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Title: Pigments, Paint and Painting - A practical book for practical men
Author: Terry, George
Language: English
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                            PIGMENTS, PAINT

                                  AND

                               PAINTING



                            PIGMENTS, PAINT

                                  AND

                               PAINTING

                 _A PRACTICAL BOOK FOR PRACTICAL MEN_

                                  BY

                             GEORGE TERRY

                       [Illustration: colophon]

                                London
                      E. & F. N. SPON, 125 STRAND
                               New York
                SPON & CHAMBERLAIN, 12 CORTLANDT STREET
                                 1893



INTRODUCTION.


In days gone by, the painter who served the usual term of
apprenticeship was deemed to have done all that was required to
qualify him for his trade. He may have learned little or much, but he
had “served his time,” and that was all that was expected of him. So
far as it went, the training was good, because it was nothing if not
practical, and practice is an essential element of skill. But nowadays
such a training can only be considered partial; mere practice, without
any scientific knowledge of the principles which underlie it, is but
half a qualification for the workman who aims at being really a master
of his trade.

When competition was unknown, and the low prices of raw material
offered no inducement for passing off inferior or fraudulent
substitutes, there was less need for a high degree of knowledge.
But under modern conditions, the painter who is unable to gauge the
qualities of the materials he uses, and who is ignorant of the rules
which govern those qualities, and of the principles which determine the
use of this and the rejection of that article, cannot long survive in
the struggle for supremacy or even livelihood.

Hence the need for a handbook such as this volume aims at being.
Granted that our technical schools and colleges are affording a liberal
and invaluable education to the workman who will avail himself of
the opportunities given him, still a man does not remain for ever at
school, and he needs a guide-book, handy of reference and accessible in
price, to refresh his memory and supplement the information gained in
the class-room and workshop.

To fulfil this useful purpose is the aim and object of this
unpretending volume.



CONTENTS.


CHAPTER I.

PRELIMINARY.

                                                                    PAGE

Colour                                                                 1

Pigments                                                               3


CHAPTER II.

BLACKS.

General                                                                5

Animal-black                                                           6

Bone-black                                                             6

Frankfort or Drop-black                                               11

Ivory-black                                                           11

Lamp-black                                                            11

Unimportant blacks--Aniline,
candle, charcoal, coal, cork,
German, iron, lead, manganese,
Prussian, prussiate,
Spanish, tannin                                                       25


CHAPTER III.

BLUES.

Cobalt blues--Cœruleum;
Cobalt blue; smalts                                                   27

Copper blues--Bremen
blue; Cæruleum; Lime
blue; Mountain blue or
Azurite; Péligot blue; Verditer                                       34

Indigo                                                                42

Manganese blue                                                        49

Prussian blue--General;
Yellow prussiate; Combination
of the cyanide and iron
solutions; Antwerp blue;
Bong’s blue; Brunswick
blue; Chinese blue; Paris
blue; Saxon blue; Soluble
blue; Turnbull’s blue                                                 49

Ultramarine                                                           70


CHAPTER IV.

BROWNS.

Asphalt or Bitumen                                                   101

Bistre                                                               101

Bone brown                                                           102

Cappagh brown                                                        102

Cassel earth                                                         102

Chicory brown                                                        102

Cologne earth                                                        102

Manganese brown                                                      103

Mars brown                                                           103

Prussian brown                                                       103

Rubens brown                                                         104

Sepia                                                                104

Ulmin                                                                105

Umbers                                                               105

Vandyke brown                                                        107


CHAPTER V.

GREENS.

Baryta                                                               109

Bremen                                                               112

Brighton                                                             112

Brunswick                                                            113

Chinese                                                              118

Chrome                                                               118

Cobalt                                                               119

Douglas                                                              120

Emerald                                                              121

Guignet’s                                                            125

Lokao                                                                129

Malachite                                                            129

Manganese                                                            130

Mineral                                                              130

Mitis                                                                130

Mountain                                                             131

Paris                                                                132

Prussian                                                             132

Rinmann                                                              132

Sap                                                                  132

Scheele’s                                                            133

Schweinfurth                                                         134

Terre verte                                                          134

Titanium                                                             135

Verdigris                                                            135

Verditer                                                             136

Verona earth                                                         136

Victoria                                                             137

Vienna                                                               137

Zinc                                                                 137


CHAPTER VI.

REDS.

Antimony vermilion                                                   138

Baryta red                                                           143

Cassius purple                                                       143

Chinese red                                                          144

Chrome orange                                                        144

Chrome red                                                           144

Cobalt pink                                                          144

Cobalt red                                                           144

Colcothar                                                            145

Derby red                                                            145

Indian red                                                           147

Lead orange                                                          147

Minium                                                               148

Orange mineral                                                       150

Oxide reds                                                           150

Persian red                                                          153

Realgar                                                              153

Red lead                                                             153

Rouge                                                                153

Venetian red                                                         153

Vermilion                                                            153

Victoria red                                                         169


CHAPTER VII.

WHITES.

Baryta white                                                         170

Blanc fixe                                                           172

Charlton white                                                       172

China clay                                                           172

Enamelled white                                                      183

English white                                                        183

Gypsum                                                               183

Kaolin                                                               183

Lead whites or White leads                                           183

Lime white                                                           245

Lithophone                                                           245

Magnesite                                                            245

Mineral white                                                        245

Orr’s enamel white                                                   245

Paris white                                                          246

Permanent white                                                      246

Satin white                                                          246

Spanish white                                                        246

Strontia white                                                       246

Terra alba                                                           246

Whiting                                                              246

Zinc whites                                                          247


CHAPTER VIII.

YELLOWS.

Arsenic yellow                                                       257

Aureolin yellow                                                      257

Cadmium yellow                                                       258

Chrome yellows                                                       258

Gamboge                                                              270

King’s yellow                                                        271

Naples yellows                                                       271

Ochres                                                               272

Orpiment                                                             280

Realgar                                                              280

Siennas                                                              281


CHAPTER IX.

LAKES.

Brazil-wood lake                                                     283

Carminated lake                                                      283

Carmine                                                              283

Cochineal lake                                                       284

Madder lake                                                          284

Yellow lakes                                                         285


CHAPTER X.

LUMINOUS PAINTS                                                      286


CHAPTER XI.

EXAMINATION OF PIGMENTS.

Fineness                                                             293

Body or covering power                                               293

Colour                                                               293

Durability                                                           294


CHAPTER XII.

VEHICLES AND DRYERS.

Generalities                                                         295

Ground-nut oil                                                       297

Hempseed oil                                                         298

Kukui or Candle-nut oil                                              298

Linseed oil                                                          299

Menhaden oil                                                         303

Poppy-seed oils                                                      305

Tobacco-seed oil                                                     306

Walnut oil                                                           307

Wood or Tung oil                                                     308

Extraction of seed oils                                              308

Dryers                                                               316

Litharge                                                             316

Cobalt and manganese benzoates                                       318

Cobalt and manganese borates                                         318

Resinates                                                            318

Zumatic dryers                                                       318

Manganese oxide                                                      318

Guynemer’s dryer                                                     319

Manganese oxalate                                                    319

Boiled oil                                                           320


CHAPTER XIII.

PAINT MACHINERY.

Wright & Co’s                                                        339

Hind and Lund’s                                                      345

Brinjes & Goodwin’s                                                  346


CHAPTER XIV.

PAINTING.

The surface                                                          351

Priming                                                              352

Drying                                                               353

Filling                                                              354

Coats                                                                355

Brushes                                                              355

Water-colours                                                        356

Removing odour                                                       356

Discoloration                                                        356

Composition                                                          358

Area covered                                                         360

Measuring                                                            360

Carriage and Car painting                                            361

Woodwork painting                                                    368

Iron painting                                                        369

Fresco painting                                                      378


INDEX                                                                383



ILLUSTRATIONS.


FIGURE                                                              PAGE

1, 2. BONE-BLACK FURNACE                                               8

3-11. APPARATUS FOR MAKING LAMP-BLACK                              12-22

12. FURNACE FOR ROASTING COBALT ORES                                  31

13. FURNACE FOR MAKING SMALTS                                         33

14-17. YELLOW PRUSSIATE FURNACE                                       60

18-20. HANNAY’S WHITE LEAD FURNACE                                   217

21-25. LEWIS’S WHITE LEAD FURNACE                               226, 230

26, 27. MACIVOR’S WHITE LEAD PROCESS                            233, 239

28. APPARATUS FOR MAKING ZINC OXIDE                                  248

29. APPARATUS FOR MAKING ZINC SULPHIDE                               253

30-32. FURNACE FOR ROASTING OCHRES                                   278

33-39. APPARATUS FOR EXTRACTING SEED-OILS                        309-315

40-44. WRIGHT & CO.’S PAINT MILLS                                340-344

45. HIND & LUND’S PAINT MILL                                         346

46-48. BRINJES AND GOODWIN’S PAINT MILLS                        347, 348

49. NOAKES & CO.’S METALLIC KEG                                      350



CHAPTER I.

PRELIMINARY.


=Colour.=--The term “colour” is inappropriately given by common usage
to material substances which convey a sense of colour to the human eye,
but is properly restricted to that sense itself. The material colour
should be called “pigment” or “dyestuff” in the raw state, and paint
when compounded with other substances for application in the form of a
coating.

The sense of colour is due to light. In the absence of light there
is no colour, only blackness; and black is really no colour, but an
absence of colour. Very many conditions combine to cause different
colour sensations, some of which are understood, while others we are
not able to explain.

For instance, take the action of heat upon a solution of chloride of
cobalt. As soon as the liquid becomes warm, the pink colour disappears
and gives place to blue; but on pouring water into it, the blue
vanishes and the pink reappears. Again, on heating the blue crystals
of sulphate of copper they become white, but the blue colour comes
back when water is added, and the solution assumes a deeper tint as it
dissolves more of the white powder.

If all the rays are cut off from an electric light except those which
are in and beyond the violet, and a flask containing a solution
of sulphate of quinine is held in that portion of the spectrum, it
will become luminous. The same thing will occur even more strikingly
on placing a piece of uranium glass in the ultra-violet rays. The
explanation of this phenomenon is that beyond those rays which give
light there are others which do not give light, i. e. which do not
cause us to experience the sensation of light; the reason being that
their vibrations are too rapid. But when certain other substances, such
as sulphate of quinine, or a thin slip of uranium glass, are placed in
the path of the rays, this rapid motion is arrested and modified, and
these rays, which in themselves are not luminous, are reflected back
to our eyes as luminous rays. The rapidity of the vibrations being
moderated, our retinas become sensible to them as rays of blue light.

Colour does not depend only upon chemical composition nor solely upon
the aggregation of the particles, but upon these and other things
besides not yet explained. All matter is in a state of motion. If
you heat a substance you communicate an increased activity of motion
to the particles of which it consists. When certain coloured rays of
light are falling upon a substance, these coloured rays of light have
a motion peculiar to themselves. It may be that the degree of motion
in that substance, either existing in it naturally without heating,
or communicated to it by artificial heating, is such that these rays
of light are precisely those which that substance is not capable of
sending back to our eyes. They are then absorbed or destroyed in some
way, by the particular state of that substance upon which they fall;
and those rays which the substance is capable of reflecting back are
mainly sent back to our eyes. Certain colours, such as blue, yellow,
and green, absorb certain rays more or less perfectly, and reflect back
in the main blue, yellow, and green to our eyes. Hence it is incumbent
on those who are studying colour, and who are interested in the purity
and permanency of colour, to comprehend at least the principles of
that science of light which tells of the action of light upon various
bodies that are used as pigments in painting.

If we put together two substances one of which destroys or modifies
the chemical condition or state of the other, then certainly one of
those substances, and very probably both, will lose the colour which it
had before it came into contact with the other. It is therefore most
important that all engaged in the preparation and use of colours should
make a study of this science of light. Of almost equal value is a study
of the science of heat. We have seen what heat can do in changing the
conditions of a substance. To give another instance. The black sulphide
of mercury, after sublimation by heat, exhibits properties, imparted
to it by the heat, which it did not possess before, i. e. it can, by
trituration, be brought to display a red colour.

On showing the spectrum on a screen, if some solution of soda or other
sodium salt be held in the course of the light, almost all the coloured
rays but one will be cut off, and a little band is seen in the yellow
part of the spectrum. This is because the sodium flame is almost
“monochromatic,” or single-lined: it cuts off all the colours but the
yellow. Again, if metallic thallium is held in the flame, the only band
remaining in the spectrum will be the green; and if a lithium salt, the
only surviving colour will be red.

=Pigments.=--The term “pigments” is applied to those colouring matters
which are mixed in a powdery form with oil or other vehicle for the
purpose of painting. They differ in this respect from the dyestuffs,
which are always employed in solution. A very large proportion of
the pigments in common use are derived from the mineral kingdom, the
most notable exceptions being found in the blacks and lakes. All
pigments are required to possess “body,” or density and opacity; to be
insoluble in water and most other solvents, except the stronger mineral
acids; and to be inert, or incapable of exercising chemical or other
influence on each other or on the vehicle or drier with which they are
mixed prior to use. They may be conveniently classified according to
their colours in the first place, reserving the consideration of their
preparation for use for a later chapter. The chief classes are Blacks,
Blues, Browns, Greens, Reds, Whites, and Yellows.



CHAPTER II.

BLACKS.


All the black pigments in use owe their colour to carbon, and all are
produced by artificial means, no natural form of carbon possessing the
requisite qualities.

Several manufactured carbonaceous substances are known in commerce
under the generic name of “Blacks.” The most important of these are
animal-black, bone-black, Frankfort-black, ivory-black, and lamp-black.
They are usually obtained by carbonising organic matter, particularly
bones, in closed vessels or crucibles, or by collecting the soot formed
by the combustion of oily, resinous, and bituminous substances. Other
blacks than those enumerated are manufactured, but only on so small a
scale as to be of no commercial importance.

Carbon, lamp, and vegetable blacks consist almost entirely of carbon,
containing usually from 98 to 99½ per cent. of that substance, the
residue consisting of a little ash, water, and occasionally unburnt
oil. Bone and ivory blacks, on the other hand, are chiefly composed of
mineral matter, which may amount to 65 or 75 per cent. and is mainly
represented by phosphate of lime. Their actual colouring matter, the
carbon, only constitutes 15 to 30 per cent. of the mass. The balance is
water and unburnt animal tissue. Blacks prepared from animal matters
other than bone and ivory carry 40 to 80 per cent. of carbon, and their
mineral matter is generally in the form of carbonates of lime and of
the alkalies.

The principal impurity to be watchful of in the vegetable and lamp
blacks is a small quantity of oily matter which may seriously
interfere with their drying qualities. They should leave very
little ash after being burned in a crucible. Bone and ivory blacks
are sometimes valued as much for their mineral matter as for their
colouring matter. The proportion of this mineral matter is ascertained
by heating a certain weight of the black to red heat in a crucible till
every trace of black has disappeared, and then weighing the residue.
The residue should next be boiled in strong hydrochloric acid till
it is dissolved; if there is any which will not dissolve it is most
probably barytes, which has been added as an adulterant and to make
the black weigh heavy. When the solution is complete, the addition
of ammonia will throw down a precipitate of phosphate of lime, which
should equal 60 to 70 per cent. of the original weight of mineral
matter. If much less than this, it is likely that whiting or gypsum
has been mixed with the pigment. As carbon is not acted upon by acids
or alkalies, it follows that all pure carbon blacks are in themselves
perfectly stable and permanent pigments, and that they exert no
influence on other pigments with which they may be mixed.

=Animal-black.=--This substance is almost identical with bone-black,
but is generally in a more finely divided state. Any animal refuse
matter may be used in its preparation, such as albumen, gelatine, horn
shavings, &c. These are subjected to dry distillation in an earthenware
retort. An inflammable gas is given off, together with much oily
matter, ammonia, and water, while a black carbonaceous mass is left
behind. This is washed with water and powdered in a mill, the product
being animal-black. It is largely used in the manufacture of paint,
printing ink, and blacking.

=Bone-black.=--When bones are heated in a retort or crucible, the
organic constituents are decomposed and carbonised. A mixture of
combustible gases is given off; some of these do not condense on
cooling, others condense in the form of a heavy oil, called bone-oil.
Also much water containing tarry water and ammoniacal salts in solution
passes over. The residue in the retort or crucible consists of finely
divided carbon, in intimate mixture with the inorganic constituents
of the bones: this mixture constitutes ordinary bone-black, or animal
charcoal, as it is sometimes called. The inorganic portion may, if
required, be removed by washing the residue in dilute hydrochloric acid.

The process, as worked on the large scale, is carried on in different
ways, according as it is desired to collect the volatile condensable
portion of the distillate, or to allow it to escape. In the latter
case, when it is required to obtain only bone-black, the apparatus
employed is of a very simple nature, and the amount of fuel needed
is comparatively small. The carbonisation is effected in fire-clay
crucibles, 16 in. high and 12 in. diameter. These are to be preferred
to crucibles made of iron, which were much used at one time, since they
do not lose their round form when subjected to a high temperature; in
consequence of this, they fit more closely together in the furnace,
less air can penetrate, and therefore less of the charcoal is consumed
by oxidation. The furnace is an ordinary flat hearth, having a
superficial area of about 40 square yards, and is covered in with a
flat arch, all of brickwork. The fireplace is situate in the middle of
the hearth; the crucibles are introduced through doors in the front,
which are bricked up when the furnace is filled; each furnace holds
eighteen crucibles. The crucibles, filled with the coarsely broken
bones, are covered with a lid luted on with clay. To economise fuel,
the furnaces should be in a row, and placed back to back.

The arrangement of the furnace and pots is shown in Figs. 1 and 2. A is
the fireplace; B, the crucibles, eighteen in number, spread over the
floor of the furnace in a single layer; _c_, _d_, _e_, and _f_ are the
flues for conducting away the heated gases arising from the calcination
of the bones, as well as the waste heat itself; the last portion of the
flue is fitted with a damper _g_. The furnaces are intended to be built
in fours, back to back, the waste heat serving in a great measure to
conduct the operation of the revivifying apparatus placed in the centre
of the group, and marked C.

[Illustration: Figs. 1 and 2.--BONE-BLACK FURNACE.]

When the furnace is filled and the doors are bricked up, the heat is
slowly raised to redness, at which point it is kept for six or eight
hours. The combustible gases are evolved and consumed in the furnace
as the bones begin to decompose, and by this means so much heat is
produced that only a small quantity of fuel is needed to maintain the
required temperature. When the carbonisation is complete, the doors are
taken down and the crucibles are removed to cool, their places being
immediately filled with fresh ones. The heat must be kept as uniform
as possible throughout the process: if it be not sufficiently high,
the bone-black will contain a portion of undecomposed organic matter,
which renders it quite unfit for use; if, on the other hand, the
temperature be raised too high, the bone-black will become dense and
compact, whereby its efficacy as a decoloriser is much reduced. When
the charcoal in the crucible has become perfectly cool, it is removed
and crushed. When required for decolorising or deodorising purposes, it
is only roughly broken up into small lumps, in which form it is most
readily applicable. The crushing is effected by means of two grooved
cylinders, consisting of toothed discs, alternately 10 and 12 in. in
diameter. These are so placed that the 10-in. discs of one cylinder are
opposite the 12-in. discs of the other, and thus, in revolving, the
carbonised bones are crushed to fragments between them, but are not
reduced to powder. They are passed successively through six of these
mills, the cylinders of each couple being nearer to each other than
the last. Finally the crushed bones are carefully sieved; the powder
is placed apart from the lumps, again passed through finer sieves, and
sorted out into different sizes.

A furnace such as that described above will carbonise four charges of
bones in one day, each charge being more than half a ton in weight.
With careful work, the bones will yield 60 per cent. of bone-black, or
more than one ton daily.

If it be required to condense the volatile gaseous products of the
carbonisation, this process is conducted in retorts similar to those
used in the manufacture of acetic acid from wood: these are so arranged
that the whole of the gaseous products are condensed and collected.
The aqueous portion of the distillate is usually evaporated down to
obtain salts of ammonia; the uncondensable gases may be employed for
illuminating purposes. The manufacture of bone-black is usually carried
on in the neighbourhood of large towns, where a good supply of bones
may be readily obtained.

Ordinary bone-black has about the following composition: Phosphate and
carbonate of lime, and sulphide or oxide of iron, 88 parts; charcoal,
containing a small quantity of nitrogenous matter, 10 parts; silicated
carbide of iron, 2 parts. The decolorising properties of bone-black are
due solely to the presence of the charcoal.

When intended for use as a deodoriser or decoloriser, bone-black should
be kept carefully excluded from the air, for by exposure it loses this
power to a great extent, and becomes almost inert. That which has been
freshly burned is therefore best for these purposes.

The cost of production of bone-black may be calculated as follows:--

                                £  _s._ _d._
  4 tons fat bones at 4_s._
    per cwt                    16   0    0
  27½ bushels coals             1   3    9
  2 firemen                     0   4    9
  4 workmen                     0   8    0
  1 carman                      0   2    4
  2 horses                      0   5    7
  Breaking up the bones         1   5    4
  Rent and taxes                0   8    0
  Interest, repairs, and
    wear and tear               0   7    2
  Contingencies and transports  0   2    4
                              -------------
                              £20   7    3

  Produce:--

    Black, 60 per cent., say 38 cwt. in grains, at
      14_s._ 3_d._                                  13 10 9
                     10 cwt. fine, at 5_s._ 6_d._    1  7 8
    Fat, 6 per cent., say 5 cwt., at 31_s._ 8_d._    7 18 4
                                                  ---------
                                                   £22 16 9   22 16 9
                                                             --------
                                      Profit                  £2  9 6

Bone-black never has the depth or brilliancy of lamp-black, but it
mixes well with either water or oil, and though a slow drier as an oil
paint, is permanent and not high priced.


=Frankfort-black or Drop-black.=--This is a black powder obtained
from dried vine-twigs carbonised to a full black and then ground very
fine. On a large scale it is prepared from a mixture of vine-twigs,
wine-lees, peach-stones, bone-shavings, and ivory refuse. It varies
in shade according as the animal or vegetable charcoal is in excess;
when the latter predominates, the powder is of a bluish colour; but
when there is an excess of animal charcoal, it has a brownish tinge.
It is customary to wash the powder well when first made, in order to
remove any soluble inorganic impurities. The finest Frankfort-black
is probably the soot obtained from the combustion of the materials
mentioned above. It makes an excellent pigment, and is extensively used
by copperplate engravers in the preparation of their ink. Drop-black
is simply Frankfort-black ground exceedingly fine, mixed with a little
glue water, and dried in pear-shaped drops for sale.


=Ivory-black.=--Ivory-black is a beautiful black pigment prepared by
carbonising waste fragments and turnings of ivory. These are exposed
to a red heat for some hours in crucibles, great care being taken
to avoid overheating or burning. When quite cold, the crucibles are
opened, and the contents are pulverised, the richest coloured fragments
being kept apart for the best quality. The powder is then levigated
on a porphyry slab, washed well with hot water on a filter, and dried
in an oven at a temperature not exceeding 212° F. The product is of a
very beautiful velvety black colour, superior even to that obtained
from peach-kernels, and quite free from the reddish tinge which so
often characterises bone-black. Ivory-black, like Frankfort-black, is
employed by copperplate printers in the preparation of their ink. Mixed
with white lead, it affords a rich pearl-grey pigment.


=Lamp-black.=--Lamp-black is an exceedingly light, dull-black powder,
formed by the imperfect combustion of oils, fats, resins, &c. It may
be prepared on a small scale by suspending a small tin-plate funnel
over the flame of a lamp fed with oil, tallow, or crude naphtha, the
wick being so arranged that it shall burn with a large and smoky flame.
Dense masses of this light carbonaceous matter gradually collect in
the funnel, and may be removed from time to time. The funnel should be
furnished with a metal tube to convey the gases away from the room, but
no solder must be used in making the connections.

[Illustration: Figs. 3 and 4.--APPARATUS FOR MAKING
LAMP-BLACK.]

An especially fine quality of lamp-black is obtained from bone-oil,
deprived of the ammonia with which it is always contaminated. It is
manufactured on a commercial scale by means of the apparatus shown
in Figs. 3 and 4. The oil is contained in the lamp A and kept at a
constant level by means of the globular vessel B, which is also filled
with oil and inverted over A. The oil flows from the lamp into the
tube C, which is bent upwards at the farther extremity on a level with
the oil in the lamp. A cotton wick is supplied to the bent end of the
tube, as well as a little spout D, for conducting away any oil that
may overflow into the receptacle E placed beneath. A conical hood _a_
surrounds the flame of the lamp and terminates in a tube _b_, through
which are conveyed the sooty products of the combustion of the oil into
the wide lateral tube _c_, arranged to accommodate the smoke from about
a dozen such lamps placed at intervals of about 6 feet, as indicated in
the figures. The effect of this wide tube _c_ is not only to cool the
smoke, but also to collect the water and other liquids condensed. The
smoke and vapours pass hence into _d_, the first of a series of sacks
made of closely woven linen, about 10 or 12 feet long and 3 feet in
diameter, closed at the bottom with a trap or slide _e_, and formed at
the upper and lower ends of sheet-copper tubing made funnel-shaped. The
upper one of these is prolonged into an additional pipe _f_, by means
of which the smoke arrives at the second sack _g_ in the series, thence
finding its way to the third, and so on till the last sack of the row
is reached. In connection with the last sack of each row is placed a
horizontal flue F, in which are arranged frames covered with wire gauze
and mounted on hinges. Their purpose is to retain the small remaining
portions of lamp-black passing out with the smoke from the sacks. The
meshes of the gauze are constantly getting filled up with soot, which
necessitates a periodical checking of the draught for its removal. This
is done by means of the rod G, which, when raised and allowed to fall
suddenly, jerks the accumulated mass off the gauze. The current of
air passing through the entire apparatus can be regulated by a damper
placed at the entrance to the chimney in which the flue F embouches.
At regular intervals, the mouthpieces in the lower ends of the sacks
are removed, and their contents are shaken out separately and collected
according to their various qualities. That gathered from the first sack
in each row should always be kept apart from the remainder, as it is
much contaminated by the presence of resinous and tarry matters.

[Illustration: Fig. 5.--APPARATUS FOR MAKING LAMP-BLACK.]

The old-fashioned method of preparing lamp-black from the incomplete
combustion of gas tar is conducted in an apparatus resembling that
shown in Fig. 5. The furnace _a_, lined with fire-brick, contains a
kettle _b_, and is surmounted by a large thick cast-iron hood _c_,
communicating with a stone or brick condensing chamber, divided
by means of perforated partition walls into three unequal sized
compartments _d_, _e_, _f_, wherein the black is deposited. A chimney
_g_ delivers uncondensed vapours into the atmosphere. In working, the
furnace is first brought to a red heat, then the kettle _b_, charged
with tar, is introduced. As a charge is finished, more tar is added,
with occasional stirring, till the kettle becomes inconveniently full
of residue, when it is withdrawn and a fresh one replaces it. The
residue is chipped out and used as fuel. The black is removed weekly
through the door _h_. It is of good quality and colour so long as the
combustion is conducted with a minimum of air, admission of which is
controlled at the furnace. The yield is about 25 per cent. of the
weight of the tar; and one furnace should treat a ton of tar in a week.
One workman can manage several furnaces.

An improved process has been introduced by Martin and Grafton for the
preparation of lamp-black from coal-tar, which affords a very good
product. The coal-tar is first stirred up energetically with lime-water
in any convenient vessel, after which the mixture is allowed to stand
until the coal-tar has subsided to the bottom, when the lime-water is
drawn off. The tar is then well washed by decantation with hot water,
and rectified in the ordinary naphtha still. Afterwards it is run into
a long iron cylinder, which is placed over a furnace, and supplied
with numerous large burners. Each burner has a metal funnel placed
immediately above it, connected with a cast-iron pipe, into which all
the fumes from each burner are conducted. The naphtha in the cylinder
is heated almost to the boiling point by the furnace beneath. A series
of smaller pipes lead away the fumes from the main pipe into a row of
chambers, and thence into a series of large canvas bags, placed side
by side, and connected alternately at top and bottom. The bags vary
in number from fifty to eighty, the last one being left open to allow
the smoke to escape, after traversing some 400 yards since leaving the
burners. The best quality of lamp-black is found in the last bags, that
near the furnace being much coarser and less pure. The bags are emptied
whenever they contain a sufficient quantity.

The process employed in Germany for the manufacture of lamp-black is
to conduct the products of the combustion of any resinous matter in a
furnace into a long flue, at the end of which is placed a loose hood,
made of some woollen material, and suspended by a rope and pulley. The
lamp-black collects in this hood, and, when a sufficient quantity has
accumulated, is shaken down and removed. In this manner about 6 cwt. of
lamp-black may be collected in twenty-four hours.

One form of the apparatus is shown in Fig. 6. The circular structure
_a_ is lined inside with hanging cloths upon which the black can
condense, and is covered with a conical roof from which depends a
movable sheet-iron cone _b_, perforated at its apex to give egress to a
current of air. This cone _b_ is supported by a rope _g_ passing over a
pulley _c_ and accessible from the outside. A fireplace _d_, containing
a small iron dish _e_ for holding the resin, is built against one of
the side walls of the structure in such a manner that it can be fired
externally. The rate of combustion is regulated by a small sliding
damper on the door of the fireplace. When the black has accumulated in
the chamber _a_ to such an extent that operations must be suspended,
the fire is let out, and the chamber is left to cool entirely, so that
the black may not ignite on contact with the air. The cone _b_ is then
lowered, and in its descent scrapes the walls of the chamber _a_ and
causes the black to collect on the floor, whence it is removed through
an iron door _f_ which at other times is kept tightly luted.

[Illustration: Fig. 6.--APPARATUS FOR MAKING LAMP-BLACK.]

In England, an inferior variety is sometimes obtained from the flues of
coke-ovens. That known as _Russian lamp-black_ is made by burning chips
of resinous deal or pine wood, and collecting the soot formed; but it
is objectionable, owing to its liability to take fire spontaneously
when left for a long time moistened with oil.

A modified form of apparatus has been introduced by Thalwitzer, a
German manufacturer, and is shown in Figs. 7 and 8. A vertical tube is
provided at its upper end with a funnel, into which cooling water is
poured and flows out through openings in the tube immediately above a
circular plate of thin cast or wrought iron arranged horizontally and

[Illustration: Figs. 7 and 8.--THALWITZER’S LAMP-BLACK
APPARATUS.]

secured at its centre to the tube. Round the periphery of this plate
is a vertical rim of tin plate, at the top of which is a pipe through
which the cooling water runs into a gutter round the top of the
cylindrical casing, the water being carried off from this gutter by
a pipe. The vertical tube is carried near its upper end in a bearing,
and at that part is attached a worm-wheel geared into it by a worm
driven by any suitable power. At the underside of the circular plate is
fixed a scraper, the edge of which is formed with a strip of leather in
contact with the lower surface of the plate. Opposite the scraper, at
the bottom of the casing, is a burning lamp, which sucks up the oil for
its consumption by a flat wide wick.

The operation is as follows:--The vertical tube is caused to revolve
by the action of the worm-wheel, the circular plate thereby receiving
a slow rotary movement; and a small stream of water being poured into
the funnel at the top of the tube, this water passes down the latter
and through the openings on to the circular plate, which is thus kept
cool. The burning lamp filled with paraffin or other oil is brought as
near to the circular plate as is necessary for the cooling of the flame
and the most perfect extraction of the carbon, which, in the form of
soot, attaches itself readily to the plate, owing to its coldness and
to the condensation of the steam produced. The revolving plate presents
continually to the flames a new and clean surface, in consequence of
the lamp-black being scraped away by the scraper as soon as deposited,
and brought away through a pipe or shoot into a collecting barrel.

The apparatus as shown in Figs. 7 and 8 consists of a round metal
plate A, provided with a flange _a_, and fixed on a vertical shaft _b_
supported by the bearing B, and carrying at its upper end a worm wheel
_d_ set in motion by a worm. The plate A is cooled by water admitted
through a pipe _g_, and the flange _a_ is provided with a discharge
pipe _h_, through which the cooling water runs into the groove D,
surrounding the whole apparatus. Underneath the plate A a number of
lamps J are applied, which are fed with oil by a common pipe _l_. H is
an oblique scratcher or blade, the working edge of which is formed by a
strip of leather, and touches the lower surface of the plate A.

For manufacturing lamp-black, a slow rotary motion is imparted to the
apparatus by means of the worm and worm-wheel, and a slight current
of water is directed upon the plate A through the pipe _g_. The lamps
J, filled with paraffin oil derived from lignite, or with any other
suitable oil, are ignited and approached to the plate A as far as
is necessary for cooling the flame, so as to deposit the greatest
possible quantity of black. The latter adheres to the cold surface of
the plate, which is also kept damp by the aqueous vapour formed during
the combustion. The revolution of the plate serves to bring the flame
continually into contact with new and clean portions of the plate, the
black being continually scraped off by the blade or scraper placed
opposite the flames, and conducted through a channel into a collecting
trough.

There is a risk of overburning, causing a grey tint and a hard and
granular texture.

A variety of lamp-black known as “carbon black” or “gas black,” has
of late years assumed an important position among black pigments. It
is produced in considerable quantities in the United States by the
combustion of the natural gas issuing from the earth in the mineral oil
regions. The soot arising from the imperfect combustion of the gaseous
hydrocarbon is made to deposit itself on cooled iron surfaces. These at
first were made stationary, but now take the form of revolving discs or
cylinders, which are automatically cleansed of the black as fast as it
is deposited. This type of lamp-black is remarkably free from mineral
impurities and unburned oil, and of a full colour.

An improved lamp-black kiln has been introduced in which the use of
water is dispensed with. It is shown in Figs. 9 and 10. The furnace A,
which is preferably built double, as shown, is constructed of brick
lined with firebrick, with a rear wall _a_ that divides the furnace
room from the condensing room, side walls _b_, front _c_, and central
dividing wall _d_, that divides the furnace into two long and narrow
fire

[Illustration: Figs. 9 and 10.--AMERICAN LAMP-BLACK KILN.]

spaces. The bottom of the fire spaces _e_ is formed by a sheet iron
plate _f_ that is supported by the walls, and the space below plate _f_
serves as an air space through which air circulates by openings _g_
in the front and side walls, this circulation of air tending to keep
the plate _f_ cool. The rear of the fire space _e_ extends upward and
communicates by an opening _h_ through the wall with the condensing
room. In the front wall _c_ is an opening to each fire space _e_ and
a door _i_ to each opening. The oil or other liquid is supplied by
pipes _k_ that enter from the outside near the rear of the fire spaces.
The outer end of each pipe _k_ is fitted with a cup-shaped receptacle
_l_, into which the oil will run from the vertical branch of the main
supply pipe _m_, so that the amount of oil running into each pipe
_k_ may be observed, and regulated by a cock. The pipe _m_ feeds the
oil to one or more furnaces, the supply of material to each furnace
being separately regulated. In the fire spaces beneath pipes _k_ are
placed shallow cast iron drip pans _o_ to receive the oil, and the oil
running in faster than it will burn will drop while on fire into the
pans _o_, and be spattered into small particles. These pans are changed
frequently, access to them being obtained by doors _i_. A slide _p_ is
provided in each door _i_ to allow of ventilation when required. The
slides and doors should close air-tight. By constructing the fire space
_e_ long and narrow, the plate _f_ is more readily kept cool, and the
space in front of the point of combustion renders the smoke less liable
to escape by the doors. The products of combustion pass through the
opening _h_ to the condensing room, which is lathed and plastered, and
if the room is sufficiently large a number of furnaces may be fitted to
discharge into the same room. This furnace is especially adapted for
burning dead oil; but by using burners of suitable construction other
oils may be burned, and a superior quality of lamp-black made from
mitigated spirits.

A very large proportion of the lamp-black now made is derived from the
combustion of creosote or anthracene oils from coal tar, or of the
residues of shale-oil distillation. The form of combustion chamber
varies in different works, but is typified by the following rough
sketch of that in use at the Stampshaw Chemical Works (Fig. 11).

[Illustration: Fig. 11.--_Apparatus for making Lamp-black from Creosote
or Shale Oil._]

At these works a horizontal brick flue _a_ about 18 inches square and
about 10 feet long is provided. At one end it enters the black-house
_b_, and is here provided with a damper _c_ to shut it off when not
working. The other end opens to the air, and here is a sliding door
_d_ which, when shut down, leaves an opening round a small pipe _e_,
which enters in this situation from a main pipe that conveys oil in a
similar manner to four burners of this description placed side by side.
At the bottom of the flue is an iron tray _f_ to catch any liquid that
falls from the tube, and in this tray the oil is burned. The burning of
oil in one of these flues is not allowed to go on for more than three
hours, and, when the combustion is over, the communication with the
black-house is closed, the entrance door of the flue is opened, and the
cover is taken off the chimney _g_ so that the flue may become cooled,
and another flue is taken into use.

The black-house is a brick chamber into which the smoke passes, and
where it deposits its sooty particles. In some works there is only
one undivided chamber; in other works there are more than one, and
the chambers communicate by flues through which the smoke passes from
one to another. At other works the chamber is divided by vertical
partitions, springing alternately from the two ends, so as to
constitute one high zigzag flue, along which the smoke must travel
to its outlet from the black-house. This chamber must needs have an
opening somewhere to the outer air. The opening is sometimes a small
chimney in the roof, and sometimes a short louvre tower. This is
necessary to produce a trifling draught, just enough to carry the smoke
into the chamber and no more.

In some works, the black from the black-house is also calcined, the
object of the “calcination” being to get rid of all greasiness, a point
of great importance when the lamp-black is to be used for making fine
pigment. This process is conducted in circular iron pans, usually about
2¼ feet high and 2¼ feet diameter, which are provided with removable
iron covers. A pan of this size will hold about 2 lb. of lamp-black. A
bowlful is first put in and lighted by a red-hot iron; more and more is
added from time to time as the ignition proceeds. When the pan, being
full, leaves off smoking, the calcination is known to be complete, and
the pan is then covered and its contents are allowed to cool. The loss
undergone in this process is about 25 per cent. The smoke which comes
off is acrid and very irritating to the eyes, like that proceeding from
boiling oil, and it is difficult for a person unaccustomed to it to
remain many minutes in the chamber where calcining is going on. This
process is sometimes conducted within a chamber, but frequently under a
shed or even in a building freely open to the air.

There are three sources from which nuisance may arise in lamp-black
making: 1. The smoke which issues from the chimney of the black-house,
small as it sometimes is, often constitutes a nuisance to near
neighbours; but the nuisance is not a very serious one, and it does not
extend very far from the works, never to a greater distance than about
50 yards. The odour, even when but little smoke escapes, is oppressive
and suffocating in character, and resembles that diffused in a room
by a smoking table-lamp. It occasions headache, but is not otherwise
injurious to health. 2. A similar nuisance of suffocating smoke
sometimes proceeds from the burners, but this is when they are leaky or
when there is a deficiency of draught through the black-house, or when
the doors of the burning chambers do not shut closely, and when there
is much wind blowing past them. This nuisance chiefly occurs when the
burners are open to the air and merely protected by an open shed. 3.
The escape of acrolein and other offensive vapours from the calcining
house.

The best mode of preventing nuisance from the black-house is so to
elongate the chamber as to give abundant opportunity for the soot to
deposit in the course of the smoke along it to the outlet, and by
taking means to consume by fire what little smoke escapes deposition. A
most effectual arrangement for the accomplishment of these ends is to
have a black-house 150 feet long, and so divided by partitions within
as to cause the smoke to traverse a distance of altogether 500 feet
before it finds an exit; the exit from the chamber communicating with a
fire, in which the last of the smoke is consumed, and which serves to
assist in regulating the draught through the chamber.

The regulation of the draught through the burner and black chamber is
of importance in order to avoid the escape of smoke from the burners.
If the draught be too great, too much black is lost from the chamber,
but if, on the other hand, it be too little, the smoke instead of
passing into the chamber will come out into the works and create a
nuisance, especially where the burners are erected in the open air,
under circumstances in which variation in the force of the wind cannot
fail to interfere with due regulation of draught. This part of the
manufacture should be conducted within a building of some sort.

The best mode of preventing nuisance from calcination is in operation
at Shackell & Edwards’ works, in Hornsey Road, Islington. At these
works the black is calcined in a chamber 20 feet square and 25 feet
in greatest height, with a paved floor and arched roof. In the centre
of the roof is the opening where a fire was formerly placed, but which
is now closed by a sky-light, capable of being raised. The calcining
pots are ranged round this chamber, and a fan, employed to draw off
the vapours from the oil-boiling pans, is further utilised to draw off
also, from the upper part of the calcining-house, the vapours arising
from the calcination, and to drive them into the boiler fire, where
they are consumed. Calcination should always be conducted in a closed
building duly ventilated so as not to create nuisance.

The transport of lamp-black is effected in barrels or bags; when in the
latter, these should be previously soaked in water containing some clay
in suspension, which stops up the pores of the sacking, and thereby
prevents loss.

The particular virtue of lamp-black as a pigment lies in its state
of extremely fine division, which could not possibly be attained by
artificial means; this quality renders it invaluable as the basis of
black pigments, all of which contain it in a greater or less quantity.
Indian ink and printers’ ink are also composed principally of this
substance.

=Unimportant Blacks.=--In addition to the recognised blacks already
noticed there are a number of other sources of black pigment which
have been drawn upon to a limited extent, or have been suggested
as substitutes for the standard articles. They only merit a short
description.

_Aniline black_ is prepared by adding an acidified (sulphuric) solution
of bichromate of potash to an aqueous solution of hydrochlorate of
aniline, and washing the precipitate. The cost is prohibitive.

_Candle black_ is candle smoke condensed on a cold plate.

_Charcoal black_ is finely-ground wood charcoal.

_Coal black_ has been suggested by grinding coal, but lacks the
requisite qualities of a pigment.

_Cork black_ is a very fine pigment prepared by calcining cork refuse.
Limited supply.

_German black_ is Frankfort black.

_Iron black_ is ground black sulphide of iron.

_Lead black_ is prepared by boiling lead fume in sulphide of soda
solution. It would probably be unstable on account of oxidation.

_Manganese black_ is ground oxide of manganese. It is costly, and dries
too quickly.

_Prussian black_ is calcined Prussian blue. It is not of a good colour,
nor economical.

_Prussiate black_ is the carbonaceous residue from making yellow
prussiate of potash. Used chiefly for decolorising syrups, &c.

_Spanish black_ is cork black.

_Tannin black_ is proposed to be made by exhausting the tannin from
refuse leather and tanning agents, and adding alum and sulphate of
iron. The colour is blue-black, weak, and unstable.



CHAPTER III.

BLUES.


=Cobalt Blues.=--Some of the compounds of cobalt with alumina,
phosphoric acid, silica, and tin, are remarkable for possessing a fine
blue colour of great permanency and indestructibility, and still find a
limited application. They are chiefly as follows:--

  Cœruleum--a mixture of the oxides of cobalt and tin.
  Cobalt blue--a mixture of the oxides of cobalt and alumina.
  Smalts--a double silicate of cobalt and potash.

CŒRULEUM.--This is a light-blue slightly greenish colour, with
no purple tendency in artificial light. It is non-granular, covers
well, mixes with water or oil, and is a good artists’ colour for sky
effects. It is permanent in strong sunlight and impure atmosphere, and
resists acids and alkalies at normal temperatures. Hot hydrochloric
acid dissolves it, and addition of water to the pale blue solution
produces a violet red; evaporation to dryness restores the original
pigment. The green tint of a nitric acid solution is due to iron and
nickel impurites. Dilute sulphuric acid causes partial decomposition
of cœruleum, but it is proof against caustic potash, acetic acid, and
concentrated sulphuric acid. Its composition is given as

  Cobalt oxide                  18·66
  Tin oxide                     49·66
  Silica and sulphate of lime   31·68
                               ------
                               100·00

There are several methods of preparing cœruleum:--

(1) A solution of stannate of potash is added to one of cobalt. A blue
precipitate is thrown down, which, on washing, becomes first light-red
and then brown. When calcined at a white heat it assumes a blue colour.

(2) A solution of stannate of soda is mixed with a solution of nitrate
of cobalt, and the resulting precipitate calcined to bright redness
forms a blue pigment.

(3) Solutions of cobalt and tin are mixed and precipitated by soda, the
precipitate washed free from soda being calcined as in all the other
cases. The silicate of soda is the most satisfactory sodium salt for a
precipitant.

COBALT BLUE.--This rich pure blue pigment is not alone
permanent, but actually develops its full intensity only after exposure
to the air. With age, however, it acquires a greenish tendency,
and in artificial light it inclines to a violet tint. It is proof
against acids and alkalies, and mixes particularly well with water.
In combination with other pigments it is unaltered and has no effect
on them. Its tones are different from those of ultramarine. It slowly
decomposes when heated in strong sulphuric acid, yielding a violet
solution and a white precipitate, which latter dissolves and affords a
blue liquid on dilution with water. It consists principally of about 80
per cent. alumina, and 15 per cent. oxide of cobalt.

There are several ways of preparing it:--

(1) By Thénard’s original process, roasted cobalt, from Tunaberg,
Sweden, is dissolved under heat in an excess of nitric acid. The
solution, evaporated nearly to dryness, is boiled in water, and the
deposit of arseniate of iron is filtered out. Into the filtrate is
poured a solution of basic phosphate of soda, which throws down
a precipitate of basic phosphate of cobalt, varying in hue from
violet to pink. This precipitate is washed on a filter, and, while
still gelatinous, 1 lb. of it is intimately mixed with 8 lb. of
hydrated alumina, recently precipitated by ammonia from a solution of
potash-alum. The mass is first dried to brittleness and then calcined
in a covered clay vessel for half an hour at a cherry-red heat. The
resultant blue pigment is stored in glass receptacles.

The preparation of the gelatinous alumina is conducted as follows. The
potash-alum is dissolved in at least three times as much water as is
necessary, and is then precipitated by an abundant excess of ammonia,
with frequent stirring. When settled, the supernatant liquor is
siphoned off, and the precipitate is thoroughly washed several times on
a filter.

Thénard blue will vary in tint according to the proportions of
alumina used. A pure colour is obtained with 4-5 parts alumina to 1
of phosphate of cobalt; a greenish hue with equal parts of alumina
and cobalt salt; and almost any intermediate tone by varying the
proportions between these limits.

(2) To 16 parts of gelatinous alumina add 1 part of arseniate of
cobalt, obtained by precipitating the solution of cobalt by arseniate
of potash.

(3) A richer and more velvety blue is got by using oxide of cobalt and
substituting phosphate of lime for the alumina (Boullai-Marillac).

(4) Binder’s process is as follows:--Dissolve by boiling 6 lb. alum,
free from iron, in a leaden or earthenware vessel, and filter it into
a vat 5½ ft. high and 3 ft. across, one-third full of clean water.
Precipitate the alumina by solution of potash, fill up the vat with
water, settle, decant the clear liquor, and wash repeatedly till
barium chloride gives no precipitate. Dissolve ½ lb. sesquioxide
of cobalt in 1½ lb. hydrochloric acid at 22° B., and evaporate to
dryness. Dissolve the residue in 3 lb. hydrochloric acid, and pass
a stream of sulphuretted hydrogen through it, to throw down any
foreign metals. Filter clear, evaporate again to dryness, and dissolve
the residue in enough water to produce 4½-5 lb. of solution. Next
precipitate the cobalt solution (3 to 6 lb., according to depth of
tint required) by ammonia, avoiding excess. Wash the precipitate, and
add it to the water, holding the gelatinous alumina in suspension,
stirring thoroughly for ½ an hour. A reddish tint in the supernatant
liquor shows that some of the cobalt has been dissolved. Add a little
ammonia, and allow the precipitate to settle. Decant and add new waters
repeatedly. Finally collect on a fine filter cloth, drain, press, stove
dry, and calcine for 2-2½ hours at red heat in clay crucibles: then
cool, grind first in a mill and then on a slab, and sift.

SMALTS.--This pigment has not maintained its position in
competition with artificial ultramarine. Formerly it was very largely
used to correct the yellow tone of cottons, papers, and pottery. It has
a pale violet-blue tint, which, however, is not constant in artificial
light. Being a silicate it is very permanent, and proof against the
action of acids, alkalies, and sunlight, besides being inert when
mixed with other pigments. It can be used with either water or oil as
a medium, but is not a successful paint owing to its weak colouring
power. It is virtually a double silicate of cobalt and potash, or a
cobalt glass, containing a few impurities, of which the chief are
aluminium, iron, and lead oxides. The colour varies somewhat according
as these impurities fluctuate, and the finest ground sample is always
the palest. It is hardly ever adulterated, and the chief point to
secure is that it be ground to the finest possible degree.

Its manufacture is most extensively and successfully carried on in
Saxony. The raw materials used are cobalt speiss (an arsenide of cobalt
and iron), potash, and sand. The ore is broken up into convenient sized
pieces and roasted at red heat in a reverberatory furnace provided with
a tall shaft for discharging the sulphurous and arsenical fumes at a
high altitude. When the evolution of these fumes has ceased and the
mass begins to assume a pasty consistence, the roasted ore is removed
from the furnace, cooled, reduced to a fine pulverulent condition (then
known as “zaffre”) and passed through a silken sieve. Should it be
necessary,

[Illustration: Fig. 12.--FURNACE FOR ROASTING COBALT ORES.]

the cobalt ore is first spalled and hand-picked to remove the ores
of foreign metals which are associated with it; and then reduced to
a very fine state in an edge runner or mortar mill, and freed from
earthy impurities by washing. The concentrated ore is then dried and
dead-roasted in small charges at a time (about 4 cwt.) in a specially
designed reverberatory furnace such as shown in Fig. 12, of which _a_
is the hearth on which the ore is spread; _b_, the fireplace, the
products of combustion from which pass over the ore on the hearth, and
thence into the flues _c_, which repeatedly circle round the furnace
so as to provide abundant opportunity for the arsenious oxide derived
from the combustion (oxidation) of the arsenic in the ore to condense;
this highly poisonous arsenious oxide is collected in a solid form from
the flues at convenient intervals by means of the doors _d_. The ore
is charged and discharged at the door _e_. The roasting should not be
carried to such a point that the whole of the sulphur and arsenic are
removed when making smalts, as by leaving a portion of these substances
in the ore at this stage, the ultimate purification is better
accomplished.

The next stage is to fuse the roasted ore with potash and silica so
as to form a blue glass. The proportions in which the ingredients are
mixed depend upon the depth of colour in the zaffre operated upon and
the tint desired in the finished smalts; hence it is always determined
by a preliminary experiment, and is then most carefully adhered to,
each material being accurately weighed out. Only the best potash can
be used, as it must be quite free from soda, and iron or other metal;
the effect of soda is to render the blue greenish tinted. Quartz
affords the requisite silica, and is hand-picked to ensure freedom from
alumina, iron, and lime, which import dullness into the colour, and
then ground to a fine powder in an edge runner mill. The duly weighed
quantities of the several ingredients are intimately mixed in wooden or
cement lined vessels, so as to preclude the possibility of any metallic
iron finding its way in; and as a further protection against this risk
a little white arsenic is often added so that the iron may be carried
down in the regulus which is formed during the fusion in the crucible.

These crucibles are of refractory earthenware quite free from lime,
and measure about 18 inches across at top, gradually diminishing to 14
inches at bottom, so that an ordinary charge is about ¾ cwt. They are
placed in rows in a furnace which generally bears a close resemblance
to a glass furnace, the operation being very similar. The form of
furnace common in Saxony, where most of the smalts is made, is shown in
Fig. 13. By means of a series of openings a in the walls of the furnace
the pots _b_ are introduced on to the hearth of the furnace, whereupon
the openings _a_ are bricked up again and remain closed during the
operation. The ingredients are charged into the pots _b_

[Illustration: Fig. 13.--FURNACE FOR MAKING SMALTS.]

by means of long iron ladles which are introduced through the small
square apertures _c_, which can be temporarily closed by a half-brick
or other simple article. The fire is then lit in the fireplace _d_,
and the products of combustion circulate around the pots _b_, and
finally escape at the orifices _e_ at the top of the furnace into
flues leading to the chimney _f_. After about 8 hours’ firing fusion
commences in the pots, whereupon the contents are thoroughly stirred by
rods inserted through the working holes _c_. The temperature is then
increased till a white heat is attained, this being necessary for the
formation of a glass. The fused mass is repeatedly sampled, and when
it has become quite homogeneous, and the regulus or speiss containing
the iron, antimony, bismuth, arsenic, copper, nickel, sulphur and other
impurities has completely separated itself and collected at the bottom
of the pots, the blue glass is ladled out and dropped at once into cold
water, by which it is disintegrated and rendered very brittle ready for
the subsequent grinding. The regulus is then drawn off from the pots
through holes provided for the purpose, and removed by the orifices
_g_, after which the pots are ready for another charge. They ordinarily
remain serviceable for about six months.

The grinding needs to be done with great thoroughness, and is
accomplished partly by stamps and partly by edge-runner mills in the
presence of water. The particles as reduced are floated off by the
water to a series of settling tanks communicating one with another.
The portion which settles in the first of the series is too coarse for
use, and is returned to the edge-runner for further grinding; while the
portion in the last of the series possesses such a weak colour that it
is rejected, or put into the crucible to undergo a second fusion. The
selected portions are dried ready for the consumer.

=Copper Blues.=--These form an unimportant class, being unstable
and not endowed with great colouring power. Their tint is pale and
greenish, and though opaque in water, they are not particularly so when
mixed with oil. Exposed to the action of sulphur or its compounds,
whether present as sulphuretted hydrogen in the air, or in combination
with a metal, forming another pigment with which they may be mixed,
copper blues undergo an important chemical change, the carbonates and
oxides of copper being converted into the sulphide, which is black.
Under the influence of heat too the blue carbonate will lose its
carbonic acid, and be turned into the black oxide. Ammonia and the
acids dissolve them, but other alkalies are resisted until heat is
applied. The chief kinds of copper blue are Bremen blue, cæruleum, lime
blue, mountain blue, Péligot’s blue, and blue verditer.

BREMEN BLUE.--This is a more or less pure hydrated oxide
of copper, varying in its qualities according to the method of
preparation. When made by precipitating a neutral salt of copper from
solution, it forms a dense and compact mass; whereas a porous and
pulverulent pigment results when basic and insoluble copper salts are
treated with alkalies.

(1) The foundation of the manufacture of this colour was waste copper
scrap, such as ship’s sheathing, from which, in various ways, was
prepared a basic chloride or oxy-chloride. Some of the methods adopted
were:--(_a_) 100 lb. scrap copper, 99 lb. powdered sulphate of potash,
and 100 lb. salt, moistened with clean water; (_b_) 100 lb. copper
fragments, 60 lb. salt, and 30 lb. diluted sulphuric acid (3 volumes
of water to 1 of acid); (_c_) a solution of copper oxide (scales) in
pure hydrochloric acid poured over the scrap copper. The method (_a_)
produces a chloride of copper which, in contact with more metal,
becomes a sub-chloride; this, absorbing more oxygen from the air, is
converted into the basic green “oxide” of the factories. By the (_b_)
process, the hydrochloric acid set free, and the atmospheric oxygen
produce the same result. In the (_c_) process a similar effect is
obtained.

It is of primary importance that no trace of this sub-chloride of
copper shall be allowed to remain, as it undergoes decomposition
by caustic alkalies, and throws down an orange-yellow sub-oxide of
copper. Hence it has sometimes been the practice to prepare the basic
oxy-chloride twelve months in advance, and to stir it frequently before
use. Complete oxidation, however, can be as satisfactorily accomplished
by alternately wetting and completely drying the mass.

An interesting phenomenon takes place during the transformation of
this green magma into a hydrated oxide of copper. On this magma being
introduced by degrees into a caustic potash or soda lye of about 22°
B., the thoroughly washed and dried product is exceedingly fine, with
great covering power, and deepens on addition of a little water. When
the magma is diluted with an equal volume of water, and the mixture at
once poured into an excess of caustic lye, with constant agitation, a
few minutes’ rest will suffice for the mass to assume a most compact
consistence. The colour thus produced, when washed and dried, is much
lighter in colour, and has less body. A blue derived from any of these
products is unsatisfactory as regards freshness and intensity of
colour; whereas by adding a small quantity of concentrated solution of
sulphate of copper to the magma before treating it with the alkaline
lye, apparently a highly basic sulphate of copper is produced which
deepens the colour.

A pigment with good body can be made in the following manner. To 100
lb. of the thick magma of basic oxychloride add a concentrated solution
of 7 lb. sulphate of copper, and then 40 lb. of a concentrated caustic
lye (32°-36° B,), with vigorous and rapid stirring, finally adding
about 150 lb. of caustic lye at 20° B. When the decomposition is quite
complete, the precipitate is carefully washed, passed through a fine
hair sieve, and filtered. Drying is effected at a low temperature, to
ensure that the hydrated state of the oxide is not changed; and the air
of the drying chamber must be free from acid or sulphuretted vapours.

(2) If neutral nitrate of copper be decomposed by an insufficiency
of potash carbonate solution, the flocculent precipitate of copper
carbonate resulting is by degrees transformed into a sub-nitrate of
copper, which goes down as a heavy green powder. On treating this
sub-nitrate with a potassic solution of zinc oxide, a dark-blue
coloured pigment is formed, which is apparently a zincate of copper
mixed with a very small proportion of a highly basic nitrate of
copper. Though very light it has great covering power. In practice the
manufacture is conducted as follows.

Calcine copper scales in a reverberatory or muffle furnace till the
sub-oxide is entirely converted into protoxide, or until it dissolves
in nitric acid without evolving red nitrous fumes. Heat is applied to
the solution of nitrate of copper, which is decomposed by addition of a
clear solution of potash carbonate. After effervescence has subsided,
small doses of potash carbonate solution are added till but little
undecomposed copper is left. This residue is recovered by decanting the
clear liquor, and repeatedly washing the green precipitate with small
quantities of clean water, collecting all the washings, and finally
precipitating by potash solution. On introducing the green carbonate of
copper into a new solution of copper nitrate, it is transformed into a
basic salt. Crystals of nitrate of potash are obtained by evaporating
the previous liquors.

To obtain an economic solution of zinc oxide, clippings of metallic
zinc are treated with a solution of caustic potash or soda in a
cast-iron vessel. The immediate result is a disengagement of hydrogen,
and saturation of the alkali with zinc oxide, which behaves as an acid.
The cleared liquor serves for decomposing the basic nitrate of copper.
The pigment produced is a handsome blue, and the potash liquor can be
evaporated down till it yields crystals of saltpetre. The economy of
this method lies in producing nitric acid cheaply from soda nitrate and
obtaining saltpetre as a bye-product.

(3) An inferior and cheaper pigment is made in the following manner.
To a solution of copper sulphate add one of barium or calcium chloride
till a white precipitate ceases to go down, and from the cleared blue
liquor all the copper is precipitated by addition of fresh milk of
lime. Usually the weight of quicklime required is 20 per cent. of the
copper sulphate. The settled, washed, and dried precipitate is the
pigment desired. The cleared barium or calcium chloride solution may be
used anew as a precipitant for the next batch.

CÆRULEUM.--This name has been given to the beautiful blue
pigment used in Egyptian and Pompeian mural paintings, and exhibiting
the same bright blue after 1000 years’ exposure to the weather as when
first used. Its composition has been given by Fouqué as approximately
63½ per cent. silica, 21 per cent. copper oxide, and 14 per cent.
calcium oxide, and he regards it as a double silicate of copper and
calcium. It is supposed to have been produced by fusing together
copper ore, sand and lime, but experiments have not yet resulted in a
successful imitation of the pigment, a difficulty being encountered
in the fact that if too high a temperature be permitted, destruction
of the blue colour ensues, and a green glass results instead. This is
unfortunate, as its remarkably bright and stable hue would make it very
popular if it could be manufactured at a moderate cost. It withstands
sulphuretted hydrogen, and even prolonged boiling with any of the acids
or alkalies.

LIME BLUE.--This pigment is essentially a mixture of hydrated
oxide of copper and calcium sulphate. It resists the action of alkalies
in the cold, but turns black when boiled in caustic soda, and is
completely soluble in hydrochloric acid. Ultramarine has largely, if
not entirely usurped its place. There are several ways of making lime
blue:--

1. Any soluble copper salt the acid of which will make a soluble salt
with lime is suitable, the only precaution necessary being that if in
the decomposition of the copper and lime salts, the combination of
the whole of the sulphuric acid with the lime is not attained, there
should be an excess of copper sulphate in the liquor rather than of
the lime salt. The resulting copper solution, containing very little
lime sulphate, is settled in a cool place for 24-36 hours, filtered,
and diluted with clean water down to about 18° B. Meantime a milk of
lime is prepared with very white and well-burned lime, slaked and mixed
with abundance of pure water, and kept stirred for a long time in a
lead-lined vat. After a short rest to permit sand, &c., to precipitate
itself, the milk is drawn off, and left to settle in lead-lined or
copper pans. The deposit is collected, ground in a mill where contact
with iron is impossible, and passed through a very fine sieve.

The mixture of lime and copper solution is made in the proportion
of 100 lb. dry lime with 175 dry copper salt, if the most intense
coloration is desired, but the proportion of lime may be much increased
without detriment to the pigment beyond lessening its intensity of
colour. After complete settlement of the precipitate, the clear liquor
is decanted; the pigment is carefully washed with clean soft water,
and drained on filter cloths till it is of a convenient consistence
forming a green paste. A definite weight of this paste calculated on
the dry pigment is taken for further incorporation, consequently it is
first necessary to ascertain how much water is in the paste. Usually
it amounts to 75 per cent., and on this basis 5 lb. of the paste are
stirred up with 1 gal. clean water in a lead-lined vat, with addition
of ½ lb. wet lime under constant agitation. Subsequently ¼ pint of
clear solution of best potash at 15° B. is well stirred in, and the
mixture is immediately taken to the mill and most thoroughly ground.

Further, for each 10 gal. of green paste is prepared a clear solution
of 1 lb. pure salammoniac in 2 gal. water and another solution of 2 lb.
copper sulphate in 2 gal. water. The liquid paste is drawn off from the
mill into a stoneware vessel, and the two solutions of salammoniac and
copper sulphate are immediately added. After complete agitation and
combination, the mixture is left for 4 or 5 days to settle, and turned
into a lead-lined vat, where it is repeatedly washed with clean waters
until turmeric paper is not discoloured.

(2) Precipitation of copper sulphate by excess of thin milk of lime in
the cold, followed by washing and drying, will give a lime blue which
will dry without turning black. Or 100 lb. of the copper sulphate may
be treated with a milk of lime prepared from 30 lb. quicklime and
addition of 12½ lb. salammoniac. When the liquor has become colourless,
the pigment is prepared from the precipitate; but the lime should be
ground after slaking, and the milk of lime left to stand for some days,
before use. The salammoniac seems to be essential to the production of
a pure full blue. Milk of lime poured drop by drop into the ammoniacal
copper solution gives a precipitate which redissolves on agitation,
and remains long in solution under heat, but finally throws down a
permanent precipitate, while the liquor on standing gives beautiful
blue crystals. From experiments it appears that out of seven atoms of
copper sulphate in the liquor, five are precipitated by milk of lime
and the last two are decomposed by ammonia. A greater proportion of
lime will produce a precipitate holding a certain quantity of less
valuable pigment. A smaller proportion of lime yields a finer coloured
and more crystalline pigment, because it crystallises partly in the
excess of solution, so that by incomplete decomposition a smaller
yield of superior pigment is obtained. The proportions necessary for
formation of the colour are 7 equivalents of copper sulphate, 5 of
lime, and 2 of ammonia, and if the 2 equivalents of ammonia be replaced
by 2 of lime and 2 of salammoniac, the proportions furnishing the
best colour will be 100 lb. copper sulphate, 24 lb. lime, and 22½ lb.
salammoniac.

Both caustic soda and caustic potash produce a fine blue precipitate
in a solution of ammoniacal copper sulphate with excess of ammonia,
but the liquor decolorises only on evaporation of the ammonia. The
precipitate becomes lighter-hued the more it is washed, and consists
of hydrated oxide of copper with a little carbonic acid; it does not
turn brown even when heated in presence of excess of potash or soda.
Moreover the presence of ammonia renders the hydrated oxide of copper
much more permanent. The composition of this pigment is given by
Gentele as 33½ per cent. copper oxide, 23½ sulphuric acid, 16 lime, and
the remainder water, &c.

MOUNTAIN BLUE OR AZURITE.--This natural blue pigment consists
essentially of a basic carbonate of copper, and is found in quartz
rocks in England, France, Bohemia, Hesse, Saxony, the Tyrol, and
Siberia. It affords a rich sky-blue paint of a permanent character,
but being comparatively costly is not largely employed. Its composition
is about 69 per cent. copper oxide, 25½ carbonic acid, and 5 water. The
only preparation needed is exceedingly fine grinding.

PÉLIGOT BLUE.--(1) Whereas the hydrated oxide of copper
precipitated from a solution of a salt of copper by excess of potash
or soda rapidly blackens even though washed with cold water, Péligot
obtains a blue hydrated oxide which resists boiling and heating at 212°
F. He uses any soluble copper salt, but preferably the sulphate. A
very dilute solution of the copper sulphate is treated with ammonia in
excess (aqua ammoniæ or an ammoniacal salt) and precipitated by soda or
potash.

(2) On adding water in excess to a slightly ammoniacal solution of
copper nitrate, the same pigment is obtained.

(3) A mixture of 73 parts silica, 16 oxide of copper, 8 lime, and 3
soda, is fused together at a temperature not much exceeding 800° F. At
higher temperatures there is risk of the pigment turning black.

VERDITER.--This sky-blue and not very durable pigment, used
in water-colour painting, closely resembles Bremen blue (see p. 34) in
composition and manufacture. It consists chiefly of copper carbonate,
mixed with a lesser proportion of hydroxide, sulphate, or oxide,
and occasionally a small quantity of sulphate of lime; and is most
satisfactorily prepared from copper chloride or nitrate, though almost
any salt of copper may be used. The mode of fabrication varies.

(1) To a solution of the nitrate or sulphate is added one of potash
or soda carbonate so long as any precipitate is formed, and this
precipitate, when filtered and washed, is treated with a weak caustic
soda solution.

(2) A hot solution of chloride of lime is added to a hot solution
of sulphate of copper at 62½° Tw. till the precipitate ceases to go
down. The solution of chloride of copper which constitutes the liquor
is filtered off, diluted with water to about 31½° Tw., and treated
with repeated small doses of slaked lime ground exceedingly fine in
water till no more copper is precipitated. The resulting green paste
is drained, filtered, washed, and put into wooden vats; here 8 lb. of
lime paste and 5 pints of potash carbonate solution at 25½° Tw. are
added for every 70 lb. of _dry_ colour contained in the green paste,
the whole mass being thoroughly agitated, then allowed to rest till the
development of the required shade is accomplished, when it is filtered,
washed, and dried.

(3) In some German works the final green paste as prepared in (2) is
put into air-tight vessels, and a solution of 3 lb. ammonium chloride
and 4 lb. sulphate of copper in 7 gal. of water is introduced for each
70 lb. of _dry_ colour in the green paste. After complete admixture of
all the ingredients, the receptacles are fastened up for several days
so that the reactions may proceed out of contact with the air, and
finally the pigment is removed, washed, and dried for use.

=Indigo.=--The well-known blue colouring matter termed indigo is
produced by a great number and variety of plants, distributed
throughout all the tropical countries of the globe. Commercially, it is
obtained chiefly from species of _Indigofera_, as _I. tinctoria_, the
cultivated species of India, furnishing the chief article of commerce,
found also in Madagascar, St. Domingo, &c.; and _I. Anil_, in the
Punjab, W. Indies, and on the Gambia river. Some is also obtained from
_I. argentea_, in Africa and America: _I. Caroliniana_; _I. disperma_,
the cultivated plant of Spain, America, and some of the E. Indies;
_I. cærulea_, the “black indigo” of India; _I. glauca_, in Egypt and
Arabia; _I. pseudo-tinctoria_, cultivated in some parts of the E.
Indies, and said to yield the best dye; _I. cinerea_, _I. erecta_, _I.
hirsuta_, and _I. glabra_, in Guinea. Considerable local supplies are
obtained from the following plants:--_Isatis tinctoria_, in Europe and
China (see Woad); _I. indigotica_, cultivated in some parts of China;
_Amorpha fruticosa_, in Carolina; _Baptisia tinctoria_, wild, in the
United States; _Gymnemia_ (_Asclepias_) _tingens_, in Burmah; _Polygala
tinctoria_, in Arabia; _Polygonum Chinense_, _P. tinctorium_, _P.
perfoliatum_, _P. barbatum_, _P. aviculare_, in China and Japan, and
introduced into Belgium; _Ruellia indigotica_, largely cultivated in
Assam, as well as in India, and at Che-king, in China; _Tephrosia
tinctoria_, and _T. apollinea_, in India and Egypt; _Wrightia
tinctoria_ (_Nerium tinctorium_), the Palas indigo of the Carnatic.

The cultivation of indigo (chiefly _Indigofera tinctoria_) is very
extensively carried on in India, especially in the district included
between 20° and 30° N. lat. The soil best suited for the culture is a
rich loam, with a subsoil which is neither too sandy nor too stiff;
alluvial soils give the best returns, but good crops are sometimes
raised on higher grounds. The land is ploughed in October-November,
after the rains; the seed, about 12 lb. to the acre, is sown in
February-April. Too rapid growth diminishes the yield of dye. In
July-September, the plants are in full blossom, and the harvest takes
place. The preparation of the dyestuff may be performed in either
of two ways, which are distinguished as the “dry-leaf,” and the
“green-leaf” process. The latter is considered the better, and is
the more general; it is conducted as follows:--The flowering plants
are cut down at about 6 in. from the ground, and immediately taken
to the steeping vats, within which they are spread out and pressed
down by beams fitted to the side posts of the tanks. Enough water is
then admitted to cover the plants; if this be delayed, fermentation
may set in and spoil the product. The duration of the steeping is
liable to considerable modification, and needs much judgment and
experience; with a temperature of 96° F. in the shade, 11-12 hours may
suffice; in cooler weather, 15-16 hours may be necessary. Moreover,
very ripe plants require less time than young and unripe ones. The
following general conditions indicate the time for suspending the
maceration:--(1) The sinking of the water in the vat; (2) the immediate
bursting of the bubbles that arise; (3) an orange tint mingled with
the green, when the surface water is disturbed; (4) the emission
of a sweetish, pungent odour, quite distinct from the raw odour of
the unripe liquor. At this point, men enter the vat, and stir up its
contents either by hand or by a wooden paddle. The agitation is at
first gentle, but increases as the fecula begins to separate; this
is known by the disappearance of the froth, and by the colour of the
liquor changing from green to blue. The “beating,” as it is called, is
continued for 1¾-3 hours, the following conditions being a guide as
to its sufficiency:--(1) The ready precipitation of the fecula from a
sample of the liquor, and the madeira-wine colour of the latter; (2)
a brownish colour observed on dipping a cloth into the liquor, and
wringing it out; (3) the appearance of a glassy surface on the liquor,
and the subsidence of the froth with sparkling and effervescence.

Next a little pure cold water, or weak lime-water, is sprinkled over
the surface of the liquor, to hasten the settlement of the fecula,
which occupies 3-4 hours. After this, the water is drained away
from the top, by means of plug-holes in the side of the vat. The
precipitated fecula is then removed to a boiler. Here it is made to
boil as promptly as possible, and is kept boiling for 5-6 hours; it is
constantly stirred, and skimmed with a perforated ladle. After boiling,
it is run off to a straining table, where it stays for 12-15 hours to
drain; next it is pressed for about 12 hours, and then cut, stamped,
and placed to dry. The ordinary dimensions of a steeping-vat are 16 ft.
by 14 ft. by 4½ ft. deep; this will contain about 100 _maunds_ (8200
lb.) of plants, which may yield from 40 lb. downwards of indigo. The
beating-vat is less deep.

Such are the methods of cultivation and manufacture most generally in
use throughout India. In limited districts, however, some modifications
are in vogue. On land subject to inundation, the plants last only one
year. South of the Ganges, the seed is sown at the beginning of the
rains, and the plants remain on the ground for two years, thus giving
a double crop, the second of which is the larger and better. In very
strong land, a third crop is sometimes secured. Occasionally, sesame
is sown on the same ground, and harvested before the indigo is cut.
Small quantities of indigo are grown on poppy lands, and irrigated. The
seed is sown in March-April, and the crop is gathered at the end of
the rains, in time for an opium crop to be taken off the land. Indigo
is sometimes manufactured by collecting the fecula, and dropping it in
cakes to harden in the sun; this is “gaud” indigo, of very inferior
quality. The fecula is improved by boiling it in coppers and pressing
it into boxes. The production of the indigo blue is the result of the
decomposition of the colouring principle of the plant, which exists as
a glucoside. Plants grown on poor soils, and in dry climates, yield
almost the whole of this glucoside to the ordinary process of steeping
and beating described above; but plants raised on rich alluvial soil,
and in damp heat, contain an amount of glucoside which cannot be
utilised by the ordinary process. In order to prevent this waste, which
causes the richest plants to give the least return, it is necessary
either to prolong the fermentation, and raise the heat to 95°-100° F.,
or to add a solution of sugar or glucose to the vat-liquor. Olphert
adopts the use of steam, to raise the temperature of the vat to 111°
F., and thus obtains 25 per cent. more colouring matter.

Japan possesses several large factories for preparing indigo from the
native _Polygonum tinctorium_. The plants, 2-3 ft. high, are cut into
three parts, the uppermost being the most valuable. The best dye is
made from the leaves alone, which, after a few hours’ exposure to air
and sun, are placed in straw bags. They are afterwards removed from
the bags and moistened with water, which must be proportioned with
the greatest exactitude. They are then spread upon, and covered by,
mats, for a few days, after which the sprinkling is repeated. The
process continues for about 80 days, the moistening being renewed
about 25 times for the best leaves, and 9 for the inferior. After
this fermentation, the leaves are pounded in wooden mortars for two
consecutive days, by which they are reduced to a pulp; this is then
formed into balls of dark-blue colour.

The central provinces of Java yield large quantities of indigo, which
are exported to Holland, and thence widely distributed. The indigo
prepared by the natives is of an indifferent quality, in a semi-fluid
state, and contains much quicklime; but that prepared by Europeans
is of a very superior quality. An inferior variety, having smaller
seeds, and being of quicker growth, is usually planted as a second
crop on land where one rice crop has been raised. In these situations,
the plant rises to a height of about 3½ ft. It is then cut, and the
cuttings are repeated three, or even four times, till the ground is
again required for the annual rice crop. But the superior plant, when
cultivated on a naturally rich soil, not impoverished by a previous
heavy crop, attains a height of 5 ft., and grows with the greatest
luxuriance. The plants intended for seed are raised in favourite spots,
on the ridges of rice-fields in the neighbourhood of the villages,
and the seed of one district is frequently exchanged for that of
another. That of the rich mountainous districts, being esteemed of best
quality, is occasionally introduced into the lowlands, and is thought
necessary to prevent that degeneration which would be the consequence
of cultivating for a long time the same plant upon the same soil. The
climate, soil, and state of society of Java seem to offer peculiar
advantages for the extensive cultivation of this plant. The periodical
droughts and inundations of the Bengal provinces are unknown in Java,
where the plant, in favoured situations, may be cultivated nearly
throughout the whole year, and where it would be secure of a prolonged
period of that kind of weather necessary for the cutting. The dye is
prepared in a liquid state by the natives, by infusing the leaves
with a quantity of lime; in this state, it forms by far the principal
dye-stuff of the country. The indigos prepared in Java by Sayers’s
process are of unusually high and constant quality. They contain an
average of 70½ per cent. of indigotine, and a minimum of 65-66 per
cent.; and an average of 2·77 per cent. of ash. Ordinary commercial
indigos seldom attain 65-66 per cent. of indigotine; and their ash
averages about 16½ per cent.

The Philippines produce considerable quantities of indigo, the best
coming from Luzon. The plants suffer from locusts and storms, but the
cultivation is very profitable. The yield of indigotine is large, but
the preparation is conducted in such a primitive manner that the value
of the product is much deteriorated.

In many parts of Africa, as Sierra Leone, Liberia, Abeokuta, the Niger
valley, Natal, Cape Colony, Tunis, and the Soudan, species of indigo
plants are found in a wild state, and from them the natives prepare an
inferior dye-stuff.

In some of the S. States of America, notably S. Carolina, indigo
culture has been attended with more or less success. The method of
preparation pursued here varies but very slightly from the ordinary
Indian process, almost the only important modification being the
addition of a little oil to the liquor in the beating-vat, when the
fermentation becomes too violent. The precipitated fecula is placed
in coarse linen bags, and hung up to drain. The drying is finished by
turning it out of the bags upon a floor of porous timber, and working
it up. It is frequently exposed to the sun for short periods at morning
and evening, and is then placed in boxes or frames, to cure till it is
fit for the market.

Several of the Central American States have figured conspicuously as
indigo producers. The dye is precipitated in the beating-vat by the sap
contained in the bark of Tihuilate (_Yonidium_), Platanillo (_Myrosma
Indica_), or Cuaja tinta. The fecula is left during the night; and,
on the following day, is boiled, filtered, pressed, and sun-dried.
In most districts, the cultivation is declining, partly owing to the
carelessness exhibited in the preparation of the dye.

Indigo is judged commercially by its lightness, by a copper gloss on
the surface, and by exhibiting no foreign ingredients when broken.
There are several ways of testing it chemically, to ascertain
the exact proportion of indigotine present; one method is as
follows:--Finely pulverised indigo, 1 part; green copperas, 2 parts;
and water containing 10 per cent. of caustic soda, 200 parts; are
well boiled in a flask, and left to cool. The clear liquor is exposed
in shallow vessels to the air, when the soluble indigo is oxidised,
and precipitated as pure indigotine. The residue in the flask is thus
treated three times; the whole indigotine is then collected on a
filter, dried, and weighed. The consumption of indigo is still very
large. Artificial indigo has not, as yet, been manufactured on a
commercial scale, nor at a commercial price; but it has been produced,
in the laboratory, from coal-tar derivatives, and further experiment
may reveal a process for preparing the article at a sufficiently low
price to compete with the natural colour.

Several preparations of indigo are in use:--(1) Sulphopurpuric acid,
phenicine, or indigo-purple, is made by mixing 1 part of indigo with 4
parts of sulphuric acid (sp. gr. 1·845), and heating for ½-1 hour; the
acid mass is thrown into 40-50 parts of water, when the purple falls
down; it is collected on a filter, and washed with dilute hydrochloric
acid. (2) Sulphindigotic acid is prepared by mixing indigotine, 1
part, with sulphuric acid (sp. gr. 1·845) 6 parts; the operation must
be performed in a leaden vessel, cooled outside, and the indigo must
be added by degrees, to avoid heating; the mixture is then left for 8
days, when the conversion will be complete. Fuming or anhydrous acid
may be used, in less proportion, but the reaction is more difficult
to manage. Weaker acid will require a longer period, say a month for
“brown acid” (145° Tw.). (3) Sulphindigotic acids are transformed into
neutral paste, or “carmine,” by neutralising with carbonate of soda,
and washing the paste, on a woollen filter, with a solution of chloride
of sodium (common salt).

=Manganese Blue.=--(1) Kuhlmann found a blue mass of manganate of lime
in furnaces used for making calcium chloride by calcining a mixture
of chalk and residues from chlorine making. The formation of this
beautiful coloured manganate he attributes to the decomposition of the
calcium chloride by steam, and to a certain solubility of the lime
in undecomposed calcium chloride. Unsuccessful attempts to reproduce
this result were apparently due to the lime not being under such
favourable conditions for acting upon the manganese oxide as when it
is in solution in the calcium chloride. As accidentally produced in
reverberatory furnaces, the manganate of lime is of an ultramarine
tint, and is insoluble in water though not permanent under its
influence; it is acted upon by the weakest acids.

(2) Bong has proposed several formulæ for making manganese blues,
the ingredients in each case being heated to redness in an oxidising
atmosphere, taking special care to avoid iron. The following are his
recipes:--(_a_) 6 parts soda ash, 5 of calcium carbonate, 3 of silica,
and 3 of manganese oxide; (_b_) 8 of barium nitrate, 2 of kaolin, and
3 of manganese oxide; (_c_) 8 of barium nitrate, 3 of silica, and 3
of manganese oxide. The tint can be varied from violet to green by
altering the proportions.

=Prussian Blue.=--This blue owes its colour to a combination of iron
and ferrocyanogen. The commercial products vary very much in tint,
depth of colour, covering power, and solubility. They are used for a
variety of purposes, nearly all of which require the blue to have some
property different from what it should have for other uses. For some
purposes a green shade blue is wanted, for others a violet shade blue;
some users want the blue to be soluble in water, others for it to be
soluble in oxalic acid, others require it to be insoluble. Ordinary
Prussian blue is insoluble in water, acids, and alkaline salts;
bleaching powder has no action on it, and therefore it is largely used
for tinting paper. It is capable of resisting acids; but alkalies,
such as caustic soda, caustic potash, the carbonates of the same
metals, lime and ammonia, decompose it, oxide of iron and a solution
of a ferrocyanide of the alkali being formed, the decomposition being
shown by a change of colour from blue to a reddish brown. On this
account Prussian blues cannot be used for colouring soaps and alkaline
products, or used as a pigment in distemper painting along with lime.
The change of colour, from blue to brown, by the action of alkalies,
distinguishes this blue from other blues.

Prussian blues require to be tested for their solubility in oxalic
acid, by taking about 20 gr. of the blue, mixing with 1 oz. of water
and 20 gr. of oxalic acid, in which a good blue ought to dissolve
completely. Some brands are soluble in strong hydrochloric acid while
others are not. It is decomposed by boiling sulphuric acid, and turned
green by boiling nitric acid. For all the ordinary uses of a pigment
Prussian blue is quite durable, and possesses a depth of colour and a
definite tint which is proof against the destructive agencies of light
and air; and though its covering powers are not great, it is one of the
most important blue pigments in use.

When a salt of the higher oxide of iron is added to a solution of
yellow prussiate of potash (or ferrocyanide of potassium) a blue
compound is formed which is called Prussian blue. But this is not the
method adopted for its commercial manufacture. In that case a ferrous
salt, the proto-sulphate of iron, is added to a potassium ferrocyanide
solution, the result of which is that a dirty bluish white precipitate
is thrown down. On adding to this a little solution of bichromate of
potash and sulphuric acid, the full deep blue is obtained. This is the
industrial method of manufacturing Prussian blue.

_Yellow Prussiate._--The first step is the preparation of the yellow
prussiate of potash. The manufacture of this substance, although
an industry of considerable importance, is comparatively little
understood, either from a scientific or a practical point of view. At
all events, many prussiate makers seem completely at sea with regard
to the most favourable conditions for carrying on the manufacture, and
there can be no doubt that in many cases great waste occurs, through
ignorance of the various reactions which take place during the process.
The raw materials usually consist of carbonate of potash, iron filings
or turnings, and organic matters containing carbon and nitrogen--such
as dried blood, woollen rags, horn, hair, leather scraps, &c. The
most suitable substances for use are, of course, those containing the
largest proportion of nitrogen. The following are the percentages of
nitrogen in various kinds of animal matter:--

  Horn             15 to 17
  Dried blood      15 to 17
  Woollen rags     10 to 16
  Sheep shearings  16 to 17
  Calves’ hair     15 to 17
  Bristles          9 to 10
  Feathers         16 to 17
  Hide clippings    4 to  5
  Old shoes         6 to  7
  Horn charcoal     2 to  7
  Rag charcoal      2 to 12

Animal matters always contain more carbon than is necessary for the
formation of cyanogen by combining with the nitrogen also present.
Consequently, when such substances are heated with pearlash, the
excess of carbon reduces a portion of the carbonate to the metallic
state, and this potassium combines with the cyanogen to produce
potassium cyanide. The manufacture of yellow prussiate of potash may
be conveniently divided into three stages: (1) The production of the
molten mass technically known as “metal”; (2) the lixiviation; and (3)
the crystallisation.

(1) The “metal” is made by fusing animal matters with pearlash, almost
invariably with the addition of iron scrap. The animal substances are
sometimes used in their original condition, whilst sometimes they are
previously charred. Generally speaking, however, a judicious mixture
of the fresh and charred materials has been found to give the best
results. The charcoal which is left on carbonising animal matters
contains a certain amount of nitrogen, decreasing in proportion as the
temperature rises; but a smaller quantity of charcoal is also thereby
produced. For example: 100 parts of rags carbonised at a certain
temperature left 75 parts charcoal containing 12 per cent. of nitrogen,
while the same rag carbonised at a higher temperature yielded 25 parts
of charcoal, which contained only 2 per cent. of nitrogen. The animal
matters employed should not leave much ash on ignition, as this would
both thicken the mass and decompose a portion of the potash. In this
respect sand is specially objectionable, for on ignition 1 part will
decompose 2 of pearlash, owing to formation of silicate of potash. It
is not necessary that the pearlash should be quite pure; in fact, a
certain proportion of sulphate is stated to be useful, as it is changed
into sulphide by ignition with the carbonaceous materials.

The theory of the formation of yellow prussiate of potash may be
briefly stated as follows: The carbonate and sulphate of potash
react with the carbon, nitrogen, and iron, forming in the first
instance sulphide of potassium, which afterwards converts the iron
into sulphide, whilst potassium cyanide is simultaneously produced.
It should be here explained that ferrocyanide of potassium (yellow
prussiate) is not formed during the ignition of the above mentioned
materials, but results from the lixiviation of the fused mass with
water, when the cyanide of potassium and iron sulphide decompose
each other, producing ferrocyanide and sulphide of potassium. It is
quite obvious that even if any ferrocyanide were produced during the
process of fusion, it would almost immediately be decomposed, at the
intense heat to which the mass is subjected, into potassium cyanide,
iron carbide, and nitrogen gas. If any doubt were felt on this point,
the experiments of Liebig conclusively prove that the formation of
ferrocyanide takes place on dissolving the ignited mass in water, but
not previously. Liebig found that if the fused mixture be allowed
to cool, and then treated with moderately strong alcohol, potassium
cyanide alone is extracted, and the residue when dissolved in water
no longer yields ferrocyanide. As ferrocyanide is not formed during
the process of fusion, the presence of iron in the preliminary stages
may appear superfluous; but such is not the case. The presence of iron
is necessary for two reasons, firstly, because the sulphate of potash
which is generally present is converted into sulphide and bisulphide,
and these, in the absence of iron, would decompose some of the cyanide
of potash into sulphocyanate, thereby causing a loss of cyanogen so
far as yellow prussiate is concerned; and secondly, because potassium
bisulphide has a very corrosive action on the iron pot in which the
fusion takes place. When iron is present it readily decomposes any
alkaline sulphides, thereby preventing formation of sulphocyanate, and
being itself converted into iron sulphide, which is again changed into
prussiate by the action of the aqueous cyanide.

Pear-shaped iron pots were formerly used for fusing the raw materials.
The arrangement now generally adopted in large English works consists
of a series of iron pots almost hemispherical in shape, set in
brickwork, and each heated by a separate fire and circular flue. These
vessels are closed by iron lids, with apertures for the admittance of
animal matters, the aperture being at once closed by a slide after each
addition. Through every lid there passes a vertical spindle, carrying a
set of blades for mixing the materials, and set in motion by a suitable
shaft worked by steam power. Instead of the ordinary iron pots,
reverberatory furnaces are often employed, especially in Germany. The
reason for this preference is, that ordinary iron vessels are worn out
in a comparatively short time, the destructive action being greatest on
the under surface of the muffle. A much larger quantity of raw material
can also be operated upon at one time if a reverberatory furnace be
used. The mode of procedure depends to some extent upon the condition
of the organic materials employed. If fresh, the muffle or furnace must
be left open, so as to permit the mixture to be well and frequently
stirred, and additions to be made at intervals until eventually
ammonia ceases to be evolved. The furnaces are arranged in such a
manner that when the carbonate of potash has once become fused the
doors of the fire-place may be shut, and no fresh firing is required
during the introduction of the animal matters. The molten mass is kept
well stirred by means of a thick iron bar, suspended by a chain, and
fixed in an aperture in the side of the furnace. By the use of this
arrangement the stirring is much more easily and thoroughly effected
than is the case with the old fashioned pots. Ordinary reverberatory
furnaces cannot be used for the fusion, because the silica in the
hearth would combine with the potash to form silicate of potash.
Gas generators with air blast are now sometimes employed instead of
ordinary fuel in the manufacture of yellow prussiate of potash. Several
advantages are gained by operating in this manner, especially that of
permitting the regulation of temperature and the admission of oxygen,
so as to obtain an ordinary, a neutral, or a reducing flame, according
to requirements. In the preparation of the “metal,” for every 100
parts of pearlash from 100 to 125 parts of fresh animal substances are
required, together with 6 or 8 parts of iron in some form or other. The
pearlash, or a mixture of 1 part of pearlash with 2 to 4 parts “blue
salt” or “blue potash” (this substance will be referred to later on),
is melted in the furnace and heated to bright redness, so that the
temperature of the mass may not be reduced too much by the addition
of the animal matters. These, in their original condition, or an
equivalent quantity of carbonised materials, together with the proper
proportion of iron, are then introduced--first pretty frequently,
afterwards at longer intervals. Each addition of animal matter causes a
somewhat violent frothing and escape of combustible gases, along with
water and carbonic acid, and the whole becomes thick--not so much owing
to the introduction of solid substances as by the fall of temperature,
resulting from the production of such large quantities of gas. In
order to hasten the decomposition, vigorous stirring must be applied.
When the reaction is at an end, the semi-fluid mass is transferred to
cast-iron dishes, and the furnace is again filled with carbonate of
potash and heated. In this way four or five charges may be accomplished
every day, and the process carried on continuously. The most favourable
conditions for effecting the melting part of the process are attained
when the heat approaches whiteness, and a bright, clear flame is
produced as soon as the raw materials are introduced. According to one
authority, woollen rags and good pearlash, with a small proportion
of waste iron, have produced the largest yield of yellow prussiate,
although even in this case two-thirds of the total nitrogen present was
lost in the form of ammonia.

(2) Lixiviation.--The fused mass, if properly prepared, should yield
about 16 per cent. of prussiate on dissolving in water. In this part
of the process, the “metal” when cold is broken into lumps and placed
in cold water mixed with the weak lyes from former operations. Heat
is then applied until the temperature rises to about 180°-190° F.,
and the liquid is stirred vigorously so as to promote rapid solution,
because some of the potassium cyanide is apt to be decomposed during
lixiviation. When the solution attains a density of 30°-40° Tw. it
is left to clarify, the heat being withdrawn. The clear solution is
decanted, and evaporated in pans, which are generally heated by the
waste heat of the furnaces. When it has a density of 54° Tw. it is run
off into the crystallisers, where it deposits the crude salt.

(3) Crystallisation.--This is a very important stage of the
manufacture, as it is the final process by which the crude prussiate
is rendered sufficiently pure to be placed on the market. The impure
substance is dissolved in warm water until the solution stands at 54°
Tw.; after all insoluble matter has deposited, the clear liquor is
placed in the crystallising vessels. These are occasionally made of
wood; but when such vessels are used, the crystallised salt generally
possesses a green colour, which is believed to be due to the tannin
present in the wood. On this account cast-iron crystallisers are more
frequently employed. The crystallisation proceeds slowly--often going
on for several weeks in large vessels. The mother liquor is then drawn
off, and if not too impure is used for dissolving fresh quantities
of the crude prussiate. The ferrocyanide is deposited in crusts in
the crystallisers; but by hanging lumps of the solid salt in the
solution, long clusters of crystals may be obtained, and by suspending
these in fresh prussiate lyes immense crystals are produced. From 100
parts crude prussiate about 90 parts pure potassium ferrocyanide are
obtained, or sometimes in the case of purer materials 97 parts.

Sulphate of potash is often present in commercial yellow prussiate.
The separation of this impurity is best effected on the large scale by
evaporating the prussiate solution to a density of 62° Tw., at which
point most of the sulphate will crystallise out. If the clear liquor be
then drawn off, diluted to 52° Tw., and allowed to cool, almost pure
potassium ferrocyanide will gradually deposit. This may be rendered
absolutely pure by gently fusing the crystals, dissolving in water, and
treating with a small quantity of acetic acid, which will decompose
any carbonates and cyanides. On adding sufficient strong alcohol, the
ferrocyanide is precipitated, and when crystallised once or twice more
from water it may be regarded as chemically pure.

Blue salt.--This substance, to which we have previously referred, is a
residue obtained in the manufacture of prussiate of potash. The last
mother-liquor contains a large quantity of carbonate of potash, along
with smaller amounts of hydrate, silicate, chloride, and sulphocyanate.
It is concentrated until the liquid has a density of 90° Tw., when most
of the chloride, silicate, &c., separates out, and the strong liquor
containing the greater proportion of the carbonate is evaporated to
dryness, and calcined in a reverberatory furnace. The dry residue
constitutes the “blue salt” or “blue potash,” and contains from 70
to 80 per cent. carbonate of potash. It may be employed instead of
pearlash, or mixed with it, for the next batch of yellow prussiate. The
composition and amount of the insoluble residue left on lixiviation of
the “metal” vary according to the proportions and character of the raw
materials used. Other conditions being equal, horn gives the lowest
percentage of insoluble matter on lixiviation.

The large proportions of potash and phosphates contained in the
insoluble residues render them well suited for use in the manufacture
of artificial manures. As already mentioned, when regarded from a
scientific or economical point of view, the yellow prussiate industry
is carried on under very imperfect conditions. In addition to the
amount of potash, there is a very considerable waste of nitrogen,
firstly, because the larger proportion of that element present in the
animal substances is not converted into cyanogen at all, but passes
off chiefly in the form of ammonia salts; and, secondly, because
part of the potassium cyanide which is actually produced is lost by
decomposition, and another portion is left in the mother liquor. It
has been calculated that out of every 100 parts of ferrocyanide which
should theoretically be obtained, 4 parts are lost when fairly pure
materials have been employed, and 14 in the case of impure ingredients.

The following analyses indicate the percentage composition of two
samples of insoluble residue:--

                                          No. 1.  No. 2.

  Sulphate of potash, &c.                  9·06    3·21
  Phosphates of lime, magnesia and iron   13·74    6·24
  Oxide of iron                           13·34   19·58
  Lime and magnesia                        5·08    7·23
  Sand and silica                         23·97   29·24
  Charcoal and moisture                   34·81   34·50
                                         ------  ------
                                         100·00  100·00

According to Karmrodt, the following proportions of the nitrogen
contained in various animal substances are actually converted into
cyanogen during the manufacture of yellow prussiate of potash:--

                                    Per cent.

  Woollen rags                         16
  Horn                                 20
  Leather cuttings                     33
  Cow hair                             14
  Dried blood                          16
  Horn charcoal                        56
  Rag charcoal                         33

As is well known, human excreta contain a considerable proportion of
nitrogen, and there seems no reason why this should not be employed
in the manufacture of yellow prussiate. It is quite possible that
municipal bodies might find this a convenient and profitable plan
of disposing of a portion of the sewage with which they have to
deal. It is obvious to all persons who have given this subject much
consideration, that the nitrogen required in the manufacture of yellow
prussiate of potash might be obtained with comparative ease from the
surrounding atmosphere. Indeed, from a theoretical point of view this
seems a charming process. About fifty years ago the Society of Arts
awarded Lewis Thompson a medal in connection with this very process.
Thompson ignited a mixture of 2 parts pearlash, 2 parts coke, and 1
part iron turnings in an open crucible for a considerable time at a
full red heat. The resulting black mass was found to contain a large
quantity of ferrocyanide, together with excess of carbonate of potash,
&c. This process, or a similar one, in which a current of air was
passed over a mixture of charcoal and iron saturated with carbonate of
potash, was tried on a large scale for two years at Bramwell’s works
at Newcastle. About 1 ton of yellow prussiate was made daily by this
process; but it was not found to work profitably, and was eventually
abandoned, chiefly, it is said, owing to the large amount of fuel
required, and because the cylinders, whether of iron or fireclay, were
not able to stand for any length of time the intense heat to which they
were subjected.

The annexed illustrations, Figs. 14 to 17, show the arrangement of a
prussiate of potash furnace at Sir E. Buckley’s works, at Clayton,
Manchester, which are well designed to prevent nuisance: A, iron pot;
B, fire-place; _a_, cover of pot; _b_, stirrer; _c_, hinged pipe
conveying vapours to the flues; _d_, flues surrounding the pot, and
leading to the chimney-shaft; _e_, chain to lift up cast-iron vapour
hood.

Brunquell, a German manufacturer, has criticised the present method
of conducting operations, and proposes that it is necessary as far
as practicable to aid the secondary formation of cyanogen by ammonia
and incandescent charcoal, and to avoid loss of potash by using pure
animal substances, and preventing contact with the solid products of
combustion from the furnace. With this view he adopts a horizontal
reverberatory furnace, the hearth of which is a cast-iron tray about
4½ ft. long, 4 ft. wide, and 3½ in. deep. The crown of the furnace is
built as flat as possible, the working space is limited, and the charge
is kept from contamination by the fire. Such a furnace, despite certain
drawbacks, presents important advantages. Fuel is economised; the
process is hastened so that seven or eight charges can be dealt with
in a day, instead of only four; and the furnaces cost less and endure
longer. The charge consists of 220 lb. potash, of which two-thirds is
from evaporated mother-liquors, and one-third fresh; 44 lb. animal
charcoal from the carbonisation of substances poor in nitrogen; 140-150
lb. of pure animal matters as dry as practicable; and 17½ lb. iron. The
firing is urged and the charge is stirred till all the potash is fused,
when the ash-pit is closed, and the damper turned on for charging half
the animal charcoal. The firing and stirring are again pushed on till
the proper consistency is attained, and potassium vapour begins to burn
off. In this state the mass is ready to receive the animal

[Illustration: Figs. 14, 15, 16, 17.--YELLOW PRUSSIATE
FURNACE.]

substances, those rich in nitrogen being first added in small portions
at a time. Their effect is to render the mass hard, dry, and difficult
of fusion, whereupon the remainder of the animal charcoal should be
promptly introduced. After thorough agitation, the working door is
closed for a short time, and the contents of the furnace are rapidly
discharged into a covered iron pan.

The character of the animal matters employed varies so much that it is
impossible to lay down hard and fast rules for the proportions of the
several ingredients, or the duration of the roasting. Nor is the value
of a raw material always in proportion to its richness in nitrogen,
because the poorer material may waste less potash, consume less fuel,
and require less labour. The addition of iron filings or turnings is
useful only in prolonging the life of the cast-iron crucibles.

_Combination of the Cyanide and Iron Solutions._--A great number of
recipes are in vogue for combining the two solutions of ferrocyanide
and an iron salt, both with reference to their proportions, and to the
addition of foreign matters of various kinds. These variations in the
formulæ give rise to distinct names for some kinds of Prussian blue,
which will be referred to below. The ordinary common Prussian blue has
a greenish tendency, and is chiefly made according to one or the other
of the following directions:--

(1) Mix a solution of 100 lb. yellow prussiate with a solution of 100
lb. green copperas (ferrous sulphate) and 18 lb. alum, to which 9 lb.
sulphuric acid has been added, and let the mixture stand for 2-3 hours,
or until the solid portion has completely settled out. Decant the clear
supernatant liquor, and well wash the precipitate with clean waters.
Finally throw it on a filter and subject it to repeated disturbance,
so as to ensure the admission of air to every particle, in order that
the requisite oxidation may take place. The proportion of alum used
is subject to very great variation according to individual fancy; it
renders the subsequent grinding of the pigment a very much easier
matter, but it causes the shade of blue to be paler than it otherwise
would be.

(2) The simple solutions of green copperas and yellow prussiate in
equal proportions are mixed together without any other ingredient being
added, and the precipitate produced is washed, filtered, and aërated as
in (1). It is, however, inferior by reason of the oxide of iron formed
in the pigment spoiling the purity of the colour, and necessitating the
treatment of the wet mass with hydrochloric acid, at some expense, for
removal of the iron oxide.

ANTWERP BLUE.--This pale variety of Prussian blue has but
little importance now. It is prepared by adding a solution of 4 lb.
yellow prussiate in 5-6 gallons of water to one of 2 lb. sulphate
of iron, and 1 lb. each of alum and sulphate of zinc in an equal
quantity of water. The resulting pigment consists of a mixture of the
ferrocyanides of iron, alumina, and zinc; it is washed, filtered,
aërated, and dried as other forms of Prussian blue.

BONG’S BLUE.--When cyanide of potassium is added to an
acid solution of a copper salt, a red colour is produced, which has
already been mentioned by different observers. The substance formed
is very changeable, at least in the liquid where it is formed. It
is decomposed by acids, alkalies, cyanide of potassium, and even
decomposes spontaneously, the colour changing to yellow. It is
precipitated by insoluble cyanides; hence when a dilute acid is added
to the red solution, the dye is at once thrown down along with the
cyanide of copper. If the precipitate thus obtained is treated with
sulphuretted hydrogen, it is decomposed and the substance is set free.
This substance can combine with iron, like cyanogen, so as to conceal
the properties of the iron. This compound is very permanent, and has
lately been studied by Bong, who gives the following directions for its
preparation:--

Cyanide of potassium is added in excess to an acid solution of a
copper salt until the red colour at first formed has disappeared,
when a ferric salt is at once added. On the addition of the iron salt,
of course, a copious precipitation of Prussian blue takes place,
and the liquid again turns to a dark purple-red. To separate the
colouring substance from the alkaline salts in the liquid, a dilute
acid is added, which precipitates it and the cyanide of copper. This
precipitate is combined with the Prussian blue, which also contains
a considerable quantity of the colouring substance, and then treated
with a boiling solution of carbonate of ammonia, in which it dissolves.
As the cyanide of copper also goes into solution, it is separated by
again precipitating it with an acid, and treating the precipitate with
sulphuretted hydrogen. The colouring substance thus liberated now
contains a certain amount of hydroferrocyanic acid, which is removed
after neutralisation by acetate of lead. It is now filtered, and the
purification is completed by precipitating with a silver salt and
treating the precipitate with sulphuretted hydrogen.

This purple-coloured compound crystallises very indistinctly. To
determine its composition, Bong precipitated it with acetate of copper.
When dried at 212° F., the rose-coloured precipitate had the following
composition: Carbon 24·31, nitrogen 28·04, hydrogen 1·88, iron 13·66,
copper 17·67, oxygen 14·44. Total 100·00. These numbers correspond to
the formula Cu, Fe Cy_{4} (HO)_{4}.

This substance is likewise precipitated by salts of zinc, mercury, and
silver. All these precipitates are pink or purple, very beautiful, and
of remarkable brilliancy. They are soluble in alkalies. Iron salts
yield no precipitate, nor do lead salts, except in the presence of
ammonia, when a blue-violet precipitate is formed. When treated with
sulphuretted hydrogen, these precipitates yield purple-red and acid
liquids, which undergo change in the air, especially if warm, forming
Prussian blue. When these liquids are neutralised with alkali, purple
compounds are formed, which are permanent in the air, soluble in water,
slightly so in alcohol, and insoluble in ether. Their colouring power
is exceptionally great. These pigments will unite with ferrocyanides,
and in its preparation such a compound is produced in considerable
quantity; it is likewise of a purple colour, and gives a rose-coloured
precipitate with acetate of lead. Both alone and in this compound it is
very permanent; it resists the action of sulphurous acid, concentrated
and boiling alkalies, and dilute acids, but is rapidly destroyed by
chlorine and nitric acid. If this pigment could be prepared cheaply
enough, it would probably be used with advantage in the arts, on
account of its resistance to chemical reagents and light, the variety
of its shades, and its brilliancy. It does not colour fibres directly,
but can readily be fixed on them from slightly acid solutions, if they
are previously mordanted with metallic oxides.

BRUNSWICK BLUE.--This pigment is made in pale, medium, and
deep shades, and is an extremely useful colour, being very fine,
requiring no grinding, thoroughly permanent in light and air, hardly
acted upon by acids, but turned brown by alkalies, and liable on
standing to separate into two portions--a white and a blue--the latter
coming to the surface while the former sinks, and necessitating a
thorough stirring of the paint before use.

It generally consists simply of barytes, or gypsum, or china clay,
coloured by a small percentage of Prussian blue, with or without the
addition of a lesser proportion of ultramarine. The barytes or other
base is very thoroughly agitated in water, while a solution of green
copperas and a solution of yellow prussiate are gradually added without
ceasing the agitation. When the incorporation of the ingredients has
been completely accomplished, the precipitate is settled, washed,
filtered, and dried. Following are a few recipes:--

    Pale. (1) 1 cwt. barytes, 1 lb. green copperas, 1 lb. yellow
    prussiate.

    Pale. (2) 1 cwt. china clay, 2 lb. green copperas, 2 lb. yellow
    prussiate.

    Pale. (3) 1 cwt. gypsum, 1½ lb. green copperas, 1½ lb. yellow
    prussiate.

    Medium. (1) 1 cwt. barytes, 3 lb. green copperas, 3 lb. yellow
    prussiate.

    Medium. (2) 1 cwt. china clay, 6 lb. green copperas, 6 lb. yellow
    prussiate.

    Medium. (3) 1 cwt. gypsum, 4½ lb. green copperas, 4½ lb. yellow
    prussiate.

    Deep. (1) 1 cwt. barytes, 5 lb. green copperas, 5 lb. yellow
    prussiate.

    Deep. (2) 1 cwt. china clay, 10 lb. green copperas, 10 lb. yellow
    prussiate.

    Deep. (3) 1 cwt. gypsum, 7½ lb. green copperas, 7½ lb. yellow
    prussiate.

In each case about 50-60 gallons of water are required.

To determine the amount of barytes present in a sample, boil about 50
gr. with caustic soda, filter, wash the residue free from soda, treat
with sulphuric acid, well wash the insoluble residue, dry, and weigh.

CHINESE BLUE.--This well-known and favourite form of Prussian
blue is prepared with great care, and is usually sold in fine powder
or little cubes. Its composition is virtually identical with that
of ordinary Prussian blue, but it is more free from impurities, and
shows a fine bronze bloom or lustre on newly fractured surfaces. Being
pure, it is entirely dissolved by oxalic acid; and its composition
is about 52 per cent. oxide of iron, 43½ cyanogen, and 4½ water. In
dyeing and calico-printing it is extensively employed. Its tint varies
from greenish to violet, according to modifications in the method of
manufacture, the chief difference being that yellow prussiate gives a
greenish tone and red prussiate a violet.

The process of preparation is mainly as follows. In about 40 gallons
of cold water dissolve 1 cwt. of green copperas selecting it carefully
for freedom from insoluble oxide; add about 5 pints of sulphuric acid.
This liquor very rapidly undergoes oxidation, by which oxide of iron
is thrown down, and the solution is rendered unfit for making the best
quality pigment. Therefore it should be prepared only immediately
before it is used. In another vessel containing about 40 gallons of
cold water, dissolve 1 cwt. of yellow prussiate (if a green shade is
desired), or of red prussiate (if a violet tint is wished for). Even
larger quantities of water may be used for the solutions, as the more
dilute they are the finer is the colour precipitated and the greater
the lustre on the surface of the finished pigment.

When the two solutions of yellow or red prussiate and acidified green
copperas are brought together, a bluish-white precipitate is thrown
down. This is allowed to completely separate itself, and then the clear
supernatant liquid is drawn off.

The next step is to thoroughly oxidise the precipitate. This cannot be
satisfactorily accomplished by utilising the oxygen of the atmosphere,
as is done in other cases, because that method entails the production
of a certain amount of oxide of iron, which prejudicially affects the
purity of colour of the finished article. Of the chemical oxidising
agents which are available, the most satisfactory in point of cost
and efficiency is chloride of lime (bleaching powder). For each cwt.
of green copperas, mix about 20 lb. of bleaching powder into a thin
cream with water, and add it, in small quantities at a time, to the
precipitate, constantly stirring so as to ensure the absorption of
the whole of the chlorine by the blue. Without the addition be made
gradually and under agitation, the chlorine will be generated more
quickly than it can be absorbed, entailing a waste of gas and a noxious
vapour to be breathed by the workmen. Sometimes the bleaching powder is
added at an earlier stage, viz. to the green copperas solution, and in
that case the blue assumes a violet tone.

After the addition of the bleaching powder solution to the bluish-white
precipitate, it is acidified with hydrochloric acid, which develops
the blue. When the whole has settled, the supernatant liquor is drawn
off, and the blue powder is well washed and strained on a filter,
then placed in pans and dried very gradually indeed in the dark, at a
temperature never exceeding 130° F. The slower the drying the better
is the gloss of the pigment. It is most essential that iron be excluded
during the final grinding operation, or it may cause ignition of the
mass, and its conversion into oxide of iron would speedily follow.

It has been proposed to treat the white precipitate (obtained in the
usual manner from green copperas and yellow prussiate) by the chlorine
contained in aqua regia (nitro-hydrochloric acid). The copperas,
however, must be as free as possible from basic sulphate (oxide), which
is ensured by keeping a little metallic iron in the acid solution
of copperas. It is also desirable to effect the precipitation with
crude prussiate, so as to avoid absorption of oxygen and premature
development of the blue colour. Habich considers that the mistake is
generally made of using too little copperas, and he has found that when
90 lb. of copperas have been added to 100 lb. of yellow prussiate, a
drop of iron solution in the filtered liquor produces no precipitate,
while the white precipitate has carried with it a certain proportion of
prussiate, which can be washed out. He therefore proposes to avoid this
waste by pouring the copperas solution into the prussiate solution,
with constant agitation, till no further precipitate goes down, then
adding one volume of the copperas solution equal to one-ninth of that
already used. After fifteen minutes stirring, it is certain that all
the prussiate carried down is decomposed.

The drained precipitate is blued (peroxidised) by adding aqua regia
prepared several days previously, and in proportions depending on the
strengths of the two acids. Generally, the aqua regia mixture will be
100 lb. of commercial nitric acid at 30° B. (containing 35·4 lb. of
anhydrous acid) and 62·2 lb. commercial hydrochloric acid at 23° B.
(containing 23·9 lb. of the anhydrous acid); and 40 lb. of this mixture
will suffice for bluing the precipitate resulting from 100 lb. yellow
prussiate. The addition of the aqua regia should take place in a
wooden vessel with constant agitation.

According to another modification, the white precipitate obtained in
the usual way is blued by adding a solution of perchloride of iron,
which may be made from a hematite ore free from clay and carbonate of
lime, or from rouge. The iron oxide, from whatever source, is ground
to a very fine state, and treated with crude hydrochloric acid in
a lead-lined tank, where the mixture remains for several days, and
is constantly stirred. When saturated with iron the clear liquid is
withdrawn for use. To receive it, the white precipitate is rapidly
heated to boiling in a copper vessel, and is then transferred to a
wooden vat, and the iron perchloride solution is stirred in till the
desired tint is produced. The pigment is washed and dried in the
ordinary way, while the supernatant liquor (essentially protochloride
of iron) is poured over old scrap iron and used instead of copperas for
a fresh batch of yellow prussiate.

A solution of perchloride of manganese may be used instead of
perchloride of iron. Inferior qualities of manganese ore can be
employed, and the residues left after treatment with hydrochloric acid
may be washed and dried for sale as purified or peroxidised manganese.

PARIS BLUE.--(1) A synonym for the violet-tinted kind of
Prussian blue.

(2) A series of compounds described below. [_a_] A thorough mixture of
2 parts sulphur and 1 part dry carbonate of soda is gradually heated
in a covered crucible to redness or till fused; a mixture of silicate
of soda and aluminate of soda is then sprinkled in, and the heat is
continued for an hour; the little free sulphur present may be washed
out by water. [_b_] An intimate mixture of 37 parts china-clay, 15
parts sulphate of soda, 22 parts carbonate of soda, 18 parts sulphur,
and 8 parts charcoal, is heated in large crucibles for 24-30 hours;
the mass is re-heated in cast-iron boxes at a moderate temperature
till the desired tint appears, and is finally pulverised, washed, and
dried. [_c_] Gently fuse 1075 oz. crystallised carbonate of soda in its
water of crystallisation; shake in 5 oz. finely-pulverised orpiment,
and, when partly decomposed, as much gelatinous alumina hydrate as
contains 7 oz. anhydrous alumina; add 100 oz. finely-sifted clay, and
221 oz. flowers of sulphur; place the whole in a covered crucible,
and heat gently till the water is driven off, then to redness, so
that the ingredients sinter together without fusing; the mass is then
cooled, finely pulverised, suspended in river-water, and filtered. The
product is heated in a covered dish to dull redness for 1-2 hours, with
occasional stirring. Colourless or brownish patches may occur, and must
be removed.

SAXON BLUE.--Following is a recipe for the preparation of this
pigment, which possesses limited importance.

Dissolve 8 lb. alum and 1 lb. green copperas in 16 gallons of
water. Add separate solutions of pearlash and yellow prussiate till
precipitate ceases to go down. Collect the precipitate when it has
completely settled; wash thoroughly, and dry.

SOLUBLE BLUE.--This term is applied to a variety of Prussian
blue which, while possessing no difference in the matter of chemical
composition, yet has the distinctive feature of being soluble in water,
which the other varieties are not. It no longer enjoys the popularity
it once had as a dye, on account of the severe competition of the
coal-tar colours. Below are some of the most satisfactory formulæ for
its preparation.

(1) Mix 10 lb. of Prussian blue thoroughly in about 10 gallons of cold
water. Then add 5 lb. of yellow prussiate and let the whole mass boil
steadily for several hours. Strain off the liquor and well wash the
precipitate on a filter. Finally dry for use.

(2) Dissolve about 1 cwt. of red prussiate in water and make the
solution hot. Prepare another solution of about 73 lb. of green
copperas in hot water. Mix the two solutions together and boil them for
about a couple of hours. Allow the solid matters to settle out, then
put them on a filter and wash with clean water until a blue coloration
manifests itself in the drainings. The blue residue is then dried as
usual.

(3) Make one solution of 10 lb. of yellow prussiate, and another of 8
lb. of green copperas, water being the solvent in both cases. Mix these
two solutions together and give them an hour’s boiling.

Add 3 lb. of a mixture of nitro-sulphuric acid, containing 2 parts of
the former to 1 of the latter. Boil for another hour. Let the solid
pigment precipitate itself thoroughly, and then filter, wash, and dry
as in the other cases.

(4) Dissolve about 1 cwt. of perchloride of iron and 10 lb. of sulphate
of soda in water. Also dissolve in another vessel 2 cwt. of yellow
prussiate and 10 lb. of sulphate of soda. Pour the first solution
into the second (never the contrary) and take care that the prussiate
solution is always preponderant. The Glauber’s salt is useful in
rendering the precipitation of the blue pigment more complete by
reason of the insolubility of the latter in saline fluids. When the
blue sediment is all thrown down it is drained off on a filter, and
repeatedly washed till a blue tint appears in the wash-waters, when it
is dried for use.

TURNBULL’S BLUE.--This is an old-fashioned name often applied,
like the term Paris blue, to the violet shades of Prussian blue which
have been prepared with red prussiate.

=Ultramarine.=--According to Rowland Williams, F.C.S., natural
ultramarine is, perhaps, the most beautiful blue pigment known. It
was formerly, and is now to a small extent, manufactured (chiefly
for artists’ use) from lapis lazuli, a blue mineral which occurs,
intermixed with limestone and iron pyrites, in Siberia, Thibet, and
China. In order to obtain ultramarine from lapis lazuli, the roughly
pulverised mineral is ignited, dipped into vinegar to remove carbonate
of lime, and then reduced to the finest possible state of division.
The powder is next mixed with a cement composed of rosin, linseed oil,
white wax, and Burgundy pitch, and the resultant paste is worked under
water until all the ultramarine is separated. The ultramarine is washed
several times with water, and afterwards with alcohol, which removes
any of the resinous compound which may have adhered. When treated in
this manner, lapis lazuli yields from 2 to 3 per cent. of ultramarine.
According to Clement and Desormes, lapis lazuli has the following
composition:--

                                              Per cent.

  Soda                                          23·2
  Alumina                                       34·8
  Silica                                        35·8
  Sulphur                                        3·1
  Carbonate of lime                              3·1
                                               -----
                                               100·0

It will be seen, therefore, that ultramarine essentially consists of
alumina, silica, soda, and sulphur, and may be regarded as a sodium
aluminium sulphate, in combination either with polysulphide of sodium
alone, or with a polysulphide and a polythionate of sodium. Clement
and Desormes believe that the iron in lazulite (lapis lazuli) is an
accidental impurity, and is neither essential to the mineral itself nor
to the ultramarine derived from it. There is still some doubt on this
point, however, many eminent chemists holding the opinion that iron is
a necessary constituent of ultramarine blue.

Natural ultramarine has been almost entirely replaced by the artificial
product, since methods have been devised for the manufacture of the
latter on a large scale. The possibility of preparing artificial
ultramarine suggested itself in a curious manner. About seventy years
ago a French alkali maker noticed the occasional appearance of a blue
coloured substance in his soda furnace. On analysis, Vauquelin found
the substance to have the same chemical composition as lapis lazuli,
and this incident led him to believe that ultramarine might be built up
from its elements. Several years passed away before Guimet succeeded in
manufacturing artificial ultramarine on anything like a large scale,
but Gmelin is said to have prepared it in small quantity half a dozen
years previously. There are four varieties of artificial ultramarine:
(1) the pure deep blue, equal in colour to average native ultramarine;
(2) pale blue; (3) violet or pink ultramarine; (4) green ultramarine.
The latter is obtained in the first stage of the ultramarine
manufacture, being the result of incomplete ignition of the materials
employed. Ultramarine is generally manufactured by one of the following
processes:--(_a_) “Sulphate”; (_b_) “Soda”; (_c_) “Silica.”

(_a_) “_Sulphate_” _Ultramarine_.--This may be prepared from sulphate
of soda (Glauber’s salt), charcoal, and kaolin (china clay). The
materials should be as free as possible from iron, and it has
been found that clay having approximately the formula Al_{2}O_{2}
(SiO_{2})_{2} gives the best results. The clay and sulphate of soda
must be thoroughly calcined. They are then intimately mixed with
charcoal in the following proportions:--

                                            Per cent.

  Clay                                       48·3
  Sulphate of soda                           43·5
  Charcoal                                    8·2
                                            -----
                                            100·0

Sometimes a portion of the sulphate of soda is omitted, and some
carbonate of soda and sulphur added instead. The composition of the
mixture then becomes:--

                                            Per cent.

  Clay                                       47·2
  Sulphate of soda                           19·3
  Carbonate of soda                          19·3
  Charcoal                                    8·1
  Sulphur                                     6·1
                                            -----
                                            100·0

Caustic soda is also sometimes used instead of carbonate. These
mixtures (whether sulphate alone or sulphate and carbonate) are made
with a view to have the soda present in sufficient amount to combine
with one-half the silica contained in the clay, and to leave sufficient
soda to form polysulphide of sodium with a portion of the sulphur.
There should then remain enough soda and sulphur to produce ordinary
sulphide of sodium (Na_{2}S). If either of the two mixtures be ignited
out of contact with air, a white compound is formed, which is sometimes
termed white ultramarine. On leaving this exposed to the atmosphere for
some time it becomes green, and on further ignition, with free access
of air, it is converted into ultramarine blue. In actual working the
carefully prepared mixture of the above mentioned materials is heated
for several hours to a high temperature in fire-clay crucibles, only a
limited supply of air being allowed to enter, and the temperature being
eventually raised to a white heat. The product of this operation, when
cool, has a grey or yellowish-green appearance. It is washed several
times with water, dried, reduced to a fine powder, and then represents
the green ultramarine of commerce. Stölzel found that green ultramarine
had the following composition:--

                                         Per cent.

  Alumina                                 30·11
  Silica                                  37·46
  Sodium                                  19·09
  Sulphur                                  6·08
  Iron                                      ·49
  Calcium                                   ·45
  Chlorine                                  ·37
  Oxygen                                   5·19
  Sulphuric acid                            ·76
  Magnesia, potash, and phosphoric acid   traces.
                                          -------
                                         100·00

Green ultramarine is transformed into blue by heating with about 4
per cent. of sulphur at a low temperature, with free access of air.
Sulphur is afterwards added, if necessary, in small quantities at a
time, and the heating is continued until the desired shade of blue is
obtained. The mass is then powdered, the soluble matter (sulphide of
soda, &c.) is removed by washing with water, and the blue is dried and
assorted according to quality.

(_b_) “_Soda_” _Ultramarine_ is sometimes made with soda alone (either
carbonate or caustic), and at others with a mixture of soda and
sulphate of soda. Rowland Williams found the following proportions of
the respective ingredients to answer satisfactorily:--

                                          Per cent.

  China clay                               36·8
  Carbonate of soda                        36·8
  Sulphur                                  22·0
  Coal                                      4·4
                                          -----
                                          100·0

The proportions for soda and sulphate of soda ultramarine have been
previously given under “sulphate ultramarine.” The ignition is carried
on in a manner similar to that already described. The resultant
green product, owing to its avidity for oxygen, is partially changed
into ultramarine blue by simple contact with the air. It is entirely
converted into the blue variety by roasting with an additional quantity
of sulphur. With care, ultramarine blue may be manufactured in one
operation, by increasing the proportions of soda and sulphur.

(_c_) “_Silica_” _Ultramarine_ is manufactured in the same way as soda
ultramarine, except that, in addition to the other materials, silica to
the extent of 5 or 10 per cent. of the weight of clay is employed. By
this process, ultramarine blue of a slightly reddish tint is obtained
in one operation. The method has, however, one decided drawback, viz.
that the materials employed are rather liable to fuse during ignition.
The faintly reddish hue of “silica” ultramarine becomes more intense
according to the proportion of silica present. “Silica” ultramarine
is said by some to be less readily attacked by acids and by strong
alum solutions than ultramarine prepared by the “sulphate” and “soda”
processes; but Rowland Williams’ experience does not confirm this
statement. He mentions that good artificial ultramarine withstands
the action of weak acids much better than is generally imagined. He
had occasion to test many samples which resisted the action of dilute
acids to a remarkable degree. Most strong acids, of course, decompose
both artificial and native ultramarine, with evolution of sulphuretted
hydrogen. Native ultramarine is, however, less susceptible to the
influence of acids (both strong and dilute) than the artificial
compound. This difference of behaviour is probably due to the fact that
the former contains considerably less sulphur than the latter, and it
is also possible that the constituents of natural ultramarine may be
combined in a somewhat different manner from those of the artificial
product.

Notwithstanding the large amount of research with reference to the
chemical composition of ultramarine, the origin of its blue colour
still remains in doubt. According to Wilkens (_Ann. Ch. Pharm._, xcix.
21), ultramarine consists of two portions, one of which is easily
attacked by hydrochloric acid, and is regarded by him as the essential
constituent, whilst the other portion is insoluble in hydrochloric
acid, and contains variable proportions of clay, sand, oxide of iron,
and sulphuric acid. From his analyses of the pure blue, Wilkens
deduces the formula (2Al_{2}O_{3} 3SiO_{2}) (Al_{2}O_{3} 4SiO_{2})
Na_{2}S_{2}O_{3} 3Na_{2}S:--

                                          Per cent.

  Silica                                   37·6
  Alumina                                  27·4
  Sulphur                                  14·2
  Soda (Na_{2}O)                           20·0
                                           ----
                                           99·2

Wilkens regards the blue colouring principle of ultramarine as a
compound of hyposulphite and sulphide of sodium. He considers the
presence of iron is not necessary for the production of the blue;
whilst Dr. Elsner, in a paper published in 1841, states that about 1
per cent. of iron (which he presumes to be in the state of sulphide)
is essential. Rowland Williams asks whether it is not conceivable
that the blue colour of ultramarine may be due to the presence of a
small quantity of black sulphide of iron, most intimately combined
with a colourless or comparatively colourless compound (such as white
ultramarine), the whole mass (owing to the dilution of the black
sulphide) showing a blue reflection.

Ultramarine is insoluble without decomposition in any known menstruum.
According to P. Ebell (Ber. 16), ultramarine, when in the most finely
divided state, will remain suspended in pure water for months. The
liquid may be filtered unchanged through several layers of Swedish
filter paper, and appears perfectly clear when examined in a ¾ in.
layer, and on evaporation deposits the ultramarine as a lustrous
coating on the sides of the vessel. Rowland Williams repeated the above
experiment, and can confirm Ebell’s statement. This result shows the
necessity of due precautions being taken during the washing of the
ultramarine in the process of manufacture, otherwise a considerable
amount of the finely divided blue may be lost. Ultramarine is largely
used in calico printing for pigment styles, being fixed on the fibre
by means of albumen. It is also employed for blueing linen and cotton,
wax candles, lump sugar, &c. Ultramarine is not adulterated to a large
extent, the chief sophistication being barium sulphate (barytes), and
occasionally chalk and china clay.--(Rowland Williams, in _Industries_.)

Another writer in _Industries_ says that the manufacture of ultramarine
has perhaps hardly received the attention it deserves in England. The
importance of the industry has been recognised in Germany, however,
and though the palmy days of the trade, when the whole production was
in the hands of a few firms, and the price was a matter of private
friendly arrangement, are gone for ever, yet the business is in a
flourishing state, and should prove lucrative if properly managed.
It is a characteristically English failing to overlook branches of
business not dealing with large quantities of staple commodities, and
thus many of the smaller but remunerative industries have passed out of
our hands. When one observes that almost every sheet of ordinary blue
official paper is decolorised when accidentally brought into contact
with an acid, betraying the fact at once that its colouring matter is
ultramarine, one realises that a very considerable consumption for
this and similar purposes must take place. Like most trades based upon
chemical principles, the manufacture of ultramarine has recently made
rapid strides, and some of the latest developments are recorded in
a paper by J. Wunder, appearing in a recent number of the _Chemiker
Zeitung_, which is worthy of some attention.

With most people not directly interested in it, the term ultramarine
is taken to mean the _blue_ pigment known under that name, the words
being reckoned almost synonymous. Others, more erudite, recognise the
existence of a green variety, but that the production of such colours
as red and violet is possible is scarcely suspected. Of course the
blue is the most important, but even that does not correspond to one
specific substance, products of different shade being prepared by
modifying the process of manufacture. As usually made, ultramarine is
formed by heating together carbonate of soda, kaolin, sulphur, and
charcoal, with limited access of air, the resulting pigment being
green; this, on roasting with sulphur, becomes blue. If the operation
be conducted with complete exclusion of air, so-called ultramarine
white (in reality _grey_) is produced, which becomes green on further
heating. Ultramarine blue capable of resisting the action of alum
is sometimes required, and may be obtained by the use of a highly
silicious charge and much sulphur, the burning being conducted in
crucibles or in mass according to the purpose for which the pigment
is required. The former process is costly, while the latter gives a
product containing a good deal of free sulphur, which is objectionable
for such purposes as calico printing. Removal of the excess of sulphur
by heat or caustic soda is not feasible, as the colour suffers in
either case, but a certain amount of success has attended experiments
with sodium sulphide, the colour often brightening noticeably.

It is curious that chemically pure sodium carbonate, or such as is made
by the ammonia-soda process, is not well fitted for the manufacture of
ultramarine; Leblanc soda, containing a little caustic, is distinctly
preferable. Sprinkling the soda with a strong solution of sodium
sulphide before use is a good plan, and one easy to adopt. The more
silicious the mixture the more difficulties are encountered, but the
product is a deeper, richer colour, and withstands the action of alum
and weak acids better. Excess of oxygen must be guarded against; many
a manufacturer has had a batch turn out a hard cold blue, instead of
a soft rich colour, solely on account of a too-excellent draught,
an accident especially liable to happen in winter time. So much
dreaded is this catastrophe that some makers habitually limit the air
supply--smothering the neighbourhood with smoke, and wasting coal. The
need for exact control here indicated points to a probable advantage
from the use of gaseous fuel. Considerable economy has resulted from
the use of the waste gases from one furnace serving for the preliminary
heating of another; a better plan would probably be the introduction of
regenerative heating.

The crude ultramarine as it comes from the furnace contains a large
proportion of soluble salts, notably 20 to 24 per cent. of sodium
sulphate, which have to be removed before it is merchantable. Usually,
after grinding, it is simply stirred up repeatedly with hot water and
the aqueous extract is siphoned off. That such a crude method should be
in vogue at the present time is very significant of the ample margin
of profit that must exist. By systematic extraction and filtration
under pressure the washing may be effected with so little water that
the solution is sufficiently concentrated to pay for evaporation by the
heat of waste furnace gases, the recovered sodium sulphate serving to
replace part of the raw material.

The quality of ultramarine largely depending upon its fineness, it
is graded by levigation, the coarser portions being filter pressed,
and the finest “floating” quality, which remains in suspension for an
inconveniently long time, precipitated by the addition of a trace of an
ammonium salt, gypsum, or even hard water, and filtered by the aid of a
suction tube on the principle of an ordinary Bunsen pump.

The first successful attempt to produce ultramarine _violet_ was made
by Professor Leykauf in 1859. By heating ordinary ultramarine with
calcium chloride in the presence of air and moisture, he obtained
a violet-toned pigment, but it was not a full colour. The active
substance in this change was probably hydrochloric acid, produced by
the decomposition of the calcium chloride. Later experiments with
other reagents, such as chlorine and gaseous hydrochloric acid, led to
the following methods being devised. In the first, ultramarine blue
is spread out on stoneware shelves in iron chambers and treated with
a mixture of chlorine and steam at a temperature of 300° F. to 480°
F. for about three hours. In the second, the plant is very similar,
but at the bottom of the chambers are stoneware dishes, into which
hydrochloric acid is poured from time to time. As the temperature is
raised, copious vapours arise from these, evaporation being aided
by a strong draught, and the ultramarine blue, after being kept at
428°-446° F. for some seven hours, becomes converted into a dull
violet, which brightens on continuing the process with a temperature
gradually falling to 320° F. The ultramarine violet produced by either
of the above methods resists the action of lime, and is of general
applicability.

The pigment produced by a third and simpler process, consisting merely
in heating ultramarine blue mixed with salammoniac and a little
sodium nitrate, is unfortunately not so stable. Another shade of
considerable interest is a pure bright light blue, formed by heating
the violet variety in hydrogen to about 536°-554° F. It has not yet
been prepared on a commercial scale, but certainly merits the attention
of manufacturers. An ultramarine red has been made by acting on the
violet produced by either of the first two methods with the vapour of
either nitric or hydrochloric acid at 275°-293° F., the sole essential
determining condition being the temperature. Iron vessels could be used
in the case of nitric acid at this temperature, but if hydrochloric
acid were employed stoneware would have to be substituted. In the
manufacture of the violet the temperature is above the limit at which
hydrochloric acid acts on iron.

It is now only necessary for some successful experimenter to put on the
market yellow and orange shades of ultramarine for almost the whole
of the spectrum to be represented. The problem of the cause of the
colour of ultramarine, attempts to solve which have been repeatedly
made, seems increasingly difficult when its protean character is
considered; but this from the industrial point of view is of secondary
importance, provided all required shades can be produced with ease and
economy. Nevertheless, it is certain that here, as in other cases,
substantial technical progress would follow from adequate scientific
investigation.--(_Industries._)

Ultramarine was also made the subject of a very interesting paper, by
Herbert J. L. Rawlins, read before the Society of Chemical Industry, in
December 1887.

After referring to the native form, lapis lazuli, Rawlins goes on
to observe that “analysis could give no clue as to the cause of
the blue colour. To prepare it artificially became a great object,
and the efforts in this direction were stimulated by the offer of
prizes, amongst which was one of 6000 francs, offered by the ‘Société
d’Encouragement’ of France, to be awarded to the discoverer of a method
of making ultramarine, provided it did not cost more than 90_s._ per
lb. How strange it seems to think of this in these days when the value
has fallen to less than half that price per cwt.!

“As early as 1814, two German chemists, Tessärt and Kuhlmann, had
observed the formation of a blue product in soda kilns and calcination
kilns, but Guimet, in 1828, first discovered how it was produced, and
gained the 6000 francs prize. He did not, however, publish his method,
and grew immensely rich, although the price sank to about 16_s._ per
lb. In 1828 he was producing at the rate of 120,000 lb. annually.

“About the same time, or, as is positively asserted by some, even prior
to Guimet, Gmelin made the same discovery and published his researches
in full, thus perhaps laying the foundation stone of the present
supremacy of Germany in this manufacture.

“In spite of the valuable discoveries of Hoffmann, Unger, and others,
our knowledge of the chemical constitution of ultramarine is very
limited and uncertain, many different theories having been advanced
regarding the cause of the blue colour.

“According to Wilkins, ultramarine is composed of two portions, one
of which consists of two silicates of alumina with sulphite and
sulphide of sodium, and is constant in its composition; the other
being a mixture of variable quantities of sand, clay and oxide of
iron, with sulphuric acid. The blue colouring principle he considers
to be a compound of sodium sulphite and sulphide. Another ingenious
theorist, Stein, in two papers published in the _Jahresberichte_ in
1871 and 1872, concludes that blue ultramarine contains sulphurous,
and not thiosulphuric acid, that neither sulphites nor thiosulphates
are necessary to its composition, and that it owes its colour to
the presence of black sulphide of sodium, which is formed at high
temperatures by the action of sulphide of sodium on alumina--admitting,
therefore, that it is not a chemical compound, but merely a mechanical
mixture, the blue colour of which is due to the bodies composing it.

“Brunner considers ultramarine to be a compound of aluminium silicate,
with sodium sulphate and sulphide; while Brünlin regards it as a double
silicate of aluminium and sodium, in combination with pentasulphide of
sodium. Green ultramarine he considers to be the same double silicate
in combination with bisulphide of sodium.

“Again, according to Ritter, ultramarine contains a double silicate,
not only associated with polysulphide, but also with thiosulphate of
soda; and Schülzenberger, on the other hand, considers that it is a
mixture of a double silicate with sulphite and monosulphide of sodium.

“Endemann considers that the blue colour is due to a ‘colour nucleus,’
consisting of unchanging proportions of aluminium, sodium, oxygen and
sulphur, in each variety of ultramarine the proportion being different,
while the rest of the sodium and aluminium and the whole of the silica
merely act as a vehicle necessary to the preparation and existence
of the colour. He considers that this ‘colour nucleus,’ in the case
of white ultramarine, which he calls the ‘mother-substance in the
manufacture of blue ultramarine,’ has the formula AlNa_{4}O_{2}S_{2}.
By the action on two molecules of this of sulphurous acid gas,
Na_{2}O is removed, and green ultramarine Al_{2}Na_{6}O_{3}S_{4}
is formed, which then, by the action of oxygen, which forms sodium
sulphate, passes into the pure green compound, having the formula
Al_{2}Na_{4}O_{3}S_{3}. In the ‘indirect process’ of manufacture, green
ultramarine is converted into blue by being burned with sulphur. By
this means Endemann considers that more sodium and sulphur are removed,
and blue ultramarine Al_{2}Na_{2}O_{3}S_{3} is formed. He considers
that the other portion, not included in the ‘colour nucleus,’ differs
in different samples. In one which he mentions it has about the
composition 3Al_{2}O_{3}.5Na_{2}O.16SiO_{2}.

“But of all chemists who have worked on this subject, none has done
more to increase our knowledge of ‘the blue marvel of inorganic
chemistry,’ as he himself has called it, than Reinhold Hoffmann.
His position of manager of the Marienberg Ultramarine Works, near
Benscheim, in the Grand Duchy of Hesse, renders his acquaintance with
the manufacture perfect, and his untiring researches on the subject
have been well rewarded by results both interesting and valuable.
He considers ultramarine to be a double silicate of sodium and
aluminium, together with bisulphide of sodium, the variety poor in
silica, characterised by its paleness and purity of tint, and easy
decomposition by acids, having the formula 4(Al_{2}Na_{2}Si_{2}O_{8})
+ Na_{2}S_{4}; while that rich in silica, characterised by its dark
and somewhat reddish tint, and more difficult decomposition by acids,
has the formula 2(Al_{2}Na_{2}Si_{3}O_{10}) + Na_{2}S_{4}. He also
considers it very doubtful whether green ultramarine is really a
chemical compound, and indeed it is now generally considered that the
colour is only due to small traces of sodium salts in very intimate
mechanical mixture with the blue variety, for by heating the green
body for some time at 160° with water in closed tubes, it is converted
into the blue product, and small traces of sodium compounds are found
in solution in the water; and further, on heating blue ultramarine
strongly with sodium sulphate and charcoal--that is, acting upon it
with sodium sulphide--the green variety is formed.

“In a paper by Knapp, an abstract of which appeared in the _Journal
of the Chemical Society_ for March 1880, there are some curious facts
recorded with regard to the colouring agent. It was noticed that when
silicic acid was replaced by boracic acid, a blue, nearly as stable in
its properties as that of ordinary ultramarine, was produced. It was
found that a blue could be obtained without alumina being introduced.
Hence silica without alumina, and alumina without silica, can be
employed with a certain amount of success. The blue, however, formed
without silica, is not so strong or stable as that formed with it.

“One very curious property which ultramarine possesses is its power of
giving up its sodium in exchange for other metals. Thus, by heating
blue ultramarine with a concentrated solution of silver nitrate
in sealed tubes to 120° for fifteen hours, a dark yellow _silver_
ultramarine is produced, containing about 46·5 per cent. of silver.
This corresponds to about 15·5 per cent. of sodium, which is just about
the amount that the original body contained.

“When this body is heated with an aqueous solution of sodium chloride
to 120° in sealed tubes, about three-quarters of the silver is replaced
by sodium, but the other quarter cannot be so replaced; in fact, blue
ultramarine, when heated with silver chloride, takes up silver, and
becomes green. But by heating silver ultramarine with sodium chloride
in the dry way, at rather a higher temperature, the whole of the silver
is replaced by sodium, but the ultramarine thus regenerated does not
equal the original body in colour. The change is probably due to the
loss of sulphur in the formation of the silver ultramarine.

“If in the above experiment potassium chloride be substituted for
the sodium salt, and the temperature not allowed to exceed 400°, a
bluish-green _potassium_ ultramarine is formed. _Barium_ ultramarine
is a yellowish-brown product, _zinc_ ultramarine is violet, and
_magnesium_ ultramarine is grey. These may all be obtained by acting on
the yellow silver ultramarine with the corresponding metallic chloride.

“From the experiments of Dollfus and Goppelsröder some very striking
differences have been brought to light between the three types of
colour which they examined--namely, the blue, green and violet--in
their behaviour with various reagents. Thus, an aqueous solution of
caustic soda or potash does not act on the blue or green, but turns
the violet to blue, and when heated with carbonic oxide the same result
ensues. Many other reagents have the same effect on the violet variety,
but when acted upon with sodium sulphide, the green turns grey, and
when heated with potassium chlorate becomes darker and loses its
brightness of colour. Dollfus and Goppelsröder attempt no explanation
of these facts, but simply state them as results of their observations,
and profess their inability to give any chemical formulæ for the three
ultramarines, though they consider that there is sufficient proof
that each has its distinct constitution. They give as their opinion,
however, that they are double silicates of aluminium and sodium, in
which a part of the oxygen is replaced by sulphur.

“Violet and red ultramarines are more bodies of scientific interest
than of any practical use, as their colouring power is not sufficiently
great. The violet variety may be prepared by exposing the underground
blue product to chlorine gas under a high temperature, while the
red may be obtained from the violet by acting on it, under a low
temperature, by dilute nitric acid fumes.

“The first artificial method of producing ultramarine was that known
as the ‘indirect process’--that is, first the manufacture of green
ultramarine; and secondly, its conversion into blue. It was carried out
as follows:--

“An intimate mixture of Glauber’s salts, china clay, and coal or rosin,
finely ground together, was placed in crucibles and baked or burned in
an oven for about six hours. It was then transferred to iron trays,
and heated with flowers of sulphur to the point where the sulphur took
fire, when it was allowed to burn itself out. By this second process
the green was converted into blue. It was then washed, ground with
water, and settled out, the first deposit being of a darker shade than
the second, and the colour becoming lighter as the powder settled was
finer in grind. This is essentially the method employed now at many
German works--those at Marienberg, for instance--and produces what is
known as “sulphate ultramarine,” distinguished by its pale shade and
almost _greenish_ blue tint.

“There are, however, some objections to the indirect process, and it
was considered advisable to find a plan by which ultramarine could be
made in bulk in a muffle furnace. The following is a method which is
employed at the present time in some of the German works:--

“A mixture of china clay, carbonate of soda, sulphate of soda, sulphur,
sand and charcoal or rosin, finely ground together, are placed upon
the floor of a muffle furnace, being pressed down so as to present an
even surface. The mixture is then entirely enclosed with fire-clay
tiles, the spaces between which are filled in with thin mortar. When
the oven is so charged, the front is built up, a small hole being left
for watching the temperature of the flue between the tiles and the top
of the furnace, and for drawing samples during the process, which is
done through a corresponding hole in the front of the fire-clay tiles,
temporarily closed with a fire-clay stopper. The oven is now heated,
slowly at first, and afterwards more strongly, so that at the end
of eight or nine hours it is at a dull red heat. It is kept at this
temperature for about 24 hours, when the heat is raised so that a clear
red glow is obtained, which is kept up to the end of the operation.

“For the purpose of taking a sample, an iron spoon borer is introduced
through the hole left in the enclosing tiles, turned round, and pulled
out. The contents are laid on a clean tile, and quickly covered with
another tile, on which a second quantity is placed, and allowed to
remain exposed to the air. If the oven has been sufficiently heated the
covered sample should appear of a bluish green, and no longer brown
or yellow, while the second sample should be rather bluer. If this be
the case, the oven is heated slowly for another hour, and then all
communication with the outer air is cut off. It is allowed to cool and
then opened, when the contents should appear as a beautiful blue mass,
the lower portion of which, however, is of a greenish tinge. Both parts
are now treated alike, but worked up separately, the greenish-blue
portion making an inferior article. The finishing process is as
follows:--

“The raw ultramarine is ground in upright mills, and then repeatedly
boiled for about ten or fifteen minutes at a time in cast-iron boilers,
being all the time agitated by a mechanical stirring arrangement. It
is then allowed to settle, and the water is drawn off with a siphon.
As soon as the powder settles into a hard compact mass, it has been
sufficiently washed, and it is then dug out. The part next to the
bottom of the boiler is generally coarse and of poor quality. It is
carefully separated from the upper portion, which is transferred to wet
mills of the ordinary description, and there ground for six to twelve
hours, during which time about 150 lb. can be treated in each mill.
The ground colour from these mills is then collected in a large tub,
and allowed to settle for four hours, during which time the coarsest
particles fall to the bottom. The liquid is then passed through a
series of tubs, in each of which it is allowed to stand for a period of
time, lengthening as the quality settled out becomes finer, the last
settling requiring about three weeks. The various qualities are then
dried and sifted, when they are ready for the market.

“The blue produced by this operation is of a good quality, but there
are some objections to the process, which have given rise to another,
in which the ultramarine is produced direct in crucibles similar to
those used in the indirect process.

“This is conducted as follows:--The mixture of raw materials consists
of about 100 parts of china clay, 90 of carbonate of soda, 110 of
sulphur, 20 of charcoal, and a quantity of infusorial earth, varying
according as the ultramarine produced is desired to be rich or poor in
silica. These are finely ground together, in which process great care
must be observed, as much depends upon its being properly carried out.
The mixture is then filled loosely into crucibles provided with flat
circular lids, which are fixed on with mortar containing clay. This
is allowed to dry, and the crucibles are then ready for firing, which
process is conducted in ovens, generally constructed so as to contain
several hundred crucibles, which are arranged in rows one above another.

“The mixture undergoes a very curious change of colour while in the
ovens. When put in it is greyish white, and during the process of
burning it becomes successively brown, green, blue, violet, red and
white, in the order named. These changes are, according to Guimet,
due to oxidation. The brown appears with the blue flames due to the
combustion of the sulphur, the green just after the sulphur flames have
ceased, and the blue is first formed at a temperature of about 700°--i.
e. a bright red heat. If, after this, heat be still applied and air
freely admitted, the mixture becomes first violet, then red or rose
coloured, and finally white. When this white body is heated to redness
with carbon or other reducing agents, the red, violet, blue, green and
brown colours (according to the amount of reducing agent employed) may
sometimes be reproduced, though the reaction is by no means a certain
one.

“If brown ultramarine be removed from the oven, and allowed to remain
exposed to the air, it immediately takes fire and burns to an inferior
blue colour. The same thing occurs with the green body. Even if the
brown product be completely cooled before being exposed to the air,
it will, as soon as the air is allowed to reach it, get hotter and
hotter, until it is glowing, when it will burst into flame and become
blue. Attempts have been made to preserve the brown colour, which is of
a beautiful chocolate tint, but have always failed. In one instance,
when this was tried, the colour was put immediately into water, and
treated like the ordinary blue variety, and as long as it was kept
moist no change was apparent. After being washed and wet ground the
moist powder was put into a cask, where for some time it was allowed to
remain undisturbed. At the end of about three weeks it was noticed that
the mass was hot, and on being turned out of the cask and broken up it
was found to be at a glowing heat in the interior.

“After the oven has been fired for several hours, it is carefully
closed at every point where air might enter, and allowed to cool
for four or five days. The exact length of time during which the
ovens are fired, and the amount of air admitted, depend upon various
circumstances, one important one being the state of the weather. Thus,
on a dull, foggy day, when the draught in the chimney is not good, a
longer time is required. Of course, no rule can be given for this,
and it is the experience required in the management of the oven that
makes the manufacture so difficult to carry out successfully, the early
efforts of a manufacturer not unfrequently resulting in the loss of
a whole ovenful of raw material. As soon as the oven has cooled, the
crucibles are taken out, and the contents of each are turned out in a
solid mass, which must be carefully cleaned with a knife of any badly
burned portions, and afterwards broken up and thrown into a cask along
with the contents of other crucibles.

“This forms what is known as crude raw ultramarine. It contains about
15 per cent. of sulphate of soda, which must be removed before the
colour is fit for sale.

“For this purpose it is washed with hot water in large tubs, after
which it is ground in wet mills to an impalpable powder, and allowed
to stand for about an hour in a large tub, in order to remove the
coarsest particles and dirt which are sure to be present. It is then
removed to another tub, where it settles for four or five hours, and
from this it passes to others, where it stands for various lengths of
time, increasing, of course, as the powder to be settled becomes finer,
the last settling occupying three or four weeks, and producing the
strongest quality that can be obtained--that is to say, it will bear
mixing with more of a reducing medium, such as mineral white, than
would a former settling for the mixture in each case to be of the same
depth of colour.

“The water, after the final settling, still contains about 5 per cent.
of ultramarine. This would take five or six months to settle, and
as this time could not generally be given to it, it is precipitated
with lime water, which has a sort of coagulating influence upon the
particles, which can then be removed by filtration. It is a curious
thing that this last quality is quite different from the one preceding
it, being very inferior in both colour and strength.

“After settling, all the various qualities are dried in kilns, and
sifted through fine brass wire sieves by means of a fan, which breaks
up the lumps and forces the particles through the meshes of the sieve,
which must be very close--about 100 to the inch--in order that the
ultramarine may be perfectly smooth and free from lumps or grit of
any sort. When finished, it should be in the form of an impalpable
powder--the finer qualities so fine, indeed, as to feel almost
_buttery_ when rubbed between the fingers. After this process the
different qualities and shades are mixed to certain standards, and are
then ready for sale.

“The uses of ultramarine in the arts and manufactures are very numerous
and important. The most important, from the point of view of quantity,
is the manufacture of ‘square blue’ for washing purposes. In the
preparation of this article the ultramarine is generally mixed with
bicarbonate of soda and some glutinous material, to help it to retain
its shape, and is then pressed into the well-known form of small square
or oblong blocks.

“It is also used largely in the manufacture of blue paint and printing
ink, and in the preparation of blue mottled soap. The way in which it
is employed in the last-named manufacture is worthy of remark. It is
added to the soap while it is in a molten state and just before it
is allowed to cool, and thoroughly mixed with it, so that the whole
mass is of a pale blue tint. If a small quantity of this be removed
from the boiler and cooled quickly, it remains of a uniform tint, but
in the case of the whole boilerful, where the cooling is very slow,
the action is entirely different. Just at the point of cooling, when
the soap is going to set hard, the ultramarine--to use a technical
expression--“strikes,” and goes into the form which gives to blue
mottled soap its well-known appearance.

“In the manufacture of paper, ultramarine also plays an important part.
It is here used not only for producing blue shades, but also as a
bleaching agent, to counteract the yellow when white paper is made.

“Another important use is in the calico manufacture, where it is used
both in the printing of blue patterns and in the finishing of goods. In
the case of calico printing, it is mixed with albumen and printed on to
the calico, which is then subjected to the action of steam, the albumen
being by this means coagulated and each grain of ultramarine surrounded
by an insoluble envelope, so that it cannot be washed out of the calico.

“The growth in the manufacture of ultramarine has been very remarkable,
especially when it is considered how little the process is understood
chemically, and what care and patience--to say nothing of the
equally important item of capital--are required in the starting of
a manufactory. Commencing less than 50 years ago in the works of
Guimet, at Lyons, who produced 120,000 lb. annually, there are at the
present day nearly 40 manufactories at work in various parts of the
world--chiefly in Germany--producing about 20 million lb. per year.
The following figures will give some idea of ten years’ growth of this
industry--from 1862 to 1872:--

                                  1862.     1872.

  Number of manufactories           24        32
  Men employed                     964      1929
  Tons manufactured               3556      8585

“From the above numbers it will be seen that in these ten years the
manufacture more than doubled itself, the fact being due, however,
not so much to the increase in the number of works, which was only
one-third, as to the enlarged capabilities of those existing in 1862.
Thus, in the works of Dr. Leverkus, near Cologne--the first works ever
started in Germany--the number of men employed had, during these ten
years, more than doubled, while the output had trebled; and in the case
of the Marienberg Works the difference was even more striking, the
number of hands employed and the quantity turned out per annum having
nearly quadrupled.”

In reply to various questions which were asked in the discussion which
ensued, Mr. Rawlins said that, with regard to the use of ammonia
soda, it had frequently been used in the manufacture of ultramarine,
and was constantly used he understood, but he himself had not much
experience of it. As far as he could make out, it certainly produced
ultramarine, but of a darker shade than that made with Leblanc soda.
It could not be supposed, in works where the Leblanc soda was used,
that ammonia soda could conveniently be substituted, for of course
a works when established had to adhere to its known standards and
shades, and it would not do to change the raw materials, though the
ammonia soda produced a very good ultramarine. As regards the discovery
of ultramarine, the first works started anywhere were Guimet’s. He
had with him a little historical list containing the dates at which
the various works established before 1866 or a little later had been
started. It was drawn out by Hoffmann, who, as he stated before, was
the manager of large ultramarine works, and he put down Guimet’s,
which were started in 1829, first on the list. Dr. Leverkus started in
1834. He knew that the discovery of ultramarine had been attributed
to different people. He had mentioned Guimet because it had generally
been considered, as far as he had heard, that Guimet and Gmelin were
the two who discovered it from a manufacturing point of view. He had
heard of crystals of ultramarine, but had never seen any, and he knew
they were very difficult to prepare, and very rare. He had mentioned
that the grinding had to be done very thoroughly, because the better it
was mixed and the finer it was ground, the better was the ultramarine
produced. If it was badly mixed it was quite fatal to getting a good
result. Grinding lightens the colour. Raw ultramarine must be ground
before it was practicable to use it at all. For instance, a coarse
ultramarine could not be used for printing calico. Therefore it was
necessary to grind it both for the sake of the colour and for the
sake of the way in which it was applied. It was increased in value by
grinding because it made it stronger and finer. Before grinding it was
of a dark colour, but after grinding it became lighter and brighter.

The materials employed in McIvor’s process for making ultramarine
are kaolin or other suitable clay, a solution of sulphide of sodium,
in which sulphur in the form of flowers of sulphur is dissolved to
saturation, and caustic or carbonate of soda.

The preparation of the solution is effected by adding the sulphur to
boiling sulphide of sodium liquor of maximum strength until it ceases
to be taken up. The clay and soda are first roasted together at a red
heat, so as to effect a partial double decomposition, and the product,
after grinding, is made into a thick paste with “sulphur liquor,” i. e.
the sulphide of sodium solution of sulphur. This latter operation may
be carried out in an ordinary pug-mill. The paste so formed is dried in
an oven or other convenient way, and the dried mass (being broken into
small pieces) is roasted without access of air in a closed earthenware
retort, first at about 480°-570° F. for an hour, then at a red heat for
eight hours, and finally at a moderate heat just below dull redness,
in presence of a slow current of air, which enters through a series of
holes or small openings in the front of the retort, the current being
regulated by means of a damper or an adjustable slide. The retort
should be allowed to become quite cold before being opened, otherwise
the tint of the product will be injured.

McIvor has found the following proportions of the raw materials used in
the process to yield excellent results, viz:--

  Sulphide of sodium             42 lb.
  Sulphur                        20 “
  Kaolin (china clay)           110 “
  Soda (as carbonate)           106 “
      or
  Caustic soda                   40 “

These quantities yield about 2 cwt. of ultramarine blue.

The following communication from the pen of J. B. Nejedly, of Vienna,
appeared in the _Chemiker Zeitung_, during 1888:--

“Animated by various articles and notes in your journal under the
heading of ‘The Present Position of the Manufacture of Ultramarine,’
I would like to draw out of obscurity a little work on this industry
which contains much that is true, and furnishes at the same time many
comparisons with regard to the present position of the ultramarine
industry in Germany.

“The work above referred to was printed in the year 1840 and bears the
title:--

    “‘Treatise on the chemico-technical preparation of Ultramarine
    colours, according to the discoveries of Leykauf and Heyne, or on
    the importance of the manufacture of blue and green Ultramarine for
    purposes of science, art, and industry. By Friedr. Wilh. Heyne,
    president of the Nürnberg Ultramarine Manufactory. Nürnberg: 1840.
    Printed at the Campe Press.’

“The preface, which I consider to be well suited to present
circumstances, I reproduce verbatim, while from the little work itself
I will only quote such sentences as would seem to be suited to the
present time, and which are the most important as bearing on the
subject.

“‘_Preface._--The incitement to this treatise was furnished by the
utility of the discovery of Leykauf, instructor of chemistry at the
technical schools in Nürnberg, of artificially preparing the well-known
blue mineral colour styled ultramarine, according to simple principles,
which discovery was supplemented by the production of the green
ultramarine, an equally genuine and beautiful green mineral colour,
by the technist Heyne in Nürnberg. A short review of the importance
of these two discoveries for mankind in general and for science,
art, and industry in particular, will form the main subject of this
treatise, which has no other object but that of arousing the attention
of all high protectors and stimulators, as well as friends of industry
and art, to a newly-born industrial branch. At this moment we are
living in a period when many industries have got into the stocks,
in consequence of far too severe competition, combined with other
influences; indeed they are barely able to support those engaged in
them. If in consequence of this state of things it already becomes of
the most vital importance that fresh sources of acquisition should be
obtained, it becomes all the more so when by their means at the same
time materials come into requisition which the Fatherland possesses
in great superfluity, and which otherwise possess no intrinsic
value beyond just the expense of extracting them from their natural
localities or deposits, and the worth that attaches to their working up
for industrial purpose. A source of acquisition in this sense is met
with in the manufacture of blue and green ultramarine colours, which in
course of time can be raised to an extremely valuable acquisition. May
the communications here made result in their being considered worthy of
a thorough many-sided investigation and consideration.’


“REGARDING THE WORK ITSELF.

“Page 21.--‘Not long since a prize of 6000 francs was offered by the
“Société d’Encouragement.” This prize was gained by Guimet, who has not
published his process, and who now furnishes ultramarine at the price
of 25 francs per oz., whereas it otherwise cost 200 francs per oz.
Latterly, in 1839, Guimet reduced his price for ultramarine, viz. No.
1, for painting, to 10 francs, a lighter shade being 6 francs per oz.
In addition, this manufacturer furnishes lower qualities for carpet and
paper manufacturers at 20 francs and 12 francs per lb.’

“Page 23.--‘Indeed, if we are able to produce ultramarine by means of
a polysulphide of sodium and common clay, then the most beautiful and
most lasting of all known blue colours would at the same time become
the cheapest of them all.’

“Page 27.--‘All faults which are known to exist in the old methods are
obviated in the new invention of Leykauf and Heyne, while the same
offers the following advantages:--

“‘(1) The materials which are treated with it can be brought into use
without any special previous chemical preparations, indeed as supplied
by Nature, while chemical treatment is entirely unnecessary. In view of
the unimportant cost of derivation of the raw material, there cannot
consequently be any questions raised with regard to waste.

“‘(2) This method is so simple that any man of sound intellect can
easily work it, without possessing any special chemical knowledge
beforehand. As the labour can be easily grappled with, errors can only
occur when the grossest carelessness is shown in conforming to the
instructions prescribed.

“‘(3) According to the said method one can work according to any
desired scale, and, what is best of all, the larger this scale the more
favourable are the results obtained, lighter work and excellence of
quality.

“‘(4) If the process is carefully conducted, everything is in your own
power, nothing depending on chance.

“‘(5) Consequently an equal product can invariably be obtained, while
this can at will be brought to the most complete stage of perfection
at but little greater cost than lower qualities entail.

“‘(6) According to this method you are master of the fire, enabling a
retention of colours in any desired shade, of the deepest tone, of the
greatest permanency.

“‘From this it appears: That this process is the easiest, the cheapest,
and the most complete. Worked according to this method the hope is
likely soon to become a reality that ultramarine may yet become the
cheapest of all mineral colours, and as in the same everything rests
upon simplicity, the preparation of the article in future will be
carried on somewhat after the fashion of baking, brewing, &c.’

“Page 32.--‘Moreover, there is not only blue ultramarine, but also a
pure green, and we may venture the hope that similar combinations in
white, black, red, and yellow will soon follow in equal perfection.
In consequence of these discoveries, Leykauf and Heyne have erected a
factory in Nürnberg, which, according to a circular dated 15th July,
1840, is in operation under the style of “Leykauf, Heyne and Co.,” and
are producing the two ultramarine colours referred to at the present
time at the rate of 50 lb. per day.’

“Page 33.--‘All the mechanical appliances of the factory are at the
present time exclusively worked by hand, the number of persons employed
being sixteen; while the establishment upon completion of the buildings
that are wanting is calculated to employ twenty operatives and two
horses. With this extension the factory will be able to turn out 5
cwt. blue and 5 cwt. green weekly, consequently annually 500 cwt. of
finished merchantable ultramarine will be brought on the market.’

“Page 35.--‘Now, as regards the prices ruling at present for Nürnberg
ultramarine, these are, as compared with those of the French, more than
500 per cent. cheaper. Blue ultramarine costs, namely, in Nürnberg,
quality No. 0, 10 florins per lb.; a lighter quality, which is
nevertheless darker than the darkest French at 100 francs, 5 fl. per
lb.; a third quality likewise darker than the seconds French at 60
francs, 3 fl. per lb. Green ultramarine, 3 fl. 10 kr. per lb.

“‘How much further, however, these low prices will be yet reduced
after the completion of the factory, may be gathered from a detailed
calculation of cost which the chief of this factory has made himself
responsible for as being the highest estimate. This calculation of
cost is based upon the weekly production of 10 cwt., which the factory
will soon be able to turn out, and upon a necessary cost of plant and
working capital of 90,000 Rhenish florins, as follows:--

“Page 36.--‘Calculation of cost of 500 cwt. blue and green
ultramarine:--

                                                             Fl.

   (1) Raw material and cost of transport                 10,000
   (2) Fuel, including cost of transport of 7200 cwt. of
          coal at fl. 1.30                                10,800
   (3) Wages of 20 operatives at fl. 250 per annum         5,000
   (4) Utensils and apparatus                              3,200
   (5) Buildings and repairs                               3,400
   (6) Keep for two horses                                   600
   (7) Expense of factory                                  3,000
   (8) Cost of administration                              2,000
   (9) Unforeseen matters and accidents                    2,000
  (10) Interest on building and working capital at 5
          per cent.                                         4,500
  (11) Public taxes, insurance, &c.                           500
                                                           ------
            Thus 500 cwt. will cost                    Fl. 45,000
              “    1  “       “                            Fl. 90
              “    1 lb.      “                                45kr.

“‘Forty-five kreutzers, therefore, in accordance with the above, will
in future be the production cost of a colour which, as is well known,
could not be obtained for several hundred guldens, while in green it
was not procurable at any price.’

“I venture to hope that the foregoing communication may yet prove of
some interest in chemical circles.”

  1 florin or gulden = 2_s._      1 kreutzer = ½_d._

Ultramarine is by far the most commonly used of the blue pigments.
It is a chemical combination of silica, alumina, soda, and sulphur,
but its exact chemical constitution is not known, the proportions of
its ingredients varying somewhat with different makes. There are two
principal varieties of ultramarine sold. One is known as sulphate
ultramarine the other as soda ultramarine, from the materials used in
the process of manufacture. In the first, silica, china clay, _sulphate
of soda_, and coal are used; in the latter, silica, china clay,
_carbonate of soda_, and sulphur are used. The sulphate ultramarine is
distinguished by its very pale greenish blue colour, while the soda
ultramarine is of a violet hue.

Ultramarine is distinguished from other blues by the fact that acids
completely decolorise it, with the evolution of sulphuretted hydrogen
and the formation of a white precipitate of sulphate.

The sulphate ultramarine is more easily decomposed by acids than
the soda ultramarine, and some makes of the latter more easily than
others. Alkalies and heat have no action on this pigment. Boiled in
strong nitric acid, ultramarine is completely decolorised, a colourless
solution being formed, and a gelatinous mass of silica being left as a
residue.

It is not as a rule necessary to make an analysis of the pigment; the
above tests serve to distinguish it from other pigments.

An assay of ultramarine should include the following points:--1st,
colour or tint; 2nd, covering power or body; 3rd, acid resisting
properties: this can be tested by making a very weak solution of
sulphuric acid--about 4 oz. in 1000 oz. of water--and adding a little
of this to the pigment contained in a glass, and noting how long it
takes to bring about decolorisation; 4th, the power of resistance to
the action of alum. When ultramarines are boiled with a solution of
alum, they are more or less reddened thereby; those which are made
with a large excess of silica are found to resist this action of
alum better than those containing a normal quantity of this compound.
Such ultramarines are preferred by the paper maker, who uses a large
quantity of alum and sulphate of alumina in the sizing of his papers,
and therefore he wants an ultramarine which shall not change in shade
when used for tinting alumina sized papers. This point is easily
tested. A solution of alum is made, and in a little of this a small
quantity of ultramarine is boiled for a few minutes, and it is noted
whether any change of shade occurs. If any sample is found to change
much, that sample must be rejected for paper tinting, although it may
be used by the painter or the laundress.

It may be worth pointing out here that ultramarine should not be used
with any other colours which have a tendency to be acid, as sooner or
later the colour will be destroyed. It should also not be used with
lead or copper pigments, as the sulphur it contains tends to react on
those metals; forming the black sulphides, thus leading to the ultimate
discoloration of the mixture.--(_Chemical Trade Journal_).



CHAPTER IV.

BROWNS.


Brown colouring matters are obtained from all three kingdoms--the
animal, the vegetable, and the mineral--but in greatest abundance from
the last named. The natural mineral brown pigments afford almost every
variety of tint, and being largely composed of silica and metallic
oxides they are remarkably permanent.

ASPHALT OR BITUMEN.--These names are applied to a variety of
black or brown resinous matters found in many parts of the world in
a mineralised state, though derived originally from organic sources.
The “Bitumen of Judæa” is supposed to be found on and around the Dead
Sea, but the bulk of the product going by that name really comes from
Trinidad. All kinds of asphalt have a pungent and peculiar smell, melt
at a low temperature, are very combustible, and while dissolving in
turpentine, and more readily in coal tar naphtha, are insoluble in
water and in alcohol. Very little asphalt is now used as a pigment,
but it continues to find a limited application in varnish making,
notwithstanding the tendency of varnishes containing it to suffer from
minute cracks with the lapse of time.

BISTRE.--This pigment is used exclusively in water-colour
painting, for which purpose it affords a fine warm yellowish tinted
brown. It is of vegetable origin, being prepared from the soot which is
deposited in the flues leading from fireplaces which consume wood fuel.
Every wood, however, does not afford an equally good sample of bistre,
and beech occupies the foremost rank in this respect. The brightest
and blackest soot is selected, and after careful grinding and sifting
through a very fine sieve, it is repeatedly stirred up, for several
hours at a time, in a series of clean hot waters, the object of which
is to dissolve out all traces of tarry and other soluble matters, which
very seriously affect its permanence, being oxidised on exposure to
air and light, and thus weakening the tint. The washing is therefore a
matter of the very first importance. The solid pigment is allowed to
settle out of each wash water, and is collected and dried, being mixed
with a small proportion of gum water to give cohesion. The drying is
effected in a stove room.

BONE BROWN.--This unimportant pigment is simply underdone bone
black (see p. 6), and is obtained by stopping the calcination of the
bones at a point which falls short of thorough charring. In consequence
it contains a proportion of unaltered animal matters, which sooner or
later may undergo decomposition, and prejudicially affect the painting.

CAPPAGH BROWN.--A mineral pigment which is only a variety of
umber, and may best be described under that head (see p. 105).

CASSEL EARTH.--Another name for Cologne earth, _q. v._

CHICORY BROWN.--This vegetable pigment is rich-coloured but
lacks permanence. It is prepared by calcining roots, such as those
of chicory, in vessels to which air is not admitted, from which then
results a fine brown powder. This is boiled in water, and the solution
is evaporated to dryness, yielding a brown pigment, which, being
soluble in water, is sometimes employed by water-colour artists.

COLOGNE EARTH.--This material, which is also known as Cassel
earth or Rubens brown, is an earthy carbonaceous substance, probably
derived from the decomposition of lignite or brown coal, readily
undergoing combustion without emitting flame or smoke. Large deposits
of it are worked in the vicinity of Cologne, whence its name. It is
blackish-brown in colour, smooth and crumbling to the touch, and very
light. To remove soluble impurities it is subjected to several washings
in water, and then collected, mixed with a little gum-water, and dried
in small moulds. The colour is used by artists, but is very variable in
composition and uncertain in durability.

MANGANESE BROWN.--One of the most durable brown pigments used
by the Romans is found to be oxide of manganese, which discovery has
led to the proposal to prepare the binoxide of that metal as a brown
pigment. The method suggested is as follows:--

The protochloride of manganese, derived from the manufacture of
chlorine, or the protosulphate resulting from the calcination of the
protoxide with iron sulphate, is dissolved in warm water (85°-105°
F.); to this is added a sodium hypochlorite solution, or a solution
of potassium hypochlorite containing a small proportion of carbonate
of soda, the addition being continued until the precipitated
manganese binoxide ceases to change colour, marking the completion
of the oxidation. The supernatant clear liquor is drawn off, and the
precipitate is washed first with acidulated water (containing 2 per
cent. of sulphuric acid), and then with pure water till all trace of
the acid is removed. The dark-brown impalpable powder of manganese
binoxide is stove dried, and forms a permanent and safe pigment with
good covering power.

MARS BROWN.--One of the products obtained by the calcination
of Mars yellow (_q. v._) at various temperatures and under different
conditions is a full-tinted and durable brown due to sesquioxide of
iron. Another method of preparing it is from alum, sulphate of iron,
and chloride of manganese. In either case the pigment is not superior
to umber or oxide of iron, while it cannot be produced as cheaply.

PRUSSIAN BROWN.--An artists’ colour known by this name is
prepared from Prussian blue, but as it has no superiority over Vandyke
brown or umber, and is higher priced, it is not in general use. It
consists essentially of carbon and ferric oxide, resembles bistre in
tone, and possesses durability and good covering powers. The operation
of calcining the Prussian blue should be conducted slowly, and is
best performed in a closed vessel, though it may also be done in the
open. The pieces of blue should not be larger than a hazel nut. They
soon split, scale off, and become red, when the heating should cease.
On breaking the cooled particles they will show a patchy coloration
varying from yellowish-brown to black. On grinding, the mass assumes
the desired brown hue.

RUBENS BROWN.--Another name for Cassel brown or Cologne earth
(_q. v._).

SEPIA.--This is one of the few pigments derived from the
animal kingdom. It is produced by several sea-inhabiting creatures
belonging to the class called _Cephalopoda_, and more particularly
by two members of the genus _Sepia_, known respectively as _Sepia
officinalis_ and _Sepia loligo_. A peculiarity of these cephalopods
is that they are provided with what is commonly called an ink bag, in
other words a gland or sac filled with a blackish-brown liquid, which
possesses intense colouring power. The object of the secretion is the
protection of the creature from pursuit by its enemies, a portion of
fluid being discharged at will, and so obscuring the surrounding water
that escape is facilitated.

For the sake of this pigment the cuttle-fish are sought after by
fishermen in the localities frequented by the animals, notably in the
warm waters of the Mediterranean. When the creatures are captured,
their glands are carefully extracted and sun-dried so as to solidify
the contents. In this state ink bags are sent into commerce. The
colourman subjects the sacs to boiling in a solution of soda or
potash, whereby the colour is dissolved out of the receptacle, and
being filtered clear of all fragments of the animal tissue, is next
precipitated by the addition of acid, collected on a filter, washed,
and dried. It then forms an exceedingly useful pigment, having,
according to Prout, the following average composition:--

                                      Per cent.

  Black pigment (melanin) about          78
  Calcium carbonate                      10½
  Magnesium carbonate                     7
  Alkaline chlorides and sulphates        2
  Organic matter                          1

It is remarkably permanent for an organic substance, suffering no
alteration on being combined with other pigments, and withstanding the
effects of exposure to air and light. Though slightly transparent, and
not quite constant in tint, it possesses very great colouring power.
Being of extremely fine texture it can be worked up equally well as an
oil colour or as a water colour, but it is especially in the latter
capacity that it forms an indispensable artists’ colour, and permits
the production of a great range of shades and tints.

ULMIN.--The pigments grouped under this name are also of
organic origin; but though they possess good colour, mix well, and flow
readily from the brush, they lack the durability which is essential to
their successful use. The following methods have been employed in their
preparation:--(1) Fused caustic potash is digested in alcohol, and the
liquor filtered and heated till a brown powder is thrown down, which is
filtered and washed with acidulated water; (2) Waste cotton, peat, or
brown coal, heated with an alkali; (3) Farinaceous matters carbonised
by mineral acids.

UMBERS.--These form a large class of natural earths of a
brown colour, differing widely in the proportions of their chief
constituents, but closely allied to the ochres and siennas in general
composition, and owing their colour mainly to the presence of hydrated
oxides of iron and manganese, the latter prevailing in the umbers to
a greater degree than in the ochres and siennas, which consequently
belong to the yellow group (_q. v._).

Beds or veins of umber of varying thickness and extent are found
in many places, especially in connection with magnesian limestone
(dolomite). Apparently they are often derived from decomposition of
this rock, perhaps due to the infiltration of carbonated water, which
has acted upon the calcium and magnesium carbonates in the dolomite,
and left the silica and the iron and manganese as oxides, forming the
bulk of the umber. Usually these beds of umber are near the surface,
though covered by an overburden of vegetable soil, and the operation
of working them may be called quarrying rather than mining, being of a
superficial and simple character, often only amounting to small pits.

As no umber is a definite body, but rather a mixture of various
substances, so the composition of every kind is peculiar to itself,
and very wide differences are noticeable. Even the same bed will not
necessarily produce always the same class of umber. The following
figures show the extent to which the proportions of the several
ingredients may vary:--

                                           Per cent.

  Water given off at 212° F.               4  to 65
  Water in combination                     5  to 11½
  Silica                                   4½ to 29½
  Manganese dioxide                        7  to 27
  Ferric oxide                             6  to 36

Calcium carbonate is sometimes present to the extent of 2½ to 6 per
cent., and at other times is quite absent, its place being taken by
½ to 1 per cent. of lime (calcium oxide); some of the English umbers
contain about 2 per cent. of calcium sulphate (gypsum) in addition to
the carbonate. Alumina may occur to the amount of 2½ to 12½ per cent.,
or may be wanting altogether. In a sample of Derbyshire umber analysed
by Hurst there appears to have been over 30 per cent. of barium
sulphate (barytes), which looks suspiciously like adulteration.

Almost every variety of shade may be found in umbers. The darkest and
richest in colour--a warm violet-brown--is the so-called Turkey umber,
mined in Cyprus, and formerly shipped viâ Constantinople; this is
of very fine quality and commands the higher price in the market. A
reddish-brown Irish umber, known as Cappagh brown, obtained from the
Cappagh mines in Cork county, is much esteemed among artists, both
for water-colour and oil painting, and especially for the latter when
it has been subjected to a preliminary desiccation at a temperature
of about 170° F. Heated to the boiling point its colour changes to a
rich red, resembling burnt sienna. Cornish umbers are of fairly good
quality. Derbyshire umbers are poor, and incline to a reddish tint,
besides being gritty. Sometimes they are adulterated with a little lamp
black, which renders the tone more like that of Turkey umber, and thus
deceives the unwary buyer.

There are three conditions in which umbers come into commerce: (1) as
raw lump, being the mineral just as it is mined; (2) as raw powdered,
when it has been ground very fine and levigated or washed in flowing
water, whereby the particles get assorted according to their several
degrees of fineness; and (3) as burnt, being the powder after it has
been subjected to calcination in a closed furnace. Some umbers are so
soft that they can be washed without any previous grinding, but this is
not generally the case. The apparatus used in grinding and levigating
is common to all pigments where these processes are employed, and will
therefore be described once for all in a later chapter. The calcination
is conducted at a red heat, and by this process the tint is made darker
and warmer, but it must not be pushed too far or the pigment will
blacken.

While different samples of umber present differences of tone and shade,
from a yellowish to a violet brown, they are alike in being very
durable and proof against the injurious influences of air, light, and
impure atmospheres; ordinary acids and caustic soda have no appreciable
effect. They mix well with other pigments without provoking any change,
and are equally satisfactory as oil or water colours. They do not
admit of much adulteration, except in the substitution of an inferior
grade for a superior one, and possibly the addition of barytes as a
make-weight.

VANDYKE BROWN.--What the original brown used so much by
the great Van Dyke was no one can tell. The pigments now sold under
the name of Vandyke brown are of varying composition, some being
simply mixtures of red oxide of iron and lamp black, others are
natural earthy substances after the character of Cologne earth, and
others again are artificial products of the partial carbonisation of
vegetable matters, such as cork waste. As it is uncertain what was the
composition of the original Vandyke brown, no standard of chemical
purity can be established.

Probably the most general sources of Vandyke brown are red oxide and
lamp black, and the quality of such a pigment will chiefly depend on
securing a good black, as any traces of unburned oily matter will
make the paint difficult to dry. Almost any variety of shade can be
produced by adjusting the proportions of lamp black and red oxide, with
sometimes the addition of a little ochre. The pigment made in this
manner forms the staple brown paint for industrial application, mixing
well with oil, and being of a durable character, but it does not mix so
well with water.

Vandyke browns of the Cologne earth type, from earthy lignites and
peaty matter, are much used in and around the localities where they are
produced, and entail nothing more than grinding and levigation to fit
them for the market. They are in general best adapted to water-colour
painting.

Warm and slightly reddish tints of Vandyke brown are obtained by
the partial carbonisation of ligneous material, in other words by
subjecting cork and bark waste to moderate calcination in closed
retorts. These mix equally well in water or oil.

All varieties of Vandyke brown are stable pigments, without any
disturbing influence when used in admixture with other colours, and
quite proof against any change on exposure to air and light. Next to
the umbers they are the most generally useful browns.



CHAPTER V.

GREENS.


Green pigments form an important and numerous class, but many of
those which possess the most brilliant and durable qualities contain
highly poisonous ingredients, and some of the most beautiful are
not permanent. All things considered they are perhaps the least
satisfactory group of colouring matters. The following list comprises
all worth notice.

BARYTA GREEN.--It is said that the manganate of baryta makes
an excellent green pigment, which may with advantage replace for
many purposes those greens which contain arsenic. Several methods of
preparing it have been published:--(_a_) One consists in igniting
together the nitrate of baryta and manganese oxide or dioxide. (_b_)
Another consists in fusing a mixture of pyrolusite or black oxide of
manganese, caustic bartya, and chlorate of potash. (_c_) According
to a third method, mix 2 parts caustic soda and 1 part chlorate of
potash, and gradually add 2 parts very finely powdered manganese; heat
gradually up to dull redness, then allow to cool, powder, and exhaust
with water; filter and cool, and add a solution of nitrate of baryta
to the filtrate; a violet-coloured baryta precipitate forms; this is
carefully washed, dried, and treated with ½-1 part of caustic baryta,
hydrated, and gradually heated up to redness, with constant stirring.
The cooled mass is powdered, and finally washed to remove any excess of
baryta.

By either process a green mass is obtained, but the second method
seems to yield a more beautiful and homogeneous product. In
experimenting with other and more direct methods for preparing a
baryta green of great purity and beauty, Fleicher has made several
observations of its properties. If a green solution of manganate of
potash be precipitated, while boiling, by chloride of barium, a heavy,
granular, but not crystalline, precipitate of manganate of barium is
obtained. This precipitate has a violet colour, approaching blue, can
be washed by decantation at first, and afterwards may be collected on
a filter. On drying the precipitate, its colour grows lighter with
the increase of temperature; and on being heated to a dark red heat,
it looks almost perfectly white, with only a shade of greyish blue.
If, then, it be heated still higher with free access of air, or in an
oxidising flame, it gradually turns green; by carrying the process
farther the colour becomes a beautiful greenish-blue, and finally,
at a very high heat, a dirty greyish-brown mass is formed from the
reduction of the manganic acid to binoxide of manganese. On adding
chloride of barium to a solution of the permanganate of potash, and
boiling, a precipitate is slowly formed of a peach-blossom colour,
while the liquid retains a deep violet colour. By decanting and
bringing the mass, diluted with water, on a filter, the precipitate is
not decomposed, and can be dried at 212° F. without changing colour.
When the dry permanganate of barium is gradually heated, its colour
also grows paler, but does not, like the manganate of baryta, acquire
a green colour at a still higher temperature; for after the colour
has once vanished, an increase of temperature soon converts it into
the greyish-brown mixture of the binoxide of manganese and baryta
or carbonate of baryta. Hence it is impossible to prepare the green
manganate of baryta from the permanganate.

In regard to the colour itself, experiments have shown that the most
beautiful green is that formed by igniting the manganate as described
above. The green prepared by Rosenstiehl’s process--fusing together
caustic baryta, chlorate of potash, and binoxide of manganese--is less
beautiful than the above; while that attained from nitrate of baryta
and binoxide of manganese is far inferior to either of the others.
Perhaps, however, this colour could be improved by preparing it in a
reverberatory furnace with a strong oxidising flame.

The blue-green baryta pigment has different shades, according to its
preparation, some being almost pure blue with only a shade of green,
and resembling the light blue quill feathers of many parrots. The
greener the colour the more it gains in intensity, but it loses in
fineness, although still surpassing the green manganate of baryta.

The production of the blue or bluish-green baryta is due entirely to
the alkaline property of the mass. Whether each definite colour is due
to a definite composition is doubtful, since the temperature, which
must not exceed that of a bright red heat, exerts a greater influence
on the colour. This much is however certain, that both manganic acid as
well as the permanganate of baryta, when mixed with about 20 per cent.
of hydrate of baryta and ignited at a red heat, will always produce
this blue-green colour. It is evident that the blue-green colour is
dependent entirely on its basic character; for on placing this powder
in weak acids, it first turns green and is then gradually decomposed.
The baryta pigment is quite permanent, and may be subjected to the
action of strong sulphuric acid for hours, at the ordinary temperature,
before the colour will be destroyed. Boiling potash solution has no
perceptible effect upon it. The permanence, especially of the blue
shade, is increased by adding a little baryta, which increases its
alkalinity. It is also worthy of remark that the pigment prepared from
the nitrate of baryta is much less permanent, because the nitrous acid
present will after a time exert a reducing action.

The baryta pigments seem especially adapted to fresco painting, because
they appear very bright and lively on stone, and especially on lime,
where many other pigments lose their beauty or are entirely destroyed.

BREMEN GREEN.--This old-fashioned pigment is a basic carbonate
of copper, and has been produced in several ways. At first a basic
chloride or oxychloride was used, its mode of preparation varying
somewhat but without affecting the character of the result, the great
essential being that no subchloride of copper should be present.
Therefore, in some factories, it was the practice to prepare the magma
of basic oxychloride even a year in advance; or, to subject it to
repeated wetting and drying in order to ensure prefect oxidation. The
method has now become obsolete, and is superseded by the following:--

When neutral nitrate of copper is decomposed by an insufficiency of
a potash carbonate solution, the flocculent precipitate of copper
carbonate formed at first is gradually changed into a subnitrate of
copper which is precipitated as a heavy green powder. In practice the
operation is conducted as follows:--Copper scales are calcined in a
reverberatory or muffle furnace, till all the suboxide is converted
into protoxide, or until a sample dissolves in nitric acid without
evolution of red nitrous vapours. The copper nitrate solution is heated
and decomposed by a clear solution of potash carbonate, and when the
effervescence subsides, small doses of potash carbonate solution are
added, till but little undecomposed copper remains in the solution. To
recover this last portion, the clear liquor is decanted, and the green
precipitate is washed several times with small quantities of water. All
the liquors are collected, and the remaining copper is precipitated by
potash solution. The green carbonate of copper is introduced into a new
solution of copper nitrate, in which it is transformed into a basic
salt. The previous liquors are evaporated till they afford crystals of
nitrate of potash, which is a valuable secondary product.

BRIGHTON GREEN.--The following recipe has been published for
making this pigment. Dissolve separately 7 lb. sulphate of copper and 3
lb. sugar of lead, each in 5 pints of water; mix the solutions, stir in
24 lb. of whiting, and when the mass is dry grind to powder.

BRUNSWICK GREEN.--(_a_) Old process.

The Brunswick green of former days was closely allied to Bremen green,
essentially consisting of a basic chloride or oxychloride of copper,
and possessing all the faults incidental to that class of copper salt.
While having fairly good covering power, and capable of being used
either as a water colour or an oil colour, it was tedious and therefore
expensive to prepare, and not thoroughly durable under exposure to
air and sunlight. Nevertheless it was a useful bluish-green pigment.
Following are some of the many methods by which it has been prepared:--

(1) Poor oxidised copper ores are moistened with hydrochloric acid,
and spread out exposed to the air. The metal is thus rendered very
susceptible to the action of chlorine, and is even attacked by
solutions of ammonium chloride and of common salt. The sub-chloride
produced is rapidly transformed into oxy-chloride, and forms a fine
light-green pigment.

(2) Place 2 parts by weight of copper-filings in a vessel capable
of being tightly closed, and over them pour 3 parts by weight of
salammoniac in the form of a saturated aqueous solution. Keep the
mixture in a warm place for some weeks and thoroughly agitate it
occasionally. In due time the newly formed oxychloride is removed from
the vessel, and separated from the non-oxidised copper by washing on a
sieve. This washing must be continued until all traces of alkali have
been destroyed, when the pigment is drained, and very slowly dried at a
low temperature to avoid decomposition.

(3) Copper scrap is covered with a concentrated solution of chloride
of copper and allowed to remain until the chloride has undergone
conversion into basic chloride. The latter is then subjected to the
straining, washing, and drying treatment prescribed in (2).

(4) In a lead-lined vessel, place a quantity of copper filings or
waste, and add to it two-thirds of its weight of common salt, and
one-third of its weight of concentrated sulphuric acid, the latter
being however first diluted by admixture with three times its volume
of water. The mass is left to stand, with occasional stirring till all
the copper has been transformed into oxychloride, when it is strained,
washed, and dried as in (2).

(5) A modification of (4) is to put the copper scrap into a wooden
vessel, and cover it with an equal weight of common salt and an equal
weight of sulphate of potash dissolved in water. After standing and
agitation as before, the oxychloride is formed, and the straining,
washing, and drying are repeated.

(6) A solution of crude carbonate of ammonia is added to a mixed
solution of alum and blue vitriol so long as any reaction takes place.
When it is completed, the precipitate is collected, washed, and dried
as in the other cases.

(7) Lighter shades are produced by the addition of alum, or of sulphate
of baryta.

(_b_) New process.

The modern Brunswick greens, which are made in a variety of shades,
and sometimes known as chrome greens, Prussian greens, Victoria
greens, and by other fancy names, really consist of a white pigment
as a basis--usually sulphate of baryta (barytes), but occasionally
also sulphate of lime (gypsum) and sulphate of lead--coloured green
of varying intensity and depth by addition of a blue pigment in the
shape of Prussian blue, and a yellow in the guise of chrome-yellow.
There are what may be called four distinct standard shades recognised
by colour-makers, viz. “pale,” “medium,” “deep,” and “extra deep”; but
inasmuch as every manufacturer adopts a formula of his own, there may
be appreciable differences among colours of the same nominal standard
if by different makers. Taken as a whole, about three-fourths of their
total weight consists of the foundation white pigment, usually barytes;
about 1 to 6 per cent. is Prussian blue, according to the shade; and 14
to 18 per cent. chrome yellow; but there are brands occasionally met
with which depart considerably from these average figures.

The actual ingredients employed to form these green pigments are
essentially different, according as the wet or the dry method of
combining them be adopted. In selecting the various ingredients the
following points must be borne in mind. The Prussian blue of every
maker is not the same in quality, and while the character of the blue
is not of the foremost importance when dark greens are being made, for
light shades of green, on the other hand, it is essential to select
only the best and brightest brands. In the same way the tint and
quality of the chrome yellow are liable to considerable fluctuation,
and it is almost impossible to ensure two lots having exactly the same
characteristics, consequently the only way in which a certain shade of
green can be ensured is by experimental trial with small quantities for
each batch. Middle chromes can be used for deep greens, but only the
lemon chromes for pale shades. Regarding the barytes which forms the
basis of the pigment, there are no special precautions to be observed;
and the same may be said of the gypsum, should that be adopted as a
substitute for the barytes, except that 1 part by weight of gypsum
takes the place of about 2½ parts of barytes. The latter, however,
is much the more commonly used. For the dry method of compounding
Brunswick greens, the above named ingredients are all that are required.

In the wet method there is this essential difference, that it is
sought to precipitate the blue and yellow colours upon the inert base
by bringing about certain reactions, and therefore while the base
remains the same as in the dry method, the colouring media are totally
distinct, consisting of lead acetate, bichromate of potash, sulphate
of iron, and yellow or red prussiate of potash. The chief condition
to be observed with regard to the lead acetate is that it shall be in
the proportion of slightly more than three to one of the bichromate
of potash; in other words, the bichromate should be a trifle less
than one-third the weight of the lead acetate. As to the iron salt,
if commercial acetate or nitrate of iron could be bought of constant
quality or purity, that would be the most convenient form; but failing
that, recourse is had to freshly made and good quality sulphate of iron
(green copperas). It is found that the best results are secured when
the weight of the sulphate of iron is exactly the same as that of the
prussiate of potash. On the score of economy, the yellow prussiate of
potash (ferrocyanide) is employed, but the red prussiate (ferricyanide
of potassium) gives better and more certain results, and should be
adopted when making a superior paint which will command a higher price.

As to the comparative merits of the wet and dry systems of mixing the
ingredients of Brunswick greens, preference must be given to the former
on the score of quality of the pigment produced, but on the other hand
it entails much more trouble and skill, and there never can be the
same degree of control over the conduct of the operation or the shade
of colour developed. The dry method, however, though much more easily
carried out, and enabling the exact shade desired to be obtained to a
nicety by adding a little more of either the blue or the yellow during
the process of manufacture, is seldom adopted, because the quality and
fineness of the tints thus secured are much inferior.

The modus operandi with the wet method is as follows:--The barytes, in
the requisite fine state of subdivision, is very thoroughly stirred up
with water in a capacious vessel fitted with an agitator, the water
being in sufficient quantity to make quite a fluid mass. In convenient
proximity to the barytes tank, and elevated above it, provide three
other tanks of lesser capacity furnished with means of discharging
their contents into the barytes tank. In one of these smaller tanks
dissolve the green copperas in cold water; in another, the sugar of
lead; and in the third the bichromate and prussiate of potash together.
When all the salts are thoroughly dissolved, and while the barytes is
kept in constant agitation, admit first of all the copperas solution,
then the lead acetate solution, and finally the combined bichromate
and prussiate solution, never allowing the stirring to slacken till
after the last drop of these solutions has been introduced. When the
commingling of all the ingredients is judged to be complete, the green
pigment formed is allowed to subside, and the clear supernatant fluid
is siphoned off. The pigment is washed several times by admitting
clean water, agitating and settling, and finally is removed, drained
on a filter, and slowly and carefully dried. Many ways of arranging
the apparatus will suggest themselves, the chief point to keep in mind
being to economise labour as much as possible.

The dry method of mixing is simplicity itself in comparison with the
above, and merely entails putting the component materials--barytes,
chrome-yellow and Prussian blue--through an edge-runner mill
simultaneously, in the proportions adapted for producing the shade
required.

In giving formulæ for compounding these Brunswick greens, it must be
understood that they are not absolute, as every manufacturer adopts
his own particular proportions for a certain shade, but they form
a sufficiently approximate basis from which to work. They are all
computed for 100 lb. of barytes forming the body of the new pigment:--

    _Pale_: Wet--1 lb. each copperas and prussiate, 12 lb. lead
    acetate, 3¾ lb. bichromate.

    Dry--80 lb. chrome yellow, 1¼ lb. Prussian blue.

    _Medium_: Wet--1½ lb. each copperas and prussiate, 12½ lb. lead
    acetate, 4 lb. bichromate.

    Dry--30 lb. chrome yellow, 2¼ lb. Prussian blue.

    _Deep_: Wet--2 lb. each copperas and prussiate, 13 lb. lead
    acetate, 4¼ lb. bichromate.

    Dry--30 lb. chrome yellow, 4½ lb. Prussian blue.

    _Extra deep_: Wet--3½ lb. each, copperas and prussiate, 14½ lb.
    lead acetate, 4½ lb. bichromate.

     Dry--30 lb. chrome yellow, 7 lb. Prussian blue.

The Brunswick greens are in the front rank of green pigments so far as
covering power is concerned, and, when made from reliable materials,
are reasonably durable under the influence of air and light, in which
respect, however, they vary considerably. They can be used as water
colours, but are superior in oil paints. Precautions are necessary
in mixing them with other pigments. By the action of sulphuretted
hydrogen, or sulphur in any form, the colour is darkened to a notable
degree; by the action of acids, the chrome is destroyed and the green
becomes blue; by the action of alkalies, both the blue and the yellow
constituents are affected, and the green gives place to a reddish hue.
The pale and medium shades are yellow greens; the deep and extra deep
are blue greens.

These colours can be distinguished by heating them with caustic soda,
which turns them brownish in tone, owing to the destruction of the
Prussian blue. If the residue be filtered, and to the filtrate some
acid and ferric chloride be added, a blue precipitate will be obtained,
indicative of the presence of Prussian blue. On washing the residue
with water and treating with hydrochloric acid, the brown colour
disappears, and, in most cases, only a white residue of barytes is
left; sometimes the residue may have a faint yellowish colour. The
solution in hydrochloric acid will give the characteristic tests for
iron. The yellow element can be recognised by boiling in hydrochloric
acid, filtering, and allowing the filtrate to cool, when crystals of
lead chloride will deposit; these, separated out and dissolved in
boiling water, will give the characteristic tests for lead, such as
a white precipitate with sulphuric acid, and yellow precipitate with
bichromate of potash. The filtrate will have a green colour, indicative
of chromium.

CHINESE GREEN.--Another name for the vegetable pigment known
in China as Lokao (q.v.)

CHROME GREEN.--This name is often applied to any green in
which chrome enters as an element, but more particularly to the modern
Brunswick greens described on pp. 114-118; and to the green which bears
the name of its first maker, Guignet, and described under the title of
Guignet’s Green, see p. 125.

COBALT GREEN.--This remarkably stable, but somewhat costly,
pigment is also known by the names of Rinmann green and zinc green,
the former after the name of the chemist who first prepared it, and
the latter because it contains a large proportion of zinc. It is in
fact a combination of the oxides of cobalt and zinc, and was originally
produced in the following manner:-½ lb. pure cobalt ore was dissolved
in 4 lb. concentrated nitric acid, and added to a solution of 1 lb.
zinc in 5 lb. nitric acid; the mixture was diluted with water, and
a solution of potash carbonate was added, throwing down a pinkish
precipitate, which was washed on a filter, dried, and calcined at a
high temperature.

Wagner found that an indispensable condition was to have a protoxide
of cobalt as free as possible from foreign metals, with which object
he practised the following method:--Cobalt oxide is dissolved in three
equivalents of hydrochloric acid, and the solution is evaporated to
dryness; the residue is dissolved in six equivalents of water, and
through the solution is passed a current of sulphuretted hydrogen
gas, so long as any precipitate is formed. This precipitate consists
of sulphides of the foreign metals. The clear solution is siphoned
off, evaporated to dryness, and the residue is dissolved in water. As
required, this solution is treated with carbonate of soda, and the
precipitate, washed, and while still wet, is mixed with zinc white. The
reddish mass produced in this way is dried and calcined. The best tone
is attained by combining 9 to 10 parts of zinc oxide with 1 to 1½ parts
of cobalt protoxide.

Louyet has shown that if the cobaltic solution be precipitated by the
phosphate or the arseniate of potash, the corresponding salt of cobalt
thus produced possesses the property of imparting a green colour to
zinc white at a much lower temperature than is required in the case
of ordinary protoxide of cobalt: moreover, the pigment gains in body,
and the colour gains in purity and brightness. If a small quantity of
arsenious acid is added to the ordinary mixture before calcination, the
calcined mass will assume a remarkably bright green colour; and its
structure being loosened by the disengagement of fumes of arsenious
acid, it will be easy to grind.

According to Barruel and Leclaire’s method, 1 lb. of pure dry sulphate
of cobalt, dissolved in hot water, is mixed with 5 lb. of zinc oxide.
The mixture is dried, and calcined for three hours at a clear red heat
in a muffle; when cooled, it is thrown into water, washed, and dried.

The composition of cobalt green has been shown by Wagner to vary
considerably, as is to be expected from the methods of its preparation.
The proportion of zinc oxide ranges from 71½ to 88 per cent., and the
cobalt protoxide from 11½ to 19 per cent.; in addition, there will be
fluctuating percentages of phosphoric acid, soda, oxide of iron, &c.,
according to the process followed.

With the single exception of its costliness, cobalt green possesses
advantages over most other green pigments. It has a bright colour,
sometimes inclining to a yellowish tint, or, when phosphates are
used in its preparation, leaning to a blue shade. But it is always
permanent, not only under the influence of air and light, but also in
the presence of alkalies and any but concentrated acids; thus it may
safely be compounded with other pigments.

DOUGLAS GREEN.--This pigment, which is fairly permanent, and
possessed of considerable covering power, owes its name to the chemist
who proposes its use, and its colour to the oxide of chromium. The
method by which it is prepared is as follows:--Solutions of barium
chloride and potassium chromate are mixed together. To the barium
chromate thus produced is added one-fifth of its weight of concentrated
sulphuric acid, whereby partial decomposition is brought about,
resulting in a mixture of barium chromate, barium sulphate, and chromic
acid. This mixture is dried, and calcined in a crucible at bright red
heat, the effect of which is that the chromic acid is converted into
green oxide of chromium, and, being scattered throughout the mass,
imbues it with a green colour.

EMERALD GREEN.--This is quite an old-fashioned pigment,
having been in use some 80 years. It is a combination of acetate and
arsenite of copper, and varies in tint from a dark to a pale green,
always with a bluish cast. It possesses good covering power, and can
be used either as an oil-or as a water-colour, but particularly as the
latter, and is much used in paper staining. In composition it varies
considerably, as there are some half-dozen industrial methods of making
it; but in general terms it usually contains over 50 per cent. of
arsenious acid, and about 30 per cent. of oxide of copper, together
with various impurities. Following are some of the processes by which
it is manufactured.

(1) According to the method introduced by Liebig, 1 part of verdigris
is heated in a copper kettle with sufficient distilled vinegar to
effect its solution, and to this is added a solution of 1 part of
arsenious acid in water. The result is a precipitate of a dirty green
colour, which is dissolved in a new quantity of vinegar and boiled for
some time. In this way is obtained a new precipitate, granular and
crystalline, and exhibiting a splendid green colour. When this has been
filtered off, washed, and drained, it is boiled with one-tenth of its
weight of commercial potash, in order to deepen and brighten the colour
and destroy the bluish tint. Should the waste liquor obtained after the
filtration of the pigment from the second boiling in vinegar contain
any remaining copper, arsenious acid is added; and if arsenious acid be
present, copper acetate is added; while if acetic acid survives it may
be used again for dissolving another lot of verdigris.

(2) Form a paste with 1 part verdigris in sufficient boiling water,
pass it through, a sieve to remove lumps, and gradually add it to
a boiling solution of 1 part arsenious acid in 10 parts water, the
mixture being constantly stirred until the precipitate becomes a heavy
granular powder, when it is filtered through calico, and dried very
carefully.

(3) Acetate of copper is mixed with a sufficient quantity of water
heated to 122° F., to make a homogeneous and liquid paste. To 10 parts
of acetate of copper in this condition is added a solution of 8 parts
of arsenious acid in 100 parts of boiling water, the whole being then
kept in a state of ebullition. The addition of a little acetic acid
helps to develop the beauty of the colour. When precipitation is
complete, the clear liquor is drawn off, and forms a convenient solvent
for the next charge of arsenic, the operation being facilitated by
adding a little carbonate of potash, forming an arsenite of potash. The
precipitate constituting the desired green pigment is filtered off and
dried at the lowest effective temperature.

(4) Dissolve 5 lb. of sulphate of copper in water, and add to it a
solution of 1 lb. of lime in 2 gallons of vinegar. Mix 5 lb. of white
arsenic with sufficient water to form a paste. Add the arsenic paste to
the copper and lime mixture, and leave the whole at rest in a moderate
degree of heat. Mutual decomposition slowly ensues, with consequent
formation of the green pigment, which is filtered off, washed, and
dried with the same precaution as before.

When sulphate of copper is used in the production of emerald green,
it is very desirable that it shall be free from sulphate of iron,
which is a common impurity in the commercial article, and greatly
detracts from the purity and brilliance of the pigment. A good method
of eliminating this iron is to add to the sulphate of copper solution
a small quantity of a gelatinous precipitate of carbonate of copper,
produced by decomposing a copper sulphate solution by a soda carbonate
solution, and washing. On adding the gelatinous carbonate of copper,
with agitation, the iron is soon thrown down in flakes of oxide, and
pure sulphate of copper may be filtered off.

(5) Braconnot proceeds as follows:--A solution of 3 lb. of sulphate
of copper is made in a small quantity of hot water; and a second
solution of 3 lb. of arsenious acid and 4 lb. of commercial carbonate
of potash in boiling water. When the evolution of carbonic acid gas
has ceased, the two liquors are mixed together while being kept
continuously stirred; the result is an abundant precipitate of a dirty
yellowish-green colour. On adding a slight excess of acetic acid, a
fine crystalline green is developed; this is washed with boiling water
on a filter, and dried very slowly and carefully.

(6) A rough and ready process is to mix white arsenic with water, and
then stir in an equal weight of verdigris, allowing the mixture to be
at rest for a time in a moderately warm temperature till the pigment is
completely precipitated, when it is washed on a filter, and dried very
gradually.

(7) A method due to Köchlin is described in the following terms:--An
aqueous solution of sulphate of copper is made by adding 100 grammes
of the salt to 500 cc. of water. To this, when solution is complete,
is added 187½ cc. of a solution of arsenite of soda, which is of the
strength represented by 500 grammes of arsenite in 1 litre of water.
The result is that a precipitate of arsenite of copper is thrown down.
This precipitate is treated with 62 cc. of acetic acid at 11° to 12°
Tw., or half that quantity of pure formic acid, for one hour, at a
temperature ranging from 104° to 122° F. The pigment thus produced
is of good colour, but its superiority would not seem to justify the
use of such an expensive article as pure formic acid, nor the minute
adjustment of the proportions of the ingredients, in an operation to be
conducted on a commercial scale.

(8) Another complicated process has been invented by Prof. Galloway,
which, under skilled supervision, and when the correct proportions of
the several ingredients have been ascertained by careful experiment,
may give good results, but several precautions have to be observed
which cannot be entrusted to ordinary factory hands. The principle
of the process is that when a quantity of sulphate of copper is
dissolved in water, sufficient carbonate of soda is added to throw down
one-fourth of that copper sulphate as carbonate of copper, and then so
much acetic acid is introduced as will convert that copper carbonate
into acetate. In order to convert the balance of the copper sulphate
into arsenite, a solution of arsenic in boiling carbonate of soda is
made and added to the copper acetate solution, both solutions being at
a boiling temperature.

Emerald green is a pigment which possesses considerable stability in
dry pure air, but in damp atmospheres it becomes brown; in the presence
of acid or ammoniacal vapours it turns blue, and under the influence
of sulphuretted hydrogen it blackens; moreover, strong alkalies
destroy it. Consequently it cannot be used in many situations, nor
in association with such pigments as contain sulphur compounds. In
decorative painting it is difficult to apply on large flat surfaces,
and necessitates stippling in order to get it to lie well; but when
stippled on a ground of proper green it develops an exceedingly
beautiful bloom-like appearance.

Its peculiar shade distinguishes it from all other green pigments,
none of which approaches it in the paleness and brightness of its
colour. It can be distinguished by the fact that it is soluble in acids
and ammonia, to a blue solution which does not change on boiling. In
caustic soda it also dissolves with a blue colour: on boiling, a red
precipitate of cuprous oxide falls down. No other green pigment answers
to all these tests.

There are a good many imitation emerald greens on the market, some
of which are offered as genuine emerald greens, others as “emerald
tint” green, which is much more honest. The composition of these
greens necessarily varies greatly, some are prepared from coal tar
greens, others by careful admixture of various green, blue, and yellow
pigments. If the tint of these substitutes is right and they are sold
for what they are, there is no reason why they should not be used
in place of the real article, over which they have the advantage of
not being poisonous, which is a great disadvantage of the genuine
emerald green. Although one authority disputes this point, certainly
the poisonous action of emerald green varies very considerably with
different individuals. The genuine emerald green may be distinguished
from the spurious by being perfectly soluble in acids and alkalies,
which the imitations are not; the character of the latter must be
inferred by the application of a few special tests, the nature of
which will be readily deduced from what is said as to the properties
of other green pigments. Emerald green should be assayed for purity
and tint; this is important, as pure emerald green has a tint of its
own, which is difficult to imitate, and sometimes really pure emerald
green offered for sale is of a defective tint, due to some fault in the
process of manufacture. Such samples should be rejected.

GUIGNET’S GREEN.--The greens of this class, which owe their
colour to chromium oxide, are also known as “chrome greens,” a name
which they share with a totally different group into whose composition
chrome yellow enters as a constituent, and which have been already
described under the synonym “Brunswick greens,” on pp. 114-118.

Though one of the simplest of chemical products, a great many ways of
preparing chromium oxides have been proposed. One of the earliest for
industrial application was that of Guignet, who has given his name to
the pigment, and this may fitly commence the long list.

(1) The first method adopted by Guignet consisted in mixing bichromate
of potash with three times its weight of boracic acid and moistening
the mass with just sufficient water to form it into a thick paste.
This paste is put on the hearth of a reverberatory furnace, which is
carefully heated to a point never exceeding a dark red heat; if this
precaution is neglected, the mass, instead of becoming porous, will
fuse entirely, and the anhydrous oxide will be produced, which has a
pale-green colour. The heated paste, while still red hot, is thrown
into cold water and washed with boiling water, in order to remove
borate of potash in solution; and this solution, when boiled down
and treated with hydrochloric acid, can be made to yield up most of
the boracic acid it contains. The filtered and washed residue is the
hydrated oxide of chromium.

(2) A modification of (1), followed by Guignet, was to replace the
bichromate of potash by chromate of soda, prepared by dissolving in
boiling water 61 parts of neutral chromate of potash and 53 parts of
nitrate of soda. For the neutral chromate of potash, also, may be
substituted a mixture of 92 parts of bichromate of potash and 89 parts
of crystallised carbonate of soda, the nitrate of soda remaining as
before. On cooling, in either case, the solution deposits much nitrate
of potash, which is commercially valuable. The chromate of soda present
in the mother liquors is obtained by evaporating to dryness. The
pigment produced by the chromate of soda process is lighter in colour
than that obtained with bichromate of potash. It may be still further
paled by adding a little alumina, baryta, or other white pigment to the
bichromate and boracic acid mixture before calcining.

(3) Equal quantities of potash bichromate and potato starch are
thoroughly mixed and then calcined in a crucible at a high temperature.
The product is washed with boiling water, to remove the potash
carbonate formed, and any remaining undecomposed bichromate. The
precipitated chromium oxide is filtered, dried, and again calcined to
drive off the water. The final result is a handsome pigment which flows
well from the brush.

(4) On heating in a crucible a mixture of 3 parts of neutral chromate
of potash with 2 parts of salammoniac, the two salts are decomposed,
the result being formation of chromium oxide mixed with potassium
chloride, which latter is removed by several washings with hot water.
The brilliancy of the chromium oxide is enhanced by calcination at a
dull red heat.

(5) Fuse together 3 parts of boracic acid and 1 part of potash
bichromate at a dull red heat on the hearth of a reverberatory furnace.
Thus is formed a borate of chromium and potash, with evolution of
oxygen. The mass is repeatedly washed with boiling water, which causes
decomposition, and consequent separation of hydrated oxide of chromium,
and a soluble borate of potash. The chromium oxide is washed, and
ground very fine.

(6) When a solution of potash bichromate is poured into a neutral
solution of mercury proto-nitrate, it forms an orange-coloured
precipitate, which is washed and gently dried, then powdered, and
heated in a stoneware retort provided with an arm dipping into cold
water, by which the mercury is distilled and condensed. The residue in
the retort is a highly comminuted chromium oxide, of a fine dark-green
colour.

(7) On calcining potash bichromate in a crucible at a very high
temperature, it is decomposed, and results in chromium oxide and
potash, the latter of which can be washed out. The chromium oxide thus
obtained is very dense and of a dark-green colour resembling (6).

(8) Equal quantities of flowers of sulphur and bichromate of potash
are thoroughly mixed, and heated to redness in a crucible, producing a
mixture of oxide of chromium with sulphide and sulphate of potash. The
latter are dissolved out by washing repeatedly with hot water, leaving
the chromium oxide as a finely comminuted dense powder of an intense
green colour.

(9) A modification of (8) consists in adding small successive
quantities of flowers of sulphur to a boiling concentrated solution of
potash bichromate. From this results a gelatinous oxide of chromium,
which is washed with boiling water, dried, and calcined in a crucible
at a red heat.

(10) Hydrochloric acid decomposes bichromate of potash, forming a
soluble chloride of potash which can be removed by washing, and a
residue of chromium oxide, which is washed on a filter and dried.

There remain for description two or three processes in which phosphoric
acid plays a part, but the greens made by these methods do not possess
the freshness of the others, and it is difficult to see what advantages
can attend this modification.

(11) According to Arnaudon, 149 parts of bichromate of potash are
thoroughly incorporated with 128 parts of crystallised neutral
phosphate of ammonia, and the mixture is heated in thin layers to a
temperature between 338° and 356° F., which brings about intumescence,
change of colour, and disengagement of water and ammonia; the heating
is continued for half an hour, but must not be allowed to exceed
392° F. When the development of the green colour is complete, the
product is washed with hot water to remove soluble salts, and the
residue constitutes an impalpable powder of chromium oxide, forming a
leaf-green pigment.

(12) Dissolve 10 lb. of bichromate of potash and 18 lb. of phosphate
of soda in boiling water, and add to the boiling mixture 10 lb. of
thio-sulphate of soda solution and a little hydrochloric acid. A
precipitate of phosphate of chromium is gradually thrown down as the
boiling is maintained.

For general utility no class of pigments can exceed the several
forms of Guignet’s green. It is capable of affording a great variety
of tints, all absolutely permanent under reasonable conditions. No
ordinary agent will decompose them, and they will stand almost any
test to which they may be subjected without losing colour. They are
quite insoluble in acids and alkalies, and are not affected even by the
extreme heat of the glass furnace. They possess good covering power,
do not suffer in brightness or purity under artificial light, and are
equally useful as oil or water colours, besides being admirably adapted
for fresco and silicious painting, and employed in making green glass
and in calico printing. They can be mixed with any other pigment.
Adulteration with Brunswick or Prussian greens is often practised, but
may be discovered by a portion being dissolved on boiling with caustic
soda, the solution giving a precipitate of chrome yellow on adding
acetic acid, and (a separate portion, of course) Prussian blue with
hydrochloric acid and perchloride of iron.

LOKAO.--This pigment, which is also known as “Chinese green,”
was first met with as a sediment left after dyeing cotton cloths
with the barks of one or more species of buckthorn, notably _Rhamnus
chlorophorus_ and _R. utilis_, and passing in China under the general
name of Lo-Kao. This sediment is spread on blotting paper and thus
dried, forming thin cakes. Latterly, the juice afforded by the berries
of the same trees is extracted by pressure, absorbed by alum, and dried
in the same form of little cakes. When first introduced into England
it was highly valued as affording a pure green, even in artificial
light. Its price on the London market in 1861 was 7_s._ 6_d._ an ounce.
So long ago as 1853 it was imported into France and used for dyeing
silk. The colouring principle appears to consist of a glucose (lokaose)
and an acid (lokaonic acid). In 1864, Chauvin obtained an identical
colouring matter from _Rhamnus catharticus_, or the common buckthorn,
a shrub which grows wild in most parts of Europe, and found a ready
market for the pigment at 37_s._ a pound. This was simply the article
known as sap green (see p. 132.)

MALACHITE.--This is one of the names applied to mountain green
(q.v.).

MANGANESE GREEN.--Several formulæ have been published for
making a green pigment from manganese, as follows:--

(1) An intimate mixture of 80 parts of nitrate of barium, 14 parts
of oxide of manganese and 6 parts of sulphate of barium, is placed
in a crucible and heated to bright redness until the green colour
is thoroughly developed. The fused green mass is poured out of the
crucible, cooled, and ground wet to a fine condition.

(2) To 3 or 4 parts of caustic baryta moistened with water are added
2 parts of nitrate of barium and 2 parts of oxide of manganese; the
whole mass is most intimately mixed, then put into a crucible in a
furnace, and subjected to a dull red heat so long as may be necessary
for securing complete decomposition. When the green colour is
satisfactorily produced, the mass in a state of fusion is poured out,
cooled, pulverised, digested in boiling water, then washed with cold
water, and finally dried in an atmosphere which is free from carbonic
acid.

(3) The oxide of manganese may be replaced by the nitrate, when the
quantities are 46 parts nitrate of barium, 30 parts of sulphate of
barium, and 24 parts of nitrate of manganese; the fusion, grinding and
washing are repeated as before.

According to some recipes the powdery pigment, consisting essentially
of manganate of barium, is mixed with a little dextrine to make sure of
its stability, but it is not clear whether this is really essential.

MINERAL GREEN.--This is only another name for the green made
from copper carbonate, and described under mountain green, see p. 131.

MITIS GREEN.--This pigment is an arseniate of copper, and
bears a very close relationship to the emerald green made according
to Braconnot’s formula, and described in the fifth paragraph of that
section, see p. 123. Mitis green is prepared by dissolving arseniate of
potash in five times the quantity of hot water and adding a solution of
an equal weight of sulphate of copper, keeping the whole in constant
agitation. A pulverulent precipitate is formed, possessing a grass
green colour. This is washed and dried. The tint can be varied by
altering the proportions of the arseniate and sulphate. The arseniate
of potash is made by boiling arsenious acid in concentrated nitric
acid, filtering, and saturating with carbonate of potash. The arseniate
is allowed to crystallise out of the liquor.

MOUNTAIN GREEN.--This pigment is also known by the names of
malachite and mineral green.

(1) In its native form the mineral malachite or green carbonate of
copper is very widely distributed in Europe, Asia, America, and
Australia, but on a commercial scale it is chiefly produced in the Ural
mountains of Siberia and in the Banat of Hungary. It only needs to be
picked clean from adhering rock and to be ground to a very fine powder
in order to render it ready for use. It is much superior to any of the
artificial substitutes referred to below, but its cost confines its
application to artistic work.

(2) Sometimes a little orpiment or chrome yellow is ground up with the
malachite.

(3) A very simple formula for making the artificial pigment is to add
solution of carbonate of soda or potash to a hot mixed solution of alum
and bluestone (sulphate of copper).

(4) Other recipes for making mountain greens have been published which
bear no relation to the composition of the original article, _e. g._
by mixing a solution containing potash and arsenic with a solution of
bluestone; or, as a much more complicated example, treating a solution
of bluestone first with slaked lime, then with a solution of arsenic
and soda obtained by boiling in water, and finally with tartaric acid.

The advantages attendant on so much trouble in producing what is at
best an unstable pigment are not very apparent.

PARIS GREEN.--This is another name, used especially in
America, for the emerald greens described on p. 121.

PRUSSIAN GREEN.--A name often applied to class _b_ of the
Brunswick greens (see p. 114), or in other words those which are
prepared from Prussian blue.

RINMANN GREEN.--The first cobalt green (see p. 119), put on
the market was made by Rinmann, and hence it is still often called by
his name.

SAP GREEN.--This vegetable pigment or lake is closely allied
to the Chinese green or lokao, described on p. 129.

It consists of the solidified juice extracted from the berries of the
common buckthorn shrub (_Rhamnus catharticus_), which is obtained
either by allowing the berries to undergo slight fermentation for about
a week in wooden tubs, then pressing and straining; or by boiling the
berries, and straining off the juice. In either case the clean juice is
boiled down to a syrupy consistence, and a little alum (about ½ oz. to
the pint of thickened juice) is added, the liquor being then evaporated
to dryness, or very nearly to that point, the drying being left to
complete itself after the pigment has been ran into bladders.

The quality of this green is liable to serious fluctuation, owing to
the neglect or ignorance of certain simple precautions. Thus, for
a true green the berries should be selected before they have quite
reached maturity. The more nearly ripe the berries are, the more yellow
will be the tint of the green afforded by them. The boiling of the
berries, if followed, and the evaporation of the juice, must be done
at a low temperature, and the final stages of the evaporation cannot
safely be done with direct fire heat, but should be effected in a water
bath. The only substance incorporated with the juice should be potash
alum. Sometimes it is replaced by carbonate of magnesia (which destroys
the transparency of the pigment); or by carbonate of potash (which
introduces a stickiness or viscosity).

Sap green possesses too little body and is too translucent for use as
an oil paint; but being non-poisonous, and in fact perfectly harmless,
it finds many useful applications outside of water colour and pastel
painting, viz. in colouring alimentary substances such as drinks and
sweets. Its true colour is a leaf green, glossy and translucent. In
durability it is not remarkable.

SCHEELE’S GREEN.--For more than a century has Scheele’s green
been a familiar pigment, but the reputation it enjoyed in its early
days has long since departed, and it is now to be classed among the
inferior green colouring matters. It consists essentially of a basic
arsenite of copper, and contains from 8 to more than 40 per cent. of
arsenic, according to the mode of preparation, of which there are
several, as follows:--

(1) A mixture of 2 parts of commercial carbonate of potash and 1 part
of powdered arsenious acid (white arsenic), are dissolved in 35 parts
of boiling water; the solution is filtered clear, and then added
gradually and while still warm to a filtered solution of 2 parts of
sulphate of copper until no further precipitate goes down. This latter
is collected, washed with warm water on a filter, and slowly dried
without excess of heat.

(2) The preceding formula is modified by making one solution of the
arsenic and the sulphate of copper, and precipitating by adding the
carbonate of potash solution till the colour is fully developed,
agitation being constantly maintained.

(3) Another variation is to mix the arsenic with soda crystals in
boiling water, and to pour the arsenite of soda solution thus formed
into the bluestone solution, the boiling being kept up for a few
minutes.

Scheele’s green has a pale yellowish cast, and mixes well with either
water or oil, but it lacks brightness, durability, and covering power,
in addition to being highly poisonous, and though once much employed in
staining wall papers, is now generally discarded.

SCHWEINFURTH GREEN.--This is an old-fashioned name for emerald
green, which has been described on pp. 121-125.

TERRE VERTE.--Rendered into English, the name _terre verte_
means “green earth.” It is applied to a number of green-coloured earths
found widely distributed in rocks of various ages, but especially in
those of a basaltic or porphyritic character. In commercial quantity it
occurs notably in Cyprus and near Verona in Italy; the latter locality
is so important that the pigment is often known as “Verona earth.”

Notwithstanding minor points of dissimilarity in samples from different
sources, there is a great family likeness among them, sufficient to
indicate that the essential constituent is a silicate of iron and
magnesia. The other ingredients vary with the locality producing the
mineral. The same may be said of the physical characteristics, some
specimens being soft and earthy, while others are hard and glassy.
All possess the peculiar soapy touch of the magnesian earths, and a
clay-like odour. Analysis of a Verona earth gave:--

                                              Per cent.

  Silica                                       51·21
  Iron protoxide                               20·72
  Magnesia                                      6·16
  Water                                         4·49
  Alumina                                       7·25
  Soda                                          6·21
  Manganese protoxide                           trace.

While a Cyprus earth showed:--

                                               Per cent.

  Silica                                        51·5
  Iron protoxide                                20·5
  Magnesia                                       1·5
  Water                                          8·0
  Potash                                        18·0

The presence of copper would point suspiciously to adulteration, and in
any case should suffice to condemn the sample for use.

Naturally there is considerable variety of tint among the many kinds
of terre verte, but they all belong to the pale greyish class, and are
more or less translucent, consequently their covering power is small.
Their value lies in their durability, and the resistance they offer to
the injurious effects of strong light and impure atmosphere. They can
be employed either as oil or water colours. The only preparation to
which the natural pigments are submitted is fine grinding and washing.

TITANIUM GREEN.--An excellent dark green pigment, though
rather costly, can be prepared from rutile or any titaniferous iron ore
by the following method:--

The ore is dressed clean, and fused with twelve times its weight of
acid sulphate of potash in a crucible. When cool, it is reduced to
fine powder, and digested at 120° F. in dilute hydrochloric acid (half
water) until solution is complete. The hot solution is filtered off
from the residue and carefully evaporated down to a syrupy consistence,
when the nearly pure titanic acid is allowed to cool in the dish and
thrown on a filter. When sufficiently drained, it is boiled in a large
volume of water containing a little ammonia, and the precipitated
titanic acid is filtered and washed.

If an ore is used containing carbonate of lime, it must first be
treated with dilute hydrochloric acid before the sulphate of potash is
applied.

The titanic acid on the filter is next mixed with a concentrated
solution of sal ammoniac, and again filtered. Then it is digested
in dilute hydrochloric acid at 120° to 140° F. till the solution is
complete. On adding ferrocyanide of potassium to the acid liquor,
and bringing quickly to a boil, a precipitate of ferro-cyanide of
titanium is thrown down. This is very carefully and slowly dried, at a
temperature never exceeding 200° F.

VERDIGRIS.--The chemical examination of verdigris shows it to
be a basic hydrated acetate of copper, containing variable proportions
of the bibasic and tribasic acetates.

Commercially it is prepared in districts where acetic or pyroligneous
acid can be had at small cost. Thin pieces of scrap copper are
subjected to the action of fermenting grape skins in mass, or cider
refuse, for a fortnight or three weeks; or to the influence of
pyroligneous acid for four or five days. By this means the copper
surfaces are attacked by the acetic acid being generated or liberated,
and become coated with acetate of copper. At intervals the pieces
are removed, and surfaces are cleaned of the accumulated acetate or
verdigris and this is repeated till the metallic copper has thus been
completely converted. The collected verdigris is washed, and carefully
dried at a very low temperature.

Its composition is subject to many irregularities, and the colour
varies from green to bluish green according to the proportion of
sesquibasic acetate present. It is one of the least permanent pigments,
especially in the presence of water, and is exceedingly poisonous.
At one time it was largely used as a pigment, but is now gradually
going, if indeed it has not already gone, out of use. It can be
distinguished by its solubility in acids and ammonia, the latter giving
a deep azure blue solution. On being heated, it turns black, owing to
its parting with acetic acid and leaving the black oxide of copper
behind. This should be entirely soluble in nitric acid, the solution
giving the characteristic tests for copper. The solution should give
no precipitate with chloride of barium or nitrate of silver, and the
original pigment should be freely soluble in any acid and in ammonia
without effervescence.

VERDITER.--Green verditer is another of the copper greens
which has practically disappeared from the modern painter’s list of
pigments. It is a yellow tinted very fugitive colour, consisting of
a basic carbonate of copper, and is manufactured by treating copper
solutions with carbonate of soda, or of potash.

VERONA EARTH.--One kind of terre verte (see p. 134), is known
by this name because it is produced in the neighbourhood of Verona.

VICTORIA GREEN.--This is a fancy name for the Brunswick greens
compounded from Prussian blue, and already described on p. 114.

VIENNA GREEN.--The aceto-arsenite of copper described under
the heading of emerald green (see pp. 121-125), is sometimes called by
this name.

ZINC GREEN.--The pigments described under cobalt green (see
p. 119), as often pass by the name of zinc greens, and in fact they
contain much more zinc than cobalt.

A handsome but not permanent green may be made by combining zinc
with iron instead of cobalt, in the form of a double cyanide. The
process is as follows:--Finely powdered Prussian blue is stirred into
a concentrated solution of chloride of zinc, and put by to allow
the decomposition to take place. After some time, the precipitated
ferro-zinc cyanide is thoroughly washed, and dried out of reach of the
light.



CHAPTER VI.

REDS.


Though the red pigments are an important class, they are not numerous,
and, with the exception of a few lakes, they are drawn from the mineral
kingdom. The most useful are compounds of the several metals, iron,
lead, and mercury.

ANTIMONY VERMILION.--This useful pigment is prepared by
several methods, as follows:--

(1) One of the earliest successful processes was that introduced by
Mathieu Plessy, which gives a scarlet product. He obtains the pigment,
a modified sulphide of antimony, by decomposition of hyposulphite of
soda in the presence of chloride of antimony. The two solutions of
hyposulphite of soda and chloride of antimony, each at 25° B., being
prepared, the next step is to pour into a stoneware vessel 4 gals. of
the antimony chloride solution, 6 gals. of water, and 10 gals. of soda
hyposulphite solutions. The precipitate caused by the water is rapidly
dissolved in the cold by the hyposulphite. The stoneware vessel is
then placed in a hot water bath, and the temperature of the contents
is thus gradually raised. At about 86° F. the precipitation of the
sulphide commences, showing orange yellow at first, but becoming darker
subsequently. When the temperature has reached 130° F., the vessel is
removed from the water bath, and the deposition of the precipitate
proceeds rapidly. The supernatant liquor is siphoned off, and the
solid residue is washed first with water acidulated by adding to it
one fifteenth of its bulk of hydrochloric acid, and then with clean
water. Finally the residue is collected on a filter, and dried. It is
exceedingly brilliant while wet, but loses a portion of its brightness
when dried.

Provision must be made for disposing of the sulphurous oxide gas driven
off during the process of manufacture.

(2) Kopp found certain disadvantages in working by the above method,
and adopted instead the reaction of antimony chloride upon a dilute
solution of hyposulphite of lime.

Experiencing much difficulty in the decomposition of antimony sulphide
by hydrochloric acid on an industrial scale, he experimented on
roasting the sulphide at a moderate temperature in contact with air
and steam, whereby most of the antimony sulphide is converted into
oxide, while the sulphurous acid driven off is utilised for making
the hyposulphite of lime. This proved a most successful plan, and
the resulting antimony oxide is readily dissolved by commercial
hydrochloric acid.

During the oxidation of the antimony sulphide, a certain proportion
of antimonious acid may be produced. This is but slightly soluble in
hydrochloric acid. It may be collected, however, by saving the residues
from the treatment by hydrochloric acid, and washing them with chloride
or hyposulphite of lime, which will dissolve the adherent antimony
chloride; they are then dried, and melted with a little antimony
sulphide and quicklime, so as to transform the whole into antimony
green, the quicklime having the effect of decomposing any small residue
of antimony chloride.

The preparation of the hyposulphite of lime is cheaply effected by the
action of sulphurous acid on sulphides of lime, the sulphurous acid
being derived either from the roasting of the antimony sulphide, or
from pyrites or brimstone in the usual way.

Calcium polysulphide is prepared by boiling finely powdered sulphur and
newly slaked lime in water. Certain advantages arise from the addition
to this solution of a little powdered calcium oxysulphide, or some
quicklime.

In the reaction of sulphurous acid on calcium sulphide and oxysulphide,
sulphur is set free and forms a sulphite of lime, which, in the
presence of sulphur and undecomposed sulphide, is soon transformed into
hyposulphite, the reaction being facilitated by the rise of temperature
which takes place in the apparatus.

As soon as the liquor has become slightly acid, it is drawn off into
a large settling tank. If, after agitating for some time, the liquor
has not become neutralised by the undecomposed calcium oxysulphide
contained in it, this is brought about by addition of a little calcium
sulphide, and is recognisable by the appearance of a black precipitate
of sulphide of iron. After due settlement, the clear liquor is
decanted, and forms a solution of nearly pure hyposulphite of lime.

The production of antimony vermilion is effected from the foregoing
solutions of antimony chloride and hyposulphite of lime, in apparatus
consisting simply of a series of wooden tanks raised conveniently above
the floor, holding about 500 gals. each, and provided with steam coils
for heating their contents.

Sufficient hyposulphite of lime solution is run into the tanks to fill
about seven-eighths of their depth; and then into the first tank is
poured the chloride of antimony solution, in quantities of a few pints
at a time. A white precipitate is formed, and rapidly dissolves at
first; when it is slow in going into solution, even though stirred,
the addition of antimony chloride should be stopped, as an excess of
hyposulphite of lime is essential. The liquor in the tank must be
perfectly clear and limpid, and should any white precipitate remain it
must be dissolved by making small additions of hyposulphite.

At this stage steam is admitted into the coils, and thereby the
temperature of the solutions is gradually raised to 120° or 140° F., or
even to 160° F., while stirring is unceasingly carried on. The reaction
is soon manifested by the successive colours of the liquor, passing
from straw-yellow to lemon-yellow, orange-yellow, orange, orange-red,
and lastly a very deep and brilliant red. The steam is shut off from
the coil before the desired tint is arrived at, as the acquired heat
and the agitation complete the development of the colour. If the
heating is carried too far, the red gradually passes to a brown and
later to nearly black. With experience, almost any desired shade of red
can be produced.

When the precipitate has attained the required colour, it is allowed to
settle, and the tank is covered. The clear and limpid liquor, having a
strong sulphurous odour, is let out through tap holes at various levels
in the sides of the tanks, and run by wooden gutters or leaden pipes
into a large reservoir holding a quantity of sulphide and oxysulphide
of lime. Here the sulphurous liquor regenerates a certain amount of
hyposulphite of lime.

The antimony chloride solution always contains a large proportion of
chloride of iron, which provides an easy means of guiding the progress
of this latter operation. All the iron remains soluble in the mother
liquors of the antimony sulphide, and as soon as they are brought into
contact with the calcium sulphide, an insoluble black precipitate of
iron sulphide is formed. So long as this remains, the mother liquors
charged with sulphurous acid have not been added in excess; but when
it disappears by conversion into soluble hyposulphite of iron, that is
a sign that the sulphurous solution is in excess. The contents of the
reservoir are then well stirred, and calcium sulphide is introduced if
necessary, until the precipitate of iron sulphide returns and remains.
It is also needful to ensure that a certain proportion of hyposulphite
of iron shall remain in solution. The clear liquor decanted off when
all the precipitate has gone down is a neutral solution of hyposulphite
of lime, containing some calcium chloride and hyposulphite of iron.

Another requisite precaution in this regeneration of hyposulphite of
lime is that no excess of calcium sulphide be left, or it will give an
orange-yellow tint to the vermilion; and if the hyposulphite of lime
solution is alkaline and yellow, sulphurous acid liquor must be run in
till all alkalinity is destroyed.

This regenerated solution of hyposulphite of lime is used like the
first. The mother liquors charged with sulphurous acid are again
neutralised in the large reservoir by new proportions of calcium
sulphide and oxysulphide, until so much calcium chloride is present
that they are useless for the purpose, say after 25 to 30 operations.

The antimony vermilion precipitated on the bottom of the first tank is
received into a conical cloth filter, and the liquor drained off is
passed to the reservoir. The first tank is then washed out with warm
water, which also passes through the filter. The precipitate of red
sulphide cannot be too carefully or completely washed, and finally is
filtered and slowly dried below 140° F.

(3) Wagner’s method of making a scarlet pigment is to dissolve 6 lb. of
tartaric acid and 8 lb. of tartar emetic in 4½ gallons of water at 140°
F., adding a solution of hyposulphite of soda at 40° Tw., and heating
the whole mixture to 180° F., whereby the red pigment is gradually
precipitated. It is collected on a filter, well washed and dried.

(4) The process adopted by Murdoch, in which a solution of antimony
chloride (prepared by dissolving black sulphide of antimony in
hydrochloric acid) is acted on by a current of sulphuretted hydrogen
gas, has disadvantages in the apparatus necessary, in the limited range
of tints which can be produced, and in the almost certain presence of
free sulphur in the finished pigment.

Antimony vermilion forms an exceedingly useful pigment, which can
be prepared in every shade of red, from orange to red-brown. It is
produced in the condition of a very fine powder, requiring no grinding,
and mixes readily with water or oil, especially the latter, and
moreover does not interfere with the drying of the oil. It possesses
great covering powers, and can be made at a low price. It undergoes
no change in strong light and impure air, and is insoluble in water,
alcohol, essential oils, weak acids, ammonia, and alkaline carbonates;
but it is destroyed by high temperatures, strong acids, and caustic
alkalies. It cannot be mixed with other pigments which are intolerant
of sulphur, nor with alkaline vehicles. When pure, it should consist of
nothing but antimony sulphide and a little water; the presence of iron
or lead indicates adulteration.

BARYTA RED.--An orange red may be prepared, according to
Wagner, in the form of a sulpho-antimonite of barium, by calcining in a
clay or graphite crucible at red heat for several hours a mixture of 2
parts of finely powdered barytes, 1 part of native antimony sulphide,
and 1 part of powdered charcoal. The calcined mass is not removed until
the crucible is quite cold, as it is liable to undergo combustion. When
cold, it is boiled in water and filtered. The residue, containing some
undecomposed sulphate and sulphide of barium, is utilised in the next
batch. The pale-yellow filtrate is treated with dilute sulphuric acid,
by which sulphuretted hydrogen is driven off, and an orange precipitate
is thrown down. This is collected, washed on a filter, and dried,
constituting the pigment.

CASSIUS PURPLE.--This costly pigment is a stannate of
protoxide of gold, much used in painting on porcelain and for
miniatures. It is the precipitate which is thrown down when solutions
of gold and tin chlorides are mixed under proper conditions, according
to one of the following methods:--

(1) Buisson prepares three solutions: [_a_] a neutral solution of
protochloride of tin by dissolving 1 part of tin in hydrochloric acid;
[_b_] a solution (bichloride) of 2 parts of granulated tin in an aqua
regia containing 3 parts of nitric to 1 of hydrochloric acid, removing
the excess of acid; [_c_] a neutral solution of 7 parts of gold in an
aqua regia composed of 1 part of nitric and 6 parts of hydrochloric
acid. The gold chloride solution is largely diluted with water, and to
it is added the solution _b_ of bichloride, and finally the solution
_a_ of protochloride is introduced, a drop at a time, until the
desired colour is produced in the precipitate. This last is rapidly
washed by decantation, and finally dried away from the light.

(2) Figuier prepares a gold bichloride solution by dissolving 20
grammes of gold in 100 grammes of an aqua regia containing 4 parts
of hydrochloric to 1 of nitric acid. The solution is evaporated to
dryness in a water bath, and the residue is dissolved in 750 grammes of
water. Into this solution, when duly filtered, pure granulated tin is
introduced, and the whole is left for some days, at the end of which
time all the gold will be in the state of stannate of protoxide; it is
collected on a filter, carefully washed, and gently dried. The residues
contain some gold, and should be preserved for subsequent operations.

CHINESE RED.--One of the many names of the chromate of lead
pigment, described under Derby red, see p. 145.

CHROME ORANGE.--A popular name for the group of yellow-red
pigments consisting essentially of lead chromate, and described under
Lead orange, on p. 147.

CHROME RED.--Another of the synonyms for Derby red, see p. 145.

COBALT PINK.--This costly and permanent artists’ colour is
a combination of oxide of cobalt with magnesia. It is prepared by
treating carbonate of magnesia with a concentrated solution of nitrate
of cobalt; the resulting paste is dried in a stove, calcined in a
porcelain crucible, and finally ground to a fine powder.

COBALT RED.--A very deep-coloured and permanent red pigment
used in oil painting is the arseniate of cobalt, which is found
native in admixture with other substances in cobalt mines, or may be
artificially produced.

The native mineral is treated with boiling nitric acid; the solution
is filtered clear, and small portions of potash are added till all
the iron has been thrown down as arseniate. After this is completed,
the mass is allowed to settle, and the clear liquor is poured off.
On adding further small portions of potash, the cobalt is also
precipitated as arseniate.

To prepare artificial cobalt arseniate, grey cobalt ore (sulph-arsenide
of cobalt), reduced to a powder, is mixed with a little sand and twice
its weight of potash, and fused in a crucible. The slag of mixed
sulphides which is formed is removed, and the remaining white arseniate
of cobalt is pulverised and subjected to another fusion with potash.
The slag is again removed, and the button of pure arsenide of cobalt
remaining is finely powdered and again roasted to effect conversion
into arseniate of cobalt. Lastly, it is ground very fine.

COLCOTHAR.--A fancy name for a kind of iron oxide pigment,
described under oxide reds (see p. 150).

DERBY RED.--As a basic chromate of lead, often known as chrome
red, Derby red is closely allied to chrome yellow, the preparation of
which is described in a subsequent chapter.

It has been asserted that all the chrome reds, from the darkest
cinnabar red to a lustreless minium red, are distinguishable simply
by the size of the crystals composing the powder, as may be easily
determined under the microscope, and that if various chrome reds of
the same hue, but with different intensities of colour, are reduced by
grinding to the same degree of comminution, the several powders will
possess exactly the same degrees of intensity of coloration, though
they lose in brightness. Therefore the conditions which give brilliancy
and intensity of colour are those which favour crystallisation.

On this supposition it is recommended by Riffault to adopt a plan which
dispenses with agitation, and he supports the following method:--

(1) Chrome yellow is precipitated in the usual manner, as described in
a later chapter, without sulphuric acid, and is carefully washed. After
draining, the mass is well stirred, and six or eight equal samples are
drawn from it and put into glass vessels of equal size and thickness
of structure. To each sample is added a different volume of caustic
soda or potash lye, marking about 20° B. For instance, to 5 volumes of
paste are added 2, 2½, 3, 3½, 4, 5, &c., volumes of lye. The different
mixtures are rapidly and thoroughly agitated, but the chemical reaction
is allowed to take place without any disturbance. After examination of
the quality of the products, the relative proportions of pulp and lye
are noted down for the best hues obtained. Too much lye will fail to
deepen the red colour; in fact, Derby red is entirely soluble in an
excess of lye, and forms needle-like crystals holding potash when the
caustic solution has absorbed carbonic acid from the air.

On the industrial scale the operation is conducted in a large tub,
which receives the mixture of pulp and caustic lye in precisely the
proportions found by experiment to give the best results. The changes
in colour soon manifest themselves, and the whole reaction is completed
in about 12 hours. At the end of that time, the lye is drawn off, and
carries with it much of the chromic acid. The precipitated pigment
is carefully washed with pure water once in the tub, and the mass is
gently stirred. The washing is continued in the filters by throwing
water upon the pulp, and in this manner there is less friction between
the crystals, which retain their deep colour. Of course a highly
crystalline dark red cannot possess great covering power.

(2) Prinvalt mixes together 100 lb. of lead carbonate and 30½ lb. of
potash bichromate neutralised with caustic potash, in 50 gallons of
water, leaving them in contact for a couple of days under repeated
agitation. About half an hour’s boiling then suffices to develop the
red colour. After settling, the supernatant liquor is drawn off, and
the precipitated pigment is washed twice with pure water and finally
with acidulated water (1 lb. sulphuric acid in 10 gallons of water),
and dried.

There are several other recipes published which differ in detail from
(2), but they do not demand a lengthy description.

(3) 100 lb. of lead carbonate (white lead) made into a paste with
water, then added to and boiled with a solution of 50 lb. of potash
bichromate and 15 lb. of caustic soda of 77 per cent. Remainder of
process as before.

(4) 4 cwt. of lead monoxide (litharge) and 60 lb. of salt dissolved
in 50 gallons of water, and left with agitation for 4 or 5 days; then
boiled for 2 hours with solution of 150 lb. of potash bichromate.

(5) 100 lb. of lead carbonate (white lead) made into a paste with
water, then added to a solution of 30 lb. of potash bichromate and 12½
lb. of caustic soda at 77 per cent., and boiled.

Derby red possesses great covering power and considerable brilliancy;
but if not very carefully washed it is liable to retain a little
alkali, which renders it unstable. Otherwise, it well resists damp,
strong light, and impure air so long as sulphuretted hydrogen is
absent. Taken altogether it is not one of the best red pigments, and
its consumption is declining.

INDIAN RED.--This is one of the names for the red pigments due
to oxide of iron, and is described under oxide red, p. 150.

LEAD ORANGE.--Equally well known as chrome orange, this
pigment may be regarded as a Derby red in which the reactions have
been curtailed. That is to say, the yellow normal lead chromate
being in excess, the red chromate formed by the action of the alkali
combines with that excess of the yellow salt and forms a yellow-red,
i.e. orange. Obviously, therefore, a great variety of tints can be
produced by altering the proportions of the alkali, and this is further
regulated by the duration of the boiling, while the tint can also be
weakened by admixture of barytes or gypsum. The better kinds of lead
orange are prepared with the aid of caustic potash or soda as the
alkali, while the cheaper sorts depend on lime. The operations are
practically identical with those adopted in the case of Derby red
(see p. 145), the chief differences lying in the proportions of the
ingredients. Thus:--

    (1) _Pale._--Add a thin cream made from 10 lb. of quicklime to a
    chrome yellow made from 100 lb. of lead acetate, 30 lb. of soda or
    potash bichromate, and 21 lb. of soda sulphate. Boil.

    (2) _Pale._--Add a thin cream of 10 lb. of quicklime to a chrome
    yellow made from 200 lb. of baryta sulphate, 100 lb. of lead
    acetate, and 35 lb. of potash bichromate. Boil.

    (3) _Deep._--Precipitate a chrome yellow by adding 35 lb. of soda
    or potash bichromate to 100 lb. of lead acetate; settle. Draw off
    supernatant liquor and admit solution of 9 lb. of caustic soda at
    77 per cent.

    (4) _Deep._--Add a cream of 10 lb. of quicklime to a chrome yellow
    made from 100 lb. of lead acetate, 75 lb. of baryta sulphate, and
    35 lb. of potash bichromate. Boil.

In characters the lead oranges resemble Derby red (see p. 145.)

MINIUM.--The important red pigment known as minium or red lead
is composed of two oxides of lead in combination, viz. about 65 per
cent. of protoxide and 35 per cent. of binoxide. In its preparation,
metallic lead is first converted by roasting into protoxide (termed
“massicot,” “dross,” or “casing”) and this protoxide is further
subjected to heat in a reverberatory furnace whereby a portion of it
is changed into binoxide. It is also possible to produce red lead by
the decomposition of the carbonate of lead (white lead) at a high
temperature, but this does not seem to be an industrial process. The
following methods are recognised:--

(1) The practice in France, as carried on near Tours, at the white lead
works using the Thénard process, is to calcine the best metallic lead
in reverberatory furnaces built in the rock. These furnaces are five
in number, with double fireplaces, four being constantly in operation,
dealing with about 4000 lb. at a charge, and using bituminous coal as
fuel. Each furnace is nearly circular in shape and about 11 feet in
diameter, with a fire-place on each side of the hearth. The latter is
constructed of fire-brick containing as little silica as possible,
and is made hollow so as to retain the metallic lead when the heat has
rendered it fluid.

The products of combustion from the side fire-places, having heated
the hearth and its contents, pass through an aperture in front of the
charging door of the hearth, and thence go to furnish heat to an upper
hearth where the conversion of the oxide into red lead takes place.

A period of about 12 hours is occupied in the oxidation of a charge,
which is repeatedly “rabbled.” Even then a considerable amount of the
metallic lead remains unoxidised and is returned to the calciner with
the next charge. Half the oxide is utilised for making white lead, as
described in a later chapter, and the other half is converted into red
lead by the method detailed hereunder.

The crude oxide is pulverised in a small mill and separated from the
unconverted metal. The mill takes the form of a flat circular cast-iron
plate on which rotates a cast-iron muller. Water and an agitating
arrangement are also provided.

As the muller revolves the material undergoes comminution, and the
small particles of oxide as formed are disturbed by the agitator and
kept in suspension in the water, by the overflow of which they are
continuously carried away into settling pits. The residual metallic
lead is not pulverised, and of course never becomes suspended in the
water, consequently it accumulates at the bottom of the mill, whence it
is occasionally withdrawn for re-calcination.

Sufficient oxide having collected in the settling pits, it is
transferred to a shallow pan heated by the waste heat from the furnaces
and is there rendered almost dry. In this state it is put into small
square dishes made of sheet iron, and adapted to hold about 30 lb. each.

A charge consists of a hundred of these dishes, which are placed in
the heated furnaces at the end of each day. The roasting is repeated
several times, and the product is accordingly known as “two fires,”
“three fires,” &c. The material at this stage is lumpy and coarse,
and has to undergo dry pulverisation, the fine particles as they are
produced being drawn off by means of a pneumatic fan, and collected.

(2) What may in contradistinction be called the English method of
making minium does not differ materially from the preceding. The
“drossing” furnace, where the metallic lead is first oxidised, receives
a smaller charge as a rule, and perhaps greater care is given to the
rabbling, and to the regulation of the temperature so that it is only
just above the melting point of the metallic lead, and not sufficient
to fuse the massicot.

Minium or red lead is one of the most important and useful red
pigments, as it mixes well with oil, has good covering power, dries
quickly, and is permanent except in presence of sulphur or sulphides.

ORANGE MINERAL.--The pigment known as orange mineral or
orange lead is simply minium which has been imperfectly calcined.
Consequently it is almost identical with red lead in composition,
qualities, and method of manufacture, the only exception being that,
as the calcination is not carried quite so far, therefore the colour
is not so fully developed, and is an orange rather than a red. As with
minium, practically the only adulterant is iron oxide red, which may be
detected by boiling the pigment to a colourless solution with nitric
acid, when addition of prussiate of potash will give a blue precipitate.

OXIDE REDS.--Under various names--such as Persian red, light
red, Indian red, scarlet red, rouge, colcothar, red oxide, purple
oxide, &c.--many pigments, of which the base is the ferric oxide
Fe_{2}O_{3}, are now made. These vary in shade from a deep scarlet red
to a dark violet. They are obtained both from natural and artificial
sources. Oxide of iron occurs naturally as the mineral hematite, and
some varieties of this are bright enough and soft enough to be used as
pigment when ground up. These are usually nearly pure oxide of iron.
Then the ochres, when calcined, yield red pigments known as light red,
Indian red, &c., and a good many reds are obtained from this source.
The composition of these is variable, being dependent upon that of the
ochres from which they are made, and these, as has already been pointed
out, vary very much. Then, in preparing fuming sulphuric acid from
copperas, oxide of iron which is specially sold as rouge, is obtained.
Colcothar is produced as a residue; this is nearly pure oxide of iron,
and usually has a red colour. In the manufacture of sulphuric acid from
pyrites, a dark violet oxide of iron is left as a residue, and much of
this is used as a pigment under the name of purple oxide. Then a large
quantity of oxide of iron reds are made artificially from waste liquors
obtained in copper refining, galvanising iron, &c. The composition of
the oxide of iron reds, therefore, is very variable.

The whole group of oxide reds is of foremost importance, by reason of
their good colour, covering power, and durability, besides which, being
mostly bye-products of much more important manufactures, their cost is
reasonable.

The methods of preparation of oxide reds vary slightly in detail
according to the material from which they are made, but the general
features of the processes are almost identical and eminently simple.
The principal sources are impure native oxides of iron, such as the
ochres, various waste liquors containing iron salts in solution, and
copperas (protosulphate of iron).

(1) Native oxides. The iron present in the ochres and similar native
earths exists in the form of hydrated oxide, and has a brown red
colour. For many purposes this hue is satisfactory, and the preparation
of such a pigment consists simply in grinding the mineral in a wet
mill, subjecting it to levigation till all grit is removed, and drying.

In order to obtain a brighter red from the native oxides they must be
calcined to effect dehydration. This can be accomplished in the most
rudimentary forms of furnace, and many kinds are in use. The colour
produced depends on the degree and duration of the heat to which the
material is exposed, the shade becoming deeper as the roasting is
prolonged or the temperature increased. As no two samples of ochre are
just alike it is impossible to fix a precise time for the length of the
operation, and therefore it is necessary to repeatedly draw samples in
order to judge of the progress of the dehydration and development of
the colour desired. When the requisite shade is attained, the charge is
drawn and allowed to cool.

(2) Waste Products. The pyrites cinders from sulphuric acid works
afford an abundance of oxide of iron. When the pyrites has contained
no copper, the cinders merely require grinding and levigating, the
iron being present as oxide. But when the pyriteshas been treated
for the recovery of the copper, by a second roasting with salt, the
liquors contain the iron as chloride and sulphate, and lime has to be
added to precipitate the oxide. This last is dried and calcined in the
same manner as the native oxides, and grinding and levigation can be
dispensed with.

The liquors from galvanising works contain acid sulphate of iron (green
copperas) in solution. To correct the acidity, more iron is added in
the form of scrap. Then lime or other alkaline substance is introduced
to throw down the iron as oxide, and this last is filtered out, dried,
and calcined in the usual way.

(3) Copperas. Where beds of common iron pyrites occur, the iron
sulphide is converted into sulphate by exposure to the oxidising
influence of the air. The result is an acid sulphate of iron, which
is leached out and neutralised by addition of more iron in the form
of scrap. The neutral sulphate is crystallised out of the liquor, and
calcined in a muffle furnace, the shade of the ultimate product being
governed by the degree or duration of the roasting. The sulphurous acid
liberated in the roasting is sometimes utilised for making sulphuric
acid, but is more often wasted, because, to be commercially successful,
the sulphuric acid manufacture must be conducted on a large scale,
demanding 100_l._ of capital for every 1_l._ necessary for the copper
and red oxide fabrication.

PERSIAN RED.--A name which is used somewhat indiscriminately
both for Derby red (p. 145) and for oxide red (p. 150).

REALGAR.--The native mineral realgar is a yellow-red
bisulphide of arsenic, often called also ruby of arsenic, or arsenic
orange. It occurs native in very limited quantities in some of the
older rocks, and then only requires to be ground and levigated. But for
painters’ purposes it is prepared artificially by heating a mixture of
sulphur and arsenic in such a way that they are melted in company and
react on each other to form the arsenic sulphide. The heating takes
place in crucibles, and the proportions are two parts by weight of
arsenious acid (white arsenic) to one of flowers of sulphur. When the
reaction has ceased, the contents of the crucible are allowed to cool,
and then reduced to very fine powder.

The pigment is exceedingly poisonous and not remarkably durable,
besides which, it cannot be mixed with any other pigment which is
affected by sulphur.

RED LEAD.--A common name for minium, see p. 148.

ROUGE.--One of the names for a particular shade of the oxide
reds, see p. 150.

VENETIAN RED.--A fancy name for a special shade of oxide red,
see p. 150.

VERMILION.--This old pigment is gradually going out of use;
the newer reds, which are more brilliant in colour and cheaper, are
gradually displacing it, although it is doubtful whether it will ever
go completely out of use. It is the mercuric sulphide HgS. When pure,
it is not attacked by acids or alkalies; only aqua regia, a mixture
of hydrochloric and nitric acids, is capable of dissolving it, when
it forms a clear solution. Heated in the flame of a Bunsen burner,
it is completely volatile, a property possessed by no other pigment
in common use, therefore any adulteration can be readily detected by
simply heating a little vermilion in a crucible; if a known weight is
taken and the residue is weighed, the amount of adulteration can be
ascertained. Vermilion is chiefly adulterated with oxide of iron and
orange lead. From the character of the residue left on heating in a
crucible, the kind of adulteration can be readily ascertained.

(1) The following notes are taken from Christy’s translation of a
brochure on the Imperial Quicksilver Works at Idria, Krain:--

In the oldest times of the existence of the present works, vermilion
was manufactured. In the beginning it was merely pure pulverised
cinnabar ore, then later it was a product made by sublimation from
this substance; and there were formerly other works for vermilion
manufacture than those for quicksilver production. When the Venetians
and Dutch began to produce better wares, the production here sank
steadily.

The researches of Christofoletti, 1681, and of Baron Richtenfels, 1726,
for the improvement of Idrian vermilion, met with as little success as
those of some Venetian women--1740-1741--who had lost their husbands in
the Venetian works and had offered themselves to manufacture vermilion
according to the Venetian method.

After Hacquet had strongly urged the manufacture of vermilion,
Oberhüttenmeister Ignaz v. Passetzky succeeded, with the Dutchman
Gussig assisting him, in making beautiful cake cinnabar in 1782, and in
1785 vermilion also, in the newly-built works on the right bank of the
Idriza.

In 1796 Oberhüttenverwalter (manager of the works) Leopold v. Passetzky
introduced the sublimate and precipitate manufacture, but it was
abandoned as unprofitable in 1824.

The many foreign attempts to manufacture vermilion in the wet way
caused similar ones here, as those of Fabriks-Controlor Rabitsch in
1838, and later of Hüttenverwalter M. Glowacki, which brought large
amounts of the vermilion so manufactured into the market. Still this
manufacture came to no full development, and became forgotten, until,
finally, in the years 1877 and 1878, experiments led to its being
discontinued on account of the costliness and uncertainty of the
method. A new set of experiments in 1878 and 1879, by Assayer E. Teuber
and Director of Works (Hüttenverwalter) H. Langer, under the direction
of the Imperial Agricultural Ministry, led to favourable results. A
new manufactory, set in operation in 1880, furnishes three sorts of
vermilion manufactured in the wet way.

The arrangements of the works for the manufacture of vermilion in
the dry way consist of:--One sulphur stamp battery. One amalgamating
plant with eighteen small barrels; both pieces of apparatus being
driven by a two horse-power water-wheel. Four sublimation furnaces,
each with six retorts of cast iron. Four vermilion mills, each driven
by a water-wheel of 2·5 horse-power. Kettles and vats for heating,
digesting, and refining the ground cinnabar. One drying hearth. The
preparation of vermilion as an article of commerce, falls into several
separate operations, viz.:

1. Amalgamation; i. e. preparation of the raw mohr.

2. Sublimation; i. e. preparation of the cake cinnabar.

3. Grinding of the cake cinnabar, refining and drying of the vermilion.

For the preparation of the raw mohr, for each charge of eighteen kegs
there are taken 80·64 kg. (117½ lb.) powdered and sifted sulphur, and
423·36 kg. (731½ lb.) of quicksilver.

The amalgamating kegs each contain 28 kg. (61½ lb.) of the charge, and
are given intermittent rotating motion by a rack and pinion driven by
a water-wheel. After an average of two and three-quarter hours, the
amalgamation is complete, and the raw mohr is taken from the casks.

For the sublimation, four furnaces are used, each with six pear-shaped
cast-iron retorts of considerable thickness. Each is charged with
58 kg. (127½ lb.) of mohr, the mouth covered with a loosely placed
sheet-iron helmet, the furnace being slowly fired; the combination
of the sulphur and the quicksilver then results in about fifteen
minutes, with a detonation. As soon as this operation (das Abdampfen)
is over, a clay helmet is placed over the retort, and the firing is
increased, so that after two hours and twenty minutes the excess
of sulphur evaporates from the tube. The condenser is now added
(Stückperiode--Cake-period) and luted, then the firing is still more
urged, whereupon the cinnabar volatilises and deposits itself upon the
glazed earthenware condensation apparatus (tube, helmet, &c.). After
four hours, the sublimation is complete, and there is furnished by
the helmet 69 per cent., by the tubes 26 per cent., by the condenser
(Vorlage) 2 per cent., cinnabar.

The grinding of the cake cinnabar takes place in four mills driven by
an undershot water-wheel. These mills have a fixed under and upper
movable stone, and the grinding is done with water. The vermilion which
leaves the spout and runs into glazed clay vessels has a temperature of
about 100° F., that of the air being 59° F. The millstones make forty
revolutions per minute, and after each passage of the charge are placed
nearer together.

(2) A German chemist named Fleck has discovered that when a warm
solution of hyposulphite of soda is added to a double salt of mercury,
such as chloride of mercury and sodium, the solution becomes acid,
and black sulphide of mercury is deposited. But if the hyposulphite
solution is added in excess, and the temperature is not allowed to
rise beyond 140° F., the solution remains neutral, and red sulphide of
mercury, or vermilion, is deposited. The least quantity of acid causes
the production of the black sulphide. The presence of a salt of zinc
facilitates the production of the vermilion. The best method is as
follows:--To four equivalents of hyposulphite of soda mixed with four
equivalents of sulphate of zinc in diluted solution, is added, drop by
drop, a solution containing one equivalent of corrosive sublimate. The
whole is gently heated for 60 hours, at a temperature of 112° to 130° F.

(3) The following account of vermilion manufacture in China appeared
over the initials T. I. B., in the _Chemical News_.

The Chinaman has no knowledge whatever of chemistry, and of the
principles of natural philosophy and statics generally his notions
are of the most rudimentary and primitive description. How, then, in
the face of these obvious disadvantages have the Chinese contrived to
place themselves in the front rank amongst nations in the matter of
certain chemical manufactures, one of the most important of which is
the subject of this article--Vermilion?

We have seen with what ingenuity and pertinacity in carrying out his
ends the Chinaman has succeeded in making perhaps the most delicate and
perfect iron castings in the world. He has succeeded in that instance,
not by any deep researches into the hidden mysteries of Nature, by
no process of thought involving an enquiry into the “reason why”; to
this the Chinaman is averse, the whole tendency of his education,
such as it is, tends to make him satisfied with observing effects; it
is sufficient to him to know that things are so, without going into
troublesome or elaborate investigations into those changeless laws of
Nature into which his philosophy teaches him that, as he cannot alter
or control, research is fruitless: but that he has in his own small,
ingenious, patient way observed effects to very good purpose, the
unrivalled excellence of some of his manufactures testifies.

We will now enter a vermilion manufactory and watch the process from
the first stage of mixing its two ingredients--mercury and sulphur--to
the final process of weighing and packing this costly and beautiful
pigment for the market.

The first objects to attract the visitor’s attention on entering the
yard attached to the works will probably be large piles or stacks of
charcoal, crates or baskets of broken crockery ware, and numerous rusty
old iron pans of somewhat similar shape to rice pans, but considerably
thicker and heavier. There will also probably be a few broken and
disused cast-iron mortars. All these articles are the cast off or worn
out implements of the manufacture, and will be described in their
proper order.

On entering the factory proper, scores of little stone mills, each
being turned by one man, and other long rows of workmen weighing out
and wrapping up the vermilion, will be seen. The furnaces are then
arrived at: there may be a score or more in number, and may be ten or
twelve in each furnace room, five or six on each side. After passing
these, the stores of quicksilver, sulphur, alum, glue, new spare iron
pans, serviceable crockery ware, and sieves and other utensils used in
the factory are arrived at, and this completes the view of the works.

The iron pans in which the vermilion is sublimed are those referred to
above; they are circular and hemispherical in shape; all are of the
same size and weight; they are cast upside down, and in the casting,
a runner or lump of iron, two and three-eighths inches in diameter
by from six-eighths to one inch in depth, is purposely left on every
pan in order to enable the workman the more readily to handle the pan
when stirring up its contents. The size of the pans proved by actual
measurement to be 29¼ inches in diameter, by 8⅞ inches deep, and the
weight 40 catties, or say about 53 lb.

These pans are set in rows of 5 or 6 on each side of a small
rectangular room, in size some 12 feet by 15 feet; the door of this
room is of wood and contains an aperture a few inches square in order
to enable the workman to watch the progress of his operation, from
time to time, without the necessity of lowering the temperature of the
apartment by opening the door. The pans are set in brickwork, each
pan having beneath it a grate to hold the charcoal used as fuel. There
is no communication between the grates or furnaces under each pan, and
no chimney, the flames and products of combustion finding exit from
the front of the grate, which is left wholly open at all stages of the
operation.

The process of manufacture is as follows:--Taking an iron pan which
is of 4 inches smaller diameter than those described, and also in
all other respects proportionally less, except the runner, which is
of the same size, a skilled workman proceeds to weigh out 17⅓ lb. of
sulphur. This he places in the pan, and adds about half the contents
of a bottle of quicksilver. The pan with its contents is then put upon
a small earthen brazier or portable furnace, the fuel used in which is
charcoal. When the sulphur is sufficiently melted, the workman, taking
an iron spatula or stirrer, rapidly stirs up the quicksilver with the
sulphur, and gradually adds the remaining contents of the bottle of
quicksilver, stirring the two ingredients together meanwhile until the
mercury has wholly disappeared, or “been killed,” as the Chinese put it.

When this takes place, the pan is removed from the fire, a small
quantity of water is added, and rapidly stirred up with the contents
of the pan, which have now assumed a dark blood-red appearance and
semi-crystalline structure. This mass is then turned out of the pan
into an iron mortar, and then broken up into a coarse powder. This
forms a charge for one of the large pans previously described, and when
sufficient material has been prepared to charge all the pans in one
furnace chamber the sublimation is proceeded with as follows:--

All the pans having received their quantum of crude vermilion, this is
covered with a number of crockery-or porcelain-ware plates, of tough,
strong manufacture, each about 8 inches in diameter; some of these
plates, however, are broken up, and are in a more or less fragmentary
condition. When these plates have been piled up into a dome-shaped
heap of the same shape as the bottom of the upper pan, to which they
should extend, the whole is covered with one of the smaller pans
previously described.

Now it will be remembered that the smaller pan was of 4 inches less
diameter than the larger one; there will consequently be a circular
space two inches all round between the circumferences of the pans.
Consequently the rim of the upper or covering pan will be about 2
inches lower than the rim of the lower pan; there will also be some 4
inches space horizontally between the rim of the large lower pan and
that portion of the smaller pan which is at the same height as the rim
of the larger one. This space is carefully filled with a clay luting
into which some holes, generally about four in number, are pierced,
extending down to the rim of the smaller pan or cover; this is done in
order to allow the heated air and other matters to escape.

All the pans in one furnace chamber being thus charged and covered, the
fires are lighted. The flames from the charcoal should occasionally
play several feet above the mouths of the furnaces. The door of the
chamber is kept closed, except when it is open for a moment in order
to enable the workmen to replenish the fires, which must be kept
up at a fierce heat for eighteen hours. During this process a blue
lambent flame is seen to play above each of the four holes which are
pierced through the clay luting of the pans, so it is evident that a
considerable quantity of either one or probably both the ingredients is
wasted. After eighteen hours the fires are allowed to go out, and the
contents of the pan cool down.

When this is accomplished, the greater portion of the vermilion will
be found adhering to the lower surface of the broken-up porcelain
plates with which the crude product is covered. The vermilion is then
carefully removed from the porcelain by means of chisels, and is now
ready for the elutriating mills. Another portion of vermilion of not
so good quality is found adhering to the upper iron pan, and that
obtained by washing the clay luting in a cradle, as diggers wash dirt
for gold. This, together with the wipings and scrapings generally,
is mixed up with alum and glue-water into cakes, and, after drying
on a brick surface heated beneath by means of wood or charcoal, is
powdered up on a mortar, and re-sublimed when a sufficient quantity has
accumulated.

The vermilion which was removed from the porcelain plates is of a
blood-red colour and crystalline structure. This is then powdered up in
a mortar and removed to the levigating mills. These are the ordinary
little horizontal stone mills used by Chinese and other natives of the
East to grind rice and other grain into flour or pulp, as the case
may be. Each stone is about 2½ feet in diameter; the lower stone is
stationary, the upper is turned by a direct-acting piece of wood having
a hole in it which works a wooden peg affixed to the upper stone,
which is made to revolve by a backward and forward movement of the
piece of wood, or handle, some 3 or 4 feet long, previously mentioned.
One man turns each mill. The upper stone has a small hole in it near
its centre, down which the workman from time to time pours a little
spoonful of the powdered vermilion, which he washes down into the mill
with water; as he turns the mill, the workman keeps continually ladling
little spoonfuls of water down the aperture or hole in the upper stone;
the ground and thus elutriated vermilion, as it escapes from between
the stones, is washed down by the water into a vessel placed beneath to
receive it.

When work is suspended for the evening, the ground vermilion is
carefully stirred up with a solution of glue and alum in water, in
the proportion of about an ounce of each to the gallon. The glue has
been made to mix with the water by previously heating it with a small
quantity of water; the earthen pots in which this process is effected
each hold about 6 gallons. The mixture is then left to settle. In
the following morning the mixture of glue and alum is poured off the
vermilion, and the upper portion of the cake of vermilion at the bottom
of the vessel--that is, the portion which remained longest suspended in
the liquid--will be found to be in a much finer state of subdivision
than the lower portion, which requires to be again elutriated as on the
previous day: this separation of the more finely divided vermilion from
that which was coarser, by suspension in a dense medium, is a really
most ingenious process, for which we should give the Chinaman every
credit.

The process of grinding, elutriation, and separation of the coarsely
ground from the fine vermilion, sometimes requires to be several times
repeated, in order to fully bring out the colour. As a final process
the damp cake of finely ground vermilion is stirred up with clean
water, and allowed to settle down until the next morning, when the
water is carefully poured off into large wooden vats to still further
deposit a small quantity of vermilion yet remaining in suspension, and
the vermilion is dried in the open air on the roof of the premises.

When quite dried, the cakes of now full-coloured pigment are carefully
powdered, and sifted by means of square muslin-bottomed sieves,
contained in a covered box some 2 feet high by 2½ wide, in which the
sieves, which slide on a framework inside the box, are jerked backwards
and forwards by means of a handle on the outside of the box or case
containing them.

The now fully-prepared vermilion is removed to the packing house, where
may be seen rows of workmen, men and boys, seated before a series of
tables. Between every two workmen is a third, with a small pair of
scales, which he holds in his left hand; and as the workmen on either
side place before him the little pieces of paper in which the vermilion
is to be wrapped up, he weighs into each paper one tael (about an ounce
and a third avoirdupois) weight of vermilion; the papers are two in
number, the inner a black or prepared paper, and the outer a piece of
ordinary white paper. After being wrapped up, the packets are placed
in rows before another workman, who stamps them with a seal containing
in Chinese characters the name and address of the manufactory in which
the article has been made, and the quantity and quality of vermilion
contained in the packet.

The rapidity and deftness of the Chinese workmen at this employment
is really surprising; the stamping, for instance, is effected at the
average rate of sixty impressions per minute, and the wrapping up is
carried on with proportionate rapidity. The mixture of alum, which is
the ordinary aluminium potassium sulphate, with the vermilion, in one
of its stages of manufacture as described above, is not added, as at
first sight we thought it might be, merely to assist in clarifying or
purifying the water by causing it to deposit its sediment, but seems
to have some peculiar effect upon the colour, although what may be the
_rationale_ of the process, or how it acts, we cannot quite clearly
see. The glue is added as described above merely to favour separation
of the finely elutriated vermilion by holding it longer in suspension
than the coarser particles, which sink first, and may therefore be
separated in their order of stratification.

The actual composition of vermilion is 100 parts of mercury to 16 of
sulphur, when both these ingredients are in a perfectly pure state;
the excess of 5⅓ lb. of sulphur added by the Chinese is probably
volatilised and lost in the process of sublimation, or, as the sulphur
used is generally not quite pure, a part may go for foreign matter
contained in the sulphur; the balance being probably the _raison
d’être_ of the blue lambent flame seen playing over the apertures
in the luting during the sublimation process. For a people having,
like the Chinese, no acquaintance with even the first rudiments of
chemistry, the proportion of ingredients taken--56¼ catties to 13
catties, or say 75 lb. to 17⅓ lb.--shows wonderfully accurate powers
of observation and a knowledge of combining proportions only to be
gained by much experience and a long extended series of careful
observations highly creditable to the manufacturers. The entire process
is one of the most ingenious and interesting to be seen in any part of
the world.--(T. I. B.)

Another and briefer account of the Chinese vermilion manufacture is
given by H. Maccallum, in the Proceedings of the Pharmaceutical Society.

He says there are three vermilion works in Hong Kong, the method of
manufacture being exactly the same in each. The largest works consume
about 6000 bottles of mercury annually, and it was in this one that the
following operations were witnessed:--

First Step.--A large, very thin iron pan, containing a weighed
quantity, about 14 lb., of sulphur, is placed over a slow fire, and
two-thirds of a bottle of mercury added; as soon as the sulphur begins
to melt, the mixture is vigorously stirred with an iron stirrer until
it assumes a black pulverulent appearance with some melted sulphur
floating on the surface; it is then removed from the fire and the
remainder of the bottle of mercury is added, the whole being well
stirred. A little water is now poured over the mass, which rapidly
cools it; the pan is immediately emptied, when it is again ready
for the next batch. The whole operation does not last more than ten
minutes. The resulting black powder is not a definite sulphide, as
uncombined mercury can be seen throughout the whole mass; besides, the
quantity of sulphur used is much in excess of the amount required to
form mercuric sulphide.

Second Step.--The black powder obtained in the first step is placed
in a semi-hemispherical iron pan, built in with brick, and having
a fireplace beneath, covered over with broken pieces of porcelain.
These are built up in a loose porous manner, so as to fill another
semi-hemispherical iron pan, which is then placed over the fixed one
and securely luted with clay, a large stone being placed on the top
of it to assist in keeping it in its place. The fire is then lighted
and kept up for sixteen hours. The whole is then allowed to cool. When
the top pan is removed, the vermilion, together with the greater part
of the broken porcelain, is attached to it in a coherent mass, which
is easily separated into its component parts. The surfaces of the
vermilion which were attached to the porcelain have a brownish red and
polished appearance, the broken surfaces being somewhat brighter and
crystalline.

Third Step.--The sublimed mass obtained in the second step is pounded
in a mortar to a coarse powder, and then ground with water between
two stones, somewhat after the manner of grinding corn. The resulting
semi-fluid mass is transferred to large vats of water and allowed to
settle, the supernatant water is removed, and the sediment is dried
at a gentle heat; when dried, it is again powdered, passed through a
sieve, and is then fit for the market.--_Proc. Pharm. Soc._

(4) Firmenich describes a process which he declares gives better
results in the beauty of colour than any other. It consists in using
sulphide of potassium, which must be in a state of great purity. Of the
various methods for preparing potassium sulphide, Firmenich rejects
those in which caustic potash lye is boiled with excess of flowers of
sulphur, on account of the simultaneous formation of a hyposulphite
or sulphate of potash, which interferes in the preparation of the
vermilion.

The process adopted by Firmenich for making pure potassium sulphide is
to reduce sulphate of potash by heating with charcoal, and subsequently
saturating the lye with sulphur to the necessary degree.

Usually about 20 parts by weight of potassium sulphate and 6 parts
by weight of charcoal are reduced to very fine powder and thoroughly
incorporated. Placed in a Hessian crucible, the mass is covered,
luted, and strongly heated. As considerable ebullition takes place
the crucible should be of such a size that the charge only occupies
two-thirds of its capacity. After fusion is complete, the mass,
which is now potassium sulphide, is allowed to cool; it presents a
reddish-brown crystalline appearance, and is very hygroscopic. It is
put into a cast-iron pan, with addition of soft water in the proportion
of 7 parts of water to every 2 parts of the potassium sulphide; after
boiling, it is filtered and on cooling, the undecomposed sulphate of
potash collects in crystals attached to the sides of the pan.

The thus purified lye is boiled a second time with flowers of sulphur,
added in small doses until saturation is indicated by bubbling and
effervescence. The simple (monosulphide) potassium sulphide in this
manner takes up four additional atoms of sulphur, and becomes the
pentasulphide.

The preparation of the vermilion then proceeds in the following
manner:--Into a series of large flasks are put 11 lb. of mercury, 5
lb. of the potassium sulphide lye, and 2¼ lb. of sulphur. The contents
are subjected to a moderate heat, and the flasks are then agitated in
a curious manner by arranging them in pairs in baskets suspended from
strings, over a straw mattress, on which the baskets bump each time
they descend.

Occasionally the flasks are turned about, and after about two hours of
this agitation they commence to grow hot, and the contents assume a
greenish-brown colour. The lye is robbed of its sulphur by the mercury,
and replenishes itself from the excess added.

Complete combination of the mercury and sulphur is accomplished in
about three and a half hours, when the colour of the mass becomes dark
brown. The next step is to cool the compound, an operation which must
proceed very slowly, and should occupy about five hours.

Development of the colour is effected by heat, for which purpose the
flasks are placed in a stove room or water bath, and subjected to a
temperature which does not fluctuate beyond 113° and 122° F., under
the influence of which the red colour appears. The greatest care is
necessary in this heating process, as it determines the success or
failure of the colour. It lasts several days, during which the flasks
should be shaken three or four times daily.

In order to separate the vermilion from the excess of sulphur, water is
added to the contents of each bottle, and, after thorough shaking, the
whole is turned out into a filter. The clear liquor escapes, and the
residual vermilion is mixed with caustic soda lye in stoneware jars,
and thus the remaining free sulphur is dissolved out. Subsequently
the lye is poured off as completely as possible, and the deposit is
repeatedly washed, first by decantation and finally on a filter. The
whole operation of filtering and washing cannot be completed in less
than two or three days. When this is finished, the drying must be
carried on at a very low temperature, till the vermilion can readily
be broken and is dry to the touch, when it is put into iron basins and
repeatedly stirred, while the temperature is allowed to reach 143° F.,
but never beyond that. The final desiccation occupies about five hours.

Vermilion made in this way is reputed more permanent and less costly
than by the usual methods.

(5) Dutch vermilion has a good name, and one method adopted in Holland
is as follows:--A mixture of 2 lb. of mercury and 1 lb. of sulphur
is thoroughly ground, and to 100 lb. of the mixture are added 2½ lb.
of minium or of granulated lead. About 2 cwt. of the compound is put
into each sublimation pot, which is duly heated. When the operation is
finished, the pots are allowed to cool for eighteen to twenty hours,
when they are broken, and their contents are ground in a mill. The lead
remains as a sulphide in the bottom of the pots.

(6) A modification of the Dutch method consists in making an intimate
mixture of 54 lb. of mercury squeezed through chamois leather and
7½ lb. of flowers of sulphur, which is then moderately heated on a
shallow iron dish, and the resulting black sulphide (“ethiops”) is
coarsely broken, ground, and kept in pots. To convert the ethiops into
vermilion, the former is put into large clay crucibles in a furnace,
and heated to dark-redness, whereupon the mass takes fire. As soon as
the flame has subsided, the crucibles are covered with a close-fitting
iron plate, and the firing is continued for thirty-six hours. The mass
is stirred every half-hour with an iron rod, and fresh additions of
ethiops are made at four or five hours’ intervals. The vermilion is
sublimed, and condenses on the cool portion of the interior of the
crucibles, whence it is collected by breaking the crucibles when cold,
and is finally ground and levigated.

(7) Kirchoff’s method requires special care, and consists in grinding
300 lb. of mercury with 68 lb. of flowers of sulphur in a mortar, the
sulphur being first moistened with a few drops of caustic potash. The
resulting black sulphide of mercury is added to 160 lb. of caustic
potash dissolved in very little water, and the whole is heated for half
an hour on a sand bath, with occasional addition of water to make up
for loss by evaporation. Gradually the mass, under constant agitation,
becomes brown and gelatinous, and finally red. Thereupon it is carried
to the stove room and still agitated at intervals. After several
washings it is drained, and dried very gently.

(8) Weshle mixes finely powdered cinnabar with 1 per cent. of antimony
sulphide, and boils the mixture several times with three parts of
potassium sulphide in a cast-iron pot. The precipitate is water-washed,
digested with hydrochloric acid, washed again, and finally dried.

(9) Jacquelin takes 90 lb. of mercury, 30 of sulphur, 30 of water, and
20 of hydrated potash; the mercury and sulphur are put into a shallow
cast-iron dish, dipping into cold water, and the potash solution
is added by degrees while the mass is kept in agitation. Then the
mixture is heated for an hour at 176° F., the evaporated water being
replenished. The vermilion is washed in an excess of boiling water, and
again several times in cold water, and finally filtered and dried.

VICTORIA RED.--One of the fancy names for Derby red. (See p.
145.)



CHAPTER VII.

WHITES.


In the whole range of pigments there is no more important class than
those to be described in this chapter. Not only are the white pigments
largely employed for the sake of their distinctive colour, but they are
probably even more extensively applied as a basis of other pigments,
both as an ingredient in the composition of the other coloured paints
and for ground coats where the final coat is to be of a delicate shade.
They are among the cheapest and most permanent pigments, and possess as
a whole remarkably good covering powers.

BARYTA WHITE.--Barytes or sulphate of baryta, the most
important of the salts of barium, is found native in large quantities,
forming the species of mineral termed barites or barytes, and commonly
known as heavy-spar, on account of its weight (sp. gr. from 4·3 to
4·7). It is found in Derbyshire and Shropshire, and often occurs in
fine tabular crystals. The massive variety found in the mountain
limestone of the above counties is sometimes called “cawk”; it is more
frequently found in white or reddish-white masses. In Saxony it occurs
as the mineral _stangen-spath_, in a columnar form; and at Bologna, a
nodular variety is found, called Bologna stone, which is notable for
its phosphorescent powers when heated.

The pure salt may be prepared artificially for use as a pigment, by
adding dilute sulphuric acid to a solution of chloride of baryta,
when a white precipitate is formed; this is well washed and dried. It
is a heavy, white powder, insoluble in water and nearly insoluble in
all other menstrua. It may also be prepared by heating the native
mineral, grinding it to powder, and well washing it, first in dilute
sulphuric acid, in order to remove any traces of iron, and afterwards
in water; the white powder is afterwards thoroughly dried. This
process is employed at several works in the neighbourhood of Matlock
Bath, in Derbyshire, but much larger quantities could be produced in
different parts of the country if the demand for the article rendered
its production more profitable. The principal use of sulphate of baryta
is to adulterate white lead, and to form the pigment known as _blanc
fixe_, or permanent white. For these purposes, the native mineral,
ground and washed as described above, is commonly employed.

Improvements in machinery and in the process of treating natural
barytes have overcome many of the objections which formerly existed
to its utilisation, and considerable attention is now being given to
the localities in the United States where it is found. The mineral, in
order to be available for the uses to which it is put, must be fairly
free from quartz grains, the stain of iron rust, and other impurities.
If the barytes is stained to any extent, it is practically valueless,
as a good white colour is essential to its usefulness. Quartz grains or
other hard substances with which it is apt to be associated injure the
machinery in grinding. The purest barytes so far produced in America
comes from Missouri, though a very fair grade is now being mined in
considerable quantities in Virginia.

The returns from all producers of crude varieties show a product in the
United States, for 1889, of 21,640 short tons, valued at 106,313 dols.,
against 20,000 short tons in 1888, valued, approximately, at 110,000
dols. The product was limited to four States, as shown in the following
table:--

                 Short tons.   Value.

  Illinois            200      $1,300
  Missouri          7,558      32,715
  North Carolina    3,000      15,000
  Virginia         10,702      57,298
                   ------    --------
        Total      21,460    $106,313

BLANC FIXE.--This name is given to baryta white when it has
been artificially prepared by adding sulphuric acid to a solution of
chloride of barium. (See p. 170.)

CHARLTON WHITE.--One of the names applied to a white pigment,
containing zinc oxide and sulphide, and described under zinc whites, p.
254.

CHINA CLAY.--This substance is also known as kaolin, porcelain
clay, and Cornish clay. It arises from the natural decomposition of
felspar in soft disintegrating granite, gneiss, and porphyry, the
rocks which are rich in soda-felspar yielding it most abundantly. The
main supplies of this country are derived from Cornwall and Devon; in
continental Europe, beds of good quality exist in France, Bavaria,
Saxony, Prussia, Bohemia, Bornholm island, and Hungary; in China, it is
very plentiful; and in the United States, it occurs in many localities.

The approximate composition of china clay may be stated as silica,
47·2; alumina, 39·1; water, 13·7 per cent. Often a little iron, lime,
and potash or soda are left in the prepared article by the imperfection
of the cleansing process. The most important characters are colour,
plasticity, and a capacity for hardening under the influence of heat.

The china-clay industry of Cornwall and Devon has been admirably
described by J. H. Collins, F.G.S., in a paper recently read before the
Society of Arts.

_Occurrence._--The natural clay rock is almost always covered with a
thick layer of stones, sand, or impure and discoloured clay, known as
“overburden.” This capping often much resembles glacial drift; but
it never contains any scratched or glaciated stones, or travelled
blocks. It varies in thickness from 3 feet to 40 feet, and must, of
course, be removed before the clay can be wrought. The clay rock,
being a decomposed granite, consists of china-clay, irregular crystals
of quartz, and flakes of mica, with sometimes a little schorl and
undecomposed felspar.

_Extraction and Preparation._--The following descriptions apply, with
more or less accuracy, to a majority of the larger works of the present
day, turning out from 2500 to 8000 tons of clay each, yearly. Two
somewhat different methods are employed, according to the situation of
the “bed” of clay in relation to the surface contour of the immediate
neighbourhood. The most general case is that in which the clay has
to be raised from a veritable pit, the bottom of which is lower than
the ground on all sides. The exact situation of the clay is first
determined by systematic “pitting,” to a depth of several fathoms, or
occasionally by boring. A shaft is then sunk either in the clay itself,
or, preferably, in the granite close to the clay. From the bottom of
this shaft, a level is driven out under that part of the clay which it
is intended to work first, and a “rise” is put up to the surface, which
should, by this time, be partially cleared of its overburden. A common
depth for such a shaft will be from ten to twelve fathoms. As soon as
the rise is completed to surface, a “button-hole” launder is placed
in it, and the remainder of the rise is again filled up with clay. In
the meantime, a column of pumps has been placed in the shaft, with an
engine to work them, unless water-power is obtainable.

For water, many works are almost entirely dependent upon that met with
in sinking the shaft and in driving levels; but, of course, this may
be, and is, eked out by catching the rain-water in reservoirs, and by
making use of such small streams as may happen to be available. A small
constant supply is sufficient even for a large work, as it is used over
and over again. The operation is begun by digging a small pit in the
clay, around the upper end of the button-hole launder, and running a
stream of water over the exposed clay, or “stope,” which is broken up
with picks. A very large quantity of sand is constantly disturbed, and
as constantly shovelled out of the way, while the water, holding the
clay and finer impurities in suspension, runs down the launder, along
the level, and into the bottom of the shaft, from whence it is pumped
up by the engine or water-wheel.

As the excavation becomes larger and deeper, more overburden is
removed, and the upper portions of the launder are taken away, until
at last the stopes reach the level, when the launder is, of course,
no longer required. At first, the sand is thrown out by one or two
“throws,” but very soon it becomes necessary to put in an inclined
road, for pulling up the sand in waggons; these are worked by a
horse-whim, or by winding gear attached to the engine or water-wheel.
As there are from three to eight tons of sand to each ton of clay,
its removal in the cheapest possible manner is a matter of great
importance. Any veins or lodes of stone, or discoloured portions of
clay, are raised from the “bottoms” in the same way as the sand. The
stream of water, holding in suspension clay, fine sand, and mica, is,
in well-arranged works, lifted at once high enough to allow of all
subsequent operations being carried out by the aid of gravity.

The stream is first led into one or two long channels, the sides of
which are built of rough stone. In these channels, called “drags,”
the current suffers a partial check, and the fine sand and rougher
particles of mica are deposited. From these drags, the stream passes on
into other channels, much resembling them, but of greater number, so
as to divide the stream still further. This second series of channels,
known as “micas,” are often built of wood, but sometimes of stone. They
differ in no essential respect from the drags, but are more carefully
constructed and better looked after, and, as the stream is greatly
divided and very gentle, the fine mica is deposited in them. The micas
are often about 11 inches wide, ten or a dozen in number, and 100 feet
or more long. Provision is made, by underground channels and plug
holes, for the periodical cleansing of the drags and micas. This may
have to be done twice a day, but generally only once.

The deposit in the drags is worthless at present, and is always thrown
away; but that from the micas is often saved, and sold as inferior or
“mica” clay. The refined stream of clay then passes on to the “pits,”
which are circular, 30 to 40 feet diameter and 7 to 10 feet deep. These
pits are built of rough masonry, and have an outlet at the bottom,
opposite the point at which the stream of clay-water is admitted. This
outlet is stopped by a gate or “hatch,” or by a plug, and is kept
closed until the pit is full of clay. In each outlet, however, is fixed
an upright launder some 4 inches square, provided with “pin-holes”
and wooden pins set close together. As the stream of clay enters on
one side, it is constantly depositing its burden, and the water is as
constantly drawn off nearly or quite clear from the pin holes, the pins
being put higher and higher as the clay rises in the pit. The effluent
water is conducted directly to small storage reservoirs, and thence
over the clay stopes, whence it does its work over again.

When the stream of clay-water enters the pits, it contains from 1½ to
3 per cent. of clay; and what is called a good washing stream will
carry about one ton of clay an hour. When the pit is full, the “hatch”
is drawn, and the clay is “landed” into the tank. The upper portion is
sufficiently fluid to run in of itself; but that near the bottom has to
be helped out by men using “shivers” of wood or iron, which resemble
large hoes; they are assisted by a small stream of water. The tanks are
commonly, but not always, rectangular, built of stone, and paved with
stone at bottom, often 60 feet by 30 feet by 6 feet or larger. Once in
the tank, the clay is left to settle, until it has the consistency of
cream cheese, the water being drawn off from time to time; it is then
ready to be trammed into the “dry.”

The “dry” is a large building erected in immediate proximity to the
tanks. It is always composed of two parts, the dry proper and the
“linhay.” The floor or “pan” of the dry is composed of fire-clay tiles
18 inches square, 5 or 6 inches thick at the fire end, and gradually
thinning off to 2 or 2½ inches at the stack end. The flues are built
of fire-brick, about 15 inches wide, 2 feet deep at the fire end, and
9 inches deep at the stack end. Each flue should be supplied with a
damper. The furnaces are built in and arched over with best fire-brick;
the fire bars run longitudinally, and are about 6 feet long. The grate
surface is about 2 feet 6 inches wide in front, and 4 feet 6 inches to
6 feet at back, according as each furnace supplies three or four flues.

The clay, brought in from the tanks in tram-waggons holding about half
a ton, is tipped on to the tiles, and spread in a layer from 9 inches
thick at the fire end to 6 inches thick at the stack end. The fire
end is loaded and cleared every day; the other end perhaps twice or
thrice a week, according to the length of the dry, thickness of tiles,
perfection of draught, &c. An average size for a first-class dry is
perhaps 15 feet wide and 120 feet long; but some have been constructed
considerably larger than this. The pan of the dry should be 6 or 8
feet above the linhay whenever possible, so as to afford storage space
for the dry clay, without expending labour in piling. The tiles should
be as porous as possible, for very much more water passes through
the tiles and into the flues than is driven upwards in the state of
steam. The temperature should never be allowed to rise so high that the
workmen cannot walk on the tiles, otherwise the clay may become baked
and damaged.

In cases where there are no means of artificial drying, as at some
old-fashioned works, the thick clay is at once transferred from the
original settling pit to shallow depressions in the ground, called
“pans.” Ten or twelve of these, each holding from 40 to 50 tons,
should be provided for each settling pit; they measure from 20 to 40
feet square, and 2 feet deep, and are enclosed by granite walls, the
interstices of which are rendered impervious by plugging with moss. The
clay, filling two-thirds of their depth, is here left exposed to the
sun and wind, by which it is partially deprived of its moisture.

In order to complete its desiccation, the clay is removed from the pans
after three or four months’ exposure. A number of parallel incisions
are made lengthwise in the clay, by means of a knife attached to a long
handle; the strips are next divided transversely, by men with spades,
who throw the blocks on to a board, upon which they are borne by women
and children to the sandy drying yard, where, in fine summer weather,
they soon become dry. They are then collected, and piled away in sheds,
under a number of thatched gates or “reeders,” or are placed in some
sheltered position where air can circulate around them without their
becoming wet from rain.

When required, the blocks are scraped by women armed with hoes, before
being despatched from the works. The transport is often effected in
small casks, holding about half a ton. A few years since, a machine for
drying china-clay was invented by a mechanical engineer named Leopoldo
Henrion, of Sampierdacena, near Genoa. It is said that, by its use, the
operation can be effected in a few hours, at a relatively small cost.

Collins was first led to adopt his arrangement in consequence of the
formation of the ground; but he is inclined to recommend it in most
cases if practicable. Very large quantities of stone are required in
the dry pits, tanks, &c. Very often this is got, in part or entirely,
in the process of excavating the pits, &c.; but if it cannot be so
obtained, a very serious expense will be incurred, in some instances
amounting to several thousand pounds. The total cost of the works may
even be doubled from this cause, if stone has to be fetched from a
distance of several miles.

Two modes of building with rough stone are adopted; they are known as
“lime building,” and “dry stone walling.” The first needs no special
remark, but the second is very ingenious and very effectual. The wall
is built up double, with a batter of about ¾ inch or 1 inch to the
foot. Moss is placed between the joints of the wall, and the space
between is filled in with sharp sand, the refuse of that or some other
clay works. A small stream of water is then made to flow over the sand,
which is well beaten in with rammers, or by treading with the feet.
This process is continued, a foot at a time, till the wall reaches the
required height, when it is either paved with rough stones set on edge,
or turfed. A wall properly built, in the manner just described, is
quite impervious to moisture, and will stand for fifty years or more.
It is, where the proper kind of sand is abundant, much cheaper than
lime walling, and is always preferred for the walls of pits and tanks.

Where the bed of clay is situated on a hill-side, with plenty of space
below, a tunnel is driven in from the hill-side or from the valley to
the required depth, and a rise is put up as before. This rise is then
divided off into two parts. In the smaller, a button-hole launder is
placed as before, and packed around with clay; but the larger is left
open. A stream of water, obtained by pumping or otherwise, is made to
run over the stope, and down the button-hole launder. It then flows
along a launder placed in the bottom of the level, until it makes its
exit in the valley. It may then be purified, settled, and dried exactly
as already described--the works being laid out at a lower level than
the adit; or, if the clear water is wanted to flow over the stope,
or it is, for any reason, necessary to place the pits and tanks at a
higher level than the stopes, the water is pumped up after partial or
complete purification.

The main difference in this mode of working is that instead of pulling
the sand and rubbish up over an incline, it may be tipped down the
pass into waggons, run out through the level, and tipped over the
hill-sides. In cases where waste water is abundant, it may even be
washed out at night, thus saving the expense of tramming. Of course,
when the workings have reached their full depth, the rise and the
launder are dispensed with, and the adit level communicates directly
with the “bottoms.” By this mode of working a considerable economy may
be effected, especially when it is not necessary to pump the clay water
for settling or repeating.

_Cost of Production._--Where the conditions of production vary so
greatly, there must necessarily be great differences of cost; but,
after having been at some pains to determine the cost under average
conditions, Collins thinks the following figures and statements may
be relied upon. A work capable of producing say 4000 tons of clay
yearly will cost from 2500_l._ to 5000_l._ To get the clay in the
linhay ready for the market will cost about 9_s._ a ton, of which about
2_s._ 6_d._ must be expended in fuel for pumping and drying, 1_s._ in
removing overburden, 1_s._ in removing sand, and 1_s._ for management
and office expenses, leaving 3_s._ 6_d._ as the net labour cost of
washing and drying a ton of clay. To the 9_s._ net cost of clay must
be added an average of 3_s._ for royalties, 4_s._ for transit and
placing on board ship, and 1_s._ for agencies, commission, bad debts,
and sundries, making the average actual cost amount to 17_s._ Some
favourably situated works can no doubt save 2_s._ or even 3_s._ on
this account; in others, the cost may amount to 20_s._ or even 22_s._
As to the selling price, this varies much more widely than the cost of
production, ranging from 14_s._ to 35_s._ _f.o.b._ Clays sold at the
lower rate are unremunerative.

_Nature and Utilisation of Waste Products._--Besides the clay proper,
there are certain waste or pseudo-waste substances produced in very
large quantities. These are as follows:--

Fine Mica.--This is deposited in the “micas”; a few years since it
was thrown away, or rather washed away, as is still the case in many
works. Sometimes, however, it is collected, dried in the manner of clay
proper, and sold to the makers of soft paper, paste-board, inferior
pottery, &c., at a low price.

Coarse Mica.--This is invariably washed away, or thrown away, there
being at present no demand for it. It, however, contains a very
beautiful material, which might be applied to many ornamental purposes.

Sand.--This consists of broken quartz crystals, mostly white or pale
brownish; when washed clean, it is the finest building sand known, as
the angles are all sharp. Mixed with one-eighth of Portland cement, it
forms a concrete as hard as stone.

Discoloured Clay.--This has to be dug out from among the good white
clay in many places. It has been successfully used in the manufacture
of white bricks for building purposes. In some instances, a quantity of
the sand already mentioned is mixed with the refuse clay, and produces
an excellent fire-brick. The same material is used in the manufacture
of the tiles used as a floor for drying the clay. The manufacture of
bricks and tiles from this debris is a growth, it is believed, of the
last twelve years.

Overburden.--The upper part of this consists of soil, or “meat earth”;
this is usually removed and carefully preserved. Underneath is a hard,
often stony or sandy layer, which, in districts where tin is worked,
often contains enough tin to pay for washing. With this stony or sandy
layer, is usually a considerable thickness of discoloured clay suitable
for brick-making.

Branches.--These are stony veins which run through the clay stopes in
various directions. Sometimes they are quite worthless; but in a few
instances they are veritable tin lodes, and contain enough tin to pay
for stamping and dressing. Thus at Carclaze, near St. Austell, each
1000 tons of clay yields something like one ton of oxide of tin, and
formerly the proportion was much greater. The proportions of these
waste materials, as compared with the fine clay procured, are thus
stated:--

For every 1 ton of fine clay there is removed--from 3 to 7 tons of
sand, average about 3½ tons; from 2 to 5 cwt. of coarse mica, average 3
cwt.; from 1 to 3 cwt. of fine mica, average 2 cwt.; from 0 to 1 cwt.
of stones, average ¼ cwt.

A cubic fathom of clay rock, of average quality, will yield about
2½ tons of fine clay; and about half a fathom of overburden must be
removed to get it.

_Suggested Improvements in Preparing._--Collins thinks that there is
still much room for improvement in the preparation of china clay, but
that such must be a growth of time and circumstances. At the present
time, about one ton of water has to be driven off from each ton of clay
in the “dry,” and this uses at least 2 cwt. of coals on an average, and
costs from 8_d._ to 10_d._ in labour. In a few modern drys, a small
economy in fuel has been effected, by lengthening the kiln; but in none
has it been brought so low as 1½ cwt. to the ton of clay.

Stocker, in 1862, suggested the use of filter beds, and also devised
a centrifugal dryer; but neither of these contrivances has come into
use, and the first would seem quite inapplicable on account of the
extreme fineness of the particles of clay, and the impermeability of
even a thin layer of that substance. Some economy might perhaps result
from the use of hydraulic filters of calico, such as are used in the
potteries for drying the slip; but it is very doubtful if any saving
would be effected, as the labour would be about the same, and, against
the 2_s._ a ton for fuel, would have to be placed the wear and tear of
the calico.

In washing the clay from the stope, some benefit might accrue from the
use of a jet of water under a pressure of from 50 to 100 lb. per square
inch, as in the so-called hydraulic mining. This could only be applied
to stopes of even quality, where very little picking out of inferior
portions was required; but it would supersede the services of the
“breakers” on the stope, and greatly lessen the labour of the washers.
It is but rarely that a natural head of water is obtainable equal to
the required pressure; but where machinery is used for pumping, the
additional cost of pumping, say 250 gals. a minute to a height of 150
feet in a standpipe, would be very slight, as the extra power required
is little more than that of one horse.

_Statistics._--From statistics obtained from many sources, it is
evident that the production has very largely increased from 1809 to
1874--2919 tons against 226,309. In 1810, Trethosa (one of the largest
works) produced 300 tons per annum, and employed thirteen persons, viz.
eight in removing burden and raising (breaking) clay (at per fathom),
three washing, two attending ponds and packing. In 1874, one of the
works near St. Austell produced 9000 tons, employing about thirty men.
Many works produced 6000 tons, employing twenty men. The quantity sent
annually from Cornwall must average at least 150,000 tons. It goes
not only to Staffordshire, but also largely to France, Belgium, and
other countries. The extensive clay works recently opened in several
departments of Northern France have done much to curtail the export of
Cornish clay to that country, and the large deposits of the island of
Bornholm have lately been worked upon to supply the needs of Denmark,
Sweden, and Germany; while similar utilisation of native clays has been
carried out in America. Nevertheless, the growth of home industries
which depend in a measure upon this article will, doubtless, counteract
the influence of decreasing exports.

_Artificial China Clay._--The principal supplies of china clay are
obtained, as has been described, through the agency of natural
decomposing influences in granite rocks. In one instance, however,
at Betleek, County Fermanagh, it is procured by calcining the red
orthoclase granite of the district. The felspar is whitened by the
process, and the iron becomes separated in a metallic state, and is
removed by magnets.

_Characters._--Being virtually a hydrated silicate of alumina, china
clay is a remarkably stable pigment. Not only is it unaffected by
prolonged exposure to strong light and impure air, but is insoluble in
water, weak acids, and alkalies. It is moreover very much lighter in
weight than any other white pigment, an advantage on the score of cost
when buying by weight. Its covering power in distemper work and as a
water colour is good; but the addition of oil reduces its capacity.
The best qualities are exceedingly fine in grain and pure in tint, but
inferior samples sometimes have their yellowness “corrected” by the
addition of a little ultramarine.

ENAMELLED WHITE.--Another name for the finest kinds of baryta
white, see p. 170.

ENGLISH WHITE.--A synonym for whiting, see p. 246.

GYPSUM.--This very common and abundant mineral is a hydrated
sulphate of lime, occurring in several forms, of which only the opaque
white variety is useful as a pigment.

The native mineral is quarried, dressed, ground, and levigated, in all
which operations there is nothing special to be noted.

Whether obtained in this way, or prepared artificially, or formed as a
bye product in other industries, gypsum affords a permanent and neutral
white pigment, mixing well with oil or water, and possessing a covering
power which ranks between white lead and zinc white. It has a bluish
tint, but less so than ordinary white lead.

KAOLIN.--One of the names applied to china clay, see p. 172.

LEAD WHITES, OR WHITE LEADS.--On the grounds of the quantity
in which it is produced and the extent to which it is applied, probably
no pigment can compare with white lead, including in that term the
various white pigments having lead as a basis.

In its commonest form white lead is lead carbonate. There are many ways
in which it is made commercially, all dependent upon certain chemical
reactions.

When a solution of normal plumbic acetate is attacked by carbonic
acid, no precipitate is produced. That normal solution is formed by
the action of acetic acid or hydric acetate upon oxide of lead. It
consists of a certain weight of lead to a certain weight of acid, which
converts it into the acetate. Carbonic acid has no power to separate
out from it the lead, and form carbonate of lead. But this acetate of
lead has the power of dissolving a considerable quantity of oxide of
lead in addition to that which was used in its first formation, and
when this additional quantity of oxide of lead is dissolved by the
acetate, a substance is formed which is termed a basic acetate, that
means an acetate which contains more of the base (the lead oxide) than
the normal acetate itself does. From such a solution we are able to
precipitate, by means of carbonic acid, a white substance, which white
substance is a carbonate of lead.

Thénard, a French chemist, proposed to make white lead in this way, but
it was found that although the colour was pure and good, yet the lead
had not sufficient body to satisfy the wishes of artists and painters.
White lead has been made for years past according to what is called
the Dutch method. Lead is cast into plates, and these plates, in some
factories, are rolled into coils. These coils then are immersed in
earthen pots; the pots are placed in a row, and a small quantity of
vinegar is put into each pot. On the top of one row of pots a board
is placed, and then other pots above, and so a stack is made. Between
the interstices of the pots is put spent tan, or some other substance
which by oxidation will evolve heat, and also carbonic acid gas. Now
the heat which is evolved in oxidation of the spent tan is useful in
volatilising the acid from the vinegar, and in the presence of this
acetate the oxygen of the air oxidises the lead. The oxide of lead is
dissolved by the acid, and the normal acetate of lead is formed. More
oxide is produced, and this is dissolved by the normal acetate, and
then you have basic acetate.

When substances containing carbon are oxidised, carbonic acid is the
product of the oxidation when the oxygen is in excess, as in this
particular case. Carbonic acid is then formed by the oxidation of the
spent tan. The carbonic acid then unites with the oxide of lead which
was dissolved in the normal acetate, and a thin film of lead carbonate
is formed. These thin films go on forming in succession, until at
last nearly the whole of the lead is converted into carbonate, which
retains the shape of the original lead. In some cases, gratings of
lead are used. When the lead is converted into carbonate, it is ground
in water and reduced to a fine powder, and is then made up into the
sort of pigments required, either with water or with oil. This is,
or rather was, an operation attended with considerable danger to the
workmen, who were subjected to what is termed lead-poisoning, to which,
unfortunately, many painters, from want of cleanly habits, are subject
now.

_Dutch Process._--In the words of Mr. Carter Bell, who has read a
most interesting paper on the subject before the Society of Chemical
Industry, the manufacture of white lead is a most ancient proceeding,
and has been pursued with but little variation in the mode of
manufacture for some hundreds of years. The Dutch seem to have been
the originators of this method of making white lead, which is now so
largely conducted in this and other countries.

In this process metallic lead is piled in stacks, and submitted to the
action of acetic acid, watery vapour, air, and carbonic acid for some
time, by which means the metallic lead becomes gradually converted into
white lead.

This method is called the “stack” or Dutch process.

The construction of a stack is a very simple and rude operation. Layers
of dung or tan, or a mixture of the two, are so arranged as to imbed
a large number of earthenware pots, each containing some acetic acid.
These pots are about 4 or 5 inches in diameter, and about 7 or 8 inches
high; a coil of lead is placed in each pot, and buckles or gratings of
lead supported on oaken bearers are laid across and on top of the pots;
boards are laid to cover the whole, and form a floor.

The stack is composed of a number of such layers of pots, bearers, and
buckles or gratings, raised one upon another.

A stack chamber is a brick enclosure 10 or 12 feet square, and 20 or
25 feet high; such a chamber will contain about 70 tons of lead when
stacked and piled. In a white lead factory several of these chambers
are built side by side, and when they are in full operation a set of
chambers will contain as much as 700 or 1000 tons of lead.

Only the purest kind of lead will be suitable for conversion in
this stack process of making white lead, the common varieties being
inadmissible. Messrs. Pontifex and Wood have furnished the following
analyses of lead used for white lead making.

    Copper.   Antimony.     Iron.     Spelter.    Silver.     Lead.

  A 0·00700    0·00490     0·00200    0·00080     0·00100    99·98430
  B 0·07580    0·00320     0·00220    0·00320     0·00200    99·91360
  C 0·00340    0·00460     0·00120    0·00070     0·00350    99·98660
  D 0·05260    0·00740     0·00150    0·00180     0·00400    99·93270
  E 0·00940    0·00210     0·00160    0·00100     0·00075    99·98515
  F 0·02360    0·00580     0·00210    0·00180     0·00100    99·96570

The presence of silver, copper and iron in the lead would damage the
colour of the white lead resulting, and other admixtures retard or
prevent the progress of conversion.

In olden times horse dung was the only imbedding material used in
the stack arrangement. This material when heated evolves gases which
seriously interfere with the colour of the resulting corrosion. Dung
has been almost superseded in this country by tanners’ refuse; in
Belgium dung is yet employed, and in some places a mixture of dung and
tan.

Where dung is used, the process of corrosion of the lead goes on more
quickly than when tan alone is employed, but the use of tan offers
great advantages, especially this one: that it does not give off gases
that damage the white lead.

The operations of charging and discharging these chambers are
principally the work of women, and are most laborious and fatiguing.

In emptying the chambers and stripping the stacks, the women are fully
exposed to the heated gases which are yielded by the decomposing tan,
and the heated and corroded lead. These gases, in themselves most
injurious to health, are not to be compared in this respect to the dust
which pervades the air and fills the chamber in which these women work.

When a stack is charged, the chamber containing it is enclosed. The tan
or dung within soon commences heating, and the heat soon causes the
acetic acid in the pots, and the water in the tan or dung, to rise in
vapour and penetrate the stack. Air is admitted to the stack through
openings left for that purpose, and carbonic acid is evolved from the
heated decomposing tan or dung, and this gas also penetrates the stack,
and the process of converting blue lead into the white lead gradually
proceeds, and the blue metal becomes corroded and incrusted with a
white crust or covering.

As to the exact chemical changes and combinations proceeding in the
working of a stack, differences of opinion exist, but we may fairly
conclude that the process resolves itself into this--first, the
formation of sub-acetate of lead, which, decomposed by the agency of
the carbonic acid gas, becomes reduced to the condition of normal
acetate by loss of a portion of its basic oxide of lead. The reduced
sub-acetate then again takes up an additional molecule of oxide of
lead, and is re-converted into its original subsalt state, to be again
attacked and reduced by the carbonic acid gas, and so on continually
during the working of the stack. It will be evident that the “nascent”
state of the various substances disengaged during the chemical changes
which are proceeding in the stack is an important factor in this
process, and must be taken into account in considering the philosophy
of the operation.

It will also be evident that the mode of proceeding in white lead
making by the stack process is most crude and clumsy, and a most
uncertain method, one governed by rule of thumb, and, by no element of
certainty or science. White lead makers, as a rule, know nothing of the
chemistry of their subject. This absence of chemical knowledge of the
subject, by those who are engaged in this manufacture, may explain the
curious circumstance that for hundreds of years this industry has been
pursued in the same old-fashioned and uncertain way, and the stack,
or Dutch process, still holds its ground and displays little or no
advance in knowledge or improvements in its method of proceeding, even
in the present age of precision in almost every branch of manufacture.
The uncertainty of the stack process is shown most clearly in this:
That stacks may work and some do not work. In the latter case all the
time and labour spent in forming the stacks, and all the acid they
contained, is lost.

No amount of foresight will avail to determine beforehand which stack
shall accomplish the conversion of its contained metallic lead, and
which will not.

The stacks are generally allowed to remain in operation, after they
are charged, three or four months; in this time it is presumed all
profitable action in the stack has ceased. The temperature of the
stack, which had risen gradually from the normal temperature to 100°
or 150° F., will have gradually fallen, and this falling temperature
is the indication that the corrosion of the lead in the stack has
terminated.

After the three months’ action of the stacks, they are stripped and
pulled to pieces. Some will be found to be done better than others, and
one part of the same stack will be done more perfectly than another.
The coating on the lead will also differ; some will be smooth, regular,
and equal in formation, some will be rough and blistered, and far from
uniform. The rough blistered casting is rejected as unfit for white
lead making: the workmen call it “dross.” The smooth laminated coating
is the one preserved for the after manufacture.

It is a curious fact connected with the consideration of the total
want of educated guidance in these matters that prevails, that in all
factories of this description some chambers are noted as always working
well, and others are equally well known to always do their work the
reverse of well. No one knows why! No one stays to seek the reason.
The factory way goes on filling and emptying these white lead chambers
whether the stacks be working well or no.

The incrustation that is most esteemed by the manufacturers of white
lead in this old-fashioned style is a hard, china-like material, formed
of thin deposits, layer upon layer, in a slow, continuous, regular way.
It is at once conceivable that in the rough-and-ready manner of stack
manufacture most irregular action must proceed.

It would be almost impossible for the contents of the pile or stack to
be submitted to the same action of the gases throughout. Some parts of
the stack and its contents will be under more favourable conditions
than others, hence the reason why, in practice, it is invariably
found that some stacks, and some parts of a stack, work better than
others. Under the microscope, this good crust of white lead, the proper
incrustation from which to prepare white lead, will be readily seen
to consist of very thin coatings or layers of white lead, which have
been slowly formed on the metallic lead and piled one upon another to
the thickness of an eighth or a quarter of an inch. This formation
constitutes the hard, china-like substance, which alone possesses the
chemical constitution and the properties to form good white lead paint.

White lead makers, recognising this peculiar incrustation as the only
one capable of fulfilling their desired purpose of making good white
lead paint, do not even recognise any other as of any service for that
purpose, be it good or bad. Such a material, if obtained, however good,
would be outside their experience and beyond their philosophy. After
the stacks have been stripped, the gratings or buckles with their
adhering coating of white lead are moistened with water and are passed
through crushing rollers to separate the unconverted lead; then the
crust which has been detached is ground under heavy edge runners with
water.

This detached crust of white lead will vary much in colour: it
will be white in some parts, yellowish or greyish in others. These
discolorations arise from various causes, but they are principally
caused by the contact of the moist wood and tan. The white lead is now
a rough crushed material, very hard, and requiring to be ground to the
finest powder. It contains, also, small fragments of blue lead which
have passed the crushing rolls, and a quantity of acetate of lead.
The presence of acetate of lead is always found in larger or smaller
quantities, which vary with every operation, and which invariably
accompany white lead produced in the stack.

To remove discolorations--to separate the fragments of metal and to
dissolve out the acetate salt--much water and washing are employed. The
material is ground with water under the heavy edge runner stones, it
then proceeds to a series of horizontal mills, each succeeding mill set
closer than its fellow, and is further and further ground to fineness
with water. From these mills it runs as a milky liquid to a series of
settling tanks, where it is allowed to subside, and the clear fluid is
run off to waste, or into tanks to be used over again. This waste water
will now contain the colouring matter removed from the incrustation,
and the principal portion of the acetate of lead which the incrustation
previously contained, and any other soluble matters removed from the
washed and ground material. The small fragments of lead which passed
the crushing roll and edge runner mills will have been previously
removed by subsidence in water.

The white lead deposited in the tanks is in some factories ladled out
into skips and agitated by a “dolly,” which further enables the heavy
powder to get free from the water in which it is entangled. The moist
powder is next placed on trays or dishes, and is conveyed to the stove
or drying chamber. Women always perform this work.

The drying or stoving room is a large enclosed space heated by a
“cockle” arrangement; rough scaffoldings are erected within this
chamber, on which women mount to stow the trays on shelves fitted
for the purpose. The trays and their contents remain in the heated
atmosphere of this chamber for two or three weeks, by which time they
become dry and ready for removal, to be packed in lumps for certain
markets, or ground to dry powder and packed in barrels for others.

Women are employed to fill and also to empty the drying or stoving
chamber, and during this work they are fully exposed to its
contaminating atmosphere. Hot and dry, and charged with fine dusty
particles of white lead, it becomes a dangerous trap, and contaminates
the blood of those engaged with its deadly poison. It is in this part
of the manufacture that the principal damage to health occurs. This is
the most laborious work; heat makes it very fatiguing, the atmosphere
within this chamber being always much above the exterior air.

Recent Government regulations have sought to curtail these and other
evils in this manufacture. Women engaged in these stoves are ordered to
wear overclothing, headdress and respirators. The general experience
of their practice, notwithstanding Government regulations, is this;
that they cannot work in them with ease and convenience, and more often
wear the respirator around their necks than in front of nose and mouth.
The excessive mortality in women who work in these stacks and stoving
houses scarcely requires assertion. Few, even of those who employ them,
know the extent of the deadly operation. Recently, medical men have
made public that cases are within their knowledge of children born
already contaminated with lead poison. Woman labour should surely be
restricted by Government enactments in all such deadly occupations.

We may sum up the whole matter as regards white lead making by the
stack or Dutch method in a few brief words: It is a most tedious
and uncertain operation; it is a most dangerous occupation for all
concerned; it is founded upon no true principles of any kind; and of
science its whole course is ignorant. White lead making is ruled by a
“happy-go-lucky” philosophy. The representatives of this manufacture
are completely ignorant of the scientific details relating to it, and
hence we may not be surprised to find amongst them an enormous amount
of ignorance and prejudice.

Good white lead will not differ materially in its composition by
whatever process it may be made, but it may differ seriously in its
physical character, and in its fitness to produce a substance adapted
to the uses to which white lead paint is applied. Good white lead is
a compound which contains hydrate and carbonate of the metal, in the
proportions either of one molecule of hydrate of lead combined with two
of carbonate, or is made up of one molecule of hydrate with three of
carbonate of lead.

If we consider the first compound roughly

  PbH_{2}O_{2},2Pb CO_{3}

white lead will be made up of one part of hydrate and two parts of
carbonate of lead.

The second compound roughly estimated

  Pb H_{2}O_{2},3Pb CO_{3}

will be one part of hydrate, combined with three parts of carbonate
or lead. The latter will be in the proportion of 75 per cent, of
carbonate and 25 per cent, of hydrate of lead, and this represents the
composition which has been assigned to good white lead by those most
acquainted with the subject. The amount of hydrate contained in white
lead should never exceed the proportion above named of 25 per cent.,
nor should its amount be much below the 25 per cent.

The hydrate contained in the substance serves to unite with the oil in
the paint; it forms therewith a drying white and elastic varnish which
embraces and holds the particles of white carbonate and prevents their
subsidence and separation in the paint. There is a chemical action of a
much more intimate character between the components of good white lead
when mixed with oil which neither of the constituents of this compound
can alone produce.

For instance, hydrate of lead and linseed oil produce a varnish-like
substance, semi-transparent and of no covering capability.

Carbonate of lead and linseed oil produce a compound which is opaque,
but has no body or covering power, and in which the white solid
carbonate is held in feeble mechanical suspension.

Neither of them constitutes a paint, but when together as white lead
they are mixed with oil, combination takes place, and serviceable paint
of good body and covering power and enduring quality is produced. Good
white lead is a dense, perfectly amorphous powder of perfect whiteness,
possessed of great body and covering power when combined with oil. When
mixed with linseed oil and used as paint it rapidly dries in the air
and assumes a varnish-like, glossy, hard surface, and is capable when
once dry of resisting the action of air and water for any length of
time. It does not weep when laid on a surface with a brush, that is,
the oil does not separate from the solid material of the paint.

Attempts have been made to produce white lead quickly and cheaply by
precipitating processes, but in all such methods the resulting compound
is deficient in certain special qualities absolutely necessary to white
lead proper and to its uses. The precipitated white lead is always of
a crystalline structure, and crystalline lead can never furnish a good
body paint--no amount of pulverising and grinding of this crystalline
material will correct this defect in its nature, and deprive it of its
crystalline form.

“Once a crystal always a crystal” has an especial application to
this point of our philosophy. Pulverising a crystal will not alter
its structure, but simply reduces the size of the crystals. Crystals
of white lead are unable to effect the necessary combination with the
oil and form the true varnish which white amorphous lead so readily
produces. Paint made with the precipitated white lead lacks body and
covering power, and this because of the absence of this chemical union
with the oil.

The manufacture of white lead by process of precipitation, even were
the resulting preparation suitable, does not correct the evils of the
present method by Dutch or stack process of making white lead.

A solution of lead may be precipitated in a few minutes, but it cannot
be made so quickly. The white lead, after its precipitation, has to be
filtered or separated, washed and dried, and ground to powder, which
processes cannot occupy less time than a few weeks for completion.

Precipitated white lead has been made in France and Germany for some
years, and it is now manufactured in those countries. It is now made in
England by one patent process, but the product lacks certain qualities,
and is consequently still open to the objections already noted.

Substitutes for white lead of a non-poisonous nature, or of such a
nature as not to produce such deadly effects in their preparation or
use as white lead does, have been proposed; their introduction has
not, however, been a great success. A mixture of sulphate, sulphide
and oxide of zinc is a patent white made by subliming galena in an
oxidising furnace or hearth. This compound lacks body.

All of these so-called substitutes are very inferior to white lead,
not only as to quality but as to cost. They cannot compete with white
lead. A committee of enquiry on these substitutes for white lead,
reporting the result of their enquiry and examination, stated that they
found that these were mostly prepared with varnishes before they were
sold for use, and that in most instances they were mixed with a large
quantity of driers, and that the drier invariably was a compound of
lead.

The principal consumption of white lead is for paint; to produce this
paint it is ground with oil in varying proportions, about 8 to 15 per
cent. This produces the ordinary white lead in oil, and is worth from
19_l._ to 20_l._ a ton, but often more than this amount.

Dry powdered white lead is chiefly made for and used by grinders
and mixers, who combine with it a variety of other cheaper
materials--chalk, clay, sulphate of lime, and sulphate of baryta, but
principal use is made of chalk and barytes. These are mixed with the
white lead, and then the mixture is ground with oil and formed into
paint, sold under various names according to quality: thus--guaranteed
white lead, firsts, seconds, thirds, and fourths, the proportion of
white lead diminishing, and that of the adulterant increasing, as we
descend from the pure material. Guaranteed and best white lead is
not pure, and does not mean pure white lead. Pure white lead can be
purchased at some makers, but its price, if pure, can never fall below
19_l._ or 20_l._ per ton.

To sophisticate white lead, and produce the various inferiors named,
dry powdered white lead is needed as a starting point, and for this
purpose principally arises the necessity for its production. If ground
in oil the adulterants cannot be properly incorporated with it. Dry
white lead is used for nothing else that could ever give rise to any
great demand for it. We have already observed that the production of
this dry and powdered white lead is the most dangerous proceeding
connected with this industry. Grinding in oil is unattended with any
important consequence to the health and comfort of those employed. A
serious drawback to the “stack” production, the china-like incrustation
to which reference has already been made, is that it requires crushing,
grinding, washing, and drying, and a second course of dry grinding
after it is dried--the most objectionable step in its preparation.

Could the corrosion of the blue lead be effected in such a way as to
prevent any discoloration of the material by the tan and wood--could
the corrosion be so produced as to be easily separated from the buckle
or grating on which it has formed--could this separation be so effected
as to prevent the breaking up of the lead skeleton, and the presence
of small pieces of metal in the detached crust of white lead, two
principal reasons for washing and drying are removed.

There is yet another consideration, that is, the presence of acetate
of lead, always found in varying quantities in the incrustation
produced, and remaining at the close of the operation and conversion.
To remove this, careful washing, and after-stoving and drying must be
accomplished. The amount of this salt present is found to differ with
each operation, and in various portions of the same make.

The washing out of the acetate is never perfect, and it involves a
large amount of labour.

Opinions differ as to the effect of this acetate if allowed to remain
in the product. White lead makers on the “stack” principle aver that
it should and must be washed out, lest it should damage the qualities
of the paint. This is questionable, and not one can produce practical
evidence of its being the cause of any damage if still contained
in white lead. Facts seem to deny its harmfulness in this respect,
inasmuch as the best prepared samples, those washed and dried from the
most careful makers, will be found upon analysis to contain more or
less of acetate of lead.

A large proportion of this salt in white lead may not be beneficial
for many reasons, but a small percentage can do no harm; nay, for many
purposes it may be good.

There is no substance used for driers for white lead that is more
esteemed than this acetate of lead, commonly known as “sugar of lead.”

A small amount of this salt present in white lead would communicate
drying properties, and this alone is what it could do.

Granting that we can discover a method of producing white lead of
amorphous character, of good density, free from all discoloration, free
from all particles of metallic lead, and free from all but a small
percentage of acetate of lead, then washing will not be needed.

Stoving and drying become unnecessary. The work of women, their deadly
occupation, so burdensome to the operatives and to all with whom they
are concerned, is done away with.

_Condy’s Process._--An improvement in the manufacture of white lead
was patented by Condy, of Battersea, in 1881, which, though giving
perfectly satisfactory results when carefully conducted, necessitated
special precautions, and led to his substituting in practice the
following additions and modifications, which are of great consequence
in rendering the process more certain in the quality of its product,
and more valuable as a commercial manufacture on the practical scale,
by virtue of its offering greatly increased facility and economy in
production.

The results of numerous and repeated experiments on the larger scale
induced Condy to qualify the recommendation contained in his first
patent, viz. that of employing a solution of tribasic acetate of lead
and bicarbonate of soda in proper proportion to precipitate nearly the
whole of the lead, and further stating that he preferred to employ
“a slight excess of tribasic lead salt rather than find carbonate of
soda in excess.” Though, when carefully conducted, if the greatest
nicety is observed, a satisfactory result is obtained; in practice, the
least variation from the exact composition of the two substances is
attended with the drawback that the white lead is liable to a slight
uncertainty of tint after it is ground in oil, whereas by the process
hereinafter described a positive and reliable result can be obtained,
as the white lead produced will be of a uniform white colour, and not
liable to turn when ground in oil. Though the earlier process was in
itself complete for the manufacture of white lead from oxide of lead,
it afterwards occurred to Condy that a great object would be attained
in rendering the process more valuable and more practical, if a method
were devised, worked out, and described for the manufacture of tribasic
acetate of lead to be made entirely from metallic lead by the action of
acetic acid or neutral acetate of lead on metallic lead, and not to be
dependent on the employment in any way of previously manufactured oxide
of lead.

This portion of Condy’s invention relating to the manufacture of
tribasic acetate of lead may be described as follows: he melts, and,
after skimming carefully, feathers the metallic lead by dropping it
into water; he places this granulated lead in wooden vessels or vats
previously fitted with perforated false bottoms under which are fixed
taps for drawing the liquor off into other vats or tanks placed on a
lower level. Having filled with granulated lead the vessels fitted with
the false bottoms, he fills up the interstices with a dilute acetic
acid composed of one part, by weight, of acid (specific gravity 1·045
at 60° F.) and 12½ parts of water, and after allowing the dilute acid
to stand for two hours, draws it off through the taps into the lower
tanks. This allows access of atmospheric air to the lead, which has the
effect of heating the lead so that oxidation takes place.

After a time (about three or four hours), this oxidation begins to
slacken, when he pumps up a second time the acid solution from the
lower vat on to the granulated lead, and allows them to stand in
contact for one hour; he then again draws off the liquid into the lower
tank, and again exposes the metallic lead to atmospheric oxidation,
allowing three or four hours for the latter operation; and if the
solution of lead has not already attained the specific gravity of
1·040, at 60° F., he again repasses the liquor over the metallic lead
partially oxidised, until it has attained that specific gravity, when
he places the dissolved lead with fresh granulated lead and recommences
the manufacture in the same way.

This operation succeeds much better on the large scale than on a
laboratory scale. In vats containing upwards of one ton of lead, the
result is all that can be desired, and can be obtained by passing the
liquor from twice to three times over the metallic lead partially
oxidised. A little practice enables the operator so to control the
process that he can obtain the solution of the desired specific gravity
with perfect ease.

This plan of manufacturing tribasic acetate of lead possesses the
advantage of producing that substance wholly or nearly wholly free from
the impurities contained in metallic lead, such as copper and silver,
which are not taken up, or soluble, in the presence of metallic lead.
In consequence of the circumstance that foreign matter is left almost
untouched, it is practicable to make white lead of a fine quality from
old lead such as lead piping, roofing, and worn out lead generally,
which can thus be utilised to greater advantage than in any other way.

Having obtained this solution of basic acetate of lead of the specific
gravity of 1·040 at 60° F., Condy proceeds as follows:--To the solution
produced by each 60 lb. of acid and 750 lb. of water previously
pumped up into another vat or tank, he adds bicarbonate of soda in
the proportion of 30 lb. for each 60 lb. by weight of acid originally
employed, and agitates the mixture. This will generally precipitate
all the white lead, but it is necessary to test the filtrate to
ascertain the exact point when all the lead is thrown down. Sufficient
bicarbonate of soda should be added to do this completely, and it would
be better to use bicarbonate of soda in excess rather than leave any
lead unprecipitated, as by this means greater certainty is obtained
in securing on the large and practical scale a white lead capable of
standing the effect of light and grinding in oil without changing. The
white lead after precipitation can be washed, pressed, and dried in the
usual way.

The following variation may be made from the method described of making
tribasic acetate of lead, thus:--To each 60 lb. of acid and 750 lb. of
water may be added sufficient of the tribasic solution to make neutral
acetate of lead, with which to recommence the manufacture of tribasic
acetate of lead by the process described. Vague reference has been made
in works on technical chemistry to the possibility of using metallic
lead in the manufacture of sugar of lead, but such references have been
practically worthless, as they contain no information of a practical
nature even for the manufacture of neutral acetate of lead, and no
process at all has ever been described for the manufacture of basic
acetate of lead from metallic lead acted on by acetic acid or acetate
of lead.

_Gardner’s Process._--The conditions observed and fulfilled in the
arrangements adopted by Prof. E. V. Gardner, are founded upon a study
of the nature, properties and behaviour of the substances concerned,
under certain methods of treatment.

There are several oxides of lead which may be formed under special
conditions.--(1) Pb_{2}O, and the same oxide combined with water
Pb_{2}H_{2}O_{2}; (2) PbO, and the same oxide combined with water
PbH_{2}O_{2}. In the hydrated form these oxides combine readily with
carbonic acid, but they do not combine readily with carbonic acid when
dehydrated. The hydrates are most readily formed at about 120°-130° F.,
and are decomposed after they are formed if heated to 212° F.

These oxides and their hydrates combine with acetic acid to form
acetate and sub-acetate of lead, and with nitric acid to form nitrate
and sub-nitrate of lead.

Lead, submitted to the action of air, watery vapour, and acetic or
nitric acid, or a mixture of these acids, with air or oxygen, with
proper precautions, forms sub-acetate or sub-nitrate of lead, and this
sub-acetate or sub-nitrate of lead readily absorbs carbonic acid and
forms carbonate and sub-carbonate of lead.

Sufficient acetic or nitric acid, or a mixture of these and air or
oxygen, and watery vapour, must be constantly supplied to form the
subsalts of lead, to carry on the operation of converting blue lead
into white lead; but an excess or an insufficiency of these agents will
in either case prevent the formation of sub-acetate or sub-nitrate, and
consequently of the sub-carbonates.

Thus--too little air, acetic acid and aqueous vapour prevents the
formation of the hydrated sub-acetate and consequently sub-carbonate
of lead. Too much acetic acid and aqueous vapour, and too little air
forms an acetate on the surface of the lead, which, by the excess of
water, dissolves and wastes, and washes the lead; it also varnishes
the surface of the lead with a coating of acetate, and checks, if it
does not completely prevent, the formation of sub-acetate of lead, and
consequently the formation of carbonate and sub-carbonate of lead.
Similar rules hold good in the case of nitric acid and the formation of
nitrate and sub-nitrate.

The process of forming the hydrated sub-acetate or sub-nitrate of
lead is most energetic at a temperature of 120°-130° F. In a lower
temperature, a much longer time is occupied in carrying out the process
of conversion; while at a higher temperature the delicate sub-acetate
or sub-nitrate of lead first formed suffers loss of water, until, at
212° F., it is completely dehydrated. At a temperature about 135° F.,
the power of forming carbonate and sub-carbonates is lessened, and at
212° F. is considerably diminished. The carbonate itself is dehydrated
at 212° F., and is decomposed at a higher temperature.

To obtain a white lead of excellent quality for its various uses, it
is necessary to produce a substance which possesses sufficient body to
cover surfaces to which it may be applied as paint, and it must possess
sufficient base to combine with the oil of the paint to form a vehicle
or varnish to retain and hold the body on the surface. This is found to
be the case with sub-carbonate of lead, or especially with a compound
constituted of two or three equivalents of carbonate of lead, with one
equivalent of hydrated oxide of that metal.

Lead, to be attacked by chemical agents, should possess a clean
surface. If the surface of the lead is _chemically_ clean, so much the
better. The surface of the lead should be extended as much as possible,
and exposed to the action of the gases and vapours at a certain heat
for its conversion into white lead.

If the most favourable conditions are sought, the temperature should
be between 120° and 130° F.; and to carry on the chemical action
most satisfactorily, the lead during its conversion should present a
granular coating--that is, the coating formed by the chemical agents
should be granular and not smooth and continuous in character. It
should be porous, and not of a continuous varnish-like or vitreous
character.

To rapidly carry on the chemical action, the chemical agents should
be well diffused and commingled throughout each other, and the blue
lead should be so exposed as to be open to their attack on all parts
of its surface equally, and all should be at, and kept at, a proper
temperature and a proper degree of humidity. Too dry a heat prevents
the process of conversion. Too moist an atmosphere wastes the materials
and arrests their action. The conditions, therefore, which are most
favourable will be a certain humid atmosphere of well diffused and
commingled vapours or gases, acting on metallic lead exposed to them
under the physical conditions described, and at a temperature between
110° and 135° F.

These favourable conditions can be further augmented by certain
electrical arrangements in connection with them.

Again, in the sources from which the carbonic acid is derived, and
in the arrangement of the apparatus for applying the carbonic acid,
favourable and unfavourable conditions can be imported. Thus, by using
paraffin, petroleum, benzine, or light oil of paraffin or petroleum,
or similar carbonaceous substances free from sulphur, or mixtures of
such carbonaceous substances either alone or mixed with air or other
oxidising substance, comparatively pure carbonic acid may be furnished
without at the same time producing any objectionable compounds.

As a result of his researches and experiments, Prof. Gardner has
proposed (Eng. Pat. 1882, No. 731) certain improvements in the method
of converting blue lead into white lead, in the apparatus employed,
and in the method of making and applying the carbonic acid used in the
process. Some of these improvements are applicable to the open stack or
chamber process, while others relate to closed chambers.

Preferably, for the conversion of blue lead into white lead, Prof.
Gardner adopts a closed, but not air-tight, chamber. This chamber is
ventilated or relieved so as to enable the incoming gases and vapours
to enter it without hindrance, and to escape by means of an exit valve
and pipe, or an exit shaft, connected with the chamber, communicating
with the exterior air, and regulated by a valve or damper. The gases
and vapours within the chamber can find their way out by this exit
shaft only, by slight pressure on expansion from within the chamber;
thus the interior of the chamber is preserved from disturbance, by
preventing the formation of currents within its atmosphere, and yet a
perfect circulation of the gases and vapours is kept up.

To warm the interior of this chamber, which is constructed of any
material that will resist the action of the vapours and gases, and
which is not too absorbent, Prof. Gardner makes the bottom of the
chamber of such a shape as to form a heating vessel to hold water
or steam, this water or steam, or both, being kept at the required
temperature by means of a steam coil. Matters are so arranged that the
contents of this coil are protected from any excess of pressure, and
consequently the temperature seldom or never exceeds about 212° F.,
unless for any special reason it is desired to raise it to a higher
point. Sometimes the sides are constructed similarly to the bottom.

The materials of which these chambers are constructed must be capable
of resisting the heated and acidified vapours within them. Cast or
wrought iron faced with glass, slate, tiles, pewter, or glazed bricks;
tinned copper, tinned brass, or pewter; timber, whether green or after
carbonising by heat or sulphuric acid; all are more or less suitable.
Means of observing the progress of the conversion, and means of lifting
the contents in or out, must also be provided, as well as thermometers
to indicate the temperature prevailing inside.

The gases and vapours which effect the conversion of the blue lead
into white lead are generated outside the chamber just described, and
are conducted into it by means of pipes, being first raised to such a
temperature in excess of that which should exist inside the converting
chamber as to allow for the cooling effect arising from the friction
and loss of heat in passing through the various pipes and distributors
attached to the converting chamber.

Owing to this extra heating, the gases and vapours are expanded,
diffused, and commingled, and do not rob the interior of the converting
chamber of any heat on entering it, so that the heat inside the
converting chamber is kept constant, and can operate to further expand
and diffuse the gases and vapours in contact with the blue lead inside
the chamber. In practice it is found that the temperature of the
converting chamber cannot be suitably controlled if any portion of the
gases or vapours be generated within that chamber.

The blue lead is arranged in the converting chamber in trays, or on
shelves or frames, so as to allow it to be completely surrounded and
attacked by the vapours or gases, and thereby be converted into white
lead, at the same time preventing the formation of direct currents
or eddies. Framed supports resembling a dinner waggon serve well for
holding the lead, and may easily be arranged to lift bodily in and out
of the chamber with their burden of blue or white lead. The surface on
which the blue lead is directly supported is made of graphitic carbon,
hard coke, platinum, or carbonised or platinised material, such as is
used for plates in electric batteries, or of other material standing
in a similar electrical relationship to lead, or capable of generating
with it an electric current.

In applying this development of electrical energy to the ordinary
“stack,” as for instance the Dutch process, the pots containing
the acetous liquid and blue lead are made of, or lined with, such
electrical carbon, or contain a portion of it in suspension, by which
the same effect is realised. Advantage may also be derived from
furnishing a supplementary supply of carbonic acid to the stack beyond
that due to the decomposing dung, &c.; as well as from injecting
a current of air or oxygen at suitable temperatures, and from the
admission of steam in a coil throughout the stack.

The generation or production of the acetic or nitric acid vapours is
attained by heat in a vessel so arranged that its contents are kept
at a certain temperature for vaporising, by having the boiling acid
solution at about 1·003 sp. gr., procured by mixing water with vinegar
or acetic or nitric acids.

The supply pipes conveying the air or oxygen and the carbonic acid, or
either of them, can easily be made to emit their contents close to the
boiling acid solution, whereby the vapours arising from the latter are
mixed and intermingled with them, and all pass together through the
pipes and distributors into the converting chamber or stack.

In preparing the carbonic acid it is necessary to observe certain
precautions, especially that it shall be pure and that no carbonic
oxide shall gain admission to the chamber. If the ordinary chalk and
carbon method be employed, the furnace must be provided at the uptake
with a series of air inlets, so as to ensure that the gases passing
from the furnace to the delivery tube shall be most thoroughly oxidised.

The purification of the carbonic acid may be effected in the usual way
by passing it through a vessel containing water; or, if requisite, it
may be cleansed, expanded, and heated in one operation by passing it
through or over hot water, or over a hot aqueous solution of carbonate
of soda, all of which, however, incur considerable cost in plant and
manipulation.

Owing to these difficulties in purifying carbonic acid, Prof. Gardner
prefers to prepare a practically pure carbonic acid in the first place,
and this he does by allowing petroleum, benzine, paraffin, or other
hydrocarbon or carbonaceous liquid to gradually fall into a retort
containing chalk or other suitable carbonate at a high temperature,
whereby relatively pure carbonic acid gas is generated; and while still
in a highly heated state it encounters a stream of air or oxygen,
ensuring its complete combustion before entering the converting chamber.

Another method of procuring fairly pure carbonic acid is by the
combustion and oxidation of any of the liquid hydrocarbons mentioned
above, in suitable lamps; and when carbonic acid gas of exceptional
purity is required, it may be obtained by heating bicarbonates to a
sufficient degree to drive off one molecule of carbonic acid, reducing
the bicarbonate to carbonate, from which it can be reproduced by
treatment with carbonic acid gas obtained from cheaper sources.

The _modus operandi_ adopted by Prof. Gardner is as follows: The
lead is granulated and prepared for conversion in one operation,
thus--an iron or a slate slab about 2-3 inches thick is placed in a
tank containing acetic or nitric acid solutions rising about 3 inches
above the upper surface of the slab. The lead is melted at low red
heat and poured from a height of 4 to 6 feet into the acid solution,
through which it falls till it encounters the slab, and thereupon
passes away into the surrounding solution, being thus converted into
a spongy condition. In this condition the lead is spread on frames or
trays, which are then lifted bodily into their places in the converting
chambers. The latter is closed and heated to 120° F. for 3-4 hours, or
until the whole contents have assumed a uniform temperature. Thereupon
the acid vapours and air are admitted and distributed, taking care that
the temperature is not thereby reduced below 110° F. nor increased
above 125° F., steam being temporarily shut off if necessary. This
treatment is continued for 24 hours.

The admission of aqueous vapour should be so regulated that while a
dry atmosphere is avoided, yet there is no appreciable condensation of
moisture in the chamber. While ensuring this condition, the temperature
may reach as high as 130° F. for a second period of 24 hours, but must
not overstep the limits of 120° F. minimum and 135° F. maximum.

After 48 hours’ treatment, the supply of duly warmed carbonic acid
gas is admitted for a period of two hours, without discontinuing the
introduction of acid vapours, air and steam; and this addition of
carbonating gas is repeated for two hours at a time with intervals
of four hours during which it is cut off. When, after four or five
days, efflorescence or exfoliation appears on the lead, the supply
of carbonic gas is increased to two hours in every four, or four in
every six; and the admission of acid vapours, air and steam may also
be augmented so long as the temperature is not allowed to exceed
130°-135°F. The whole operation is completed in seven to fourteen days.

Of this process, Mr. Carter Bell, in his paper before referred to,
speaks in the highest terms. No washing or drying is necessary. No
women are engaged in the manufacture, and but few men. The white lead
thus produced by the aid of electricity is deposited in a peculiar
state of disintegration, it is perfectly amorphous and non-crystalline,
of the purest quality; its density is 5·8. When ground in oil and made
into paint, it possesses great body and a covering power inferior to no
other paint, if not superior to them all.

Painters who have used the paint, practical men, and amongst these
we may observe coach painters, have pronounced its excellence and
superiority to the best ordinary white lead paint.

By this electrical process of manufacture, not only is the time
consumed in the making and in the preparation of this material greatly
shortened, but the cost of preparation is reduced, and added to this
is the important fact, the vital factor in our consideration. The
labour of women is unnecessary. No lives are sacrificed to its working
requirements.

Prof. E. V. Gardner, who has been for some years occupying his
attention with the subject of white lead making, with a view especially
to remedying its existing evils, has invented his electrical chamber
process of manufacture, and an entirely new course of after treatment.
He has for the last seven or eight years been more or less occupied
perfecting his conception, and accommodating it to practical and
commercial claims.

Chamber processes are not new, there have been several patents
enrolled for making white lead in closed chambers, but none has proved
commercially convenient or practically successful in its adaptation,
and none has survived to the present time.

In Germany, white lead is made in chambers at the present day. The lead
in gratings or sheets is supported on wooden rods, saddle fashion, the
chamber is then filled, and its contents are submitted to currents
of acetic acid vapour, air, steam, and carbonic acid gas; the time
needful for conversion is six or seven weeks. The after steps in the
separation of the incrustation, and its preparation for trade purposes
are much the same as in the “stack” product preparation. It is washed,
stoved, dried, and ground. The white lead made on the German plan does
not differ in any material degree, save it be in price, from the best
English commodity. We may assume that the washing and drying in Germany
consumes a like period of time to that process in English works, viz.
two or three weeks. We then see that the German plan of making white
lead cannot be perfected in less than eight or ten weeks’ time from the
commencement of the corroding action in the chamber. To compare these
facts and the efficiency of the different plans described:--

The time required to complete the corrosion in the stack is at least 14
or 16 weeks.

The Gardner’s electric process requires for the same purpose only 14
days.

As to point of time, the German plan excels the stack, and can be
carried out in one-half the time required in the stack method of
conversion.

The Gardner’s electrical method excels the German, and can be perfected
in one-third the time needed for the German chamber operation, and
one-sixth the time required the stack.

These figures open out a most important matter when we regard the
capital invested and lying dormant in stack lead works.

It is well known that we have in electricity a most powerful agent
by which to effect the chemical combination of various substances on
the one hand, or on the other, by its means to break up and disrupt
a chemical compound. Professor Gardner’s main principle of action in
his new process is founded upon these facts, and he takes advantage
of electrical power to cause the combination of the lead with the
necessary elements to build up white lead in his chambers. He either
employs electrical discharges to energise and render active in their
chemical affinities the various materials engaged, or he so disposes
of them as to form an electric or galvanic combination in the chamber.
In the latter arrangement the chamber and its contents represent a
gas battery on an extensive scale. In practice he prefers the latter
plan; it is more simple, more manageable in the hands of the ordinary
workmen. The original plan was to have graphite or graphitic carbon
plates or supports for the buckles or gratings of lead within the
converting chamber. These carbon plates and the lead to be converted,
were so placed as to form a collection of galvanic “couples” or
“pairs,” and in this condition were submitted to the gases entering
the chamber from without. It will be understood that various modes of
arrangement would occur without departing from the principle concerned.

In practice this method answered very well, but presently a
difficulty arose; not only were the graphitic carbon or graphitic
plates expensive, but they were easily broken, and became friable
in use. Carbon plates of an especial kind were manufactured to meet
these failures and remedy these defects, not without success, but
still open to objection. In looking round for some substitute to
replace the carbon, two points were to be kept in view, to seek some
electro-negative to lead like the carbon, and some electro-conductive
like it, and some material that would bear rough handling such as
workmen give, and be practically convenient in its adaptation to a
working chamber.

Gold or platinum would excel in these particulars, but there are
considerations which debar their use. Pure tin presented itself, and
tin was tried with great success. Tin would at first be thought,
on account of its close electrical relationship to lead, far from
favourable to the purpose. Graphite or carbon would appear far more
suitable. Practice pronounced the tin to act as efficiently as carbon.
This may at first seem contradictory and strange, but if we consider
that while the carbon is certainly more highly electro-negative to
lead than tin, yet the tin is the more conductive, and offers the less
resistance to the electric current of the two; in this manner the tin
compensates by its conductive power all it may lack, as compared with
carbon, in electrical energy when coupled up with lead.

Tin plate is now used as the electric-negative element in the chamber
of the Gardner plan. Tin plate means pure tin. Ingot tin is rolled out
into plates, the bottom of the chamber is covered with this pure tin
plate, so are the bearers and the shelves, or supports which are to
hold the lead during its conversion. Tin pipes and tin fittings, which
resist the action of acetic acid, are also used to conduct the gases to
the chamber to carry on the converting operation, and to preserve the
product from any source of discoloration.

When a chamber is prepared for the converting operation, the whole of
the lead it contains will be in metallic communication with the tin
supports, and these with the tin covered bottom of the chamber.

The chamber when working is kept at a certain temperature by a steam
coil beneath the floor of the chamber. The process is simple. The lead
buckles or gratings are placed on tin-covered stands, somewhat in form
and make like a dinner-waggon. The whole is hauled up and dipped into
a bath of acetic acid and acetate of lead; it there remains for one or
two minutes, it is then hauled out, drained and lifted bodily through
the top into the chamber. Other stands filled with buckles are so
dipped and so placed till the space of the chamber is fully occupied.

This dipping cleanses the surface of the metal, and when it is exposed
to the air it is speedily coated with a hydrated oxide of lead. This is
the first step in the process of conversion. The chamber when filled is
closed, and its temperature is brought to about 100° F.; then vapour
of acetic acid and vapour of water and air are supplied from without
to the interior of the chamber. This is continued for 15 or 20 hours.
The lead buckles within the chamber will now possess a whitish coating,
consisting of subhydrate and subacetate of lead, and they will present
a uniform colour. Carbonic acid generated in any convenient manner is
next passed into the acid generator; it mixes with the other gases and
vapours, and with them goes on its way to supply the chamber. Speedily
the action of the carbonic acid is observed, the surface lead becomes
quite white and presents the appearance of a snow shower having fallen
within the chamber. The formation of white lead is now speedily
effected. This treatment is continued throughout the space of 13 days;
at the end of this time the supply of acetic acid vapour is stopped,
and the supply of air, steam and carbonic acid is continued, according
as it is desired to obtain white lead rich in oxide or in carbonate.

After a short further period, steam and air only are sent into the
chamber, which is varied in temperature to 120° or 130°F., and lastly
the steam supply is stopped; air alone enters the chamber, which is
kept heated by the coils beneath the floor. The contents of the chamber
are now in a dry state, and the operation is terminated.

It will occur to most readers that these terminal proceedings amount
in effect to a convenient method of washing and drying the white lead
while it is still attached to the parent lead, and this it is in fact.

The contents of the converting chamber are lifted out through the
opened top, and the buckles or gratings with their crust of white lead
are turned into the agitator. This agitator is an iron cage revolving
inside a closed chamber of the same material. During the revolution
of this cylinder or cage, the contained lead gratings fall from side
to side, and the incrustation on their surfaces becomes detached and
broken up. It falls in, this broken state through the bars of the cage
or cylinder into a receptacle beneath. The denuded buckles or gratings
are retained in the cylinder and are removed. These gratings or buckles
are cast of such a thickness as to withstand two or three converting
operations in the chamber before they are recast.

This crude white lead is carried by an elevator, or it falls into the
hopper of a pair of granite crushing rolls, also enclosed; and from
these it passes into the mixer or incorporator from which it can be
removed in a dry state or mixed with oil.

The incrustation of white lead will be found upon examination to be
possessed of some peculiarities, the result of the electrical action
which has been going on within the chamber. It is quite white. It
falls from the lead buckle or grating which it coats, if the grating
be struck against a piece of wood with but a slight blow. It is easily
friable, and can be rubbed to the finest powder between the thumb and
finger, or on the palm of the hand.

Now, we may explain, as we conceive it, the philosophy of its
production in this state of disintegration.

We know that a feeble and prolonged current of electricity will in
time deposit metals from their solutions in a crystalline condition,
and that if we quicken the current of electricity and cause it more
energetically to act on the same solution, we can precipitate the metal
from that solution in a state of powder.

It is to similar action of electricity as that to which we last refer
that we ascribe the formation of the crust on the gratings of lead in
the non-adherent and disintegrated condition in which it is produced,
and by reason of which it is so easily detached from the lead and
broken up to powder. No edge runner grinding, such as is required by
the stack process, is in this case necessary.

The crude white lead and crushed material, whether in a dry state, or
incorporated with oil, is finished and ground in a granite roller paint
mill, from which it issues as dry white lead, or as white lead in oil.

Paint made from this electric white lead has been sent to America, to
France, to Belgium, to Germany for trial, and has also been largely
tested in this country by painters, engineers, and others unacquainted
with its precise nature, and it has been productive of good results.

Of its density, body, and covering power, there can be no doubt, and
never once have these qualities been called in question.

The cost of the manufacture of white lead by the stack process is about
3_l._ to 3_l._ 10_s._ per ton. By the German method, the cost is about
the same as by the stack. By Gardner’s electric process, the cost of
conversion is 10_s_. a ton. To this 10_s._ must be added the cost of
labour expended in preparation, an item which cannot be well estimated
on the present limited scale of manufacture; it could not exceed an
additional 15_s._ a ton. This would bring the cost of manufacture of
electric white lead to 25_s._ a ton.

In this electric process inferior lead can be operated upon with
success. Brands of that metal such as white lead makers by the stack
method dare not employ, may be successfully converted in an electric
chamber, and with fair results as to the quantity and quality of the
white lead produced.

By the use of Gardner’s electric process it would appear that we not
only preserve health, but save lives; we not only save time, but
interest on large capitals, which lie idle for long periods at a time;
and we can economise and simplify the whole manufacture and preparation
of white lead, divesting it of all its present cumbersome and unhealthy
stages. Gardner’s process, we believe, must take a prominent position
as one of the most necessary, valuable, and scientific inventions of
modern times.

_Hannay’s Process._--Mr. J. B. Hannay, whose name is well known in
connection with various chemical and engineering inventions and
processes, has recently brought out a process for the manufacture of
white lead. The old method of producing white lead or carbonate of
lead is one involving much time and labour, together with no small
risk to the health of the workpeople. By the new process brought out
by Mr. Hannay, sulphate of lead is manufactured direct from galena or
lead ore, without the necessity of the intermediate process of the
reduction of the ore and the extraction of metallic lead. It is said
that the sulphate of lead produced by the new process is whiter and
more permanent than the carbonate.

The process is described as follows:--A furnace, 36 in. by 30 in., and
48 in. deep, contracting to a narrow chamber about 36 in. long by 14
in. wide, communicating with the main flue, is charged with coke and
brought to a red heat. The bed of coke is so thick as to be almost
up to the level of the sill of the furnace door. The lead ore is not
charged in large quantity and left for an indefinite time, but is
thrown in in quantities of a few shovelfuls at a time, the object being
to effect extremely rapid volatilisation and consequent oxidation. The
proportion of ore to fuel is 1 ton to 1 ton, and the result is said to
be the conversion of 95 per cent. of the charge into its equivalent
in white lead. In the first or wide chamber, the coke is oxidised,
carbonic oxide being the resulting gas, the rapid volatilisation of
the galena or sulphide of lead also taking place in the same chamber.
On entering the inner portion of the furnace, the volatilised sulphide
of lead and the carbonic oxide are converted into carbonic acid and
sulphate of lead.

Forced blast has been employed to cause the high temperature necessary
for the volatilisation of the ore; but it has been found unnecessary,
admission of air at atmospheric pressure through tuyere holes being
quite adequate. After leaving the inner chamber of the furnace, the
gases pass into a flue, level with the furnace, and about 40 feet long
by 16 square feet in sectional area. From this flue the gases pass into
a tower about 20 feet high, and from thence into wrought-iron flues 3
feet in diameter. These flues terminate in a wrought-iron chest, in
which are fitted two steam injectors. The gases are forced by these
injectors into the central chambers of the condensers. These condensers
are two in number, and contain a central chamber, 16 feet by 12 inches,
into which the gases are forced as described. The gases escape through
interstices into the outer chambers of the condenser, and are there
condensed, a continuous stream of water occupying the lower part of
the condenser. The waste gases escape by a downcast leading to a tall
chimney. The temperature of the gases as they enter the condenser is
about 840° F.

The product is pumped from the condensers to a settling vat. Here it
settles for an hour, the deposit being pumped into a second vat and
washed with dilute sulphuric acid, in order to remove impurities. The
resulting product is washed several times with water, and is then
passed through a filter press. From the press it drops into bogies,
which carry the pulp to the drying house, where it is dried by hot air.
The process would thus appear an extremely simple and practical one.
Several chemical authorities of repute, as well as manufacturing firms
and others who have used the new white lead, have expressed themselves
strongly as to its merits. One point immensely in favour of sulphate
of lead as opposed to carbonate, is that the former is almost entirely
innocuous, whereas the poisonous properties of the other are well
known. It is claimed that the sulphate is not acted upon by coal gas or
by the atmosphere of towns, which is always more or less impregnated
with gases such as sulphuretted hydrogen, whose reactions with various
metals have only too good reason to be known.

According to the patent specification bearing the names of French and
Hannay, they employ lead ores, lead fume, or lead slags containing
sulphur; and when these materials do not contain sufficient sulphur to
form a sulphate with all the lead which sublimes in the process, they
add to them pyrites or other sulphur-yielding substance to make up the
deficiency. They heat the materials mixed with a suitable proportion
of coke in an air-blast cupola furnace, which is by preference of
an improved and special construction shown in Figs. 18 to 20, and
hereinafter described; and they thereby produce sulphite of lead as a
sublimate, provided that there are no chlorides such as common salt
present in the charge, in which case sulphate and chloride of lead will
be formed.

The sublimate is carried forward with the current of gases through
flues to a fume condenser, which is by preference of the kind known
as Wilson’s and French’s. As the gases and sublimate pass through the
flues, hydrochloric acid is mixed

[Illustration: Fig. 18.

Fig. 19.

Fig. 20.

FRENCH AND HANNAY’S WHITE-LEAD FURNACE.]

with them, being by preference formed in a chamber in connection with
the flue, by introducing a solution of chloride of sodium in spray,
and by providing a sufficient excess of sulphurous acid beyond that
required for forming the sulphite of lead. Air is also present, and
a well known reaction takes place, yielding hydrochloric acid and
sulphate of soda, the operation taking place at a part of the flue near
enough to the furnace to be always at a red heat. The hydrochloric
acid thus mixed with the gases and sublimate causes the formation
of chloro-sulphite of lead, or other combinations of lead, sulphur,
oxygen, and chlorine of variable constitution, depending on the
proportions of the several constituents, but in most cases the product
is a body which forms a white pigment of extremely good quality.

Fig. 18 is an elevation of their improved cupola furnace; Fig. 19 is a
corresponding vertical section; and Fig. 20 is a horizontal section as
at the level of the principal tuyeres.

This cupola furnace is formed with a deep hearth _a_ (like the American
lead smelting cupola) having a siphon outlet _b_, for withdrawing
molten lead when necessary. At a level a little above the siphon outlet
_b_, an outlet _c_ for slag and scoria is provided, and the main blast
tuyeres _d_ enter at about the same or a slightly higher level. With
this arrangement, a considerable depth of melted lead is constantly
maintained, and the choking of materials is thereby avoided at the
level of the tuyeres. For a short distance above the main tuyeres _d_,
the interior of the furnace is made with the sides _e_ moderately
and gradually widening upwards, and is afterwards continued upwards
of uniform diameter or width. The charging door _f_ is between 3 and
4 feet above the main tuyeres _d_, and the space above the charging
door is crossed by two or more arched diaphragms _g h_, of brickwork,
having irregular openings in them, there being doors _i k_ in the side
of the furnace above each diaphragm, for inspection and cleaning. The
purpose of these diaphragms is to cause the air, gases, and sublimate
to become thoroughly intermixed and to ensure the complete combustion
of any black smoke and the oxidation of any sulphide of lead that may
be present.

An upper series of small tuyeres or jet pipes _l_ is provided for
admitting air a little below the lowest diaphragm _g_, these jet pipes
_l_ being supplied by a branch pipe _m_ from the main air blast pipe
_n_, it being found that the introduction of the air and the production
of heat can be best regulated by supplying a portion at the upper part
of the furnace in this way, in addition to that supplied by the main
tuyeres _d_; and a tap or valve _o_ is fitted on the branch pipe _m_
for adjusting the supplementary supply thus admitted. Above the highest
perforated diaphragm _h_, the interior of the furnace communicates with
a lateral flue _p_, through which the gases and sublimate pass; and at
a part of this flue _p_, sufficiently near the surface for the gases to
be still hot enough, an enlargement or chamber _r_ forming a descending
part of the flue, is constructed. Into this chamber _r_ a solution
of chloride of sodium is introduced as a spray from a number of jet
pipes _s_, for the purpose hereinbefore explained. This chamber _r_ is
provided with a door _t_ at its lower part, for periodically removing
matters that become deposited in it. The continuation _u_ of the flue
communicates with the lower part of the chamber _r_.

Preferably the spray of chloride of sodium is fine enough to allow of
all or nearly all of the water being instantly evaporated, so as to
leave the salt in fine particles, and in a favourable condition for
being acted upon by the sulphurous acid, steam, and oxygen present in
the gaseous currents. The temperature of the chamber _r_ should not be
allowed to fall below red heat. The proportion of chloride of sodium
used will depend on the amount of chlorination of the lead that may be
desired, in addition to what is necessary for converting the zinc and
other metals into chlorides. In practice a proportion of salt 2½ to 5
per cent. of the weight of the sublimate formed answers the purpose
and yields a good product, but a larger proportion may be used without
injury.

It is of great importance to keep the temperature of the upper part
of the furnace steadily at a red heat and flaming, as the colour of
the sublimate will be inferior if the temperature is either too high
or too low. To facilitate the proper regulation of the temperature,
a pyrometer (which may be similar to the kind used in ironworks) is
employed, which pyrometer is placed in the flue at a distance from the
furnace where it cannot be injured; and by a few trials is ascertained
what temperature should be indicated by the pyrometer when the
temperature in the furnace is what it should be. This point having been
ascertained, a glance at the pyrometer will at any time show whether
the furnace is working properly or not. The inventors also provide
for rapidly cooling the upper part of the furnace without interfering
with the lower part, in the event of the heat becoming too great, by
arranging a water pipe _w_, with a set of jets, round the top of the
furnace, so that on turning a tap on the supply pipe a spray of water
may be applied to the outside of the furnace; and as it is desirable
that water applied in this way should not run down to the lower part of
the furnace they build gutter plates _x_ into the sides of the furnace
just above the charging door _f_, to lead off any surplus water to a
drain pipe.

Sufficient hydrochloric acid may be formed or introduced, as
hereinbefore described, not only for forming chloro-sulphite of lead
in the condenser, but also for saturating all the free oxide of
lead, and for combining with and rendering soluble any iron, zinc,
antimony, silver, or other metals. The chlorides thus formed become
dissolved in the water of the condenser, and the solution, separated
from the insoluble white pigment, may be treated by known processes
for recovery of the metals. When the lead ores or other lead-yielding
materials contain silver to a greater extent than 5 oz. per ton, a
notable quantity of the silver is volatilised, and if it is left in the
white pigment it renders the latter sensitive to sunlight; whereas if
rendered soluble in the manner hereinbefore described, and separated
by any of the known processes, it becomes a source of profit.

The white pigment is washed in the ordinary way; and when chloride
of zinc is not completely removed by washing, a small quantity of
sulphuric acid may be mixed with the pigment, by adding the same to
the last washing water, to convert the chloride of zinc into sulphate,
which is not hygroscopic.

The white pigment made as hereinbefore described is a very good and
economical material for manufacturing into chrome yellow, this being
done by mixing a solution of any suitable chromate or bichromate
with the wet pigment; whilst the chrome yellow thus obtained may be
converted into chrome orange or red by treating it in the usual way.

_Italian Process._--The precise period of the introduction of
white-lead manufacture in Italy is unknown, but it was certainly
previous to the beginning of the present century. Prior to 1881, the
Dutch process was exclusively used in Italy. In 1881 the so-called
Brumlen and Dahn process was introduced into Liguria. Somewhat later
the Rhenish process was introduced. The Rhenish process is one in use
in nearly all the Italian white-lead manufactories. It is employed in
a large manufactory at Cogoleto, as follows:--Lead in thin sheets of
about 3 feet in length, and 4 inches in width are placed in a clay
chamber having the form of a cube, of about the capacity of 5800 cubic
feet. In this chamber there is a wooden framework, upon which are hung
the sheets of lead. Three of these sheets weigh together about 4½ lb.,
and as the capacity of the chamber is about 20 tons, it can hold about
30,000 sheets of lead.

On the floor of the chamber are placed twenty-four copper receptacles,
each having four circular apertures. These receptacles are all in
direct communication with a large pipe of masonry, which, by means of a
copper tube, receives the gas coming from a boiler and furnace placed
under the chamber. In the boiler, which is also of copper, is placed a
mixture of 900 parts of water, and 80 parts of acetic acid concentrated
to 40°, and the capacity of the boiler is about equivalent to 25,882
gallons. The furnace serves the purpose of producing carbonic acid.

The gaseous mixture, consisting of volatilised acetic acid, carbonic
acid, and aqueous vapour, is admitted from the boiler and furnace into
the chamber in quantity and proportions best adapted to each stage of
the work. The chemical reactions resulting are analogous to those which
take place in the Dutch process, and the Brumlen and Dahn process.
Each operation lasts six weeks, and gives a product of about 20 tons
of white lead, with a consumption of 7 per cent. of acetic acid, and
9 tons of coke. The residuum of lead is about 10 per cent. of the
quantity placed in the chamber. At the end of each operation the white
lead taken from the chamber is washed and purified in large tubs, some
of which are furnished with filters. Finally, it is packed in earthen
vessels, and dried, when it is ready for the market, and is sold either
in cakes or powder.

The establishment at Cogoleto, above referred to, is able to produce
annually about 2000 tons of white lead, of which 1200 tons are produced
by the Rhenish process, and 800 tons by the Brumlen and Dahn process.
There is also in a manufactory in Milan, the process of revolving
heaters. The process of precipitation has not yet been used in Italy.
The Rhenish process, as above described, furnishes the greater part
of Italian white lead. The smaller manufacturers still use the Dutch
process, but the Brumlen and Dahn process is generally regarded as
unsatisfactory.

It is believed that the production of white lead in Italy during the
first ten years of this century was about 300 tons per annum. The total
annual production at present is, in round numbers, about 3500 tons, of
which 2800 are produced in the Ligurian manufactories, and about 300 in
those of Naples. Milan also produces about 300 tons.

The product finds a market almost exclusively in Italy. The annual
amount exported is about 300 tons, and goes chiefly to Constantinople.
Of the quantity of 2000 tons per annum produced at Cogoleto, about 300
tons is sold mixed with oil. No exact statistics are attainable as to
the total amount of Italian white lead which is annually sold other
than in the dry state, but it is believed to be little, if any, larger
than 300 tons.

When the price of white lead was high, sulphate of baryta was mixed
with carbonate of lead to produce white lead of inferior quality, but
in consequence of the present price of white lead the use of sulphate
of baryta has almost entirely ceased. No other methods of adulteration
are known to be in use. In Italy, white lead is universally known as
“_biacca_” when it is sold in cakes. When sold in powder it is known as
“_carbonato di piombo_” (carbonate of lead). The lead from which all
the white lead made in Italy is manufactured comes from the lead mines
of the island of Sardinia, with the exception of a very small quantity
of argentiferous lead coming from Spain. The acetic acid used in the
process of manufacture comes from France. The market price of white
lead in Italy is now about 45 lire the quintal (equivalent to 18_s_.
per cwt.).

_Lewis’s Process._--Many attempts have been made to substitute for
carbonate of lead--the ordinary poisonous white lead, that slowly but
surely induces paralysis in those who come in contact with it for any
considerable period--some less deleterious pigment. Zinc white has
often been put forward as a substitute, and is indeed largely employed;
but it is open to the objection of not possessing sufficient body or
opacity. Sulphate of lead is not poisonous; but, when prepared in
the ordinary way by precipitation, is of a crystalline nature, and,
therefore, wanting in both these qualities.

A sulphate of lead has, however, been produced by John T. Lewis, of
Philadelphia, by sublimation, which, when treated by the Freeman
process, is stated to possess a body and colour superior to the best
white lead made by the ordinary process, with the additional merit of
being cheaper, while it is also non-poisonous.

In the smelting of lead ore into pig lead, 15 per cent. goes off
in fumes, and 10 per cent. are all that it has hitherto been found
possible to recover; by this process, however, the whole 15 per cent.
are recovered. The sulphate may also be produced from a low quality of
galena, or lead ore, which is not suited for smelting into metallic
lead, and from the slag formed in the process of smelting.

The plant consists of simple subliming furnaces, iron cooling pipes,
suction fans, and a series of flannel or calico bags, arranged
vertically in a building, well ventilated, so as to allow the filtered
gases to escape, leaving behind the sublimed white lead, which is then
merely shaken down from the bags into barrels placed beneath them. The
manufacture is carried on at Joplin, Missouri, with four subliming
furnaces, about 500 feet of cooling pipes and towers, with suction
fans, which drive the fumes into 300 bags, 20 inches in diameter,
and 38 feet long, arranged vertically. About 50 tons weekly of white
lead are now being produced from waste fumes, slag, and poor ore, by
this establishment alone, at a cost not exceeding that given in the
following figures:--

                                                     £   _s._  _d._

  20 cwt. of galena (with 82 per cent, of ore)       8    0     0
  Cost of subliming and catching, including repairs,
    labour and all expenses                          1   10     0
  Casks                                              0    6     0
                                                     ------------
                                                     9   16     0

Freeman’s process (see p. 254) consists of grinding together, in a dry
state, under great pressure, and consequently with great friction,
sulphate of lead and sulphate of zinc. While neither of these two
substances alone possesses good body or opacity, when treated by this
process they are so changed in character that the new substance is
stated to be superior in these respects to the best form of ordinary
white lead.

The white lead thus obtained mixes well with oil, and has also the
advantage of not becoming blackened when exposed to the fumes of
sulphuretted hydrogen, and of not peeling off in a saline atmosphere.
As a basis for coloured paints, it is recommended on the ground that,
being decomposed with greater difficulty, it can be mixed with almost
any colouring substance; and, being free from acid, it does not change
the tints of other substances.

The details of Lewis’s process, and the plant employed, are more fully
set forth below.

Heretofore the manufacture of dry white lead from galena or the
native sulphurets has been effected by roasting or desulphurising the
galena, and then mixing the residue, after roasting, with carbon, and
subjecting the mixture to the action of heat in a compound reducing or
subliming and oxidising furnace, and collecting the resulting fumes in
textile bags.

Lewis’s process, however, is based upon the discovery that by subliming
unroasted galena, or the native or raw sulphuret of lead, and then
oxidising the volatile products, cooling the fumes, and collecting them
by means of a textile fabric, a superior basis of a pigment can be
obtained. The admixture of carbon with the raw ore will facilitate the
subliming process, or it may be carried on in a muffle or reverberatory
furnace without the previous admixture of carbon.

The furnace, which has been found to answer well for the purposes
above mentioned, is commonly known as the Wetherill furnace, and is
represented in plan, in Fig. 21; in front and back elevations, in Figs.
22, 23; and in central longitudinal section, in Fig. 24.

_a_ represents the main chamber, the bottom _b_ of which is composed
of iron bars perforated with small holes of about ¼ inch in diameter
and about 1 inch apart, and preferably made slightly conical, with the
larger diameter downward, that is to say, the said holes are of such a
size as to prevent

[Illustration: Fig. 21.

Fig. 22.

Fig. 23.

Fig. 24.

LEWIS’S WHITE LEAD PLANT.]

the escape of the crushed ore and coal. These perforated bars are
suitably sustained at the ends on the front and back walls of the
furnace. The ash pit below the perforated bottom is of equal area
therewith, and is provided with a door _e_ in front, and with a hole
_f_ at the back for the reception of a pipe from suitable blowing
apparatus.

The walls _g_, and arch on the top should be built of some refractory
substance, such as fire-brick.

The front of the furnace is entirely open, and is provided with sliding
doors _i_, by which it can be closed when working, and opened to remove
the residuum.

At the back of the furnace there are two slides _j_, to permit access
to the main chamber, for stirring the charge and for inspection.

At the back, near the arch, there is a hole _k_, governed by a sliding
damper leading to a chimney, for carrying off smoke and impure gases at
the commencement of the operation upon a new charge.

In the centre of the roof there is an aperture _l_, governed by a
damper or sliding door _m_, the said aperture leading to a suitable
apparatus for the collection of the oxidised vapours of lead.

The exterior walls _n o p q_ may be built up to form two feeding
troughs _r_, one on each side of the arch or roof, and each provided
with an aperture or passage _s_, communicating with the inside or main
chamber, and each aperture or passage is provided with a cover to be
put on after the furnace has been charged.

In working with this furnace, crushed ore (native sulphuret of lead)
and carbon, preferably in the state of pea or dust anthracite coal,
are mixed in equal proportions; the mixture is ignited, and the fumes
are oxidised by the blast through the mixture, which also promotes the
combustion. Dense white vapours or fumes pass off, and are conveyed to
a separate chamber, where they are strained by passing through a screen
or series of screens of muslin or other textile fabric. Lime may also
be employed in the furnace, in the proportion of 200 lb. of lime to 400
lb. of galena, although the addition of the lime is not necessary in
all cases.

Instead of the furnace above described, a muffle furnace may be
employed, in which the heat is applied indirectly to the ore, with
the precaution of constructing the sole or bottom of this furnace of
a material not rapidly acted on by the constituents of the ore; or
a reverberatory furnace may be used in which the heat is directly
applied. In both of these two cases reducing carbon may or may not be
mixed with the ore.

Sometimes, generated gas is employed in place of coal, to effect the
same result.

In a later specification Lewis remarks that when white lead pigment
is manufactured from galena or other lead ores, in the raw state, or
even in the roasted state, by subjecting them to the joint action of
heat and air, either with or without reducing means, according to the
quality of the lead ores used, the fumes are discoloured by particles
of carbon or sulphuret of lead, or both, when they are caught in bags
of textile fabric, and are unfit when in this state for use as a white
pigment.

The fumes which are produced by this action of heat and air on galena
or other lead ore are cooled and then collected in bags, and Lewis
prefers to expose the so-collected products to the joint action of
heat and air, to destroy or to burn out all the particles of carbon
or sulphuret of lead, or both, by either throwing the said fumes on
a bright clean anthracite or coke fire, with a blast from the sides
or from below, or by throwing them over such fires or into a cupola
furnace, or by throwing or blowing them into a generator gas flame, or
through externally heated retorts. He then in either case collects the
escaping fumes from the furnace or retort in bags or screening chambers.

The best process to be adopted depends upon the kind of fuel in the
locality where the fumes are refined, and also upon the purity of the
lead fumes. If they contain iron, clay, or the like, it is best to
throw them into a coal fire; but if they are pure, one process is about
as effective as the other, and the degree of purity of the fuel decides
the kind of heating apparatus to be used.

There being such great difference in the purity of fuels, and this
irregularity not allowing of uniform results, Lewis prefers to use a
furnace in which the flame and heat are produced by burning gaseous
fuel with air, which is forced into the furnace with the fumes which
have been collected in a previous process.

Fig. 25 represents a furnace which may be advantageously employed for
the purpose when gas is used as fuel.

_a_ represents a blower, into which the fumes are fed from the hopper
_b_. The fumes, being thoroughly mixed with air in this blower, are
forced into a chamber _c_, and then through a series of tuyeres _d_.
At the same time, gas from a generator or producer is admitted by the
flue _e_, and is burned by the incoming blast from the tuyeres _d_; the
volatile fumes produced in the furnace _f_ pass through it and out of
the flue _g_, and are collected in bags or screening chambers. By using
gas fuel, which is easily and fully burned, and clean to handle, a fine
white pigment is produced.

_MacIvor’s Processes._--The name of MacIvor is a familiar one in
improvements in manufacturing chemistry, and not the least in
connection with the subject of pigments, sometimes in conjunction with
other inventors.

In 1889 MacIvor and others introduced some modifications in the process
of producing white lead or carbonate of lead, by the treatment of
oxide of lead (litharge or massicot) with a solution of acetate of
ammonia, whereby the oxide of lead is transformed into hydrate and
acetate, which are subsequently converted into carbonate of lead by
the injection of carbonic acid. The hydrate and acetate of lead, in
presence of free ammonia formed in the reaction, are quickly decomposed
by

[Illustration: Fig. 25.--LEWIS’S WHITE LEAD PLANT.]

the carbonic acid, yielding the final product, namely carbonate of
lead, and re-forming the acetate of ammonia.

The rapidity with which the conversion of oxide of lead into hydrate
can be effected depends upon the strength of the acetate of ammonia
solution employed, that is to say, the weaker the solution the slower
will be the conversion. It has been found that, for commercial
purposes, the solution of acetate of ammonia may be used with advantage
of a strength which need not be more than 25 per cent. nor less than 5
per cent. A strength of 25 per cent, operates in a comparatively short
space of time; but a strength as low as ½ per cent. will effect the
hydration if there be a sufficient quantity of the weak solution, the
lead oxide being in a fine state of division, and time of no object.
This ½ per cent. strength, however, or any strength below 5 per cent.,
is not recommended for commercial purposes, having regard to the time
required for completing the operation.

The conversion of the oxide of lead into the hydrate and acetate of
lead is effected in the cold (heat may be used, but for commercial
operation it is not recommended). The conversion is facilitated by
employing a mechanical arrangement, similar in many respects to that
adopted in a previous specification, but modified by connecting the
digesting vat with a cistern or vat containing acetic acid, whereby any
free ammonia carried over during the operation may be absorbed. Also
by introducing carbonic acid to the mass of hydroxide and acetate of
lead formed by the oxide of lead and acetate of ammonia, either by a
series of concentric rings perforated with small holes in the bottom
of the vat itself, or by passing carbonic acid down a hollow shaft to
which are attached stirrers, and through perforated tubes attached to
the blades of the stirrers, or by any other means that may ensure the
thorough saturation of the hydroxide and acetate of lead so as to form
carbonate of lead.

The solution of acetate of ammonia may be recovered from the white
lead, and be repeatedly used, for the conversion of further quantities
of oxide of lead into hydrate and acetate of lead, which hydrate and
acetate are converted into carbonate of lead by the injection of
carbonic acid. Theoretically, a given weight of acetate of ammonia in
solution, used in conjunction with carbonic acid, should be capable
of converting an unlimited quantity of oxide of lead into carbonate
of lead; but during the manufacture of white lead by this process, it
may be reckoned that there will be a loss of ammonia acetate varying
with the strength of the solution employed, but it should not exceed 10
per cent. on each charge. This loss arises in the washing of the final
product, and through the escape of some ammonia during the process.

Fig. 26 is a vertical section of the apparatus employed.

_a_ is a vat, which may be made of wood or other material capable of
resisting the chemicals employed; it may conveniently be 6 feet in
diameter and 4 feet deep, and it is provided with a closely fitting
cover, _b_ is a cistern situate at a higher level, and intended to
contain a solution of acetate of ammonia, _c_ is a pipe by which the
solution can be drawn down from the cistern _b_ into the vat _a_; a
cock is provided upon this pipe, as the drawing indicates. There is a
man-hole _d_ in the cover of the vat, and the vat contains an agitator
_e_, with a vertical shaft which can be turned by gearing _f_ as shown,
or the agitator may be driven by any suitable motor. The shaft of the
agitator _e_ is hollow, and pipes _g_, which stand immediately behind
the stirring tines, are connected with the hollow shaft, to deliver
the carbonic acid gas into the vat; or this may be effected by means
of coils of pipe laid at the bottom of the vat, and pierced with small
holes. The pipes _g_ are open at their lower ends. _h_ is a cock by
which the liquor can be drawn off from the vat into the receiver _i_.
A pump _k_ is provided upon the cover of the receiver, by which the
liquor may be returned into the vat _a_. _l_ are outlets by which the
white lead is discharged from the

[Illustration: Fig. 26.--MACIVOR’S WHITE LEAD PLANT.]

vat into the washing cisterns _m_ and _n_. _o_ is a man-hole, which may
be opened to facilitate the emptying of the vat. _p_ is a plug which
is removed to let the white lead run out of the vat. _r_ is an ammonia
catch box, charged with acetic acid.

All the metal-work of the apparatus with which the acetate solution
comes into contact should be of such a character as to resist
corrosion, or should be coated with a material capable of withstanding
attack by the chemicals employed in or formed during the process.

The operation is by preference conducted in the following manner, but
the details admit of variation. The charge of monoxide of lead for an
apparatus of the dimensions indicated may weigh about 1120 lb.

The monoxide should be in fine powder, and may be either moist or dry.
Having received this charge, which is introduced by the upper man-hole
_d_, the vat _a_ is closed, and a solution of acetate of ammonia is
let down upon the charge from the cistern _b_, or pumped out of the
receiver _i_. The vat _a_ should be charged with the solution of
acetate of ammonia in the proportion of three parts of said solution
to one part of lead monoxide by weight. It is convenient to employ
a solution containing 5 per cent. of acetate of ammonia, and the
quantities above stated are suited to a solution of this strength; but
the strength of the acetate solution may be varied within wide limits,
as hereafter explained.

The charge of monoxide of lead and acetate of ammonia in the vat should
be kept constantly stirred by the agitator until it becomes whitish
in colour, when it will be found that the monoxide of lead has become
converted into hydrate and acetate. The workman will know that this
change is complete when the reddish or yellowish appearance of the
monoxide of lead disappears, and the mass in the vat becomes whitish in
colour.

The hollow axis of the agitator is connected with a pipe, by which
carbonic acid gas is supplied to it under pressure sufficient to cause
the gas to pass through the contents of the vat. This gas escapes at
the lower ends of the pipes _g_, and ascends through the liquid in
which the hydrate and acetate of lead are held suspended. A free flow
of this gas should be maintained, and some excess allowed to pass away
by a pipe on the cover into the ammonia catch box _r_. The pipe dips
down into the acetic acid which this box contains, and any ammonia
passing off with the carbonic acid gas is caught by the acid, and forms
acetate of ammonia. When the hydrate and acetate of lead are completely
converted into basic carbonate (and this the workman will know by its
changed appearance), the motion of the agitator is caused to cease, and
the white lead is allowed to settle.

There is a gauge glass on the side of the vat _a_, and in this glass
the changes of appearance can be recognised by which the workman
regulates the process.

When the white lead is deposited, the liquor is drawn off into the
receiver _i_ by opening the cock _h_. The plug _p_ is then displaced,
and the white lead is allowed to descend into the washing cistern _m_.
Below the cock _h_ is another cock or cocks, in order that any further
quantity of liquor may be drawn off into the cistern _i_ if required.
The taper plug _p_ is then raised, by being pushed up from underneath
the vat _a_, through pipe _l_.

The white lead requires to be well washed with clean cold water.

When a fresh charge of monoxide of lead has been placed in the vat
_a_, the liquor is pumped up on to it from the receiver _i_, and then
a further quantity of solution is drawn from the cistern _b_, and also
from the ammonia catch box _r_, until the proper quantity has been
supplied, which is determined by the gauge.

Quite recently, in conjunction with Mr. Watson Smith, Mr. MacIvor has
introduced further improvements in the production of “white lead” or
basic carbonate of lead, by the treatment of oxide of lead (litharge or
“massicot”) with a heated solution of acetate of ammonium, in a closed
vessel (called the digestor), with agitation and under pressure, so
that the oxide of lead is transformed with exceeding rapidity into
basic acetate of lead, chiefly consisting of the tribasic acetate,
ammonia being set free; and the subsequent treatment of the basic
acetate thus obtained, after filtering or otherwise removing insoluble
impurities, and after cooling with carbonic acid gas in a separate
vessel (called the carbonator).

The acetate of ammonium is by preference first charged into the
digestor, and then the oxide of lead (litharge or massicot) in fine
powder, and in the equivalent proportions calculated to be at least
somewhat slightly in excess of the quantity necessary to form with the
acetic acid of the acetate of ammonium the tribasic acetate of lead
Pb(C_{2}H_{3}O_{2})_{2} + 2 PbO or Pb_{3}O_{2} (C_{2}H_{3}O_{2})_{2},
is added (this being in allowance for the certain amount of insoluble
and unconvertible matters in the litharge), the acetate of ammonium
liquor of a strength preferably not below 5 per cent., though it may be
stronger, being first, before the addition of the oxide of lead, set in
vigorous agitation and circulation by the pump, and heat having been
applied by means of a steam heater as shown in the drawing and as will
be explained later on.

The digestor is closed, and the temperature rises. As the temperature
rises, and approaches 212° F., and as the pressure, due more especially
to the tension of vapour of ammonia set free during the reaction with
the litharge, increases, so is the rapidity of the conversion of the
oxide into basic acetate increased, and more and more of this basic
acetate becomes dissolved, whilst at or about 212° F. it is entirely
in a state of solution. The degree of heat, or the prolongation of the
heat, depend of course upon the state of dilution of the acetate of
ammonium used. Something more specific will be said later on regarding
this question of degree of heat.

The clear liquor, together with extraneous coloured particles, red
lead, dirt and undissolved matters, is pumped out through a suitable
filter to one of the carbonators, being carried en route through a
cooling-pipe system, and being let cool further if necessary in the
carbonator itself. After cooling thoroughly, the cold basic acetate,
which may now have crystallised or separated out more or less,
according to the strength of the solution, is treated with carbonic
acid gas, whereby the basic acetate, in the presence of the ammonia set
free, is converted into basic carbonate of lead of exceptionally white
colour, and with high basicity and covering power, proper care being
taken as specially indicated later on. There are several reasons why
greater whiteness of the product is secured, the principal one being
that filtration from solid and insoluble coloured impurities has taken
place previous to the carbonating process.

The advantages of the employment of heat and pressure for the formation
of the basic acetate of lead from oxide of lead, and the advantage of
thus carrying out the conversion of oxide into basic acetate of lead
separately from the conversion of that basic acetate of lead into basic
carbonate or white lead, are--

Firstly--That separation by filtration from impurities and so forth
left by the oxide of lead used, and insoluble in the acetate of
ammonium, is made possible, and thereby a pigment of exceeding
whiteness and purity can be obtained, besides high basicity, with
corresponding body and covering power.

Secondly--That the rapidity of the conversion of oxide of lead into
basic acetate of lead is immensely increased--it becomes in fact almost
instantaneous, much time being saved.

Thirdly--That the conversion is effected quickly and in a perfectly
closed vessel; no chance of the escape of ammonia occurs, and, in the
carbonating stage of the process, the free ammonia present in the
liquid assists in securing the formation of basic carbonate of lead,
and the maintenance of this basicity throughout the conversion.

This free ammonia is converted by the carbonic acid into the less
volatile but still alkaline carbonate, and, later on, this ammonium
carbonate reacts upon the basic acetate of lead, converting it into
basic carbonate of lead, acetate of ammonium, still less volatile,
being simultaneously produced, ready for use over again.

It must be understood then, that as the carbonic acid passes into
and through the ammoniacal mixture in the carbonator, it continually
precipitates or forms basic carbonate of lead, in presence of more or
less of the volatile alkali, which however continually diminishes in
quantity as the conversion proceeds.

Were no ammonia present, but only tribasic acetate of lead, as in
the case of the earlier methods of precipitating white lead, unless
a very slow current of carbonic acid gas were passed through, some
of the first formed basic carbonate of lead would be in danger of
being over-carbonated and losing its basicity, being converted into
mono-carbonate of lead. This danger is much lessened, and consequently
a much greater possibility of rapidity of carbonating is conferred,
in the case of the process as above described, and by virtue of the
ammonia which is present.

Nevertheless the carbonic acid gas must not be passed, even into the
ammoniacal liquid containing the tribasic acetate of lead, with such
rapidity that distinct alkalinity to the usual tests ceases to be
maintained, and the process must be terminated whilst the liquid is
still alkaline. If a little lead salt on the one hand, and a little
ammonia on the other, be left in the mother liquors ultimately obtained
on filtering from the white lead, they will be returned and circulated.

Fig. 27 shows an elevation (partly in section) of the apparatus
employed.

_a_ is an iron vessel made of boiler plate, and lined internally with
lead. It may be here added that all the apparatus is thus lined, or
is constructed of material invulnerable to the action of lead salts,
ammonia, or acetate of ammonia.

The vessel _a_, which is termed the digestor, should be furnished
with, a thermometer, pressure gauge, safety valve, charging hole _b_,
and suitably arranged sampling pipe with cock.

[Illustration: Fig. 27.--MACIVOR’S WHITE LEAD PLANT.]

Agitation of the contents of the vessel is effected by means of the
circulating pump _c_, which, drawing off supernatant liquor along
with air and ammonia vapour from the upper part of the digestor _a_,
by the pipe _d_, forces it through the pipe _e_ to the heater _f_,
consisting of a coil suitably heated by steam in a steam vessel _g_,
or by similar means, and then is forced through a pipe _h_, leading to
the bottom of the digestor _a_, where it terminates in a cone-spreader
_i_, provided with a regulating valve _j_. By means of this spreader,
the liquid is forced through the heavy and dense mixture containing the
oxide of lead, which it vigorously agitates and keeps more or less in
circulation as indicated.

The charges of litharge and acetate of ammonium will vary according
to the strength of the latter. For a 5 per cent. strength of acetate
of ammonium solution, however, it will be best to calculate the
proportions as follows, viz. about 1200 gallons of 5 per cent. acetate
of ammonium liquor for 1 ton of litharge, which should be finely ground.

The temperature of the liquor may vary between 140° F. and 212° F.;
but within these limits, the lowest temperature consistent with
the sufficiently rapid conversion and solution of the litharge is
preferable. The reason of this is simply that the less the heat
employed, the less is the tension of the ammonia, and the chances of
the loss of ammonia are thus minimised; in addition to this, less
delay is involved and less refrigeration or cooling is needed before
carbonating.

The object of the violent agitation of the litharge amongst the heated
acetate of ammonium in the digestor _a_, is that caking of the former
may be prevented, and a most rapid conversion of the oxide of lead into
basic acetate of lead, with minimum expenditure of heat, be secured.

The effect of the heat in the closed space is greatly aided by that of
the pressure due to the tension of the ammonia and aqueous vapour at
the increasing temperatures.

When the litharge is converted into tribasic acetate of lead, and
brought into solution, the liquor and sludge of insoluble matters is
preferably pumped through pipe _k_, by means of the filter-press pump
_l_, and forced through the filter-press _m_, which removes and retains
the insoluble matter, allowing the clear liquor to pass to the cooler
_n_, and through this to the carbonator _o_, similar in construction to
the digestor _a_.

Here the cooled liquid is circulated by means of pump _p_ (in a similar
manner to that adopted in the digestor process already described),
carbonic acid being simultaneously pumped in through the pipe _q_,
which is perforated as shown, or introduced in any other way.

When carbonated to the desired extent, the white magma, consisting of
basic carbonate of lead and mother liquor, is drawn through the pipe
_r_, by the pump _s_, into the filter press _t_.

The clear liquid flows through _u_, into the covered mother liquor tank
_v_, whilst the press-cakes of white lead, after sufficient washing
with water, are removed and dried in a suitable manner.

The washings are run off to a weak liquor tank (not shown) for
concentration for use over again. Any inert gases accompanying the
carbonic acid, or the latter alone in excess, pass from the carbonator
through pipe _w_, into the catch-box or other condensing and absorbing
apparatus _x_, containing either dilute acetic acid or cold water, in
order to retain any ammonia carried over, and furnished with perforated
trays or baffle-plates.

This catch-box is, if necessary, also connected with a further
condenser, so as to remove all ammonia from the displaced air, or
inert gases (if impure carbonic acid has been used) of the carbonator.
It is a lead lined vessel preferably. The ammonia carried from each
charge thus is tested by measuring the volume of the solution from the
catch-box or other condensing and absorbing apparatus, and estimating
the ammonia present in the solution. This amount of ammonia in a
sufficiently concentrated form is then added to the charge in the
carbonator, when cold, so as to produce a completely neutral solution.
The ammonia in the catch-box is either added to the ammonia stock
in the ammonia department, used for dilution of strong ammonia in
making fresh acetate, or strengthened up to further absorption in the
catch-box or condenser, until strong enough to add to a freshly run and
cooled charge in the carbonator, to replace any ammonia driven off by
heat.

The sludge in the filter-press _m_ is washed with water to remove
acetate liquors and ammonia, the weak liquors being run to a separate
closed vessel similar to the carbonator _o_, but smaller, and not shown
in the drawing.

Here white lead is precipitated by carbonic acid, and the product is
passed into the filter-press _t_, the weak liquors associated with it
serving to give a preliminary washing to a freshly received charge
of white lead already in the press. The weak filtrate liquors thus
obtained are preferably sent to the weak liquor tank already mentioned,
for subsequent concentration, instead of to the stronger liquor tank
_v_.

The sludge-cakes from the filter-press _m_, connected with the
digestor, are suitably treated for the recovery of lead therefrom.

The mother liquors contained in tank _v_ from the white lead
filter-press _t_, are directly returned to the ammonium acetate tank
_y_, by the pump _z_, for use over again.

The carbonators may be operated singly as described, or two or three
may be connected together so as to be worked in rotation, the partially
absorbed carbonic acid from one carbonator being completely, or more
or less completely, absorbed in the one, or two, with which it is
connected. The last of such series of carbonators would of course be
connected with the catch-box arrangement previously described, or other
condensing and absorbing apparatus.

_Characters._--The advantages and disadvantages in the employment of
white lead have been described pretty fully by Prof. Barff in one of
the Cantor series of lectures which form such an important feature in
the publications of the Society of Arts.

Probably the fact that white lead possesses the body it has is the
reason why it has been so largely used, and why so many paintings
which have been painted with it have come to a most untimely end. Prof.
Barff says he knows of no pigment so liable to change of colour as
white lead. In saying this he expects that there are many who will not
agree with him. They know that white lead works well and easily, and
they like it because it covers down well; but then he points out some
of the great defects under which it labours.

If you take some oil, and if to that you add lime-water, the oil will
mix with the lime-water, and form a kind of emulsion. Again, if you
boil oil or fat with soda, a kind of soap is formed, and the process
of manufacturing soap is termed the process of saponification. Now if,
instead of boiling fat with soda or alkali, we boil it with plumbic
oxide or oxide of lead, we shall form a soap, and that soap goes by the
name, amongst medical men, of _emplastrum plumbi_, or lead plaster.
This is a substance made by the saponification of oil with the oxide
of lead. Because this oxide and carbonate of lead have the power of
saponifying oils, you get in white lead that peculiarly smooth easy
working which you do not get with any other white pigment; and it is on
this account, for one reason, that it is liked by artists and painters.
Taking a piece of paper coated with some of this lead plaster,
if you throw a light upon it, you will see that the substance is
semi-transparent. This is a peculiarity of lead that it will saponify
and form this sort of transparent substance.

The famous landscape painter, Mr. Wilson, made an addition to a room in
his house. The old part of the room had been painted a dark colour; the
new part, of course, when it left the workman’s hands, was perfectly
white, and therefore the painters painted down the dark colour with
white lead, until the whole room displayed one uniform tint. After
a while, however, it was found that the part which was originally
painted dark became dark again; the dark paint, in fact, showed through
the white lead. Sometimes, possibly, when an artist wishes to put in
figures upon a dark background that he has painted, he uses white
lead, and the figures will stand out well and brilliantly at first,
but after a time the dark colour upon which they are painted strikes
through the lead, and the figures of course recede. Now, this striking
through is owing to a slight process of saponification, no doubt owing
to an interchange between the carbonic acid of the lead carbonate and
the stearic and oleic acids of the oil with which the lead is mixed; so
that, in time, the white lead, which has a body which makes it so great
a favourite with artists, loses that body, and becomes a transparent
or semi-transparent substance, something like lead plaster. Here is
a reason why white lead should not be used unless the ground has
previously been brought to a light colour.

There is another objection to the use of white lead, and really a
very valid one it is. Persons go on year after year laying out sums
of money for having their houses painted with white lead, when other
pigments which will keep their colour might well be employed. A house
painted with white lead after some time darkens in tint considerably;
the colour is changed by some influence that is acting upon it through
the air, and that influence is sulphuretted hydrogen gas. If you paint
with white lead doors placed near a drain from which this gas escapes,
those doors will become browned and blackened. White lead is very
often, particularly that procured at ordinary shops, adulterated with
a substance called sulphate of baryta, or, commonly, barytes. This is
much more transparent when ground with oil than white lead itself, and
it will materially impair that property for which white lead is valued,
viz. that of covering down well and solidly. White lead adulterated
with barytes has, generally speaking, a bluish sort of look; it is
semi-transparent. It has not that opacity that pure white lead has.
If you take a small piece of white lead and put it into a test-tube,
and add to it a little nitric acid, or aquafortis, and some water, if
the lead is pure the whole of it will dissolve in the liquid, and you
will have a pure solution. If it does not dissolve there is a white
precipitate, which will fall down to the bottom of the tube, and that
precipitate is sulphate of baryta. Sulphate of baryta is insoluble in
aquafortis, but carbonate of lead, and most lead salts, are soluble in
it.

There is another excellent test for the purity of white lead, which
is this. If you take a small portion, and grind it up with a little
carbonate of soda into a small pellet about the size of a pea, and
then put it upon a piece of charcoal and hold it in the middle flame
of a blow-pipe for some short time, the sulphate of baryta becomes
decomposed, and you get sulphide of sodium formed. If this sulphide
of sodium be acted upon by an acid liquid, sulphuretted hydrogen is
given off, which could not be formed from carbonate of lead, for in it
there is no sulphur at all; and inasmuch as sulphate of baryta is the
impurity for which we have to look, the presence of sulphide after this
treatment indicates that it was with the white lead which was examined.

LIME WHITE.--A name sometimes given to the white pigment
prepared from sulphate of barium. See baryta white, p. 170.

LITHOPHONE.--This is a fancy title for one of the several
varieties of white pigment having the metal zinc as a basis, and
described under zinc whites on p. 247.

MAGNESITE.--The mineral known by this name is a natural
carbonate of magnesia, just as limestone is a natural carbonate of
lime. Where sufficiently abundant it is quarried, ground, and levigated
much in the same manner as barytes, which it greatly resembles in
its qualities as a pigment, and for which it constitutes a suitable
substitute. It is very white, heavy, and opaque; permanent in ordinary
situations; neutral with other pigments, mixes equally well with oil or
water, and possesses good covering power.

MINERAL WHITE.--One of the names applied to the pigment
prepared from gypsum, see p. 183.

ORR’S ENAMEL WHITE.--A name derived from the maker of a
certain variety of the zinc sulphide pigments, described under zinc
whites, p. 254.

PARIS WHITE.--Another name for the best brands of whiting, see
below.

PERMANENT WHITE.--This name is often bestowed upon baryta
white (see p. 170), on account of its durability as compared with white
lead.

SATIN WHITE.--There is a certain amount of confusion in the
application of this term, for while it is sometimes referred to baryta
white (p. 170), it is also a synonym for fine gypsum (see p. 183).

SPANISH WHITE.--The most carefully prepared samples of whiting
(see below), are often known by this name.

STRONTIA WHITE.--Though much less common than the closely
similar sulphate of barium, the natural sulphate of strontium is
equally suitable for employment as a pigment, and is prepared in
exactly the same way as baryta white (see p. 170). The artificial
product is also used. Both possess qualities remarkably akin to those
of baryta white.

TERRA ALBA.--An old-fashioned name for levigated gypsum (see
p. 183).

WHITING.--This material is simply prepared chalk. It should
be soluble in hydrochloric acid with effervescence, leaving at the
most but a small residue. Sometimes samples of whiting are found which
are more or less alkaline or caustic in their properties. This is a
serious defect for many purposes. It can be detected by treating the
sample with water, and adding to the liquor a little phenolphthalein.
If a brilliant red colour is obtained, caustic lime is present, and the
sample should be rejected, if to be used for mixing with chromes or
Brunswick greens, where a neutral product is required.

Chalk itself is too familiar to need any description beyond saying
that it essentially consists of carbonate of lime, with always a small
percentage of silica associated with it.

Its preparation consists in hand selection to exclude the silica
which, occurs in the more pronounced form of flints, then grinding in
several stages, levigation and drying. The levigation is effected by
having a series of settling pits into which the ground material flows
with water, and deposits according to its degree of fineness. The
drying is performed in chambers provided either with pipes carrying
steam or heated air, or by fires beneath the floor, and thoroughly
ventilated so that the moist air can escape as fast as it is saturated.
Finally the dried whiting is again ground very fine. The drying must be
done with great care and at a low temperature, so as to ensure avoiding
calcination, whereby the carbonate of lime is changed into oxide
(quicklime).

Whiting is a permanent and useful pigment mixed with water in
distempers, but is not applicable as an oil colour.

ZINC WHITES.--Originally and properly the term zinc white was
reserved for the white pigment consisting of zinc oxide; but latterly
many kinds of white pigment have been introduced containing a large
proportion of sulphide of zinc, sometimes associated with more or less
oxide, and sometimes without any oxide, and these are also by many
people called zinc whites, to which name they are perhaps as well
entitled as the original zinc oxide. It will therefore be convenient to
arrange them all under the same general heading of zinc whites.

(1) _Oxide._--Under the influence of a white heat metallic zinc is
volatilised, and if the vapour is thus brought into contact with
oxygen, either in the pure state or as air, combustion takes place,
and the oxygen unites with the metal to form zinc oxide. On this very
simple principle is based the manufacture of zinc oxide white.

The operation is conducted in plant similar to that shown in Fig. 28,
which consists essentially of two departments, that in which the zinc
is volatilised and that in which the oxidised vapour is deposited for
collection.

The volatilising process takes place in a series of oblong fire-clay
retorts _a_, varying somewhat in form but always with a contracted and
rising neck. Ordinary dimensions are about 2 feet long and 9 inches in
diameter each way, with walls about 1½ inches thick. These are heated
to whiteness and then charged with ingots of metallic zinc.

The retorts are arranged in double rows in reverberatory furnaces _b_,
two furnaces being arranged back to back so as to economise heat. The
furnaces are fired at the side, and the heat is conveyed around the
retorts by means of the flues _c_, the products of combustion of the
fuel finally escaping by the chimney stack _d_.

[Illustration: Fig. 28.--APPARATUS FOR MAKING ZINC OXIDE.]

As the vaporised zinc is emitted at the mouths of the retorts _a_ in
a partially ascending current, it immediately encounters a plentiful
supply of air, and thereupon takes fire (undergoes combustion or
oxidation). In this condition it enters the lower and funnel-shaped end
of the sheet-iron flue _e_, by which it is conveyed into the series of
settling compartments _f_.

While the bulk of the zinc oxide thus formed passes into the settling
chambers, a portion of it is too heavy to do so; its specific gravity
is such that the force of the draught is not sufficient to carry it
up. This portion falls at once into a receptacle placed beneath the
mouth of the flue _e_.

In order to obtain the necessary draught, the conduits _h_ are open
to the outside atmosphere, and introduce a supply of air just below
the mouths of the retorts _a_, so that it impinges against the current
of escaping zinc vapour. After passing through the settling chambers
_f_, the superfluous air finds an outlet at _i_ into a sufficiently
capacious flue _k_, which communicates with the chimney stack _d_. Thus
the draught created by the fuel consumed in the reverberatory furnaces
is made to assist the current through the settling chambers.

These settling chambers _f_, are constructed of wood, and are usually
about three in number, intercommunicating of course. The zinc oxide
enters the first compartment through an aperture in the top of the side
to which the discharge end of the iron flue _e_ is attached. After
traversing the first chamber, the stream of air and such oxide as has
not yet settled passes into the second chamber through the orifice _l_,
near the bottom of the partition dividing the two chambers. To reach
the next compartment, the stream has to ascend again, the aperture
being at the top of the partition, and this alternation is carried
on to the end of the series, thus checking the through draught and
facilitating the settlement of the zinc oxide. The floors of all the
chambers are made funnel-shaped, with a door at the lowest point, so
that the discharge of their contents may be as automatic as possible.
The flue _k_ contains screens hung at intervals for the purpose of
hindering, as far as possible, the escape of minute particles of zinc
oxide into the chimney, and thence into the outer air, whereby they
would be lost.

A description of the process as conducted in Belgium, says that ingot
zinc is placed in a series of retorts within one furnace, and the oxide
is formed in an exhaust chimney, and then passes through a long series
of passages and condensing chambers, in which are ranged tanks of sheet
iron or cloth to collect deposits. At a certain hour of the day it is
collected into casks, and after being tested as to quality it is ready
for delivery. According to the purity of the metal various qualities
are produced. The best is called “blanc de neige,” or snow white, and
is of a very superior quality; No. 1 white is the most used, the ores
for this quality being selected and purified by remelting; No. 2 white
is the common variety. In the process of manufacture there is more or
less waste material, imperfectly oxidised, deposited in the retorts and
passages. This residuum is carefully ground, washed and dried, and is
employed in painting in the place of lead.

In the American method of making zinc white they use the ore direct.
This is cheaper than the Liège method, but its product is of inferior
quality to that produced by sublimation. There are but two works in
Belgium for making zinc white, the Vieille Montagne Company (at the
Valentine Cocq works), which produce yearly 3000 tons by sublimation;
the other is at Ougree, near Liège, where the American method is
employed, but at present it is idle.

There are many other modifications in detail in different works. One
may be noticed here as it is claimed for it that the pigment is gifted
with greater covering power or body, the limited degree of which is the
only drawback to zinc oxide whites. The plan consists in this, that the
oxide is allowed to collect in the condensing chambers till it is of
such a depth that a man entering stands waist deep in the pigment. The
latter is gathered in pieces of sacking, which are drawn together and
squeezed up tightly, so that the oxide, when newly prepared, is pressed
into hard dense masses.

(2) _Sulphide._--Prof. Phipson, in a paper read before the
International Health Congress, at Paris, remarked that for several
years efforts had been made to discover some white substance to
replace white lead for painting buildings, ships, &c. He himself had
devoted several months to this important subject, but without success.
There has been found, it is true, in oxide of zinc a substance less
poisonous than lead, and serving very well as a white pigment in oil
painting; but its production is very expensive, and its mechanical
properties as a colour in oil are not pronounced enough to allow
it to compete in commerce with white lead. Such is not the case,
however, with an invention of Mr. Thomas Griffiths, of Liverpool,
who has succeeded in obtaining a very interesting product. This new
preparation, which is being manufactured at the present time on a
pretty extensive scale, has for its base sulphide of zinc (or an
oxysulphide of that metal), the properties of which as an oil colour
are of the most remarkable character. It is prepared by precipitating
one of the salts of zinc by a soluble sulphide, and washing and drying
the precipitate. The latter is then calcined at a red heat, with some
precautionary measures, then taken from the furnace, and, while still
warm, thrown into cold water. It is afterwards levigated and dried. The
result is a white pigment, very fine, and of great beauty. Regarded
from a hygienic point of view, Griffiths’ new white is infinitely
superior to white lead, as it also is in its practical bearing; it
possesses no injurious qualities; its manufacture and use do not affect
the health of workmen; its durability in climates of the most diverse
kinds is, so to speak, illimitable; it is altered neither by gaseous
emanations nor by dampness; and its price is comparatively low. The
most remarkable thing about this new white is that it covers as well as
white lead, while it withstands the effects of all kinds of weather,
so that its use is not only deprived of all danger to health, but it
is much more economical than white lead. Prof. Phipson stated to the
Congress that he regarded this new chemical preparation as being among
the most ingenious and useful products that have been discovered in our
time.

A later method, introduced by Griffiths and Cawley, consists in making
an artificial sulphide of zinc by bringing the vapours of zinc and
sulphur into intimate contact.

In carrying out this process, sulphur is melted in a jacket pan heated
preferably by high pressure steam. The melting vessel is connected with
a cast-iron still by means of a jacketed pipe, and the connection is
regulated by means of a valve in the bottom of the melting pan. The
latter should be at such a height above the still that the pressure due
to the column of sulphur in the conduit pipe may be greater than the
tension of the sulphur vapour in the still, so that when the valve is
opened, the sulphur in the melting pans may descend into the still. The
still is kept during the process at a temperature of incipient redness,
so that when the sulphur reaches it, the sulphur is immediately
vaporised and the resulting current of vapour passes to the chamber
described below.

Metallic zinc is melted in a retort or crucible, heated preferably by
means of a furnace on the Siemens or a similar principle, and raised to
such a temperature that it begins to volatilise freely. When this takes
place, the resulting zinc vapour is met by a current of sulphur vapour
obtained as above described, and in excess of that required to form
with the zinc sulphide of zinc. The reaction takes place according to
the chemical equation Zn + S = ZnS.

The sulphide of zinc is in the form of white extremely light powder,
which is carried along by the current of sulphur into the collecting
chamber, such as is used for the manufacture of oxide of zinc. This
allows of the separation of the different constituents of the products
into different qualities; those parts that are carried the farthest
are the whitest and best generally. Those portions nearest to the part
of the apparatus where combination takes place may sometimes contain
metallic zinc, if the sulphur supply has not been carefully attended
to, but this may be separated from the sulphide by levigation.

The collecting apparatus should be kept at a temperature slightly
superior to that of the boiling point of sulphur, in order that
sulphur, which is necessarily in excess, may not be deposited with the
sulphide of zinc, but may pass on in the vaporous form to a suitable
condenser, where, after condensation, it may be collected and used
again.

Before the sulphur vapour reaches the condenser, the last traces of
sulphide carried along with it are collected by the interposition of
metallic screens or sieves, placed between the sulphur condenser and
the apparatus.

In carrying out the process, care must be taken to keep the collecting
apparatus as cool as possible consistently with the fulfilment of the
conditions above mentioned, viz. that no sulphur be condensed therein.
In practice, this object can be effected with little difficulty.
Impurities in the zinc and sulphur are of little consequence provided
they are not volatile, and not of such a nature that they would detract
from the whiteness of the sulphide of zinc formed.

[Illustration: Fig. 29.--APPARATUS FOR MAKING ZINC SULPHIDE.]

The accompanying diagram, Fig. 29, is given as an example of a plant
that may be employed with good results.

_a_ is the sulphur melting pan with its steam jacket _b_, and steam
pipe _c_; _d_ is a cast-iron still, arranged within a gas furnace _e_;
_f_ is a crucible for melting and volatilising zinc, the said crucible
being contained in a gas furnace _g_; _h_ is an automatic apparatus for
freeing the mouth of the zinc vessel from deposited sulphide of zinc;
_i_ is the collecting apparatus; and _k_, the condensing apparatus.

(3) _Mixtures._--There are a number of compound pigments commonly known
as zinc whites, which only deserve the name in so far as they contain
a proportion, greater or smaller, of some zinc salt. At the same time
it must be admitted that some of these combinations possess very good
qualities, and that the foreign ingredients largely correct the weak
points of the zinc compounds.

Freeman’s.--This pigment, when ground with oil in the customary way,
forms a paint equal in body and covering power to the best white lead,
while it is superior in colour, permanence, and density, and is free
from odour and noxious qualities.

It is produced by grinding together “zinc white” (either oxide or
sulphide), lead sulphate and barium sulphate, in certain proportions,
in the dry state, in an edge-runner mill. By thus grinding the several
pigments together, their particles become intimately incorporated and
undergo changes in character. The barium sulphate not only cheapens the
product by reason of its low cost, but also imparts a distinct feature
in rendering the paint more free working. The proportions generally
adopted, calculated by weight, are 5 parts lead sulphate, 2 of zinc
white, and 1 of barium sulphate. The duration of the grinding will
necessarily vary in accordance with various governing conditions, but
it should be continued until the mixture has a density of about 200 lb.
per cubic foot.

Orr’s.--The pigment known as Orr’s enamel or Charlton white, is a
compound of oxide and sulphide of zinc and sulphate of strontia, or of
baryta. It is prepared in two ways.

(_a_) Barytes is calcined for some hours at white heat with charcoal,
and the calcined mass is lixiviated with water to wash out the barium
sulphide; to one-half of this solution is added zinc chloride, which
produces a precipitate of zinc sulphide, leaving barium chloride in
solution. To this mixture of zinc sulphide and barium chloride is added
the remaining half of the barium sulphide and some zinc sulphate, the
result of which is a double precipitate of zinc sulphide and barium
sulphate. This is water-washed, filter-pressed, dried, calcined at red
heat, thrown immediately into cold water, ground very fine, and finally
dried.

(_b_) The second process closely resembles the first, but celestine
takes the place of the barytes.

Either form of Orr’s enamel is a good useful white pigment, very
permanent, mixing well, of excellent covering power, and pure in colour.

_Characters._--Zinc oxide, being an expensive pigment, is liable to
adulteration; fortunately, all such adulterations are easily detected,
and their nature ascertained by a few simple tests. Zinc oxide, if
pure, should dissolve entirely without effervescence in nitric acid;
any residue would indicate adulteration with barytes or china clay;
the former may be distinguished by its weight and the yellowish green
colour it imparts to the Bunsen flame, the latter is lighter and gives
no colour to the Bunsen flame. Boiled with strong sulphuric acid,
barytes is not acted on, while china clay is. If, after cooling, the
mass be diluted with water, and ammonia be added to the liquor, if
barytes is present no precipitate will be obtained, while if china clay
is present a white precipitate is produced.

If the zinc oxide dissolves with effervescence, white lead or whiting
may be present; the solution should give no precipitate of black
sulphide of lead on passing sulphuretted hydrogen through it. On
neutralising the solution in nitric acid with sufficient ammonia, and
adding ammonia sulphide to precipitate all the zinc (the precipitate
should be white, any other colour would show some impurity), filtering
off and adding a little oxalate of ammonia, no white precipitate of
calcium oxalate should be obtained; such a precipitate would show
presence of whiting or gypsum.

The white pigments having as a base the sulphide of zinc, also contain
barytes, oxide of zinc, sulphate of strontium, &c. They can be
distinguished by evolving sulphuretted hydrogen gas, recognisable by
its odour, on treatment with an acid. They are not entirely soluble in
acids, the residue being mostly barytes, but may also be sulphate of
strontium; it is immaterial whether the two be distinguished or not.



CHAPTER VIII.

YELLOWS.


The yellow pigments do not form a large or important group, and beyond
a few organic colouring matters which have a limited use in artistic
painting, they are chiefly confined to the ochres of natural origin and
to the chromes, and one or two other kinds artificially prepared from
mineral substances.

ARSENIC YELLOW.--Another name for Orpiment (see p. 280).

AUREOLIN YELLOW.--This colour is prepared by precipitating
carbonate of cobalt from a solution of a cobalt salt with carbonate
of potash; this precipitate is dissolved in acetic acid, and to it is
added nitrite of sodium; nitrite of potassium and cobalt are thrown
down as a yellow powder. Experiments made on the freshly-precipitated
colour prepared in this way, have proved it to be invariably destroyed
by caustic potash; but aureolin yellow prepared in another way, put
into a flask with some caustic soda, has remained unchanged for several
days, and proved to be perfectly stable under this treatment. Samples
of the same yellow, exposed to the action of ammonia, soda, and potash
for a considerable length of time, have not changed. This is really a
most important fact, because aureolin yellow is so beautiful a pigment
that one wishes it would stand for ever. It is, in fresco and silicious
painting, a great desideratum to have colours that will stand the
action of lime and caustic alkalies. Our colours for fresco painting
are very limited, and if aureolin will stand the action of caustic
alkalies, it may be safely used both for fresco and silicious painting.

CADMIUM YELLOW.--The pigment known as cadmium yellow is
important on account of its permanence and brilliance. It is prepared
by passing a stream of sulphuretted hydrogen gas through a slightly
acid solution of a salt of cobalt, usually the nitrate or sulphate,
whereby the sulphide of cadmium is precipitated as an impalpable
powder. It is filtered off, well washed with water, and finally dried
at a low temperature.

By this method there is considerable difficulty in producing any
precise, shade of yellow, which is dependent on the proportion of
precipitant used, and can only be controlled after long practical
experience. Therefore some modifications have been introduced with
the object of securing definite tints. The ordinary colour is a pure
chrome yellow. For a lemon-yellow shade, a solution of yellow sulphide
of ammonium is added to one of sulphate of cadmium. For an orange
tint, the cadmium solution (chloride or sulphate) is made very acid by
addition of hydrochloric acid before passing the sulphuretted hydrogen;
or the cadmium solution, as for lemon-yellow, is boiled, and receives
the ammonium sulphide solution while still boiling. But none of these
modifications results in a really durable pigment, on account of the
presence of traces of free sulphur or acid.

The best brands of this pigment being somewhat expensive, there is
great inducement for adulteration, which usually takes the form of
orpiment or of chrome. The former can be detected by the ordinary tests
for arsenic, and the latter by the process described on p. 266.

CHROME YELLOWS.--A very important family of yellow pigments
are the “chromes,” consisting mainly of chromic acid in combination
with lead, iron, or zinc.

Chromates of lead, says Prof. Barff, are produced by precipitating a
lead salt with a salt of chromic acid, and the difference in tint is
owing to the different quantity of the chromic acid which is present
in the salt. The orange chrome is a basic chromate of lead, and basic
chromate of lead contains more of the chromic acid than is present in
the lemon chrome. The lightest chrome contains some sulphate of lead
precipitated with the chromate. All these colours contain lead, and
are therefore liable to the influence of sulphuretted hydrogen. Now,
if a chromate of lead is bought hap-hazard anywhere, it may or may not
be pure, but generally speaking, unless the chromates are obtained
from makers who are careful in the preparation of their colours, they
contain many other substances besides chromate of lead. For instance,
they contain a quantity of the solution, in a dry state, from which
they have been precipitated, and are by no means pure. Obviously, also,
there is an inducement to men who sell cheap colours to adulterate
them, to bring them down with whiting, and so forth. In that case the
chrome loses its body; if it is brought down with lead sulphate it has
more body; but when chrome yellows are prepared and mixed with good
oils, and put on carefully, and those oils have time allowed them to
become oxidised, perfectly dry in fact--not dry in the sense in which
an artist considers a painting dry, but perfectly hard--then the action
of sulphuretted hydrogen in the atmosphere will not be of such great
moment as if the colours are impure, or if they are submitted to the
influence of that deleterious gas before they have become perfectly
hard.

Another objection to the chromes, on the authority of Prof. Barff, is
that they are soluble in alkali, and so are many other colours. If
a painting is painted with chromes, and if that painting be washed
with an alkaline soap, it is quite certain that some of the chromates
will be dissolved. Consequently the minute and delicate touches, upon
which the artist depended for some of the best effects of his picture,
are removed with soap and water. No painting should ever be washed
with soap and water at all; but there are certain colours which will
withstand the action of soap, even if it is intensely alkaline, while
others will not. Taking a precipitate of lemon chrome, if we act upon
this with potash or soda, it dissolves up the yellow precipitate, and
destroys it altogether.

According to Weber, the preparation of chrome yellow presents
difficulties in practice, because products differing in shade and
structure, although of uniform chemical composition, are obtained
according to modifications of the method of manufacture or nature of
materials employed. A special difficulty is the turning of colour,
whereby a “turned” yellow has a dirty orange-yellow colour, which when
mixed with barytes gives a yellowish-brown leather-coloured shade and
not a light pure yellow. Other derived colours are similarly affected.

_Lead Chromates._--The acetate and nitrate of lead are the soluble
lead salts generally used. Basic carbonate of lead (white lead) is
also much used, the yellows prepared from this material being cheaper,
having a large covering capacity, and being particularly adapted for
ordinary greens; but these have not the smoothness and lightness so
much prized in some of the yellows obtained from soluble lead salts.
The oxide, sulphate, and chloride of lead have been proposed for the
manufacture of chrome yellow, but their treatment would be very tedious
and the products obtained only of medium quality. In the manufacture of
chrome yellow from acetate of lead and bichromate of potash, different
proportions of these materials are given by different authorities, but
some of these are obviously wrong where an excess of bichromate is to
be used, for every light chrome yellow is liable to turn by the action
of chromic acid or a chromate and thereby become of little value.
Proportions should be used, whereby the acetate of lead remains in
excess, and the reaction should take place in a solution as cold and
dilute as possible. In this way a brilliant yellow tint is obtained.

For the lighter chrome yellows, lead sulphate is precipitated
simultaneously with the chromate by adding sulphuric acid or a soluble
sulphate to the solution of the bichromate; these yellows have less
tendency to change colour than the pure chromate of lead, if the above
precautions are observed. Chrome yellow, precipitated from an excess of
lead acetate solution, by means of potassium bichromate, corresponds
to the formula PbCrO_{4}. When dried it forms light pieces, which show
a conchoidal fracture. A still more voluminous product corresponds to
the formula PbCrO_{4}, PbSO_{4}. The yellow having the composition
PbCrO_{4}, 2PbSO_{4}, is very heavy and shows a smooth fracture.
Lightness is often imparted to chrome yellow by the addition of
magnesium carbonate.

Nitrate of lead offers no advantages over the acetate, and is generally
more expensive to use. Free nitric acid is more objectionable than
free acetic acid, because it may act as a solvent on chrome yellow
and liberate free chromic acid, which is liable to “turn” the yellow.
When using lead nitrate it is preferable to neutralise potassium
bichromate with soda, to avoid the presence of free nitric acid. The
yellows mostly in demand are the inferior qualities, prepared by mixing
pure chrome yellows with white mineral matters, generally barytes,
gypsum, and kaolin, usually stirred in with the bichromate solution
before adding the lead salt. Barytes injures the colour least, but
kaolin has the advantage that it does not increase the weight of the
colour so much. Gypsum occupies an intermediate position; it is much
more voluminous than barytes, and does not injure the colour so much
as kaolin, but it is generally used together with barytes. It is not
advisable to use this combination, for the reason that the colour on
drying forms very hard pieces, which offer difficulties in grinding.
Gypsum, too, from being more easily acted on by reagents than barytes
or kaolin, tends to take part in the reaction by decomposing the
potassium bichromate before the addition of the lead salt, and this is
objectionable. (Weber.)

A writer in the _Chemical Trade Journal_ gives the following formula
for the production of various yellows:--

(1) For soluble lead salts:--

                            Kilos.

  Lead acetate               100
  Potassium bichromate        18
  Sulphuric acid (66° B.)     12

This mixture yields a yellow of the formula PbCrO_{4}, PbSO_{4}. To
obtain good shades, the amount of water employed should not be less
than 1000 litres, or double as much with the nitrate. In the latter
case, it is better to neutralise the lead nitrate and to replace the
acid by the sulphate of an alkali or of magnesium, or preferably of
aluminium; for the neutralisation of the bichromate, the best substance
is magnesite. The formula thus becomes--Lead acetate and bichromate as
before; magnesite, 6 kilos.; aluminium sulphate, 27 kilos.

(2) The Basic Acetate Method.--Litharge, 76 kilos.; acetic acid (30 per
cent.), 42; bichromate, 21·5; sulphuric acid, 21·5; water, 2000 to 3000
litres. To obtain a denser chrome, 10 kilos. of soda should be added to
the acetate, and 5 of the sulphuric acid replaced by 10 of aluminium
sulphate. For the production of an orange chrome, the following
formula is given: Litharge, 76 kilos.; acetic acid (30 per cent.), 42;
bichromate, 24; Solvay’s soda, 15; and caustic soda (100 per cent.), 5.
Care must be taken that the temperature does not rise sufficiently high
to spoil the shade.

(3) The White Lead Method.--In this case the white lead must be in
the finest state of subdivision possible, and suspended in the water:
White lead, 100 kilos.; nitric acid (36° B.), 12; bichromate, 13;
and aluminium sulphate, 10: or nitric acid (40° B.), 44; bichromate,
24; sulphate, 20, the latter giving the more fiery shade. For the
production of an orange: white lead, 100 kilos.; nitric acid (36° B.)
18; bichromate, 28; and caustic soda, 8, the latter being best added to
the bichromate before precipitation, and the temperature kept between
150° and 165° F.

(4) The Basic Chloride Method.--The same proportions and temperature
are suitable here as in the case of the white lead.

(5) The Sulphate Method.--Lead sulphate, 100 kilos.; bichromate, 24
to 25; Solvay’s soda, 8·75 to 16; ammonia (24 per cent.), 1 to 2; and
acetic acid (30 per cent.), 5 to 10. The sulphate, in the form of a
cream, is gradually added to the other ingredients after solution.

A new and improved process for manufacturing from galena chemically
pure chrome yellow having great colouring power, according to the
_Paper Trade Journal_, consists in first dissolving pulverised
galena with nitric acid to produce liquid nitrate of lead, and then
precipitating the chromate of lead by subjecting the nitrate of lead
to the action of bichromate of potash, neutral chromate of potash or
chromate of potash-soda.

The galena (sulphide of lead) is first pulverised by suitable means,
and in case it contains foreign minerals or other impurities, it is
washed or otherwise treated in a suitable manner to remove these
substances. The pulverised galena is then placed in acid-proof vessels
and is dissolved by adding nitric acid diluted with water, the entire
mass being well stirred. A slow dissolving takes place at the ordinary
temperature; but when the mass is heated artificially, either by
heating the vessel, or by using hot water added to the nitric acid,
or by the use of steam, more rapid solution is effected. The product
obtained is nitrate of lead in a liquid state.

The quantity of nitric acid necessary for dissolving a certain quantity
of galena depends on the percentage of lead contained in the ore, and
to a certain extent on the amount and nature of impurities present in
it, and also on the length of time in which the dissolving takes place.
In treating 100 lb. of galena having 80 per cent. of metallic lead,
about 90 to 100 lb. of nitric acid of 36° to 38° B. are used, and the
nitric acid is diluted with 100 to 200 lb. of water. This mixture is
left for about 24 to 36 hours, and is stirred up occasionally, as above
stated.

After the galena is dissolved by the nitric acid, and the sulphide of
lead is changed into liquid plumbic nitrate, then the sulphur which
floats occasionally on the surface of the solution is removed, and the
substance which remains undissolved is washed out and is also removed.
The liquid nitrate is then passed through filters of felt, linen, hemp,
flannel, &c., or is left standing for about 12 to 18 hours for settling
and clearing.

Now in order to produce the chrome yellow from this nitrate of lead,
bichromate of potash is dissolved in water, and a sufficient quantity
of this solution is poured into the plumbic nitrate solution until all
the plumbic nitrate is changed into chromate of lead, called “chrome
yellow.” Instead of the bichromate of potash, neutral chromate, or
chromate of soda may be used, and for the purpose of obtaining lighter
tints they may be tempered with sulphuric acid or any other compound
of sulphur. The liquid nitrate of lead is placed, preferably, in
large open receptacles of wood, clay, earthenware, or other suitable
material, and the chromate of potash solution is put into similar
vessels, and then placed above the receptacles containing the plumbic
nitrate. The chromate of potash can then easily be run into the lower
receptacles containing the liquid nitrate of lead, and this mixture is
constantly agitated by similar means until all the plumbic nitrate is
changed into chromate of lead, which is precipitated on the bottom of
the larger receptacles.

The chemical action which takes place by this changing of nitrate of
lead into chromate of lead is that the chromic acid of the potash
assumes the place of the nitric acid, which parts from the lead and
combines with the potassium, so that the lead as chromate of lead is
precipitated on the bottom of the receptacle, while the nitric acid of
the plumbic nitrate remains with the potassium, which latter has parted
with its chromic acid, and a quantity of water as solution above the
chromate of lead.

To change the nitrate of lead recovered out of the 100 lb. of galena
above mentioned into chromate of lead, about 56 lb. of bichromate of
potash are used. This change usually takes place in from about 10 to
30 minutes, after which the chrome yellow (chromate of lead) is left
for a few hours to settle, and then the solution standing on top of the
chrome yellow is drawn off by suitable means, or run out of the vessel
by opening a cock placed above the level of the chromate of lead.

The latter is then washed by adding pure water, which is poured
upon the chrome yellow, and the mixture is stirred up, so that all
the remaining liquid nitrate of potash is removed. After this is
accomplished, the mass is left to settle, and the water is again drawn
off from the precipitate, which then settles on the bottom of the
receptacle. This washing is repeated as often as is deemed necessary.

The chrome yellow is next placed in suitable receptacles, and dried in
the open air or in specially constructed drying rooms, after which it
is packed in boxes, kegs, &c., and is then ready for use. The liquid
nitrate of potassium, or saltpetre lye, removed from the receptacles
in which the chrome yellow is precipitated, and the first water used
for washing the chrome yellow, as above described, are placed in
large open flat receptacles or excavations, so as to be exposed to
the action of the air and sun; or the liquids may be operated on by a
small graduation work, so that a great portion of water evaporates. The
residue is then heated in suitable vessels or troughs by a slow heat
until a salt crust is formed, which, when cooled off and left to dry,
is nitrate of potassium or saltpetre in a pure state.

From 100 lb. of galena having 80 per cent. metallic lead, some 28 to
30 lb. of pure and dry saltpetre are produced by the above described
process. The sulphur produced by the dissolving of the galena by nitric
acid is melted in a small stove or furnace in the usual manner, and
then refined, so as to produce bars of sulphur called “brimstone.”
About 10 lb. of such sulphur are produced from 100 lb. of such galena
treated in the manner described.

The chrome yellow thus produced is said to be chemically pure, and of
great covering power, equal to the best chrome yellow in the market.

The process is very simple, and the crude lead ore is transformed into
chrome yellow in from three to four days.

_Characters._--Pure chromate of lead has an orange-yellow colour, and
in whatever manner it be made it always has this tint. Commercially,
chromes are made of a great variety of tints, from a pale-lemon chrome
to a deep scarlet, through all the intermediate shades of yellows and
oranges; in fact, most colour makers produce not less than eight, and
some more shades of chromes, whence it is obvious that they cannot be
chemically pure, but must be mixed with some other bodies.

In commerce, chromes are distinguished as “pure” and “common”: the
distinction between them is that the “pure” chromes are made from a
lead base, and consist of chromate of lead mixed to a larger or smaller
amount with sulphate of lead, the paler shades containing most of the
latter; while the “common” chromes are mixed with china clay, barytes,
gypsum, or similar bodies.

To distinguish the two kinds of chromes, treat with boiling
hydrochloric acid. If pure, the chrome will completely dissolve, the
solution usually having a green colour. On cooling, crystals of lead
chloride separate out. The liquor will give a white precipitate with
barium chloride.

By taking a weighed quantity of chrome, dissolving in hydrochloric
acid, adding excess of barium chloride, filtering, thoroughly washing
the precipitate with _boiling_ water, drying and weighing it, and
making the necessary calculations, every one part of the precipitate
being equal to 1·3 parts of sulphate of lead, the quantity of the
latter in the chrome is obtained.

The “common” chromes, which mostly contain barytes, are not completely
dissolved on boiling in hydrochloric acid, the barytes they hold being
left as an insoluble residue. By taking a weighed quantity of the
chrome, and filtering off, drying and weighing the residue, the amount
of barytes present can be ascertained; the solution can be tested for
sulphate of lead, which they sometimes contain, as described above.

Chrome yellows and oranges should be assayed for colour and tint,
care being taken to compare them with a thoroughly reliable standard
sample. Their colouring power and body should also be assayed. Another
property which should be tested is the colour that they yield on mixing
with Prussian blue. A great deal of chrome is used for making greens
by mixing with Prussian blue, and as there is a very considerable
difference between them in the shade of green they give, it is rather
important to test this property, which can readily be done by mixing
100 grains of the chrome with 10 grains of Prussian blue, grinding them
together in a mortar, and observing the shade of the green which is
produced thereby; if the green is not bright and pure, the chrome is
not fit to be made into greens, and should be rejected for that purpose.

_Iron Chromate._--If a solution of chloride of iron acidulated with
hydrochloric acid be added to neutral chromate of lead, a light orange
powder is precipitated, which is chromate of iron. Dried at 104° F.,
it is found to consist of 65 to 65·11 chromic acid, and 34·58 to 34·78
oxide of iron. Chromate of iron is insoluble in water, dissolves easily
in hydrochloric, nitric, and sulphuric acids, and decomposes when
mixed with soda lye. Under strong heat, it melts into a brownish mass.
It may be used in painting in oils, as a substitute for chromate of
lead. Although inferior to the latter in brilliancy, it has certain
advantages over it--it does not blacken with exposure to sulphuretted
hydrogen, is not injurious to the user, and withal is cheaper.

_Zinc Chromates._--The before-quoted writer in the _Chemical Trade
Journal_, speaking of zinc chromates, says that although it is possible
to prepare lead chromates having shades varying imperceptibly from
the palest lemon to a deep granite red, in the case of the zinc
compounds scarcely any variation from the normal is possible, this
being, however, different from anything obtainable with lead. Zinc
yellows fall considerably below the ordinary chromes in their colouring
power, but they are faster in light and are less poisonous. More
than 80 per cent. of the amount annually made in Germany is used for
the production of zinc green by mixing with Prussian blues, of which
substance Holland, Switzerland, and Hungary are the greatest consumers.
The zinc yellows met with in commerce vary in their constitution
considerably, being usually acid chromates of zinc and potassium, basic
zinc chromates being rare. Ordinary salts of zinc invariably contain
small amounts of iron, which must be removed before they are used in
the manufacture of colours. The simplest method is to heat them with
the quantity of permanganate theoretically necessary to convert all the
iron present into the ferric state, adding zinc hydrate, which need not
be free from iron; after thorough stirring, the whole is allowed to
settle, and filtered, when the liquid will be found to contain not a
trace of iron.

On the addition of chromate to such solutions, a yellow precipitate
(ZnCrO_{4}) falls, but owing to its great solubility in the liquid
this process is valueless. By using an excess of bichromate, the zinc
chromate combines with some of the alkaline salt, forming the compound
(ZnCrO_{4})_{3}.K_{2}Cr_{2}O_{7}, which may be washed without loss;
but on drying yields an extremely hard, sandy powder, possessing, in
spite of its fine colour, no value as a pigment. By neutralising the
two solutions before precipitating, a much higher yield of chromate is
obtained, but still so much chromic acid is lost as to make the process
too expensive to pay. Formerly an addition of calcium chloride was
made to the neutral solutions, so as to precipitate as calcium chromate
some of the acid which remained mixed with the zinc salt. The best
results, both in regard to yield and colour, are obtained by adding to
the zinc salt sufficient alkali to decompose one-quarter of it, so that
the chrome may have the formula (ZnCrO_{4})_{3}.ZnO. To the bichromate,
enough alkali should also be added to convert it into the normal
salt. It is to be remarked that the nature of the metal combined with
the chromic acid has the greatest influence on the shade of the zinc
yellow, so much so that in manufacturing “acid” zinc yellow, the use of
sodium bichromate is inadmissible. A basic zinc yellow prepared from
the sodium salt has a redder and more cloudy shade than one made from
the potassium compound, but the difference is hardly noticeable when
sodium-potassium chromate is employed.

Modern zinc yellows are invariably prepared from acid solutions, and
consist of a double salt of zinc chromate and potassium bichromate,
mixed with a varying amount of unchanged zinc oxide, which must not
be regarded as an adulteration of the pigment, for its presence gives
the substance “body.” As previously stated, sodium bichromate is
inadmissible, as it does not form similar double salts.

The raw material is usually zinc oxide, which is met with in a state
of great purity; by the addition of sulphuric acid, this is converted
into basic zinc sulphate; potassium bichromate solution is added, and
the whole is stirred vigorously for an hour. At the end of this time,
the zinc chromate, which was previously in a state of partial solution,
begins to separate out in the form of a brilliant yellow scum on the
surface of the liquid, consisting of (ZnCrO_{4})_{3}.K_{2}Cr_{2}O_{7},
while the solution rapidly becomes almost colourless. Suitable
proportions are:--Zinc oxide, 100 parts; sulphuric acid (66° B.) 60;
and potassium bichromate, 100. Although it is hardly possible that any
hydration takes place, it is found advisable to soak the zinc oxide in
water for 24 hours before the other reagents are added, the sulphuric
acid after dilution being added gradually. Great care must be taken
that the solutions are all cold, and the stirring is continuous, to
avoid the pigment being deposited in a hard sandy form. Zinc yellows
thus prepared are not liable to change during washing in a manner
analogous to the lead compounds.

Zinc chrome is not much used, partly because it is expensive, partly
because it cannot compete with the lead chromes in brilliance, depth of
colour, and body. Still, owing to the fact that it can be mixed with
sulphur pigments without change, it is often employed in the place of
the lead chromes. Pure zinc chrome is completely soluble in sulphuric
acid without any effervescence, but a slight effervescence may be
disregarded. Any residue may be put down as adulteration, and its
character can be ascertained by a few simple tests: it may be chrome
yellow, barytes, &c.

GAMBOGE.--Gamboge is a product of several trees of Eastern
Asia: viz. _Garcinia Morella_ var. β. _pedicellata_ [_G. Hanburyi_],
a native of Cambodia, the province of Chantibun in Siam, the islands
on the east coast of the Gulf of Siam, and the south parts of
Cochin China; _G. Morella_, growing in the moist forests of Ceylon
and Southern India; and _G. pictoria_, of Southern India, by some
considered identical with _G. Morella_. _G. travancorica_, of the
southern forests of Travancore and the Tinnevelly Ghâts, is capable
of affording small supplies of the pigment for local use, but not for
export.

When the rainy season has set in, parties of natives start in search
of gamboge-trees, and select those which are sufficiently matured.
A spiral incision is made in the bark on two sides of the tree, and
joints of bamboo are placed at the base of the incision so as to catch
the gum-resin as it exudes with extreme slowness during a period of
several months. It issues as a yellowish fluid, but gradually assumes a
viscous and finally a solid state in the bamboo receptacle. It is very
commonly adulterated with rice-flour and the powdered bark of the tree,
but the latter imparts a greenish tint. Sand is occasionally added.
The product from a good tree may fill three bamboo joints, each 18 to
20 inches long and 1½ inches in diameter. The trees flourish on both
high and low land. Annual tapping is said to shorten their lives, but
if the gum-resin is only drawn in alternate years, the trees do not
seem to suffer, and last for many years.

Dr. Jamie, of Singapore, who has gamboge-trees growing on his estate,
says that they flourish most luxuriantly in the dense jungles. He
considers the best time for cutting to be February to April. The filled
bamboos are rotated near a fire till the moisture in the gamboge has
evaporated sufficiently to permit the bamboo to be stripped from the
hardened gum-resin. The gamboge is secreted by the tree chiefly in
numerous ducts in the middle layer of the bark, besides a little in the
dotted vessels of the outermost layer of the wood, and in the pith. It
arrives in commerce in the form of cylinders, 4 to 8 inches long and 1
to 2½ inches in diameter, often more or less rendered shapeless. When
good, it is dense, homogeneous, brittle, showing conchoidal fracture,
scarcely translucent, and of rich brownish-orange colour. Inferior
qualities show rough, granular fracture, and brownish hue, and are
sometimes still soft. The pigment consists of a mixture of 15 to 20
per cent. gum with 85 to 80 per cent. resin. Its chief uses are in
water-colour painting, and in varnishes.

KING’S YELLOW.--A familiar name for the trisulphide of
arsenic, also known as orpiment (see p. 280).

NAPLES YELLOWS.--This group of pigments embraces several
combinations of the oxides of lead and antimony, derived from various
sources, and prepared by sundry methods. Two of the most useful formulæ
are as follows:--

(_a_) Mix 3 lb. powdered metallic antimony, 1 lb. oxide of zinc, and 2
lb. red-lead; calcine, grind fine, and fuse in a closed crucible; grind
the fused mass to fine powder, and wash well.

(_b_) Grind 1 part washed antimony with 2 parts red-lead to a stiff
paste with water, and expose to red heat for 4 or 5 hours.

There are a great many modifications both in the ingredients and the
processes, and a great variety of shades in consequence.

Taken as a whole, the Naples yellows are unsatisfactory pigments, very
prone to deteriorate in impure air, and necessitating great care in
their preparation to avoid contact with iron, which turns them green.
They cover well, are fairly brilliant, and mix readily with water or
oil, but their application is declining rapidly.

OCHRES.--The large class of mineral pigments known
collectively as ochres or sienna earths possess considerable
importance, notably on account of their remarkable durability and
their reasonable price. They all consist essentially of an earthy base
coloured by oxide of iron or of manganese, or of both. Some authorities
differentiate between ochres and siennas, and ascribe the latter name
only to those earths which contain manganese, but this seems to be
an arbitrary proceeding, because the term sienna, or more properly
Siena, is derived solely from the name of the Italian province in
which these minerals are worked. They are of widespread occurrence,
both geographically and geologically, and the methods of mining and
preparing them are not subject to much variation.

Consul Colnaghi, in his report on the mineral products of the province
of Siena, says that Siena earths, known also under the names of
ochre, bole, umber, &c., are considered by some mineralogists to be
ferruginous clays, by others, minerals of iron. They are chiefly found
in large quantities in the communes of Castel del Piano and Arcidosso.
The yellow earths and bole found on the western slopes of Monte Amiata
are true lacustrine deposits found amid the trachytic rocks, of which
it is principally composed. They lie under, and are entirely covered
by, the vegetable soil. Varying in compactness and colour, they are
termed yellow earths when of a clear ochreous tint, and terra bolare,
or bole, when of a dark chestnut colour. Each deposit consists for the
greater part of yellow earth, beneath which bole is found in strata or
small veins. The mineral being very friable, its excavation is easy,
and is generally conducted in open pits.

The different qualities are separated during the process, the bole,
which has the higher commercial value, being the more carefully
treated. After the first separation the bole is further classed into
first, second, third, and intermediate qualities--_boletta_, _fascia_,
_cerchione_, &c. Its most important characteristic is termed, in
commercial language, _punto di colore_, or tint. The value of the bole
rises as its tint deepens. Thus bole of the third quality is lighter
than that of the second, and the second than that of the first. After
the third quality comes the _terra guilla_. The yellow earths, after
excavation, are exposed to the open air for about a year, by the pit
side, without classification. The bole, on the contrary, is placed in
well-ventilated storehouses to dry for about six months. This diversity
of treatment is owing to the fact that exposure to the elements
brightens the colour of the yellow earths, and raises their value,
while it would damage the bole by turning its darker tint first into
an orange yellow, and, if continued, into an ordinary yellow earth. It
also loses in compactness and crumbles up under exposure.

In addition to the _punto di colore_, the size of the pieces influences
the commercial value of the bole, which increases with their volume.
Thus the classification is _bolo pezzo_, _bolo grapolino_, and _bolo
polvere_. The yellow earths are classed as _giallo in pezzo_, _giallo
commune_, and _giallo impalpabile_, the impalpable being worth more
than the common yellow. The production of the Siena earths is estimated
at about 600 tons per annum, of which amount about 50 tons are
calcined, and the rest sold in the natural condition. The value of the
trade is estimated at from £4000 to £6000.

The European trade in these earths is very large. Rouen exports some
5000 tons yearly, and Havre about 1500 tons.

Similar deposits occur in America, where they are known as
“paint-beds,” and the earths are called “metallic paints.” A prominent
example is the paint-bed at Lehigh Gap, Carbon County, Pennsylvania,
which was originally opened as an ironstone mine. The mineral proved
valueless metallurgically, but remarkably useful as a pigment, since
it contains about 28 per cent. of hydraulic cement, which hastens the
drying and causes the paint to set without any addition of artificial
dryers, thereby making it eminently fitted for all outdoor application.

Along the outcrop of the paint, the beds are covered by a cap or
overburden of clay, and by the decomposed lower portion of the
Marcellus slate, which is 50 feet thick at the Rutherford shaft.

Beginning with the Marcellus slate, the measures occur in the following
descending order:--

_a._ Hydraulic cement (probably Upper Helderberg), very hard and
compact.

_b._ Blue clay, about 6 inches thick.

_c._ Paint-ore, varying from 6 inches to 6 feet in thickness.

_d._ Yellow clay, 6 feet thick.

_e._ Oriskany sandstone, forming the crest and southern side of the
ridge.

East of the Rutherford shaft the sandstone forms the top-rock of the
bed. This is due to an overthrow occurring between the Rutherford
tunnel and shaft.

The paint-bed is not continuous throughout its extent. It is faulted
at several places; sometimes it is pinched out to a few inches and
again increases in width to 6 feet. A short distance south of Bowman’s
there is a fault striking; north-east in the Marcellus slate, which has
produced a throw of about 200 feet. The measures dip from 10° to 90°.
The dip at the Rutherford shaft is about 79° south, whereas at the
tunnel it is 45° north. The ore is bluish-gray, resembling limestone,
and is very hard and compact. The bed is of a lighter tint, however,
in the upper than in the lower part, and this is probably due to its
containing more hydraulic cement in the upper strata. The paint-ore
contains partings of clay and slate at various places.

At the Rutherford shaft there are fine bands of ore, alternating with
clay and slate, as follows--Sandstone (hanging-wall), clay, ore, slate,
ore, clay, ore, clay, ore, slate, ore, cement, slate (foot-wall).
These partings, however, are not continuous, but pinch out, leaving
the ore without the admixture of clay and slate. Near the outcrop the
bed becomes brown hematite, due to the leaching out of the lime and to
complete oxidation. Occasionally, streaks of hematite are interleaved
with the paint-ore. In driving up the breasts, towards the outcrop, the
ore is found at the top in rounded, partially oxidised and weathered
masses, called “bombshells,” covered with iron oxide and surrounded by
a bluish clay. In large pieces the ore shows a decided cleavage.

The method used in mining is a variation of panel-work. Nearly the
same system of working is employed by all of the companies who have
developed their mines either by means of tunnels or shafts. Tunnels are
preferred whenever equally convenient, because they involve no expenses
for pumping and hoisting machinery, fuel, repairs to machinery, &c.

The following description of the operation of the Rutherford mines is
typical of all the workings in the vicinity.

The Rutherford tunnel is 6 feet high and 600 feet long. The gangways
are driven along the foot-wall of the cement side, 6 feet high, and
are heavily timbered and lagged at the top and on the clay side. The
sets of timbers are 3½ feet apart, and usually of 9-inch timber. The
width at the top is 3½ feet, with a spread of 5 feet at the bottom, the
extra width being cut from the clay. Where the cement-rock is firm,
the collar is hitched 6 inches into it and supported by a leg on the
clay side. The cost of the timber is 54 cents (2_s._ 3_d._) per set,
including the lagging. The monkey gangway, which carries the air along
the top of the breast from the air-shaft, is 2½ feet high, 1½ feet wide
at the top, with a spread of 2½ feet at the bottom. Wooden rails with a
gauge of 18 inches are spiked to the cross-ties.

The gangway is not driven continuously, but after being driven about 55
feet on either side of the shaft, the breasts are started 25 feet from
the shaft, a pillar being left to protect it. The breast is then opened
up to the face of the gangway, and when one ore-breast is worked out,
the gangway is driven ahead about 30 feet, and a new breast is opened
and worked out before commencing a third. The air-hole is first driven
to the surface, then the breast is opened to its full width of 6 feet.
The thickness of the bed of ore here varies from 4 to 6 feet, depending
upon the thickness of the partings of clay and slate. The clay and
slate are left on the bottom, which is made sloping to allow the ore to
roll down to the shute; this is 6 feet wide and 4 feet long and heavily
timbered. Small props or sprags are hitched into the cement, and wedged
with a lid on the clay side to prevent falls of rock.

The holes are drilled by hand in the clay-partings. They vary in depth
from 1 to 4 feet, and the charge of dynamite is varied correspondingly,
according to the amount of ore it is desired to throw down. The loose
ore is wedged down with crowbars and picks, and is then freed from
any adhering clay and thrown down the shute. It is there loaded into
boxes holding about half a ton each, which are pushed to the shaft on
a truck. The ore-boxes have four rings at the corners, to which are
attached four chains, suspended from the wire hoisting-rope. At the top
of the shaft the boxes are detached and placed on a truck, which is run
to the dump. Thirty cars, averaging 15 tons, are extracted in a day
of two shifts, the day-shift working nine hours and the night-shift
eleven. The pay of the miners is 5_s._ per shift. The cost of mining
the ore averages 7_s._ per ton.

The ore, as it comes from the mines, is free from refuse, great care
having been taken to separate slate and clay from it in the working
places. It is hauled in 2-ton wagons to kilns, which are situated on a
hill-side for convenience in charging. The platform upon which the ore
is dumped is built from the top of the kiln to the side of the hill.
The ore is first spalled to fist-size and freed from slate, and is then
carried in buggies to the charging-hole of the kiln.

The slate, when burned, has a light yellowish colour, which would
change the colour of the product. Figs. 30 to 32 represent a front
elevation of the kiln and two sections at right angles to each
other. The kiln is 22 feet high and 16 feet square on the outside.
The interior is cylindrical, 5 feet in diameter, with a fire-brick
lining _a_ of the best quality. The interior lining slopes from the
fire-place _b_ to the door _c_, by which the charges are withdrawn;
this facilitates the removal of the calcined ore. The casing _d_ is of
sandstone, 5½ feet thick, and tied together with the best white-oak
timber _e_. When charged, a kiln holds 16 tons of ore, and the kiln
is kept constantly full. The heat passes from the fire-places _b_--of
which there are two, placed diametrically opposite each other--through
a checker-work _f_ of brick into the centre of the charge. The charge
enters at _g_ and is withdrawn by a door _c_ in the front wall, 2 feet
long and 18 inches high. The ashpit is at _i_. The fire is kept at a
cherry-red heat, and about one cord of wood is burned every twenty-four
hours.

The kiln works continuously, calcined ore being withdrawn and
fresh charges made without interruption. The ore is subjected for
forty-eight hours to the heat, which expels the moisture, sulphur and
carbon-dioxide. About 1½ tons of calcined ore are withdrawn every three
hours during the day. The outside of the lumps of calcined ore has a
light brown colour, while the interior shows upon fracture a darker
brown. Great care is necessary to regulate the heat

[Illustration: Figs. 30, 31, 32.--RUTHERFORD AND BARCLAY KILN.]

so that the ore is not over burned. When this happens, the product has
a black scoriaceous appearance, and is unfit for the manufacture of
metallic paint, as it is extremely hard to grind.

The calcined ore is carried from the kiln in wagons to the mill, where
it is broken to the size of grains of corn in a rotating crusher. The
broken ore is carried by elevators to the stock-bins at the top of the
building, and thence by shutes to the hoppers of the mills, which grind
it to the necessary degree of fineness. Elevators again carry it to the
packing-machine by a spout, and it is packed into barrels holding 500,
300, or 100 lb. each.

_Characters._--Ochres owe their colour to hydrated oxide of iron,
besides which body they contain clayey matter (silicate of alumina),
earthy matters, barytes, carbonate and sulphate of calcium, &c.,
dependent upon the locality from whence they are obtained; thus
Derbyshire ochres contain mostly calcareous earthy matters, barytes,
gypsum, &c., while Oxford ochres and French ochres contain clayey
matter; Welsh ochres are variable, and usually contain a good deal of
silicious matter.

Crude ochres should first be assayed for actual colouring matter and
grit or refuse. This can be done by a kind of levigation method: 200
grains of the crude ochre are crushed in a mortar; the grinding must
not be too well done, or otherwise faulty results will be obtained. The
crushed ochre is put into a tall conical glass; a long glass funnel
passes to the bottom of the glass, and the whole is arranged in a large
glass basin or dish. A current of water is now caused to flow down the
glass funnel; this washes the fine particles of ochre away from the
grit, and they are carried over the sides of the glass into the dish.
Here they are allowed to settle, and are collected and weighed after
drying, an operation which gives the amount of ochre in the crude
material.

Levigated and prepared ochres can be tested for colour and covering
power, by the usual methods. These are the most important points about
ochres to which attention should be paid.

ORPIMENT.--Orpiment, king’s yellow, or trisulphide of
arsenic, is a lemon or orange-yellow coloured substance, found native
in Hungary, the Hartz, and other places. The finest samples used by
artists (golden orpiment) come from Persia. The commercial article is
artificially prepared for use as a pigment in the following way:--

A mixture of arsenious acid and sulphur is placed in an iron
subliming-pot, similar to those used in the preparation of crude
white arsenic. The mixture is then heated until the sublimate which
immediately forms upon the rings fixed above the pot begins to
melt. The proportions of the two ingredients used vary largely, the
best colours being probably produced when the mixture contains from
one-third to one-fifth of sulphur; for the lighter colours, a smaller
proportion of sulphur is employed. Orpiment made in this manner
consists of a mechanical mixture of sulphide and oxide of arsenic.

Orpiment is also employed as a dye, in the preparation of fireworks,
and in some depilatories. The native sulphide is preferred to the
artificial variety by artists and dyers, by reason of its richer
colour; but it is a colour which in reality is hardly ever used now.
Sometimes it is employed in water-colours, but as a pigment it is
worthless. If it comes in contact with white lead it is decomposed in
time, and a brown or black sulphide of lead is formed. While it endures
it is a very brilliant colour.

REALGAR.--Realgar or disulphide of arsenic, is a deep
orange-red substance, soluble in water, and highly volatile and
poisonous. It is found native in some volcanic districts, especially
in the neighbourhood of Naples; but the commercial article is made by
distilling, in earthenware retorts, arsenical pyrites, or a mixture
of sulphur and arsenic, or of orpiment and sulphur, or of arsenious
acid, sulphur, and charcoal, in the proper proportions; it has not the
brilliant colour of the native mineral, and is much more poisonous.

On a large scale, the manufacture is carried on in the following
way:--The ingredients are mixed together in such proportions that the
mixture shall contain 15 per cent. of arsenic, and from 26 to 28 per
cent. of sulphur, in order to make allowance for the volatilisation
of a portion of the latter substance. The mixture is then placed in a
series of earthenware retorts, which are charged every twelve hours
with about 60 lb.; this quantity should fill them three parts full.
These are then gradually heated to redness for from eight to twelve
hours, during which time the realgar distils off, and is collected
in earthen receivers, similar to the retorts, but perforated with
small holes to permit the escape of these gases. After the operation,
the receivers are emptied, and the crude product is remelted. This
is performed in cast-iron pots, the contents being well agitated,
and the slag carefully removed. The requisite amount of sulphur or
arsenic is then added, according to the colour of the mixture, or else
a proper quantity of realgar containing an excess of the required
constituent, and the mass is again stirred. When, on cooling, it
exhibits the correct colour and compactness, it is run off into conical
moulds of sheet iron, cooled and broken up; it is sometimes refined
by re-sublimation. The chief use of realgar is as a pigment; and in
pyrotechny in the preparation of white fires.

As a pigment it possesses the same features and faults as its close
ally orpiment.

SIENNAS.--Another name for ochres, described on p. 272.



CHAPTER IX.

LAKES.


Organic colouring matters for use as pigments are mostly made in the
form of “lakes,” by one of the three following methods:--

(_a_) To a filtered solution of the colouring matter is added a
solution of alum; the whole is agitated, and the colour is precipitated
by a solution of carbonate of potash.

(_b_) A solution of the colouring matter is made in a weak alkaline
lye, and precipitated by adding a solution of alum.

(_c_) Recently-precipitated alumina is agitated with a solution of the
colouring matter as before, until the liquid is nearly decolorised,
or the alumina assumes a sufficiently deep tint. The first method is
generally adopted for acidulous solutions of colouring matter, or those
injured by alkalies; the second for those not injured by alkalies; the
third, for those whose affinity for gelatinous alumina enables them to
combine with it by mere agitation.

Alumina in a state suitable for the preparation of the pigments known
as “lakes” may be produced in the following manner:--Dissolve 1 lb. of
alum in ½ gallon of water, and add 75 grains of sulphate of copper,
and about ¼ lb. of zinc turnings; leave the mixture for three days in
a warm place, renewing the water lost by evaporation. The copper is
first deposited upon the zinc, the two metals thus forming a voltaic
couple sufficiently strong. Hydrogen is disengaged, sulphate of zinc
is formed, and the alumina gradually separates in the state of a
very fine powder; the action is allowed to continue until there is
no more alumina left in solution, or until ammonia ceases to give a
precipitate. If the reaction is prolonged beyond this point, oxide of
iron will precipitate if present. The alumina washes easily, and does
not contract upon drying.

BRAZIL-WOOD LAKE.--(_a_) Digest 1 lb. ground Brazil-wood in
4 gal. water for 24 hours, boil ½ hour, and add 1½ lb. alum dissolved
in a little water; mix, decant, strain, add ½ lb. tin solution, again
mix well, and filter; to the clear liquid cautiously add a solution of
carbonate of soda while a precipitate forms, avoiding excess; collect,
wash, and dry. The shade will vary according as the precipitate is
collected.

(_b_) Add washed and recently-precipitated alumina to a strong filtered
decoction of Brazil-wood.

CARMINATED LAKE.--(_a_) The cochineal residue left in making
carmine is boiled with repeated portions of water till exhausted;
the liquor is mixed with that decanted off the carmine, and at once
filtered; some recently-precipitated alumina is added, and the whole
is gently heated, and well agitated for a short time. As soon as the
alumina has absorbed enough colour, the mixture is allowed to settle,
the clear portion is decanted, and the lake is collected on a filter,
washed, and dried. The decanted liquor, if still coloured, is treated
with fresh alumina till exhausted, and thus a lake of second quality is
obtained.

(_b_) To the coloured liquor obtained from the carmine and cochineal
as just stated, a solution of alum is added, the filtered liquor is
precipitated with a solution of carbonate of potash, and the lake is
collected and treated as before. The colour is brightened by addition
of tin solution.

CARMINE.--Boil 1 lb. cochineal and 4 dr. carbonate of potash
in 7½ gal. water for ¼ hour. Remove from the fire, and stir in 8 dr.
powdered alum, and allow to settle for 20 to 30 minutes. Pour the
liquid into another vessel, and mix in a strained solution of 4 dr.
isinglass in 1 pint water; when a skin has formed upon the surface,
remove from the fire, stir rapidly, and allow to settle for ½ hour,
when the deposited carmine is carefully collected, drained, and dried.

COCHINEAL LAKE.--(_a_) Digest 1 oz. coarsely powdered
cochineal in 2½ oz. each water and rectified alcohol for a week;
filter, and precipitate by adding a few drops of tin solution every 2
hours, till the whole of the colouring matter is thrown down; wash the
precipitate in distilled water, and dry.

(_b_) Digest powdered cochineal in ammonia water for a week; dilute
with a little water, and add the liquid to a solution of alum as long
as any precipitate (lake) falls.

(_c_) Boil 1 lb. coarsely powdered cochineal in 2 gal. water for 1
hour; decant, strain, add solution of 1 lb. cream of tartar, and
precipitate with solution of alum. By adding the alum first and
precipitating the lake with the tartar, the colour is slightly changed.

MADDER LAKE.--(_a_) Tie 2 oz. madder in a cloth, beat it well
in a pint of water in a stone mortar, and repeat the process with about
5 pints of fresh water till it ceases to yield colour; boil the mixed
liquor in an earthern vessel, pour into a large basin, and add 1 oz.
alum dissolved in 1 pint boiling water; stir well, and gradually pour
in 1½ oz. of strong solution of carbonate of potash; let stand until
cold, pour off the yellow liquor from the top, drain, agitate the
residue repeatedly in 1 qt. boiling water, decant, drain, and dry.

(_b_) Add a little solution of acetate of lead to a decoction of
madder, to throw down the brown colouring matter; filter, add solution
of tin or alum, precipitate with solution of carbonate of soda or
potash, and proceed as before.

(_c_) Macerate 2 lb. ground madder in 1 gal. water for 10 minutes;
strain and press quite dry; repeat a second and third time, and add
to the mixed liquors ½ lb. alum dissolved in 3 qt. water; heat in
water-bath for 3-4 hours, adding water as it evaporates; filter first
through flannel, and when cold enough through paper; add solution of
carbonate of soda as long as precipitate falls; wash the latter till
the water comes off colourless, and dry.

YELLOW LAKES.--(_a_) Boil 1 lb. Persian berries,
quercitron-bark, or turmeric, and 1 oz. cream of tartar, in 1 gal.
water till reduced to half; strain the decoction, and precipitate by
solution of alum.

(_b_) Boil 1 lb. of the dyestuff with ½ lb. alum in 1 gal. water, and
precipitate by solution of carbonate of potash.

(_c_) Boil 4 oz. annatto and 12 oz. pearlash in 1 gal. water for ½
hour; strain, precipitate by adding 1 lb. alum dissolved in 1 gal.
water till it ceases to produce effervescence or a precipitate; strain
and dry.

For information concerning the numerous coal-tar colours now largely
manufactured into lakes, the reader is referred to “Spon’s Encyclopædia
of Industrial Arts.”



CHAPTER X.

LUMINOUS PAINTS.


The luminosity of minerals has an obvious practical value in the case
of such substances as can be conveniently applied in the form of a
paint to surfaces which are alternately exposed to light and darkness,
such exposed surfaces emitting at one time the light which they have
absorbed at another. Familiar illustrations are street plates, buoys,
and interiors of railway carriages having to traverse many tunnels. The
light absorbed may be either daylight or powerful artificial light.
With this object, several compositions are prepared under the generic
name of luminous paints. They are chiefly as follows:--

(1) Balmain’s.--This consists of a phosphorescent substance introduced
into ordinary paint. The phosphorescent substance employed for the
purpose is a compound obtained by simply heating together a mixture of
lime and sulphur, or substances containing lime and sulphur, such as
alabaster, gypsum, &c., with carbon or other agent, to remove a portion
of the oxygen present; or by heating lime in a vapour containing
sulphur. In applying this phosphorescent powder, the best results are
obtained by mixing it with a colourless varnish made from mastic and
turpentine; drying oils, gums, pastes, sizes, &c., may, however, also
be used.

(2) A French compound.--100 lb. of a carbonate of lime and phosphate
of lime produced by the calcination of sea-shells, and especially
those of the genus _Tridacna_ and the cuttle-fish bone, intimately
mixed with 100 lb. of lime rendered chemically pure by calcination,
25 lb. of calcined sea-salt, 25-50 per cent. of the whole mass of
sulphur, incorporated by the process of sublimation, and 3-7 per cent.
of colouring matter in the form of powder composed of monosulphide of
calcium, barium, strontium, uranium, magnesium, aluminium, or other
mineral or substance producing the same physical appearances, _i.
e._ which, after having been impregnated with light becomes luminous
in the dark. After having mixed these five ingredients intimately,
the composition obtained is ready for use. In certain cases, and
more especially for augmenting the intensity and the duration of the
luminous effect of the composition, a sixth ingredient is added, in the
form of phosphorus reduced to powder, which is obtained from seaweed by
the well-known process of calcination. As to proportion, it is found
that the phosphorus contained in a quantity of seaweed, representing
25 per cent. of the weight of the composition formed by the five
above-named ingredients, gives very good results.

The phosphorescent powder thus obtained and reduced to paste by the
addition of a sufficient quantity of varnish, such as copal, may serve
for illuminating a great number of objects, by arranging it in more or
less thick coatings, or by the application of one or more coatings of
the powder incorporated in the varnish, or by varnishing previously
and sprinkling the dry powder upon the varnish. The amount of powder
applied should not exceed the thickness of a thin sheet of cardboard.

The dry phosphorescent powders are also converted into translucent
flexible sheets of unlimited length, thickness, and width, by mixing
them with about 80 per cent. of their weight of ether and collodion in
equal parts in a close vessel, and rolling the product into sheets,
with which any objects may be covered which are intended to be
luminous in the dark. The powders may also be intimately mixed with
stearine, paraffin, rectified glue, isinglass, water glass, or other
transparent solid matter, in the proportion of 20 to 30 per cent. of
the former with 50 to 80 per cent. of either of these substances,
and this mass is then reduced into sheets of variable length, width,
and thickness, according to their intended applications. A luminous
glass is also manufactured by means of the powders, by mixing them in
glass in a fused state in the proportions of 5 to 20 per cent. of the
mass of glass. After the composition has been puddled or mixed, it is
converted into different articles, according to the ordinary processes;
or after the manufacture of an object still warm and plastic, made of
ordinary glass, it is sprinkled with the powders, which latter are then
incorporated into the surface of the article by pressure exerted in the
mould, or in any other suitable way.

It has been observed, after various trials, that the passage of an
electric current through the different compositions augments their
luminous properties or brilliancy to a great extent; this peculiarity
is intended to be utilised in various applications too numerous to
describe, but of which buoys form a good example. The current of
electricity is furnished by plates of zinc and copper mounted on
the buoy itself, when the latter is used at sea; but in rivers and
fresh-water inlets the battery will be carried in the interior of
the buoy. To secure the full effect, 10 to 20 per cent. of fine
zinc, copper, or antimony dust is added to the phosphorescent powder
described.

(3) Take oyster-shells and clean them with warm water; put them into
the fire for ½ hour; at the end of that time take them out and let them
cool. When quite cool, pound them fine, and take away any grey parts,
as they are of no use. Put the powder in a crucible with alternate
layers of flowers of sulphur. Put on the lid, and cement with sand
made into a stiff paste with beer. When dry, put over the fire and
bake for an hour. Wait until quite cold before opening the lid. The
product ought to be white. You must separate all grey parts, as they
are not luminous. Make a sifter in the following manner:--Take a pot,
put a piece of very fine muslin very loosely across it, tie around
with a string, put the powder into the top, and rake about until only
the coarse powder remains; open the pot, and you will find a very
small powder. Mix it into a thin paint with gum water, as two thin
applications are better than one thick one. This will give paint that
will remain luminous far into the night, provided it is exposed to the
light during the day.

(4) Sulphides of calcium, of barium, of strontium, &c., give
phosphorescent powders when duly heated. Each sulphide has a
predominant colour, but the temperature to which it is heated has a
modifying effect on the colour. Calcine in a covered crucible, along
with powdered charcoal, sulphate of lime, sulphate of baryta, or
sulphate of strontia; there is produced in each case a greyish white
powder, which, after exposure to strong light (either sun-light or
magnesium light), will be phosphorescent, the colour depending on the
sulphate used and the degree of heat employed.

(5) Five parts of a luminous sulphide of an alkaline earth, 10 of
fluorspar, cryolite, or other similar fluoride, 1 of barium borate;
powdered, mixed, made into a cream with water, painted on the glass or
stone article, dried, and fired in the usual way for enamels. If the
article contains an oxide of iron, lead, or other metal, it must be
first glazed with ground felspar, silica, lime phosphate, or clay, to
keep the sulphur of the sulphide from combining with the metal. The
result is an enamelled luminous article. (Heaton and Bolas.)

(6) Boil for 1 hour 2¼ oz. caustic lime, recently prepared by calcining
clean white shells at a strong red heat, with 1 oz. pure sulphur
(flowers) and 1 qt. soft water. Set aside in a covered vessel for a
few days; then pour off the liquid, collect the clear orange-coloured
crystals which have deposited, and let them drain and dry on bibulous
paper. Place the dried sulphide in a clean graphite crucible provided
with a cover. Heat for ½ hour at a temperature just short of redness,
then quickly for about 15 minutes at a white heat. Remove cover, and
pack in clay until perfectly cold. A small quantity of pure calcium
fluoride is added to the sulphide before heating it. It may be mixed
with alcoholic copal varnish. (_Boston Jl. Chem._)

The luminous calcic sulphide (also called sulphide of calcium),
now obtainable in the market, has a yellowish white tint, which
considerably limits its direct application as a paint. On the other
hand, the calcic sulphide, or the luminous paint obtained therefrom,
loses its luminous property, if it is directly mixed with the ordinary
commercial paints.

Schatte, of Dresden, produces durable white or coloured paints,
containing a luminous substance which causes them to shine in the dark,
without changing or neutralising in daylight the tint of the colouring
substance or substances contained in such paints.

For this purpose, Zanzibar or cowrie copal is melted over a charcoal
fire, 15 parts of this melted mass are dissolved in 60 parts of French
turpentine, and the resulting mixture is filtered, whereupon 25 parts
of pure linseed oil are added, which linseed oil has been previously
boiled and allowed to cool a little. The lake varnish thus obtained is
carefully treated in a paint mill with granite rollers, and worked into
a luminous paint by one of the processes hereinafter described.

Iron rollers capable of giving off under great pressure small particles
of iron, which might affect the luminous power, should not be used.
Lake varnish as obtained in commerce contains nearly always lead
or manganese, which would destroy the luminous power of the calcic
sulphide.

The proportions given are as follows:--

_Pure White_: By mixing 40 parts of lake varnish obtained as described
with 6 parts of prepared baric sulphate, 6 parts of prepared calcic
carbonate, 12 parts of prepared zinc sulphide white, and 36 parts of
calcic sulphide in a luminous condition, in an oil vessel, and worked
into a coarse emulsion, which is then ground fine between the rollers.

_Red_: 50 parts of the said lake varnish are mixed with 8 parts of
prepared baric sulphate, 2 parts of prepared madder lake, 6 parts of
prepared realgar (diarsenious disulphide), and 34 parts of calcic
sulphide in a luminous condition, and the mixture is worked in the same
way as described for white.

_Orange_: 46 parts varnish are mixed with 17·5 parts prepared barium
sulphate, 1 part prepared India yellow, 1·5 parts prepared madder lake,
and 38 parts luminous calcium sulphide.

_Yellow_: 48 parts varnish are mixed with 10 parts prepared barium
sulphate, 8 parts barium chromate, and 34 parts luminous calcium
sulphide.

_Green_: 48 parts varnish are mixed with 10 parts prepared barium
sulphate, 8 parts chromium oxide green, and 34 parts luminous calcium
sulphide.

_Blue_: 42 parts varnish, 10·2 parts prepared barium sulphate, 6·4
parts ultramarine blue, 5·4 parts cobalt blue, and 46 parts luminous
calcium sulphide.

_Violet_: 42 parts varnish, 10·2 parts prepared barium sulphate,
2·8 parts ultramarine violet, 9 parts cobalt arsenate, and 36 parts
luminous calcium sulphide.

_Grey_: 45 parts of the varnish are mixed with 6 parts prepared barium
sulphate, 6 parts prepared calcium carbonate, 0·5 part ultramarine
blue, 6·5 parts grey zinc sulphide.

_Yellowish-brown_: 48 parts varnish, 10 parts precipitated barium
sulphate, 8 parts auripigment, and 34 parts luminous calcium sulphide.

Luminous colours for artists’ use are prepared by using pure East India
poppy oil, in the same quantity, instead of the varnish, and taking
particular pains to grind the materials as fine as possible.

For luminous oil-colour paints, equal quantities of pure linseed
oil are used in the place of the varnish. The linseed oil must be
cold-pressed and thickened by heat.

All the above luminous paints can be used in the manufacture of
coloured papers, &c., if the varnish is altogether omitted, and the dry
mixtures are ground to a paste with water.

The luminous paints can also be used as wax colours for painting on
glass and similar objects, by adding, instead of the varnish, 10 per
cent. more of Japanese wax and one-fourth the quantity of the latter of
olive oil. The wax colours prepared in this way may also be used for
painting upon porcelain, and are then carefully burned without access
of air. Paintings of this kind can also be treated with water glass.



CHAPTER XI.

EXAMINATION OF PIGMENTS.


Besides the chemical tests for purity and adulteration, which
necessarily must vary with each pigment, there are certain other
examinations which partake rather of a mechanical nature, and which
are applicable to practically all pigments without any modification.
They are directed chiefly to ascertaining fineness, body, colour, and
durability.

FINENESS.--Fineness may be tested for as follows:--A tall
glass cylinder is filled with clean water, and about ½ oz. of the
pigment under examination is well shaken in the water; the glass is
placed on one side to settle out, and the length of time taken to
settle may be noted for future reference. The finer the sample, the
longer the time it takes to settle out; and the time in seconds may be
taken as an approximate estimate of the fineness of the sample.

BODY OR COVERING POWER.--An equal and exact quantity, say 50
gr., of the sample under examination and of a standard sample of the
same degree of fineness is weighed out, and placed on two separate
sheets of paper. To each sample is added an equal quantity, say 15
gr., of vegetable black or of very finely ground barytes, according
as the pigment is a light or a dark-tinted kind. The ingredients of
each sample are most intimately and completely mixed, and the tints
of the two mixtures are compared by observation with the naked eye.
The pigment which most nearly retains its own colour, possesses the
greatest body or covering power.

COLOUR.--The colour or tint of a pigment can only be
estimated by comparing it with a standard sample; this is done as
follows:--A sheet of black paper, with, a dead surface, is spread
out on a table in front of a window, a small heap of the standard is
placed on the paper, and next to it a similar heap of the sample to
be compared; by means of a palette knife the surface of the two heaps
is flattened out; on now carefully looking at the two heaps, the one
which has the purest colour can readily be picked out. The heaps should
be looked at from several points of view before a final judgment is
arrived at. If the pigment is dark-coloured, it should be spread on
white paper, taking care that the same kind of paper is used for the
two samples.

_Durability._--Durability is not a difficult point to test, but it
takes some time to make a complete test. The best method is to mix a
small quantity of the pigment in question with raw linseed oil, cover
a piece of glass with the mixture, and expose it outside to the action
of the sun and air for some time, noting at intervals how it behaves.
It is well for the sake of comparison to coat a second piece of glass
with the mixture, and keep this in a dark place. The difference between
the two from time to time will show how the pigment behaves under the
influence of light and air. As the durability of pigments is decidedly
different according as they are used in oil or water-colour painting,
the oil in the former case acting as a protective agent, it is well
to use a similar test, using a little gum water as a vehicle to mix
the pigment with. It will take from two to three months at least to
properly test the durability of a pigment in summer, while in winter
the time will be increased considerably. Glass is the best substance to
use, as it is quite neutral, and does not of itself introduce into the
test any injurious element, as wood or paper might do, although these
bodies may be used if thought desirable.



CHAPTER XII.

VEHICLES AND DRYERS.


Paint consists essentially of two parts, the pigment (see Chapters I.
to VIII.), and the vehicle or medium. In the case of oil paints, a
third substance termed a dryer becomes necessary, to facilitate the
“drying,” or solidification of the vehicle.

A perfect vehicle should mix readily with the pigment, forming a mass
of about the consistency of treacle. It should itself be colourless,
and have no chemical action upon the pigments with which it is mixed.
When spread out in a thin layer upon a non-porous substance, it should
solidify, and form a film not liable to subsequent disintegration or
decay, and sufficiently elastic to resist a slight concussion.

Unfortunately, we possess no vehicle which complies with all these
conditions; those which most nearly approach them are the drying
oils. Oils are compound bodies containing acids and a base. Some oils
oxidise very rapidly, while others do not oxidise at all. When oils
oxidise they change their colour, and however white they may be at
first, they gradually turn yellow and finally brown. The advantages of
oils are that they mix kindly with most pigments, can be dissolved in
turpentine, and can be used in almost any desired state of fluidity.
Against these have to be set the disadvantage of the oxidation of the
oil, to which oxidation the use of oil in paint is entirely due.

The use of oil in painting is said to have been invented in the 14th
century, and, in a short time, it reached a considerable degree of
perfection. We have only to compare a Van Eyck with a painting by
a modern master--Turner, for instance--to see that even the best of
recent painters have not succeeded in giving to their works that
durability which the originators of the method attained. All organic
substances are liable to a more or less rapid oxidation, especially if
exposed to light and heat. Oil is no exception to this rule; but it
seems that, in its pure state, it is much more durable than when mixed
with other substances. Although ground-nut-and poppy-oils are sometimes
employed by artists where freedom from colour is essential, yet
linseed-oil is the vehicle of by far the larger proportion of paints
used both for artistic and general purposes.

Oil-paint appears to have been unknown to the ancients, who used
various vehicles, chiefly of animal origin. One of these, which was
in high repute at Rome, was the white of eggs beaten with twigs of
the fig-tree. No doubt the india-rubber contained in the milky juice
exuding from the twigs contributed to the elasticity of the film
resulting from the drying of this vehicle. Pliny was aware of the fact
that when glue is dissolved in vinegar and allowed to dry, it is less
soluble than in its original state. Many suggestions have been made
in modern times for vehicles in which glue or size plays an important
part. In order to render it insoluble, various chemicals have been
added to its solution, such as tannin, alum, and a chromic salt. None
of these vehicles, however useful for special purposes, has become
sufficiently well known to warrant description here.

Substitutes which do claim attention are wax and dammar gum, or
paraffin wax, dissolved in turpentine. The colours must then be ground
in turpentine and not in oil. Such a vehicle is very pleasant to work
with, and gives good results; moreover, it permits alterations or
corrections to be made by rubbing out with turpentine. Nevertheless,
both the wax and the turpentine undergo oxidation to some extent, and
are therefore not altogether free from the same objections as oils. But
benzol, especially when carefully prepared, answers all the purposes
of turpentine without undergoing oxidation. The only drawback that
can be urged against benzol is its odour, which some people have an
aversion to; but it really has very little smell, and it evaporates
away completely in a very short time. A mixture of wax, dammar, and
benzol forms an excellent vehicle. The wax may be replaced by paraffin
wax with advantage.

It is desirable to be able to ascertain whether the oil intended for
use is, or is not, adulterated with non-drying oil. The distinction of
non-drying oils is that they solidify when acted upon by peroxide of
hydrogen, or by sub-nitrate of mercury--the oleic acid is concreted,
and a substance called elaidin is formed. This does not take place with
the drying oils.

The oils used in paint making are chiefly--

  Ground-nut.
  Hempseed.
  Kukui or candle-nut.
  Linseed.
  Menhaden.
  Poppy-seed.
  Tobacco-seed.
  Walnut.
  Wood or Tung.

GROUND-NUT OIL.--The ground-nut or pea-nut (_Arachis hypogæa_)
is very widely cultivated in the tropics for the sake of its oily
seeds. In Java, the oil is extracted by drying the seeds in the sun,
and then subjecting them to pressure. In European mills, the nuts are
first cleaned, then decorticated and winnowed, by which the kernels are
left perfectly clean. These are crushed like any other oil seed, and
put into bags, which are introduced into cold presses; the expressed
oil is refined by passing through filter-bags. The residual cake is
ground very fine, and pressed under 3 tons to the inch, in the presence
of steam-heat; this affords a second quantity of oil, inferior in
quality to the cold pressed. The usual product is 1 gal. of oil from 1
bush. of nuts by the cold process, besides the extra yield by the hot
pressing. In France, where the oil is most largely prepared, three
expressions are adopted, as with some sorts of gingelly: the first
gives about 18 per cent. of superfine oil, fit for alimentary purposes;
the second, after moistening with cold water, affords 6 per cent. of a
fine oil, suitable for lighting and for woollen-dressing; the third,
after treating with hot water, yields 6 per cent. of _rabat_, or oil
applicable only to soap-making. In India, the total mean yield is 37
per cent. at Pondicherry, and 43 in Madras.

The cold-pressed oil is almost colourless, of agreeable faint odour,
and bland olive-like flavour. The best has a sp. gr. of about 0·918,
or 0·9163 at 59° F.; it becomes turbid at 37½° F., concretes at
26½°-25°F., and hardens at 19½° F. By exposure it changes very slowly,
but thickens with time, and assumes a rancid odour and flavour. It is
not a good oil for paint.

HEMPSEED-OIL.--The seeds of the hemp plant, so well-known
as a fibre-producer, are valued for their oil. It is from Russia and
Lorraine that the seed for expressing mostly comes. When the fibrous
stems are tied in bundles, the seed is rudely threshed out, and spread
in thin layers under cover to dry. The extraction of the oil is
performed in the same manner as with other seed oils, described on p.
308. The proportion of oil contained in the seed is about 34 per cent.
on an average; the yield varies from 25 to 30 per cent. The oil is
at first greenish or brownish-yellow, deepening with exposure to the
air; the flavour is disagreeable, and the odour is mild. It has a sp.
gr. of 0·9252 at 59° F.; it thickens at 5° F., and solidifies at-13°
to-18° F.; it dissolves in 30 parts of cold alcohol and any proportion
of boiling; it saponifies with difficulty, forming a soft soap, but
less soft than that from linseed oil. It is inferior for the painter’s
purposes.

KUKUI OR CANDLE-NUT OIL.--An oil bearing a multitude of names
is obtained from the candle-nut (_Aleurites moluccana_). It is the
most important product of the tree, and constitutes about two-thirds
of the entire weight of the kernel of the nut. A great obstacle to
its wider development is the difficulty encountered in extracting the
kernels from the shells, both on account of the extreme hardness of
the latter, and the obstinacy with which the two adhere. Boiling is
out of the question, as the kernels are cooked long before the shells
are affected; but there is every reason to suppose that a slight
roasting would have the desired effect, inasmuch as this plan seems
to be adopted successfully by the Samoans. The weight of the shells
necessitates this treatment being performed on the spot, and, as the
kernels quickly become rancid and dark-coloured after liberation, they
must also be operated upon without removal. The local cheapness of
labour is an additional argument in favour of preparing the oil at the
places where the nut grows. The extraction of the oil is very simple.
In Jamaica, Polynesia, and the East Indies, 50 per cent. is obtained by
boiling the kernels in water; by reducing the kernels to meal, heating
in a water-bath, and placing the mass in bags under hydraulic pressure,
the yield is about 60-66 per cent. The shells are themselves excellent
fuel. The oil is completely clarified by mere filtration. As ordinarily
prepared, it is amber-coloured, tasteless and odourless; slightly
viscid at the temperature of the air in England, congealing at 32°
F.; its sp. gr. is 0·923; it is insoluble in alcohol, and saponifies
readily, giving a very soft soda-soap. It dries less rapidly than
linseed oil, and is used for mixing paints and making oil-varnishes. It
is said to corrode tin plate and even platinum.

LINSEED-OIL.--The flax plant, so well known as yielding a
textile fibre, affords a valuable oil-seed. The supplies of linseed for
crushing are furnished chiefly by Russia and India. It is found that,
as a general rule, the colder the climate in which the seed is grown,
the greater are the drying properties of the oil, but the worse is its
colour. In India, preference is given to white seed, as yielding 2 per
cent. more oil, affording it more freely, and giving a softer and
sweeter cake, than the red seed; the latter, moreover, always comes
to market largely mixed with rape-seed, which is very difficult of
separation, and greatly depreciates the market value. Oil from unripe
seed is watery. The seed should always be kept for 3-4 months in a
dry place, as the oil furnished after this lapse of time is much more
abundant than when the expression takes place immediately after the
harvest. The seed is crushed and pressed in the manner described on
p. 308. The best and finest oil is that which is “cold-drawn”; it is
paler, less odorous, and less flavoured, but the yield is only 21-22
per cent. of the seed. By the aid of a temperature not exceeding 200°
F., and powerful and long-continued pressure, as much as 28 per cent.
of very good oil can be obtained. The cake forms a valuable cattle
food. The Italian variety is said to have a much more highly oleaginous
seed than the Russian.

Linseed-oil has a faint colour, and mild odour and flavour when pure,
but the commercial article is dark-yellow, with sharp repulsive flavour
and odour. Its sp. gr. is 0·930; at 0° F., a little solid fat separates
out; at-4° F., it solidifies. By exposure to the air, after heating
with oxide of lead, it rapidly dries up to a transparent varnish.
The fresh oil saponifies readily, giving a yellow and very soft soap
with soda; by saponification, it yields 95 per cent. of fatty acids,
chiefly linoleic, with a little oleic, palmitic, and myristic acids. It
dissolves in 1·6 parts of ether, and in 32 parts of alcohol at 0·820
sp. gr. The oil is very extensively used in the manufacture of paint
and oil-varnishes. For artists’ use it is purified by shaking up with
whiting, and warming. Linseed-oil is never met with in commerce really
pure, nor even the seed itself. Previous to the Crimean War, it was a
recognised custom at the Black Sea ports to add one measure of hemp
or other seed to every 39 of linseed. Since then the proportion has
advanced to 1 in 19, in addition to which the Indian seed is grown
mostly as a mixed crop with mustard and colza: pure linseed oil can
only be obtained by picking out the seeds individually.

Linseed-oil, to be suitable for painting, must dry well. A reliable
test is to cover a piece of glass with a film of the raw oil, and to
expose it to a temperature of about 100° F. The time which the film
requires to solidify is a measure of the quality of the oil. If the
oil has been extracted from unripe or impure seed, the surface of the
test-glass will remain “tacky” or sticky for some time, and the same
will happen if the oil under examination has been adulterated with
either an animal or vegetable non-drying oil.

Until recently, linseed oil was frequently adulterated with cotton-seed
oil, extracted from the waste seeds of the cotton plant. Where the
admixture was considerable, it could easily be detected by the sharp,
acrid taste of the cotton-seed oil. Now, however, means have been found
for removing this disagreeable taste, and the consequence has been that
cotton-seed oil is so largely used for adulterating olive-oil, or as a
substitute for it, that its price has risen above that of linseed oil.

Another adulterant which is rather difficult to detect is rosin. Oil
containing this substance is thick, and darker in colour than pure
oil. When the proportion of rosin is considerable, its presence may be
ascertained by heating a film of the oil upon a metallic plate, when
the characteristic smell of burning rosin will be perceptible. When the
percentage of rosin is too small for detection in this manner, a film
of the oil should be spread upon glass and allowed to dry. When quite
hard, the film should be scraped off, and treated with cold turpentine,
which will dissolve any rosin which may be present, without materially
affecting the oxidised oil. The presence of rosin may also be detected
by the following simple chemical test. The oil is boiled for a few
minutes with a small quantity of alcohol (sp. gr. O·9), and is allowed
to stand until the alcohol becomes clear. The supernatant liquid is
then poured off, and treated with an alcoholic solution of acetate of
lead. If the oil be pure, there will be but a very slight turbidity,
while the presence of rosin causes a dense flocculent precipitate.
Should linseed oil be adulterated with a non-drying oil, it will remain
sticky for months when spread out in a thin film upon glass or any
other non-absorbent substance.

The sp. gr. of linseed oil is, in some cases, of value in estimating
its quality; but as the variations are slight, it would be difficult to
detect them in so thick a liquid by means of an ordinary hydrometer. A
simple method of obtaining an approximate result is to procure a sample
of oil of known good quality, and to colour it with an aniline dye.
A drop of this tinted oil will, when placed in the oil to be tested,
indicate, by its sinking or swimming, the relative density of the
liquid under examination. Freshly-extracted linseed oil is unfit for
making paint. It contains water and organic impurities, respecting the
composition of which little is known, and which are generally termed
“mucilage.” By storing the oil in tanks for a long time, the water and
the greater part of the impurities are precipitated, forming at the
bottom of the cistern a pasty mass known as “foots.”

To accelerate the purification of the oil, and to remove at least a
portion of the colouring matter, various methods are in use. The action
of sulphuric acid upon linseed-oil is not so favourable as upon other
oils. It is, however, sometimes employed, in the proportion of two
parts of a mixture of equal volumes of commercial sulphuric acid and
water to 100 parts of oil. The dilute acid is poured gradually into
the oil, and the mixture is violently agitated for several hours. It
is then run into tanks, and allowed to settle. A concentrated solution
of chloride of zinc has been substituted for sulphuric acid in the
proportion of about 1½ per cent. of the weight of the oil. When the
reaction is complete, steam or warm water is admitted into the liquid,
in order to clarify it. Oil treated in this way loses a considerable
proportion of the colouring matter which it originally contained.

When the oil is to be used for white paint, it is sometimes bleached by
exposing it to the action of light. On a large scale, this is done by
placing it in shallow troughs, lined with lead and covered with glass.
The lead itself appears to have some influence upon the bleaching of
the oil, for the decoloration is not so rapid if the troughs be lined
with zinc. For small quantities, a shallow tray of white porcelain or
earthenware, similar to those in use for photographic purposes, gives
very good results, the white surface increasing the photo-chemical
action. It is not quite clear whether the presence of water accelerates
the bleaching of oil by this method; some manufacturers consider its
presence necessary, others omit it. Various salts are added to the
water, the one most in use being copperas.

However the oil may have been prepared, it will, if kept a long time,
deposit a sediment. At first this contains mucilage; but the sediment
from old oil consists chiefly of the products of decomposition of
the oil itself. The presence of oxygen is not necessary for this
decomposition; but it is increased by the action of light. Raw
linseed-oil dries more slowly than boiled; but the resulting film is
more brilliant and durable. Raw and boiled oil are therefore usually
mixed in proportions varying according to the time which can be allowed
for the paint to dry, or to the properties required of the film. For
the ordinary kinds of paint, equal parts of boiled and raw oils are
customary. Linseed-oil heated to a temperature of 350°-400° F. dries
much more rapidly than in its raw state.

MENHADEN OIL.--A fish eagerly sought for its oil on the
Atlantic coast of America’ is the “Menhaden” or “porgie” (_Alosa
[Brevoordia] Menhaden_), a member of the herring family, about 8-14
in. long. The fishery is carried on all along the coast from Maine to
Maryland. The fish leave the Gulf Stream and strike the coast of New
Jersey in April, reaching the coast of Maine in May-June, and remaining
till October-November. They migrate in enormous schools, and are
caught in seines, carried by the fastest and smartest yachts. Very few
of the fish are sent to the table; nearly all are boiled down for their
oil.

This is performed in the following manner:--The fish are shot into
receiving-tanks situated outside the building; thence a sliding door
opens into the boiling-tanks, which are long, watertight, uncovered
boxes, of varying capacity, provided with a coil of perforated pipe
for the admission of steam, and a plug-hole for the exit of the liquid
after boiling. Some water is put into the tanks ready for the fish, and
as soon as the latter have been introduced, steam is turned on, and the
whole mass is boiled for 20-40 minutes. When the cooking is completed,
the liquor, containing a portion of the oil of the fish, is drawn
off into settling tanks, for the recovery of the oil. The “pomace”
or cooked fish is raked into “curbs,” perforated cylinders fitted
with hinged bottoms, and these, when full, are placed under hydraulic
presses. Pressure is applied so long as water and oil continue to
escape from the mass. The remaining solid matters, called “scrap,” are
treated for the preparation of a fertilising compost. The oil and water
pass by gutters into settling tanks, where the oil soon rises to the
surface, and is skimmed off, or allowed to escape over a separating
partition.

The oil is still crude, and requires clarifying and bleaching before
it becomes a saleable commodity. This is effected in several ways. It
is first boiled, to free it completely from water. It is purified from
solid matters by running it into filter-bags suspended over casks, and
then subjecting it to pressure in bags, the oil escaping while the
sediment remains in the bags. This refuse, termed “foots,” is bleached
and used for soap-making. The oil thus refined is termed “traits,” and
is ready for barrelling. “Bank” is an inferior grade. Bleaching is
sometimes performed by exposure to the sun in shallow tanks, having
glass covers to exclude dust when a superior quality is desired.

Its principal application in America is for tanning and currying
purposes. In France, it is largely employed as a substitute for
cod-liver oil. In this country, it is often passed off as olive-oil,
and considerable quantities of it are mixed with linseed-oil for
painters’ use. The rapidity with which it oxidises, and its good body,
render it not unsuitable as a vehicle for paint.

POPPY-SEED OILS.--Oil is yielded by the seeds of three kinds
of poppy--the opium-poppy (_Papaver somniferum_), the spiny-poppy
(_Argemene mexicana_), and the yellow-horn poppy (_Glaucium luteum_).

In Asia Minor and Persia, after the collection of the opium from the
poppy-heads, the latter are gathered, and the seed is shaken out
and preserved. It is black, brown, yellow, or white; some districts
produce more white seed than others. The seed is pressed in wooden
lever presses to extract the oil, which is used by the peasants for
culinary and illuminating purposes. Some of the seed is also sold to
Smyrna merchants, who ship it to Marseilles, where it is employed in
soap-making, and as a substitute for linseed-oil. The average yield of
oil is 35-42 per cent., the white seed being considered the richest.

The same economy takes place in India, where the plant is also grown
for the sake of its seed alone in some districts. In this latter case,
the sowing takes place in March-April, about 2 lb. of seed being sown
broadcast to one acre. The seed vessels ripen in August; the heads are
then cut off, sun-dried, sorted, and trodden out in bags, or threshed.
The seed is immediately crushed and pressed, the yield of oil being in
proportion to the freshness of the seed, and amounting to 14 oz. from 4
lb. under favourable conditions. The oil readily bleaches by exposure
to the sun in shallow vessels, and is then transparent and almost
tasteless. The natives use it very largely for cooking purposes, and
as a lamp-oil. The cake is consumed as food by the poorer classes. The
unpressed seed is largely exported from India.

France grows a large quantity of poppy-seed at home, over 100,000
acres having been returned as under this crop some few years since.
The French oil is of two kinds, a white cold-drawn oil, and a coarser
oil obtained by a second expression and from inferior seed, the total
yield being 40 per cent. The finer oil is fit for alimentary purposes,
and is largely used to adulterate olive-oil; it is also employed as a
lamp-oil, and very extensively by artists for grinding light pigments,
as, though possessing less strength and tenacity than linseed-oil, it
keeps its colour better. The pure oil has a golden-yellow tint and
agreeable flavour; its sp. gr. is 0·924 at 59° F.; it solidifies at
0° F., and remains long in this state at 28½° F., is slow to become
rancid, and saponifies readily; dissolves in 25 parts cold and 6 parts
boiling alcohol, and dries in the air more rapidly than linseed oil.

_Glaucium luteum_ is a common plant on the sandy shores of the
Mediterranean, the western coast of Europe as far as Scandinavia, and
some parts of North America. It is very hardy and cultivated with
little trouble. It prefers stony and chalky soils, where few other
plants will thrive, and has therefore been recommended for culture
on such otherwise waste land. Under cultivation, it affords about
10 bush. of seed per acre. The seed contains 42½ per cent. of oil,
and yields about 32 per cent. by pressure. The oil obtained by cold
expression is devoid of odour and flavour, and has a sp. gr. of 0·913.
It is applicable to culinary and illuminating purposes, as well as
for soap-making and paint. The cake is a good phosphatic manure. It
seems to have been very little utilised, probably on account of the
comparatively small yield of seed.

TOBACCO-SEED OIL.--The seeds of the tobacco-plant contain
about 30 per cent. of a fatty oil, which is extracted by powdering
them, kneading them into a stiff paste with hot water, and pressing
hot. The oil is clear, limpid, golden-yellow in colour, inodorous, and
mild flavoured; its density is 0·923 at 59° F.; it remains liquid at 6°
F., dissolves in 168 parts of alcohol at 0·811 sp. gr., and saponifies
readily. One authority excludes it from the drying oils; another
considers its drying quality to be unusually developed, and recommends
it for paints and varnishes.

WALNUT OIL.--The common walnut (_Juglans regia_) is found
native from Greece and Asia Minor, over Lebanon and Persia, along the
Indu Kush to the Himalayas, and from the Caucasus almost throughout
China, besides having been introduced generally throughout temperate
Europe. In portions of the Alps and Apennines, it is very abundant, and
is fairly plentiful in the forests of Lazistan, on the Black Sea, but
is perhaps most common in Cashmere, whence come the walnuts imported
into the plains of India.

The albuminous kernel of the walnut affords some 50 per cent. of oil.
It is said that it furnishes one-third of all the oil made in France;
it is extensively prepared in the central and southern departments,
notably Charente, Charente-Inférieure, and Dordogne, where it is
commonly met with in barrels of 50 _kilo_. In both Spain and Italy,
outside the olive-region, walnut-oil is largely expressed. It is of
considerable importance in the hill districts of India, but is seldom
seen in the plains. Cashmere and Circassia also include it among their
industrial products.

The oil should not be extracted from the nuts until 2-3 months after
they have been gathered. This delay is absolutely necessary to secure
an abundant yield, as the fresh kernel contains only a sort of emulsive
milk, and the oil continues to form after the harvest has taken place;
if too long a period elapse, the oil will be less sweet, and perhaps
even rancid. The kernels are carefully freed from shell and skin,
and crushed into a paste, which is put into bags and submitted to a
press; the first oil which escapes is termed “virgin,” and is reserved
for feeding purposes. The cake is then rubbed down in boiling water,
and pressed anew; the second oil, called “fire-drawn,” is applied to
industrial uses. The exhausted cake forms good cattle-food.

The virgin oil, recently extracted, is fluid, almost colourless, with
a feeble odour, and not disagreeable flavour. Its sp. gr. is 0·926 at
59° F., and 0·871 at 201° F.; it thickens to a butter-like consistence
at 5° F., and solidifies to a white mass at-17½° F. In the fresh
state, it is largely used in Nassau, Switzerland, and other countries,
as a substitute for olive-oil in salads, &c., but is scarcely to be
considered as a first-class alimentary oil. The fire-drawn oil is
greenish, caustic, and siccative, surpassing linseed-oil in the last
respect and exhibiting the property more strongly as it becomes more
rancid. On this account it is preferred by many artists before all
other oils.

WOOD-OIL OR TUNG-OIL.--This fatty oil is a product of the
so-called “oil tree” of China, Cochin China, and Japan (_Aleurites
cordata_ [_Elæococca vernicia_, _Dryandra cordata_]), and must not be
confounded with the Malayan article, which is an oleo-resin. The fruit
capsules of the _t’ung_ are filled with rich oil-yielding kernels, from
which 35 per cent. by weight of oil may be obtained by simple pressure
in the cold. The sp. gr. of the oil is 0·9362 at 59° F. It possesses
several remarkable properties: heated to 212°-392° F. out of contact
with the air, it retains its original limpidity after cooling, but in
contact with the air it solidifies almost instantaneously, melting
again at 93° F, and exhibiting the same elementary composition; the
cold expressed oil rapidly solidifies by light in the absence of air;
and its drying qualities exceed those of any other known oil. It is
devoid of colour, odour, and flavour. The oil is produced in immense
quantities in China; in the provinces of Ichang and Szechuen, it is one
of the principal articles of native manufacture.

In China the oil is universally employed for caulking and painting
junks and boats, and for varnishing and preserving woodwork of all
kinds. The oil is unknown to European commerce, but an attempt to
naturalise the tree in Algeria has been projected. Its industrial value
has been too long neglected.

EXTRACTION OF SEED-OILS.--The old-fashioned crude apparatus
for extracting oil from seeds, which answered the purposes of our
forefathers, has had to give way to modern improved machinery, such as
that manufactured by Rose, Downs & Thompson, of Hull, and shown in the
subjoined illustrations.

[Illustration: Fig. 33.--SEED-OIL MILL.]

The arrangement of the mill is shown in plan in Fig. 33 and in
elevation in Fig. 34. The seed or other material passes through the
following course:--It runs from an upper floor through the roll frame
A, by which it is crushed three or four times; it is then taken by
the elevators B to the kettle C, where it is heated and damped. From
beneath the kettle, it is drawn, in quantities sufficient to make a
cake, by a box which conveys it to the moulding machine E. Here it
undergoes preliminary compression, the objects of which are (1) to
increase the number of cakes which may be inserted in the presses at
one time, enabling 18 12-lb. cakes to be made where 4 8-lb. cakes were
formerly made, and (2) to ensure uniform size and weight, and uniform
density or consistence throughout.

[Illustration: Fig. 34.--SEED-OIL MILL.]

The cakes are removed from the moulding machine, and put into the
press F, 3 or 4 of which are required to each moulding machine. The
pressure is applied either by means of hydraulic pumps, or by a high
and low pressure accumulator; but unless extreme care is used with
the latter, it gives too rapid a pressure, squirting out the seed at
the side of the plates, and exercising a destructive effect upon the
cloth employed. The pulsation caused by the pumps working directly to
the press cylinder is more akin to the action of a wedge, and seems to
extract the oil better than the dead pressure given by the accumulator.
If the latter is used, a small cylinder may be applied to give the
preliminary pressure in the moulding machine, in lieu of a cam. After
remaining under pressure about 25 minutes, the cakes are withdrawn, and
after being stripped of the cloth, are pared by the machine H, which
completes the manufacture of the cakes. The parings fall under a very
small pair of edge-running stones J, which automatically discharge them
when sufficiently ground, into an elevator conducting to the kettle,
where they are worked up with fresh seed. In a mill with 4 presses, 2
men and a boy in the press room can make 6 tons of cake in 11 hours, a
rate of production requiring 6 men by the old process. The saving in
steam power is about 30 per cent., chiefly due to the absence of the
heavy edge runners, which also effects an economy of space. About 2 per
cent. more oil is extracted, and the cakes are improved in appearance
by not having the structureless texture caused by the trituration of
the seed under edge-runners.

[Illustration: Fig. 35.--SEED-CRUSHING ROLLS.]

Having described the general routine of the process, some details
may be added concerning the working of the several machines. The
roll-frame, Fig. 35, consists of 4 or 5 chilled-iron rolls, each 3 ft.
6 in. long by 16 in. in diameter, placed one above the other. These
rolls are used for crushing all the seed that passes through one set
of presses, making 5½-6¼ tons linseed-cake per spell of 11 hours. The
seed passes into the hopper in the usual manner, and is distributed
to the crushing-rolls by a fluted feed roll the same length as the
crushing-rolls, placed at the bottom of the hopper. When the seed
passes the feed roll, it falls on a guide-plate that carries it between
the 1st and 2nd roll. After passing between these rolls and being
partly crushed, it falls on a guide-plate on the other side, which
carries it back between the 2nd and 3rd rolls, where it is crushed more
fully. It then falls on another guide-plate, which carries it between
the 3rd and 4th rolls, where it is ground more fully. Then it falls on
a 4th guide-plate, and is conveyed between the 4th and 5th rolls to
receive the finishing touch. It is thus crushed four times.

The kettle is shown in Fig. 36, which represents one capable of heating
sufficient seed to keep four 16-plate presses occupied, or to make 6
tons of cake per 11 hours. It is steam jacketed and furnished inside
with a damping apparatus. The inside diameter is 5 ft., and the depth 2
ft. 6 in. The seed introduced is kept in motion by the stirring gear,
and when sufficiently heated and damped, is withdrawn by the box A in
quantities to form one cake, and transferred at once to the moulding
machine, attached or separate.

This machine is illustrated in Figs. 37, 38. Its purpose is to
measure the quantity of seed required to make each cake, to shape it
as required, and to press it so much, without extracting any oil, as
will enable the greatest number of cakes to be put into the press. The
measure of seed is placed on a strip of woollen cloth, spread upon a
thin iron tray, sliding on the guides B; the bottomless hinged mould
C, having the exact shape of the intended cake, is closed upon it, and
the measure A (Fig. 36), which is also bottomless, is drawn over guides
in the upper surface of the mould C, thus accurately distributing
the seed. The mould is next thrown upon its hinge (Fig. 37), and the
ends of the strip of cloth are folded over the seed, the thickness of
which is about 3½ in. The thin iron tray, with the mould of seed upon
it, is then pushed along the guides B, beneath the die D. This action
gives motion to a cam, shown above in the illustrations, but which
may be placed beneath if necessary. This cam brings down the die and
compresses the mould of seed to a thickness of 1¼ in.; its revolutions
are so timed that the seed is under pressure long enough (about
one-third of a minute) to let the workman have another cake ready.

[Illustration: Fig. 36.--SEED-BOILING KETTLE.]

When the die of the moulding-machine rises, the cake and tray are
removed and placed in the press (Fig. 39), the tray being withdrawn.
The plates of the press are slightly thickened towards the edges, and
bear the name of the

[Illustration: Fig. 37.--OIL-CAKE MOULDING MACHINE.]

[Illustration: Fig. 38.--OIL-CAKE MOULDING MACHINE.]

manufacturer in reverse. The press is suitable for extracting oil from
linseed, rape-seed, cotton-seed, hemp-seed, niger-seed, sunflower-seed,
gingelly-seed, castor-seed, ground-nuts, coco-nuts, olives, &c. It is
made in various sizes. The No. 1

[Illustration: Fig. 39.--OIL-SEED PRESS.]

double press (not shown) is furnished with 4 cake boxes, suitable for
making 4 tapered cakes at one pressing, each about 2 ft. 5 in. long, by
10½ in. wide at one end, and 7½ in. at the other, when using linseed,
48 lb. of Bombay seed being required to charge the press, and giving
a cake weighing about 8 lb.; the maximum and minimum weights of its
charges are 60 lb. and 40 lb., of the cakes, 13 lb. and 6½ lb. The
charges vary from 3 to 6 an hour, being 4 for cotton-seed and 5 for
linseed; most other seeds are worked the same as linseed, but rape and
gingelly are worked twice. By using 2 presses for the first time and 3
for the second, 3 presses will crush as much seed as 5. These presses
are made of a capacity to take 270-320 lb. of seed at a charge, giving
cakes of 9-15 lb., and requiring 30-45 minutes for the operation. In
all these presses, the hair wrappers, weighing some 26 lb., used in the
old process, are dispensed with.

A very complete account of oils and fats will be found in Spon’s
‘Encyclopædia of the Industrial Arts,’ to which the reader is referred
for further information.

DRYERS.--The maximum of drying power in oils is obtained by
the addition of certain metallic oxides, which not only part with some
of their own oxygen to the oil, but also act as carriers between the
atmospheric oxygen and the heated liquid. This heating of the oil with
oxides is known as boiling, although the liquid is not volatilised
without decomposition, as is the case with water. At about 500° F.,
bubbles begin to rise in the oil, producing acrid, white fumes on
coming into contact with the air. The gas thus given off consists
chiefly of vapour of acrolein mingled with carbonic oxide. There is
no advantage in heating the oil to a higher temperature than 350° F.
Accurate experiments have shown that the drying properties of the oil
are not increased by heating it beyond this point, while its colour is
considerably darkened.

For the finer qualities of boiled oils, it is essential that the raw
oil should have been stored for some time, so that it may be free from
mucilage. This mucilage is the chief source of the dark colour of some
boiled oils; when heated, it forms a brown substance, which is soluble
in the oil itself, and extremely difficult to remove.

The oxides usually added to the oil during boiling are litharge or
red-lead, the former being preferred on account of its lower price.
About 2-5 per cent. by weight of the oxides or dryers is gradually
stirred into the oil after it has been slowly raised to a temperature
of about 300° F. The stirring should be continued until the litharge is
dissolved, or it would cake on the bottom of the pan, and cause the oil
to burn. Litharge may even be reduced to a cake of metallic lead when
the fire is brisk. Some pans are furnished with stirrers and gearing by
which the latter can be worked, either by hand or steam. The material
of which the pans are made is either wrought or cast iron. Copper pans
are sometimes used with the object of improving the colour of the oil.

Little is known respecting the chemical reactions which take place
during the boiling of oil. Even when the air is excluded during the
process, the drying properties are greatly increased, and, if boiled
long enough, the oil is converted into a solid substance. The loss of
weight which ensues is dependent upon the temperature and the time
during which the operation continues. It is less when the air is freely
admitted than if the pan is covered with a hood. The vapours given off
by the oil are of an extremely irritating character, and should be
destroyed by passing them through a furnace. As their mixture with air
in certain proportions is explosive, this furnace should be situated
at some distance, and the gases be conducted into it by means of an
earthenware pipe.

Since zinc oxide has been introduced as a substitute for white lead in
painting, researches have been made to replace litharge as a dryer,
because it is not logical to discard the lead pigment and then use a
lead dryer with a zinc pigment.

Several metallic oxides and salts, especially zinc sulphate, manganese
oxide, and umber, have the property of combining with oils, which they
render drying. To these may be added the protoxides of the metals of
the third class, i. e. iron, cobalt, and tin. But these oxides are very
unstable and difficult of preparation; hence it became desirable to
discover some means by which they might be combined with bodies which
would enable them to be prepared cheaply, and at the same time leave
unimpaired their desiccating powers. Moreover, it is acknowledged that
dryers in the dry state are preferable in many respects to drying oils.
Following are some of the recently introduced dryers:--

_Cobalt and Manganese Benzoates._--Benzoic acid is dissolved in
boiling water, the liquid being continually stirred, and neutralised
with cobalt carbonate until effervescence ceases. Excess of carbonate
is removed by filtration, and the liquor is evaporated to dryness.
The salt thus prepared is an amorphous, hard, brownish material,
which may be powdered like rosin, and kept in the pulverulent state
in any climate, simply folded in paper. Painting executed with a
paint composed of 3 parts of this dryer with 1000 of oil and 1200 of
zinc-white, dries in 18 to 20 hours. Manganese benzoate is prepared
in the same way, substituting manganese carbonate for that of cobalt.
Applied under similar circumstances, it dries a little more rapidly,
and a little less is required. Urobenzoic (hippuric) acid is equally
efficacious.

_Cobalt and Manganese Borates._--These salts also, in the same
proportions, are found to be of equal efficacy. The latter is extremely
active, and requires to be used in much smaller proportions.

_Resinates._--If an alkaline resinate of potash or soda be dissolved
in hot water, and this solution be precipitated by a solution of a
proportionate quantity of a cobalt or manganese chloride or sulphate,
an amorphous resinate is formed, which, after being collected on cloth
filters, washed, and dried, forms an excellent drier.

_Zumatic (Transparent) Dryer._--Take zinc carbonate, 90 lb.; manganese
borate, 10 lb.; linseed-oil, 90 lb. Grind thoroughly, and keep in
bladders or tin tubes; the latter are preferable.

_Zumatic (Opaque) Dryer._--Manganese borate, as a dryer, is so
energetic that it is proper to reduce its action in the following
way:--Take zinc-white, 25 lb.; manganese borate, 1 lb. Mix thoroughly,
first by hand, then in a revolving drum; 1 lb. of this mixed with 20
lb. paint ensures rapid drying.

_Manganese Oxide._--Purified linseed-oil is boiled for 6 or 8 hours,
and to every 100 lb. boiled oil are added 5 lb. of powdered manganese
peroxide, which may be kept suspended in a bag, like litharge. The
liquid is boiled and stirred for 5 or 6 hours more, and then cooled and
filtered. This drying oil is employed in the proportion of 5 to 10 per
cent. of the zinc white.

_Guynemer’s._--Take pure manganese sulphate, 1 part; manganese acetate,
1 part; calcined zinc sulphate, 1 part; white zinc oxide, 97 parts.
Grind the sulphates and acetate to impalpable powder, sift through a
metallic sieve. Dust 3 parts of this powder over 97 of zinc oxide,
spread out over a slab or board, thoroughly mix, and grind. The
resulting white powder, mixed in the proportion of ½ or 1 per cent.
with zinc-white, will enormously increase the drying property of this
body, which will become dry in 10 or 12 hours.

_Manganese Oxalate._--A writer in the _Moniteur de Produits Chimiques_
draws attention to the properties possessed by manganese oxalate as
a drier. This salt has hitherto not had any important industrial
uses, but it can be readily prepared in a state of purity from the
native carbonate by the action of oxalic acid; the author is of the
opinion that it will be found of use for this purpose. If prepared
from carbonate free from iron and lime, it can be obtained as a fine
crystalline white powder, and two-fifths per cent. suffices to bring
about the change. The oxalate is resolved by heat into manganese oxide,
carbonic acid and carbon monoxide, and in the presence of fatty acids
the manganese oxide formed combines with them, the decomposition taking
place at about 130°. The operation is carried out by mixing in a mortar
the oxalate with two or three times its weight of oil, and then adding
the mixture to the main portion of the oil. The heat should be applied
gradually, and the decomposition is known to be complete when there
is no further evolution of gas. The boiled oil, under this treatment,
preserves its limpidity and also remains colourless. Manganese oxalate
has the advantage over oxide of lead, which is commonly employed for
this purpose, in causing the oil to remain transparent when exposed
to sulphur vapours. Manganese acetate has also been used, but it
likewise causes a darkening in the colour of the oil, and the nitrate
is dangerous owing to the possible action of nitric acid on the fats
present in the oil. Manganese borate appears to be next in value to the
oxalate as an oil drier.

In a paper recently read before the Society of Arts, Prof. Hartley
remarked that paint, such as is used for ordinary purposes, is
essentially composed of three materials, without taking into account
the coloured pigments.

(1) White lead, or sublimed zinc-white.

(2) An oil, generally linseed or poppy oil, which is ground up with the
white lead or zinc-white until it becomes a soft paste. This is mixed
with variable preparations of linseed oil and spirit of turpentine.

(3) A substance called dryers, or siccative materials; it may be
linseed oil in which litharge is dissolved, or it may be linseed oil
containing a compound of manganese.

Paint owes to the dryers its property of drying more rapidly than it
would do without it; and it is considered indispensable in buildings
in all cases where paint applied to wood, stone, or metal, would not
be quite dry in 48 hours, or at most in 72 hours, after the first
application.

The first question which requires an answer is, what chemical process
takes place when a paint dries.

BOILED OIL.--Linseed oil absorbs oxygen; and, when the oil
contains manganese, it absorbs oxygen much more greedily; and when a
manganese oil--that is to say, a boiled oil containing manganese--is
mixed with linseed oil, the substance absorbs oxygen, from a limited
supply of air contained in a closed space, until no trace of any other
gas but nitrogen remains. The power of absorbing oxygen possessed
by 100 volumes of linseed oil, compared with that of 100 volumes of
a mixture of linseed oil and so-called manganese oil, is as 9·4 to
100. This may be termed the measure of its drying power. A mixture
of linseed oil, with a little more than one-fourth of its volume of
manganese oil, has a power of absorbing oxygen four and a half times
greater than either of the components of the mixture taken separately.
In this case Chevreul argues that linseed oil may be considered as a
“dryer” to manganese oil.

Linseed oil, without any addition whatever, if boiled for three hours,
becomes a better drying oil than it was previous to the action of heat.

Oil boiled with 10 per cent, of litharge for three hours, is a much
better dryer than when heated without this oxide.

Oil boiled alone for five hours is an inferior drying oil to one heated
for only three hours.

Oil previously boiled alone for five hours, and boiled alone again for
three hours, is scarcely altered in drying power, but it becomes a
better drying oil if it is boiled the second time with litharge. It is
inferior to a drying oil which has been boiled only three hours with
litharge, without being submitted to a previous boiling.

Oil boiled alone for five hours, boiled for a further period of three
hours with manganese dioxide which has already been used for one
operation, is very nearly as strong a dryer as that which has been
boiled with litharge under the same conditions; but it is superior
to an oil which has been boiled with manganese dioxide for eight
hours. This no doubt arises from the longer boiling with manganese
having caused a larger quantity of manganese to dissolve, and that the
quantity dissolved is in excess of that which yields the best result.

Finally, oil boiled for five hours, and then boiled alone once more
for eight hours, becomes viscous, and the first coat requires a
considerable time to dry. We thus see that the oxides of lead and of
manganese in certain proportions concur with heat in increasing the
drying power of linseed oil. This drying of oils is a process of slow
oxidation.

The following points of Chevreul’s appeared to be difficult of
satisfactory explanation, and suggested to Prof. Hartley an examination
_de novo_ of the facts, as well as an investigation of the chemistry of
the subject generally:--

1. Linseed oil not boiled acted as a dryer to the same oil boiled with
manganese dioxide.

2. Linseed oil, boiled with either litharge or manganese dried more
rapidly when mixed with turpentine.

3. Oil, mixed with white lead, zinc white, antimony white, and
arseniate of tin, acts differently, thus:--The white lead dries most
rapidly, the zinc white next, but antimony white and arseniate of tin
are incapable of acting as dryers, in fact, they retard the drying
process.

4. Oil boiled alone for five hours, and boiled for a further period of
three hours with manganese dioxide becomes a superior drying oil to one
which has been boiled with manganese dioxide for eight hours.

From a series of experiments, which were continued for two years, on
twenty-five weighed quantities of raw linseed oil, Prof. Hartley draws
the following deductions:--

1. The chemical action of a manganese compound, when dissolved in
linseed oil, is that of a carrier of oxygen from the atmosphere to the
oil. Manganese oxide takes up oxygen from the air, and transfers it to
the oil, and in so doing it suffers alternately the opposite processes
of oxidation and reduction.

2. To obtain the best result, the amount of manganese present must not
exceed a certain small proportion of the oil.

3. Oil to which turpentine has been added dries more rapidly than oil
without such addition, because the oil being diluted and rendered
thinner, it spreads over a larger surface, and is in contact,
therefore, with a much larger quantity of oxygen.

4. Turpentine does not act as a dryer, that is, as a carrier of oxygen
to linseed oil.

5. Different white pigments behave differently when drying, because
the more powerfully basic the properties of the pigment, the more
powerful is its action as a dryer. Lead oxide and white lead (basic
lead carbonate) combine more easily with the acids of linseed oil than
zinc oxide does. But zinc oxide dries better than antimony oxide,
because it is a stronger base, while arseniate of tin has no basic
properties, therefore does not act as a dryer.

Different substances, that is to say, those without chemical action
on oil, such as lamp-black, sulphate of baryta, and sulphate of lead,
cannot act as “dryers.”

Linseed oil is a glyceride of a peculiar acid, called linoleic acid.
Whatever the exact constitution of linoleic acid may be, linseed oil
for the most part is composed of trilinolein. Raw linseed oil contains
the following constituents:--

1. Glyceride of linoleic acid or trilinolein

  {C_{18}H_{31}O}O_{3}.
  {  C_{3}H_{5} }

2. Water.

3. Mucilage, with the composition _n_(C_{6}H_{10}O_{5}). On boiling
with dilute acids this yields a gum and a sugar.

4. An essential oil, present in minute proportions, and of unknown
composition.

5. A mixture of colouring matters of intense tinctorial power, viz.
blue and yellow chlorophylls and erythrophyll.

The only useful and desirable substance is the trilinolein.

The effect of oxidation upon linseed oil is to destroy all the
glycerine, and to produce therefrom carbonic, formic, and acetic acids,
together with some acrolein. When boiled at a high temperature without
the addition of any metallic oxide, the glyceride is decomposed,
acrolein is formed, and linoleic acid is set free. In fact, whether
oil is oxidised by air or by metallic oxides, or whether it be simply
heated, the action in each case first leads to the destruction of the
glycerine and the liberation of linoleic acid. But linoleic acid very
readily absorbs oxygen, and the oxidised substance becomes a tough
elastic solid, which is essentially a varnish.

In fact, the process which an oil undergoes in drying is not
desiccation, or depriving it of moisture or of glycerine, but
solidification, and the technical term “drying” is a misnomer. That,
however, is of little consequence if we really know what is the
chemical action of the “drying” process. When oxidised even at a low
temperature, the glycerine is destroyed, and the oxidised products form
a tough varnish.

There are various methods of converting linseed oil into a drying oil
or varnish:--

1. Heating it to a high temperature with litharge.

2. Heating with red oxide of lead.

3. Heating with metallic lead.

4. Heating to a high temperature with manganic oxide.

5. Heating with manganese borate.

6. Heating with manganese oxalate.

7. By the joint action of air and heat upon the oil and manganous
oxide, or a solution of manganese dioxide or manganous oxide in the oil.

In the processes 1, 2, 3, there can be no doubt that a lead of linoleic
acid is produced, and that this facilitates further oxidation in air,
by forming salts with some of the acid products of such oxidation,
while the oxidation of the linoleic acid continues. Heating with red
lead favours oxidation, by the compound itself conveying oxygen to the
oil. In the case of metallic lead, it must be noted that the metal is
dissolved. Under certain circumstances, metals become dryers to oils;
thus sheets of metallic lead are capable of acting as dryers to linseed
oil.

Linseed oil is pre-eminent in its capacity for absorbing oxygen.
This action of metallic lead as a dryer is due to the metal becoming
oxidised at the expense of the glycerine of the oil, and so passing
into solution by combining with the linoleic acid, or with acetic
or formic acid, caused by the oxidation of the glycerine. It is the
destruction of the glycerine with concurrent oxidation of the fatty
acid which causes the drying or hardening of the oil.

When a drying oil which has been treated with metallic lead, or with
litharge, is shaken up with a solution of zinc sulphate, all the lead
is precipitated from the oil, and zinc passes into solution therein. By
manganese sulphate or copper sulphate, the lead is removed by manganese
or copper. Oil charged with lead dries in 24 hours when spread out in a
thin layer on glass; it will dry completely in 5 or 6 hours if charged
with manganese, in 30 or 36 hours with copper, zinc, or cobalt; and it
requires more than 48 hours with nickel, iron, chromium, &c.

Although solidification of a drying oil charged with manganese
takes place in from 5 to 6 hours when spread in thin films, the
solidification of thicker films requires a longer time. A temperature
of 122° to 140° F. accelerates the oxidation of the drying oils, partly
because the oil becomes more fluid, and partly because the oxygen is
more active at a higher temperature. Hence, oil which has been mixed
with an equal volume of turpentine, or a light hydrocarbon, such as
benzene, dries more rapidly than oil without such admixture.

When a boiled oil, prepared with manganese, is dissolved in an equal
volume of benzene, and shaken up with air in a bottle, rapid absorption
of oxygen occurs, especially about 120° F. If fresh air is repeatedly
provided, the oxidation is sufficient to cause the liquid to become
thick, and, on distilling off the sapient, a perfectly dry and elastic
solid remains.

An oil containing manganese is a very superior drying oil to one which
has been prepared with lead. This fact, however, is to be noted,
that though a large proportion of manganese in an oil may hasten its
drying, yet it is disadvantageous, because it does not form so tough a
film. This arises from the film becoming hard upon the surface, and so
protecting the oil underneath from absorbing oxygen from the air.

Though the oils containing large quantities of dryers dry, they
afterwards lose weight, and become viscous under the same conditions.

Pure linoleates of lead and of zinc are not dryers; but if heated
until it has turned brown, or begun to blacken, a lead dryer becomes
effective, although it contains less of the lead compound.

In this case, some compound of lead is formed by absorption of oxygen,
which either itself actually oxidises or causes the oxidation of
ordinary linseed oil.

Having treated of the materials used for producing boiled oil, and of
their action upon the oil, let us now consider how the operation is
brought about.

_Process 1._--Oil is boiled at a high temperature, that is to say, it
is heated until frothing and bubbles of gas escape, when litharge or a
manganese compound is added.

_Process 2._--Oil is boiled at a steam heat, with litharge or a
manganese compound, in conjunction with a blast of air.

_Process 1._--The chemical action in the first process is doubtless one
which takes place in three stages. It commences by depriving the oil of
water; in the second stage, it destroys the mucilage, by charring it;
in the third, it destroys, in part, the glycerine, and sets free the
fatty acids. After the litharge or manganese compound is added, there
is formed in the oil a solution of lead salts of the fatty acids, or a
manganese salt of the fatty acids.

The oil then, at the high temperature, loses glycerine by oxidation
caused by the air, such oxidation being greatly facilitated by the
presence of manganese compounds, which are repeatedly oxidised by the
air and reduced by the oil, that is to say, they absorb oxygen and pass
it over to the oil with great facility.

It matters little, so long as the ultimate action is oxidation, what
salt of manganese or what oxide is used, if it be capable of undergoing
processes of an alternate character called oxidations and reductions.

It is, however, certain that some manganese compounds are more
suitable than others, owing to their more or less complete solubility
in the oil, and their more readily undergoing the two different
processes of oxidation and reduction in presence of air and of oil.

_Process 2._--The credit of being the first to boil oil without
resorting to the dangerous expedient of using an open fire and a high,
temperature in the manufacture, is due to Vincent. He used manganese
compounds, or both manganese salts and litharge. His method of boiling
oil for the manufacture of printing inks is, with some modifications
in technical details, carried out on a large scale at the present time
in the preparation of ordinary boiled oil. The essential parts of the
plant are a steam-jacketed close boiler with agitating gear, and a pipe
for conducting a current of air into the oil by means of a blowing
engine. From the head of the boiler there passes a funnel under the
back of the furnace fire, by which the disagreeable products of the
chemical action are conducted to a place where they are destroyed.
These products, as already mentioned, are volatile fatty acids and
acrolein.

Oil boiling, as ordinarily carried out, is conducted by means of
litharge along with compounds of manganese; in some processes these
are mixed with salts of alumina and zinc. The oil so produced is brown
and not clear, but it is clarified by keeping. Many samples of such
boiled oil deposit insoluble matter when stored for some time, even
although they may have become clear previously. This is not a desirable
property. Sometimes rosin is added to hasten its drying.

The defects to be noticed, even in the best samples of boiled oil, are
the following:--

1. The oil causes a brownish or yellow colour to be communicated to
white lead or zinc white.

2. The oil darkens pigments containing brilliantly coloured metallic
sulphides, such as vermilion, cadmium yellow, and ultramarine blue.

3. Delicate colours are darkened by the oil when exposed to ordinary
town air, that is to say, air which, is not quite pure. This is the
case even when the oils themselves may not injure the paints.

The causes of such alterations is, in nine cases out of ten, the use of
lead dryers.

1. In the first place, boiled oil which contains litharge or other lead
compounds takes a permanent brown colour, which affects the purity of
white lead, zinc white, and delicate pale tints.

2. Lead forms, with extreme ease, lead sulphide, which, in very
minute proportions, is yellow or brown; in larger quantity its colour
is black. The lead sulphide is readily formed by contact with other
sulphides, as, for instance, vermilion, cadmium yellow, and ultramarine.

3. Boiled oil, containing lead, is coloured brown by exposure to air,
owing to the presence of minute quantities of sulphuretted hydrogen,
which causes the formation of lead sulphide.

The remedy is obvious: no oil should be used which has been boiled with
dryers containing lead. In other words, oil should be boiled with pure
manganese compounds only.

In cases where it is desirable to have information of the presence or
absence of lead in a boiled oil, the following test will be found most
useful:--A mixture is made of 4 oz. of glycerine with 1 oz. of ammonium
sulphide, the liquid being kept in a stoppered bottle. Or glycerine is
mixed with an equal volume of water, and saturated with sulphuretted
hydrogen. Half an ounce of the oil to be tested is placed in a white
basin, with the addition of two or three drops of the glycerine
solution. The two liquids are thoroughly incorporated, by stirring with
a strip of glass. A brown or black colour, which gradually appears,
indicates the presence of lead. A pure manganese oil simply becomes
slightly yellow. It is true that, if iron is present, a black colour
might appear, but iron is also an undesirable impurity. Should it be
required to ascertain that the coloration is or is not caused by iron,
two or three drops of glacial acetic acid may be stirred into the oil,
when, if the black colour remains, it is certainly not caused by iron.

Under the old process of oil-boiling at a high temperature, the brown
colour of the oil was, to some extent, an indication that the oil had
been sufficiently heated--that is to say, properly boiled; but in
the modern processes, so largely used, in which oxidation is aided
by a blast of air, this coloration is no indication whatever of the
excellence of the oil; it may be, in fact, the very reverse.

This fact appears to be unknown, or, at any rate, is not a matter
of common knowledge among practical men in this country, who, being
uninformed as to the methods of preparing the oils, consider that a
brown colour is desirable, if not essential.

When oil-boilers were compelled to adopt some expedient to give a
reddish-brown colour to the oil, they added a small amount of litharge,
the introduction of which actually spoils the oil, and makes it
unsuitable for many purposes to which it is otherwise applicable. Of
late years, pale boiled oils have been more largely manufactured for
special purposes. It is obvious that, for decorative house painting, in
which delicate tints are a leading feature, they may be advantageously
employed.

Notwithstanding that some of the brown oils, when mixed with white
lead, do not entirely retain the brownish tint, but, to some extent,
lose it upon drying, yet they never preserve the whiteness of white
lead. It follows, therefore, that a pale colour in the oil, provided
it is not the yellow colour of raw oil, is greatly to be preferred.
Moreover, when paints are mixed with zinc white, no trace of lead
should be contained in the oil, otherwise, one of the valuable
properties of zinc white pigments is destroyed, namely, its power to
retain its whiteness in the atmosphere of a town, because its colour is
not affected by sulphuretted hydrogen.

Very generally, zinc white and white lead paints are not mixed with
drying oils, but with refined linseed or bleached oil. This, at any
rate, is the practice on the Continent. That is to say, the pigments
are mixed with an oil from which the impurities, and the natural yellow
and red colouring matter, have been removed, so that the colour of the
paint is white. If ordinary oil be used, the paint is more or less
yellow. In order to render such paint quick drying, a certain amount
of dryers, in a solid or liquid form, is added. These dryers almost
invariably contain lead, so that zinc white paint is contaminated by
lead in another way, which may not be suspected, or which is overlooked.

Now as to the chemical action of dryers on oils. Raw oil contains water
and mucilage; the former can be absorbed by anhydrous zinc salts and
by dried alum, and solutions of the salts and the salts themselves are
capable of precipitating mucilage from the oil; hence these substances
cause the impurities to become insoluble, so that they are carried
down as “foots.” Heat greatly facilitates this action, particularly
by causing the oil to become more fluid; and by the action of the
anhydrous salts water is withdrawn from the oil. On the “drying” or
oxidation of the oil, they exert no chemical action whatever.

Zinc linoleate and lead linoleate do not act as dryers when simply
added to the oil. Though the former is soluble in hot oil it is
insoluble in cold oil, and it therefore separates from the oil as it
cools. The latter is very soluble in linseed oil, but only adds to its
drying power when heated therewith.

In conjunction with a high temperature, lead dissolves in oil at the
expense of the glycerine, which is decomposed into acrolein, while lead
linoleate is formed.

When litharge is heated with linseed oil, the action is somewhat
similar, the substances formed being acrolein, lead linoleate, and
linoleic acid.

If we consider the action of red lead on trilinolein, we have not
only the formation of these lead linoleates, but an excess of oxygen
available for the oxidation of glycerine to acrolein and acrylic acid,
or to acetic and formic acids.

These equations serve to show the effect of lead and lead oxides in
what may be termed the initiation of the chemical action upon the oil.
Subsequent changes, no doubt, depend upon the conditions which obtain
at the time, notably upon the temperature and upon access of air to the
oil. It is probable that acid linoleates are formed, and that compounds
formed from the polymerisation of linoleic acid result eventually.

Whatever doubt there may be as to the action of lead salts, there can
be none whatever as to that of manganese compounds. In the first place,
manganous oxide is a powerful base, which readily dissolves in oil;
manganic oxide is also readily soluble, yielding fatty acid salts of
manganese, and causing oxidation of glycerine. Manganese borate and
manganese oxalate are both soluble in oil, the former much more readily
than the latter, but they are both salts of little stability at high
temperatures in contact with oils. They both dissolve by the aid of
heat, forming fatty acid salts of manganese. Borate liberates boric
acid under these circumstances, but oxalate yields a mixture of carbon
monoxide and carbon dioxide.

Of manganese oleate and linoleate nothing more may be said than that
both are extremely soluble in oil, and both easily oxidised from
colourless to brown compounds when submitted to the action of air.

The chief adulterants of linseed oil and of boiled oil are cotton-seed
oil, rosin oil, and linoleic acid. Cotton-seed, which is to some extent
a drying oil, can act as such when mixed with linseed, but when added
to olive oil, it behaves as a non-drying oil. In fact, its behaviour
is anomalous, and of such a character that it greatly facilitates its
extensive use as an adulterating material for the more expensive oils.

Rosin oil is a deleterious adulterant, but one which may be more
readily detected than cotton-seed oil. Rosin is added to boiled oil to
hasten its drying; this also is an injurious substance. Of late years
glycerine has become an article of greater value than formerly, and
this may account for the manufacture of linoleic acid and its use as an
adulterant of oleic acid and of linseed oil.

Lastly, it may be mentioned that certain samples of “pale boiled oil”
have been found to contain what is practically a raw oil mixed with
dryers. Although such oils will dry, their efficiency is nothing like
so great as that of an oil “boiled” with a blast of air at a suitable
temperature, and, moreover, such oils are deficient in body.

In bleaching vegetable oils, it is necessary to consider the nature
of the colouring matters naturally contained in them. These consist
of a mixture in varying proportions of the colouring matters known to
exist in the leaves of plants, but which, in the case of oils, are
derived from the fruit or seeds from which the oils are expressed.
There can be no doubt that these substances are closely allied in
chemical constitution; they all possess an intensely powerful colouring
property, by which is meant that though the colour of some of them may
not be dark, yet a very minute weight is capable of imparting a tint to
a very large quantity of material.

The names of these substances are:--

  Xanthophyll--yellow.
  Yellow chlorophyll--yellow.
  Blue chlorophyll--blue.
  Erythrophyll--red.

In some oils only the xanthophyll and yellow chlorophyll are present;
in others, such as olive oil, the yellow and blue chlorophylls occur,
and give the liquid a green tint; while in linseed erythrophyll is
always present with more or less of the yellow and blue chlorophylls,
and some xanthophyll. According to the different proportions of these
colouring matters the oil varies in colour. For instance, linseed oil,
when brown, contains a mixture of erythrophyll with yellow and blue
chlorophylls; when greenish brown, the yellow and blue chlorophylls are
present in somewhat larger proportion, but mixed with erythrophyll;
while, generally speaking, a bright yellow or pale yellow oil contains
xanthophyll only. These substances appear to be combined with the oils,
or to be substances of a fatty nature. They are neither dissolved nor
acted upon by water, nor by acids diluted with water, when naturally
contained in the oils. They are freely soluble in alcohol, and an
alcoholic solution is not only susceptible of being destroyed by the
joint action of air and water, but by very dilute aqueous solutions of
mineral acids, and by acetic acid. In aqueous and alcoholic solutions,
light speedily modifies the blue, and eventually destroys all these
colours. A solution in turpentine of the isolated colouring matters
is also easily destroyed. But, on the other hand, a solution of the
colours in melted paraffin wax is comparatively stable.

Zinc hydroxide, copper hydroxide, baryta, potash, and soda easily
combine to form metallic salts with blue chlorophyll, less readily,
though readily enough with yellow chlorophyll, but far less readily
with xanthophyll and erythrophyll. The following facts will serve
to show that this is the case. When a solution of the colouring
matters contained in green leaves is made by extracting dry, but
freshly-gathered, leaves with absolute alcohol, an addition of a
saturated solution of baryta water, to the intensely green extract,
precipitates first the compound of blue chlorophyll with baryta, then a
further addition precipitates the yellow chlorophyll, also as a baryta
salt; but xanthophyll and erythrophyll either remain in solution, or
require a much larger addition of the base in order to be precipitated.
A crystalline compound of blue chlorophyll with soda is comparatively
stable. This substance is, no doubt, formed in green vegetables when
they are boiled in water to which some carbonate of soda has been added
to maintain their fresh appearance. The addition of a small trace
of copper sulphate to peas and to pickles forms a very permanent
copper compound with the colouring matter, which gives an attractive
appearance to these articles. Such being an outline of the chief
chemical properties of the natural colouring matters contained in oils,
the facts mentioned will serve to render the processes for removing the
colour from oils more intelligible than they otherwise would be.

Vegetable oils are decolorised, either partially or completely, by the
application of one of the following processes:--

1. By the action of light, or by the joint action of light and air.

2. By acids.

3. By saponification.

4. By the action of chlorine.

1. By exposing raw linseed oil to the action of sunlight, it slowly
becomes pale in colour, and finally colourless. It is in the highest
degree probable that, as oxygen is absorbed by the oil and acid
substances are thereby produced, these acids effect the destruction of
the colouring matters. In such wise castor oil is bleached.

2. By treating linseed oil with moderately strong sulphuric acid. As
the oil and sulphuric acid are of very different specific gravities, it
is essential that they be very rapidly and thoroughly mixed by violent
agitation. The impurities, such as mucilage and albuminous matters,
are thus deprived of water, and more or less charred, and along with
them the colouring matters are destroyed by the acid. It is essential
for the success of the process that the oil and the acid be not long
in contact without undergoing dilution, otherwise the oil itself may
become charred. It is, however, possible to obtain oil by this process
in a fairly colourless condition, after it has been thoroughly washed
with water and allowed to settle.

3. Both rape oil and cotton oil may be rendered of a pale yellow,
and even almost colourless, by a process of partial saponification
with caustic alkali of a suitable strength. The colouring matters are
saponified, and the resulting soap is of a dark yellow or brown colour,
from the colouring matter having combined with the alkali.

4. By the action of chlorine produced in contact with the oil when, for
instance, an aqueous solution of bleaching powder is acidified with a
cheap mineral acid, such as dilute sulphuric. In this case rapid mixing
and violent agitation are essential to the success of the process,
otherwise chlorinised products are retained in the oil, which not only
confer upon it a distinct flavour and odour, but also cause the oil to
solidify with a very moderate lowering of the normal temperature. It is
very questionable whether drying oils can with advantage be submitted
to such treatment.

5. A variety of methods may be merely mentioned, such as treatment with
sulphurous acid, with ferrous sulphate (green vitriol), and potassium
dichromate and sulphuric acid.

6. Lastly, the method of Binks, to which reference will be made farther
on.

Prof. Hartley next gives an account of certain improvements in the
process of oil-boiling, designed with the object of producing a drying
oil absolutely free from lead, and, as compared with ordinary oils,
absolutely free from colour.

The operations have been carried out, on a manufacturing scale, by
Mr. W. E. B. Blenkinsop and himself, and there is no doubt of the
practicability of the process.

The process consists in, first, refining the oil, by the removal
therefrom of water and mucilage; second, boiling and bleaching the oil
at one operation.

It is a fact that water and mucilage can be removed from linseed oil by
the action of certain dehydrating substances and solutions of metallic
salts, as, for instance, by alum, by strong sulphuric acid, and by a
solution of zinc chloride.

There are certain objections to each of these methods, which are of
a practical nature: thus, in treating the oil with strong sulphuric
acid, there is too frequently a charring of something, either the oil
itself, or of some impurity therein, and this charring, though it may
be very slight, has the effect of giving a pale brownish tinge to the
oil, which cannot be completely removed by the bleaching process to
which the natural colouring matters in the oil are amenable. It is
quite true that this brown colour separates sometimes, but it is only
after storage for a long period, when a finely divided flocculent
matter separates by subsidence. Treatment with zinc chloride is
satisfactory but expensive. Perfectly pure manganese sulphate, which
is a neutral salt, has been used by Hartley and Blenkinsop in very
strong solution, and where there is an objection to using an acid.
For ordinary purposes, perfectly satisfactory results are obtained
by the use of a dilute sulphuric acid containing about 30 per cent.
of H_{2}SO_{4}, since, though it possesses the power of withdrawing
water from the oil, it may remain in contact therewith without causing
any charring, and at the same time it causes the precipitation in a
complete and rapid manner of all the mucilage. A purified linseed
oil is thus produced which is bright, clear, and slightly yellowish
in colour, though somewhat paler than the ordinary oil. It is
important that the strength of the oil should not exceed that degree
of concentration which is sufficient for the purpose for which it is
intended. The oil having been so treated, and the impurities separated
by subsidence or otherwise, it is next submitted to the bleaching and
oxidising treatment.

Binks bleached oils with oxides of manganese dissolved in the oil, but
difficulty was experienced in carefully regulating the quantity of
the manganese compounds which were to be introduced into the oil. For
instance, he precipitated manganous hydroxide in contact with oil, and
added the mixture to the bulk of the material, and he also modified the
treatment by dissolving manganous hydroxide in ammonia, and added the
solution to the oil.

Hartley and Blenkinsop prepare manganese linoleate, and dissolve
this in a hydrocarbon, and add a sufficient quantity of the solution
to the oil, whereby it dissolves easily and mixes completely. By this
treatment, the colouring matter of the oil forms a compound with
the manganese which, while it remains in solution, is very speedily
oxidised in contact with air, especially when a current of air or
oxygen is blown through. The oxidation destroys the colouring matter,
and the manganese compound is deoxidised; subsequently it undergoes
oxidation again, and the products of such oxidation taking place in the
oil are acrolein, formic and acetic acids. After, or concurrently with,
the oxidation of the colouring matters, the oil is oxidised, and, at a
suitable temperature below 132° F., the oil is bleached, increased in
density, and converted into a pale drying oil. By limiting the amount
of the manganese linoleate to that which is capable of just oxidising
the colouring matters, oils may be bleached with very little further
oxidation.

Excellent drying oils have been produced by this process, of a very
pale colour. The oil has been used for decorative house painting, for
both indoor and outdoor work, on wood and on metal. It has also been
used as a coating for iron work, without the addition of a pigment.
The plant used in its production is the same as that employed in
oil-boiling by the usual processes when a blast of air is used.

The advantages of a pale boiled oil, containing no lead, are the
following:--

1. Zinc white retains its pure white colour.

2. Delicate tints, and colours containing sulphides, are not darkened
in course of time.

It may be suggested that for indoor decoration, for the painting of
ships, railway carriages, railway semaphores, signs, and stations,
such oil is free from liability to alter the colours with which it is
mixed, owing to its freedom from lead, which is darkened by traces of
sulphuretted hydrogen in the air to which such paints are exposed.

Gasometers in gas-works may be painted an unalterable white with such
oil and zinc white. But in this case also the zinc white must be free
from lead carbonate or oxide.

In commenting on Prof. Hartley’s paper, Mr. Laurie said he had never
used linoleate of manganese for boiling with oil, but by the use of
borate one did get a boiled oil paler than the oil with which one
started. If you take linseed oil which has been already bleached in the
sun to a golden yellow, and convert it into boiled oil with manganese,
a further bleaching process undoubtedly takes place. An oil prepared
with manganese salts, spread on a glass plate, and allowed to dry in
the dark, will remain almost colourless, whereas if it were boiled with
a lead salt it quickly darkens, even if it is kept away from impure
air. Even in a dark room, in pure air, a picture painted with oil
boiled with lead will darken. That is another argument in favour of
manganese, and he should say it ought always to be used in preparing
oil for artistic purposes.



CHAPTER XIII.

PAINT MACHINERY.


In Fig. 40 is shown a complete set of paint grinding and mixing
machinery, made by Wright & Co., 157 Southwark Bridge Road, London,
S.E., which has given highly satisfactory results in efficiency and
economy. It has a set of three granite rollers 30 inches by 15, and two
mixing cylinders or pugs, 24 inches in diameter by 25 inches deep, the
whole mounted on a strong cast-iron frame. It is made in the following
five sizes:--

 ---+----------------+---------+---------+----------+-------------------------
    |                |         |         |          | Work turned out per day.
 No.| 3-roller Mills.| Diam. of| Speed of| Weight.  +------------+------------
    | Size of Roller.| Pulleys.| Pulleys.| (Approx.)|  Ordinary  |  White
    |                |         |         |          |  Colours.  |  Lead.
 ---+----------------+---------+---------+----------+------------+------------
    |   in.    in.   |   in.   |         |   cwt.   |    tons.   |   tons.
  1 |   16  ×   9    |    18   |    70   |     12   | ½   to ¾   |    1½
  2 |   22  ×  12    |    24   |    60   |     16   | 1   “  2   |    4
  3 |   22  ×  14    |    26   |    55   |     22   | 3   “  4   |    8
  4 |   30  ×  15    |    30   |    50   |     30   | 4   “  5   |   10
  5 |   36  ×  16    |    30   |    50   |     35   | 5   “  6   |   12
 ---+----------------+---------+---------+----------+------------+----------

The utility of having the pug mills placed above the granite rollers
is to save labour and space, and the roller mill is kept constantly at
work. There are two pugs, one of which is always ready to deliver to
the rollers, whilst the other is getting ready by the time the first is
being emptied; by this means the output is always going on, and hence
great saving both of time and labour. The pugs are

[Illustration: Fig. 40.--WRIGHT’S PAINT MILL.]

easily fed from a wooden stage fixed to the frame at the back of pugs.
The drawing shows the gear driven by a cog-wheel, but if required it
can be driven by fast and loose pulleys. The whole of the machine is
very compact.

A handy little grinding machine by the same well-known firm is shown
in Fig. 41; a liquid-paint mixer, in Fig. 42; and a single pug mill
for paint or putty, in Fig. 43. Neither of these machines requires any
special description, as the mode of application is evident from the
illustrations.

[Illustration: Fig. 41.--WRIGHT’S SMALL PAINT MILL.]

This firm are also the manufacturers of Clark’s patent paint mill,
illustrated in Fig. 44. This mill differs from the ordinary mills in
having five instead of three rollers. The material is fed in between
the two uppermost rollers, being prevented from spreading too much by
means of wooden cheeks shaped so as to fit between the rollers and form
a kind of hopper in which the material to be ground is placed. From
these two rollers it passes to a third and

[Illustration: Fig. 42.--LIQUID-PAINT MIXER.]

fourth placed below, and receives a final grind from the fifth roller
in front of the machine, from which it is delivered to the spout as
shown. The rollers are all of granite turned truly cylindrical. They
are 15 inches in diameter and are mounted on strong steel spindles.
These spindles run on bearings working in guides, by means of which the
distance

[Illustration: Fig. 43.--SINGLE PUG MILL.]

apart of the rollers and the fineness of the grinding can be adjusted.
It will be seen that the driving-shaft of the machine is the lowest of
the six shown. It drives by means of spur gearing roller number three,
and through it the other rollers of the mill. No two rollers working
together revolve at the same speed, and hence one rubs over the other,
and they thoroughly grind the material between them. In the common
three-roller mills it is necessary to pass the material to be ground
through the mill twice, but in this machine one passage is sufficient,
so that both time and floor space are saved, as the machine, it is
claimed, does the work of two ordinary mills. Another noteworthy
feature of this mill is that on No. 2 roller is a

[Illustration: Fig. 44.--CLARK’S PATENT GRANITE-ROLLER MILL.]

lateral motion giving a sidewise movement of ¾ inch. This is also
applied to No. 4 roller, and gives the same movement. Each roller can
be separately and easily adjusted by means of adjusting screws. The
fifth roller is the delivery roller. The weight of No. 2 roller is
carried by a strong spring fixed between the bearings of rollers Nos. 2
and 3, so that by the movement of the adjusting screw, the No. 2 roller
can be brought down upon No. 3 with the required pressure.

The principle of mounting the rollers of most machines, is that the
centre roll shall revolve in fixed bearings, and the two outer ones
shall revolve in bearings made to slide backward and forward in grooves
in the framework. There is no means of adjusting the rollers in order
to keep them in perfect parallel plane, or to compensate for the wear
of the brasses. The consequence is, that immediately the bearings begin
to wear, the rolls are not then in parallel plane with each other, and
the longer they work the worse they get; until they only grind a small
distance in the centre of the roll, causing the machine not only to
perform imperfect work, but the rolls to wear hollow in the centre.

Hind and Lund, Limited, of the Atlas Works, Preston, Lancashire, claim
that in their machine (Fig. 45) these defects are entirely obviated.
The centre roll is mounted in a similar manner to other machines, but
the two outer rolls are hung upon excentric studs at each side. Should
the journals wear at all unevenly, this is at once detached, and by a
single turn of the excentric stud at either one side or the other as
the case may be, the rolls are kept always in perfect parallel plane.

This machine is fitted with relieving apparatus for throwing the two
outer rolls apart whilst working, in case of accident, or for cleaning
purposes. Thus the machine may be run for an indefinite period without
the granite touching, hence no wear can take place; while the rollers
may be instantly put back to their working distances, just exactly as
they were before the rolls were thrown apart.

As will be readily understood, this is a very valuable improvement, as
the machine can be put in and out of actual grinding work as often as
may be desired, without once altering the pressure upon the springs or
the grinding distances.

[Illustration: Fig. 45.--HIND AND LUND’S PATENT PAINT MILL.]

The bearings are all self-lubricating, and the most delicate colours
or white lead can be ground without fear of being deteriorated. The
machine too is almost noiseless. Many of them can be seen at work, and
giving great satisfaction.

Brinjes and Goodwin’s machine is shown in Figs. 46, 47, and 48. The oil
and pigments, having been measured or weighed, are placed in the trough
_h_. This is provided with

[Illustration: Figs. 46 and 47.--BRINJES AND GOODWIN’S PAINT
MILL.]

[Illustration: Fig. 48.--BRINJES AND GOODWIN’S PAINT MILL.]

stirrers, similar to those in a pug-mill, which are driven by means
of the pulley _l_, _m_ being a loose pulley; by shifting the strap on
to this, the machine can be stopped at once. When the oil has been
thoroughly incorporated with the pigment, the mixture is allowed to
run through the spout _g_ on the roller _a_. Working against _a_ is a
second roller _b_, and this in its turn bears upon a third roller _c_.
In order to prevent the grooving of the faces of the rollers, which
always takes place when they revolve in the same plane, there is an
arrangement by which a slight lateral motion is communicated to _b_,
in addition to the rotary motion. A pin fixed upon the rigid bracket
_k_ works in the grooved cam _i_, which is keyed on the shaft of the
roller _b_. The grinding power of the machine is considerably increased
by this modification. The rollers are worked from the pulley _d_;
the loose pulley _e_ receives the strap when a pause in the working
of the machine becomes necessary. The details of the construction of
the grinding machine are given in Fig. 48. The rollers _a b c_ are
constructed of granite or porcelain; for fine grinding, the latter
substance is preferable. They are adjusted by means of the screws _g
h_. These are furnished with spiral springs, so that should a nail
or other hard substance get between the rollers, these can rise in
their bearings, letting the nail fall down at the back. The “doctor”
or scraper _f_ removes the paint from the surface of the roller _c_;
_a b_ are also provided with smaller scrapers, which remove any paint
that may cake upon their surfaces. Where extreme fineness is requisite,
the paint is again passed through the machine, and this operation is
sometimes repeated several times.

In working these or any other form of grinding rollers, great care
must be taken to clean them thoroughly immediately after use. If the
paint be allowed to dry upon the surface of the rollers it is difficult
of removal, and interferes with the perfect action of the machine.
Should the working parts become clogged with solidified oil, a strong
solution of caustic soda or potash will remove it. By means of the
same solutions, porcelain rollers may be kept quite white, even if
used for mixing coloured paints. Although the colour of most pigments
is improved by grinding them finely in oil, yet there are some which
suffer in intensity where their size of grain is reduced. Chrome red,
for instance, owes its deep colour to the crystals of which it is
composed, and when these are reduced to extremely fine fragments, the
colour is considerably modified.

PACKING.--When paint is not intended for immediate use, it
is packed in metallic kegs. The construction of these, as made by B.
Noakes & Co., is shown in Fig. 49. For exportation to hot climates, the
rim of the lid is sometimes soldered down, a practice which effectually
prevents access of atmospheric oxygen. White-lead paint is frequently
packed in wooden kegs; these prevent the discoloration sometimes caused
by the metal of iron kegs. When paint is mixed ready for use, it will,
if exposed to the air, become covered with a skin, which soon attains
sufficient thickness to exclude the atmospheric oxygen, and prevent
any further solidification of the oil. The paint may be still better
protected by pouring water over it, or it may be placed in air-tight
cans. If it has been allowed to stand for some time, it must be well
stirred before using, as the pigments have a tendency not only to
separate from the oil, but also to settle down according to their
specific gravity.

[Illustration: Fig. 49.--PAINT KEG.]



CHAPTER XIV.

PAINTING.


The successful application of paint, whether for artistic or
preservative purposes, necessitates careful attention to a number of
considerations, some of a mechanical and others of a chemical character.

_The Surface._--Of whatever the surface may be to which the paint is
to be applied, great care must be taken that it is perfectly dry. Wood
especially, even when apparently dry, may on a damp day contain as much
as 20 per cent. of moisture. A film of paint applied to the surface
of wood in this condition prevents the moisture from escaping, and it
remains enclosed until a warm sun or artificial heat converts it into
vapour, which raises the paint and causes blisters. Moisture enclosed
between two coats of paint has the same effect. Paint rarely blisters
when applied to wood from which old paint has been burnt off; this is
probably due to the drying of the wood during the operation of burning.

When the surface to be painted is already covered with old paint, this
should be either removed or rubbed down smooth before applying the
new. When the thickness of the old coat is not great, rubbing down,
accompanied by a careful scraping of blisters and defective parts,
will suffice. When the thickness of the old paint necessitates its
removal, it may either be burned off, or softened by a solution of
caustic alkali, and afterwards scraped. The burning process is the
most effective, and leaves the wood in a fit condition to receive the
fresh coat of paint; but it is not applicable in the case of fine
mouldings. When caustic potash or soda is used, the paint is left in
contact with, it for some time, when the linoleic acid of the oxidised
linseed-oil becomes saponified, and can easily be scraped or scrubbed
off the surface of the wood.

Whenever an alkali is employed, it is of the greatest importance that
the wood should afterwards be thoroughly washed several times with
clean water, in order to remove every trace of the solvents. Any soda
or potash remaining in the pores of the wood would not only retain
moisture and cause blistering, but would also have an injurious action
upon the vehicle of the paint subsequently applied, and in many cases
upon the pigment itself. The remarks already made as to the necessity
of an absolutely dry surface should be borne in mind in this instance.
When the surface of the paint is to be protected by a coat of varnish,
the latter should not be applied until the whole of the oil contained
in the paint has solidified. The wrinkling of varnish upon paint is
frequently erroneously attributed to the bad quality of the varnish,
when the real cause is the incomplete oxidation of the paint itself.

_Priming._--The first coat of paint applied to any surface is termed
the “priming-coat.” It usually consists of red lead and boiled and raw
linseed-oil. Experience has shown that such a priming not only dries
quickly itself, but also accelerates the drying of the next coat. The
latter action must be attributed to the oxygen contained in the red
lead, only a small portion of which is absorbed by the oil with which
it is mixed.

Kali, of Heidelberg, prepares a substitute for boiled oil by mixing 10
parts whipped blood, just as it is furnished from the slaughter-houses,
with 1 part of air-slaked lime sifted into it through a fine sieve.
The two are well mixed and left standing for 24 hours. The dirty
portion that collects on top is taken off, and the solid portion is
broken loose from the lime at the bottom; the latter is stirred up with
water, left to settle, and the water is poured off after the lime has
settled. The clear liquid is well mixed up with the solid substance
before mentioned. This mass is left standing for 10 or 12 days, after
which a solution of potash permanganate is added, which decolorises it
and prevents putrefaction. Finally the mixture is stirred up, diluted
with more water to give it the consistence of very thin size, filtered,
a few drops of oil of lavender are added, and the preparation is
preserved in closed vessels. It is said to keep a long time without
change. A single coat of this liquid will suffice to prepare wood or
paper, as well as lime or hard plaster walls, for painting with oil
colours. This substance is cheaper than linseed oil, and closes the
pores of the surface so perfectly that it takes much less paint to
cover it than when primed with oil.

_Drying._--The drying of paint is to a great extent dependent upon the
temperature. Below the freezing point of water, paint will remain wet
for weeks, even when mixed with a considerable proportion of dryers;
while, if exposed to a heat of 120° F. the same paint will become solid
in a few hours. The drying of paint being a process of oxidation, and
not evaporation, it is essential that a good supply of fresh air should
be provided. When a film of fresh paint is placed with air in a closed
vessel, it does not absorb the whole of the oxygen present; but after a
time the drying process is arrested, and the remaining oxygen appears
to have become inert.

Considerable quantities of volatile vapours are given off during the
drying of paint; these are due to the decomposition of the oil. When
the paint has been thinned down by turpentine, the whole of this
liquid evaporates on exposure to the air. There must, therefore, be
a plentiful access of air, to remove the vapours formed, and afford
a fresh supply of active oxygen. The presence of moisture in the air
is rather beneficial than injurious at this stage. Especially in the
case of paints mixed with varnish, moist air appears to counteract the
tendency to crack or shrink. Under the erroneous impression that the
drying of paint is a species of evaporation, open fires are sometimes
kept up in freshly-painted rooms. It is only when the temperature is
very low that any benefit can result from this practice; as a rule, it
rather retards than hastens the solidification of the oil, which cannot
take place rapidly in an atmosphere laden with carbonic acid.

The first coat of paint should be thoroughly dry before the second is
applied. Acrylic acid is formed during the oxidation of linseed-oil,
and unless this be allowed to evaporate, it may subsequently liberate
carbonic acid from the white lead present in most paints, and give rise
to blisters. Sometimes a second priming-coat is given; but usually
the second coat applied contains the pigment. This, as soon as dry,
is again covered by another coat, and subsequently by two or more
finishing coats, according to the nature of the work.

_Filling._--Before the first coat is applied to wood, all holes should
be filled up. The filling usually employed is ordinary putty; this,
however, sometimes consists of whiting ground up with oil foots of a
non-drying character, and when the films of paint are dry, the oil from
the putty exudes to the surface, causing a stain. The best filling for
ordinary purposes is whiting ground to a paste with boiled linseed-oil.
For finer work, and for filling cracks, red lead mixed with the same
vehicle may be employed. For porous hard woods, use boiled oil and
corn starch stirred into a very thick paste; add a little japan, and
reduce with turpentine. Add no colour for light ash; for dark ash and
chestnut, use a little raw sienna; for walnut, burnt umber and a slight
amount of Venetian red; for bay wood, burnt sienna. In no case use
more colour than is required to overcome the white appearance of the
starch, unless you wish to stain the wood. This filler is worked with
brush and rags in the usual manner. Let it dry 48 hours, or until it
is in condition to rub down with No. 0 sand-paper without much gumming
up; and if an extra fine finish is desired, fill again with the same
materials, using less oil, but more japan and turpentine. The second
coat will not shrink, being supported by the first. When the second
coat is hard, the wood is ready for finishing up by following the usual
methods. This formula is not intended for rosewood.

_Coats._--There is no advantage in laying on the paint too thickly.
A thick film takes longer to dry thoroughly than two thin films of
the same aggregate thickness. Paint is thinned down or diluted with
linseed-oil or turpentine. The latter liquid, when used in excess,
causes the paint to dry with a dull surface, and has an injurious
effect upon its stability. Sometimes the last coat of paint is mixed
with varnish, in order to give it greater brilliancy. In this case,
special care must be taken that the previous coats have thoroughly
solidified, or cracks in the final coat may subsequently appear. The
same remark applies when the surface of the paint is varnished. The
turpentine with which the varnish is mixed has a powerful action
upon the oil contained in the paint, if the latter is not thoroughly
oxidised. The exterior of the paint is thus softened, and the varnish
is enabled to shrink and crack, especially in warm weather.

_Brushes._--The bristles are frequently fastened by glue or size, which
is not perceptibly acted upon by oil, and if brought into contact with
this liquid alone, there would be no complaints of loose hairs coming
out and spoiling the work. It is a common practice to leave the brushes
in a paint-pot, in which the paint is covered with water to keep it
from drying. The brushes are certainly kept soft and pliant in this
way; but at the same time the glue is softened, and the bristles come
out as soon as the brush is used. After use, brushes should be cleaned,
and placed in linseed-oil until again required, when they will be found
in good condition. Treated in this way, they will wear so much better
that the little additional trouble entailed is amply repaid. When
brushes will not again be required for some time, the oil remaining in
them should be washed out by means of turpentine, after which they may
be dried without deterioration. On no account should oil be allowed
to dry in a brush, as it is most difficult to remove after oxidation
has taken place. The best means are steeping in benzoline for a few
days, or in turpentine, with occasional washing in soda-water and with
soft-soap, avoiding too violent rubbing.

_Water-colours._--The manufacture of water-colour paints is more simple
than that of oil paints, the pigments being first ground extremely
fine and then mixed with a solution of gum or glue. The paste produced
in this manner is allowed to dry, after having been stamped into the
form of cakes. As soon as the hardened mass is rubbed down with water,
the gum softens and dissolves, and if the proportion of water be not
too great, the pigment will remain suspended in the solution of gum,
and can be applied in the same manner as oil-paint. To facilitate the
mixing with water, glycerine is sometimes added to the cake of paint,
which then remains moist and soft.

_Removing Odour._--(1) Place a vessel of lighted charcoal in the room,
and throw on it 2 or 3 handfuls of juniper berries; shut the windows,
the chimney, and the door close; 24 hours afterwards the room may be
opened, when it will be found that the sickly, unwholesome smell will
be entirely gone. (2) Plunge a handful of hay into a pail of water, and
let it stand in the room newly painted.

_Discoloration._--Light-coloured paints, especially those having white
lead as a basis, rapidly discolour under different circumstances. Thus
white paint discolours when excluded from the light; stone colours lose
their tone when exposed to sulphuretted hydrogen, even when that is
only present in very small quantity in the air; greens fade or darken,
and vermilion loses its brilliancy rapidly in a smoky atmosphere like
that of London.

Ludersdorf thinks that the destructive change is principally due to
a property in linseed-oil which cannot be destroyed. The utility of
drying oils for mixing pigments depends entirely on the fact that
they are converted by the absorption of oxygen into a kind of resin,
which retains the colouring pigment in its semblance; but during
this oxidation of the oil--the drying of the paint--a process is set
up which, especially in the absence of light and air, soon gives
the whitest paint a yellow tinge. Ludersdorf therefore proposes
to employ an already formed but colourless resin as the binding
material of the paint, and he selects two resins as being specially
suitable--one, sandarach, soluble in alcohol; the other, dammar,
soluble in turpentine. The sandarach must be carefully picked over, and
7 oz. is added to 2 oz. Venice turpentine and 24 oz. alcohol of sp.
gr. 0·833. The mixture is put in a suitable vessel over a slow fire
or spirit-lamp, and heated, stirring diligently, until it is almost
boiling. If the mixture be kept at this temperature, with frequent
stirring for an hour, the resin will be dissolved, and the varnish is
ready for use as soon as cool. The Venice turpentine is necessary to
prevent too rapid drying, and more dilute alcohol cannot be employed,
because sandarach does not dissolve easily in weaker alcohol, and,
furthermore, the alcohol, by evaporation, would soon become so weak
that the resin would be precipitated as a powder.

When this is to be mixed with white lead, the latter must first be
finely ground in water, and dried again. It is then rubbed with a
little turpentine on a slab, no more turpentine being taken than is
absolutely necessary to enable it to be worked with the muller; 1 lb.
of the white lead is then mixed with exactly ½ lb. of varnish, and
stirred up for use. It must be applied rapidly, because it dries so
quickly. If when dry the colour is wanting in lustre, it indicates the
use of too much varnish. In such cases, the article painted should be
rubbed, when perfectly dry, with a woollen cloth to give it a gloss.
The dammar varnish is made by heating 8 oz. dammar in 16 oz. turpentine
oil at 165° to 190° F., stirring diligently, and keeping it at this
temperature until all is dissolved, which requires about an hour. The
varnish is then decanted from any impurities, and preserved for use.
The second coat of the pure varnish, to which half its weight of oil of
turpentine has been added, may be applied. It is still better to apply
a coat of sandarach varnish made with alcohol, because dammar varnish
alone does not possess the hardness of sandarach, and when the article
covered with it is handled much, does not last so long.

_Composition._--The composition of paints should be governed--

(1) By the nature of the material to be painted: thus the paints
respectively best adapted for wood and iron differ considerably.

(2) By the kind of surface to be covered: a porous surface requires
more oil than one that is impervious.

(3) By the nature and appearance of the work to be done: delicate tints
require colourless oil, a flatted surface must be painted without oil
(which gives gloss to a shining surface), paint for surfaces intended
to be varnished must contain a minimum of oil.

(4) By the climate and the degree of exposure to which the work will be
subjected: for outside work, boiled oil is used, because it weathers
better than raw oil; turps is avoided as much as possible, because it
evaporates and does not last; if, however, the work is to be exposed to
the sun, turps is necessary, to prevent the paint from blistering.

(5) The skill of the painter affects the composition: a good workman
can lay on even coats with a smaller quantity of oil and turps than
one who is unskilful; extra turps, especially, are often added to save
labour.

(6) The quality of the materials makes an important difference in the
proportions used: thus more oil and turps will combine with pure than
with impure white lead; thick oil must be used in greater quantity than
thin when paint is purchased ready ground in oil, a soft paste will
require less turps and oil for thinning than a thick.

(7) The different coats of paint vary in their composition: the first
coat laid on to new work requires a good deal of oil to soak into the
material; on old work, the first coat requires turpentine to make it
adhere; the intermediate coats contain a proportion of turpentine to
make them work smoothly; and to the final coats the colouring materials
are added, the remainder of the ingredients being varied according as
the surface is to be glossy or flat.

The exact proportions of ingredients best to be used in mixing paints
vary according to their quality, the nature of the work required,
the climate, and other considerations. The composition of paint for
different coats also varies considerably. The proportions given in the
following table must only be taken as an approximate guide when the
materials are of good quality:--

TABLE showing the COMPOSITION of the different
COATS of WHITE PAINT, and the QUANTITIES
required to cover 100 yd. of NEWLY-WORKED PINE.


  --------------+-----+-------+------+------+-----+------+---------
                |Red  | White |  Raw |Boiled|Turp.|Dryers|REMARKS
                |lead | lead. | Lins.|Lins. |     |      |(see
                |     |       |  oil | oil  |     |      |footnotes)
  --------------+-----+-------+------+------+-----+------+---------
  _Inside work, |     |       |      |      |     |      |
  4 coats not   |     |       |      |      |     |      | [A]
   flatted._    | lb. |  lb.  |  pt. | pt.  | pt. |  lb. |
                |     |       |      |      |     |      |
  Priming       |  ½  | 16    | 6    | --   | --  |   ¼  |
  2nd coat      | [B] | 15    | 3½   | --   | 1½  |   ¼  |
  3rd coat      | --  | 13    | 2½   | --   | 1½  |   ¼  |
  4th coat      | --  | 13    | 2½   | --   | 1½  |   ¼  |
                |     |       |      |      |     |      |
  _Inside work  |     |       |      |      |     |      |
  4 coats &     |     |       |      |      |     |      |
  flatting._    |     |       |      |      |     |      |
  Priming       |1½   | 16    | 6    | --   | ½   |  1-8 |
  2nd coat      | --  | 12    | 4    | --   | 1½  |  1-10|
  3rd coat      | --  | 12    | 4    | --   | 0   |  1-10|
  4th coat      | --  | 12    | 4    | --   | 0   |  1-10|
  Flatting      | --  |  9    | 0    | --   | 3½  |  1-10|
                |     |       |      |      |     |      |
  _Outside work |     |       |      |      |     |      | [C]
  4 coats not   |     |       |      |      |     |      |
   flatted._    |     |       |      |      |     |      |
  Priming       |  2  | 18½   |  2   | 2    | --  |  1-8 |
  2nd coat      | --  | 15    |  2   | 2    | ½   |  1-10|
  3rd coat      | --  | 15    |  2   | 2    | ½   |  1-10|
  4th coat      | --  | 15    |  3   | 2½   | 0   |  1-10|
  ---------------------------------------------------------------

[A] Sometimes more red lead is used and less dryer.

[B] Sometimes just enough red lead is used to give a flesh-coloured
tint.

[C] When the finished colour is not to be pure white, it is better to
have nearly all the oil boiled oil. All boiled oil does not work well.
For pure white a larger proportion of raw oil is necessary, because
boiled oil is too dark.

_Area Covered._--For every 100 sq. yd., besides the material enumerated
in the foregoing, 2½ lb. white lead and 5 lb. putty will be required
for stopping. The area which a given quantity of paint will cover
depends upon the nature of the surface to which it is applied, the
proportion of the ingredients, and the state of the weather. When the
work is required to dry quickly, more turpentine is added to all the
coats. In repainting old work, two coats are generally required, the
old paint being considered as priming. Sometimes another coat may
be deemed necessary. For outside old work exposed to the sun, both
coats should contain 1 pint turpentine and 4 pints boiled oil, the
remaining ingredients being as stated in the foregoing table. The
extra turpentine is used to prevent blistering. In cold weather, more
turpentine should be used to make the paint flow freely.

According to another authority, it is found that in painting wood,
one coat takes 20 lb. lead and 4 gal. oil per 100 sq. yd.; the second
coat, 40 lb. lead and 4 gal. oil; and the third the same as the second;
say 100 lb. lead and 16 gal. oil per sq. yd. for the three coats. The
number of square yards covered by one gallon of priming colour is found
to be 50; of white zinc, 50; of white lead paint, 44; of lead colour,
50; of black paint, 50; of stone colour, 44; of yellow paint, 44; of
blue colour, 45; of green paint, 45.

_Measuring._--Surface painting is measured by the superficial yard,
girting every part of the work covered, always making allowance for
the deep cuttings in mouldings, carved work, railings, or other work
that is difficult to get at. Where work is very high, and scaffolding
or ladders have to be employed, allowances must be made. The
following rules are generally adopted in America in the measurement
of work:--Surfaces under 6 in. in width or girt are called 6 in.;
from 6 to 12 in., 12 in.; over 12 in., measured superficial. Openings
are deducted, but all jambs, reveals, or casings are measured girt.
Sashes are measured solid if more than two lights. Doors, shutters and
panelling are measured by the girt, running the tape in all quirks,
angles or corners. Sash doors measure solid. Glazing, in both windows
and doors, is always extra. The tape should be run close in over the
battens, on batten doors, and if the stuff is beaded, add 1 in. in
width for each bead. Venetian blinds are measured double. Dentels,
brackets, medallions, ornamented iron work, balusters, lattice work,
palings, or turned work, should all be measured double. Changing
colours on base boards, panels, cornices, or other work, one-fourth
extra measurement should be allowed for each tint. Add 5 per cent. to
regular price for knotting, puttying, cleaning, and sand-papering. For
work done above the ground floor, charge as follows:--Add 5 per cent.
for each story of 12 ft. or less, if interior work; if exterior work,
add 1 per cent. for each foot of height above the first 12 ft.

CARRIAGE AND CAR PAINTING.--The following is the substance of
an address delivered by McKeon, secretary and treasurer of the Master
Car-Painters’ Association of the United States:--A first-class railway
coach costs, when complete, about 1200_l._ To protect this work, the
painter expends 60_l._ to 120_l._ The latter figure will make a first
class job. The car has been completed in the wood-shop, and is turned
over to the painter, who is responsible for the finish. He is expected
to smooth over all rough places or defects in the wood, which requires
both patience and skill to make the work look well. Twelve weeks should
be the time allowed to paint a car, and it cannot be done in any less
time, to make a good job that will be a credit to the painter and all
other parties interested in the construction and finish of the car. Too
much painting is done in a hurry: proper time is not given the work to
dry or become thoroughly hardened before it is run out of the shop, and
consequently it does not always give the satisfaction it should; nor
can it be expected that hurried work will be so lasting or durable as
that which has the necessary time given to finish it.

_Priming Paint._--The priming coat of paint on a car is of as much
importance as any succeeding one, and perhaps more. Good work is ruined
in the priming by little or no attention being given by the painter to
the mixing and application of the first coat. The foundation is the
support, and on that rests success or failure. The priming should be
made of the proper material, mixed with care from good lead and good
oil, and not picked up from old paints which have been standing mixed,
and must necessarily be fat and gummy, for such are unfit for use on
a good job, and will have a decided tendency to spoil the whole work.
Special care should be exercised, both in mixing and applying the
priming, and it should be put on very light, so that it may penetrate
well into the wood. Too much oil is worse than not enough. Good ground
lead is by far the best material for the undercoats on a car. Two coats
should be given to the car before it is puttied, as it is best to fill
well with paint the nail-holes and plugs, as well as defects in the
wood, so that moisture may not secure a lodgment, which otherwise will
cause the putty to swell, although sometimes unseasoned lumber will
swell the putty; and as it shrinks, the nail remains stationary, and of
course the putty must give way.

_Best Putty._--In mixing putty, which may be a small matter with some,
take care to so prepare it that it will dry perfectly hard in 18 hours.
Use ground lead and japan, stiffening up with dry lead, and whatever
colouring you may require in it to match your priming coats.

The next coats, after the work is well puttied, should be made to dry
flat and hard. Two coats should be applied, and, for all ordinary
jobs or cheap work, sand-papering is all that is necessary for each
coat; but when a good surface is required, I would recommend one coat
to be put on heavy enough to fill the grain; and before being set,
scrape with a steel scraper. The plain surface is all that requires
coating and scraping with the heavy mixture. For this coat, which is
called filling, use one-half ground lead and any good mineral which
experience has shown can be relied on. This scraping of the panel work
will fill the wood equal to two coats of rough stuff, and saves a great
amount of labour over the old process, when so much rubbing with lump
pumice was done. Sand-paper when the filling is thoroughly hard, and
apply another coat of paint of ordinary thickness, when, after another
sand-papering, you have a good surface for your colour.

Rough-coating on cars has gone almost out of use, and few shops are now
using it to any extent. My experience is that paint has less tendency
to crack where rough stuff is left off. I do not claim that the filling
was the principal cause of the cracking, if it was properly mixed; but
I believe the water used in rubbing down a car with the lump pumice
injures the paint, as it will penetrate in some places, particularly
around the moulding plugs.

_Finishing Colour._--The car being ready for the finishing colour, this
should be mixed with the same proportion of dryer as the previous coat,
or just sufficient to have it dry in about the same time. A great error
with many car painters is using a large portion of oil in the under
coats, and then but little, if any, in the finishing coats; this has a
decided tendency to crack, the under coats being more elastic. Always
aim to have colour dry in about the same time, after you have done
your priming; by this plan you secure what all painters should labour
to accomplish--namely, little liability to crack. Work will of course
crack sometimes after being out a few months, or when it has repeated
coatings of varnish; and using a quick rubbing varnish on work will
cause it to give way in fine checks quicker than anything else. Many of
the varnishes used are the cause of the paint cracking, and no painter
has been wholly exempt from this trouble.

_Cause of Cracking._--The most common cause of cracking is poor japan,
which is the worst enemy that the car-painter has to contend with. The
greater part of the japan is too elastic, and will dry with a tack,
and the japan gold size has generally the same fault, although the
English gold size is generally of good quality; but its high price
is an objection to its use. A little more care in the manufacture
of japans would give a better dryer, and few would object to the
additional cost. Japan frequently curdles in the paint; it will not mix
with it, but gathers in small gummy particles on the top. Work painted
with such material cannot do otherwise than crack and scale, and the
remedy lies only in getting a good pure article of turpentine japan.

In regard to using ground lead, car-painters differ, as some prefer
to grind their own in the shop. I use the manufactured lead, and my
reasons for doing so are that it is generally finer than any shop can
grind it with present facilities, and it has age after grinding, which
improves its quality. You can also get a purer lead and one with more
body than you can by grinding in the shop, which is a fact that I think
most painters must admit. I have tested it very fully, and am convinced
on this point.

_Mixing the Paints._--Permit me to make a few suggestions here in
regard to the mixing of paint, which may not fully agree with others’
views. There is just as much paint that cracks by putting it on too
flat as by using too much oil. Some painters mix their finishing colour
so that it is impossible to get over a panel of ordinary size before
it is set under the brush, and consequently the colour will rough up.
Colour should be mixed up so that it will not flat down for some time
after leaving it, and then you have got some substance that will not
absorb the varnish as fast as it is applied to the surface. This quick
drying of colour is not always caused by want of oil in it, but because
there is too much japan, and a less quantity of the latter will do
better work, and make a smoother finish. Give your colour 48 hours to
dry between coats; always give that time, unless it is a hurried job,
and experience has fully demonstrated that it is poor economy to hurry
work out of the shop before it is properly finished.

_Oils, Dryers, and Colours._--In car-painting, both raw and boiled oils
are used, and good work may be done with either, but I recommend oil
that is but slightly boiled, in preference to either the raw or the
boiled. After it is boiled, if it is done in the shop, let it stand
24 hours to settle, then strain off carefully; this takes out all the
impurities and fatty matter from the oil, and it will dry much better,
nor will it have that tack after drying that you find with common
boiled oil. Use the proper quantity of dryer in mixing your paint,
and a good reliable job will be the result. In car-painting, never
use prepared colours which are ground in oil, as nine-tenths of such
colours are ground in very inferior oil, and they may have been put up
for a great length of time, in which case they become fatty, and will
invariably crack. These canned colours do not improve with age, as lead
and varnish do.

Finishing colours should all be ground in the shop, unless special
arrangements can be made with manufacturers to prepare them; and the
colour should be fresh, not over 6 or 8 days old after being mixed,
and open to the air. Enough may be prepared at a time to complete the
coating on a job; but when colour stands over a week it is not fit to
use on first-class work, as it becomes lifeless, and has lost that
free working which we find in freshly-mixed colours. Such colour may,
however, be used upon a cheap class of work, or on trucks, steps, &c.,
so that nothing need be wasted in the shop.

_Varnishing._--Three coats of varnish over the colour are necessary on
a first-class coach. The first coat should be a hard drying varnish
put on the flat colour; the quick rubbing that some use I would not
recommend, but one that will dry in five days (in good drying weather)
sufficiently hard to rub, is the best for durability. After striping
and ornamenting the car, and when thoroughly washed, give a coat of
medium dry varnish. Let this stand 8 days; then rub lightly with curled
hair or fine pumice, and apply the finishing coat, which is “wearing
body;” this will dry hard in about 10 days, after which the car may
be run out of the shop. It should then be washed with cold water and
a soft brush, and is ready for the road. In varnishing, many will
apply the varnish as heavy as they can possibly make it lie, when, as
a consequence, it flows over and runs or sags down in ridges, and of
course does not harden properly; this also leaves a substance for the
weather to act on. It is better to get just enough on at a coat to make
a good even coating which will flow out smooth, and this will dry hard,
and will certainly wear better than the coat that is piled on heavily.

Varnishing, we claim, can be over-done, despite some painters’ opinions
to the contrary. We have heard of those who put 2½ gal. on the body
of a 50-foot car at one application, and we have also listened to the
declaration, made by a member of the craft, that he put 2 gal. on the
body of a locomotive tank. Such things are perhaps possible, and may
have been done; but if so, we know that the work never stood as well as
it would if done with one-half the quantity to a coat. In varnishing
a car, care should be taken to have the surface clean; water never
injures paint where it is used for washing; and a proper attention
to cleanliness in this respect, and in the care of brushes used for
varnishing, will ensure a good-looking job.

Perhaps your shop facilities for doing work are none of the best, but
do the best you can with what you have. Select if possible, a still,
dry day for varnishing, especially for the finishing coat. Keep your
shop at an even temperature; avoid cold draughts on the car from doors
and windows; wet the floor only just sufficient to lay the dust, for
if too wet, the dampness arising will have a tendency to destroy the
lustre of your varnish. Of course we cannot always do varnishing to our
perfect satisfaction, especially where there are 25 or 30 men at work
in an open shop, and 6 or 8 cars are being painted, when more or less
dirt and dust are sure to get on the work.

A suggestion might here be made to railroad managers, which is that
no paint-shop is complete where the entire process of painting and
finishing a car is to be done in one open shop. A paint-shop should
be made to shut off in sections by sliding doors, one part of the
shop being used exclusively for striping and varnishing. I know from
experience that nine-tenths of the railroad paint-shops are deficient
in this particular, and still we are expected to turn out a clean job,
no matter what difficulties we are compelled to labour under.

_Importance of Washing Cars._--In regard to the care of a car after it
has left the shop, more attention should be given to this than is done
on many roads. The car should not be allowed to run until it is past
remedy, and the dirt and smoke become imbedded in the varnish, actually
forming a part of the coating, so that when you undertake to clean the
car you must use soda or soap strong enough to cut the varnish before
you succeed in removing the dirt. Cars should be washed well with a
brush and water at the end of every trip. This only will obviate the
difficulty, and these repeated washings will harden the varnish as well
as increase its lustre. We know that, in washing a car, where soap is
required to remove the dirt and smoke, it is almost impossible to get
the smoke washed off clean; and if it is not quite impossible, the hot
sun and rain will act on the varnish and very soon destroy it.

_Re-varnishing._--Cars should be taken in and re-varnished at least
once in 12 months; and if done once in 8 months it is better for them,
and they will require only one coat; but where they run a year, they
will generally need two coats. Those varnished during the hot months
will not stand as well as if done at any other time. Painting done in
extremely cold weather, or in a cold shop, is more liable to crack than
if done in warm weather.

_How to Dry Paint._--Paint dried in the shop where there is a draught
of dry air passing through, will stand better than that dried by
artificial heat; and you will find, by giving it your attention, that
work which has failed to stand, and which cracked or scaled, was
invariably painted in the winter season, or in cool, wet weather. I
have paid some attention to this matter, and know the result.

WOODWORK PAINTING.--One of the attendant drawbacks of houses
that are newly built, or have been hastily finished for letting, is
the inferior painting of the woodwork, and its speedy destruction. The
wood is not thoroughly dry, and the consequence is the preparatory coat
does not adhere; the pores being full of dampness, it is impossible for
the oil to sink into them, especially as oil and water are unmiscible.
Another equally injurious condition is the gum-resin which exudes from
the knots of new pine and other timber. Painted over before it has
time to come to the surface, the coat is destroyed by the action of
the gum. Now, these evils have to be endured so long as the wood has
no time to get seasoned. The painter follows the carpenter without any
interval of time, and before the action of the weather can bring out
the moisture and resinous substances. A coating of shellac is usually
given to the knots, though this is often so thin as to be worthless.
Crude petroleum, as a preservative coat, is found to be an admirable
preparation for the painting. The petroleum is thin, and penetrates the
wood, filling up the pores, and giving a good ground for the coats of
paint.

According to one American authority, the preparation is of great value.
The priming coat should be thin and well rubbed in, and it is better
to use a darker colour than white lead as a base. White lead forms a
dense covering to the surface, though it has its disadvantages. When
petroleum has formed the first coat, two other coats will suffice,
one being the priming coat, and a third coat may be given after the
work has stood for a season. It is a very desirable plan to leave
the painting, or rather finishing coats, for a time, so that any
imperfections in the wood or work may be discovered; it also allows
time for any change of colour that may be made. After the priming
coat, it is usual in good work to stop all cracks, nail-holes, and
other defects with putty; but in the commoner class of paintings, the
coats are laid on quickly, the preceding coat has hardly time to dry
before the next is put on, and all the defects of wood, bad seasoning,
exudation of gum, &c., quickly begin to show themselves through and
disfigure the work.

A good paint ought to possess body, power of covering, and flow evenly
from the brush, and become hard. Though zinc white has less body than
white lead, it is more durable, and will stand sulphur acids without
blackening. Some colours stand better than others; the ochres, Indian
and Venetian reds, burnt and raw umber, are reliable, and may be
used without scruple. It is also worthy of notice that salt air acts
injuriously on white lead, and zinc white is therefore preferable in
situations exposed to the sea-air. (_English Mechanic._)

IRON PAINTING.--The decay of iron becomes very marked in
certain situations, and weakens the metal in direct proportion to the
depth to which it has penetrated; and although where the metal is in
quantity this is not very appreciable, it really becomes so when the
metal is under ¾ inch in thickness. The natural surface of cast iron is
very much harder than the interior, occasioned no doubt by its becoming
chilled, or by its containing a large quantity of silica, and this
affords an excellent protection.

But should this surface be at all broken, rust immediately attacks
the metal and soon destroys it. It is very desirable that the casting
be protected, and a priming coat of oil or paint should be applied
for this purpose; the other coats, though requisite, can be given at
leisure.

The following is a process to which all cast-iron water pipes should be
submitted. It was introduced by Dr. Smith, and is equally applicable to
any other casting that can be manipulated:--Each casting is thoroughly
dressed, and made clean and free from the earth and sand which cling
to the iron in the moulds, hard brushes being used in finishing the
process to remove the loose dust. Every casting must be likewise free
from rust when the paint is applied. If the casting cannot promptly be
dipped after being cleansed, the surface must be oiled with linseed oil
to preserve it until it is ready to be dipped. No casting is on any
account to be dipped after rust has set in. The coal-tar pitch used
as a paint in this process is made from coal-tar distilled until the
naphtha is entirely removed and the material is deodorised. In England
it is distilled until the pitch is about the consistence of wax. The
mixture of 5 or 6 per cent. of linseed oil is recommended by Dr. Smith.
Pitch which becomes hard and brittle when cold will not answer for
this use. Pitch of the proper quality having been obtained, it must be
carefully heated in a suitable vessel to a temperature of 300° F., and
must be maintained at not less than this temperature during the time
of dipping. The material will thicken, and deteriorate after a number
of pieces have been dipped; fresh pitch must, therefore, frequently be
added, and occasionally the vessel must be entirely emptied of its old
contents and refilled with fresh pitch. The refuse will be hard and
brittle like common pitch, and consequently worthless for the purpose.
Every casting must attain a temperature of 300° F., either by previous
heating or during the immersion before being removed from the vessel of
hot pitch. It may then be slowly removed, and laid upon skids to drip.
In the case of water pipes, all those of 20 inches diameter and upwards
will have to remain at least 30 minutes in the hot fluid to attain this
temperature. The coating when cold should be tough and tenacious, and
not brittle nor have the slightest tendency to scale off.

In considering the painting of wrought iron it must be noticed that
when iron is oxidised by heating in contact with the atmosphere, two
or three distinct layers of scale form on the surface, and, unlike
the skin upon cast iron, can be readily detached, as by bending or by
hammering the metal. The outer layer of this scale is more highly
oxidised than the inner, and is slightly redder in tinge from the
presence of a variable excess of ferric oxide over that contained in
the inner layer. The oxide occurring in the outer scale is fusible only
at a high temperature, is strongly magnetic, and slightly metallic in
lustre; while the inner layers are more porous, dull, and non-metallic
in lustre, less brittle, and also less powerfully magnetic. It will be
seen that the iron has a tendency to rust from the moment it leaves the
hammer or rolls, and that the scale above described must come away. One
of the plans to preserve the iron has been to coat it with paint when
still hot at the mill; and although this answers for a while, it is
a very troublesome method, which iron-masters cannot be persuaded to
adopt, and the subsequent cutting processes to which it is submitted
leave many parts of the iron bare. Besides, a good deal of the scale
remains, and until this has fallen off, or has been removed, any
painting over it will be of little value. The only effectual way of
preparing wrought iron is to effect a thorough and chemical cleansing
of the surface of the metal upon which the paint is to be applied, that
is, it must be immersed for three or four hours in water containing
from 1 to 2 per cent. of sulphuric acid. The metal is afterwards rinsed
in cold water, and, if necessary, scoured with sand, put again into the
acid bath or pickle, and then well rinsed.

The real value of any paint depends upon the quality of the linseed
oil, the quality and character of the pigment, and the care bestowed
on the grinding and mixing, and as all this is entirely a matter of
expense, cheap paints are not to be relied upon. The superiority of
most esteemed paints is due to the above causes rather than to any
unknown process or material employed in the manufacture.

The following excellent article on the preservation of iron and steel
structural work appeared in the ‘Illustrated Carpenter and Builder.’ It
conveys the most recent American opinion.

One of the most important economic questions of the present day is the
preservation of iron and steel structures. In America many millions of
dollars are annually spent in the erection and construction of these,
and the amount is increasing in rapid proportion. Bridges, buildings,
viaducts, ships, and machinery are now being made of these materials,
to almost the entire exclusion of others. From their very nature iron
and steel are peculiarly subject to decay from atmospheric and other
influences. The question of preserving them is not merely one of
finance, but, especially in the case of railroads, one affecting the
lives and property of a large portion of the community. What, then, is
the best method of preserving them?

The almost universal method is by means of paints, or the application
of substances to their surfaces which will resist or retard the
influence of air, water, and other destructive agencies. The requisites
of a good paint for this purpose are that it shall adhere firmly to
the surface, and not chip or peel off, thereby leaving portions of
the surface exposed. It must not corrode the iron, else the remedy
may only aggravate the disease. It must form a surface hard enough to
resist influences which would remove it by friction, yet elastic enough
to conform to the expansion and contraction of the metal by heat and
cold. It must be impervious to and unaffected, as far as possible, by
moisture, atmospheric and other influences to which the structure may
be exposed.

The paints that have been used for this purpose are principally asphalt
and coal-tar paints, consisting of mineral and artificial asphalt or
coal-tar, either applied alone or combined with each other, and, more
or less, with metallic bases, and iron oxide paints and lead oxide
paints, especially red lead, in all of which the pigment is held to
the surface of the iron or steel by combination with linseed oil. The
choice of paints must lie, so far as our present practical experience
goes, between these three classes, zinc oxide being found to be
entirely unsuitable on account of a propensity to peel off. What,
then, is found to be the experience in actual practice with these?
Asphalt and coal-tar paints “run” when exposed to the sun and other
sources of heat, which is a serious matter with vertical surfaces;
and after a time become extremely brittle and scale off entirely,
leaving the under surface exposed, unless the paint is constantly
renewed. In the meantime the exposed iron and steel are being corroded
by rust. Iron oxide paints, including “metallic brown,” are paints
made from iron ore, or by some chemical process with an iron base.
These are invariably iron in a greater or less degree of oxidation,
or in other words, rusted iron. Now it is well known that one of the
most active promoters of rust or decay in iron is the rust itself.
Under the combined influences of the moisture and carbonic acid of the
atmosphere, iron oxide or iron rust becomes a carrier of oxygen from
the air to the metal, rust begetting rust. It is therefore evident
that this material alone has no preserving effect on iron; in fact, it
promoted its decay.

How is it when combined with linseed oil in the form of paint? In the
economy of nature, iron oxide is a great disinfectant. When in contact
with organic matter and moisture, even at a low temperature, under
favourable conditions, it readily gives up oxygen, destroying more or
less the organic matter, and being itself reduced to a lower oxide.
When thus reduced, with equal readiness it absorbs oxygen from the
atmosphere, and again passes it on, thereby promoting and eventually
ensuring the destruction or transformation of the organic matter with
which it may be in contact, either in the soil or elsewhere. The same
process appears to take place when combined with linseed oil in the
form of paint and exposed to atmospheric influences, the oil being the
organic matter. If linseed oil in drying formed an air-and water-proof
film, it might be urged that the oxide of iron would be entirely
protected from the direct influences of oxygen of the air and moisture.
Such however is not the case.

The most eminent authorities have recently shown that the dried film
of linseed oil, unless united with a pigment that combines chemically
and forms a water-proof coating with it, actually absorbs water very
much like a sponge. Where water will go air will also go, and we thus
have in direct contact with the iron oxide of the paint, which does
not combine chemically with oil, those elements, air, moisture, and
organic matter, which cause the iron to become a carrier of oxygen and
a destroyer of what it is in contact with. It is well known that iron
paint darkens with age; this is caused largely by the iron oxide losing
oxygen, which is partly transferred to the oil, burning it up and
destroying its tenacity, as may be seen by examining iron structures
painted for some time with iron paint or metallic brown, the paint
being found extremely brittle and in feathery scales. This is not
all the damage that is done. The iron oxide in the paint becomes a
carrier of oxygen to the very metal it was designed to protect, and the
process of corrosion is commenced and carried on under the paint, which
eventually peels or scales off, the surface of the metal being found
more or less oxidised and corroded.

Asphalt and iron oxide being thus shown to be entirely incapable of
preserving the iron, it remains for us to consider the effect of red
lead. This pigment has the property of forming with linseed oil a hard
elastic coating, clinging with great tenacity to the metal. It has no
oxidising effect on iron, and does not act as a carrier of oxygen from
the atmosphere after the paint has set, neither does it render the
oil brittle nor promote rust. When red lead fails, it is principally
by gradual wear or friction from the outside. It does not scale or
blister, which both asphalt and iron oxide paints will do, thereby
requiring a thorough scraping and removal of old material before a new
coat can be applied. Any red lead pigment adhering to the metal forms
a permanent base for subsequent paintings, and is utilised in further
preserving the metal. The U.S. Government specifications for ironwork
in the new Library Building of Congress provide that “all the work
not Bower-Barffed must be given one coat of pure red lead paint--not
metallic paint of any kind, but pure red lead--before leaving the shop
and becoming rusted.”

The experiments of the U.S. Navy Department on the preservation and
fouling of plates covered with different pigments may be interesting.
A plate of iron covered with asphalt paint was immersed in sea-water
for eight months and six days at the U.S. Navy Yard, Portsmouth, N.H.
At the end of that period it was found to be covered with scum and mud,
and very badly rusted. A plate coated with iron paint, immersed at
Key West, Fla., was found to be covered with branch shell and coral,
but little paint remaining, and very badly pitted and rusted. A plate
with two coats of red lead, at the Norfolk Navy Yard, was found to
have a few barnacles attached, but to be in fair condition, with no
rust whatever on the iron after the paint was removed. It will be seen
that not only did the red lead protect the iron better than the other
pigments referred to, but that the plates were in far better condition
as regards barnacles and fouling. The superiority of red lead being
thus established, it is adopted for use on hulls of U.S. Government
warships.

On the Dutch State railroads a series of experiments extending over a
period of three years were made with the above pigments on scrubbed
plates, as well as those which had been pickled in acid to remove the
scale. It was found that the red lead was superior in each case to the
others.

If red lead is thus proved to be the best pigment for preserving iron
and steel structures, what is the proper method for applying it? We
have seen above that the value of red lead depends upon its forming
certain combinations with the oil, and actually setting very much the
same as plaster of Paris or cement sets when mixed with water. To
successfully work with the latter substances, it is necessary to put
them in shape as quickly as possible after mixing with water before the
setting takes place. If the chemical action of setting has partly taken
place, the material may be moulded, but it is known that good results
will not be obtained. Red lead, like these substances, must be applied
to the work before it sets with the oil. It is on this point that
failures in the use of pigment have generally occurred, because if it
be applied after the combining or setting process has taken place, the
hard, elastic, clinging coating will not be formed on the iron surface.

The following is the practice of one of the largest shipbuilding
establishments in applying red lead to the hulls of vessels. The plates
are first pickled in a dilute solution of muriatic acid; then passed
between rapidly revolving wire brushes, which remove all scale and
dirt, leaving the iron with a bright, smooth surface; then thoroughly
washed with pure water, and rubbed entirely dry; and immediately
coated with red lead and pure raw linseed oil. The red lead is first
thoroughly mixed with just enough linseed oil to form a very thick,
tough paste, which will keep for several days without hardening. This
paste, as wanted for use, is thinned down to the proper consistency for
spreading with pure linseed oil, and applied at once, care being taken
to leave paint-pots empty at night. A gallon of paint thus prepared
contains about 5 lb. of oil and 18 lb. of red lead, and will cover on
first coat about 500 square feet. In this way the red lead and oil
get their initial set on the surface of the iron, and the closer the
pigment is brought to the iron the more durable will it be found. Some
parties prime iron with iron oxide paint or metallic brown before
applying red lead, which appears to be a mistake, as this paint readily
scales from iron, and, of course, carries the lead with it. Others coat
the iron with oil before applying the red lead. This, too, prevents the
adhering paint from coming in contact directly with the surface, and
should be avoided, provided the iron is properly prepared by thorough
cleaning and removal of any scale and moisture, which is a matter of
the greatest importance.

In priming wood surfaces which are absorbent of oil, the best practice
favours the putting on of a coat of pure oil, or oil thinned with
turpentine, which shall penetrate the surface, and form a binder for
the subsequent coats. With iron, the case is quite different, provided
we have a paint which, from its very nature, can attach itself firmly
to the surface, because it is out of the question for it to hold on
to the surface of iron by any process of absorption into the pores of
the metal, as linseed oil will not penetrate to any extent. Such a
paint should be put directly on the surface of the clean, dry metal,
as is done in the cases of Government vessels referred to, without the
intervention of a coat of oil or other substances.

The rusting of iron before the application of paint, which is sometimes
recommended, should by all means be avoided, as it not only prevents
the contact of the paint with the metal, but induces a chemical action
which may go on with its corroding work under the applied paint.

As to the relative cost of iron oxide paints and red lead, there is no
doubt that the first cost of painting structures with iron oxide is
somewhat less than with red lead. The best railroad authorities state,
however, that labour in painting structural work costs twice as much as
material. The true economy must therefore be sought in the durability
of the paint as well as the preservation of the structure from rust.
Actual experiments have shown that structures painted with iron paint
had to be repainted in the third or fourth year, those with red lead
not until the sixth year.

In the second painting with iron paint, the old material must be
entirely removed before a fresh coat can be properly applied, entailing
considerable increased cost, whereas with the red lead no such expense
is necessary, but, as before stated, a portion of the pigment remains
on the iron, continuing to protect the surface, and is the very best
base for the new coat, besides contributing materially towards it,
lessening the expense of each repainting. It will therefore be easily
seen that, although in first cost red lead may be slightly dearer than
the iron paint, yet in the long run it will be greatly cheaper, besides
giving assurance, for the reasons above stated, that the structure is
not deteriorating from the effects of the atmosphere and paint.

Before closing, it might be well to allude to the effect of lampblack
when mixed in small quantities, say an ounce to the pound of red lead.
It changes the colour to a deep chocolate, a possible advantage in
some cases, and also prevents the red lead from taking its initial set
with linseed oil as quickly as when mixed with oil alone. Experiments
recently made showed that this compound would remain mixed in paste
form with linseed oil some thirty days without hardening. Thorough
mixture is of the greatest importance, and should be done in the dry
state before adding the oil. If rapid drying is desired, Japan dryer
can be mixed with the oil used in thinning the paste before application
with the brush.

Too much stress cannot be laid on the great importance of having the
metallic surface perfectly clean and as free as possible from scale
and rust before the application of the paint. Where pickling with acid
is impracticable, as is frequently the case in railroad and other
structural work, thorough brushing with wire brushes should be resorted
to.

FRESCO PAINTING.--The following observations are due to Prof.
Barff, who dealt with the subject in one of the Cantor Lectures given
at the Society of Arts.

The ground upon which fresco is painted is a lime ground; and in order
to have a permanent picture, it is clear we must have a firm and stable
ground. In order to prepare that ground, first of all the wall must be
absolutely dry; there must be no leakage of moisture from behind. Lime
which has been “run” (as it is technically called by builders) for a
year or a year and a half is best to be employed, for in proportion
as the lime has been carbonated (though it must not be so to too great
an extent) by the action of the carbonic acid of the air it makes a
better and a harder mortar. With this lime must be mixed sand, and a
great deal depends on the selection of the sand. It must be river sand,
and it should be of even grain; the sand should be mixed with water,
and allowed to pass along down a small stream, so that in the centre of
the stream you would have sand grains pretty nearly equal in size. This
is a point of considerable importance. The reason why new lime cannot
and ought not to be used is because it blisters; small blisters appear
on the surface, and that of course would be ruinous to a picture. A
well-plastered wall should not have a blister or crack in it, and this
is secured by having your lime run for some time, of good quality to
start with, and mixed with good sand. There is no chemical process that
takes place in fresco painting other than this, that silicates are
formed by the action of the lime upon the sand, and carbonates by the
action of the carbonic acid of the air upon the lime.

In painting a fresco picture, inasmuch as there is no retouching
the work when it is finished, the artist must make his drawing very
carefully. The cartoon is made upon ordinary paper, then it is fixed
against the wall where the picture is to be painted. The part where
the artist decides to begin his work is uncovered, that is to say, a
portion of the paper is turned down and cut away, but in such a manner
that it may be replaced. Then the plasterer puts fresh plaster, about
an eighth of an inch thick, upon the uncovered portion of the wall, and
the plasterer’s work is of the utmost importance in fresco painting.
The workman ought to practise it well before he attempts to prepare
the ground for a large picture, and it is of the greatest importance
to allow the man to practise for several weeks before he is allowed to
prepare any portion of the ground, even for decorative: painting. In
this way he becomes accustomed to the suction of the wall, and upon
the suction of the wall depends the soundness of the ground and the
success of fresco painting.

When the plaster is first put on, of course it is very soft; the piece
of cartoon is replaced upon it, and the lines of the picture are gone
over with a bone point, so that an indentation is made, and then the
artist begins his painting. At first he finds his colours work greasy;
you cannot get the tint to lie on, it works streaky; but you must not
mind that, you must paint on, but you must only paint on for a certain
time, for if you go on painting too long you will interfere with the
satisfactory suction of the ground, which is so necessary to produce a
good fresco painting. Of course, nothing but practice can tell any one
the period at which he ought to stop. After some practice, you know
perfectly well by the feel when you ought to stop. If you feel your
colour flowing from your brush too readily, you ought to stop at this
period. You must then leave your work for a time, and go back to it
again. And then you will find, as the plaster sucks in the colour which
you have first laid on, that there will be--it may be in the course of
half-an-hour, it may be an hour, that depends upon the temperature of
the atmosphere--a pleasant suction from your brush, the colour going
from it agreeably, and you will find that it will cover better. Now
is the time to paint rapidly, and complete the work you have in hand.
When the colour leaves your brush as though the wall was thirsty for
moisture, you should cease painting; every touch that is applied after
that will turn out grey when it dries, and the colour will not be fast
upon the wall.

You will see, then, how impossible it is, with such materials, to
paint in the same style in which you paint a picture in oil-colour.
Fresco painting involves the adoption of an entirely different style
from oil painting. The frescoes of the old masters are not highly
wrought up, highly-finished works. They depend for their effect upon
the juxtaposition of tints, the shadows being intensified by lines and
cross-hatching. If you look at those reproductions of some of the most
valuable fresco paintings of the old masters, you will see the method
adopted by them depended upon the juxtaposition of tints, not upon
covering over, and over, and over again. That juxtaposition of tints
produces roundness, transparency, and has a very pleasing effect upon
the eye. If two tints are put against one another, they do not appear
to us as if they were single, but each adds something to the effect
of the other, and together they produce a pleasurable and agreeable
effect, if they have been properly selected. We all know how tempting
it is to go back to a piece of painting and do something to it that
ought to have been done before. We think that a touch will improve
it, and we go and make it: but in fresco painting the temptation must
be resisted, for it will be absolutely fatal to the permanency of the
work.



INDEX


African indigo, 47

Alkali for removing old paint, 351

Alumina for lakes, 282

American indigo, 47

---- lamp-black furnace, 20

Aniline black, 25

Animal black, 5, 6

---- matters, nitrogen in, 51

Anthracene oil black, 21

Antimony vermilion, 138

Antwerp blue, 62

Area covered by paint, 359, 360

Arnaudon’s green, 128

Arsenic from cobalt ores, 31

---- orange, 153

---- yellow, 257, 280

Asphalt brown, 101

---- for painting iron, 373

Aureolin, 257

Azurite blue, 40


Balmains luminous paint, 286

Baryta blue, 111

---- green, 109

---- red, 143

---- white, 170

Barytes, 170

Benzoates as dryers, 318

Benzol vehicles, 296

Binder’s cobalt blue, 29

Bistre, 101

Bitumen, 101

Blacks, 5-26

---- adulterations, 6

---- aniline, 25

---- animal, 5, 6

---- anthracene oil, 21

---- bone, 5, 6

---- bone-oil, 12

---- candle, 25

---- “carbon,” 19

---- charcoal, 25

---- coal, 25

---- coal tar, 14, 15, 21

---- coke-oven, 16

---- composition, 5

---- cork, 25

---- creosote oil, 21

---- drop, 11

---- fat, 11, 12, 21

---- Frankfort, 11

---- “gas,” 19

---- gas-tar, 14, 15, 21

---- German, 26

---- impurities, 5

---- iron, 26

---- ivory, 5, 11

---- lamp, 5, 11

---- lead, 26

---- manganese, 26

---- mineral gas, 19

---- ---- matter in, 6

---- natural gas, 19

---- oil, 11, 12, 21

---- oily matter in, 5

---- organic, 5

---- paraffin, 16

---- peach-stone, 11

---- Prussian, 26

---- prussiate, 26

---- resin, 11. 15, 16

---- shale oil, 21

---- Spanish, 26

---- tannin, 26

---- tar, 14, 15, 21

---- testing, 6

---- unimportant, 25

---- vegetable, 5, 11, 25, 26

---- vine twig, 11

---- wine lees, 11

Blanc fixe, 172

Bleaching oils, 332

Blenkinsop and Hartley’s dryer, 335

Blood in priming coat, 352

Blue luminous paint, 291

---- potash, 54, 56

---- salt, 54, 56

---- washing, 90

Blues, 27-100

---- Antwerp, 62

---- azurite, 40

---- baryta, 111

---- Binder’s cobalt, 28

---- Bong’s, 62

---- Bremen, 34

---- Brunswick, 64

---- cæruleum, 27

---- Chinese, 65

---- cobalt, 27, 28

---- cœruleum, 37

---- copper, 34

---- indigo, 42

---- lime, 38

---- manganese, 49

---- mountain, 40

---- Paris, 68

---- Péligot, 41

---- Prussian, 49

---- Saxon, 69

---- smalts, 27, 30

---- soluble, 69

---- Thénard’s cobalt, 28

---- Turnbull’s, 70

---- ultramarine, 70

---- verditer, 41

---- zaffre, 30

Body of pigments, 293

Boiled oils, adulterants of, 331

---- ---- defects, 327

---- ---- substitute for, 352

---- ---- testing for lead in, 328

Boiling oil, 316, 320

Bole, 272

Boletta, 273

Bologna stone, 170

Bone black, 5, 6

---- ---- composition, 10

---- cost of production, 10

---- ---- furnaces, 7

---- ---- qualities, 10

---- brown, 102

---- oil black, 12

Bones, carbonising, 7

---- gases from, utilising, 9

Bong’s manganese blue, 49

---- Prussian blue, 62

Borates as dryers, 318

Braconnot’s green, 123

Bramwell’s prussiate, 58

Brazil-wood lake, 283

Bremen blue, 34

---- green, 112

Brighton green, 112

Brinjes and Goodwin’s paint mill, 346

Brown luminous paint, 291

Browns, 101, 108

---- asphalt, 101

---- bistre, 101

---- bitumen, 101

---- bone, 102

---- cappagh, 102

---- Cassel earth, 102

---- chicory, 102

---- Cologne earth, 102

---- manganese, 103

---- Mars, 103

---- Prussian, 103

---- Rubens, 102

---- sepia, 104

---- ulmin, 105

---- ultramarine, 88

---- umbers, 105

---- Vandyke, 107

Brunquell on prussiate, 59

Brunswick blue, 64

---- green, 113

---- ---- characters, 118

---- ---- dry method, 117

---- ---- testing, 118

---- ---- wet method, 115

Brushes, 355

---- keeping, 355

Buckley’s prussiate works, 59


Cadmium Yellow, 258

Cæruleum, 27

Calcining lamp-black, 23

Calcium hyposulphite, preparing, 139

---- polysulphide, preparing, 139

Candle black, 25

Candle-nut oil, 298

Cappagh brown, 102, 106

“Carbon” black, 19

---- pigments, 5

Carbonising bones, 7

Carminated lake, 283

Carmine lake, 283

Carriages, painting, 361

---- varnishing, 365

Cars, painting, 361

---- re-varnishing, 367

---- washing, 367

Cassel earth, 102

Cassius purple, 143

Cawk, 170

Central American indigo, 47

Cerchione, 273

Chalk, 246

Charcoal black, 25

Charlton white, 172, 254

Chicory brown, 102

China clay, 172

---- ---- artificial, 182

---- ---- characters, 182

---- ---- drying, 175, 181

---- ---- waste products, 179

---- ---- works, 173

Chinese blue, 65

---- green, 118, 129

---- red, 144, 185

---- vermilion, 157

Chromates, iron, 267

---- lead, 260

---- zinc, 268

Chrome greens, 113, 118, 125

---- orange, 144

---- red, 144, 145

---- yellows, 258

Chromes, characters, 266

Clark’s paint mill, 341

Coaches, painting, 361

---- re-varnishing, 367

---- varnishing, 365

---- washing, 367

Coal black, 25

Coal-tar black, 14, 15, 21

Coats of paint, 355

Cobalt benzoate, 318

---- blues, 27, 28

---- borate, 318

---- greens, 119

---- ore furnace, 31

---- oxide blue, 28

---- pink, 144

---- red, 144

Cochineal lake, 284

Cœruleum blue, 37

Coke oven black, 16

Colcothar, 145, 150

Cologne earth, 102

Colour and aggregation of particles, 2

---- ---- chemical composition, 2

---- ---- light, 1

---- causes, 1

---- defined, 1

---- destruction, 2

---- of pigments, testing, 293

---- sense, 1

Condenser, fume, 216

Condensing smoke, 13, 15, 17

Condy’s white lead, 197

Copper blues, 34

---- carbonate, 40

---- greens, 121, 130, 131

---- hydrated oxide, 34, 41

---- nitrate, 36

---- sulphate dryer, 325

Cork black, 25

Cornish umber, 107

Cotton seed oil in linseed oil, 301

Covering power of pigments, 293

Cracking, causes, 363

Creosote oil black, 21

Crucibles for roasting smalts, 32

Cuttle fish brown, 104

Cyanogen in prussiate making, 58

Cyprus earth, 134


Dammar vehicles, 296

Derby red, 145

Derbyshire umber, 106

Discoloration of paint, 356

Douglas green, 120

Drop black, 11

Dryers, 295-338, 365

Dryers, benzoates, 318

----  Blenkinsop and Hartley’s, 335

----  borates, 318

----  chemical action of, 322

---- ---- ---- in oils, 330

----  cobalt benzoate, 318

----  copper sulphate, 325

----  Guynemer’s, 319

----  lead, 325

----  litharge, 316

----  manganese benzoate, 318

---- ---- oxalate, 319

---- ---- oxide, 318

---- ---- sulphate, 325

----  red lead, 316

----  resinates, 318

---  zumatic, 318

Drying china clay, 175, 181

----  paint, 353, 367

----  white lead, 191

Durability of pigments, 294

Dutch vermilion, 167

----  white lead, 185

Dyestuff defined, 1, 3


Egyptian cœruleum, 37

Emerald green, 121

---- ---- characters, 124

---- ---- testing, 124

---- tint greens, 124

Enamel white, 245, 254

Enamelled white, 170, 183

English white, 183, 246

Extraction of oils, 308

Eye, colour effect on, 1


Fascia, 273

Fat-black, 11, 12, 21

Ferric oxide reds, 150

Figuier’s red, 144

Filling before painting, 354, 362

Fineness of pigments, testing, 293

Finishing painting, 363

Firmenich’s vermilion, 165

Fleck’s vermilion, 156

Frankfort black, 11

Freeman’s white, 224, 254

French and Hannay’s white lead, 216

Fresco colours, 257

---- painting, 111, 378

Fume condenser, 216

Furnaces, bone-black, 7

---- cobalt ore, 31

---- galena, 225

---- lamp-black, 12, 14, 16, 17, 18, 19, 20, 21

---- ochre, 277

---- prussiate, 53, 59, 60

---- smalts, 32, 33


Galena, chromates from, 263

---- white lead from, 225

Galloway’s green, 123

Gamboge, 270

Gardner’s white lead, 200

“Gas” black, 19

Gases from bones, utilising, 9

Gas-tar black, 14, 15, 21

Gaud indigo, 45

German black, 26

---- lamp-black, 15

Gmelin’s ultramarine, 81

Green luminous paint, 291

Greens, 109

---- Arnaudon’s, 128

---- baryta, 109

---- Braconnot’s, 123

---- Bremen, 112

---- Brighton, 112

---- Brunswick, 113

---- Chinese, 118, 129

---- chrome, 113, 118, 125

---- cobalt, 119

---- copper, 121, 130, 131

---- Cyprus earth, 134

---- Douglas, 120

---- emerald, 121

---- ---- tint, 124

---- Galloway’s, 123

---- Guignet’s, 125

---- Köchlin’s, 123

---- lokao, 129

---- malachite, 129, 131

---- manganese, 130

---- mineral, 130, 131

---- mitis, 130

---- mountain, 131

---- Paris, 132

---- Prussian, 132

---- Rinmann’s, 132

---- sap, 129, 132

---- Scheele’s, 133

---- Schweinfurth, 134

---- terre verte, 134

---- titanium, 135

---- ultramarine, 73

---- verdigris, 135

---- verditer, 136

---- Verona earth, 134, 136

---- Victoria, 137

---- Vienna, 137

---- zinc, 137

Grey luminous paint, 291

Griffith’s zinc white, 251

Grinding Chinese blue, 67

---- smalts, 34

Ground-nut oil, 297

Guignet’s green, 125

Guimet’s ultramarine, 81

Guynemer’s dryer, 319

Gypsum, 183


Hannay’s white lead, 214

Hartley & Blenkinsop’s dryer, 335

---- on dryers, 322

Hempseed oil, 298

Hind and Lund’s paint mill, 345

Hyposulphite of lime, preparing, 139


Idrian vermilion works, 154

Indian indigo, 43

---- red, 147, 150

Indigo, African, 47

---- American, 47

---- blue, 42

---- carmine, 48

---- Central American, 47

---- cultivation, 43

---- gaud, 45

---- Indian, 43

---- Japanese, 45

---- Javan, 46

---- maceration, 43

---- manufacture, 43

---- Philippine, 47

---- plants, 42

---- preparations, 48

---- qualities, 48

---- testing, 48

Irish umber, 106

Iron black, 26

---- chromates, 267

---- decay, 369

---- oxide for painting, 373

---- ---- reds, 150

---- painting, 369

---- ---- experiments, 375

---- pitching, 369

Italian white lead, 221

Ivory black, 5, 11


Jacquelin’s vermilion, 168

Japanese indigo, 45

Javan indigo, 46


Kaolin, 172, 183

Keeping brushes, 355

Kegs, paint, 349

Kilns, 7, 12, 14, 16, 17, 18, 19, 20, 21, 31, 32, 33, 53, 59, 60, 225, 277

King’s yellow, 271, 280

Kirchoff’s vermilion, 168

Köchlin’s green, 123

Kukui oil, 298


Lakes, 282

---- alumina, 282

---- Brazil-wood, 283

---- carminated, 283

---- carmine, 283

---- cochineal, 284

---- madder, 284

---- yellow, 285

Lamp-black, 5, 11

---- ---- anthracene oil, 21

---- ---- apparatus, 12, 14, 15, 16, 17, 19, 20, 21, 22

---- ---- calcining, 23

---- ---- creosote oil, 21

---- ---- gas tar, 14, 15, 21

---- ---- German, 15

---- ---- in iron painting, 378

---- ---- Martin & Grafton’s, 15

---- ---- natural gas, 19

---- ---- nuisance in making, 23

---- ---- paraffin, 16

---- ---- qualities, 25

---- ---- Russian, 16

---- ---- Shackell & Edwards’ Works, 24

---- ---- shale oil, 21

---- ---- transport, 25

Lapis lazuli, 70

Lead black, 26

---- chromates, 260

---- dryers, 325

---- for white lead making, 186

---- in boiled oil, testing for, 328

---- orange, 147

---- red, 148

---- sulphate, 223

---- whites, 183

Levigating ultramarine, 79

Lewis’s white lead, 223

Light and colour, 1

Lime blue, 38

---- hyposulphite, preparing, 139

---- manganate, 49

---- polysulphide, preparing, 139

---- white, 170, 245

Linseed oil, 299

---- ---- adulterants, 331

---- ---- bleaching, 303

---- ---- impurities, 301

---- ---- purifying, 302

Litharge as a dryer, 316

Lithophone, 245, 247

Lokao, 129

Luminous paints, 286


MacIvor’s ultramarine, 93

---- white lead, 229

Madder, lake, 284

Magnesite, 245

Malachite green, 129, 131

Manganese benzoate, 318

---- black, 26

---- blue, 49

---- borate, 318

---- brown, 103

---- green, 130

---- linoleate, 336

---- oxalate dryer, 319

---- oxide dryer, 318

---- sulphate dryer, 325

Manure from prussiate residue, 57

Mars brown, 103

Martin & Grafton’s lamp-black, 15

Massicot, 148

Measuring painting, 360

Menhaden oil, 303

Metallic paints, 274

Mica, 179

Mills, oil, 309

---- paint, 339, 341, 345, 346

Mineral gas black, 19

---- green, 130, 131

---- matter in blacks, 6

---- orange, 150

Minium, 148

Mitis green, 130

Mixer for paint, 341

Mixing paint, 364

Mountain blue, 40

---- green, 131

Murdoch’s antimony red, 142


Naples yellows, 271

Natural gas black, 19

Nitrogen converted into cyanogen in prussiate making, 58

---- in animal matters, 51

Nuisance from making lamp-black, 23

---- prevention, 24


Ochres, 272

---- characters, 279

---- kilns, 277

---- working, 274

Odour of paint, removing, 356

Oil-black, 11, 12, 21

---- cake, moulding, 313

---- mill, 309

---- seed crushing rolls, 311

---- ---- kettles, 312

---- vehicles, 295

Oils, bleaching, 332

---- boiled, defects, 327

---- ---- substitute for, 352

---- ---- testing for lead in, 328

---- boiling, 316, 320

---- ---- chemistry of, 326

---- candle-nut, 298

---- chemical action of dryers on, 330

---- drying and non-drying, 297

---- extraction, 308

---- ground-nut, 297

---- hemp-seed, 298

---- kukui, 298

---- linseed, 299

---- menhaden, 303

---- pea-nut, 297

---- poppy, 305

---- porgie, 303

---- refining, 335

---- tobacco, 306

---- tung, 308

---- walnut, 307

---- wood, 308

Oily matter in blacks, 5

Old paint, removing, 351

Orange, chrome, 144

---- lead, 147

---- luminous paint, 291

---- mineral, 150

Organic blacks, 5

Orpiment, 280

Orr’s white, 245, 254

Oxide reds, 150

Oxygen in making ultramarine, 78


Packing paint, 349

Paint beds, 274

---- composition, 358

---- covering power, 359, 360

---- cracking, 363

---- defined, 1

---- discoloration, 356

---- drying, 367

---- estimating quantity required, 359, 360

---- machinery, 339

---- mills, 339, 341, 345, 346

---- mixer, 341

---- mixing, 364

---- old, removing, 351

---- packing, 349

---- priming, 354, 362

Painting, 351

---- brushes, 355

---- carriages, 361

---- coats, 355

---- drying, 353

---- filling, 354, 362

---- finishing, 363

---- fresco, 378

---- iron, 369

---- measuring, 360

---- plaster, 378

---- priming coat, 352, 362

---- removing odour, 356

---- surface, 351

---- walls, 378

---- water-colours, 356

---- woodwork, 368

Paraffin black, 16

---- wax vehicles, 296

Paris blue, 68

---- green, 132

--- white, 246

Peach-stone black, 11

Pea-nut oil, 297

Péligot blue, 41

Permanent white, 170, 246

Persian red, 145, 150, 153

Philippine indigo, 47

Phosphorescent paints, 286

Pigments, carbon, 5

---- defined, 1, 3

---- examining, 293

---- requirements, 3

---- testing body, 293

---- ---- colour, 293

---- ---- covering power, 293

---- ---- fineness, 293

Pink, cobalt, 144

Pitching iron, 369

Plaster painting, 378

Pompeian cœruleum, 37

Poppy seed oils, 305

Porgie oil, 303

Potash, blue, 54, 56

---- prussiate, 50

Prevention of nuisance, 24

Priming coat, 352

---- paint, 362

Prussian black, 26

---- blue, 49

---- ---- Bong’s, 62

---- ---- common, 61

---- ---- qualities, 49

---- ---- recipes, 61

---- ---- testing, 50

---- brown, 103

---- green, 132

Prussiate, 50

---- black, 26

---- Bramwell’s works, 58

---- Brunquell on, 59

---- Buckley’s works, 59

---- crystallisation, 55

---- formation, 52

---- furnaces, 53, 59, 60

---- metal, lixiviation, 55

---- of potash, 50

---- pots, 53

---- residue as manure, 57

---- separating: impurities, 56

Pug mill, 341

Purple of Cassius, 143

Putty, 354, 362

Puttying, 354, 362


Realgar, 153, 280

Red lead, 148

---- ---- as a dryer, 316

---- ---- for painting iron, 374

---- luminous paint, 291

---- ultramarine, 77, 80

Reds, 138-169

---- antimony vermilion, 138

---- baryta, 143

---- Cassius purple, 143

---- Chinese, 144, 145

---- chrome, 144, 145

---- ---- orange, 144

---- cobalt, 144

---- ---- pink, 144

---- colcothar, 145, 150

---- Derby, 145

---- Indian, 147, 150

---- iron oxide, 150

---- lead orange, 147

---- minium, 148

---- orange mineral, 150

---- oxides, 150

---- Persian, 145, 150, 153

---- realgar, 153

---- rouge, 150, 153

---- tin, 144

---- Venetian, 150, 153

---- vermilion, 153

Refining oils, 335

Removing old paint, 351

Resin black, 11, 15, 16

Resinates as dryers, 318

Rinmann green, 132

Roller mills for paint, 339, 341, 345, 346

Rosin in linseed oil, 301

Rouge, 150, 153

Rubens brown, 102

Ruby of arsenic, 153

Russian lamp-black, 16

Rutherford & Barclay kiln, 277


Sap green, 129, 132

Satin white, 246

Saxon blue, 69

Schatte’s luminous paint, 290

Scheele’s green, 133

Schweinfurth green, 134

Seed-crushing rolls, 311

---- kettles, 312

---- oils, extracting, 308

Sepia, 104

Shackle and Edwards’ lamp-black works, 24

Shale oil black, 21

Ships’ hulls, painting, 376

Siena earths, 272

Siennas, 272

Silica ultramarine, 74

Smalts, 27, 30

---- crucibles, 32

---- furnace, 32, 33

---- grinding, 34

Smith’s pitching process for iron pipes, 369

Smoke, condensing, 13, 15, 17

Soda for making ultramarine, 78, 92

---- sulphate from ultramarine, 79

---- ultramarine, 74

Soluble blue, 69

Spanish black, 26

---- white, 246

Square blue, 90

Stack white lead, 185

Stampshaw lamp-black furnace, 21

Stangen-spath, 170

Strontia white, 246

Sulphate ultramarine, 72

Surface for painting, 351

Tannin black, 26

Tar black, 14, 15, 21

Terra alba, 183, 246

---- bolare, 273

---- guilla, 273

Terre verte, 134

Thalwitzer’s lamp-black apparatus, 16

Thénard’s cobalt blue, 28

---- white lead, 184

Tin red, 144

Titanium green, 135

Tobacco-seed oil, 306

Transport, lamp-black, 25

---- paint, 349

Tung oil, 308

Turkey umber, 106

Turnbull’s blue, 70


Ulmin, 105

Ultramarine, 70

---- adulterations, 76

---- ancient treatise on, 94

---- artificial, 71

---- brown, 88

---- Brunner on, 82

---- chemistry, 81

---- colouring principle, 75

---- composition, 81

---- direct crucible process, 87

---- Dollfus on, 84

---- effect of silica, 83

---- Endemann on, 82

---- faults, 100

---- German process, 86

---- Gmelin’s, 81

---- Goppelsröder on, 84

---- green, 73

---- Guimet’s, 81

---- Heyne on, 94

---- history, 80

---- Hoffmann on, 83

---- indirect process, 85

---- levigating, 79

---- Leykauf process, 94

---- McIvor’s, 93

---- natural, 70

---- Nejedly on, 94

---- oxygen in making, 78

---- properties, 76

---- red, 77, 80

---- removing soluble salts, 78

---- resisting alum, 75, 77

---- Ritter on, 82

---- Schülzenberger on, 82

---- silica process, 74

---- soda for, 78, 92

---- ---- process, 74

---- ---- sulphate from, 79

---- statistics, 91

---- Stein on, 81

---- sulphate process, 72

---- testing, 99

---- uses, 76, 90, 99, 100

---- varieties, 99

---- violet, 77, 79

---- Wilkins on, 81

Umbers, 105, 272


Vandyke brown, 107

Varnishing coaches, 365, 367

Vegetable blacks, 5, 11, 25, 26

Vehicles, 295-338, 365

Venetian red, 150, 153

Verdegris, 135

Verditer, 136

---- blue, 41

Vermilion, 153

---- antimony, 138

---- Chinese, 157

---- Dutch, 167

---- Firmenich’s, 165

---- Fleck’s, 156

---- Jacquelin’s, 168

---- Kirchoff’s, 168

---- Weshle’s, 168

---- works at Idria, 154

Verona earth, 134, 136

Victoria green, 137

Vienna green, 137

Vine-twig black, 11

Violet luminous paint, 291

---- ultramarine, 77, 79


Wagner’s antimony red, 142

Wall painting, 378

Walnut oil, 307

Washing blue, 90

Water-colour painting, 356

Wax vehicles, 296

Weshle’s vermilion, 168

Wetherill furnace, 225

White lead, 183

---- ---- adulterations, 195

---- ---- characters, 242

---- ---- chemistry of, 187, 192, 193, 200

---- ---- Condy’s, 197

---- ---- cost, 213

---- ---- dangers of, 191

---- ---- dry, 195

---- ---- drying, 191

---- ---- Dutch, 185

---- ---- French & Hannay’s, 216

---- ---- from galena, 225

---- ---- Gardner’s, 200

---- ---- Hannay’s, 214

---- ---- Italian, 221

---- ---- lead for, 186

---- ---- Lewis’s, 223

---- ---- MacIvor’s, 229

---- ---- powdered, 195

---- ---- precipitated, 194

---- ---- stack, 185

---- ---- substitutes, 194

---- ---- testing, 244

---- ---- Thénard’s, 184

---- luminous paint, 286, 290

Whites, 170-256

---- baryta, 170

---- blanc fixe, 172

---- chalk, 246

---- Charlton, 172, 254

---- china clay, 172

---- enamel, 245, 254

---- enamelled, 170, 183

---- English, 183, 246

---- Freeman’s, 224, 254

---- Griffith’s, 251

---- gypsum, 183

---- lead, 183

---- lime, 170, 245

---- lithophone, 245, 247

---- magnesite, 245

---- mineral, 183, 245

---- Orr’s, 245, 254

---- Paris, 246

---- permanent, 170, 246

---- satin, 246

---- Spanish, 246

---- strontia, 246

---- terra alba, 183, 246

---- whiting, 246

---- zinc, 247

Whiting, 246

Wilson’s fume condenser, 216

Wine-lees black, 11

Wood charcoal black, 25

-- oil, 308

Woodwork, painting, 368

Working ochre beds, 274

Wright’s paint machinery, 339


Yellow luminous paint, 291

---- prussiate, 50

Yellows, 257-281

---- arsenic, 257, 280

---- aureolin, 257

---- hole, 272

---- cadmium, 258

---- chrome, 258

---- gamboge, 270

---- King’s, 271, 280

---- lakes, 285

---- Naples, 271

---- ochres, 272

---- orpiment, 280

---- realgar, 280

---- umbers, 272


Zaffre, 30

Zinc chromates, 268

---- green, 137

---- oxide, 37, 247

---- sulphide, 250

---- whites, 247

---- ---- characters, 255

Zumatic dryer, 318


LONDON: PRINTED BY WILLIAM CLOWES AND SONS, LIMITED, STAMFORD STREET
AND CHARING CROSS.





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