracts, and
which, in its turn, attracts and fixes the colours. In many cases,
however, the function of mordants is more complex, not merely fixing the
dyestuff, but often modifying, or even producing its colour. Thus tannin
dyes black on an iron mordant, though it is itself colourless. Such
mordants may be applied _after_ the colouring matter, where it has
sufficient attraction for the fibre to be taken up alone, but does not
produce the required colour. This process is often called “saddening,”
as the colour is generally darkened. A familiar instance is the use of
iron solutions to darken or blacken tannin or logwood. There is scarcely
any distinction in theory between mordants of this class and the
constituents of dyes which are successively applied to the leather in
order to produce the colouring matter on the fibre. Among these may be
mentioned several mineral salts which were formerly employed in leather
dyeing, though their use is now nearly obsolete. Iron salts are easily
fixed by leather, whether tanned or tawed, and in the former case
produce a dark colour by action of the tannin. On subsequent treatment
with a solution of potassium ferrocyanide, a deep blue is formed
(Prussian blue). If copper acetate or ammoniacal solution of copper
sulphate be substituted for the iron salt, a deep red-brown ferrocyanide
is produced. Yellows are sometimes dyed by first treating tanned
leathers with lead acetate, which is fixed by the tannin, and then with
potassium bichromate, by which yellow lead chromate is produced. A more
important use of lead is in the so-called “lead-bleach,” which is really
a white pigment-dyeing with lead sulphate. The tanned leather, after
washing, is first treated with a solution of lead acetate (usually
“brown sugar of lead” of about 4 grm. per liter), and subsequently with
a dilute sulphuric acid of about 30 grm. of concentrated acid per litre,
and then thoroughly washed to free it from acid. The process is often
used as a preparation for dyeing pale shades, as many of the aniline
dyes are easily fixed on the bleached leather, but is subject to the
disadvantage attendant on all pigments containing lead, of becoming
rapidly darkened by traces of sulphur or sulphuretted hydrogen, such as
are constantly contained in lighting gas, or arise from the putrefaction
of organic matters. The use of acid is also liable to cause early decay
of the leather.
A large proportion of the coal-tar colours contain amido-groups (NH₂
groups) which, when treated on the fibre with nitrous acid (or an
acidified solution of sodium nitrite), become “diazotised” (converted
into --N : N-- groups with elimination of OH₂). On further treating the
diazo-compound with solutions of amines or phenols, combination takes
place, and new azo-colours are formed in or on the fibre, often
remarkably fast to washing or rubbing. Since these qualities are less
important in leather than in textiles, and the process is moreover
somewhat delicate, and the nitrous acid is apt to injuriously affect the
leather, these processes have been little used in leather-dyeing, and
are only mentioned here for the sake of completeness.
The use of the natural polygenetic colours in dyeing leather of
vegetable tannage, which was once universal, is gradually disappearing,
except for the production of blacks. Leather cannot be very
satisfactorily mordanted for these colouring matters; but they have some
natural attraction for the leather itself, and are generally dyed first,
and their colours afterwards developed by metallic mordants such as
iron, chrome, tin salts, and alum, which act not only on the absorbed
dyestuff, but frequently on the tannin and colouring matters derived
from the tanning materials. For black-dyeing, the use of coal-tar
colours, either alone, or to deepen the colours produced by iron, is
gradually extending. Claus and Rée’s “Autho-black,” the “Corvolines” of
the Badische Co., and Casella’s “Naphthylamine Black,” “Aniline Grey,”
and “Naphthol Blue-black” may be mentioned as useful colours. As
coal-tar blacks are mostly dark violets rather than dead blacks, their
colour may be deepened by the admixture of suitable yellows or browns,
and this has already been done in one or two of the colours named. Apart
from the coal-tar colours, black dyeing is generally produced by the
action of iron (and chrome), either on the tannin of the leather itself
or on logwood. As the leather is frequently greasy, and the satisfactory
formation of a tannin- or logwood-lake can only take place in presence
of a base to absorb the liberated acid of the iron salt, the skins are
either brushed with, or plunged in, a logwood infusion, rendered
alkaline with soda or ammonia, or the tanned leather receives a
preliminary treatment with weak soda or ammonia solution. As such
solutions act powerfully on tanned leathers, rendering them harsh and
tender, great care must be taken to avoid excess. The effect of this
alkaline treatment is not only to assist the wetting of the greasy
surface, but to prevent too deep penetration of the dye, by causing
rapid precipitation of the colour-lake. In recent times, however,
leathers are sometimes demanded in which the colour goes right through,
and in this case it might be well to reverse the treatment, beginning
with a weak solution of a ferrous salt, perhaps with addition of sodium
acetate or potassium tartrate, and finishing with alkaline logwood, as
without alkali the full colour is not developed. The use of iron salts
is not very satisfactory in regard to the permanence of the leather; and
in this respect it is of great importance that they should not be used
in excess, and that any strong acids they contain should be saturated
with permanent bases, and if possible washed out. Leather-surfaces
blacked with iron almost invariably ultimately lose their colour,
becoming brown if tannins, and red if logwood has been employed, and at
the same time the leather surface usually becomes brittle or friable.
This is to a large extent due to the effect of iron oxides as
oxygen-carriers. Exposed to light, they become reduced to the ferrous
state, oxidising the organic matters with which they are combined, and
in the dark they re-oxidise, and the process is repeated. It is
therefore of the first importance that excess of the organic colouring
matter should be provided, and that the quantity of the iron should be
as small as possible, and in stable combination. These points are
greatly neglected in practice, especially where blacking is done by the
application of iron salts without logwood, when the evils mentioned are
intensified by the actual removal of part of the tannin of the leather,
and perhaps by the combination of ferric oxide with the skin-fibre
itself, forming a brittle iron-leather. Treatment with alkaline sumach-,
gambier- or logwood-solutions, both before and after the application of
the iron, would lessen the evil. Iron-logwood blacks are much less
permanent, and fade more rapidly under the influence of light and air
than iron-tannin blacks. The use of iron-blacks on curried leathers
seems considerably to increase the tendency to “spueing,” a defect due
to oxidation of the oils (see p. 390). Copper salts mordant logwood a
very dark blue, which is much more stable than the iron compound, and
hence are often used advantageously in mixture with iron salts. In
practice, iron blacks are generally oiled in finishing, and this renders
them more permanent, both by protecting the lake from air and by forming
iron soaps which are stable. The use of actual soaps in blacking and
finishing is not unknown, and probably deserves more attention. Hard
soaps of soda and stearic acid,[173] form an excellent finish where a
moderate glaze is required, the soap jelly being applied with a brush
very thinly, allowed to dry thoroughly, and polished with a flannel or
brush, or glassed. Many acid colours are soluble in such soap jellies,
which may thus be employed for staining. Similar but harder finishes,
and capable of being glazed to a high polish, are made by dissolving
shellac with dilute borax or ammonia solutions.[174] Both of these
finishes are useful in lessening the tendency of iron blacks to smut or
rub off, a failing which is due to the precipitation of loose iron-lakes
on the surface, instead of in combination with the fibre, and is
particularly obvious where “inks” or one-solution blacks are employed,
or where the mordant and the colouring matter solutions are allowed to
mix on the surface of the leather. Such “inks” are generally made with a
ferrous salt and logwood or tannin, together with some aniline black,
and the colour-lake should only be formed on oxidation. Chrome is not
much employed in blacks with vegetable tannages, as it only produces
blacks with logwood, the chrome compounds of tannins having no colouring
value; and bichromates used at all freely being very injurious to the
leather.
[173] 1 of caustic soda in 10-15 of water, boiled with 8 of stearic
acid till clear, cooled to 25° C. and diluted with 400-800 water, with
constant stirring, till the white jelly of suitable consistence is
obtained. Somewhat similar, but harder preparations may be made with
waxes, or fatty acids still higher than stearic.
[174] 5 parts of shellac digested warm with 100 water and 3 of ammonia
fort., or 1 of borax. If the solution is used as a “seasoning” for
glazing, the waxy matter which separates on standing should be mixed
by shaking before use. As a varnish, a stronger solution should be
used and the wax skimmed off.
In dyeing blacks on other than vegetable tannages, however, chrome
becomes of importance, as logwood is principally employed, though
sometimes in conjunction with tannin, and often with addition of
quercitron or fustic, to correct the bluish shade of the logwood-chrome
or logwood-iron lake. It must not be overlooked in practice, that if
ferrous salts are mixed with bichromate solutions, the latter are
reduced, and the iron is oxidised to the ferric state.
In alumed leathers the fixing power of the original hide-fibre is much
less affected than in vegetable tannages. Whatever may be the truth with
regard to the latter, there is little doubt that physical influences are
at least as important as chemical ones in the production of mineral
tannages. The amount of the tanning agent absorbed is greatly influenced
by the concentration of the solutions, and in ordinary alum tawing much
of the alumina may again be removed by free washing. In this case, the
sulphate of potash present takes no part in the operation, but the
alumina salt is absorbed apparently as a normal salt. Alum or alumina
sulphate alone is incapable of producing any satisfactory tannage
without the assistance of common salt, the quantity absorbed being
small, and the fibre becoming swollen by the action of the acid. In
presence of salt the absorption is greater, and the swelling is
prevented. The explanation of this is not to be found in the formation
of aluminium chloride, for though this undoubtedly takes place, it has
been shown that the action of aluminium chloride without salt is not
more satisfactory than that of alum. It has long been known that salt
prevents the swelling action of acids on skin, although it does not
lessen the absorption of acid; and the fact is capable of explanation on
modern osmotic theories (cp. p. 89). The skin so treated is found to be
converted into leather, but if the salt be washed out, the acid is
retained by the skin, which returns to the state of acid-swollen pelt.
It is probable, therefore, that although the acid and alumina are
absorbed in equivalent proportions to each other, they are really
dissociated, and attached to different groups in the gelatine molecule,
and that the effect of the salt is to allow the absorption of the acid
without swelling, and, osmotically, to increase the dissociating power
of the pelt. If, in place of a normal alumina salt, a basic salt is
employed, such as may be obtained by partial neutralisation of the
sulphuric acid with soda, satisfactory tannage may be accomplished
without salt, a basic compound is absorbed, and the leather is much less
affected by washing. In the analogous case of chrome tannage, this basic
compound may be still further deprived of its residual acid, by washing
the tanned skin with alkaline solutions, leaving a leather which is
extremely resistant even to hot water; and a somewhat similar result may
be obtained with alumina, though with more difficulty, as apparently a
very small excess of alkali destroys the qualities of the leather. (Cp.
p. 187.)
The results on dyeing are almost what might have been foreseen. While
ordinary alumed leather absorbs both acid and basic dyes readily, the
basic chrome leather has practically lost its affinity for the latter.
Both chrome and alumina leathers readily absorb vegetable tannins, thus
supporting the view that the acid-fixing groups of the gelatine molecule
are still unsaturated (tannins are capable of tanning pelt swollen with
sulphuric acid and apparently of expelling the acid). In the case of
chrome leather the effect of re-tanning with tannins is greatly to
lessen its stretch, and if carried too far, to destroy its toughness,
but it at once becomes capable of fixing basic dyestuffs. This property
is frequently made use of in dyeing, but the effect on the leather must
not be disregarded where softness and stretch are important, as in the
case of glove-leathers. Polygenetic dyes are, of course, fixed on alum-
or chrome-leathers by the alumina- or chrome-mordant, though apparently
the bases are not present in the most favourable condition for fixing
colours. Thus logwood extracted without alkali dyes tanned leather
yellow, alumed leather violet-blue, and chrome leather blackish-violet,
and some of the alizarine group dye very well on chrome as its
resistance to hot water allows much higher temperatures to be used than
with most other leathers. The tannin contained in dyewoods has the
effect of lessening the stretch of chrome leathers.
Something should perhaps be said on the dyeing of oil and aldehyde
leathers, but the subject has as yet been scarcely treated
scientifically, and our practical knowledge of the subject is
insufficient to justify theorising. (See, however, p. 496.)
Defects in the colour of the finished leather are due to a variety of
causes, but many are produced by want of cleanliness and system during
the dyeing itself. The greatest care is needed in this respect, and in
brush-dyeing a different brush should be used for each different colour,
as it is impossible to thoroughly remove all traces of dye by the
ordinary methods of cleansing.
Irregular and surface dyeing sometimes occurs owing to too rapid
fixation of the colours; while in other cases the affinity of the dye is
too small to allow of reasonable exhaustion of the bath. Addition of
salts of weak acids, such as potassium hydrogen tartrate (tartar), or of
those like sodium sulphate, which form hydric salts, lessen rapidity of
dyeing with acid colours; while acids generally increase it, and it is
also often increased by addition of common salt, which lessens the
solubility of the dye. Weak acids, such as acetic or formic, or acid
salts, such as sodium bisulphate, are generally to be preferred to
sulphuric acid as an addition to the dye-bath; and if the latter is
used, great care is desirable in its complete removal. There is no doubt
that the rapid decay of leather bookbindings and upholstery is largely
due to the careless use of sulphuric acid in “clearing” and dyeing the
leather;[175] and even if it is fully removed, it has saturated all
bases such as lime, which are naturally present in leathers in
combination with weak acids, and which would otherwise act as some
protection from the sulphuric acid evolved in burning coal gas.
[175] See Report of Committee of Society of Arts on Bookbinding
Leathers, 1901.
“Bronzing,” the dichroic effect produced by light reflected from the
surface of many colouring matters, complementary to that transmitted by
them and reflected by the surface of the dyed material, is not peculiar
to basic colours, but is generally more marked in them than in acid
ones. Basic colours, from their great affinity for tannins, and
consequent rapid dyeing, are apt to dye irregularly, and without
sufficiently penetrating the leather, and if the soluble tannin is not
wholly washed out of the skins previously to dyeing, it bleeds in the
dye-bath, and precipitates insoluble tannin-lakes, which waste colour
and adhere to the surface of the leather. The inconvenience of basic
colours due to their too rapid fixation may sometimes be lessened by
slight acidification of the dye-bath with a weak acid, such as acetic or
lactic. The acid may be still further “weakened” if desired, by the
addition of its neutral (sodium) salt. The precipitation of tannin-lakes
in the bath may be prevented by previous fixation of the tannin with
tartar emetic, titanium potassium oxalate or lactate, or some other
suitable metallic salt.
The fading of the colours of dyed goods by exposure to light is a defect
which has been much more investigated in the textile industries than in
leather manufacture, though in the latter case, and especially with
regard to bookbinding and furniture leathers, it is of even greater
importance. It is probable that no colours are actually unaffected by
strong sunlight, but in many cases the action is so slight that it may
practically be disregarded; some of the coal-tar colours, and especially
some of the alizarines, being practically permanent, while others, and
particularly the aniline colours belonging to the triphenylmethane
group, such as magenta, are so fugitive as to be practically bleached by
a week of strong sunlight. Chrysoidine and the eosins are also very bad
in this respect. The fastness of colours to light is a good deal
influenced by the material on which they are dyed, and but little has
yet been published of the results of direct experiments on leathers, but
Mr. M. C. Lamb has been for some time engaged in a research of this
nature,[176] and the subject is now receiving a good deal of attention
in other quarters. Experiments are easily made by exposing samples to
sunlight under glass or in a south window, a part of the leather being
covered with wood or thick brown paper for comparison. The results are
often complicated by the tendency of all leathers tanned with tannins of
the catechol group, and especially with turwar bark (p. 298), mimosa and
quebracho, to darken and redden in sunshine, or even by exposure to
diffused light. Pure sumach tannages are nearly free from this defect,
and are also much less easily destroyed by the action of gas fumes
(sulphuric acid), and the other injurious influences to which books and
furniture are often subjected.[177]
[176] See App. D., p. 488, 498, and Journ. Soc. Chem. Ind., 1902, pp.
156-158.
[177] Cp. Report of Society of Arts Committee on Bookbinding Leathers,
1901.
Want of fastness to friction or rubbing is a defect generally more
important in textiles than in leather, where it is often prevented by
glazings or other finishes applied to the surface; but in some cases,
and, especially in black leather, it is apt to be annoying. If suitable
colours are used, the defect is generally due to the precipitation of
loose colour on the surface, either by the too free use of mordants, or
the dyeing of basic colours on leathers which have not been sufficiently
freed from loose tannin. It is also often caused by “flaming” or the
application of colour mixed with the “seasoning” used in glazing, to
hide imperfections in the dyeing, or vary its colour. Colour applied in
this way is only mechanically fixed on the leather, and is easily
removed by moisture, staining articles with which it comes in contact.
A very similar defect may be caused by incomplete washing of the dyed
leather, which leaves loose dye from the dye-bath in the goods. To avoid
it in glove-leathers, where its occurrence would be particularly
annoying, the natural mordant colours are still largely in use, which
being precipitated on the fibre in an insoluble form by the mordant or
“striker” (generally a metallic salt) are little liable to come off.
Basic colours may be fixed by a subsequent treatment with tannin, or by
topping with certain acid colours such as picric acid. Some few colours,
and especially Martius or “Manchester” yellow (dinitronaphthol) are
volatile at a low temperature, and therefore liable to “mark off” or
stain any materials with which the dyed fabric, even in a dry state, is
placed in contact.
[Illustration: FIG. 91.--Dyeing in the Tray.]
The practical dyeing of leathers varies considerably according to
whether they are tanned with vegetable materials, chrome, alumina salts,
or chamoising. Vegetable-tanned leathers are dyed either by hand in the
“dye-tray,” or in the drum or paddle, the two latter methods being now
largely employed. The dye-tray is a shallow vat, about 10 inches deep,
and large enough for the goods to be laid flat in it. In the English
method, one or two dozen skins, or even more, are dyed at a time, being
turned over in the tray by hand, the undermost pair being drawn out and
placed on the top (Fig. 91). The method is convenient where only a small
number of skins are to be dyed to one particular shade, which is more
easily matched as the goods are always under observation, and it has the
further advantage that, if desired, the grain sides only of the skins
can be coloured, by “pairing” or “pleating” them before dyeing. For this
purpose two skins of equal size are laid together flesh to flesh
(pairing), or each skin is doubled down the back, flesh side in
(pleating), and pressed firmly together with a sleeker on the table,
when the skins adhere so closely that if carefully handled, no colour
penetrates between them during the dyeing, except a little round the
edges. This effects considerable economy of dye-stuff, as the fleshes
would absorb a good deal, and for some purposes, an undyed flesh is
preferred. In dyeing in the paddle or drum, the skins are merely placed
loose in the dye-liquor, so that the fleshes are dyed equally with the
grain sides. Paddle-dyeing has the advantage of effecting a considerable
saving of labour, as compared with the dye-tray, in which constant
handling, which often lasts an hour or more, is required. It also allows
of almost equal facility in examining the colour of the skins, which is
very important when dyeing to shade; but it is less economical in
dye-stuff, as not only the flesh sides are dyed but a much larger
volume of liquor is used, and as the dye-bath can never be entirely
exhausted, more dye is run away in the used liquor. Drum-dyeing is much
less expensive in this respect, as the volume of liquor may be very
small, and from the efficiency of the motion, the dyeing is very
thorough, and penetrates deeply into or through the skin, which in many
cases is advantageous, but it is difficult to dye to exact shade, since
the skins can only be examined by stopping and opening the drum. Most
dyes are more readily fixed at high temperatures, and in this respect
the drum has an advantage over all other methods, as once heated it
retains its heat with very little loss to the end of the operation,
while both in the paddle and the dye-tray the liquor is rapidly cooled,
and special methods of maintaining the temperature complicate the
apparatus, and require great care to avoid overheating. It is usually
best to work at the highest temperature which the goods will safely
bear, and this varies to some extent with the class of goods, chrome
tannages and chamois leather being peculiar in standing almost any
temperature short of boiling. With vegetable tanned leather 50° C. may
be taken as a maximum; but cold wet skins may safely be introduced
rapidly into a liquor heated to 60°, as they will cool it sufficiently.
The Continental method of dyeing in two trays may be mentioned here, as
it produces very rapid and even dyeing, with considerable economy of
dye-stuff, and the principle is capable of application to other methods
where a large number of skins have to be dyed to the same colour. As
generally carried out, two trays are employed, each about 4 feet long,
18 inches wide, and 10 inches or a foot deep, and these are usually made
with a sloping bottom, or propped up in such a way that the dye-liquor
all runs to the further side of the tray. A single pair of skins is
usually dyed at once (in about 6 liters (5 qt.) of liquor for sheep and
goat). To begin with, the first tray is filled with a very weak liquor,
and the second with one of about half strength. The goods are entered in
the first tray, turned a few times, and passed into the second; the
liquor in the first is run away, and it is re-filled with one of the
full strength, to which the goods are then transferred, and dyed to
shade. The second tray is much reduced in strength by the skins, and now
serves as the weak liquor for a fresh pair, which in its turn passes
into that from which the goods have been dyed out, and then into a new
liquor; each pair of goods thus passing through three baths, of which
the last is of full strength, and which quickly brings up a full and
even colour. In the ordinary English method, the goods must, for the
sake of economy of dye-stuff, be dyed out in a nearly exhausted bath,
which is a tedious operation, the last stage of dyeing often taking a
time far longer than that required to bring the goods nearly up to
shade, and even then failing to produce a good and full colour. This
evil may be lessened by adding the dye-stuff in several successive
portions, as the bath becomes exhausted, but cannot be altogether
avoided with a single tray, if any reasonable exhaustion of the bath is
to be attained. At first sight it seems a very slow process to dye the
goods in single pairs, but this is to a great extent compensated by the
rapidity with which they take on colour. In the Continental system, the
dyes, mostly of the coal-tar series, are used as strong solutions, and
each new dye-bath is made up by filling the tray with a definite volume
of hot water and adding a measured quantity of the dye-solution.
The re-use of partially exhausted dye-baths is generally limited to
cases where either single dyes, or mixtures of very equal affinity for
the leather are employed, since where dyes of unequal affinity are
employed, one is more rapidly removed than the other, and the shade of
the dye-bath is altered. Many dyes sold as single colours are really
mixtures,[178] and alter in shade if successive quantities of leather
are dyed in their solutions. Basic dyes are also apt to be precipitated
by traces of tannin washed out of the goods, and thus rendered unfit for
use a second time. This may be avoided by suitable preparation of the
goods (see p. 411).
[178] Such mixtures may often be detected by putting a drop of their
solution on blotting-paper, when the dyes form differently coloured
rings according to their more or less rapid fixation by the paper, or
by dusting the dry dye _very_ thinly on wet blotting-paper, when each
particle produces its separate spot.
Much of the success of practical leather-dyeing depends on proper
selection and preparation of the goods. Sound uninjured grain is a
matter of first importance; no satisfactory dyeing can be expected on
skins which through carelessness in soaks, limes, or bates, are tainted
by what is known as “weak grain,” caused by destruction or injury of the
delicate hyaline layer, which forms the natural glaze and outer surface
of the skin (p. 50). For such goods, “acid” are to be preferred to
“basic” dyes, the latter having an especial tendency to dye darker and
deeper where the grain is imperfect. Goods of different tannages and
colours should never be dyed together, as they are certain to produce
different shades in the same dye-bath. Tanned skins which have been
dried, especially if they have been in stock for some time, should be
thoroughly softened by soaking in tepid water and drumming, a
temperature of between 40° and 45° C. being most advantageous. Skins,
such as calf of mixed or bark tannage, must now be freed from all bloom
by scouring with brush and if necessary with slate or stone, but great
care is requisite to avoid injury to the grain. A little borax or other
weak alkaline solution assists in removing bloom. Fresh sumach-tanned
skins merely require setting out with a brass sleeker, but those which
have been long dried often dye more evenly and readily if they are
re-sumached.
Dark coloured tannages, such as Australian bazils, and East India sheep
and goat tanned with cassia bark, are always improved by sumaching, and
if for light colours, by first stripping a portion of the original tan
by drumming for a quarter of an hour with a weak (¹⁄₄ per cent.)
solution of soap powder or borax at a temperature of 30° to 35° C. and
then passing (after well washing in warm water, but with as little
exposure as possible to the air) through a weak sour of sulphuric acid
of 1-2 per cent. The acid should now be as thoroughly removed as
possible by washing in water, and the goods should be sumached. The
process, and especially the use of sulphuric acid, is always deleterious
to the skins, and is one of the causes of the early decay of coloured
bookbindings and furniture leathers. Lactic, formic, or acetic acid may
be substituted for sulphuric with safety, and the risk of injury from
sulphuric, which generally is only apparent after the lapse of a
considerable time, is a good deal lessened by adding to the sumach
liquor a small quantity of potassium tartrate, sodium acetate or
lactate, or some other salt of a weak organic acid, which is thus
substituted for the much more dangerous sulphuric. Except in cases of
absolute necessity for the production of light shades, the use of
sulphuric acid should not be resorted to, and then only for goods which
are not expected to possess great permanence. For light shades for
bookbinding and upholstery, good sumach-tanned leathers and organic
acids only should be employed. Alkaline treatment also demands great
caution, as excess of strong alkalies is very injurious to the leather.
Another objectionable method for the preparation of leather for very
light shades, is the use of the lead-bleach described on p. 399.
The sumaching is best done in a drum, at a temperature of about 40°.
Lamb advises that 1 to 2 lb. of sumach per dozen is sufficient for calf,
and recommends running in this liquor for two or three hours. The skins
are then rinsed in water to free them from adhering sumach, and set out
on a table with a brass sleeker, and are now ready for dyeing with
“acid” dye-stuffs. If “basic” dyes are used, thorough washing in several
tepid waters is necessary to free them from the loose tannin; and if
deep colours are to be dyed, it is better, instead of too much washing,
to fix the tannin, which then serves as a mordant for the colour. For
blues, blue-greens, or violets, this is done with a solution of “tartar
emetic” (antimony potassium tartrate, of 5 to 20 grm. per liter
according to the amount of tannin to be fixed, often with addition of
some common salt), which produces no alteration in the colour. For
browns, yellows, deep reds, or yellow-greens, it is advantageous to use
titanium-potassium lactate or oxalate (2 grm. per liter), which in
combination with the tannin produces a very permanent yellow coloration
on which the basic colours dye freely. In many cases the titanium salt
is best applied after dying with one of the dyewoods (Dreher).
The basic colours usually require simple solution in hot water before
adding to the dye-bath, and are used in quantities of 0·5 to 2·5 grm.
per liter of dye-bath, according to their colouring power, which varies
a good deal, and to the depth of shade required. The solutions should
not be boiled, and some colours are injured by too high a temperature.
Some colours dissolve incompletely, and require filtration through a
cotton cloth. As basic colours are precipitated by calcium carbonate, it
is important that “temporary” hard waters should be neutralised with
acetic or lactic acid till they faintly redden litmus; and in the case
of colours which, from their attraction for the leather fibre, dye too
rapidly, and consequently unevenly, better dyeing is often obtained by
the use of a small excess of acetic acid, which also increases the
solubility of the colour. Too much acid, however, will prevent the
proper exhaustion of the bath. Some few colours, now little used,
require to be dissolved in the first instance in a little methylated
spirit; and the addition of spirit will often assist dyeing and staining
where the leather is slightly greasy, though considerations of cost
generally prevent its use. Sodium sulphate is not unfrequently added to
dyeing baths to improve equality of dyeing; and with some of the cotton
dyes common salt is used to lessen their solubility and facilitate the
exhaustion of the dye-bath.
“Acid” colours usually dye better if acid is added to the bath, to
liberate their colour-acids, and for this purpose sulphuric acid is
generally used in weight about equal to that of the colour used. Its use
is, however, objectionable, in this case, for the same reasons as in
bleaching, since it is impossible by mere washing to remove it entirely
from the leather, which it ultimately rots when concentrated by exposure
to a dry atmosphere or high temperature; and it is better to use formic
or acetic acid to the extent of two or three times the weight of the
dye-stuff. Sodium acid sulphate may also be used, but is probably more
objectionable than an organic acid. Many acid colours, however, dye
quite satisfactorily from a neutral bath. The acid colours are used in
somewhat similar quantities to the basic, but are generally inferior in
colouring power, though they dye more evenly, especially on defective
grain, and are often more permanent to light.
Mention has already been made of the polygenetic or mordant dye-stuffs,
which are still used to some extent for dyeing glove-leathers, and of
which logwood is important in dyeing blacks. Fustic and Brazil-wood
(peach-wood) are not quite gone out of use among old-fashioned dyers,
even for dyeing moroccos and other coloured leathers of vegetable
tannage. Peach-wood, with a tin mordant (generally a so-called “tin
spirits” made by dissolving tin in mixtures of hydrochloric and nitric
acid) was formerly much used in dyeing cheap crimsons, but is now quite
displaced by the azo-scarlets. The acid tin-solutions were frequently
very injurious to the leather.
The wood-infusion, rendered slightly alkaline with soda, ammonia or,
formerly, with stale urine, is usually dyed first on the leather, and
followed by the mordant “striker”; ferrous or ferric solutions, and
potassium bichromate being used for dark colours, and tin salts, or
sometimes alum, for the brighter ones. The mordant is sometimes added
to the dye-bath towards the end of the operation, but is better used as
a separate bath, as it is apt to produce a precipitate of colour-lake on
the surface of the skin, which rubs off on friction. In some cases, and
especially in black dyeing, the strong infusion of dye-wood, and the
necessary “striker” are successively applied by brushing instead of in
the dye-tray.
Logwood and Brazil wood are both Cæsalpinias closely allied to
divi-divi. Logwood is _Cæsalpinia_ (see p. 287) _Campechianum_. Its
colouring matter is hæmatoxylin, a substance nearly allied to tannins,
and almost colourless; which on oxidation gives hæmatin, which dyes a
yellow-brown, only developing other colours by the aid of mordants.
Logwood chips are extracted by boiling or heating under pressure for
some time with water; and as hæmatin gives dark purplish-red compounds
with alkalies, soda or stale urine is frequently added under the
mistaken belief that it produces a better extraction, but really leads
to waste of colouring matter by oxidation. It is best to extract with
water alone, and add any necessary alkali to the infusion before use.
1-2 lb. of wood per gallon is frequently employed in making the
infusion, and as this proportion of water is quite insufficient to
properly extract the wood, the residue should be boiled with one or more
further quantities, which are employed in turn for extracting fresh
portions of wood. Logwood dyes best at high temperatures, and especially
in the case of chrome leather with which a temperature of 80° C. may be
safely used. The presence of a trace of a salt of lime is advantageous,
and with very soft waters a little lime water or chalk may be added to
the logwood liquor.
In blacking skins, the strong infusion is rendered slightly alkaline
with sodium carbonate or ammonia, and brushed undiluted on the leather.
If employed as a bath, a somewhat weaker infusion is used, and the
leather is frequently treated first in an alkaline bath, to which a
small quantity of potassium bichromate is often added. The object of the
alkali is not only to assist in the formation of the colour-lake, by
saturating the acid set free from the iron-salt used as a striker, and
thus to prevent the colour from penetrating the leather too deeply, but,
at the same time, to overcome the resistance to wetting caused by grease
or oil which the leather may contain. It must thus be used more freely
when stuffed leather is to be blacked, but excess should be carefully
avoided, as it easily renders the leather tender and brittle. The
potassium bichromate oxidises the hæmatoxylin, or the ferrous salt
subsequently applied, and forms a nearly black chrome-logwood lake.
The iron solution is generally either of ferrous sulphate of perhaps 5
per cent. strength, or commercial “iron-liquor,” which is a
“pyrolignite” or crude acetate of iron, containing catechol-derivatives
and other organic products from the distillation of wood, which act
advantageously, both as antiseptics, and in preventing the rapid
oxidation which occurs when pure ferrous acetate is used. Iron-liquor is
generally to be preferred to ferrous sulphate (“green vitriol”), as the
sulphuric acid of the latter, unless completely neutralised by the
alkali employed in preparation, acts in the end disastrously on the
leather. Commercial iron-liquor is often adulterated with ferrous
sulphate, which may be detected by its giving a precipitate with barium
chloride. Great care should be taken not to use iron in excess of the
logwood or tannin present, as it otherwise takes tannin from the leather
itself, making it hard and liable to crack, while any uncombined iron
acts as a carrier of oxygen, giving up its oxygen to the colouring
matter or tannin with which it is in contact, and again oxidising from
the air, and so causing “spueing” or oil-oxidation, and other evils.
Good blacks which are more permanent than those with logwood, may be
obtained by merely treating leather containing an excess of oak-bark
tannin or sumach, first with an alkaline solution (not at the most
stronger than 2¹⁄₂ per cent. of liquid ammonia, or 5 per cent. of soda
crystals), and then with iron-liquor. If it is not certain that the
leather contains excess of a suitable tannin, a tannin-solution must be
employed like the logwood infusion, or the leather must be sumached. The
addition of some sumach to logwood liquor is often advantageous, and a
blacker (i.e. less blue) black, especially on alumed leathers, is
obtained by using a proportion of fustic. Solutions made by boiling 10
per cent. of cutch with 5 per cent. of sodium carbonate give good blacks
with iron-liquor, and do not make the leather tender, and they can be
used in mixture with logwood. Many commercial logwood extracts contain
chestnut-wood extract as an adulterant.
Instead of dyeing in the bath, it is very common, especially for the
cheaper leathers such as linings, and coloured leathers of the commoner
sort, to apply the colour by brushing (commonly called “staining”). Many
colours, however, which dye well with time and warmth, are inapplicable
in this way, and only those should be used which have a strong
attraction for the leather, and hence go on well in the cold. If “acid”
colours are employed, it is essential to select those which can be used
in neutral solution, or at most with addition of some mild organic acid
such as formic or acetic, since, as the leather is not washed after
staining, the sulphuric acid would remain in it, and would ultimately
destroy it. Where leathers have a hard and repellent surface, the
addition of a little methylated spirit to the dye is often very useful.
The colours are used in solutions of from ¹⁄₄ to 1 per cent., which
should be quite clear and free from sediment. Difficultly soluble
colours must be used in weak solution, or the dye kept warm while in
use. Dye-solutions will not generally keep for any great length of time
without change.
Before staining, the leather must be carefully “set out,” or otherwise
made as smooth as possible, and the staining is generally done after
most of the other operations of currying or dressing have been
completed. Staining is best begun with the leather in a slightly damp or
“sammied” condition, and the colour is applied evenly with a softish
brush in two or three coats, the leather being slightly dried after
each. As a rule the more coats are applied, the more even is the work;
but to save cost of labour it is common on cheap goods to be content
with two, of which the first is given, preferably with a weaker
solution, to the dry leather. Where the leather is “weak-grained” it is
sometimes advantageous to size it first with a weak solution of
gelatine, gum tragacanth, or linseed mucilage, and similar solutions are
often used to fix the colour and give a higher gloss. The stearine-glaze
mentioned on p. 401 may also be used for this purpose, and a weak
solution of it is sometimes employed as a vehicle for the acid colours.
Acid yellows and browns may also be dissolved in the undiluted glaze
where only a pale colour is required, or to heighten the colour of
leather already stained. A list of suitable colours for staining is
given in the Appendix, p. 486.
It rarely happens in leather dyeing that the required colour can be
given by the application of a single dye, most of the shades now
required being produced by mixtures. It is, therefore, necessary to say
a few words on the theory of colour combinations.
White light is of course composed of a mixture of all the
spectrum-colours, and can be separated into them by the prism. It is
probable, however, that the eye is only capable of three distinct
colour-sensations, and that all the colours we perceive are represented
by the excitement of these in different proportions, the actual
colour-sensations being red, blue-green, and violet.[179] If we
interpose a piece of yellow glass between the eye and white light, the
violet and blue are absorbed, and the remaining red and green rays
combine to produce the sensation of yellow. If pure blue glass is used,
the red is absorbed, and we have blue as the result of the remaining
mixture of green and violet. Red glass absorbs the whole of the green,
and greenish-blue, allowing red and much of the violet to pass. Thus, if
we combine blue and yellow glass, only the green is allowed to pass, and
similarly with red and blue glass, green and blue is cut out, and only
the violet remains. Thus red, yellow, and blue are frequently called the
primary colours, and by combining all three in equal proportions all
colours are cut out, and black or grey results. The blue and violet
which are stopped by yellow glass are those colours which would produce
the sensation of violet-blue, and hence the latter is called the
“complementary colour” of yellow, and so on with the rest. It will be
noted that all the colours of coloured objects are produced by
absorption of a part of the light, and therefore coloured bodies are
always darker than white ones, and where a colour is mixed with its
complementary in suitable proportion, all colours are absorbed and black
or grey is produced.
[179] The subject of colour is too complicated to be adequately
treated here; and for fuller information, readers are referred to
Abney’s ‘Colour Measurement and Mixture,’ S.P.C.K., London, 1891. It
may, however, be pointed out that, while the true primary
colour-sensations are unquestionably red, blue-green and violet, and
by mixture of _light_ of these colours, all other colours, including
white, can be produced; the primary pigments or dyes are red, yellow,
and blue; the effect being produced in the former case by the addition
of colours, and in the latter by their subtraction.
Colours which are made by mixing two primary colours are generally
called “secondary”; while the duller tints made by the addition to these
of black, or of a complementary colour which produces black, are called
“tertiary.” Any primary colour is complementary to the secondary colour
produced by mixing the other two primaries and _vice versa_. The
following tabular arrangement shows at once the effect of colour mixing.
PRIMARY. SECONDARY. TERTIARY.
Red }
} Orange with Black. Brown.
Yellow}
} Green „ Olive, Sage.
Blue }
} Purple (Violet). „ Puce, Maroon.
Red }
Theoretically, _any_ colour may be obtained by mixture of the primaries,
and that this is possible to a great extent is shown in the success of
modern “three colour” printing, by which pictures are obtained in
natural colours by the use of three primaries only; but in practice few
colours are quite pure, and if two very different colours are mixed, it
is difficult to avoid the production of tertiaries. The most brilliant
colours are generally produced by dyeing with the nearest colour which
can be obtained to that required, and shading with another which is
near, but on the other side of the desired tint.
Thus if we want to produce bright shades in dyeing, we must avoid the
introduction of complementary colours. A bluish red mixed with a reddish
blue will produce a bright shade of violet, but if we mix an orange-red
with a greenish-blue, we introduce yellow into the mixture, and obtain a
dull maroon or puce according to the proportion of the other colours. In
a similar way, the introduction of a blue dye will dull a bright orange
to a brown, and a little of a yellow dye will dull a bright purple to a
maroon. This fact is frequently used in producing the quiet shades of
colour often required from the most brilliant dyes. If to a bright
orange we add black, or a blue dye which as its complementary produces
black, we convert it into a brown. If instead of blue we use green for
dulling, we give the brown a yellower shade, since the green produces
black at the expense of the _red_ of the orange. Violet similarly used
gives a redder brown, since it produces black by combination with the
_yellow_. This shading, if small in amount, is frequently done by direct
mixture of a suitable dye, but if considerable, it is generally better
to top one colour with another. Thus a blue, topped with a powerful
orange, will produce a Havanna brown. For dark colours, it is frequently
convenient to produce a dark ground with some cheap dye, such as logwood
and iron or chrome, and to top it with a bright shade of the colour
required. In this way cheap dark blues and greens can be easily
produced. For reds and browns, mixtures of logwood and Brazil-wood, or
Brazil-wood and fustic may be used, topped with coal-tar colours.
Tanning materials, such as quebracho and mangrove extracts, which give
browns with bichromate, are also employed on cheap goods. It is also
frequently wise to dye with a basic colour and top with an acid one, or
_vice versa_; as in many cases the one fixes and combines with the
other, and an increase of fastness is obtained.
Morocco and many other coloured leathers are finished by damping the
surface of the dried leather with a very dilute “seasoning” of water,
milk, and blood or albumen, allowing the leather to become quite or
nearly dry, and polishing by friction under a cylinder of agate, glass,
or wood in the glazing machine. Many leathers are also grained by
printing from engraved or electrotype rollers, or by “boarding,” or a
combination of the two. “Boarding” consists in pushing forward a fold in
the leather on a table with a flat board roughed underneath, or lined
with cork, in a way which is difficult to describe, but which in skilful
hands wrinkles or “grains” the skin in a regular pattern.
The colour of a dyed skin is much altered by finishing and especially by
glazing, which always darkens and enriches the colour. In dyeing to
pattern, it is useful to glaze a little bit of the rapidly dried skin by
friction with a smooth piece of hard wood for comparison, and a portion
of the pattern may also be wetted for comparison with the wet skin.
Colours which look full and even in the dye-bath, often go down in a
most disappointing manner on drying, though to some extent they regain
intensity on finishing.
In comparing the dyeing value of colours, the most practical way is to
make actual dyeing trials with equal or known quantities of the colours
and of water. Such trials may be made, either by “turning” the samples
in photographic porcelain trays, kept warm in a water-bath (a “dripping
tin” may be used for the purpose, the trays being supported a little
above the bottom on tin supports soldered to the tin), or the leather
may be hung from glass rods, by hooks of copper wire, in glass vessels
(square battery jars), also placed in a water-bath. The leather samples
should be of equal _surface_ in every case; for suspension, pieces of
“skiver” (sheep-grain) of 8 by 4 in. or 20 by 10 cm. are very
convenient. These may either be “pleated” or suspended by the two ends
grain side out, with a short glass rod to weight the fold, and keep them
flat. The weight of colour used for a sample 8 in. by 4 in. multiplied
by 54 times the area of a single skin in feet, will give approximately
the weight of colour needed per dozen; which is, however, a good deal
influenced by the mode of dyeing, and the quantity of water used.
In dyeing on the large scale, iron, zinc and even copper are to be
avoided, the latter acting very injuriously on many colours, and on the
whole wooden vessels are to be preferred. Though these become deeply
dyed, they become very hard, and if well washed with hot water, and
occasionally with dilute acid, they may be cleansed so as to give up no
colour in subsequent dyeing operations, though of course it is not
desirable, if it can be avoided, to use the same vessel for very
different colours. Zinc rapidly bleaches many colours, especially while
wet and slightly acid, and discharge-patterns may often be produced by
pressing the wet leather on perforated zinc plates.
CHAPTER XXVI.
_EVAPORATION, HEATING AND DRYING._
Questions of evaporation, whether for raising steam, or for the
concentration of tanning extracts and other solutions are of
considerable importance in the tanning industry, and as the same natural
laws which apply to these equally govern the drying of leather, it is
convenient to study the theory of the whole subject in one chapter,
rather than to divide it, and place each part in a different portion of
the book.
The modern conception of evaporation and vapour pressures has been
described on page 75, but it will be necessary to recapitulate a little.
It is a well-known fact that most liquids, if left exposed in an open
vessel, gradually disappear by evaporation into the air, even at
ordinary temperatures. If the vessel is heated sufficiently, the liquid
“boils”; that is, bubbles of vapour are formed in it, and escape, and
the evaporation is therefore much more rapid. To avoid complication, let
us first imagine a liquid sealed in a glass flask, which contains no
air, but which is only partially filled by the liquid. It has been
pointed out that the motion of heat by which the molecules of the liquid
are agitated, enables some of them to break away from the attraction by
which liquid particles are held together, and pass into the form of gas
or vapour, which will fill the empty part of the flask. This evaporation
will, however, soon reach a limit, since the vapour cannot escape from
the flask. The flying molecules of vapour produce pressure by striking
the walls of the flask, while a proportion of them will strike the
surface of the liquid, and again be caught and retained by its
attraction; and as the pressure rises, the number of these necessarily
increases till a point is reached when as many fall back and are
retained (or “condensed”), as those which evaporate, and the pressure
will then remain constant. The amount of the pressure will vary with
the nature of the liquid, and will be the greater the more volatile it
is, or, in other words, the less the power of its internal attraction.
It will also increase with rising temperature, which, by increasing the
velocity of motion of the molecules, renders their escape from the
liquid easier, and their recapture more difficult. It will not be at all
affected by the volume of vapour or the size of the flask, but so long
as any liquid is present, it will depend merely upon the nature of the
liquid, and the temperature. If the flask is large, more of the liquid
will evaporate till the same pressure is reached. If at the outset the
flask is not empty, but filled with air, it will make no difference to
the pressure or quantity of the _vapour_ in it, which will be added to
that of the air, whatever that may be. If the sealing of the flask is
broken so that it is open to the atmosphere, air and vapour will escape,
or air will pass in, till the total pressure is equal to the atmospheric
pressure outside, (about 15 lb. per square inch). As, however, the
vapour in the flask is always renewed by evaporation, so that the full
vapour-pressure of the liquid is maintained, the “partial” pressure (as
it is called) of the _air_ in the flask will be less than that of the
outer atmosphere by the amount of the vapour-pressure, which makes up
the difference. Once this balance is attained, evaporation will go on
very slowly in the flask, as it can only replace the small quantity of
vapour which escapes. If, however, the vapour is removed by blowing
fresh air into the flask, it will rapidly be replaced in the old
proportion by fresh evaporation. Thus goods in a close room will dry
only very slowly, even if the temperature is high, unless the moistened
air is replaced by dryer air from the outside by some effective system
of ventilation. In absence of this, evaporation only becomes rapid when
the temperature of the liquid is raised to its “boiling point,” that is,
when the vapour-pressure becomes slightly in excess of that of the
atmosphere, so that the freshly formed vapour can push out that already
in the flask or chamber into the outer air, and at the same time,
bubbles can be formed in the interior of the liquid by the escaping
vapour. As the vapour-pressure of a liquid rises continuously with
increasing temperature, and its boiling point is defined as that
temperature at which it is equal in pressure to the air (or vapour) in
contact with it, it is evident that the boiling point must entirely
depend on the pressure. Thus the boiling point of water in a boiler at
a pressure of 55 lb. per square inch above the atmosphere is 150°C., and
in a partial vacuum equal to 5·8 inches of barometric pressure, is only
60° C., a fact which is made use of in the concentration of extracts and
other liquids at a low temperature in the vacuum-pan. (Atmospheric
pressure is taken at 30 inches or 760 millimeters of the barometer or
14·7 lb. per inch, or 1·033 kilos per square centimeter.)
If a piece of iron is placed over a powerful gas-burner, it will go on
getting hotter till its temperature is nearly or quite equal to that of
the gas-flame. On the other hand, a pan of water, in the same condition,
once it has reached its boiling point, becomes no hotter till all the
water is evaporated. It is evident that the whole available heat or
energy of the gas-flame is consumed in converting the water into steam.
We might convert a proportion of this energy into mechanical work, by
using the steam in a steam engine; but even without this, work is
actually being done by the escaping steam in raising the weight of the
atmosphere, and in overcoming the attractive force which holds the
particles of water together in the liquid form. It is of course known to
everyone, that energy may change its form, as from heat to work, but
that it cannot be destroyed, diminished or increased; and therefore the
whole of the work performed in converting the water into steam is again
recovered as heat when the steam is condensed. In this connection a
clear distinction must be made between _quantity_ of heat, and
_temperature_, which in popular language are often confused. It is for
instance obvious that if we mix a pound of water at boiling temperature
with another pound at freezing point, the temperature is altered to 50°
C., but the total _quantity_ of heat is unchanged. It is equally clear
that no change in quantity of heat takes place when 1 lb. of mercury at
100° is mixed with 1 lb. of water at 0°, though in this case, owing to
the small capacity of mercury for heat, the common temperature would
only be raised to about 3°. We must therefore have some measure of heat
apart from the mere direct indications of the thermometer, and that most
generally used is the quantity of heat required to raise 1 kilo of water
1° C. (kilogram-calorie).[180] In England the heat required to raise 1
lb. of water 1° F. is also in use as a unit. The k.-calorie is equal to
3·97 (very approximately 4) lb. × F. units. For our purpose it may be
taken that 100 k.-calories of heat are required to raise 1 kilo or liter
of water from freezing to boiling temperature. If, however, the water is
actually frozen, we require 80 k-calories merely to melt the kilogram of
ice without perceptibly raising its temperature, and when the water is
raised to 100°, 536 calories of heat are still necessary merely to
convert it into steam at the same temperature. To melt 1 lb. of ice
requires 144 lb. × F. units, to raise it to boiling point 180 more, and
to evaporate it 965 additional. The quantity of heat required for actual
evaporation varies a little at different temperatures, being somewhat
larger at lower temperatures, but the total heat required to raise water
from the freezing point, and convert it into steam at _any_ pressure is
nearly constant, being 635 calories at atmospheric pressure, and only
about 650 calories, or 1180 lb. × F. units at 50 lb. per sq. inch. The
quantity of heat evolved by the combustion of 1 lb. of good coal is
13,000 to 15,000 lb. × F. units; or of 1 kilo, 7200 to 8300 k-calories,
but in raising steam in a good boiler coal will only evaporate 10 times
its weight of water at 100° (5360 calories or 9650 lb. × F. units), the
remaining heat being lost. 1 horse-power (33,000 foot-pounds per
minute)[181] in the best engines requires about 1¹⁄₂ lb. of coal or 15
lb. of steam per hour, but in those of worse construction may run up to
many times that amount. As, even theoretically, not 20 per cent. of the
total heat can be converted into mechanical work in a “perfect” engine
working at 75 lb. pressure, it is often economical to use waste steam
for heating or evaporation, and where this can be done profitably, the
additional cost of the mechanical power is very small.
[180] A gram-calorie of one-thousandth part of the above is also in
use for some scientific purposes, but the kilogram-calorie only is
used in the following pages.
[181] This is equal to 76·04 kilogrammeters per sec., but the metrical
horse-power is only taken at 75 kilogrammeters in France and Germany.
In evaporating liquids in the open pan 536 calories is required to
evaporate 1 kilo of water already raised to boiling temperature, and a
larger amount for salt-solutions, and it makes comparatively little
difference whether this is done at 100° or at a lower temperature.
Where, however, evaporation is done in _vacuo_, considerable economy can
be effected by what are known as multiple “effects,” in which the steam
from one vacuum-pan is employed to boil a second under a reduced
pressure, and consequently boiling at a lower temperature. This
principle can be practically applied to as many as five or six
successive “effects,” the weaker liquor being usually evaporated at the
highest temperature and lowest vacuum in the first “effect,” by the
exhaust steam of the engine used for the vacuum pumps, while the steam
from the first effect heats that of the next higher concentration, and
so on. In the Yaryan evaporator (p. 339), the boiling liquid is sprayed
through coil-tubes, thus exposing an enormous surface to evaporation,
and the whole concentration of any given portion of liquid takes place
as it passes through the apparatus, which does not, even in multiple
effects, occupy more than 4 or 5 minutes; and without the temperature of
the liquid ever rising above 60° or 70° C. In the case of liquids, like
sugar- and tannin-solutions which are liable to chemical change from
continued heating, the shortness of the time is a very great advantage.
The number of effects which it is desirable to use depends greatly on
the cost of fuel as compared to the largely increased cost of the
apparatus. 1 lb. of coal employed in raising steam will evaporate 8¹⁄₂
lb. in a single-effect Yaryan, 16 lb. in a double-effect, 23¹⁄₂ lb. in a
triple, 30¹⁄₂ lb. in a quadruple, and 37 lb. in a quintuple-effect
apparatus.
Where liquids are evaporated in the open air at temperatures below
boiling, it is advisable by some means to spread the liquid in a thin
film, so as to expose a large surface, which must be continuously
removed by agitation, so as to prevent the formation of a skin. A good
apparatus for this purpose is the Chenalier evaporator (Fig. 92), which
consists of steam-heated copper discs rotating in a trough containing
the liquid, which is taken up by buckets attached to the rims of the
discs, and poured over their heated surfaces. In other forms, the liquid
is allowed to trickle over steam-heated pipes or corrugated plates. Such
evaporators should be placed in a current of air so as to rapidly carry
off the vapour formed. Their use is very objectionable for liquids, like
tannin-liquors, which are injured by oxidation, and they are not nearly
so economical as vacuum-pans.
_The drying of leather_ depends on the same laws as the evaporation of
liquids, but demands special consideration from its very different
conditions of temperature and supply of heat. It is important to
remember that evaporation cannot go on unless the vapour-pressure of the
liquid to be evaporated is higher than that of the vapour in contact
with it, and that air-pressure does not prevent evaporation, so that if
we sweep away the stagnant vapour with dry air, evaporation will go on
as quickly as in vacuo, except that the liquid cannot boil. We must also
bear in mind that evaporation consumes quite as much heat at low
temperatures as in a steam boiler, and that this heat must generally
come from the surrounding air, the temperature of which it reduces.
[Illustration: FIG. 92.--Chenalier Evaporator and Glue Coolers.]
The rapidity of evaporation, and the quantity of moisture which can be
taken up by a given volume of air depends on the vapour-pressure, which
increases with temperature. The relation between the two, and the weight
of water in grams per cubic meter which can be dissolved in dry air is
given in the following table. (Grams per cubic meter is practically
equivalent to ounces per 1000 cubic feet. Vapour-pressure is given in
millimeters of mercury of the barometer, p. 422.)
VAPOUR PRESSURE OF WATER.
----------------+---+---+---+---+---+----+----+----+----+----+----
Temperature, °C |-10|- 5| 0 | 5 | 10| 15 | 20 | 25 | 30 | 35 | 40
„ °F | 14| 23| 32| 41| 50| 59 | 68 | 77 | 86 | 95 |104
Pressure, mm. |2·2|3·2|4·6|6·5|9·1|12·7|17·4|23·5|31·5|41·9|54·9
Grams per cb. m.|2·4|3·4|4·9|6·8|9·3|12·8|17·2|22·8|30·1|39·2| ..
----------------+---+---+---+---+---+----+----+----+----+----+----
Air is practically never dry, and in damp weather is frequently
saturated with moisture to the full extent corresponding to its
temperature. In England the average quantity of moisture contained in
the air throughout the year is 82 per cent. of the total possible, and
even in the driest summer weather it is never less than 58 per cent. So
long as the water is in the form of vapour, the air remains quite clear
and does not feel damp; in fogs, the air is not only saturated with
moisture, but contains small liquid particles floating in it. Of course
when the air is really saturated with moisture, it has no drying power
whatever.
As is evident from the table, the amount of water which can be dissolved
in a given volume of air rapidly increases with temperature. Air at 0°
C. is only capable of containing 4·9 grams per cubic meter, or not much
more than 20 per cent. of what it can contain at 25° C. It hence rapidly
increases in drying power as it is warmed, and consequently the air in a
warm well-ventilated drying room in winter is generally much drier, and
has greater capacity for absorbing moisture than the open air in the
driest summer weather. This is the principal cause of the tendency to
harsh and irregular drying by the use of artificial heat; and may be
remedied by a proper circulation of the air by a fan without too
frequent change with the colder air outside. On the other hand the use
of a little artificial heat in damp summer weather, when the air is
saturated with moisture, may be quite as necessary as in winter. The
amount of moisture in the air is most easily ascertained by a device
known as the “wet and dry bulb thermometers.” This consists of two
thermometers mounted on a board; one of which has the bulb covered with
muslin, and kept moist by a lamp-wick attached to it, and dipping in a
vessel of water. The temperature of the wet bulb is lowered by the heat
consumed in evaporation, and the difference of its temperature from that
of the dry bulb is proportionate to the drying power of the air. This
may be approximately calculated in grams per cubic meter by multiplying
the difference by 0·64 for Centigrade or 0·35 for Fahrenheit degrees;
and if deducted from the total capacity for moisture corresponding to
the temperature of the _wet_ bulb as given in table, p. 426, will give
the actual moisture in grams contained in a cubic meter of air; but for
practical purposes, all that is necessary is to find by experience the
temperature and difference between the wet and dry bulbs, which gives
the best result for the drying required, and to maintain it as nearly as
possible by regulation of the heating and ventilation. Cheap forms of
the instrument are made for use in cotton-mills, where it is necessary
to maintain a certain degree of moisture; or it may be improvised from
two chemical thermometers which agree well together. Distilled (rain or
steam) water should be used to moisten the bulb, or it will quickly
become coated with lime salts, and it should be placed in a draught, or
its indications will not be accurate.
It is of course obvious that not only the wet thermometer, but the wet
hides or skins are cooled by evaporation, and they, in their turn, cool
the air with which they are in contact, which not only becomes
moistened, but is lessened in its capacity for moisture by cooling, and
thus rapidly reaches a condition when it can absorb no more moisture. It
is thus necessary to maintain its temperature by artificial heat, or to
replace it constantly by fresh air from the outside, and which of these
expedients is most economical will depend on the temperature of the air
outside as compared with that which it is required to maintain. If the
outside air is sufficiently warm, and not saturated with moisture, it is
generally best to use it in large quantities without artificial heat,
wind usually supplying the necessary motive power for its circulation.
Wet goods from the pits may thus be dried to a “sammed” condition by any
air which is not saturated, and above freezing point; though the drying
will often be slow. For drying “off,” artificial heat is generally
necessary, since the attraction of the fibre for the last traces of
moisture is very considerable, and to remove it the drying power of the
air must be considerably higher than that required for the evaporation
of free water.[182] In drying stuffed leather a temperature must
generally be maintained sufficient to keep the fats employed in partial
fusion, and so permit their absorption by the leather, while at the same
time the drying must be gradual, or the water may be dried out before
the fats have time to take its place. This is generally best attained by
the use of artificial heat, and ventilation by circulating the air by a
fan without its too frequent renewal, especially in cold weather.
Frequently air which has been heated and used for drying off finished
goods, and so partially saturated with moisture, may be used with
advantage for wet goods, or for other purposes where a more gentle
drying is required. If the temperature is low outside, the amount of
heat consumed in heating cold air to the temperature required may be
very considerable. The weight of a cubic meter of air at 0° C. and
atmospheric pressure is 1·293 kilos, and its specific heat at constant
pressure is 0·2375 of that of water. Therefore to heat a cubic meter of
air at ordinary pressure and temperature 1° C. will require the same
amount of heat as that used to heat 0·3 kilo of water to the same
extent, or in other words 0·3 of a k.-calorie. If steam-heating is used,
1 kilo of good coal burnt under the boiler should heat about 1800 cubic
meters 10° C., or 1 lb. should heat 52,000 cubic feet 10° F., assuming
that the condensed water is not cooled below 100° C. These seem large
volumes, but if we reflect that a 48-inch Blackman fan may move 30,000
cubic feet per minute, we shall realise that the cost of coal in heating
air is not inconsiderable.
[182] Commercially-dry leather generally, if unstuffed, contains about
15 per cent. of residual moisture, which varies in amount with the
weather, and can be more or less completely removed by drying at high
temperatures. If leather has been over-dried, it only slowly regains
its weight on exposure to cold air. Commercial disputes not
unfrequently arise on the dryness of leather. In the opinion of the
writer, a customer can only claim that the leather should be
sufficiently dry not to lose weight when exposed to dry air at the
ordinary temperature and degree of dryness of a warehouse or factory,
and claims based on re-drying in hot drying rooms are distinctly
fraudulent.
We must now consider the heat consumed by the actual evaporation of the
water in the leather. The actual evaporation of water already raised to
100° C. consumes 536 k.-calories, but the evaporation of water which has
not previously been heated so far consumes more heat, and we may take
that required at ordinary temperatures as in round numbers 600
k.-calories per kilo, or 1080 lb. × F. units per lb. Disregarding small
fractions, this is equivalent to the cooling to the same temperature of
an equal weight of steam in the heating pipes, and this, as we have
seen, demands about ¹⁄₁₀ of its weight of coal for its production from
water already heated to 100° C.
The cooling takes place, in the first instance, in the leather, the
temperature of which is reduced like that of the wet-bulb thermometer;
and this in its turn cools the air in contact with it. Thus in
air-drying without artificial heat, the whole heat must be supplied by
the air and the loss reduces its capacity for moisture, greatly
increasing the volume required. This is not of much consequence in
open-air drying, since even a light wind will supply air in enormous
volume. A moderate breeze of ten miles an hour moves about 15 feet or
4¹⁄₂ meters per second. When, however, the air must be moved by fans,
the power required becomes important. The evaporation of 1 kilo of water
at summer temperature will cool about 2000 cubic meters, and that of 1
lb. 32,000 cubic feet of air 1° C.
In calculating the ventilating and heating power required in fitting up
drying rooms, it is usually necessary to ascertain that required under
the most unfavourable circumstances, and then add a liberal margin to
cover errors and accidents. As the calculations are, in consequence of
the many varying conditions, somewhat complex, it may be convenient to
give as examples the quantities of air and heat required to evaporate 1
kilo (2·205 lb.) of water under different ordinary conditions, and these
may serve as a basis of calculation of the drying power which must be
provided for different tanneries.
1. _Indifferent Open-Air Drying._--Air at 10° C. (50° F.), wet-bulb
thermometer 7° C. (44·3° F.), indicating a total capacity for moisture
of about 2 grm. per cubic meter; air not to be cooled beyond 7·75° C.
(46° F.), leaving a residual capacity for moisture of 0·5 grm. per cubic
meter. Each cubic meter will therefore take up 1·5 grm. of moisture, and
as 1 kilo contains 1000 grm. we have
1000
----
1·5
= 666 cubic meters per kilo required to absorb moisture; and
600
-----------
2·25° × 0·3
= 888 cubic meters reduced 2·25° to furnish the 600 cal. required for
evaporation. Total air used 1554 cubic meters or 54,900 cubic feet.
2. _Drying with Heat._--Outside-air at 10° saturated with moisture,
heated to 20° C. (68° F.) acquires a capacity for 7·9 grm. per cubic
meter. If we assume that a drying capacity of 2 grm. per meter is
required to complete the drying, we have an effective capacity of 5·9
grm.
1000
----
5·9
= 170 cubic meters or 6000 cubic feet, and to heat this 10° C. will
require 510 cal. Evaporation of 1 kilo will consume 600 cal. Total heat
1110 cal.
3. _Drying with Heat._--Outside-air at 10° as above, heated to 25° C.,
giving an effective capacity for moisture of 13·5 - 2·0 = 11·5 grm. per
cubic meter.
1000
----
11·5
= 87 cubic meters or 3070 cubic feet. To warm this 15° requires 391
cal.; and 600 cal. added for evaporation gives a total of 991 cal.
Comparing 2 and 3 we see that the higher temperature is more economical,
where it can be allowed, than the lower, both in air and heat, though
this is partly compensated by the greater loss of heat by cooling of the
building, etc., which it entails.
4. Air at 0° C. heated to 20° requires about 97 cubic meters, or 3430
cubic feet of air, and a total of 1180 cal.
5. Air at 0° C. and heated to 25° C. requires 63 cubic meters or 2230
cubic feet, and a total of 1075 cal.
6. Air at -15° C. (5° F.) requires 4·5 cal. per cubic meter to raise it
to 0° C., and acquires a capacity for drying of about 2 grm. per meter.
We will apply these figures to a drying room arranged with a screw-fan
with a central division, or two floors, so that the air can be either
circulated or replaced with fresh air from the outside at will (see Fig.
94, p. 435). Such a room with 100 feet of length clear of space required
for fans, air passages, and heating pipes, and 20 feet × 8 feet in
section, should hang about 800 medium butts, weighing say 12¹⁄₂ kilo (27
lb.) each, and when wet from the yard, containing the same weight of
water. A 48-inch Blackman fan, under these conditions would probably
move say 20,000 cubic feet (565 cubic meters) of air per minute, at the
cost of 2 or 2¹⁄₂ horse-power. This, in a room of the section named,
would give an average velocity of 125 feet per minute or rather under
1¹⁄₂ miles an hour; not at all too much to keep the air freely
circulating among closely hung leather. If we assume that these butts
are to be dried in a week (practically 10,000 minutes) under the
conditions of No. 2, the 10,000 kilos of water they contain will require
1,700,000 cubic meters of air, or about 170 cubic meters per minute, or
about ³⁄₁₀ of the air must be fresh every time it passes through the
fan. 1 kilo of water requiring 1110 cal. must be evaporated per minute.
Under the conditions of No. 4, only 97 cubic meters of air per minute
would be required, or about ⁵⁄₆ might be circulated without change, but
the total heat required would be about the same, 1180 cal. Under the
conditions of Nos. 4 and 6 some 1620 cal. per minute would be employed.
It is hardly necessary to provide for the full amount of heat required
by No. 6, since in this country such conditions occur but seldom, and
never for more than a few days at a time, and during such a period, much
less heat would suffice to carry on the drying at a slower rate, and
keep out the frost.
Beside the heat required for actual drying, it is necessary to provide
for that lost by the building during cold weather, and this is much more
difficult to calculate. If, by arranging the outlet for moist air on the
pressure side of the fan, the internal pressure of the building be kept
a little lower than the outside, there can be no loss by escape of hot
air, any leakage being inwards, and supplying a part of the change of
air which, we have seen, is necessary. In a brick building with glass
windows, the loss of heat is far less than in the old-fashioned wooden
louvre-boarded structure, and where fan-drying is in constant use, the
brick structure is much to be preferred. Frequent windows, with
casements horizontally pivoted at the centre, will supply enough air for
favourable conditions of air-drying, and when the weather is bad, resort
is had to the fan. Most modern drying rooms in the Leeds district are
built upon this plan. Where louvre-boarded structures must be used for
fan-drying, the sides should be made as tight as possible in winter by
sheets of canvas or sail-cloth nailed on, for which purpose old sails
can be bought in seaport towns at reasonable rates, a few louvre-boards
only being kept open for the admission of air in suitable positions.
Box, in his ‘Practical Treatise on Heat’[183] puts the loss through
walls in brick buildings for a difference of 30° F. (16·6° C.) between
inside and outside temperatures, at the approximate amounts shown in the
following table.
[183] E. & F. N. Spon, Ltd., London.
LOSS OF HEAT THROUGH WALLS.
----------+------------+----------------------------------------------
Thickness| K.-calories| --
of Wall |per Sq. Foot|
in Inches.| per Hour. |
----------+------------+----------------------------------------------
4·5 | 1·76 |Stone walls must be about one-half thicker, to
9 | 1·44 |afford equal warmth with brick ones.
14 | 1·20 |The loss from glass windows amounts to 3 or 4
18 | 1·06 |k.-calories per square foot per hour.
----------+------------+----------------------------------------------
If the building is of several stories, the loss to the roof in the
intermediate ones need hardly be taken into account, but if the ceiling
is not tight, and open to the roof, the loss may be great, but difficult
to estimate. If we consider the drying room already described, the total
area of the walls and ceiling is about 4000 feet, and to maintain its
temperature 30° F. above the atmosphere at 1·2 cal. per sq. foot would
require 4800 cal. per hour or 80 cal. per minute, a very small amount
compared to that consumed in drying.
The following table calculated from data given by Box will give some
idea of the amount of steam or hot-water piping required for heating.
The sizes given are for the internal diameter of the pipe, allowance
being made for the increased heating surface of pipes of ordinary
thickness. Small pipes are considerably more effective in proportion to
their surface than large ones, and for high-pressure heating 1¹⁄₂ or
2-inch wrought-iron pipes are to be recommended as in many ways
preferable to cast iron. The gilled or ribbed pipes now often used are
also advantageous as giving a greatly increased heating surface.
HEAT GIVEN BY STEAM-PIPES.
------+---------+-----------+-----------------+------
| Steam |Temperature| K.-calories per |
|Pressure,| of |hour per foot run|
| lb. per | Pipe. | of Pipe. |
| sq. in. | | |
------+---------+-----------+-----+-----+-----+------
| | °F. |2 in.|3 in.|4 in.|
| 52 | 300 | 102 | 137 | 169 |
| 35 | 280 | 92 | 121 | 148 |
| 21 | 260 | 81 | 106 | 130 |
| 10 | 240 | 68 | 92 | 113 |
| 2·5 | 220 | 59 | 81 | 97 |
| | 210 | 54 | 72 | 89 |
| | 200 | 49 | 66 | 81 |
| | 190 | 45 | 60 | 74 |
| | 180 | 40 | 54 | 67 |
| | 170 | 36 | 49 | 60 |
------+---------+-----------+-----+-----+-----+------
The temperature of the air to be heated is understood to be 60° F.; at
lower temperatures the quantity of heat given off by the pipes would be
greater, and at higher temperatures less; the amount being approximately
proportional to the difference of temperature between the air and the
hot pipes. It is also important to note that the table refers to
steam-pipes in still air, and that if placed in a powerful draught, (as
immediately before or behind the fan), their heating effect may be at
least doubled. This has not been considered in the following
calculations.
Applying these figures to the estimate of 1110 calories per minute
required for drying in our building, and assuming 80 calories per minute
for the loss of heat through the walls, we have a total of about 71,400
calories per hour, and to obtain this would require 736 feet of 4-inch
pipe at 220° F. (heated by exhaust steam) or 700 feet of 2-inch pipe
heated to 300° F. by steam at 52 lb. pressure.
If we adopt the estimate of 1620 calories of No. 5 and 6, we shall
require 1050 and 1000 feet of the two pipes respectively, and this
covers approximately the worst conditions. We must, however, remember
that these estimates are made for continuous drying during the
twenty-four hours, and that if the fan and steam are only applied during
a portion of this time, the supply both of air and steam must be
proportionately increased, or the time of drying correspondingly
lengthened.
It is very desirable, however, that the fan should be driven by a small
separate engine, the steam for which will only form a small proportion
of that required for heating, and of which the whole of the heat will be
recovered, since even that utilised in driving the fan will again be
converted into heat by the friction of the air, and will therefore cost
nothing. This arrangement will enable the drying to proceed so long as
the necessary steam is maintained, which in bad weather can easily be
done by the night watchman. It may also be pointed out that, during a
great part of the year, the goods can be dried to a “sammied” condition
without heat, or in the open air, or in the case of dressing leather, a
considerable part of the water can be removed by pressing or squeezing,
effecting a further economy.
[Illustration: FIG. 93.--Blackman Fan.]
It must be left to the reader to apply the same calculation to other
sorts of leather than sole, but it may be pointed out that the essential
point, as regards heating and ventilation, is the weight of water to be
evaporated in a given time, and that the actual size and shape of the
drying room is unimportant, so long as adequate heating and circulation
of the air between the leather is secured; and these remarks also apply
to the particular form of fan or other ventilation employed, and to the
means of heating. As the quantity of heat consumed is very considerable,
it is well to look out for sources of waste heat which can be employed,
or for means by which the heat of the fuel can be more directly and
completely utilised than it is in raising steam. Thus a large amount of
heat can sometimes be obtained by passing air through pipes or
“economisers” fitted in a chimney-flue;[184] or gilled stoves or
“calorifers” may be used in a separate chamber to directly heat the air
which is drawn in by the fan.
[184] These pipes should be provided with scrapers to remove soot as
in Green’s economiser, or their efficiency will be much diminished.
[Illustration: FIG. 94.--Section of Drying Rooms with Fan.]
Figs. 93 and 94, furnished by the James Keith and Blackman Co., Ltd.,
give a good idea of the construction of screw fans, and the general
principle of arrangement of fan drying rooms, the air in this case being
circulated in opposite directions on two floors, and the amount of
change being regulated by the shutters at A, etc. The grouping of pipes
at the ends of the two floors which it shows is in general a good
arrangement, but the length between them should not be too great, or the
drying will be unequal in different parts of the room. Sometimes this is
convenient; thus if most of the heat be supplied to the air coming fresh
from the inlet on the upper floor, the damper and colder air of the
lower room can be continuously used for drying wet goods from the yard,
and the upper reserved for drying off the finished leather. A
disadvantage of this plan is that open air drying can seldom be utilised
except in an elevated building; and even when it is adopted, means
should be provided for heating the lower room in cold weather. In place
of two floors, it is obvious that a single floor may be divided into two
compartments by a longitudinal partition. Whatever pipes are grouped at
the ends of the building, it is advisable to arrange sufficient to
prevent frost, against the walls, or in the old-fashioned way on the
floors beneath the leather, but not too close to it, and protected by a
wooden lattice on which the workmen can stand, which removes the risk of
accident from wet leather falling on the hot pipes. The latticed space
should be open at the end facing the air current, so as to receive a
portion of the draught, which will become heated and ascend, its place
being taken by damp and cold air from the leather, to be re-warmed.
Water-vapour in itself is lighter than air, but the contraction produced
by the cooling of evaporation more than compensates this, and the damp
air is therefore heavier than the dry. The arrangement of hot pipes near
the ceiling of a drying room, which has been borrowed from some American
tanneries, is wrong in principle, unless the air is forced in at the
upper part of the room, or the upper floor is latticed, and only acts in
other cases when the air is thoroughly mixed and circulated by
mechanical ventilators; while pipes near the floor will continue to
produce a certain amount of circulation of the air, even when the fan is
not running. In protecting pipes by lattices care should be taken not to
confine them too closely, or their heating effect will be seriously
diminished. In fan-drying, leather should be hung edgeways to the
current of air, so as to allow of its free and uniform passage between.
In the case of sole leather the butts or bends are conveniently
suspended by S-hooks of brass or iron wire, to hooks or nails fixed in
the joists. If gangways between the leather must be left in the
direction of the draught, they should be closed at intervals in the
length of the room by curtains or shutters, so as to deflect the
air-current into the leather.
Screw fans like the Blackman can be used either to suck or to blow the
air, though the former is preferable where it can be arranged, because
it produces a more uniform current in the room. On the blowing side the
air issues with considerable velocity in a sort of cone, but little
coming through the centre of the fan, while that near the edges spreads
rapidly from its centrifugal motion. This is rather advantageous where
the fan blows into an open room, but involves waste of power where it
discharges into narrow and square air-ways. The ends of the vanes of the
Blackman are turned in at the rim of the fan to prevent this tangential
discharge, but it is probable that where a fan is to _blow_ into a room,
it would be more advantageous to put it on the inner side of the wall,
and without curved ends to the vanes, so as to distribute the air as
widely as possible. A somewhat similar result would be attained with a
Blackman, by placing it in a position the reverse of that for which it
is intended, and running it also the reverse way; but its “efficiency”
might possibly be lessened.
Screw-fans are good for moving large volumes of air at comparatively low
velocities, and against little or no resistance, but they are quite
unsuitable for forcing air against high resistance, or through narrow
channels, and for this purpose centrifugal fans like the Capel (Fig. 95)
are much more suitable, and mechanically more efficient. In any case
there is much loss of power in forcing air through narrow airways, and
if a screw fan must be employed for the purpose, the channel should be
as large in section as the area of the fan, and all sharp angles in its
course should be avoided. There is great loss of power where a current
of air or water has to pass suddenly either from a wider to a narrower
channel, or the reverse, and in both cases the resistance is diminished
by making the enlargement or contraction gradual or “bell-mouthed.” Thus
a pipe conveying water at a given head into or out of a cistern will
discharge a much larger quantity, if the ends are bell-mouthed, than if
it terminates abruptly. For the same reasons, air suffers considerable
resistance if it has to pass suddenly into, or out of a larger space,
such as a drying room; and unnecessary partitions, and other abrupt
changes of dimension in the current should be avoided. Curves should
also take the place of angles as much as possible.
[Illustration: FIG. 95.--Capel Centrifugal Fan.]
Systems in which air is drawn or forced over systems of heating pipes by
a centrifugal fan, and then distributed through comparatively small
airways among the leather which is to be dried are in some cases
convenient and advantageous. Among these may be mentioned the Sturtevant
and the Seagrave-Bevington. There can be no valid patent on the general
principle of heating by distributing air in this way, but only on the
particular arrangement or appliances used in the special case.
Centrifugal fans should be considerably larger in diameter than in axial
length, those with long vanes of small radius being wasteful in power
from the insufficient supply of air to the centre. There is also no
reason why, in some cases, centrifugal fans should not be substituted
for screw-fans in drying on the system which I first described,
especially in cases where the air has to encounter considerable
resistance, as for instance in traversing a filter to remove dust. One
of the best filters for this purpose is a table of wire-gauze covered to
a depth of 3 or 4 inches with loose wool. Hair or cheaper fibrous
materials may be substituted for the wool, but are less efficient. The
air must of course be sucked downwards through the gauze. When the wool
becomes dirty, it may be washed, if possible in a wool- or hair-washing
machine, and again spread on the table in a damp condition, as it will
quickly be dried by the current of air. Flannel is also useful where the
wool-filter is impracticable, but requires frequent washing.
Apart from wind, natural ventilation is seldom to be relied on for
drying on any considerable scale. Heated air is, of course, lighter than
cold, and this is the cause of chimney-draught, but to get a good
circulation in this way, a high shaft, and high temperature is required.
Nevertheless, in one of its best forms, the method has been a good deal
used in America, in the so-called “turret-dryer,” a building of seven or
eight stories in height, constructed of wood with latticed floors, and
heated by steam-piping at the bottom, where the air is admitted. The
method is not likely to be much used in this country, as apart from the
questions of cost of building, fire-risk, and trouble of raising and
lowering the leather, a good draught will only be obtained when the
outer temperature is low in comparison to that inside, and in our milder
and moister climate the conditions are not nearly so favourable as in
the United States. As the air is rendered heavier by the cooling of
evaporation to a larger extent than it is lightened by the water vapour,
there is a tendency in drying by upward ventilation for the warm air to
form local upward currents, while the cold and damp air falls back; and
from this irregularity of flow, it is difficult to saturate the air
equally. This may be avoided by downward ventilation, in which the warm
air is admitted at the top of the drying room and the cold and damp air
allowed to escape at the bottom. This fact suggests that in using
systems of drying such as the Sturtevant, it would be better to place
the distributing pipes at the top rather than the bottom of the room,
but in this case care would have to be taken that there were no openings
left by which the air could escape at the top of the room without
descending through the leather. If this be avoided, the warm air will
float on the top of the colder and damper, and press it uniformly down
and out. I believe the merit of first having applied the principle of
downward ventilation to leather-drying is due to Edward Wilson of
Exeter. It is necessary that the hot air should be _forced_ in at the
top, or the cold air _sucked out_ from the bottom; and the mere placing
of hot pipes near the top of the room (p. 436) will not cause the
required circulation. Wilson placed his heating pipes in a partitioned
space at the side of the room, at the bottom of which cold air was
admitted from the outside, which escaped into the room at the top. As
the temperature of this side chamber was high and the air consequently
light, an upward current was produced in it, though probably somewhat
inefficiently, as the height of the column of heated air could only be
small. Assisted by a fan, and circulating a part of the air, the method
should give good results, especially over two (latticed) floors. As the
air could not be satisfactorily heated in its downward course, the
method would not be suited for more than about two floors, and the
drying in the lower room would be cool and gentle.
One or two points in the practical arrangement of steam-pipes may be
mentioned, as they are often overlooked even by professional engineers.
The steam must _always_ be admitted at the highest point in the system,
and there must be a steady descent, without hollow places where
condensed water can accumulate, to the steam-trap by which it is
removed. In horizontal pipes, about 1 inch descent in 100 is sufficient.
If water accumulates, there is not merely serious danger in case of
frost, but during use, by the sudden condensation of the steam, a vacuum
is frequently formed, into which the water is shot like the liquid in a
“water hammer,” producing violent and noisy concussions, and in some
cases even fracture of the pipes, or loosening of their joints. If
high-pressure steam is used, a very small supply-pipe will feed a
considerable system of heating pipes or radiators, but with exhaust
steam, great pains should be taken to have pipes of ample size, to avoid
back-pressure on the engines. In both cases it is often convenient to
arrange the pipes, not as a continuous line, in which drainage is
generally difficult, but in parallels like the bars of a gridiron. With
high-pressure steam, there need be no fear, if the pipes are kept clear
of air by allowing a little escape through small air-taps, of the steam
failing to find its way to all parts of the pipe, as a vacuum is
produced by condensation in proportion to the heat given off. With
exhaust-steam, no steam-trap is desirable, but any steam not condensed
should escape freely into the open air or a chimney (after separating
condensed water), and it is well to render the resistance in all the
pipes of a gridiron approximately equal, which may be done by admitting
steam at one corner, and allowing it to escape at the opposite
(diagonal) one. In the arrangement of steam-pipes in parallels, the
practicability of repair to one pipe or joint without interfering with
the others must always be considered. If screwed wrought-iron pipes are
used, each parallel must be provided with a bolted flange, or “running
socket,” to permit of unscrewing. The difficulty of accurately adjusting
the lengths of the several parallels must be considered, especially with
flanged metal pipes, and also their motion by expansion when hot, which
amounts to 1 or 2 parts per 1000 of length according to the temperatures
of steam and air. Expansion-joints with stuffing boxes are costly and
troublesome, and apt to leak, and may in many cases be avoided by
suitable arrangement of the pipes. Thus instead of having the pipes
rigidly fixed at both ends, one end of the system may be left free to
move, each pipe being separately returned to an exit pipe at the same
end but lower in level than the supply; or a single exit pipe may be
thus returned, its expansion and contraction being practically the same
as that of the heating pipes. In moderate lengths of wrought-iron pipe,
sufficient relief may often be obtained from the flexure of the pipe, if
in some part of its course it is carried at right angles to its general
direction, which is often necessary for other reasons. If pipes are laid
in long lengths, the loose end should be supported on rollers or short
pieces of pipe, so as to avoid moving the supports or straining the pipe
in expansion.
It is useless to attempt to regulate the temperature of low pressure
steam-pipes by turning down the steam, since, so long as the pipe is
supplied with sufficient steam to fill it, its temperature cannot be
less than 100°, and even with high-pressure pipes, the power of
regulation by altering the steam-pressure is very limited. It is far
better to arrange the pipes or radiators in groups, from some of which
the steam can be turned off entirely when less heat is needed. It must
not be forgotten that if these discharge into a common steam-trap, it
will be necessary to turn off their exits as well as their steam supply,
or steam will come back into them from the other pipes, and probably
prevent the escape of condensed water. In some cases it is more
convenient to give the several sections independent exits or
steam-traps.
Many good steam-traps are now on the market, depending either on the
expansion and contraction of metals, or on floats in a closed box, which
open a valve as the water accumulates. Traps of the latter class with
closed copper balls are to be avoided, as the ball is sure eventually to
become filled with water. Several traps have been devised in which an
open vessel is used as a float, which is always kept empty by the
discharge of the water through a pipe dipping into it.
The condensed water from steam-pipes is rarely suitable for use in the
tannery, from the dissolved and suspended iron-oxide which it contains,
from which it can only be freed by boiling and filtering, or treatment
with precipitants (p. 95). Its most appropriate use is generally return
to the boiler. Systems were formerly in vogue by which it was allowed to
run back to the boiler as it condensed, but these could only answer when
the pressure in the pipes was equal to that in the boiler, which is
rarely the case. It must generally be forced in by the feed-pump or
injector.
Hot water has often been advocated in preference to steam for heating,
but is more costly, as it requires a separate boiler, and much larger
pipe-surface for the same effect. Its only important advantage is that
the pipes maintain their heat for some time, even when the fire has gone
down, while steam-pipes cool at once if steam is allowed to go down in
the boiler. In any considerable tannery, however, this will seldom or
never be the case, since if a good pressure of steam is up at night,
when the fires are banked up, the boiler will in itself contain a large
reserve of heat, and, of course, working pressure will be required
before the engines can start in the morning. Hot water systems require
careful planning to obtain reliable and uniform circulation.
CHAPTER XXVII.
_CONSTRUCTION AND MAINTENANCE OF TANNERIES._
As few architects have specially studied the construction of tanneries,
and in most cases much of the arrangement depends on the knowledge of
the tanner himself, a short chapter on the subject will not be out of
place.
In the selection of a site, a clay or loamy soil is to be preferred to a
gravelly or sandy one, as lessening the liability to leakage, and waste
of liquor. Perhaps, however, the first consideration of all is the
possibility of drainage and disposal of effluent waste liquors and
washing waters, since it is now rarely possible to run these, without
previous treatment, into a river or stream. Some information is given in
Chapter XXVIII. on the methods of partial purification which are
available to the tanner, but these are always costly and troublesome,
and the possibility of running direct into a sewerage system, or a tidal
river is of great advantage. Under the Public Health Act, authorities
are bound to receive manufacturing effluents into their sewers if the
latter are of sufficient capacity, and the effluents not such as either
to damage the sewers, or interfere with the processes of purification
adopted by the authority. This act is in many districts practically
superseded by special legislation, but tanners’ effluents are generally
received into sewers if freed from solid matter. When mixed with other
sewage, they do not interfere with irrigation or bacterial treatment. In
selecting a site within a sewered district, regard must be had to the
possibility of causing a nuisance to the neighbourhood by foul smells.
Really injurious smells should not be caused by a properly conducted
tannery, but it is difficult to avoid odour, and a single badly disposed
neighbour may cause infinite trouble and expense.
Another important consideration is the water supply, since for the large
quantities used in a tannery, town water is generally very expensive.
With regard to quality and impurities of water information may be found
in Chapter X.; but, as a general rule, the softer and purer the supply
the better. It is also of great advantage when the source is at such a
level that the water can flow into the tan-yard, or at least into the
beam-house, without pumping. Filtration too, when needed, is much
facilitated by a sufficient head of water.
Commercial facilities, such as nearness to markets and sources of supply
of raw materials, and the availability of rail and water carriage are of
an importance at least equal to the points already considered, but
hardly come within the scope of this work.
The site chosen, the next question is the arrangement of the buildings.
It is very doubtful, where ground is not inordinately expensive, whether
it is wise to erect drying-sheds over the pits. In case of fire, very
serious damage is done to liquor and leather by the heat and burning
timber. If the turret form of drier be decided on, strong foundations
are required, and the ground-floor or basement is occupied with heating
apparatus; if fan-drying, no lofty buildings are needed, and the drying
rooms are conveniently placed over the finishing and currying shops;
and, on the other hand, the tan-house may be easily and cheaply covered
with slated roofs, with nearly vertical sections of glass, to the north
if possible, like a weaving-shed, through which sufficient light for
convenient work and cleanliness is admitted. The direct rays of the sun
should be avoided, but in the writer’s opinion the balance of advantage
is largely in favour of a liberal supply of light. Iron roofs are
unsuitable, since the moisture condenses on, and rusts them; and
particles of oxide fall into the liquors, and cause iron-stains.
Good ventilation along the ridge of the roof should be provided,
wherever there is any steam or hot liquor used; or the condensed
moisture soon leads to decay.
In arranging the general plan of the buildings, much depends on local
circumstances; but as far as possible, they must be so arranged that the
hides and leather work straight forward from one department to another
with as little wheeling or carrying as possible; that the buildings
where power is used be near to the engine so as to avoid long
transmissions, which are very wasteful of power; and that the different
buildings be so isolated as to diminish the risk of the whole being
destroyed in case of fire.
A chapter on the construction and maintenance of tanneries and leather
works would be incomplete if it did not refer to the very important
question of Fire Insurance.[185] To an extent this may be regarded as a
fixed charge against any business, very much in the same way as local
and imperial rates. It is not, however, to be lost sight of, that to
some considerable extent the amount of insurance premium is regulated by
the insured himself. If a man conducts his business in unsuitable and
badly constructed buildings; if attention is not paid to some of the
elementary hazards connected with a fire outbreak; he must not blame the
insurance companies for the demand of what he considers an excessive
premium. If this faulty construction and imperfect equipment of
buildings pertain to any considerable extent throughout a given trade
where the process is more or less hazardous, it is futile to appeal to
insurance companies, which, after all, are merely commercial and not
charitable institutions, for a reduction in the rates. The only standard
to guide the company is the loss-ratio, and given a high loss-ratio,
there must be a corresponding premium paid.
[185] With regard to fire insurance, I am much indebted to Mr. A. W.
Bain, of Leeds for valuable information.
There is, however--thanks to modern science--a method available whereby
the great bulk of fires may be checked in their inception; an appliance,
automatic in its operation, and of proved efficiency. This appliance is
known as the sprinkler. A system of water-pipes is fixed under the
ceilings of the building to be protected, to which are attached
sprinkling jets at suitable intervals, each of which is closed by a
valve held in place by a joint of fusible metal, which gives way if the
temperature rises beyond a given point. There are two or three
recognised patterns approved by the Fire Offices Committee after patient
investigation and practical test. These appliances have now been at work
for something like fifteen years in this country. One of the first
trades to recognise their utility was that of the cotton-spinner. At one
time serious fires in the cotton trade were of frequent occurrence.
Now--owing to the efficient fire appliances--while fires may be as
frequent in their inception as formerly, they are stopped at such a
stage as to prevent any considerable loss. The consequence has been that
the cotton-spinner, at one time the owner of a highly-rated risk, and
one which few companies cared to insure, is now in the position of
having his business eagerly sought after, and large discounts offered
him off the charges he was once called upon to pay.
More important still is the consideration to him that his business is
not so liable to be interfered with or stopped as the result of fire.
There are, it is estimated, at the present moment, no less a proportion
than 90 per cent of the cotton-spinners whose premises are protected by
sprinkler installations.
Other hazardous risks such as corn-millers’, woollen and worsted
manufacturers’, saw-millers’, engineers’, are adopting these appliances
freely, and it is a matter of surprise that so very few tanneries or
currying shops--so far as I have been able to learn, not more than
twelve--have done the same. The consequence is that the loss-ratio in
tannery risks still retains its unenviable notoriety: the rates for fire
insurance have risen considerably, and as a result the tanners’ profits
are correspondingly less. Considering the extent and importance of many
of the tannery risks throughout Great Britain, one can only express
surprise that these appliances have been so little adopted.
The construction of a new tannery demands serious attention from an
insurance standpoint. The boiler-house should be a detached building;
the grinding of bark and myrobalans should be conducted in buildings
isolated from the general works; in fact no better advice could be given
to a tanner, either in the construction of new premises, or the
rearrangement and remodelling of old, than to consult an experienced
insurance man, whether official or broker, as to the best means of
constructing and arranging to secure the most favourable terms.
Another point which should be provided for, and which is often
overlooked, is the feasibility of future extension without serious
changes of arrangement. It may be taken as a probability of the future,
even if it be not already a fact, that small tanneries cannot be made to
pay, and that if a business succeeds, its extension will prove
desirable; and in an ill-planned yard this may involve either entire
reconstruction of a very expensive and inconvenient sort, or the
separation of new departments, so as to involve serious increase of
carrying. A good arrangement is that of a long front building serving to
connect the whole, behind which the various departments are erected at
right angles leaving room for extension backwards as required.
As regards the first of these conditions, if the various soaks, limes,
bates, and handlers are well arranged, it is hardly necessary to do more
than draw the goods from one pit into the next throughout the whole of
the process. To, and from the layers, the goods must generally be
carried or wheeled. In the sheds, if it be a sole-leather tannery, the
butts should first come into turrets or open sheds for the rough drying;
then into a room sheltered from draughts to temper for striking. The
striking machines or beams should be in an adjoining room, or
immediately below; then a small shed-space for drying before rolling;
next the roller room; and then the warm stove for drying off. If two of
the latter can be provided to be used alternately, it will allow the
goods to be aired off without taking down, and they may then be
immediately handed or lowered into the warehouse, without fear of
over-drying, which is sometimes difficult to avoid where leather must be
taken direct out of the hot drying-room. The same principles are easily
applied in yards for lighter leathers.
To lessen loss of power in transmission, the engine should be near the
centre of the main range of buildings, with perhaps the grinding
machinery on one side, and the leather finishing on the other; but this
would be rather liable to increase the fire-risk. A very good plan would
be to have the engine-house in the centre as suggested, but separated
from the buildings on each side by brick gables; and with the
boiler-house behind it, and under a separate roof, say of corrugated
iron. If it be impossible to have the engine near its work, it is in
most cases better to employ a separate high-pressure engine, which may
be within a glass partition, and will work all day with scarcely any
attention. The loss of power in carrying steam for moderate distances
through sufficiently large and well-clothed pipes is much smaller than
that of long lines of shafting. The writer has known cases where fully
half the indicated power of the engine was consumed in friction of the
engine, shafting and belts. High-pressure engines are as a rule to be
preferred to condensing for tannery use, since the waste steam can
generally be employed for heating, and both the first cost and that of
maintenance are smaller. Where much fuel is used, it is quite worth
while to have the cylinders indicated occasionally, both running light,
and driving the machinery; much information is gained in this way as to
the power spent on the various machines, and very frequently large
economy is effected by proper adjustment of the valves. To work
economically, an engine should be of ample power for all it has to do;
and adjusted to its work, not by lowering the pressure of steam, or by
checking it at the throttle-valve, but by setting the slide-valves to
cut off as early in the stroke as may be. As to how early this is
possible, an indicator-diagram will at once give information. If the
whole of the waste steam can be used profitably for heating purposes,
economy in the working of the engine is of little consequence, but,
otherwise, it is very injudicious, for the sake of a little saving in
first cost, to put in an old or inferior engine, which has to be dearly
paid for in waste of fuel. In the choice of an engine, the advice of an
expert engineer is desirable, since many engines which are mechanically
well made, are uneconomical through the faults of a rule-of-thumb
design. In this respect the English engine-builder is frequently
inferior to his better trained continental competitor.
In place of using small steam engines to distribute power, electric
driving deserves consideration. For long drives the loss of power is
much less than that of shafting, and by concentrating the whole
production of the power in one large and well-constructed engine, the
cost per horse-power can be much reduced. While large and
well-constructed engines may develop 1 horse-power at a cost in coal of
1¹⁄₂ lb. per hour, it is not uncommon to use 12 lb. for the same output.
In tanneries, however, the power used bears a much less proportion to
total expenses than it does in the textile and many other trades. The
first cost of electric driving is somewhat high. Motors of the
“armoured” or iron-cased type must be used in all positions where they
are subject to wet or dust. It must be borne in mind that an electric
motor will not start against a heavy load, as it only develops its full
power at a high speed, and if it receive the full pressure of the
current before this is attained, its coils will probably be burnt out,
unless saved by the melting of its safety-fuse. A similar danger is
incurred, if the motor is brought up by overloading while the current is
on. It is therefore generally necessary to connect a motor with its work
by a belt which is only brought on to the working pulley when its full
speed is attained.
In some cases the use of gas-engines is convenient and economical; for
though gas from town-supplies is an expensive fuel, the best gas engines
give a higher mechanical efficiency than steam-engines, and they work
with very little attention.
In arranging shafting, moderate speeds, say 100-150 revolutions per
minute, should be chosen for main lines, and when higher speeds are
necessary, they should be got by light and well balanced counter-shafts,
with wrought iron or wooden pulleys. (Cp. p. 452.) In calculating
speeds, it must be remembered that they vary inversely as the size of
the pulleys. Thus a 3-feet pulley running at 100 revolutions will drive
a 2-foot pulley at 150 revolutions, and a 12-inch one at 300. Of course
the higher its speed, the more power any shaft will transmit, but
increased friction and wear and tear soon limit this advantage. The
velocity of a belt in feet per minute is obtained by multiplying the
number of revolutions per minute by the girth of the pulley in feet or
by its diameter multiplied by 3¹⁄₇, or more accurately, 3·1416.
Pulleys should always be of ample breadth for the power they have to
transmit; and it is more economical, both in power and cost, to use
broad single belting than the same strength in double. If the pulley
will not take a belt broad enough for the work it has to do, a second
belt may be made to run on the top of the first, as suggested by Mr. J.
Tullis, and will do its share of the work. Belts should be washed
occasionally with soap and tepid water, and oiled with castor or
neatsfoot oil; but if of sufficient breadth, should not require the use
of rosin, or adhesive materials, to make them grip the pulley.
Chrome-leather belts should be kept thoroughly oiled. They have a much
greater adhesion than vegetable tannages, and this is increased by
oiling. Good chrome belting is much stronger than bark-tanned; and is
unaffected by damp or steam, but generally stretches somewhat more.
Makers of machines often err in constructing their driving pulleys too
small both in breadth and diameter.
The horse-power which a belt is capable of transmitting obviously varies
extremely with circumstances, but may be approximately calculated by the
formula
_a_ . _v_
---------,
66000
where _a_ is the area of contact of the belt with the smallest pulley,
and _v_ its velocity in feet per minute. Another rule is, that at a
velocity of 1000 feet per minute, each inch of breadth of belt should
transmit 2¹⁄₂ horse-power on metal pulleys, or 5 on wooden ones, on
which the adhesion is greater. Adhesion may also be increased by
covering the pulleys with leather or indiarubber. Both rules assume that
the belt is of ample strength. One horse-power would be transmitted by a
belt running 1000 feet per minute with a pull of 33 lb. A good single
belt should not break with a much less stress than 1000 lb. per inch of
breadth, and should stand about ¹⁄₁₀ as much as a working stress.
The following table gives the experimental breaking stresses and
extensions of some leathers. It may be noted that 1 square inch
sectional area is equal to a belt 4 inches wide × ¹⁄₄ inch thick; and
that _kilos per cm²_ × 14·22 = lb. per inch².
BREAKING STRESSES OF LEATHER.[186]
-----------------------------+------------+---------+---------
-- |Kilo per sq.| Lb. per | Stretch
| centimetre.|sq. inch.|per cent.
-----------------------------+------------+---------+---------
Belting leather, layer system| 283 | 4,030 | 25·4
„ „ Durio system| 298 | 4,240 | 21
Well-tanned chrome leather | 740 | 10,500 | 32·5
Over-tanned chrome leather | 234 | 3,330 | 23
Stuffed alumed leather | 835 | 11,900 | 38·3
Alumed “rawhide” | 921 | 13,100 | 31·4
-----------------------------+------------+---------+---------
[186] ‘Gerber,’ 1900, p. 73.
Good English tanned belting leather breaks at from 4500 to 5500 lb. per
sq. inch sectional area.
Over-tanned leathers are less tough, whether of vegetable or mineral
tannage, than those somewhat lightly tanned, and the tensile strength of
leather varies considerably with the part of the hide from which it is
taken, that from approximately over the kidneys being the strongest.
Even thick and tough leather is easily torn if a cut or nick is once
started, and all holes used in jointing belts should be carefully
rounded. Glucose, and the use of acid in bleaching both lessen the
toughness of belts, and they may also be rendered tender by the heat
evolved in slipping on a pulley.
Countershafting and high-speed machinery, such as disintegrators,
striking machines of the Priestman type, etc., should run without
material jar or vibration. If this occurs, it is generally a sign that
the running part is not equally balanced. In this case the shaft or
spindle must be taken out of its bearings, and supported on two exactly
horizontal straight-edges, on which it will roll till the heaviest part
is downwards; and weight must then be taken off or added till it will
lie in any position. In this way the writer has had to add fully 2 lb.
of iron to balance the drum of a striking machine before equilibrium was
secured, and a most troublesome vibration prevented. Of course all
machinery should be supported as solidly as possible; and if
circumstances permit, most machines are better on a ground floor. In
placing bark mills, however, it is frequently convenient to fix them at
a higher level, so that the ground material may be sent down shoots by
its own weight to the required places. An alternative plan is to set the
mill on the ground over a pit, and to raise the ground material with a
bucket-elevator. This may be done successfully by letting the material
fall directly from the mill into the buckets; but otherwise it must be
thrown in with a shovel, as buckets will not pick up ground bark, even
from a hopper; and in any case such elevators are apt to be troublesome.
In a grinding plant designed by the writer, the unground material was
filled on the basement floor into an iron barrow, which was wheeled into
an iron sling working between upright guide-rails like a hoist. On
pulling a brake line, the barrow was raised to the top of the building,
and its contents were tipped into a large hopper, after which the barrow
righted itself, and descended for another load. In the bottom of the
hopper was a sliding shover, which forced the material on to vibrating
screens, by which it was guided either into a disintegrator, or
crusher-rolls, at pleasure. Both these discharged through iron spouts
into large hoppers on the outside of a brick gable, from which powdery
materials like myrobalans and valonia could be run direct into barrows
or trucks. It is very desirable that such hoppers should be separated
from the main building by a fireproof partition. Fires may occur from
hard substances getting into disintegrators along with the bark, etc.
and if this occur with a dry and dusty tanning material, it is not
unlikely that it may result in an explosion such as sometimes happens in
flour mills, in which the fire is rapidly conveyed along spouts, and
into chambers filled with dusty air. Insurance companies generally
charge an extra rate for disintegrators, and it is very desirable to
keep the mill-house structurally apart from other buildings, either by
actual separation or by the introduction of brick gables dividing the
roofs. On the whole, however, mills of the coffee-mill type are probably
quite as dangerous as disintegrators; since if they become partially
choked, the heat caused by friction is very great.
In America, the fire-risk from mills is often lessened or prevented by
the introduction of a jet of steam into the chamber or spout by which
the mill discharges, but this is only permissible if the tanning
material is conveyed at once to the leaches or yard.
The use of chain-conveyors for handling tanning material both wet and
dry is practically universal in America, though comparatively rare in
England. Various forms are used, the most common consisting of a chain
of square links of malleable cast iron which hook into each other, so
that a broken link can be immediately replaced (see p. 325). At
intervals special links are inserted, which can be had of various
patterns, for the attachment of scrapers or buckets. The endless chain
runs in a trough of rectangular or V-shaped section, and is driven by a
toothed wheel, over which it runs like a belt. In some cases the
returning half of the chain can be utilised to bring back the spent tan
on its way to the boiler house. For dry materials, cotton or leather
belts with short wooden cross-laths attached, may often be used
satisfactorily in place of the chain.
For lubricating purposes, mineral oils of high density are not more
dangerous than animal or vegetable, but rather the reverse; as, though
they are possibly more inflammable, and make more smoke, their mixture
with cotton-waste and other porous materials is not spontaneously
combustible, as those of vegetable and animal oils occasionally are. The
danger of spontaneous combustion is very considerable when heaps of
leather shavings or cuttings containing fish-oils are allowed to
accumulate in warm workshops, and, especially near steam-pipes. Heavy
mineral oils should always be used as cylinder-oils in high-pressure
engines, in preference to other oils or tallow, since they are not
decomposed by steam, and do no harm if blown into the feed-water, but
serve to loosen and prevent scale and deposit. Ordinary oils and tallow,
on the other hand, when submitted to the action of high-pressure steam,
are separated into glycerin and fatty acids (see p. 351), and the
latter corrode the valve faces and seatings, and are liable with
“temporary hard” waters to form a very dangerous porous deposit in the
boilers, which often leads to overheating of the tubes.
Next to the machinery, the pits demand special consideration. The
chapter on the subject in the late Mr. Jackson Schultz’s book on
‘Leather Manufacture,’ is well worth attentive study as giving American
practice on the subject.
The old-fashioned method of sinking pits is to make them of wood, and
carefully puddle them round with clay, which should be well worked up
before use. It is of no use to throw it in in lumps and attempt to
puddle it between the pits, which will not be made tight, but probably
displaced by the pressure. Such pits, if made of good pine and kept in
constant use, are very durable, some of the original pits at Lowlights
Tannery, constructed in 1765, having been in use till 1889. Loam mixed
with water to the consistence of thin mortar may also be employed, the
pits being filled up with water, to keep them steady, at the same rate
as the loam is run in. Probably the best materials for pit-sides are the
large Yorkshire flagstones. Where these are not attainable, very durable
pits may be made of brick, either built with Lias lime, and pointed with
Portland cement, or built entirely with the latter. Common lime cannot
be used, as it spoils both liquors and leather; and even cements with
too large a percentage of lime are unsatisfactory. Brick and common
mortar are, however, suitable for lime-pits, and for these Mr. C. E.
Parker’s plan of constructing the bottom of cement, the ends and sloping
hearth of brick, and the sides of 3-inch planks bolted together is also
very satisfactory (Fig. 96).
The writer has constructed wooden pits in two ways. In the one case,
after making the excavation, beams were laid in a well-puddled bed of
clay; on these a floor of strong tongued and grooved deals was laid, and
on this the pits were constructed of similar wood to the floor, and
puddled round with clay. In the second case the pits were built like
large boxes above ground, and when finished, lowered on to a bed of clay
prepared for them, and then puddled both around and between. It may have
been due to defective workmanship in the first case, but those made on
the last-named plan, which is that adopted from very early times,
certainly proved the tightest and most satisfactory. Mr. Schultz
describes a plan as the Buffalo method, in which a floor is laid as just
described, and grooves cut with a plane for the reception of the sides,
which are formed of perpendicular planks, each end and side being
finally tightened up by the insertion of a “wedge plank.” Owing to the
perpendicular position of the side-planks such pits would be difficult
to repair in the common case of decay at the top.
[Illustration: FIG. 96.--Mr. C. E. Parker’s construction of Lime-Pits.]
If bricks be used, great care must be taken that the cement is not
merely laid so as to fill the joints towards the two surfaces of the
wall, as is the habit of modern bricklayers, but actually floated into
all the joints so as to make the wall a solid mass; or leaks can hardly
be avoided. Hard pressed bricks are best, and should be tested as to
whether they discolour liquor. Cement-pits are very good, and, though
not particularly cheap in material, which must be of the best, are
readily made by intelligent labourers under good supervision. The first
step is to lay a level floor of good concrete, in which glazed pipes for
emptying the pits may be embedded; care being also taken that all joints
in these are thoroughly tight, since future repairs are impossible. The
next step is to make frames, the exact length and breadth of the pits
required, and perhaps 15 inches deep. These are arranged on the floor
where the pits are to be, and the intervening spaces are filled with
concrete of perhaps 1 of cement to 3 or 4 of crushed stone or brick.
Rough stones and bricks may also be bedded in the concrete as the work
goes on, to help to fill up. After the first layer has set, the frames
may be raised and a second added, and so on. The work is generally
finished by floating over it, while still damp, a little pure cement, to
give a smooth surface. Before using, the cement should be tried on a
small scale, to be sure that it does not discolour leather or liquors,
and the pits should always be seasoned with old or cheap liquor before
actual use.
[Illustration: FIG. 97.--Cleaning Rod Joint.]
If possible, both leaches and handler-pits should be provided with plugs
and underground pipes, communicating with a liquor-well some feet below
their levels. Glazed fire-clay is very suitable both for pipes and
plug-holes, which should be in the pit corners. If fire-clay blocks for
plug-holes cannot be obtained, they may be cast in good cement, the
wooden mould being soaked with hot paraffin wax to prevent adhesion.
Means must be provided for the ready clearing of the pipes when choked
with tanning materials. A good plan is to let each line of pipes end in
a liquor-well large enough for a man to go down. As it is almost
impossible to make plugs fit without occasional leakage, it is not well
to run pits with very different strengths of liquors to one well, but
the layers, handlers, and different sets of leaches should each have
their own, so as to avoid mixture. A good means of clearing pipes
consists in a series of iron rods 3-4 feet long, connected by hooks
fitting into double eyes, as shown in Fig. 97. It is obvious that in a
narrow pipe or drain, these cannot become disconnected. Pipes may often
be forced out by fitting a strong delivery-hose of a steam-pump into one
of the plug-holes.
It is, as Schultz points out, of questionable advantage to lay wooden
troughs under the alleys for supplying liquor to each pit, since it is
almost impossible to preserve them from decay; but the same objection
would not apply to glazed pipes, jointed with pitch or cemented. A good
and cheap plan in practice, is to let the liquor-pump, or a raised
liquor-cistern, discharge into a large and quite horizontal trough
raised 6 or 7 feet above the level of the yard, and provided with
plug-holes at intervals, from which the liquor can be run into the
various pits by short spouts or sailcloth hose. In place of plugs in the
raised trough, a simple and convenient valve devised by the writer may
be advantageously employed. A lead weight is made by casting in a
hemispherical tin basin of about 5 inches diameter and 2 inches deep in
the centre, a loop of strong brass wire with turned up lower ends, being
suspended in the middle, so as to become fixed in the lead. To prevent
adhesion, the tin must be previously burned off, and the basin well
blackleaded. This weight forms the valve, which rests in use on a 6-inch
washer of good indiarubber with a 4-inch hole, which is held by a wood
block against the bottom of the trough, through which a 5-inch hole is
cut. The valve is raised by a lever or cord, and is absolutely
water-tight in use. It is shown in section in Fig. 79, p. 333.
It is very advantageous in practice, instead of pumping direct into the
pits, to have one or more tanks, into which liquor can be delivered by
the pump, and which are sufficiently raised to allow it to be run from
them into the horizontal distributing troughs which have been mentioned.
This is specially important with regard to liquors for leaches and
suspenders which are worked on a circulating system, since they do not
run very quickly, and much time is lost in pumping out pits, if the
speed of the pump has to be regulated by the rate at which the liquor
will circulate. It also enables liquors to be run through suspender- and
rocker-pits during the night or at meal-times while the machinery is
standing; and it is often useful on beginning work in the morning, to
have an empty tank into which the first liquor can be pumped.
Direct-acting steam-pumps without fly-wheels are very unsatisfactory for
tan-yards, since they are usually uncertain in their action, difficult
to run slowly, and apt to “hammer”; and they are also costly in steam,
which cannot be used expansively. Steam-pumps with fly-wheels, operating
the steam-valve by an eccentric, are free from these defects, and though
more costly at the outset, soon save the difference in lessened repairs
and consumption of steam. Pumps with a capacity of 8000 gallons per hour
are very suitable, and can be used with a 3-inch hose pipe; smaller
sizes are decidedly more liable to choke with tanning material. Rubber
mitre-valves work satisfactorily, and do not choke frequently, but are
costly, and easily damaged by hot liquors. On the whole brass
clack-valves are the most satisfactory, but the hinge-pins, instead of
fitting neatly in circular sockets, should be held in slots, allowing
the back of the valve to rise half an inch, when it will clear itself of
small hard myrobalan stones and suchlike things, which getting under a
more tight-fitting hinge would prevent the valve closing, and so stop
the pump. Whatever valves are employed, means should be provided for
easy access without unscrewing too many bolts. If the several
valve-chambers of the pump are closed by a single cover with an
indiarubber washer, the spaces between them which make the joint should
be faced with brass or gun-metal, as, if the least leakage takes place
over an iron surface, the friction and solvent power of the liquors soon
eat away the metal and render a good joint impossible. Where colour is
of first importance, it is well to have the whole pump of gun-metal, but
in any case the working cylinder should be brass-lined, and the piston
and rod, and the valves and seatings should be of brass or gun-metal.
Spring-rings are far better than pump-leather and are unaffected by hot
liquors; chrome leather, however, will stand a good deal of heat.
Double-acting force-pumps have practically superseded the older
single-acting double or triple pumps. Instead of direct driving with a
steam cylinder, it is sometimes advantageous to drive by belt, but at
least one steam pump should be provided, so that pumping can be done
when the main engine is not running, and the speed of the pump can be
regulated to the work, which is impossible in a belt-driven pump. Steam
pumps are sometimes very useful as fire engines.
Centrifugal pumps are very suitable for tannery work, where the liquor
is drawn from a well, but are not well adapted for use with
suction-pipes. If the form with vertical spindle is adopted, which is
sunk below the liquor in the well, the pump fills itself, and needs no
foot-valve, but unless the well is very large, or some convenient means
is devised of withdrawing the pump, repair or cleaning is difficult. If
the horizontal pattern is used, which is above the ground, repair,
cleaning, and driving is much easier, but a foot-valve is necessary,
which may itself give trouble, and some convenient means, such as a pipe
from a raised tank, should be provided for filling the pump with liquor,
as, unlike suction pumps, centrifugals will not start unless full,
although they raise very large quantities when running, and from their
steady flow, will deliver much more through a given pipe than an
ordinary reciprocating pump with the same power. In selecting the pump,
care should be taken that the pattern allows ready access, not only to
the foot-valve, but to the body of the pump.
It is seldom satisfactory to use windbores or strainers to prevent
tanning material getting into a pump, as they speedily become choked;
and it will be found better, after taking such precautions as are
possible, to have the pump and valve of ample size and suitable
construction to pass what comes with the liquor. The writer has known a
mop-head pumped and delivered through a 3-inch hose without stoppage, by
a Tangye fly-wheel steam-pump with brass clack-valves such as have been
alluded to.
Pulsometers have not, in the experience of the writer, proved
satisfactory in tanneries, warming and diluting the liquor, consuming
much more steam than a pump of the same power, and becoming easily
choked. For the same reasons, steam-jet water-raisers are not to be
recommended except where raising is to be combined with heating, as in
some leaching devices (p. 334).
CHAPTER XXVIII.
_WASTE PRODUCTS AND THEIR DISPOSAL._
The products which are of no direct value to the tanner and currier in
the manufacture of leather, and which are nevertheless obtained in
fairly large quantities, are of very varying characters. In the present
chapter, the most important of them will be described, and some of their
uses mentioned.
_Hair_ is removed from the skin of the animal in the process of
depilation (p. 143) in the form of a wet sodden mass, containing a
considerable amount of lime when the skin has been through the
lime-pits.
As white hair is the more valuable, care should be taken in the
unhairing to keep it separate from the coloured. It is washed first in
plain water to get rid of as much of the lime as possible, and then in
water containing a little acid. Hydrochloric acid is often used for this
purpose, but sulphurous acid (p. 25) is preferable as it has a slight
bleaching action on the hair. The acid neutralises and renders soluble
the lime which still remains in the hair, so that it can be easily
removed by washing with water. In many tanneries, hair-washing machines
are used. The washed hair is dried by laying it out on frames; or
preferably, the greater part of the water is first removed by a
centrifugal drier, or by pressing, and the drying is completed in a
drying room, the temperature of which is a few degrees higher than that
of the outside air, and which is provided with a fan or some other
appliance for mechanical ventilation. Tables of wire gauze on which the
hair is spread, and through which the warm air of the room is drawn by a
centrifugal fan, are the most effective.
Coloured hair is sometimes washed and treated like the white hair, but
is usually sold direct to plasterers, in which case there is no
necessity to remove all the lime and other impurities which the hair
contains. A considerable amount of hair is also sold to iron founders,
who use it in preparing cores and in loam-casting. The loose lime may be
effectively beaten from dried hair by passing it through a disintegrator
with one of the grates removed.
_Fleshings and Glue-stuff._--The various scraps of fat and flesh, more
or less free from actual hide substance, are usually worked up for glue,
though if they cannot be sold for a fair price it will pay to boil them
in order to recover the fat they contain. If this is to be done, the
fatty portions may be thrown out at the beam and not mixed with the
fleshings as in the ordinary way. Before boiling, the fat is treated
with sulphurous, sulphuric or hydrochloric acid, sufficient to
neutralise the lime present. The boiling should be carried on very
gently, so as to allow the fat to rise without emulsifying with the
gelatinous matter. For boiling, open steam may be used, but in this case
the size formed will have little value; on the other hand, if sulphurous
acid has been used and a wooden vat with a copper steam-coil be
employed, really good glue may be obtained, and the slight trace of
bisulphite which it may contain will prevent its putrefaction. Except
under special conditions it will not pay to make glue on a small scale
in England, as its value depends much on its appearance, and the
necessary plant is somewhat expensive. In some places, however, size can
be sold to advantage. Fig. 98 shows a glue-boiling plant.
After separation of the fat by skimming, the clear size is run off from
the residual matter into wooden cooling troughs about 5 feet long by 9
inches deep and 15 inches wide, in which it is allowed to set (Fig. 92,
p. 425). Great care is required that both size and coolers are quite
sweet and free from putrefaction, the coolers being frequently washed
with sulphurous acid solution or fresh milk of lime. The jelly is cut
out in blocks, and sliced into cakes of appropriate thickness by means
of a series of frames like slate-frames which fit over the block of
glue, and between which a wire or thin blade stretched on a saw-frame is
inserted to cut the glue into sheets. In some factories a machine is
used, with a series of parallel blades against which the glue-block is
pushed. The sheets are afterwards separated by girls and laid to dry on
nets, on which they are frequently turned. When dry, the cakes may be
washed with warm water to remove any adhering dirt, but this causes
some loss of weight, and in many cases it pays better to dry in a stove
until quite hard, then grind in a disintegrator and sell as
“size-powder,” in which appearance counts for little if the colour and
strength of the size are good.
[Illustration: FIG. 98.--Glue Boiling.]
_Fat._--The fat, whether obtained in the manufacture of glue, or by
boiling the fleshings and shavings for its recovery alone, is skimmed
from the surface of the heated liquor, and should afterwards be freed
from gelatinous matter by washing it with hot water in a tub and running
off the upper layer after allowing the water to settle out. The fat thus
obtained is a light-coloured grease of buttery consistence.
There are various other sources of waste fats which may be considered
here. If glue is made from dried glue-stuff without previous treatment
with acid, the fat skimmed off the pans, though dark in colour, will be
neutral or alkaline, and a considerable additional quantity of fat and
free fatty acids may be obtained by reboiling the “scutch” or refuse
with open steam in lead pans with the addition of water and enough
sulphuric acid to render the contents of the pan distinctly acid. This
grease will be dark and of unpleasant smell from volatile fatty acids,
but its odour may be to a considerable extent improved by blowing air
and steam through it, and washing with water, or by heating to a
temperature somewhat above the boiling-point of water for a considerable
time. The same sort of treatment may be applied to the fat pressed out
of sheepskins, and to that obtained by boiling currier’s shavings with
water and a little acid.
Recovered fats may be separated into a tolerably firm grease suitable
for use instead of tallow in currying, and an oil not unlike neatsfoot
oil, by melting, allowing to cool slowly to a soupy consistency to
promote the crystallisation of the harder fats, and forcing the mixture
through flannel cloths in a filter press. The temperature at which the
filtration should take place is generally 20-25° C. The oil is, of
course, “tender,” or liable to solidify in cold weather; and the more so
the higher the temperature at which filtration takes place. The tallow
is obtained in cakes. If from fresh fleshings, it will be white and with
little odour, but that from dried glue-stuff is usually brown and of
unpleasant smell, while recovered grease from curriers’ shavings or
“moisings” is always dark in colour.
If the fleshings are to be sold wet, they should be preserved in a sweet
lime liquor; if to be dried, they are washed carefully in a fresh lime,
spread on frames, and frequently turned over so that they may dry evenly
and rapidly. Heat, if employed at all, is in most cases only used at the
end of the drying operation, but some tanners dry from the first in a
room the temperature of which is a few degrees higher than the normal,
and which is provided with good ventilation. For the purposes of the
glue manufacturer, the roundings and larger pieces are more valuable
than the fleshings, and should be treated with correspondingly greater
care by the beamsman and his assistants.
_Bate-Shavings_ are very valuable as sizing materials. They should be
well washed in water, or with a very dilute solution of sulphurous acid,
and are then laid out in thin layers to dry. They may also be partially
dried by pressing between latticed boards in a screw or hydraulic press,
and are then best finished as cakes. On the manufacture of sulphurous
acid compare p. 25.
_Horns_ are usually kept until the “slough,” “pith,” or internal bone
can be knocked out, having become loosened through drying and
putrefaction. If kept dry, practically no longer time is required, and
the smell and other annoyances incidental to storing in a damp place are
avoided. The sloughs may be removed by steaming, but the horns are
somewhat damaged by this treatment. The sloughs are principally ground
for “bone-meal,” but some are boiled for glue, either without
preparation, or after decalcifying with dilute hydrochloric acid.
The actual horn itself, which is quite incapable of making glue, is used
chiefly in the manufacture of combs, buttons, and similar articles. The
value of horns is to a considerable extent dependent on their size,
small horns being unprofitable to work up for the articles above
mentioned.
_Spent Tan._--The tan as it is obtained from the leaches after
extraction has, naturally, no value for the tanner except as a fuel.
Spent tan cannot be profitably sold as manure, as its worth in this
respect is extremely small. In those places where white lead is still
made by the Dutch process, oak-bark is used to cover up the earthen
pots, and commands a good price. It is, however, essential that oak-bark
only should be used, as many other tanning materials give off products
which injure the colour of the white lead. The quantities of tan used
for hot-beds, and for deadening the noise of traffic in the streets, are
so small that they are of no practical account in the disposal of this
product. Spent tan is not nearly so good as wood for the manufacture of
paper, and an attempt to distil it and thereby obtain pyroligneous acid
and wood-spirit did not result in any commercial success. On the
Continent, fine-ground tan is usually pressed into briquettes for use as
domestic fuel, but it would be hard to obtain a market for these in
England.
On the whole, in spite of its low heating value, spent tan is best
utilised as a fuel. For this purpose specially constructed furnaces are
necessary on account of the dampness of the tan, and its low calorific
value, which varies, however, with the particular materials: thus while
oak-bark and valonia are only poor fuels, hemlock and myrobalans are
much better on account of the resin and lignine they contain.
The first successful furnaces for raising steam with wet tan were
introduced in the United States, and consisted of large arched
combustion chamber with abundant grate-area, and with four or six
feed-holes in the fire-brick top which formed a floor on which the spent
tan was laid, and where to some extent it was dried by the waste heat.
The flames and furnace gases were conducted under the boilers, the flue
being very large and deep so as to collect the light ash which was drawn
in great quantities from the furnace, and the gases then returned
through the tubes of the boiler, afterwards passing down the sides and
going to the chimney. The wet fuel was fed in through the firing holes
alternately, so that only a part of the grate-space was covered at once
with wet fuel; which was speedily ignited by the heat from other parts
of the furnace, and especially from the vaulted arch.[187] The large
grate-area was a necessity not only on this account, but because of the
light weight of the fuel and its low calorific power, which involved the
need of burning a large volume. Fig. 99 represents a furnace of similar
principle constructed by Messrs. Huxham and Browns. Furnaces of this
type are, the author believes, still largely in use in the United
States, but in Germany “step-grates” sloping from the furnace-doors
towards the back, are now preferred. In these the combustible material
rests upon the flat surfaces of the grate, while the air enters by the
spaces between the steps without the fuel being able to fall through.
Fig. 100 represents the furnace on this principle constructed by the
Moenus Co. of Frankfort.
[187] Detailed drawings and particulars are given in Jackson Schultz’s
‘Leather Manufacture in the United States,’ New York, 1876.
[Illustration: FIG. 99.--Huxham and Browns’ Furnace.]
The essential conditions which are to be observed in the proper burning
of the tan are a sufficiently large grate-area, a correct and sufficient
supply of air, and a combustion-chamber of very high temperature. It is
consequently not possible to burn tan very successfully in an ordinary
Lancashire or Cornish boiler, since not only the grate-space is too
limited, but the water of the boiler prevents the upper part of the
furnace from attaining a high temperature; and it is therefore difficult
to get the damp tan rapidly into vigorous combustion. The difficulty may
to some extent be overcome by mixing the tan with a proportion of coal,
and by closing the ash-pit and employing a forced draught unless the
chimney is a very powerful one. In this way large quantities of tan may
be burnt, but without effecting any great saving of coal. The heating
power of the tan is improved by the partial removal of its water by
pressing, and this is almost essential where a special furnace is not
employed.
[Illustration: FIG. 100.--Moenus Step-grate Furnace.]
The answer to the question as to whether tan should be used as fuel in
the wet state in which it is obtained from the leaches, or whether it
should be previously pressed, depends upon the nature and quantity of
the tan. Where abundant quantities of a fairly good material such as
hemlock bark are to be disposed of, the cost of pressing is an
unnecessary expenditure; but if it is desirable to obtain the highest
value from the fuel, or if the furnaces are not well constructed for
burning very wet fuels, it will be profitable to press the tan.
Hydraulic presses have been used for this purpose, but those now
commonly employed consist of powerful rollers arranged in the same way
as those of the valonia-crusher (p. 322). The pressure is given by
levers loaded with weights or fitted with powerful springs. The liquid
which runs from these presses is of little value, as it contains such
large quantities of finely divided material that it is almost impossible
to filter it, and if run upon the leaches it chokes them and prevents
their proper circulation. Much of the cost of pressing is caused by the
labour of feeding it to the press, and this may be greatly reduced by
the use of mechanical conveyors (p. 325) from the leaches. A tan press
is shown in Fig. 101.
[Illustration: FIG. 101.--Tan Press.]
_Sewage and other Waste Liquids._--The waste liquors from the different
liming, bateing, puering, tanning, washing and other soaking processes
are, without any doubt, the most troublesome of any of the side-products
which are obtained in the manufacture of leather. In former times they
were simply run into the nearest stream, but nowaday the various
sanitary authorities and other similar bodies will only permit
comparatively pure waters to be turned into a public stream or
watercourse.
Various methods of effecting the necessary purification of the waste
liquors from tanneries have been proposed at different times, and have
been used with varying degrees of success. These methods may be divided
into three heads: precipitation, followed by filtration or sedimentation
land-treatment; and bacterial purification.
The first of these depends on the power of certain substances, such as
alumina and oxide of iron, to carry down organic matter with them if
precipitated in solutions containing it. The method usually consists in
adding a sufficient quantity of lime to render the waste liquid slightly
alkaline, and then treating it with some crude salt of aluminium or of
iron. By this means a precipitate of aluminium or iron hydrate is
formed, which encloses within itself a considerable proportion of the
organic matter of the liquid, and after settling to the bottom of the
precipitation-tank is drawn off as “sludge.” Various chemicals are sold
under fancy names, such as “alumino-ferric,” “ferrozone,” etc., and have
a composition not very dissimilar to that of crude sulphate of iron or
alumina. In some cases by-products, such as the acid liquors used in
preparing iron articles for “galvanizing,” can be used with advantage.
In the case of the waste liquors from a tannery, the use of these
chemicals may often be avoided if sufficient care be taken in regulating
the proportion of the various liquids which are to be mixed together and
run into the settling tank. As tanning matter combines with lime and
dissolved hide-substance to form a heavy brown insoluble precipitate, it
is clear that if care be taken to have rather more waste lime-liquor
mixed with the waste tan-liquors than is necessary to throw all the tan
out of solution, a very considerable amount of purification of the
effluent will have taken place without any cost whatever to the tanner.
Hence, if the proportion of waste lime is small in comparison to that of
the tanning liquors, an extra addition of lime may be necessary in order
to precipitate the tannin.
The precipitation- or settling-tanks are usually square or rectangular
vessels or pits, the size of which varies with the quantity of liquid to
be treated, but the depth of which rarely exceeds six feet. They may be
divided into two classes--the “intermittent,” and the “continuous.” In
the former class the tank is filled with the mixed waste liquids, taking
care that such a sufficiency of lime is present that the mixture is
faintly alkaline to phenolphthalein paper, and is then allowed to rest
until the suspended matter has settled down to the bottom of the tank,
when the clear, or almost clear upper liquid is drawn off, the remainder
being the “sludge”; some means must also be employed to prevent the
passage of scum and floating matters. In the case of the intermittent
process it is advisable to have two tanks, one of which is being filled
while the other one is settling or being emptied. With the continuous
process the liquids are run into the tank in the proportions calculated
to give a maximum amount of purification, as described above, but as
they enter very slowly the undissolved matter soon settles, and
consequently the liquid may be continuously run out at the further end
of the tank. This plan, though it does not yield such good results in
the hands of unskilled workmen, is yet useful in many cases, as only one
tank is absolutely necessary. It is desirable that in running off the
tanks, the effluent should be taken as near the surface as possible, by
means of a hinged pipe attached to a float, or some equivalent device;
and care is required, as the tank gets low, to avoid the escape of any
of the sludge.
For continuous settling the tanks are usually long and somewhat shallow
rectangular ponds, into which the previously well-mixed precipitating
liquid flows through a wooden trough fixed across one end and as long as
the breadth of the tank, and perforated with holes to allow the uniform
and quiet influx of the liquid, which finally escapes by a similar
trough crossing the opposite end of the tank. In front of the
exit-trough a “scum board” must be placed, which is a simple plank
dipping slightly below the surface of the liquid, so as to prevent any
oil, scum or other floating matter from passing out of the tank along
with the clear effluent. Whether the intermittent or continuous system
is employed, the effluent should in most cases be afterwards passed
through a bacterial filter-bed, or treated by land filtration before it
is allowed to flow into a stream or river. Tannery effluents are usually
received into sewers without further treatment than mixing and settling
to remove solid matter, and many authorities are satisfied with the
removal of merely such coarse suspended matters as might choke the
sewers. Where continuous precipitation-tanks are used, they must be
emptied at frequent intervals, and the sludge run on to cinder-filters,
to part with most of its water. These filters are conveniently placed at
a lower level than the settling tanks, and it is generally necessary to
return the effluent from them for further precipitation and settling.
Several types of continuous settling tank with upward flow have been
devised by Mr. Candy and others, which are very suitable for use where
space is limited; but otherwise less costly constructions are often
sufficient. Apart from the question of obtaining an effluent
sufficiently good to satisfy the sanitary authority, the treatment of
the sludge is one of the greatest difficulties in the purification of
effluents. It is usually very bulky, easily putrescible, and therefore
difficult to dry; it is of little value for manure; and if allowed to
remain long wet, its smell is very offensive.
It has been mentioned that in most cases the liquid, and in every case
the sludge, must be freed from solid undissolved matter by filtration.
This may take place through open filters or through filter-presses. The
open filters generally consist of a pit with an exit at the bottom for
the filtered liquid. This pit is filled with either stones and sand,
with clinker, ashes or coke. Most tanners use clinker and ashes, as they
do not cost anything; and the material should be so arranged that while
the lowest layers are very coarse, the surface of the filter-bed should
be of the finest material. As soon as this has become covered with so
thick a layer of solid matter that the filtration proceeds too slowly,
the top surface of the filter may be removed with a rake (taking care to
remove as little of the ashes or sand as possible), and burnt, or dried
and used as manure. In some cases, filter-presses are used which are
composed of grooved or perforated plates with cloths between them
through which the liquid is forced by pressure. The solid matter remains
behind in the form of a comparatively dry “cake.” The filter-cake, dried
if desired, is sold as manure, for which it is in many ways very
suitable. Although they work much more rapidly than do the open filters,
the cloths so soon become rotten and have to be replaced, that the open
ash-filter is on the whole the most convenient for the tanner’s use. It
will be readily understood that apparatus of this kind, though very
efficient on a small scale, is quite out of the question when many
thousand gallons of liquid have to be filtered daily, and so can only
be effectively applied to “sludge.”
No system of chemical precipitation has as yet proved entirely
satisfactory. Undoubtedly a great deal of purification is effected by
this means, but in most cases the “purified” liquid is still too impure
to be turned into a stream, though for various reasons this is often
permitted by the authorities.
A great advance was made in the purification of effluents when
manufacturers were compelled by law to allow the effluent from the
precipitation-tank to filter through land set apart for that purpose. In
this case certain hardy cereals were sown on the land, which was watered
as often as possible with the effluent. This latter, after soaking
through the land, was drained off into the nearest stream. Although in
many ways this treatment was satisfactory, it had the disadvantage of
being very expensive, especially in the neighbourhood of large towns
where the price of land is high, and, in addition to this, the
conditions necessary for success were far from being correctly
understood, so that the land often became “sewage-sick” or waterlogged,
and ceased to purify the effluent. It was not until the researches of
bacteriologists proved that the purification by land-filtration was
mainly due to the bacteria in the soil, that any really satisfactory
solution of the problem could be found, but the question has now been to
a considerable extent simplified by the introduction of “bacterial
treatment.”
Bacteria, considered from the point of view of their action on organic
matter, are often classified as “anaerobic” and “aerobic,” though many
species are capable of existing under both conditions (Cp. L.I.L.B.,
Section XXIV.). The anaerobic bacteria thrive only in the absence of
air, and their chemical action consists in breaking down the organic
matter on which they feed into simpler, and generally more soluble
forms, by processes which do not involve oxidation. The aerobic
bacteria, on the other hand, require air or oxygen for their existence,
and produce changes which are generally of a less complex character, but
result in the complete oxidation and conversion of the organic matter to
simple compounds, such as nitrates and carbonic acid, which are
perfectly harmless and inoffensive. The two classes therefore are to a
large extent complementary to each other, the anaerobic bacteria
converting the animal or vegetable substances into more soluble and
simple compounds which are adapted to the needs of the aerobic, which
complete the destruction of the organic matter.
In harmony with what has just been said, bacterial treatment of sewage
is of two kinds, each of which may be used alone, or in conjunction with
a preliminary precipitation-process, but which are generally best used
successively. The oldest form of bacterial purification depends mainly
on the action of anaerobic bacteria, and is known as the “septic tank.”
This originally consisted of a tank sometimes filled with small pieces
of coke, but generally containing the liquid only, and which was tightly
closed to prevent access of air and escape of foul gases. It has,
however, been found that if deep tanks (6 to 10 feet) are employed, they
soon become in continuous use so covered with scum and floating matter
as effectually to prevent access of air and light, or any serious escape
of smell. The liquid to be purified is allowed to flow very slowly
through a tank or series of tanks of this description, entering about a
foot below the surface through a distributing trough at one end, and
flowing out similarly at the other, at such a rate as to change the
contents of the tank about once in twenty-four hours; and when the tank
is in working order, the liquid is much purified by the process, and
most of the solid organic matter has become liquefied and disappears. It
not unfrequently happens, especially where the septic tank treatment is
not very prolonged, that the liquid which escapes has a stronger and
more offensive odour than it had on entering the tank. It is
nevertheless really purer than before, the increased smell being due to
the volatile products of the partially decomposed organic matter; and,
by passing the liquid through an open coke-filter, the smell will be
effectually removed. In all cases it must be borne in mind that as
septic tanks and bacterial filters depend for their efficiency on the
organisms they contain, time must be allowed for these to develop and
accumulate before good results are obtained; and for this about six
weeks’ use is generally necessary, after which they will continue to act
for an indefinite period until they become choked by sand and inorganic
matter.
It must not be supposed that the action in the septic tank is wholly
anaerobic; and with weak sewage, most of the organic matter may under
favourable circumstances be converted into nitrates and carbonic acid
by this means only; but generally a much more complete purification is
effected by the subsequent use of “bacterial filters.” These in their
simplest form consist of tanks of about 4 feet deep, filled with coke,
broken bricks, or clinkers, and fitted with drain pipes at the bottom,
by which they can be easily emptied. These tanks, often known as
“contact-beds,” are filled with the sewage or septic tank effluent,
which is allowed to remain on them two hours, and the tank is then
emptied, and allowed a rest of six hours for oxidation and aeration. In
most cases the sewage requires two such treatments, the last often
through a bed with finer coke, in order to be completely freed from
putrescible matter. In place of the intermittent process, as applied on
the contact-beds, continuous aerobic filtration is often employed, the
bed being so constructed as to allow of free admission of air at the
bottom and sides, and the liquid to be purified being distributed on the
surface by a sprinkler, or some similar device, and allowed to trickle
through the bed. The continuous process seems likely to supersede the
intermittent one, as the beds are not only capable of treating a much
larger quantity of sewage in proportion to their area, but are also less
liable to choke. About six weeks is required, with either contact-beds
or continuous filters, before the material they contain becomes coated
with the necessary bacterial layer and they get into full working order.
The results as regards the effluent are perfectly satisfactory, and the
great difficulty and cost consists in the slow but inevitable choking of
the beds, which involves the replacement of the porous material. This is
considerably delayed by the use of a settled or precipitated sewage, and
in this respect, beside its bacteriological function, the septic tank
serves a useful purpose in settling insoluble matter, which is much more
cheaply removed from it than from the filter-beds. It will be obvious
that ordinary settling-tanks, if deep, fulfil many of the functions of
the septic-tank, and both lead to the production of a much more uniform
liquid from the different effluents which the tanner produces, which is
important in the subsequent bacterial purification. A good deal of
interesting information on these subjects will be found in a paper by
Mr. W. H. Harrison on the ‘Bacteriological Treatment of Sewage.’[188]
[188] Journ. Soc. Chem. Ind., 1900, p. 511.
There are a good many patents in connection with the various methods of
sewage purification, and some caution is necessary to avoid their
infringement, though of course the general principles of settling and
filtration, and the destruction of organic matter by bacterial action,
are open to all.
As a general rule the waste-liquors from a tan-yard or leather dye-works
are exceedingly impure. They contain the organic matter (in a state of
great putrefaction) from the soaks, bates and puers; other organic
matter, also more or less putrefied, from the tan-pits; the lime
liquors, with their large proportion of lime and of dissolved
hide-substance, and in addition the various dyes and other chemicals
which may have been used in the conversion of the raw hide into the
finished leather; and hence their efficient purification has presented
difficulties which do not occur in most other trades.
The different waste liquids are best run into a capacious tank, and,
after being thoroughly mixed up together, are allowed to settle for some
hours. By this means the greater part of the tanning matter will combine
with the lime also present to form a heavy, brown insoluble substance;
some of the dye and other organic matter will become entangled in this,
and thus be removed from the liquid. The clear liquid is next run off
into a bacterial filter (preferably a septic tank, followed by an open
coke filter), and then into the nearest stream. If the tannery is near
to a town, and the corporation sewers can be utilised, it is probable
that a filter made of spent tan may be substituted, as this material
will not only remove all excess of lime from the liquid but will also
fix much of the colouring matter as well (Koenig). The tan, after being
used for this purpose, contains so much lime in its pores that it is
said to be useful as manure.
In tanneries where large quantities of disinfectants such as mercuric
chloride, carbolic acid, etc., are used, it is necessary that the mixed
liquids shall contain so much lime as to make them distinctly alkaline.
In this way most of the disinfectants will be either precipitated or
rendered inactive. Where arsenic is used in the limes it may be
advisable to add a little ferrous sulphate (green vitriol or copperas),
in order that the arsenic may form an insoluble compound with the iron,
and so be removed along with the sludge. The ink produced by the action
of the iron salt on the tan liquors will be completely removed by the
bacterial filter.
APPENDICES.
APPENDIX A.
METHOD OF THE INTERNATIONAL ASSOCIATION OF LEATHER TRADES CHEMISTS FOR
THE ANALYSIS OF TANNING MATERIALS. INCLUDING ALTERATIONS ADOPTED AT THE
LEEDS CONFERENCE IN 1902.
SECTION I.--SAMPLING FROM BULK.[189]
[189] See London Report, pp. 22-29 and 124.
1. _Liquid Extracts._--In drawing samples, at least 5 per cent. of the
casks must be taken, the numbers being selected as far apart as
possible. The heads must be removed, and the contents mixed thoroughly
by means of a suitable plunger, care being taken that any sediment
adhering to sides or bottom shall be thoroughly stirred in. All samples
must be drawn in the presence of a responsible person.
2. _Gambier and Pasty Extracts._--Gambier and pasty extracts should be
sampled from not less than 5 per cent. of blocks, by a tubular sampling
tool, which shall be passed completely through the block in seven
places. Solid extracts shall be broken, and a sufficient number of
portions drawn both from the inner and outer parts of the blocks to
fairly represent the bulk. In both cases samples shall be rapidly mixed
and enclosed at once in an air-tight bottle or box, sealed and labelled.
3. _Valonia, Algarobilla, Divi-divi, and General Tanning Materials._
Valonia, algarobilla, and all other tanning materials containing dust or
fibre, shall be sampled, if possible, by spreading at least 5 per cent.
of the bags in layers one above another on a smooth floor, and taking
several samples vertically down to the floor. Where this cannot be done,
the samples must be drawn from the centre of a sufficient number of
bags. While valonia and most materials may be sent to the chemist
ground, it is preferable that divi-divi, algarobilla, and other fibrous
materials shall be unground. Bark in long rind, and other materials in
bundles, shall be sampled by cutting a small section from the middle of
3 per cent. of the bundles with a saw.
4. _Samples for more than one Chemist._--Samples to be submitted to more
than one chemist must be drawn as one sample, and well mixed; then
divided into the requisite number of portions, not less than three, and
at once enclosed in suitable packages, sealed and labelled.
SECTION II.--PREPARATION FOR ANALYSIS.[190]
[190] See London Report, p. 40 _et seq._
1. _Liquid Extracts._--Liquid extracts shall be thoroughly stirred and
mixed immediately before weighing, which shall be rapidly done to avoid
loss of moisture. Thick extracts, which cannot be otherwise mixed, may
be heated to 50° C., then stirred and rapidly cooled before weighing,
but the fact that this has been done must be noted in the Report.
2. _Solid Extracts._--Solid extracts shall be coarsely powdered and well
mixed. Pasty extracts shall be rapidly mixed in a mortar, and the
requisite quantity weighed out with as little exposure as possible, to
avoid loss of moisture. Where extracts are partly dry and partly pasty,
so that neither of these methods is applicable, the entire sample shall
be weighed and allowed to dry at the ordinary temperature sufficiently
to be pulverised, and shall then be weighed, and the loss of weight
taken into calculation as moisture.
In such cases as gambier, in which it is not possible to grind, or by
other mechanical means to thoroughly mix the constituents of the sample,
it is permissible to dissolve the whole, or a large portion of the
sample, in a small quantity of hot water, and immediately after thorough
mixing to weigh out a portion of the strong solution for analysis.
3. _Barks, and other Solid Tanning Materials._--The whole sample, or not
less than 250 grms., shall be ground in a mill until it will pass
through a sieve of 5 wires per centimetre. Where materials such as barks
and divi-divi contain fibrous materials which cannot be ground to
powder, the ground sample shall be sieved, and the respective parts
which do and do not pass through the sieve shall be weighed separately,
and the sample for analysis shall be weighed so as to contain like
proportions.
SECTION III.--PREPARATION OF INFUSION.
1. _Strength of Solution._--The tannin solution employed shall contain
from 0·35 to 0·45 grms. per 100 c.c. of tanning matters absorbed by
hide. (Paris 1900.)
2. _Solution of Liquid Extracts._--A sufficient quantity shall be
weighed into a covered basin or beaker, from which it shall be washed
into a liter flask with boiling water and well shaken, and the flask
shall be filled to the mark with boiling water. The neck being covered
with a small beaker, the flask shall be placed under a cold water tap or
otherwise rapidly cooled to a temperature between 15° and 20° C., and
made up accurately to the mark, after which it shall be thoroughly
mixed, and the filtration at once proceeded with.
_Note._--Tannin infusions may be kept from fermenting by the addition of
3 to 5 drops of essential oil of mustard per liter. (F. Kathreiner.)
3. _Filtration._--The filtration of the solution for analysis may take
place through any paper which may be considered most suitable for the
particular case, and with or without the use of kaolin, absorption of
tanning matter, if any, being corrected for by an amount determined by a
similar filtration of a clear solution. Perfectly clear solutions need
not be filtered.
To determine the correction, about 500 c.c. of the tanning solution of
the strength prescribed for analysis is obtained perfectly clear,
preferably by the method of filtration which is to be corrected for.
After thorough mixing, 50 c.c. is evaporated to determine “total soluble
No. 1.” A portion of the remainder is then filtered in the manner for
which correction is to be made, and 50 c.c. of the filtrate is
evaporated for “total soluble No. 2.” Deducting No. 2 from No. 1 the
difference is the correction required, which must be added to the total
soluble found by analysis. It is generally advisable, both in analysis
and in the second filtration for correction, to filter first 150 c.c.
(which in analysis may be used for the determination of non-tannins),
and then to employ the next 50 c.c. for evaporation, keeping the filter
full during the operation; but whatever procedure is adopted must be
rigidly adhered to in all analyses to which the correction is applied.
Where kaolin is employed, a constant weighed quantity (1 or 2 grm.) must
be used, which is first washed with 75 c.c. of the liquor by
decantation, and then washed on to the filter with a further quantity of
liquor, of which 200 c.c. is filtered as above.[191]
[191] It is obvious that in the first instance it will be necessary to
determine the correction for each particular material employed, but it
will soon be found that the correction is practically constant for
large groups of tanning materials, so long as the same method of
filtration is rigidly adhered to.
4. _Solid Extracts._--Solid extracts shall be dissolved by stirring in a
beaker with boiling water, the undissolved portions being allowed to
settle, and treated with further quantities of boiling water, and the
solution poured into a liter flask. After the whole of the soluble
matter is dissolved, the solution is treated similarly to that of a
liquid extract.
5. _Extraction of Solid Materials._--Such quantities shall be weighed as
will give an infusion of the strength already prescribed. (_Preparation
of Infusion_, Resolution 1.) Not less than 500 c.c. of the infusion
shall be extracted at a temperature not exceeding 50° C., after which
the temperature shall be gradually raised to 100° C.,[192] and the
extraction continued till the percolate is free from tannin and the
whole made up to one liter, the weaker portions of the solution being
first concentrated if necessary by evaporation in a flask, in the neck
of which a funnel is placed.
[192] In substances which, like canaigre, contain a large quantity of
starch, the extraction should be completed at a temperature of 50°
C.--H. R. P.
SECTION IV.--DETERMINATION OF TANNING MATTERS AND NON-TANNINS; ETC.
1. _Total Soluble Matter._--100 c.c.[193] of the clear filtered tanning
solution, or a smaller quantity if the balance employed is of sufficient
delicacy, shall be evaporated in an open weighed basin of platinum, hard
glass, porcelain, or nickel, on the water-bath, and the basin shall
afterwards be dried till constant in an air-oven, at a temperature of
100° to 105° C., or at a temperature not exceeding 100° C. _in vacuo_
till constant, care being taken that no loss occurs by splintering of
the residue. The use of the vacuum-oven for drying the residues is
recommended when possible.
[193] 50 c.c. is sufficient, and is the quantity now generally
employed.
2. _Determination of Non-Tannins._--That the filter method shall remain
the official method until the next Conference, but that members be
permitted to employ the chromed hide-powder method of the American
Association of Official Agricultural Chemists of 1901 (Appendix C) where
it is desired, the fact being clearly stated on the report that the
A.O.A.C. method has been employed, and not that of the I.A.L.T.C.
(Leeds, 1902, see note, p. 480.)
That the “bell form”[194] of filter, as described by Professor Procter,
shall be employed; not less than 5 grms. of hide-powder be used; the
hide-powder should be so packed in the tube that the detannised liquor
shall come over at a rate of about one drop in two seconds; and the
filtrate be rejected so long as it gives a turbidity with a clear
tanning solution. The filtrate may be used for the determination of
non-tannin so long as it gives no reaction with salted gelatine
solution.[195] The first 30 to 35 c.c. should be thrown away, and the
next 50 c.c. of detannised solution, or an aliquot part of it,
evaporated in a weighed basin on a water bath, and then dried till
constant in an air-oven at a temperature of 100° to 105° C., or, _in
vacuo_, not exceeding 100° C.
[194] It is obvious that the exact form and dimensions of the filter
must be adapted to the character of the hide-powder available, as
considerable differences exist in the absorptive power of different
samples.
[195] 8 to 9 grams of good gelatine are dissolved in 500 c.c. of hot
water, 100 grams of salt added, and the whole cooled and filtered.
3. _Hide-Powder._--That the hide-powder must be sufficiently absorbent
for use in the filter, and that in a blank experiment conducted with
distilled water in the same way as an analysis, the residue from the
evaporation of 50 c.c. should not exceed 5 milligrams.
The Freiberg Hide-Powder, made by Mehner and Stransky, containing
between 10 and 20 per cent. of cellulose (as suggested by Cerych), is
recommended by the Conference (Liège, 1901) and is very suitable for the
filter method; but the powder, when analysed by the Kjeldahl method and
calculated to 18 per cent. of moisture, must not contain less than 11·5
per cent. of nitrogen (Leeds, 1902).
4. _Determination of Moisture and “Total Dry Matter.”_--That the
moisture in the sample be determined by drying a small portion at the
temperature adopted in the determination of the “total soluble.” In
extracts yielding turbid solutions which can be thoroughly mixed, it is
generally preferable after mixing the solution and before filtration, to
measure off and evaporate 50 c.c. for the determination of total dry
matter (and moisture) in the same manner as the “total soluble.”
5. _Statement of Results._--It is recommended, when full analysis is
given, that the Statement should be made in the following manner:--
(1) _Tanning Matters Absorbed by Hide._--Obtained by deducting the
“soluble non-tanning matters” found by evaporating the hide-powder
filtrate from the “total soluble.”
(2) _Soluble Non-Tanning Matters._--Found by evaporation of filtrate
from hide-powder filter.
(3) _Insoluble._--By deducting “total soluble matter” from the “total
dry matter.”
(4) _Moisture._--Determined by drying a portion at the temperature
adopted in the determination of “total soluble.”
If other determinations are given they shall form a separate additional
statement.
_Density._--The statement of densities of extracts, etc., should be
given as specific gravity in preference to arbitrary degrees, such as
Baumé, Twaddell, etc.
SECTION V.--COLOUR MEASUREMENT.
_Colour Measurement._--It is recommended that the method used by English
chemists, namely, measuring with Lovibond’s Tintometer (as described by
Professor Procter and Dr. Parker, Journ. Soc. Chem. Ind., 1895, 125),
shall be used, and the results stated in units of red, yellow and
black. The measurement may be made on the solution used for analysis,
but must be calculated to one containing 0·5 per cent. of tanning
matter, in a centimetre cell.
ANALYSIS OF USED LIQUORS.
It was decided at Liège, 1901, and Leeds, 1902,[196] that the “Shake
Method” with chromed hide-powder, of the American Association of
Official Agricultural Chemists, 1901 (A.O.A.C.), should be employed in
the detannisation of used tanning liquors, as with these the filter
method is apt to give too high results owing to the amount of
non-volatile acids which they contain. The method of the A.O.A.C. is
given in Appendix C.
[196] Procter and Blockey quoted experiments at the Leeds Conference,
proving that gallic acid and some other non-tanning substances were
largely absorbed by the hide-powder filter, though probably not
permanently retained by leather; while the error, though still
considerable, was much less when the chromed hide-power shake method
was employed. Where only gallotannic and gallic acids are present, as
in the case of sumach and commercial gallotannic acids, the most
accurate quantitative estimation is probably that by the Löwenthal
method carried out as described L.I.L.B., p. 123, but considerable
skill is required in its execution.
ANALYSIS OF SPENT TANS.
It was decided at Leeds, 1902, that spent tans must be analysed like
fresh tanning materials; but where the prescribed strength of solution
cannot otherwise be obtained it is permissible to concentrate the entire
solution by evaporation. It is advisable, where suitable apparatus is
available, to concentrate _in vacuo_; but failing this, an ordinary
flask may be used, in the neck of which a funnel is placed.
APPENDIX B.--THE DECIMAL SYSTEM.
The metrical system of weights and measures, and the Centigrade
thermometer scale have been generally used throughout the book, as more
international and scientific than the complicated systems still
unfortunately in use in this country. They have been fully explained in
the Author’s ‘Laboratory Book,’ p. 2; but as this is not always at hand,
a short sketch may be permitted here.
The basis of the metrical system is the “meter,” which is approximately
¹⁄₁₀,₀₀₀,₀₀₀ of the distance from the earth’s pole to the equator, and
is equal to 39·3708 English inches, and for many practical purposes may
be roughly reckoned as 40 inches. The meter is divided into 10 parts or
“decimeters,” 100 parts or “centimeters,” and 1000 parts or
“millimeters.” The standard of capacity is a cube of 1 decimeter, or
about 4 inches, and consequently contains 1000 cubic centimeters, and is
denominated a “liter.” The standard of weight is 1 cubic centimeter of
water (at 4° C.), which is called a “gram.” Hence 1 liter of water
weighs 1 “kilogram,” or 1000 grams. 1 cubic meter of water contains 1000
liters, and weighs 1000 kilograms, or 1 metrical ton (2200 lb. English).
For purposes of reduction, the following figures may be given:--
1 gram = 15·431 grains. 1 lb. av. = 453·6 grams.
1 liter = 0·22 gallon. 1 gallon = 4·543 litres.
Actual reduction is, however, generally unnecessary if the question be
treated as one of proportion. Thus a solution of 1 gram per liter is of
the same strength as one of 1 lb. per 100 gallons (1000 lb.), and very
approximately, as one of 1 oz. avoirdupois per cubic foot. In the case
of pits, it is often simplest to measure them directly with a meter
rule; length, breadth and depth, measured in decimeters and multiplied
together, giving the contents in liters, and, in the case of water, the
weight in kilograms.
The Centigrade or Celsius thermometer divides the difference between the
freezing and the boiling points of water into 100°. The following table
gives the points at which its scale agrees without fractions with that
of Fahrenheit:
COMPARISON OF CENTIGRADE AND FAHRENHEIT DEGREES.
°C. °F.
-20 -4
-15 +5
-10 14
-5 23
0 32
5 41
10 50
15 59
20 68
25 77
30 86
35 95
40 104
45 113
50 122
55 131
60 140
65 149
70 158
75 167
80 176
85 185
90 194
95 203
100 212
105 221
110 230
115 239
APPENDIX C.
OFFICIAL METHOD FOR ANALYSIS OF TANNING MATERIALS, ADOPTED AT THE
EIGHTEENTH CONVENTION OF THE AMERICAN ASSOCIATION OF OFFICIAL
AGRICULTURAL CHEMISTS, 1901.
I. PREPARATION OF SAMPLE.
Barks, woods, leaves, dry extracts, and similar tanning materials should
be ground to such a degree of fineness that they can be thoroughly
extracted. Fluid extracts must be heated to 50° C., well shaken, and
allowed to cool to room-temperature.
II. QUANTITY OF MATERIAL.
In the case of bark and similar material, use such quantity as will give
about 0·35 to 0·45 gram tannins per 100 c.c. of solution, extract in
Soxhlet or similar apparatus at steam-heat for non-starchy materials.
For canaigre and substances containing like amounts of starch use
temperature of 50° to 55° C., until near complete extraction, finishing
the operation at steam-heat. In case of extract, weigh such quantity as
will give 0·35 to 0·45 gram tannins per 100 c.c. of solution, dissolve
in 900 c.c. of water at 80° C., let stand twelve hours, and make up to
1000 c.c.
III. MOISTURE.
(_a_) Place 2 grams, if it be an extract, in a flat-bottom dish, not
less than 6 cm. in diameter, add 25 c.c. of water, warm slowly till
dissolved, continue evaporation and dry.
(_b_) All dryings called for, after evaporation to dryness on
water-bath, or others, shall be done by one of the following methods,
the soluble solids and non-tannins being dried under similar, and so far
as possible identical conditions:
1. For eight hours at the temperature of boiling water in a steam bath.
2. For six hours at 100° C., in an air bath.
3. To constant weight _in vacuo_ at 70° C.
IV. TOTAL SOLIDS.
Shake the solution, and without filtering immediately measure out 100
c.c. with a pipette, evaporate in a weighed dish, and dry to constant
weight, at the temperature of boiling water. Dishes should be
flat-bottomed, and not less than 6 cm. in diameter.
V. SOLUBLE SOLIDS.
Double-pleated filter paper (S. and S., No. 590, 15 cm.) shall be used.
To 2 grams of kaolin add 75 c.c. of the tanning solution, stir, let
stand fifteen minutes, and decant as much as possible. Add 75 c.c. more
of the solution, pour on filter, keep filter full, reject the first 150
c.c. of filtrate, evaporate the next 100 c.c. and dry. Evaporation
during filtration must be guarded against.
VI. NON-TANNINS.
Prepare 20 grams of hide-powder by digesting twenty-four hours with 500
c.c. of water, and adding 0·6 gram chrome alum in solution, this
solution to be added as follows. One-half at the beginning and the other
half at least six hours before the end of the digestion. Wash by
squeezing through linen, continue the washing until the wash-water does
not give a precipitate with barium chloride. Squeeze thoroughly by hand,
and remove as much water as possible by means of a press, weigh the
pressed hide, and take approximately one-fourth of it for moisture
determination. Weigh this fourth carefully and dry to constant weight.
Weigh the remaining three-fourths carefully and add them to 200 c.c. of
the original solution; shake ten minutes, throw on funnel with cotton
plug in stem, return until clear, evaporate 100 c.c. and dry. The weight
of this residue must be corrected for the dilution caused by the water
contained in the pressed hide-powder.[197] The shaking must be done in
some form of mechanical shaker. The simple machine used by druggists,
and known as the milk-shake, is recommended.
[197] For method of correction, see p. 313.
PROVISIONAL METHOD.--To 14 grams of dry chromed hide-powder in a shaker
glass add 200 c.c. of the tannin solution, let stand two hours, stirring
frequently, shake fifteen minutes, throw on funnel with a cotton plug in
the stem, let drain, tamp down the hide-powder in the funnel, return the
filtrate until clear and evaporate 100 c.c.
VII. TANNINS.
The amount of these is shown by the difference between the soluble
solids and the corrected non-tannins.
VIII. TESTING HIDE-POWDER.
(_a_) Shake 10 grms. of hide-powder with 250 c.c. of water for five
minutes, strain through linen, squeeze the magma thoroughly by hand;
repeat this operation three times, pass the last filtrate through paper
(S. and S. No. 590, 15 cm.) till clear, evaporate 100 c.c. and dry. If
this residue amounts to more than 10 mg. the hide must be rejected.
(_b_) Prepare a solution of pure gallo-tannin by dissolving 6 grams in
1000 c.c. of water. Determine the total solids by evaporating 100 c.c.
of this solution and drying to constant weight. Treat 200 c.c. of the
solution with hide-powder exactly as described in paragraph 6. The
hide-powder must absorb at least 95 per cent. of the total solids
present. The gallo-tannin used must be completely soluble in water,
alcohol, acetone and acetic ether, and should not contain more than 1
per cent. of substances not removed by digesting with excess of yellow
mercuric oxide on steam-bath for two hours.
IX. TESTING NON-TANNIN FILTRATE.
(_a_) _For Tannin._--Test a small portion of the clear non-tannin
filtrate with a few drops of a 1 per cent. solution of Nelson’s gelatin.
A cloudiness indicates the presence of tannin, in which case repeat the
process described under VI., using 35 instead of 20 grams of
hide-powder.
(_b_) _For Soluble Hide._--To a small portion of the clear non-tannin
filtrate add a few drops of the filtered tannin solution. A cloudiness
indicates the presence of soluble hide, in which case repeat the process
described under VI., giving the hide-powder a more thorough washing.
The temperature of solutions shall be between 16° and 20° when measured
or filtered. All dryings should be made in flat-bottomed dishes of at
least 6 cm. diameter, S. and S. No. 590, 15 cm. filter paper should be
used in all filtrations.
APPENDIX D.
The following Lists of Colours have been furnished by Mr. M. C. Lamb,
Director of the Leather Dyeing Department of Herold’s Institute, London,
who has devoted much time to testing the various dyes with regard to
their permanence and suitability for leather. Many of the colours have
also been tested and found satisfactory in the Leather Department of the
Yorkshire College. The following abbreviations of makers’ names are used
in the lists:--
B. BASLER CHEMISCHE FABRIK, A. G. Basle, Switzerland.
B.A.S.F. BADISCHE ANILIN UND SODA FABRIK. Ludwigshafen a. Rhine,
Germany.
Ber. BERLIN ANILINE CO. Berlin S.O., Germany.
B.S. Spl. BROOKE, SIMPSON & SPILLER. Atlas Dye Works, Hackney Wick,
London, N.E.
By. FARBEN-FABRIKEN, late BAYER & CO. Elberfeld, Germany.
C. L. CASSELLA & CO. Frankfort a. Main, Germany.
C.A. FRENCH ANILINE COLOUR WORKS. Vieux-Conde (Norde), France.
C. & R. CLAUS & RÉE. Clayton, near Manchester.
D. DAHL & CO. Barmen, Germany.
D. & H. DURAND, HUGUENIN & CO. Basle, Switzerland.
G. R. GEIGY & CO. Basle, Switzerland.
Ger. GERBER & CO. Basle, Switzerland.
K. KALLE & CO. Bierbrich a. Rhine, Germany.
Leon. A. LEONHARDT & CO. Muhlheim a. Main, Germany.
Leitch J. W. LEITCH. Milnsbridge Chemical Works, Huddersfield.
Lev. LEVINSTEIN LTD. 21 Minshull Street, Manchester.
M.L.B. MEISTER, LUCIUS & BRUNING. Hoechst a. Main, Germany.
Mo. GILLIARD, P. MONNET & GARTIER. Lyons, France.
N. NOETZEL, ISTEL & CO. Griesheim a. Main, Germany.
O. K. OEHLER & CO. Offenbach a. Main, Germany.
P. ST. DENIS DYESTUFF CO., late POURIER. St. Denis, near
Paris.
R. SOCIÉTE CHIMIQUE DES USINES DU RHONE. Lyons, France.
R. H. & S. READ, HOLLIDAY & SONS. Huddersfield.
S.C.Ind. SOCIETY OF CHEMICAL INDUSTRY. Basle, Switzerland.
Uer. CHEMISCHE FABRIKEN. Uerdingen a. Rhein, Germany.
W. Bros. WILLIAMS BROS. & CO. Hounslow, Middlesex.
STAINING.
SINGLE ACID DYES SUITABLE FOR STAINING VEGETABLE TANNED LEATHER.
_Browns._
Solid brown. (M.L.B.)
Acid brown. (W. Bros.)
Brown A2. (B.S. Spl.)
Brown A1. (B.S. Spl.)
Mikado brown B. (Leon.)
New acid brown. (B.S. Spl.)
Bronze acid brown. (By.)
Golden brown Y. (C.), (By.)
Acid anthracene brown R. (By.)
Fast brown N. (B.A.S.F.)
Nut brown A. (C.)
Fast brown. (By.)
Fast brown G. (Ber.)
Resorcin brown. (Ber.)
Resorcin brown. (D.)
Acid brown. (Ber.)
Dark nut brown. (W. Bros.)
Acid brown R. (C.)
Acid brown R. (Uer.)
Acid brown R. (R. H. & S.)
New golden brown A1. (C.)
Dark brown. (C.)
Acid brown L. (B.A.S.F.)
Acid brown D. (C.)
_Yellows._
Azo yellow. (Uer.)
Phosphine subst. (B.S. Spl.)
Chrysoine. (W. Bros.)
Azo-acid-yellow. (Ber.)
New phosphine G. (C.)
Cuba yellow 2072. (S.C. Ind.)
Cuba yellow (W. Bros.)
Azo-flavine RS. (C.) and (B.A.S.F.)
Azo-flavine 3R. (B.A.S.F.)
Indian yellow R. (By.)
Turmeric yellow. (G.)
Solid yellow G. (Leon.)
Solid yellow B. (Leon.)
Indian yellow R. (C.)
Cuba yellow. (C.)
Napthol yellow S. (By.), (C.), (B.A.S.F.)
Turmeric yellow. (C.), (G.)
Fast acid yellow. (C.A.)
_Reds and Oranges._
Scarlet R. (By.)
Crocein scarlet 3BN. (By.)
Orange 2. (M.L.B.), (S.C. Ind.), (C.) and (B.A.S.F.)
Orange 2B. (By.)
Mandarin G extra. (Ber.)
Brill. crocein M.O.O. (C.)
Bordeaux G. (By.)
Atlas orange. (B.S. Spl.)
Bordeaux cov. (Ber.)
Fast red 21528. (By.)
Fast red A. (Leon.), (By.), (Ber.), and (B.A.S.F.)
Bordeaux B. (M.L.B.)
_Greens._
Acid green extra conc. (C.)
Guinea green B. (Ber.)
Guinea green G. (Ber.)
Acid green GG. (By.)
Acid green BB. (By.)
Acid green B. (By.)
Acid green G. (By.)
Acid green 000. (Leon.)
Acid green extra. (By.)
Acid green (Uer.)
Acid green (R. H. & S.)
Light green SF. (B.A.S.F.)
Erioglaucine. (G.)
_Violets._
Acid violet 4RS. (Ber.)
Acid violet 7B. (Ber.)
Acid violet 6B. (By.)
Formyl violet S4B. (C.)
_Blues._
Bavarian blue D.B. (Ber.)
Marine blue o. (K.)
Solid blue. (M.L.B.)
Blue 1. (Lev.)
Blue 2. (Lev.)
Blue 3. (Lev.)
SINGLE BASIC DYES SUITABLE FOR STAINING VEGETABLE TANNED LEATHER.
_Browns._
Bismark brown GG. (C.)
Chrysoidine AG. (O.)
Bismark brown 2B. (K.)
Bismark brown (By.)
Bismark brown R.C.E. (Lev.)
Bismark brown M. (By.)
Vesuvine conc. (M.L.B.)
Vesuvine conc. (B.A.S.F.)
Bismark brown C extra. (Leon.)
Bismark brown RS. (B.S. Spl.)
Bismark brown 3762. (W. Bros.)
Rheonine A. (B.A.S.F.)
Rheonine N. (B.A.S.F.)
Brown R. (G.)
Brown G. (G.)
Manchester brown. (C.)
_Yellows._
Acridine yellow NC. (Leon.)
Phosphine N. (Ber.)
Patent phosphine R. (S.C. Ind.)
Leather yellow 6730. (C.A.)
Auramine 2. (By.)
Chrysoidine cryst. (B.S. Spl.) and (By.)
Chrysoidine diamond cryst. (W. Bros.)
Leather yellow o. (M.L.B.)
Chrysoidine. (R. H. & S.)
Leather yellow G. (M.L.B.)
Leather yellow 6730. (C.A.)
Patent phosphine G. (S.C. Ind.)
Leather yellow DRR. (Ber.)
Xanthine. (O.)
Cannella G. (W. Bros.)
Pure phosphine. (C.)
New phosphine G. (C.)
Cori-phosphine o. (By.)
Para-phosphine R. (C.)
Para-phosphine G. (C.)
Leather yellow 374. (D.)
Leather yellow 375. (D.)
Homo-phosphine G. (Leon.)
Phosphine ABN. (Leon.)
Auramine 2 patent. (S.C. Ind.)
_Greens._
Methyl green cryst. (Ber.)
Methylene green. (M.L.B.)
Solid green. (Leon.)
Malachite green. (Ber.), (M.L.B.), (P.), (C.A.), (S.C. Ind.), (R. H.
& S.), (Lev.), (C.), (B.S. Spl.) and (K.)
_Reds._
Safranine. (M.L.B.), (B.A.S.F.), (S.C. Ind.) and (K.), (Ber.), (By.),
(C.A.), (Leon.), (Uer.)
Russian Red. (By.) and (Ber.)
_Violets._
Methyl violets. (Ber.), (By.), (M.L.B.), (R. H. & S.), (B.S. Spl.),
(C.) (S.C. Ind.), (P.) and (D.).
_Blacks._
Corvoline B. (B.A.S.F.)
Corvoline G. (B.A.S.F.)
DYEING.
SINGLE ACID COLOURS SUITABLE FOR DYEING VEGETABLE TANNED LEATHERS.[198]
[198] For explanation of Roman numerals see end of Appendix D.
_Yellows._
II. Napthol yellow S. (Ber.), (B.A.S.F.), (By.) and (C.).
VII. Quinoline yellow. (Ber.), (By.) and (B.A.S.F.).
II. Citronine. (Leon.)
IV. Solid yellow G. (Leon.)
IV. Solid yellow B. (Leon.)
V. Indian yellow S.
V. Azo-acid yellow.
IV. Indian yellow T. (C.)
VII. Indian yellow R. (By.) and (C.).
IV. Indian yellow G. (By.) and (C.).
IV. Cuba yellow. (C.), (W. Bros.) and (S.C.Ind.).
V. Azo-flavine RS. (B.A.S.F.) and (C.).
V. Azo-flavine 3R. (B.A.S.F.) and (C.).
VI. Circumein extra. (Ber.)
VII. Tartrazine. (B.A.S.F.)
_Oranges._
V. Orange 2. (B.A.S.F.), (C.), (M.L.B.), (S.C. Ind.), (P.) (W.
Bros.) and (By.).
V. Mandarin G extra. (Ber.)
V. Crocein orange. (K.) and (By.).
VI. Ponceaux 10RB, 4R, Bo, 4RB, 6RB. (Ber.), (By.) and (D.).
_Bordeaux._
VII. Azo bordeaux. (By.)
IV. Bordeaux B extra. (By.)
VI. Bordeaux G. (By.)
V. Bordeaux Y. (W. Bros.)
V. Acid maroon. (M.L.B.) and (B.S. Spl.).
VIII. Chromatrop 6B. (M.L.B.)
_Reds._
V. Fast red A. (Ber.), (By.), (B.A.S.F.), (B.S. Spl.) and (Leon.)
VIII. Fast red S. (M.L.B.)
VI. Fast red 21528. (By.)
_Scarlets._
V. Crocein scarlet R. (By.) and (K.).
V. Crocein scarlet 2R. (By.)
VII. Fast scarlet B. (B.A.S.F.), (W. Bros.) and (K.)
_Browns._
IV. Acid brown R. (C.)
V. Acid brown L. (B.A.S.F.)
V. Acid brown Y. (S.C. Ind.)
IV. Acid brown D. (C.), (B.A.S.F.)
IV. Acid brown (R.H. & S.)
IV. Acid brown 4601. (B.S. Spl.)
V. Acid brown D. (C.)
VII. Resorcin brown. (Ber.)
V. Acid brown. (Uer.)
IV. Acid brown R. (Ber.)
VIII. Acid brown Y. (M.L.B.).
VI. Solid brown o. (M.L.B.)
V. Fast brown. (By.)
V. Fast brown. G. (Ber.)
V. Fast brown. N. (B.A.S.F.)
IV. Fast brown. 3B. (Ber.)
V. Bronze acid brown. (By.)
VIII. Acid anthracine brown R. (By.)
V. New acid brown. (B.S. Spl.)
VI. Dark nut brown. (Uer.)
IV. New golden brown A1. (C.)
_Blacks._
IV. Napthol blue black. (C.)
V. Napthylamine black 4B. (C.)
V. Napthylamine black 6B. (C.)
VII. Phenol black S. (By.)
IV. Phenylamine black 4B. (By.)
VII. Victoria black B. (By.)
_Blues._
VIII. Fast blue R. (Ber.)
VIII. Bavarian blue DB. (Ber.)
V. Erioglaucine. (G.)
IV. Cyanole ext. (C.)
IV. Marine blue. (K.)
VII. Water blue N. (B.A.S.F.)
VIII. Water blue 4 B. (Ber.)
VII. Cotton blue II. (By.).
VII. Toluidine blue. (B.A.S.F.) and (By.).
VII. Water blue R. (Leon.)
VII. Water blue 3R. (Leon.)
VII. Water blue BTR. (B.A.S.F.)
_Violets._
Acid violets (Lev.), (B.A.S.F.) and (By.)
IX. Acid violets 4R. (B.A.S.F.)
V. Acid violets R. (C.)
V. Acid violets R. (B.A.S.F.)
VI. Acid violets 3BA. (M.L.B.)
IV. Acid violets 3BN. (Lev.)
II. Acid violets 6B. (By.) and (C.).
III. Formyl violet S4B. (C.)
_Greens._
IV. Acid green extra conc. (C.)
IV. Guinea green B and G. (Ber.)
IV. Acid green ext. (By.)
IV. Acid green GG ext. (By.)
IV. Acid green 225. (By.)
IV. Acid green BB. ext. (By.)
IV. Acid green o. (M.L.B.)
IV. Acid green 5677. (B.S. Spl.)
V. Capri green 2G. (Lev.)
SINGLE BASIC DYES SUITABLE FOR DYEING VEGETABLE TANNED LEATHERS.
_Browns._
IV. Vesuvine ooo ext. (B.A.S.F.)
II. Vesuvine B. (B.A.S.F.)
II. Vesuvine (C.)
III. Vesuvine conc. (M.L.B.)
III. Bismark brown ext. (Ber.) and (B.S. Spl.).
III. Bismark brown ext. M. (By.)
III. Bismark brown F. (By.)
IV. Bismark brown YS. (B.S. Spl.)
III. Bismark brown PS. (C.)
III. Bismark brown GG. (C.)
III. Bismark brown O. (L.)
III. Bismark brown G. (O.)
III. Bismark brown (S.C. Ind.)
III. Bismark brown NYY. (W. Bros.)
III. Bismark brown o. (M.L.B.)
II. Cannella. (B.S. Spl.)
II. Cannella. (B.A.S.F.)
II. Cannella. (C.)
V. Cannella. (S.C. Ind.)
II. Cannella S. (Ber.)
III. Cannella P. (W.)
IV. Nanking. (B.A.S.F.)
IV. Nanking. (R.H. & S.)
IV. Nanking. (S.C. Ind.)
III. Lavilliere’s 122. (By.)
II. Rheonine. (B.A.S.F.)
IV. Xanthine. (O.)
_Yellowish Oranges._
III. Chrysoidines, (Leitch); R, (R.H. & S.)
IV. Chrysoidines ext. (W.).
II. Chrysoidines (S.C.Ind.); GG,(C.)
III. Chrysoidines G. (Leon.)
II. Chrysoidines RE. (Lev.)
III. Chrysoidines YY. (C.)
III. Chrysoidines cryst. (B.S. Spl.)
III. Chrysoidines G. (By.)
V. Chrysoidines cryst. (C.A.)
_Yellows._
III. Auramine 2. (B.A.S.F.)
III. Auramine. (S.C. Ind.)
III. Auramine. (G.)
III. Auramine. (Ber.)
III. Auramine. (By.)
III. Auramine. (L.)
III. Auramine. (W.)
III. Auramine. (C.)
III. Auramine. (W.)
V. Auramine conc. (M.L.B.).
IV. Phosphine E. (B.A.S.F.)
IV. Phosphine L. (B.A.S.F.)
IV. Phosphine G. (Ber.)
IV. Phosphine. (O.)
IV. Phosphine. (C.)
IV. Phosphine Ext. (M.L.B.)
IV. Phosphine B ext. (S.C. Ind.)
III. Phosphine III., II., I. (Leon.)
III. Phosphine N. (Ber.).
V. Cori-phosphine. (By.)
V. Homo-phosphine. (Leon.)
V. Para-phosphine. (C.)
_Greens._
III. Methyl green cryst. (By.) D.
V. Methylene green o. (M.L.B.)
II. Diamond green B and G. (B.A.S.F.)
II. Benzal green. (O.)
II. Brillt. green cryst. (M.L.B.)
II. Brillt. green cryst. (By.)
II. Brillt. green cryst. (O.)
II. Brillt. green cryst. (L.)
II. Brillt. green cryst. (Lev.)
II. Brillt. green cryst. (Uer.)
II. Brillt. green cryst. (S.C. Ind.)
II. Malachite green. (B.S. Spl.)
II. Malachite green. (Ber.)
II. Malachite green. (C.A.)
II. Malachite green. (K.)
II. Malachite green. (M.L.B.)
II. Malachite green. (Lev.)
II. Malachite green. (G.)
II. Malachite green. (O.)
_Blues._
VII. Methylene blue B, 2B and R. (Ber.)
VII. Methylene blue. (B.A.S.F.)
VII. Methylene blue. (M.L.B.)
VII. Methylene blue. (Lev.)
VII. Methylene blue. (C.)
VII. Methylene blue. (C. & R.)
VIII. New methylene blue. GG. (C.)
VIII. New methylene blue. BB. (C.)
IV. New blue R. (Ber.)
V. New blue R. (By.)
VI. New patent blue 4B. (By.)
_Violets._
IV. Methyl Violet 4B. (B.A.S.F.)
IV. Methyl Violet 4R. (K.)
IV. Methyl Violet 4R. (C.)
IV. Methyl Violet 3B. (By.)
IV. Methyl Violet 3B. (Ber.)
IV. Methyl Violet 2B. (M.L.B.)
IV. Methyl Violet (D.)
IV. Methyl Violet 6B. (Leon.)
IV. Neutral violet ext. (C.)
_Bordeaux._
IV. Magenta WB. (Leon.)
IV. Magenta 3B. (Ber.)
IV. Magenta RE. (Leon.)
IV. Magenta WBG. (Leon.)
IV. Magenta. (M.L.B.)
IV. Magenta. (K.)
IV. Magenta. (B.A.S.F.)
IV. Magenta 4128. (B.S. Spl.)
_Reds._
VIII. Rhodamine B extra. (Ber.)
VIII. Rhodamine B. (B.A.S.F.)
VIII. Rhodamine B. (By.)
VIII. Rhodamine (S.C. Ind.)
VIII. Rhodamine (M.L.B.)
VII. Safranine. (B.A.S.F.)
IV. Russian red G. (B.A.S.F.).
IV. Russian red B. (C.)
IV. Russian red (Ber.)
IV. Russian red (Uer.)
IV. Russian red B. (B.A.S.F.)
IV. Russian red G. (C.)
IV. Russian red (Ber.)
IV. Russian red R. (By.)
IV. Cardinal 4B. (By.)
VIII. Rhoduline red. (By.)
V. Safranine G ext. (C.)
VII. Safranine BS. (By.)
Safranine G ext. (Ber.)
ACID MIXTURES SUITABLE FOR DYEING AND STAINING VEGETABLE TANNED
LEATHERS.
Orange 2. (M.L.B.)
Azo-yellow o. (M.L.B.)
Patent blue V. (M.L.B.)
Resorcin brown. (Ber.)
Circumein ext. (Ber.)
Nigrosine 105. (Ber.)
Acid brown R. (C.)
Indian yellow G. (C.)
Pure soluble blue. (C.)
New acid brown. (B.S. Spl.)
Phosphine subst. (B.S. Spl.)
Induline. (B.S. Spl.)
Acid brown R. (C.)
Azo-flavine R.S. (C.)
Naphtol blue black. (C.)
Resorcin brown, (Ber.)
Fast brown G. (Ber.)
Napthylamine black D. (C.)
Fast brown G. (Ber.)
Circumine ext. (Ber.)
Nigrosine 105. (Ber.)
Fast brown. (By.)
Indian yellow R. (By.)
Fast green blue shade. (By.)
Acid anthracene brown. (By.)
Indian yellow R. (By.)
Fast green blue shade. (By.)
Fast brown N. (B.A.S.F.)
Azo-flavine RS. (B.A.S.F.)
Light green S.F. (B.A.S.F.)
Dark nut brown. (Uer.)
Azo-yellow. (Uer.)
Acid green. (Uer.)
Acid brown. (D.)
Crocein orange. (D.)
Cotton blue 3R. (D.)
Resorcin brown. (D.)
Cotton blue 3R. (D.)
Acid brown B. (S.C. Ind.)
Cuba yellow 2072. (S.C. Ind.)
Acid green. (S.C. Ind.)
Resorcin brown. (W. Bros.)
Cuba yellow. (W. Bros.)
Acid green. (W. Bros.)
Napthol brown. (Leon.)
Citronine A. (Leon.)
Acid green 000. (Leon.)
Acid brown R. (R.H. & S.)
Acid yellow. (R.H. & S.)
Nigrosine cryst. (R.H. & S.)
Orange 2. (P.)
Yellow oS. (P.)
Acid green J3E. (P.)
Acid brown. (C.A.)
Acid yellow S. (C.A.)
Pure blue cryst. (C.A.)
Resorcin brown. (Ber.)
Azo-acid-yellow or Circumine ext. (Ber.)
Bavarian blue DB, or Guinea green G. (Ber.)
Indian yellow R. (C.)
Acid brown R. (C.)
Pure soluble blue. (C.)
Azo-acid-yellow conc. (M.L.B.)
Solid brown o. (M.L.B.)
Fast blue o sol. (M.L.B.)
Bronze acid brown. (By.)
Indian yellow R. (By.)
Fast green blue shade. (By.)
Acid anthracene brown. (By.)
Indian yellow R. (By.)
Fast green blue shade. (By.)
Orange 11. (B.A.S.F.)
Scarlet GL. (B.A.S.F.)
Light green SFYS. (B.A.S.F.)
Azo-flavine RS. (B.A.S.F.)
Acid brown L. (B.A.S.F.)
Light green SFYS. (B.A.S.F.)
Chocolate. (Uer.)
Tartrazine, (B.A.S.F.); or Azo-yellow, (Uer.)
BASIC MIXTURES SUITABLE FOR DYEING AND STAINING VEGETABLE TANNED
LEATHERS.
Bismark brown M. (By.)
Auramine 2. (By.)
Methylene blue BB. (By.)
Rheonine A. (B.A.S.F.)
Vesuvine B2. (B.A.S.F.)
Diamond green G. (B.A.S.F.)
Bismark brown O. (Leon.)
Auramine 2. (Leon.)
Solid green P. (Leon)
Bismarck Brown ext. (Ber.)
Philadelphia yellow R. (Ber.)
Malachite green cryst. (Ber.)
New phosphine G. (C.)
Chrysoidine. (C.)
New blue B. (C.)
Phosphine ext. (F.)
Chrysoidine diamond cryst. (F.)
Bright green cryst. ext. (F.)
Bismark brown GG. (O.)
Aniline yellow ext. (O.)
Neutral violet ext. (O.)
Dark brown B. (By.)
Auramine 2. (By.)
Emerald green cryst. (By.)
Phosphine 3RB. (Ber.)
Philadelphia yellow R. (Ber.)
Russian green 36784. (Ber.)
Bismark Brown RS. (B.S. Spl.)
Cannella. (B.S. Spl.)
Malachite green. (B.S. Spl.)
Vesuvine conc. (M.L.B.)
Auramine conc. (M.L.B.)
Methylene green. (M.L.B.)
Cutch brown. (Leitch.)
Lemon yellow G. (Leitch.)
Russian green 3 B. (Leitch.)
Bismark brown 2 B. (K.)
Yellow for leather ext. (K.)
Malachite green cryst. (K.)
Auramine. (G.)
Brown R. (G.)
Malachite green. (G.)
Auramine o. (Lev.)
Bismark brown R.C.E. (Lev.)
Brill. green. (Lev.)
Bismark brown Y40. (R.H. & S.)
Canary 2. (R.H. & S.)
Green cryst. Y. (R.H. & S.).
Leather brown A. (S.C. Ind.)
Auramine 2. (S.C. Ind.)
Leather black 1. (S.C. Ind.)
Leather black R. (Uer.).
Yellow 4803. (Uer.)
Blue black S. (Uer.)
Bismark brown NYY. (W. Bros.)
Cannella G. (W. Bros.)
Brown for leather 375. (D.)
Fast yellow 168. (D.)
Methyl green G ext. fine. (D.)
Brown N. (D.)
Leather brown P. (D.)
Paris violet o. (D.)
CHROME LEATHER.
The following dye-stuffs are suitable for dyeing chrome leather. The
leather after tanning, is boraxed in the usual manner and then mordanted
by drumming or paddling in a tannin solution; for dark shades 3 per
cent. gambier and 3 per cent. fustic extract (the weight being
calculated on the leather struck out after boraxing) is suitable; for
light shades 1¹⁄₂ per cent. gambier is to be recommended. The leather,
after mordanting, is fat-liquored and dyed, adding a weight of sodium or
potassium bisulphate equal to that of the dye-stuff, to the dye-bath.
The following is not by any means a complete list of the dyes which will
dye chrome leather well, but merely representative.
After the goods are dyed, they should be well washed in tepid water to
which has been added a little common salt; one pound to every three
dozen skins being a suitable amount to use. When the goods have been
washed, they are struck out by machine and are then ready for shaving,
if the operation has not been performed previous to dyeing. The skins
are afterwards nailed out flat, grain-side up, on boards, and a mixture
of glycerine and water--3 lb. of glycerine dissolved in one gallon of
water being a suitable strength--is well sponged on the grain-side; the
goods are now lightly oiled (using a good sperm, neat’s-foot or mineral
oil), before being taken to the drying room. When thoroughly dry they
are taken off the boards, and placed with layers of damp sawdust between
each skin, for a few hours in order to allow the goods to become
suitably damp for staking. The skins should now be well staked by
machine, the Haley (England), Slocomb or Vaughn (America) being good
machines for this purpose (p. 192).
After staking, the goods are “soft-boarded,” and a thin coat of a weak
linseed mucilage is applied to the skins, which are afterwards dried out
and seasoned with the following mixture:--
“Soak 10 to 15 oz. of dry egg albumen for four hours in 1 gallon of cold
water, with occasional stirring, strain off any insoluble matter and add
1 gallon of milk. A little carbolic acid (phenol) may be added to the
above if it is desired to keep the finish for more than two or three
days--1 oz. of phenol previously dissolved in a little water, added to
each gallon of the finish, being a suitable amount.” A little dye should
be added to the mixture.
After seasoning, the skins are dried out in the stove, glazed twice
round and re-seasoned with the above mixture diluted with its own volume
of water. The goods are dried out and again glazed, perched lightly, and
finally boarded up from neck to tail in order to raise the popular
straight-grain. Should the glaze be too bright the albumen solution may
be reduced to half-strength.
When the goods have been glazed they are rubbed over on the grain side
with a flannel cloth which is slightly damp with linseed oil, trimmed
up, and are ready for sale.
DYES SUITABLE FOR DYEING CHROME-TANNED LEATHER.
_Browns._
Resorcin brown. (Ber.)
Chocolate. (Uer.)
Fast brown. (BY.)
Fast brown. (Ber.)
New golden brown A.1. (C.)
Fast brown. (B.A.S.F.)
Acid brown Y. (S.C. Ind.)
Acid brown B. (S.C. Ind.)
Golden brown. (Leitch.)
Bronze acid brown. (By.)
Light nut brown. (Uer.)
Resorcin brown. (W. Bros.)
Acid brown 5210. (W. Bros.)
New acid brown. (B.S. Spl.)
Light nut brown. (R.)
Brown 2Y. (R.)
Azo-phosphine. (Uer.)
Golden brown Y. (W. Bros.)
_Yellowish Browns and Yellows._
Citronine. (Leon.)
Azo-flavine RS. (B.A.S.F.)
Cuba yellow 2072. (S.C. Ind.)
Phosphine substitute. (B.S. Spl.)
Azo-yellow conc. (M.L.B.)
Azo-flavine. (R.)
Golden orange R. (Leitch.)
Circumein ext. (Ber.)
Indian yellow G. (By.)
Turmeric substitute (W. Bros.).
Azo-yellow R. (M.L.B.).
Chrysophenin G. (Leon.)
Indian yellow T. (C.)
Quinoline yellow. (Ber.)
Cuba yellow. (C.) and (W. Bros.)
Indian yellow. G. (C.)
Chrysoine ext. (W. Bros.)
Azo-flavine. (B.S. Spl.)
Turmeric yellow B. (Leitch.)
Azo-yellow FY. (R.H. & S.)
Orange 4. (R.H. & S.)
Naphtol yellow S. (B.A.S.F.)
Turmeric yellow Y. (Leitch.)
Azo-flavine 7032. (S.C. Ind.)
Turmeric yellow. (G.)
Solid yellow Y. (Leon.)
Solid yellow B. (Leon.)
Milling brown G. (Leon.)
Napthamine yellow 3 G. (K.)
Orange GG. (C.)
Resorcin yellow. (Ber.)
_Greens._
Acid green conc. (M.L.B.)
Acid green ooo. (Leon.)
Guinea greens G and B. (Ber.)
Acid green ext. conc. (C.)
Fast acid green BN. (C.).
Erioglaucine. (G.)
Acid green 5677. (W. Bros.)
Acid green (Uer.)
Light green SF. (B.A.S.F.)
Acid green ext. GG. (By.)
_Violets and Blues._
Bavarian blue DB. (Ber.)
Blue R. (Lev.)
Water blue TR. (B.A.S.F.)
Fast blue O. (M.L.B.)
Water blue 4B. (Leon.)
Cyanole extra. (C.)
Acid blue. (C.A.)
Acid violets 3BN and 6BN. (Lev.)
_Oranges._
Orange 2, (S.C. Ind.); C, (M.L.B.); B, (By.) and (B.A.S.F.).
Orange A. (Leon.)
Orange G. (R.H. & S.)
Ponceaus. (Ber.) and (By.)
Crocein oranges. (K.) and (By.)
Mandarine G ext. (Ber.)
Atlas oranges. (B.S. Spl.)
_Scarlets and Reds._
Most acid scarlets and reds dye chrome leather well on the mordant,
particulars of which are given above.
_Blacks._
These are dyed direct without any mordanting.
Leather black V. (By.)
Leather black 1. (S.C.Ind.)
Naphthylamine blacks 4B and 6B. (C.)
French black. (Uer.)
Chrome leather black. (C. & R.)
Coomassie black 4BS. (Lev.)
Phenylamine black 4B. (By.)
Titanium salts (potassium titanium oxalate and tanno-titanium oxalate)
may be employed in conjunction with the coal-tar colours for dyeing
chrome leather, with many advantages over the ordinary mordants, the
colour produced being faster to light, rubbing, fuller in shade, and
with much less tendency to “grinning.” When employing titanium mordants,
the leather should be first lightly mordanted with some tannin solution
and afterwards dyed with the titanium and dye-stuff in the same bath, in
which case only “acid” dyestuffs may be employed. If desired the goods
may be mordanted with the tannin mordant, afterwards treated with the
titanium salts, washed and dyed; in this case the dyeing and application
of the titanium mordant being carried out separately, the leather may be
dyed with either the acid or basic dye stuffs. The titanium and tannin
mordants may also be applied in the same bath.
DYEING CHAMOIS LEATHER.
The following colours dye chamois leather well, after washing the
leather in a weak soda solution, mordanting with 3 per cent. basic
chrome alum solution, and transferring to the dye-bath without washing.
Equal weight of bisulphate of soda to that of the dyestuff is added to
the dye-bath.
BASIC COAL-TAR COLOURS.
Bismark brown extra. (Ber.)
Philadelphia yellow R. (Ber.)
Pure phosphine. (C.)
Leather blue V. (G.)
Leather brown Y. (S.C. Ind.)
Leather brown A. (S.C. Ind.)
Philadelphia brown. (Ber.)
ACID COAL-TAR COLOURS.
Circumine extra. (Ber.)
Resorcin brown. (Ber.)
Induline NN. (B.A.S.F.)
Orange 2. (M.L.B.)
Golden brown. (Leitch.)
Fast brown. (By.)
Azo-yellow R. (M.L.B.)
Napthylamine black 4B. (C.)
Chocolate. (Uer.)
Azo-flavine RS. (C.)
Azo-phosphine. (Uer.)
Acid anthracene brown R. (By.)
Acid green conc. (M.L.B.)
Acid brown Y. (S.C. Ind.)
Acid brown B. (S.C. Ind.)
Napthylamine black 4B. (O.)
Jet black cryst. (C.)
Anthracene brown R. (By.)
Anthracene brown GG. (By.)
Anthracene brown W. (By.)
Dark nut brown. (Uer.)
Orange 2. (M.L.B.)
NATURAL DYESTUFFS.
Peachwood extract.
Sapan ext.
Logwood ext.
Fustic ext.
Turmeric ext.
A variety of shades may be obtained on chamois leather by mordanting in
a 1 per cent. solution of the titanium salts above mentioned and then
transferring without washing to the dye-liquor, which is best used in
the drum. The colours which are most suitable are the Alizarin colours,
Janus colours and the natural dyestuffs.
_Alizarin Colours._
Alizarin black produces light slate.
Alizarin orange „ bright orange.
Alizarin blue „ blue.
Azo-alizarin black „ brownish maroon.
Azo-alizarin brown „ reddish violet.
Alizarin red „ bright scarlet red.
Azo-alizarin blue „ slate blue.
Coerulein „ yellowish green.
Azo-alizarin yellow „ bright yellow.
Anthracene brown „ fawn brown.
Acid anthracene brown G „ brownish orange.
Acid anthracene brown R „ dull chocolate brown.
Anthracene blue „ pale blue.
Mordant yellow „ lemon yellow.
_Janus Colours._
Janus yellow G. produces bright orange.
Janus yellow R. „ reddish orange.
Janus red „ dark maroon.
Janus claret red „ bluish maroon.
Janus brown R. „ dark reddish chocolate.
Janus blue B. „ bluish black.
_Natural Dyestuffs._
Barwood produces salmon pink.
Logwood „ dull reddish brown.
Fustic „ bright yellow.
Turmeric „ yellow.
Brazil wood „ reddish brown.
Sapan wood „ light nut brown.
Sumach „ buff yellow.
Persian berries „ light orange yellow.
Madder „ red.
Quercitron bark „ light orange yellow.
Cutch „ fawn brown.
Campeche „ canary yellow.
Peach wood „ pale reddish tint.
Divi-Divi „ buff yellow.
The leather is run in the dyestuff solution at a temperature of about
45° to 50° C. for about half an hour, and then lightly fat-liquored, if
desired, and afterwards dried.
In addition to the dyestuffs mentioned above many basic colours may be
employed after the treatment with titanium, some of these producing a
colour lake with the titanium mordants.
As regards the permanency of the various colours to light, the reader
is referred to an important paper by Mr. Lamb,[199] but in many cases
the probable permanency is indicated by a number prefixed to the name of
the colour in Roman figures, I. corresponding to the lowest, and X. to
the highest permanency. In the research referred to, about 1500 samples
of leathers dyed with coal-tar dyes were exposed to light for a series
of “periods,” each equal in actinic power to nine days of the brightest
summer sunshine. The most fugitive colours faded completely, even in the
first “period,” and the most permanent before the end of the tenth. The
prefixed numerals indicate to which of these “periods” the colour
survived.
[199] Journal of Society of Chemical Industry, 1902, p. 156.
INDEX.
_Abies_, 246
_Acacia_, 288
-- _arabica_, 165
Acetic acid, 154, 221, 410
Acid, acetic, 154, 221, 410
-- amido-acetic, 61
-- -- -caproic, 61
-- -- -propionic, 61
-- -- -succinic, 61
-- arsenious, 26
-- aspartic, 61
-- benzoic, 29
-- boric, 155, 162, 221, 229
-- butyric, 61
-- carbolic, 26, 295
-- carbonic, 99, 105, 161
-- chromic, 200
-- cresotinic, 29, 162
-- digallic, 295
-- ellagic, 231, 296
-- ellagitannic, 231, 297
-- formic, 154, 159, 410
-- gallotannic, 295
-- hydrochloric, 154, 157
-- lactic, 154, 158, 221
-- linolenic, 355
-- oleic, 240, 354, 360
-- oxalic, 155, 221
-- oxynaphthoic, 30, 163
-- perchromic, 200
-- protocatechuic, 295
-- pyroligneous, 154
-- salicylic, 28, 295
-- stearic, 351
-- sulphuric, 114, 154, 157, 410
-- sulphurous, 23, 114, 338
-- xanthoproteic, 67
Acids, action on hide, 84
-- amido-, 61, 66
-- in tanning liquors, 20
-- mineral, 23
-- use in softening, 114
Acrilene bating acid, 163
Adipose tissue, 53
_Æthalium septicum_, 10
African oak, 257
Ageing, 188
_Ailantus_, 272
Air, capacity for moisture, 426
-- cost of heating, 428
-- -filters, 439
-- -passages, 437
-- weight of, 428
Alanine, 61
Albumin of hide, 65
Albumins, 56, 66
Alcohol, action on hide, 83
Alcoholic fermentation, 13, 16
Alder, 250
Allen, 366
Aleppo pine, 248
Algarobilla, 286, 293
Alizarine colours, 403
Alkalies, action on gelatine, 89
-- -- on hide, 84
Alkaline carbonates, 138
_Alnus_, 250
Alsop, 313
Alum, 159, 339, 185
Alumed leathers, 2, 4, 9
-- -- dyeing, 402
Alumina, 185
-- in water, 103
-- soap, 352
Aluminium, 185
Amido-acetic acid, 61
-- -acids, 61, 66
-- -caproic acid, 61
-- -propionic acid, 61
-- -succinic acid, 61
Amines, 173
Amœba, 10
Ammonium chloride, 157, 159
-- sulphate, 159, 184
Analysis of tanning materials, 300, 475, 482
_Anacardiaceæ_, 269
Andreasch, 272
Angicabark, 293
Anhydrides of tannins, 297
Aniline dyes, 394
Anion, 80
_Anogeissus_, 293
Anticalcium, 29, 157
Antiseptics, 21
A.O.A.C. method, 300, 312, 482
_Apocynaceæ_, 279
Apples of Sodom, 261
Arata, 269
Arbutus, 279
Archbutt and Deeley, 95
_Arctostaphylos_, 279
Arsenic, 26
-- cures, 39, 42
-- -limes, 194
-- sulphide, 139, 142
Arsenious acid, 26
Aspartic acid, 61
_Aspidospermum_, 279
Association of Official Agric. Chem., 300, 312, 482
Atmospheric pressure, 422
Attractions of molecules, 74
Avidity of acids, 81
Azo-colours, 399
Bablah, 289
Babool, 165, 228, 288
Babul--_see_ Babool
_Bacillus erodiens_, 175
_Bacteria_, 14, 15
-- aerobic and anaerobic, 471
Bacterial filters, 469, 472
-- products, 18, 19
_Bacterium furfuris_, 166
Badamier bark, 282
Bag-tannage, 235
Bakau bark, 283
Balance, analytical, 310
Balaustines, 285
Bali-babilan, 288
_Balsamocarpon_, 286
Band-knife splitting machine, 384, 387
_Banksia_, 268
Barbed wire scratches, 43
Barium chloride, 391
-- sulphydrate, 142
Bark, 243, 244
Bark mills, 316, 452
Barytes, 390
Basic chrome liquors, 211, 241
-- salts, 187, 199
Bast, 243
Bastin, 244
Bate-shavings, 463
-- -stains, 176
Bating, 8, 19, 152, 170, 233
-- effect of water on, 107
Baudouin’s test, 365
Bearberry, 279
Becker, 172, 174
Bedda nuts, 282
Beeswax, 371
Beetle attacking hides, 42
“Bell”-mills, 317
Belting, 450
Benzene, 295
Benzoic acid, 29
Bernardin, 242
Betel nut, 248
_Betula_, 250
_Betulaceæ_, 250
Bichromate of potash, 201
Biernacki, 21
Bilberry, 280
Birch, 250
-- -tar oil, 32
Bistort, 266
Bisulphites, 25, 338
Biuret reaction, 67
Black-dyeing, 398, 399, 413
Bleunard, 57
Blood-albumen, 337
Bloom, 231, 297
Bluebacking, 217
Boarding, 233
Boiler incrustations, 99, 101
Boiling point, 75, 421
Bone-oil, 62
Book-binding leathers, 234
Boral, 155
Borax, 156, 216
Borgman, 181
Boric or boracic acid, 156
Bottle-tannage, 235
Bourgois, 57
_Brabium_, 268
Bran-drench, 166, 195
Brands, 43
Brazil-wood, 287, 413
Breaking stress of leathers, 451
Breed, effect on skin, 45
Brick pits, 455
Brining hides, 38
Briquettes of tan, 464
Bronzing, 395, 404
Brunton, 61
Brusca, 272, 280
Brushing machine, 226
Buff-leather, 378
“Buffalo” method, 129
Burns and Hull, 163
_Butea_, 285
Butyric acid, 61
_Byrsonima_, 269
C. T. bate, 163
Calcium sulphydrate, 140
Calculation of tannin analysis, 314
Calf-kid, 189
Calorie, 422
_Cambium_, 243
Camphor, 31
Canaigre, 264
-- root, extraction, 348
Carbolic acid, 26, 295
Carbolineum, 28
Carbon disulphide, 30
Carbonic acid, 99, 105, 161
Carboxyl, 295
Carr’s disintegrator, 319
Carter’s disintegrator, 319
Cascalote, 286
Casein, 68
Cassia, 235, 287, 299
_Castanea_, 251
_Casuarina_, 249
Catechins, 298
Catechol, 295
-- tannages, 234, 295
Catechu, 277, 289
Caustic alkalies, 22
-- soda, 114, 136
Cavallin, 202
Cavallo, 285
Cebil, 293
Cells, 10
Cellulose, 12
Centigrade thermometer, 481
Centrifugal pumps, 458
_Ceriops_, 283
Chain-conveyors, 325, 453
Chamois leather, 9, 378
-- -- dyeing, 496
Chamoising, 369, 378
Chemical deliming, 153
Chenailier evaporator, 424
Chestnut, 251
-- -oak, 254, 263
-- -wood extract, 222, 231
Chlorides in water, 104
Chondrin, 63
Chromalin, 212
Chrome-alum, 201
-- -blacks, 402
-- combination tannages, 215
-- -iron ore, 200
-- leather, 4, 9
-- -- dyes for, 494
-- tannages, 200
Chromic acid, 200
Chromium, 185, 200
Churco bark, 280
Clark, 95
_Cleistanthus_, 293
Coal-tar or C. T. bate, 29, 163
-- -- dyes, 394
_Coccoloba_, 267
_Cocos_, 249
Cockle, 45
_Cæsalpinia_, 285
Coffee-mill, 317
Cohn’s solution, 177
Colloids, 77, 396
Colour, theory of, 416
-- -measurement, 479
Colouring matters, 299
Colours, primary, 416
-- secondary, 416
-- tertiary, 416
Combination-tannages, 4, 236
_Combretaceæ_, 280, 293
Concentration of extracts, 339
Concrete pits, 455
Cone-mill, 317
_Coniferæ_, 246
Connective tissue, 50
_Conocarpus_, 283
Contact-beds, 472
Conveyors, 325, 327, 453
Copper in water, 104
-- sulphate, 26
_Coriariaceæ_, 277
_Coriaria_, 272, 277
_Corium_, 46
Coriin, 64
Cork, 244
_Cork-cambium_, 243
Cork oak, 257
Corrosive sublimate, 25
_Cortegia rossa_, 248
Couperus, 277
Creasote, 28
Creolin, 28
Cresotinic acid, 29, 162
Crown leather, 381
Crystallisation, 77
Crystalloids, 77
_Cupuliferæ_, 251
Curtidor bark, 280
Curupy bark, 293
Cutch, 289
Cuticle, 46
_Cutis_, 46
Cylinder-oils, 100
Danish glove-leather, 236, 238
_Daphne_, 267
Daphnoidæ, 267
Decoloration of extracts, 337
Degrees of hardness, 94, 105
Dégras, 368, 380
“Dégras-former,” 370, 385
Deliming by acids, 154
-- by washing, 154, 160
De Lof, 242
Denaturised salt, 23
Dennis, 163, 211
Depickling, 91
Depilation, 7, 54, 119
Depletion by puers and bates, 91
_Derma_, 46
_Dermestes vulpinus_, 42
“Devil disintegrator,” 318
Dextrose, 16
Diazo-compounds, 399
Digallic acid, 295
Diffusion, 78
Dippel’s oil, 62
Disc-mill, 317
Disinfectants, 21, 474
Disintegrators, 318
Dissociation, 85
Distilled grease, 359
Divi-divi, 285
Djaft, 263
Docks, 264
Dog-dung, 174, 179, 181
“Dogskin,” 197
Dongola, 197
-- imitations, 241
-- leather, 236, 239
Doornbosch, 293
Drench fermentation, 19
Drenching, 8, 20, 152, 166, 195, 233
_Drepanocarpus_, 285
Dressing-leather tannage, 232
-- leathers, 8
Driers for oils, 363
Drum-stuffing, 388
Drumming, 234
Drums, 117
Dry hides, 110
Drying, effect on skin, 112
-- hides, 41
-- leather, 424
-- oven, 308
-- rooms, 431
-- of sole leather, 232
Dry-salted hides, 110
-- -salting, 38
Dschigh dschighe, 286
Dubbing, 386
Dust-prevention, 325
Dye-testing, 419
Dyeing alumed leathers, 402
-- chrome leather, 403, 493
-- Continental method, 408
-- defects in, 404
-- in drum, 408
-- in paddle, 407
-- in tray, 406, 408
-- kid, 196
-- oil-leathers, 404
-- selection of goods, 409
-- theories of, 396
Dyes, acid, 395, 412
-- basic, 395, 411
-- lists of, for leather, 486, _et seq._
-- list of manufacturers, 485
-- mixtures for leather, 491
-- -woods, 412
Earp, W. R., 143
East India skins, 235, 238, 241
Eberle, 212
Edge-stones, 316
Effluents, 467
Egg-albumin, 67
-- -yolk, 68, 393
Eglinton Tanning Co., 203
Egyptian leathers, 2
_Einbrennen_, 390
Eitner, 105, 109, 112, 114, 131, 168, 205, 212, 216, 252, 366, 382,
392
Elandsboschjes, 293
Elastic fibres, 53, 69
Electric driving, 449
Electrolytes, 79
Electrolytic dissociation, 79
_Elephantorrhiza_, 293
Ellagic acid, 231, 297
Ellagitannic acid, 231, 297
Emulsifying, 240
Enzymes, 15, 16, 171, 173
_Epidermis_, 46, 68
_Epithelium_, 46
-- -cells, 13
_Erector pili_, 50
_Ericaceæ_, 279
“Erodin,” 174
Espinillo, 293
Essential oils, 31
_Eucalyptus_, 284
Eudermin, 27
_Euphorbiaceæ_, 293
Evaporation, 421
-- for analysis, 307
-- in _vacuo_, 423
Evaporator, Yaryan, 339
Evergreen oak, 256
Excise-duties on leather, 3
Extraction, _optimum_ temperature, 344
Extracts, liquid, analysis, 301, 305, 475, 476, 477
-- solid, analysis, 301, 305, 476, 477
-- use of, 342
Fading of colours, 405
Fahrion, 61
Faller-stocks, 116
Fan, Blackman, 428, 430, 434
-- Capel, 437
-- centrifugal, 437, 439
-- drying by, 433
-- screw, 430, 434, 437
Fat, 461, 462
-- -cells, 52
-- -glands, 48
-- -liquoring, 217, 237, 239, 393
-- -liquors, 100
-- -tanned leathers, 1, 4
Fats and oils, 350
-- solvents of, 353
Fatty acids, liquid, 354
-- -- saturated, 354
-- -- unsaturated, 355
Fellmongering, 34
Fermentation, 13, 15
Ferric chloride, 86
-- salts, 198
Ferrocyanides, 339
Ferrous salts, 198
Fibre-bundles, 50
Fibres, elastic, 53
Fibrils, 50
_Filao_ bark, 249
Filter method, 311, 478
Filtration for analysis, 307, 477, 483
-- of sludge, 470
“Fine-hairing,” 180
Finishes for leather, 401
Fire insurance, 325
Fish-tallow, 368
Flaying, 42
Fleshing, 8, 146
-- machines, 147
Fleshings, 461
Flückiger and Hanbury, 277
Fluorides, 26
“Foots,” 356
Formaldehyde, 30, 380
Formalin, 31, 380
Formic acid, 154, 159, 410
Frizing, 378
_Fuchsia_, 284
Fuchsine, 395
Fungi, 15
_Fusanus_, 267
Gall oak, 261
Gallotannic acid, 295
Galls, 261, 280
Gambier, 222, 231, 277
-- extraction, 349
_Garcinia_, 293
Gas-engine, 449
-- -lime, 141
Gaseous state of matter, 74
_Gaultheria_, 251, 373
Gelatin, 56, 58
-- action of bacteria on, 61
-- analyses, 57
-- chemical constitution, 57
-- decompositions, 60
-- determination, 59, 60
-- properties of, 58
-- reactions, 62
-- swelling of, 82
Glaeser mill, 318
Glassy layer, 50, 398
Glazing, 418
Globulin, 67
Globig, 17
Glove-kid, 194
Glucose, 13, 16, 177, 390
Glue, 461
Gluestuff, 461
Glutin, 56, 58
Glycerin, 351
“Golden spoon,” 269
-- tan bark, 248
Gonagra, 264
Grain, 233
-- “drawn,” 228
-- -layer, 51
-- microscopic examination, 52, 55
-- pattern of, 52
Grains of various skins, 52
_Granataceæ_, 285
Grease, recovery, 462
-- refining, 463
Greases, 357
“Green leather,” 197, 239
_Grevillia_, 268
Griffith, 231
Grinding machinery, 452
-- samples, 303, 476
Grounding, 189
Guano, 177
Gum tree, 284
Gumming, 20
_Guttiferæ_, 293
_Gunnera_, 284
_Gunneraceæ_, 284
Glycocine, 61
Glycocoll, 61
_Hæmatoxylon_, 286
Hair, 68, 460
-- -bulb, 49
-- -cuticle, 48, 49
-- -muscle, 50
-- -papilla, 49
-- structure and growth, 47
Hand-stuffing, 386
Handlers, 221
Hardness determination, 94
-- effect on dyeing, 100
-- effects on tanning, 98, 105
-- of water, 93
Harrison, 273, 473
Hauff, 157, 162, 163
Heal, 203
Heat, capacity for, 422
-- consumed in evaporation, 423, 428
-- -- in melting ice, 423
-- given by pipes, 432
-- loss through walls, 431
-- measure of quantity, 422
-- of combustion of coal, 423
Heating by hot water, 442
-- by steam, 432, 436, 440
Heath honeysuckle, 268
Heaths, 279
Hehner, 94
Heinzerling, 203
Helvetia leather, 381
Hemicollin, 60
Hemlock-bark, 222
Hen-dung, 179, 181
Henry, 95
“Hickory” bark, 291
Hide-fibre, analysis, 57
-- -markets, 33
-- -mill, 117
-- powder, 310, 312, 479, 484
-- -- filter, 311, 478
-- -- chromed, 313, 483
High-speed machinery, 451
Hofmeister, 56, 57, 60
Holbrook system, 331
Holden fat, 359
Horns, 464
Horn-sloughs, 464
Horny structures, 50
Horse-fat, 357
-- -flesh, 177
-- -power, 423
Hruschau, 214
Hull, 163, 165
Hummel, 203, 250, 285
Hunt, 188
Hyaline layer, 50, 176, 398
Hydrochloric acid, 86, 154, 157
Hydronaphthol, 30
_Hypoderma bovis_, 43
Hyposulphite of soda, 204
_Hyphæ_, 14
I.A.L.T.C. method, 300, 311, 475
Ice, heat to melt, 423
Ilex, 256
Immiscible liquid, 76
_Inga_, 293
Inks, 402
“Inoffensive,” 143
International Association of Leather Trade Chemists, 300, 311, 475
Internal pressure of liquids, 76
Invertase, 16
Iodine-value, 353
Ionisation-pressure, 81
Ions, 80
Iron-alum, 199
-- -bark tree, 284
-- -blacks, 398, 413
-- in water, 102
-- stains, 22, 38
-- tannages, 198
Izal, 28
Jamrosa bark, 282
Japans, 355
Jeye’s fluid, 28
Jellies, 77
Jensen, 241
Jones fleshing machine, 148
Juniper, 248
Kaspine leather, 380
Kath, 289
Kathreiner, 301
Kation, 80
Kent, 236
Keratin, 14, 56, 68
Kermes oak, 258
Kid-leather, 9
Kilogram, 481
Kips, soaking, 113
Kjeldahl’s method, 70, 179
Klemm, 381
Knapp, 74, 188, 199, 202, 210, 382
_Knoppern_, 262
Knotted tree, 268
Koch, 251
Koerner, 82, 91, 283
_Krameria_, 269
Lace-Leathers, 197
Lactic acid, 154, 158, 221, 410
-- fermentation, 18
Lamb, 218, 273, 405, 485
Land filtration, 470
Lanoline, 359
Lanosoap, 218
Larch, 247
_Larix_, 247
_Lauraceæ_, 267
Layers, 222, 231
Leach-bottom, 329
Leaching, 328
Lead-bleach, 399
-- in water, 104
Leather Industries Laboratory Book, 5
_Leguminosæ_, 288
Leidgen unhairing machine, 145
Lentisque, 269
_Leucadendron_, 268
Leucine, 61
_Leucospermum_, 268
Levulose, 16
Lewkowitsch, 366
Lietzmann, 382
L.I.L.B., 5
_Liliaceæ_, 248
Lime, 21, 120
-- action on hide, 125
-- analyses, 124
-- “available,” 125
-- -burning, 121
-- -liquors, analysis, 143
-- pits, 127, 455
-- quantity used, 129
-- solubility in water, 123
-- -water, 123
Limes, age of, 130
-- bacterial action in, 134
-- plumping, 133
Liming, 126
-- loss of hide-substance, 132
-- Pullman’s method, 137
-- sheep-skins, 34
-- temperature, 129
Linolenic acid, 355
Lipowitz, 59
Liquor-pipes, 456
Liquid state of matter, 74
Liquor tanks, 332, 457
-- -troughs, 333, 457
Liter, 481
Logwood, 286, 413, 398, 401
_Loxopteryngium_, 269
Lubrication, 453
Lufkin, Prof., 142
Lymph corpuscules, 10
Lysol, 28
McFadyen, 61
Madder, 239, 277
Maiden, 290
Magenta, 395
Magnesia, 95
_Malpighia_, 269
_Malpighiaceæ_, 269
Manganese, 185
_Mangifera_, 277
Mangle, 283
Mango, 277
Mangosteen, 293
Mangrove, 283
-- bark, extraction, 348
Mangrutta, 269
Market-hides, 33, 108
“Marking off,” 406
-- weight of hides, 33
Marsh Rosemary, 268
Mather and Platt, 97
Mauve, 394
Maynard, 114
“Mellowing” of liquors, 82
Mellowness of liquors, 229
Mercuric chloride, 25
-- iodide, 26
Metabisulphite of soda, 25, 114, 160
Meter, 481
Metrical system, 481
Milk-shaker, 313
Millon’s reagent, 67
Mill for samples, 303
Mills, arrangement of, 452
-- construction, 316
Mimosa, 231, 290
-- extraction, 346
_Mimoseæ_, 288
Mineral acids, 23
Moellon, 368, 380
Moisture in tanning materials, 314, 315, 479, 482
Molecules, 74
Mordant colouring matters, 238
Mordants, 398
Moulds, 14, 15, 20
Mountain ash, 285
Mucous layer, 47
Mud, 102
Muir, John, 141
Multiple effect, 342, 423
Munkwitz, 145
Muscle, voluntary, 53
_Mycoderma_, 14, 20
_Myrica_, 250
_Myricaceæ_, 250
Myrobalans, 231, 280, 293
-- crusher, 322
-- extraction, 345
_Myrtaceæ_, 284
_Myrtus_, 284
Nancite, 269
Naphthalene sulphonic acid, 29
Naphthols, 29
_Nauclea_, 277
Neb-neb, 289
Nesbitt’s bating process, 161
“Neutralising” chrome leathers, 216
Nihoul, 104
Nitrates and nitrites in water, 104
Nitrogen estimation, 70
“Non-tannins,” 310, 478, 483
Nucleolus, 11
Nucleus, 11
Oak-Bark, 222, 253
-- - -- extraction, 344
-- -twigs, 244
Oakwood, 254
-- extract, 222, 231, 256
Oaks, 252
_Œnothera_, 284
Oil, Arctic sperm, 371
-- birch, 372
-- black birch, 373
-- boiled linseed, 363
-- bottlenose, 371
-- castor, 353, 355, 360
-- cod-liver, 365
-- cottonseed, 364
-- egg-yolk, 393
-- from fats, 463
-- herring, 368
-- Japanese, 368
-- linseed, 362
-- menhaden, 367
-- neatsfoot, 358
-- olive, 359
-- porgie, 367
-- Russia, 372
-- sandalwood, 373
-- sardine, 368
-- sassafras, 373
-- seal, 367
-- sesamé, 364
-- shark-liver, 366
-- sod, 368, 380
-- sperm, 353, 371
-- Straits, 367
-- Three-crown, 367
-- Turkey-red, 361
-- vaseline, 375
-- whale, 367
-- wintergreen, 373
Oils and fats in currying, 384
-- blown, 355, 361
-- drying, 353
-- essential, 350, 372
-- fixed, 350
-- lubricating, 453
-- mineral, 374
-- non-drying, 353
-- resin, 376
-- volatile, 372
Oleic acid, 240, 354, 360
Oleine, 359
Oleostearine, 356, 359
_Onagraceæ_, 284
One-bath chrome process, 211
Organised ferments, 15
Origin of leather manufacture, 1
Osmotic pressure, 78
_Osyris_, 267
_Ovum_, development of, 46
Oxalates, 159
Oxalic acid, 155, 221
_Oxalideæ_, 280
_Oxalis_, 280
Oxynaphthoic acid, 30, 163
Ozokerit, 376
Paal, 57, 62, 66
Paessler, 56
Pairing, 407
Palmer, A. N., 167
Palmer, T., 176
Palmetto, 245
_Palmæ_, 248
Pancreatin, 172
_Panniculus adiposus_, 53
_Papilionaceæ_, 285
Paraffin, 375
Paraform, 31
Parapeptones, 66
_Parenchym_, 243
Parker, J. G., 162
Parker, C. E., 455
_Pars papillaris_, 51
“Partial” pressure, 76
Paypay, 293
Payne and Pullman, 137
Peaty waters, 106
_Penicillium_, 14
Pepsin, 171
Peptones, 60, 61, 62
Perching, 188
Perchromic acid, 200
Perkin, 250, 394
-- and Allen, 276
-- and Gunnell, 269
Permanent hardness, 100
_Persea_, 267
“Persians,” 235, 287, 299, 365
Phenol, 26
-- for deliming, 162
Phenols, 295
Phenolphthalein, 155
_Phloem_, 243
Phlobaphenes, 231
Phloroglucol, 295, 297
Phosphates, 159
_Phyllanthus_, 293
_Phyllocladus_, 248
Pickling, 23, 89, 187
Pigeon-dung, 178
Pilang, 290
_Pinus_, 246
Pipes, arrangement of, 436, 440
-- heat from, 432
_Piptadenia_, 293
_Pistacia_, 269, 272
Pits, construction of, 454
Plaster-cure of Indian kips, 39
Pleating, 407
_Plumbaginæ_, 268
_Podocarpus_, 248
Poison ivy, 274
_Polygalaceæ_, 269
Polygenetic colours, 399, 403
_Polygonaceæ_, 264
_Polygonum_, 266
Polysulphides, 165, 211
“Polysulphin,” 139
Pomegranate, 285
Poplars, 264
Popp, 172, 174
Porcelain, to mark, 304
Porter-Clark, 98
Potassium dichromate, 201
-- hydrate, 136
_Potentilla_, 285
Precipitation tanks, 468
Preller, 381
Press-leaches, 330
“Pricking,” 169
Primitive leather manufacture, 1, 73
Printing, 418
_Protaceæ_, 268
Procter, 203
Procter’s extractor, 306
_Prosopis_, 286
_Protea_, 268
Protocatechuic acid, 295
Protoplasm, 10
Pseudopodia, 11
_Pterocarpus_, 285
Puering, 8, 19, 152, 170, 233
Pulleys, 450
“pulling down,” 157
Pullman, 380
Pullman’s liming method, 137
Pulsometers, 459
Pumps, 457
_Punica_, 285
Putrefaction, 15, 19
Putrid soak, 113, 137
Putz, Dr., 69
Pyrogallol, 295
-- tannages, 234
-- tannins, 295
Pyroligneous acid, 154
Pyrrol, 62
_Pyrus_, 285
Quebracho, 231, 269, 277
-- wood, extraction, 347
Quercetin, 263
_Quercus_, 252
Quicklime, 122
Rabinowitsch, 17
Raw hide leather, 381
Realgar, 142
Reddening of leather by light, 405
“Reds,” 231, 297, 339
Reimer, 58, 64
Resin, 376
_Rete malpighi_, 47
Rhatany, 269
_Rhizophoraceæ_, 283
_Rhus_, 270
Riems, 381
Roans, 235
Rollet, 64
Rolling machines, 224
Roman leathers, 2
_Rosaceæ_, 285
Rosin, 376
Rounding, 151
_Rove_, 261
_Rubiaceæ_, 277
_Rumex_, 264
Rusma, 139
“Russia” leather, 251
_Sabal_, 248
_Saccharomyces_, 14, 20
Saccharomycetes, 15
Saddening, 398
_Salicaceæ_, 263
Salicylic acid, 28, 295
Saliva-corpuscles, 10
_Salix arenaria_, 238, 263
-- _caprea_, 239, 264
Salomon, 178
Salt, 22
-- and acids, action on gelatine, 88
-- -stains, 22, 38
Salts, action on hide, 84, 92
Salted hides, 109
Salting, 35
Sampling tanning materials, 301, 475
-- tool, 301
_Santalaceæ_, 267
Saponification, 351
Sappan-wood, 287
Saturated solutions, 77
_Saxifragaceæ_, 280
Schilling, 62
Schinia, 272
_Schinus_, 270
Schmeija mill, 318
Schulze, 178
Schultz, A., 203, 204
-- Jackson S., 112, 454, 456
Schutzenberger, 57
_Scilla_, 248
_Scorza rossa_, 248
Scouring, 384
-- machine, 384
Screening tanning materials, 323
Scudding, 180
Sea lavender, 268
Seagrave-Bevington dryer, 438
Seaside grape, 267
Seasonings, 418
Sebaceous glands, 48
Semiglutin, 60
Semipermeable membranes, 78
_Senna_, 288
_Septa_, 14
Septic tank, 472
Settling tanks, 468
Sewage, 467
-- purification, 468
Shafting, 448, 450
Shake-method, 312, 483
Shaving, 384
-- machine, 384, 386
-- mill, 323
Sheep-skins, 34
Shellac glaze, 401
Silent boiling jets, 334, 343
Silicic acid in water, 104
Silver fir, 246
Skens, 272
Skin, structure, 46
Skivers, alumed, 197
Skutch, 462
Slaking of lime, 122
Slocomb staking machine, 192
Sludge, 470
Smoked leathers, 2
Snakeweed, 266
Snoubar bark, 248
Soaking and washing, 7
-- of hides, 108
-- with caustic soda, 114
Soaps, 351
-- cold process, 352
Soap test, 94
Society of Arts, 234
Soda in water, 103
Sodium bisulphate, 155
-- carbonate, 139
-- hydrate, 136
-- silicate, 216
-- sulphate, 23, 41
-- sulphide, 114, 139
-- thiosulphate, 204, 213, 216
Sole-leather tanning, 220
Solid solution, 83, 396
-- state of matter, 74
Solubility of liquids, 76
Solution-pressure, 76, 78
Soluble phenyl, 28
_Sorbus_, 285
Souring, 410
Spent tan, 329, 464
-- tans, analysis, 480
Spermaceti, 371
Splitting, 384
Sprinkler, 446
-- -leach, 336
Spruce fir, 246
Spueing, 20, 353, 355, 390
Squill, 248
Staining, 415
-- dyes suitable for, 486, 491
Stains caused by hard water, 99
-- on sole-leather, 227
Staking, 188
-- machines, 192
Staling, 34
Stanhope, 98
_Statice_, 268
Steam-engine, 423, 433, 448
-- - -- indicator, 449
-- -pumps, 457
-- -traps, 442
Stearic acid, 351
Stearin, 351, 359
Stearine glaze, 401, 415
Stenhouse, 296
Sterilisation, 18
Stinco, 272
_Stippen_, 109
Stocking, 180
Stocks, 116
-- for unhairing, 145
Stone-cells, 245
-- pits, 454
Striking, 223
-- machine, 223
Stringy bark tree, 284
Structure of skin, 46
Stuffing, 386
-- drum, 388
Sturtevant dryer, 438
Sudoriferous glands, 49
Sugar-bush, 268
Sulphate of soda, 23
Sulphates in water, 104
Sulphide of sodium, 34, 132, 139, 165
Sulphides, 139
Sulphur dioxide, 23
-- in chrome-leather, 216
Sulphonated oils, 361
Sulphuric acid, 114, 154, 157, 410
Sulphurous acid, 23, 114, 338
Sumach, 234
-- Cape, 267
-- extraction, 347
-- French, 277
-- Sicilian, 270
-- Venetian, 276
Sumachs, American, 273
Sumaching, 410, 411
Supersaturated solutions, 77
Surface-tension, 76
Suspenders, 221, 227, 232
Suspension lime-pit, 128
Swan, 202
Sweat-glands, 49
-- -pit, 119
Sweating, 1, 19, 34, 119
Swedish glove-leather, 236, 238
Swelling, 82, 84
Tallow, 356
-- fish, 368
_Tamarix_, 272, 280
_Tamarisciniæ_, 280
Tamwood, 269
Tan as fuel, 464
-- -burning furnaces, 464
-- -press, 467
_Tanekahi_ bark, 248
Tanghadi bark, 287
Tank-waste, 141
Tannery, construction, 445
-- extension, 447
-- fire risks, 446, 452
-- selection of site, 444
Tannin colour-lakes, 397
-- materials, extraction, 305
-- -- sampling, 302
“Tanning matters,” 311, 478, 483
Tannins, 242, 294, 397
-- pathological, 298
-- physiological, 298
Tari pods, 286
Tartar emetic, 411
Tawing, 191, 196
Temperature, 422
-- in leaching, 343
Temporary hardness, 94, 154, 411
Tengah bark, 283
Teri pods, 286
_Terminalia_, 280
Terra japonica, 277
Thann tree, 282
Thermophilic bacteria, 17
Tinian pine, 249
Tintometer, 315, 479
Titanium, 185, 218, 411, 495
Tjamara laut, 249
Topping, 406
_Tormentilla_, 285
_Torula_, 14
“Total soluble” matter, 307, 477, 478
Treacle, 177
Tri-formol, 31
Trimble, 263
Trioxymethylene, 31
Trypsin, 171, 172
_Tsuga_, 246
Tub-leaches, 331
Tugwar, 293
Turkey oak, 255, 256
-- -red oil, 217, 240
Turwar bark, 235, 287
Turret-dryer, 439
Two-bath chrome process, 204, 213, 216
Tyrosin, 66
Unhairing, 7, 54, 143
-- machines, 144
Unorganised ferments, 15, 16
Used liquors, analysis, 480
Vacciniæ, 280
_Vaccinium_, 280
Vacuoles, 13
Vacuum-oven, 308
-- pans, 342
Valdivia leather, 267
Valonia, 222, 231, 258
-- extraction, 345, 346
Valve, 457
-- for liquor troughs, 333
Van Tieghem, 243
Vapour-pressure, 75, 421
Vaseline, 375
Vaughn fleshing machine, 145, 148
Vegetable-tanned leathers, 2, 3
Velocity of gaseous molecules, 75
Ventilation and heating, 429
-- downward, 440
Verbeek and Peckholdt, 310
Vibration, 452
Vitellin, 67
_Vitis_, 272
Von Höhnel, 242
-- Schroeder, 56, 129, 134
Wagner, 298
Warbles, 43
Ward, H. Marshall, 243
Wash-wheel, 108, 118, 180
-- -- for unhairing, 145
Washing of hides, 108, 111
Waste liquids, 467
Water, condensed, 442
-- hardness, 93
-- impurities, 93
-- oven, 308
-- raisers, 459
-- softening, 95, 101
Wattles, 290
Wax, bees-, 371
-- Brazilian, 372
-- carnauba, 372
-- Japan, 372
-- mineral, 374
-- paraffin, 375
Waxes, 350, 353, 370
-- liquid, 350, 353, 371
Weak grain, 108
Weighing for analysis, 304
_Weimannia_, 280
Weiske, 63
“Wet and dry bulb” thermometer, 426
“White bark,” 293
-- leathers, 197
-- or gelatinous fibres, 50
-- spruce, 247
Wichellow and Tebbutt, 241
Wild almond, 268
Williams’ patent crusher, 320
Willow-bark, 238
Willows, 263
Wilson, 223, 224, 440
Wind, 427, 439
Windbores, 459
Wood, J. T., 166, 171
Woolfat, 358
Woolly butt tree, 284
“Working,” 180
Xanthoproteic acid, 67
Yaryan evaporator, 339, 424
Yeast, 12
Yeasts, 15
Yellow fibres, 53, 69
Yolk of egg, 68
Yorkshire grease, 359
Youl, 231
Young hair, 144
Zinc chloride, 26
-- sulphate, 26, 159
Zollickoffer, 160
Zymases, 15, 16, 17, 61, 296
LONDON: PRINTED BY WILLIAM CLOWES AND SONS, LIMITED,
GREAT WINDMILL STREET, W., AND DUKE STREET, STAMFORD STREET, S.E.
Transcriber’s Notes
Inconsistent and unusual spelling, hyphenation, etc. (including in
names and non-English words) have been retained, except as mentioned
below.
The original work does not have a Fig. 17.
Page 199, (It must be distinctly understood ...: the closing bracket
is missing in the source document.
Page 223: ... tool of triangular section shown in Fig. 29: the
reference should possibly be to Fig. 27 (or 30).
Page 267, kruppelboom: should possibly read kreupelboom.
Changes made:
Footnotes, illustrations, and tables have been moved outside text
paragraphs.
The following phrases have been standardised: C. T. bate and C.T. bate
to C.T. bate (cole-tar); c.c. and cc. to c.c. (cubic centimetre);
Liége and Liège to Liège. Pullman and Pullmans to Pullman; Huxham and
Brown and Huxham and Browns to Huxham and Browns; Kjehldahl and
Kjeldahl to Kjeldahl; Körner and Koerner to Koerner.
Where useful for clarity’s sake, Ibid. in literature references has
been replaced with the actual (abbreviated) title.
Some obvious minor typographical errors have been corrected silently;
accents in French and German words have not been corrected or added.
Page 40: In the re-renewal ... changed to In the renewal ...
Page 64: Chrondrin changed to Chondrin
Page 74: Die Natur und Wesen ... changed to Natur und Wesen ...
Page 139: Na₂S,9OH₂ changed to Na₂S·9OH₂
Page 248: footnote anchor removed from after ... by cork lamellæ
(there is no footnote in the original work).
Page 252: ... and the more colouring matter is contained ... changed
to ... the more colouring matter is contained ...
Page 253: ... and the better the bark changed to ... the better the
bark
Page 272: Ailantus gladulosa changed to Ailantus glandulosa
Page 310: Verbeck changed to Verbeek
Page 329: ... is shown in Fig. 86 changed to ... is shown in Fig. 77
Page 361: Benedict changed to Benedikt
Page 400: Claus and Ree changed to Claus and Rée
Page 429: 44°3 F. changed to 44·3° F.
Page 451: kilos per cm² × 14·22 = lb. per inch changed to kilos per
cm² × 14·22 = lb. per inch²
Page 486: Acid green B.B. changed to Acid green BB.
Page 501: Blue-backing changed to Bluebacking
Page 507: Oxalic acid, page number 155 corrected
Page 508: Pay-pay changed to Paypay; Phylocladus changed to
Phyllocladus; Protacæ changed to Protaceæ
Page 509: Schutzenberger changed to Schützenberger
Page 512, Zymases: page numbers 17, 61, 296 inserted
Index: some entries moved to their proper place.
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