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Title: Glass and Glass Manufacture
Author: Marson, Percival
Language: English
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*** Start of this LibraryBlog Digital Book "Glass and Glass Manufacture" ***
GLASS
------------------------------------------------------------------------
_Reprinted June, 1918._
_Reprinted June, 1919._
------------------------------------------------------------------------
[Illustration:
AN OLD GLASS HOUSE, A.D. 1790
_Frontispiece_
]
------------------------------------------------------------------------
_PITMAN’S COMMON COMMODITIES
AND INDUSTRIES_
GLASS
AND GLASS MANUFACTURE
BY
PERCIVAL MARSON
CONSULTANT UPON REFRACTORY MATERIALS, ETC.,
HONOURS AND MEDALLIST IN GLASS MANUFACTURE.
LONDON
SIR ISAAC PITMAN & SONS, LTD., 1 AMEN CORNER, E.C.4
BATH, MELBOURNE AND NEW YORK
------------------------------------------------------------------------
PRINTED BY SIR ISAAC PITMAN
& SONS, LTD., LONDON, BATH,
NEW YORK AND MELBOURNE
------------------------------------------------------------------------
PREFACE
Who is not acquainted with glassware in some form or other? From the
early days of the Ancient Egyptians the art of glassmaking was known,
and it is now one of our most important industries, supplying as it does
many articles for our common domestic use and convenience. Glass windows
have introduced comfort and convenience into every home; for by their
means light is admitted into our dwellings without the wind, rain and
cold, and we enjoy the blessings of the one without the inconveniences
of the others. The purposes for which glass can be used are manifold;
and in domestic articles it contributes largely to our cleanliness and
health. In the use of spectacles, table glass, mirrors, bottles, and
many other goods our dependence upon glass becomes very evident. The
degree of proficiency attained in the manufacture of glass is still more
remarkable when we consider the various kinds of glassware used in
physical, chemical, astronomic, medical, and other scientific
investigations. Many of the wonderful results of the present times would
not have been attained without the aid of glass in supplying the needs
of our scientific investigators. Before August, 1914, few people
realised the important part glass occupies in the production of war
munitions. The importance of optical glasses for telescopes, gun sights,
and microscopes is well known. Again, glass plays an essential part in
every ship, locomotive, motor-car, aeroplane, and coal mine, and if
defective glasses were supplied there would be a great loss in our
industrial efficiency. The manufacture of high explosives or special
steels could not be carried on without the supplies of laboratory
glassware to enable the chemist to carry out his delicate tests.
Upon the outbreak of the present war our supplies of certain types of
glassware were not made in Great Britain, but imported from abroad, and
it was owing to the energy and enterprise of a Scottish glass
manufacturer, with some assistance from a well-known scientist, that a
start was made in making these much-needed goods, and what might have
been a serious crisis was averted. Professor Herbert Jackson and the
Institute of Chemistry placed at the disposal of glass manufacturers
numerous formulas for the special glasses that were urgently required,
and later on this work was recognised by the Government; and now the
investigations are being continued by a committee, with the assistance
of the Government, under the control of the Ministry of Munitions. This
committee is now rendering the greatest assistance to manufacturers in
the general development of the glass trade and the reclamation of the
ground lost in previous years. There is now every hope that Britain may
raise again to eminence and perfection this very important industry of
glassmaking. One of the chief objects of this volume is to supply within
a small practical treatise the general available information upon glass
manufacture, much of which, although familiar to many manufacturers or
those engaged in glass works, will be of great assistance to those who
are commencing a study of this very interesting and complex subject.
Few people have any idea of the vast and enormous trade done on the
Continent in the manufacture of glassware for export to Great Britain
and British Possessions abroad, and on this account it is essential that
so important a subject as glass manufacture should form some part in the
technical education of our universities and trade schools, so that a
section of the rising generation may be taught to understand the
manufacture of such a necessary commercial product, and assist in
recapturing the trade from the Continental glass works in supplying our
needs. That some progress has been made along these lines is evident by
the establishment at Sheffield University of a school in Glass
Technology, and it is to be hoped that similar schools will be
established in other centres, staffed by capable instructors and
supported by the co-operation of the glass manufacturers.
The author gives in an Appendix the literature accessible to those who
wish for further information upon the subject, and trusts that, in the
presentation of these notes, in response to the demand for such a book,
a useful purpose will have been served by introducing the first
principles of glass manufacture to those interested.
It affords me great pleasure to acknowledge the valuable aid that has
been rendered me by Mr. S. N. Jenkinson, Professor Herbert Jackson, and
Mr. Frederick Carder, to whom I am much indebted.
My thanks are also due to the following firms: Messrs. Melin & Co.,
Crutched Friars; The Hermansen Engineering Co., Birmingham; The Glass
Engineering Co., Edinburgh; and Banks & Co., Edinburgh, who have kindly
supplied me with illustrations.
PERCIVAL MARSON.
CRAIGENTINNY,
EDINBURGH.
------------------------------------------------------------------------
CONTENTS
-------
CHAP. PAGE
PREFACE V
I. HISTORY 1
II. THE CHEMISTRY OF GLASS-MAKING AND THE 4
MATERIALS USED
III. THE CHEMICAL AND PHYSICAL PROPERTIES OF 15
GLASS
IV. THE COMPOSITION OF THE DIFFERENT KINDS 24
OF GLASS
V. COLOURED GLASS AND ARTIFICIAL GEMS 28
VI. DECOLORIZERS 32
VII. THE REFRACTORY MATERIALS USED 36
VIII. GLASS HOUSE FURNACES 43
IX. GLASS-MELTING POTS AND THEIR MANUFACTURE 59
X. LEHRS AND ANNEALING 71
XI. THE MANIPULATION OF GLASS—GLASSMAKERS’ 76
TOOLS AND MACHINES
XII. CROWN, SHEET, AND PLATE GLASS 89
XIII. TUBE, CANE, AND CHEMICAL GLASSWARE 96
XIV. OPTICAL GLASS 104
XV. DECORATIVE GLASSWARE 108
XVI. ENGLISH AND FOREIGN METHODS OF GLASS 118
MANUFACTURE COMPARED
APPENDIX 123
INDEX 125
------------------------------------------------------------------------
LIST OF ILLUSTRATIONS
-------
PAGE
AN OLD ENGLISH GLASS HOUSE, A.D. 1790 _Frontispiece_
HORIZONTAL CRACKING-OFF MACHINE 1
INTERIOR VIEW OF AN ENGLISH 44
GLASS-MELTING FURNACE
EXTERIOR VIEW OF AN ENGLISH 46
GLASS-MELTING FURNACE
SIEMENS SIEGBERT REGENERATIVE
GLASS-MELTING FURNACE—
FIG. A. CROSS SECTION 48
FIG. B. SECTIONAL PLAN 49
Fig. C. SECTIONAL ELEVATION 50
A MODERN GLASS HOUSE. HERMANSEN’S 52
CONTINUOUS RECUPERATIVE GLASS-MELTING
FURNACE, COVERED POT TYPE
HERMANSEN’S CONTINUOUS RECUPERATIVE 53
GLASS-MELTING FURNACE, 8-POT TYPE
HERMANSEN FURNACE—
FIG. A. SECTION THROUGH GAS PRODUCER 54
FIG. B. CROSS SECTION THROUGH GAS 55
PRODUCER
FIG. SECTIONAL PLAN 56
“THE HARLINGTON” BOTTLE-MAKING MACHINE 79
GLASS WORKER’S CHAIR 81
GLASSWARE BLOWN IN MOULDS, FIG. A. AND 85
B.
VERTICAL CRACKING-OFF MACHINE 87
FOUR STAGES IN CROWN GLASS MAKING (A, B, 90
C, D)
SIX STAGES IN SHEET GLASS MAKING (A, B, 91
C, D, E, F)
MACHINE FOR SMOOTHING BOTTOMS OF 110
TUMBLERS
GLASS ENGRAVING 113
------------------------------------------------------------------------
GLASS AND GLASS
MANUFACTURE
-------
CHAPTER I
HISTORY
The discovery of making glass is attributed to the early Phoenicians.
Pliny relates that certain mariners who had a cargo of soda salt, having
landed on the banks of a river in Palestine, started a fire to cook
their food, and, not finding any stones to rest their pots on, they
placed under them some lumps of the soda from their cargo. They found
that the heat of their fire had melted the soda and fused it with the
sand of the river bank, producing a transparent glass. The natives in
the vicinity where this discovery was made in process of time carried on
the practice of fusing sand with soda and other materials to make glass,
until they succeeded in improving and bringing the art to a high degree
of excellence. Discoveries amongst the ruins of Pompeii and Herculaneum
present some first-rate examples of the skill attained by the ancients
in glassmaking: glass was found to have been used there, admitting light
into dwellings in the form of window glass.
The ancient Egyptians have left us many distinct proofs that glassmaking
was practised in Egypt. At the same time, the glazing of pottery was
also carried out, proving that they knew the mode of mixing, fusing, and
melting the proper ingredients for glassmaking. Among the tombs of
Thebes many specimens of glass and glazed pottery beads have been found,
which suggests a date about 3,500 years ago.
From the Egyptians, the Greeks and Romans acquired the art of
glassmaking, which in Nero’s time was so highly developed that clear
crystal glasses were produced in the form of drinking cups and goblets,
which superseded the use of gold cups and were much prized by the
Emperor in those days.
Many specimens of old Roman glass discovered have been preserved in the
British Museum, and, although many valuable pieces have been lost by
disintegration and collapse due to the influence of years of exposure,
there still remain some very fine examples which show that the Romans
were highly skilled in glassmaking. One of the finest examples of the
work of the ancient Romans in glassmaking is the Portland Vase, which
was unearthed near Rome. This is an ornamented vase showing white opaque
figures upon a dark blue background. The white opal appears to have been
originally cased all over the blue and the beautiful figures carved out
in cameo fashion, with astonishing patience and skill upon the part of
the operator.
The Venetians and Muranians followed the Romans in the art, and examples
of old Venetian glassware show rare skill and ingenuity. To the
Venetians belongs the honour of first making glass at a cost to allow of
its being more generally used, and they also introduced the art of
making window glass and drinking vessels into this country. Jacob
Verzelina, a Venetian, introduced such glassmaking into England, working
at a factory in Crutched Friars, London, between 1550 and 1557, where he
made window glass, afterwards carrying on similar work in other places
about the country until his death in 1606.
Not until 1619 were glass works started in the neighbourhood of
Stourbridge. There we find some remains of a factory worked by Tyzack
about that date in making window glass in the village of Oldswinford.
That Stourbridge should have been selected as one of the early centres
for glassmaking is probably due to the presence in that locality of the
so necessary and important to glass manufacturers in building their
furnaces and pots, and the coal used for maintaining the fires for
melting their glass.
Stourbridge was known for a long time before this as a centre for the
mines producing , and eventually this clay was adopted for making
glass-house pots; now many other sources are available for these
fire-clays. Much of the antiquity of the glassmaking of England is
hidden in the neighbourhood of Stourbridge, and the writer has himself
found a few antique specimens of old green devitrified window glass
embedded in the subsoil of some fields near Oldswinford, probably relics
of the Huguenots, who practised and extended the art of glassmaking in
that district. Other important centres for glassmaking now are York,
London, Manchester, Edinburgh, Newcastle, and Birmingham; but, although
glassmaking has reached a high degree of excellence in this country,
there is nothing yet comparable with the extensive factories which exist
abroad. The conservatism of many English manufacturers, and the adverse
influence of the Glass Makers’ Society, considerably restrict the
progress of this trade compared with the broad and progressive manner in
which it is carried on abroad.[1]
Footnote 1:
_See_ article “Trade Unionism,” in last chapter.
------------------------------------------------------------------------
CHAPTER II
THE CHEMISTRY OF GLASS-MAKING AND THE MATERIALS USED
The term “glass,” in a general sense, is applied to the hard, brittle,
non-crystalline, transparent, opaque or translucent vitreous substance
which results from fusing silica with active mineral solvents or fluxes,
such as the alkalies, earthy bases, or metallic oxides. Silica exists in
great abundance, in a free natural state, in the form of flints, quartz,
and sand; and in the latter form it is now most generally used for
glassmaking. When sand alkali and lead oxide are heated together to a
high temperature, the sand is dissolved by the solvent action of the
fused alkali and lead oxide until the whole becomes a molten mass of
glass. The solvent action of the alkalies, soda potash or lead oxide, is
very energetic whilst being heated, and the mass boils with evolution of
gases until, at last, the solution, becoming complete, settles down to a
clear quiescent molten liquid metal, which is quite soft and malleable,
after the nature of treacle. In this condition it is ready for working.
The time and temperature necessary for melting such mixtures vary
according to the proportions and composition of the ingredients.
=Silica=, combined with alumina and other oxides, is freely distributed
in nature in the form of clays, granites, and feldspars, which are also
available for use in glassmaking. Originally glass was made by using
crushed and ground flint stones as the source for the silica: hence is
derived the old name of “flint” glass; but now the large extensive
deposits of white sand present a much more convenient and less expensive
source, and sand has become universally used. Fine white sand is
obtained from Fontainebleau, near Paris; other sources are Lippe, Lynn,
Aylesbury, Isle of Wight, Holland, and Belgium.[2] These are the sources
preferred by crystal glass manufacturers and makers of fine quality
glass, such as chemical ware pressed glass, tube, cane, and medical
bottles, on account of their greater purity. The commoner varieties of
sand from Reigate and Bagshot and even red sand are being used in the
manufacture of the lower grades of glass such as beer bottles and jam
jars, where a greater latitude in the chemical impurities present is
permissible. Only the best and purest silica sands are used for making
cut crystal and optical glasses. In these trades the sand is always
cleaned by washing it in water to clear it from any salt, chalk, or
other impurities which may possibly be present. The sand, after washing,
is heated to redness, or “burnt,” in order to burn off any organic or
vegetable matter, and when cold it is sifted through a fine screen to
take out any coarse grains or lumps. In this prepared state, the sand is
ready for weighing out into the proportions desired for mixing with the
other materials, and is stored for use in covered wooden compartments
situated in or near the mixing rooms, along with the other materials
which may be used in the glass mixtures.
Footnote 2:
_See_ “British Glass Sands” (Boswell), “British Glassmaking Sands”
(Peddle); papers read at the third meeting, Society of Glass
Technology, Sheffield, for further information.
The alkalies, potash or soda, or a mixture of both, are commonly used in
making glass in the form either of carbonates, sulphates, or nitrates.
The soda and potash silicates form very fusible glasses, but they are
not permanent, being soluble in water; therefore they cannot be used
alone. In making glassware for domestic use, other bases, such as lead
oxide, barium, or lime, have to be added to form more insoluble
combinations with the silica or sand.
=Carbonate of Potash= or =Pearlash=, which before the war was imported
into this country by glass makers from Stassfurt, is much prized by
crystal glass makers on account of the colourless silicate it forms when
fused with the best white sand. It is now very expensive and difficult
to get, and is less used on this account. Potash carbonate is very
hygroscopic and absorbs much moisture from the air; therefore it is
necessary to keep it within sealed chests while in store.
Potash and soda each have an influence upon the colour of the resulting
glasses in which they are respectively used. The potash silicate gives
better and clearer glasses than the soda silicate.
=Carbonate of Soda=, or =Soda Ash=, is now more generally used. Being a
less expensive form of alkali, it constitutes a base in most of the
commoner varieties of glassware. Carbonate of soda is manufactured in
England from common salt, of which there are large deposits in the
Midlands. This common salt, or chloride of sodium, is treated chemically
and converted into the carbonate, in which form it is supplied to the
glass manufacturers as soda ash.
=Sulphate of Soda= (=Salt Cake=) is the form of alkali used in window
and bottle glassmaking. In mixtures containing sulphate of soda it is
necessary to use a small proportion of carbon in some form, such as
charcoal or coal, in order to assist the decomposition of the salt and
the formation of the sodium silicate. Sulphate of soda is used in this
class of glassware on account of its cheapness. Glasses made from
sulphate of soda mixtures are not so clear and colourless as those in
which the source of alkali is potash or soda carbonate. On this account,
the best crystal glasses cannot be made from sulphate of soda.
=Potash Nitrate= (=Saltpetre=) is used in glass mixtures to oxidise the
molten metal and improve the colour of the glass. In fusing it
disengages oxygen gas, which purifies the glass while melting, and
assists the decolorizers in their action by keeping up an oxidising
condition within the molten mass.
=Sodium Nitrate=, or =Chili Nitre=, is the corresponding soda salt to
potash nitre. It is much cheaper, but less pure; it has a similar but
not nearly so powerful an oxidising action in the glass as potash nitre.
It is exported from Chili, where it exists naturally in a crude state as
“Caliche,” from which the nitrate is refined by recrystallisation.
=Boric Acid= acts as an acid in glass, as does silicic acid. It renders
glass more fusible and brilliant; it has a searching action upon the
colourising properties of certain metallic oxides when they are
dissolved in the glass. It is an expensive ingredient, but is
considerably used in optical and special chemical glassware in replacing
a portion of the silicates ordinarily used and forming borates. It
cannot be used in large amounts, as an excess produces glass of a less
stable nature.
=Borax=, or =Borate of Soda=, consists of boric acid combined with soda.
It is a very useful glassmaking material and is an active fluxing agent.
If used in excess in glass mixtures it causes considerable ebullition,
or boiling of the metal. In moderate proportions it is used in the
manufacture of enamels for glass, as it helps to dissolve the colorific
oxides and diffuse the colouring throughout the enamel mass.
=Tincal=, and =Borate of Lime=, are other forms in which borates may be
introduced into glass.
=Carbonate of Lime=, =Limespar=, =Limestone=, =Paris White=, or
=Whitening= are all forms of =Calcium Carbonate=. It is an earthy base
and is added to the simple alkaline silicates and borates to form
insoluble combinations or double silicates of soda and lime. By the use
of lime, glasses are rendered more permanent and unchangeable when in
use. Lime forms a very powerful flux at high temperatures. The quantity
used must be carefully regulated according to the proportion of other
bases present; otherwise an inferior or less stable glass may be
produced. In excess it causes glass to assume a devitrified state.
=Dolomite= is a _Magnesium Limestone_, and is a natural stone which is
available for use in making glass in tank furnaces.
=Fluorspar=, or =Fluoride of Lime=, is used in giving opacity and
translucency to glass. It can only be used in small amounts, as the
presence of any large proportion attacks the clay of the pots, causing
serious damage by the sharp cutting chemical action due to the evolution
of fluorine gas.
=Phosphate of Lime= is another material which produces opacity and
translucency, but does not seriously attack the pots. Bone ash is a form
of phosphate of lime, and is procured by calcining bones until all
organic matter is consumed.
=Carbonate of Barium=, or =Witherite=, is a very heavy, white powder,
and is a form of earthy base available for use in glassmaking. It can be
used to replace lime, with similar results. By replacing other elements
in the glass which are of lower density, barium can be used to increase
the density of glass. Like lime it is a very powerful flux in glass at
high temperatures. It gives increased brilliancy and little coloration.
For this reason it is very useful in the manufacture of pressed
glassware, giving a glass which leaves the moulds with better gloss than
is found to be the case with lime glasses.
=Magnesia= and =Strontia= are other bases which are less used in
glassmaking.
=Zinc Oxide= is a base used in the manufacture of many optical glasses.
With boric acid it gives silicates of a low coefficient of expansion and
special optical values. Used with cryolite, it forms a very dense opal
suitable for pressed ware. It is rather more expensive than the other
bases used.
=Cryolite= is a natural opacifying ingredient used in making opal
glasses. It consists of a combination of the fluorides of aluminium and
sodium, and is one of the most active fluxes known to glass and enamel
makers. Its cutting chemical attack on the pots is very intensive. It is
imported from Greenland. An artificially manufactured form of cryolite
is known, which is a little cheaper than the natural variety and gives
similar results in opacifying glass.
=Alumina.= This is sometimes present to a small extent in glass makers’
sands. As such it is not a dangerous impurity. It exists in combination
with silica and potash to a large extent in feldspars, china clays, and
granites. Alumina, when used, has a decided influence upon the viscosity
and permanency of glass. In large proportions it noticeably diminishes
the fusibility of glass, and makes it more or less translucent. Owing to
the refractory nature of alumina it is with difficulty that it can be
diffused in alkaline silicates, borates, or lead silicates; consequently
any considerable proportion present in glass may cause cords or striae,
which are objectionable defects in the glass.
=Oxide of Lead.= _Red Lead_, or _Minium_, is much used in the
manufacture of enamels, table glassware, and heavy optical glass. It
gives great brilliancy and density to all glasses in which it is used,
but if used in excess the glass is attacked readily by mineral acids and
becomes unstable. Red lead is a powerful flux, even at low temperatures,
and forms the chief base in making best crystal ware and enamels. The
red oxide of lead used by glass manufacturers is a mixture of the
monoxide and peroxide. Glass manufacturers, in buying red lead, should
realise that it is the peroxide present which is the active oxidising
agent, and that at least 27 per cent. should be present. A dull, dark
red oxide shows a low percentage of peroxide; a bright orange red a high
percentage. Impure red oxides of lead may be adulterated with barytes,
finely divided metallic lead, or added water. Such impure varieties
should be avoided. The red oxide of lead is preferred to the other
oxides and forms of lead for glassmaking, on account of its greater
oxidising action, which is desirable in producing crystal glassware.
=Tin Oxide= and =Antimony Oxide= are used as opacifiers. When used they
generally remain suspended in a finely divided form in the glass. Used
in small quantities they have a favourable influence in the development
of ruby-coloured glasses.
=Manganese=, =Arsenic=, and =Nickel Oxides= are used in glassmaking as
“decolorizers,” which will be treated in a later chapter.
=Cullet.= In all glasses a proportion of “cullet,” or broken glass
scrap, is used. This cullet is usually of the same composition as the
glass mixture or “batch.” The use of cullet facilitates the melting, and
assists in giving homogeneity to the resultant glass by breaking up the
cords and striae which tend to develop in most glasses.
In the commoner varieties of bottle glass =Basalt= and other igneous
rocks are crushed and used. These are naturally occurring silicates
containing lime, alumina, alkalies, iron, and other elements in varying
proportions. They are used more on account of their cheapness, and
produce dark, dirty-coloured glasses, which in the case of common
bottles are not objected to. In some instances iron, manganese or carbon
is added to produce black bottle glass.
Of the various silicates used in glassmaking, the silicate of alumina is
the most refractory. The silicates of lime and barium are rather
refractory, but under a strong heat and in the presence of other
silicates they can be readily formed. The silicates of the alkalies,
lead, and many of the other metals are formed at much lower
temperatures. In the case of the silicate of iron, manganese, or copper,
a strong affinity is shown between the metal and the silica, and a black
or dark-coloured slag with a very low melting point is formed. Such
slags are very active in corroding the masonry and pots of the furnace.
No single silicate is entirely free from colour. Each gives a slight
distinctive coloration, the lead silicate being yellowish and the soda
silicate greenish, but by the judicious mixture of different silicates
and the use of decolorizers, such as manganese, nickel, etc., compound
silicates are obtained, giving less perceptible colours or crystal
effects. In optical glassmaking the use of the ordinary decolorizers is
not permissible, and the purity of the materials used becomes the most
important factor.
The raw mixture of the various materials used in making glass is termed
a “batch.” The mixing is usually done by hand, but in many cases
mechanical batch mixers are used. If the mixing is done by hand, the
materials are first weighed out in their correct proportions by means of
a platform weighing-machine. As they are weighed out, one by one, they
are introduced into a rectangular wooden arbour or box, large enough to
hold the whole unit weight of the batch and allow of its being mixed and
turned from side to side. The batch is then sieved, and all the coarse
materials reduced or crushed to a size not coarser than granulated
sugar. By sieving and turning the batch several times a thorough mixture
of the ingredients is obtained. A few ounces of manganese dioxide are
then added, according to the unit weight of the batch weighed out, and
the proportion of decolorizer necessary; which varies according to the
heat of the furnace and the amount of the impurities present.
The whole batch is then put into barrels and conveyed to the glass
house, where the furnace is situated. Here it is tipped into another
arbour or box in a convenient position near to the melting pot, and, a
proportional quantity of “cullet” being added, the mixture is then ready
for filling into the pots. The stopper of the pot mouth is taken away
and placed aside, and a man shovels the mixture or batch into the hot
pot until it is full. He then replaces the stopper, and, after a few
hours, when the first filling has melted and subsided, another filling
of batch into the pot takes place until it becomes full of glass metal
in its molten state. The batch melts with considerable ebullition, owing
to the chemical reactions taking place under the heat of the furnace,
giving off at the same time large quantities of gas. By the evolution of
these gases the batch shrinks in volume so that it becomes necessary to
fill a pot more than once with the batch before it becomes full of
molten metal. The capacity of the pots varies between 250 and 1,200
kilogrammes, according to the type of glass and nature of the goods
made.
Much care is required in mixing and sieving batches containing lead and
other poisonous ingredients, to prevent the inhalation of the dust by
the mixer. Therefore, where such materials are used, exhaust fans and
ventilating ducts should be provided and fitted in the mixing rooms. A
proper respirator should be worn by the mixer in charge to prevent any
absorption into his system of the poisonous dust. Cases of poisoning are
not unknown, but these are due to gross carelessness. A small regular
weekly dose of Epsom salts should be taken by the mixers who have to
prepare lead batches. This salt tends to remove any lead salts absorbed
in the system by converting them into insoluble lead sulphate.
------------------------------------------------------------------------
CHEMICAL FORMULAE AND MOLECULAR WEIGHTS.
───────────────┬───────────────┬─────
│ │_Molecular
_Materials._ │ _Formulae._ │Weight._
───────────────┼───────────────┼─────
Alumina │Al_{2}O_{3} │ 102
Antimony Oxide │Sb_{2}O_{3} │ 287
Arsenic │As_{2}O_{3} │ 197
Bismuth Oxide │Bi_{2}O_{3} │ 468
Boric Acid │H_{3}BO_{3} │ 62
Borax │Na_{2}B_{4}O_{7}10H_{2}O│ 382
Calcined Borax │Na_{2}B_{4}O_{7}│ 202
Calcined Potash│K_{2}CO_{3} │ 138
Carbon │C │ 12
Carbonate of │BaCO_{3} │ 197
Barium │ │
Carbonate of │MgCO_{3} │ 84
Magnesia │ │
China Clay │2SiO_{2}Al_{2}O_{3}2H_{2}O│ 258
Chrome Oxide │Cr_{2}O_{3} │ 153
Cobalt Oxide │Co_{2}O_{3} │ 105
Copper Oxide │Cu_{2}O │ 143
(Red) │ │
Copper Oxide │CuO │ 79
(Black) │ │
Cryolite │6NaFAl_{2}F_{6}│ 210
Dolomite │CaOMgO2CO_{2} │ 184
Fluorspar │CaF_{2} │ 78
Gold Chloride │AuCl_{3}2H_{2}O│ 339
Iron Oxide │Fe_{2}O_{3} │ 160
Lime │CaO │ 56
Lime Spar │CaCO_{3} │ 100
Manganese Oxide│MnO_{2} │ 87
Nickel Oxide │NiO_{2} │ 75
Nitrate of Soda│NaNO_{3} │ 85
Phosphate of │Ca_{3}(PO_{4})_{2}│ 310
Lime │ │
Potash │K_{2}CO_{3}(2H_{2}O)│ 174
Carbonate │ │
Potash Felspar │6SiO_{2}Al_{2}O_{3}K_{2}O│ 556
Red Lead │Pb_{3}O_{4} │ 683
Saltpetre │KNO_{3} │ 101
Sand │SiO_{2} │ 60
Soda Carbonate │Na_{2}CO_{3} │ 106
Sodium Fluoride│NaF_{3} │ 61
Sulphate of │Na_{2}SO_{4} │ 142
Soda │ │
Tin Oxide │SnO_{2} │ 150
Uranium Oxide │UO_{2} │ 272
Zinc Oxide │ZnO │ 81
───────────────┴───────────────┴─────
------------------------------------------------------------------------
CHAPTER III
THE CHEMICAL AND PHYSICAL PROPERTIES OF GLASS
The main essential and peculiar property of glass is its transparency.
When subjected to a gradually increasing temperature, glass becomes
softened, and whilst hot it is plastic, ductile, and malleable, in which
state it can be cut, welded, drawn, or pressed. A thread of glass can be
drawn so thin and fine that it can be twisted and bent to a remarkable
extent, showing that glass is flexible.
The above properties shown by glass while softened under heat permit it
to be shaped and formed by a variety of methods, so that in the
manufacture of the different kinds of glass we find goods pressed,
blown, drawn, moulded, rolled and cast from the hot metal. Upon cooling,
the form given to them is retained permanently.
Another property of glass is its conchoidal fracture and liability to
crack under any sudden change of temperature. Advantage is taken of this
peculiarity in dividing or cracking apart glass when necessary, during
the stages of the manufacture of any glass article.
[Illustration:
_By permission of_
_Melin & Co._
HORIZONTAL CRACKING-OFF MACHINE
]
If a glass worker, in making an article of glass, desires to detach or
cut apart certain sections, he applies a cold wet substance, such as an
iron file wetted with water, to any portion of the hot glass, which
causes it to fracture at the point of contact with the cold metal, and a
slight jar is then sufficient to break the two portions apart. This
method of chilling heated glassware to divide it is applied in the
mechanical process of cutting up the long cylindrical tubes of glass
into short sections for use as miners’ safety lamp chimneys. Wherever it
is desired to cut them through, a narrow section or line round the
cylinder is first heated by a sharp, hot pencil of flame projected from
a burner against the rotating cylindrical tube of glass at equidistant
short sections, and the divisions chilled by contact with a cold, steel
point, or the heated area may be gently scratched with a diamond point,
when a clean, sharp fracture results exactly where the chill or scratch
has been applied and spreads round the whole circumference in a circle,
giving neat, clean-cut divisions. In cutting narrow tube and cane, the
fracture caused in the structure of the glass by scratching its surface
with a steel file or diamond is sufficient to cause it to break apart
without the application of heat.
A piece of hot glass will weld on to another piece of hot glass of
similar composition. The glass maker uses this method of welding for
sticking handles on to jugs, etc., during the process of making table
glassware.
The density of glass varies according to its composition. Certain
classes of lead and thallium glass for optical work are of very high
density. The specific gravities of such glasses may vary from 3·0 to
well over 4·0. In soda-lime glasses the density is less and approaches
2·4. Ordinary crystal glass approximates to a specific gravity of 3·1.
The elasticity and thermal coefficient of expansion of glass can be
regulated within normal limits. Glasses are now manufactured which can
be perfectly sealed to copper, iron, nickel, and platinum wires.
Glass, if kept heated for any length of time at a temperature just short
of its softening or deformation point, becomes devitrified and loses its
transparency, becoming opaque and crystalline. In this state it has much
of the nature of vitreous porcelain and is totally different to
manipulate, being tough and viscid on further heating. This devitrified
state may occur during glassmaking, where the metal is allowed to remain
in the pot or tank furnace for a considerable time under low
temperature. Small stars or crystals first develop throughout the glass
and continue to grow until it becomes a stony, white, opaque, vitreous
mass. “Réaumur’s Porcelain” is a glass in a devitrified state, and is
used for pestles and mortars, devitrified glass being less brittle than
ordinary glass and similar to vitrified porcelain.
Glass can be toughened to an extent which is surprising. Bastie’s
process consists of plunging the finished glass article whilst hot into
a bath of boiling oil, which toughens the glass so much as to make it
extremely hard and resistant to shocks, losing most of its brittle
nature. Strong plates of glass are produced by a process of toughening
under pressure. These plates of glass are used for ship porthole lights
and in positions where great strength is required. Toughened or hardened
glass is of great value in the production of miner’s lamp glasses and
steam-gauge tubing. Glass, when hardened, is difficult to cut even with
the diamond, and difficulty is experienced in finding suitable means to
cut it into shapes to suit commercial requirements.
“_Prince Rupert drops_,” or tears, exhibit the state in which unannealed
glass physically exists. These are made as a curiosity by dropping a
small quantity of hot metal from the gathering-iron into a bath of water
and then taking the pear-shaped drops out quickly. These pear-shaped
drops of glass will stand a hard blow on the head or thicker portion
without breaking, but, if the tail is pinched off or broken, the whole
mass crumbles and falls to powder. This well illustrates the latent
stresses or strains apparently in a state of tension and thrust within
the structure of unannealed glass.
Glass is not a good conductor of heat. This accounts for the necessity
of slow cooling or annealing glassware, and also applies when re-heating
glass, which must be done slowly and evenly to allow time for the
conduction of the heat through the mass gradually. Glass is a
non-conductor of electricity, and is used to a considerable extent in
the electrical trades for insulation purposes. Most glasses are attacked
slightly, but not readily, by water and dilute mineral acids. Continued
exposure to a moist, humid atmosphere causes slight superficial
decomposition, according to the stability and chemical composition of
the glass. Old antique specimens of glass show the superficial
decomposition caused by long continuous exposure to atmospheric
moisture. Many antique specimens have been known to collapse instantly
upon being unearthed. The first change in antique glass is exhibited by
a slight iridescence forming on the surface, gradually increasing
towards opacity afterward disintegration sets in, until it finally
collapses or crumbles to powder. Glasses high in lead are readily
attacked by the acid vapours met with in the atmosphere, but the harder
soda-lime glasses are more resistant. An excess of boric acid, soda, or
potash also renders glass subject to disintegration and decay.
Hydrofluoric acid attacks all silicate glasses, liberating silicon
fluoride. Use is made of this acid reaction in decorating glasswares in
“Etching,” by exposing the surface of glass to the fumes of hydrofluoric
acid gas in some form.
The most permanent glasses are those containing the highest proportion
of silica in solution, but the available heat necessary to decompose
such highly silicious mixtures is limited by the present known
refractory materials which can be procured for constructing the
furnaces. Quartz glassware is a highly silicious glass. It is now made
and used in the manufacture of special chemical apparatus and laboratory
ware such as crucibles, muffles, etc., which have to withstand severe
physical and chemical tests. This quartz glass possesses remarkable
features in its low coefficient of expansion and resistance to heat
changes. It is highly refractory. Articles made of this glass can be
heated to red heat and plunged directly into cold water several times
without fracturing. Several varieties of quartz glass are now
manufactured, and a new field for investigation is presented in applying
the features and properties of this glass for use in chemical processes.
In a purely physical sense glass is a supercooled liquid, the silicates
are only in mutual solution with each other, and they appear to be
constantly changing. Glass cannot be described as a homogeneous or
definite chemical compound. Many of the after effects and changes which
occur in glass, and the formation of crystals in the devitrification of
glass tend to prove the above assertion. The colour changes which take
place when ruby and opalescent glass is re-heated, and even the change
in colour of glass going through the lehr, cannot be explained unless in
the above sense of viewing these remarkable changes. Glasses with an
excess of lime in their composition are more subject to devitrification
than lead glasses or those of moderate lime content constructed from
more complex formulas. The presence of a small proportion of alumina in
glass prevents this tendency to devitrification and ensures permanency.
Those glasses which have the highest silica content, and which have been
produced at the highest temperatures, show the greatest stability in
use. Bohemian glasses of this type contain as much as 75 per cent.
silica, and are produced in gas-fired regenerative or recuperative
furnaces, where the heat approaches 1,500° Centigrade. Such glass is
much sought after for enamelling on, being harder and less easily
softened by the muffle heat firing on the enamels used. Taking two
corresponding glasses of the same basicity, or proportion of silicic
acid to the bases present, those formulae which have the greater
complexity of bases produce the more fusible glasses. A multiple of
bases constituting a more active flux than a single base content, it
follows that a compound mixture of silicates fuses or melts at a lower
temperature than the respective simple silicates would. These facts are
useful in constructing commercial formulae for glasses.
Glasses containing lead oxide as an ingredient are subject to reduction
when exposed to flames of a carbonaceous nature. The carbon partially
reduces the lead oxide to its metallic state, forming a black deposit.
On this account, lead glasses cannot be used in blow-pipe working with
the ease with which soda-lime glasses can be worked, without reduction
taking place. English crystal glass, which contains a high percentage of
lead, is usually melted in hooded or covered pots to prevent the
carbonaceous flames of the furnace reducing the lead and otherwise
destroying the clearness of the glassware. Soda-lime glass and others
without the presence of lead can be melted in open pots without any fear
of reduction. Modern gas-fired recuperative furnaces, in which more
complete combustion of the carbon takes place, can now be used for
melting lead glasses in open pots, thus presenting a great saving in the
fuel required to melt and produce such glass, besides permitting the use
of a cheaper form of pot. This cannot be done with the ordinary English
coal-fired furnaces.
Advantage is taken of the reducing action of the coal-gas flame when
producing lustre and iridescent glassware. A small proportion of easily
reducible metal, such as silver or bismuth, is introduced into the glass
and first melted under oxidising conditions. It is then reduced in
after-working by flaming, which deposits the metal in a thin sheen upon
the surface of the glass, where it comes in contact with the reducing
flames. An example of this effect is shown in Tiffany lustre ware, in
which silver chloride is used and reduced within the glass, giving a
pretty coloured iridescence on the surface, due to the reflection of
light from the particles of metal deposited under the surface.
“Aventurine” is a form of glass in which copper and iron oxides are
introduced under reducing conditions during melting. The glass is then
allowed to cool slowly. The metallic copper tends to separate out in
small spangled crystals, which give a pretty sparkling effect. The use
of strong reducing agents with very slow annealing is necessary to
produce this effect. Copper and gold ruby-coloured glass presents other
instances of partial precipitation of the metal by reduction within the
glass. According to the extent of reduction, the glass ranges in colour
from yellow, ruby, to brown.
The manganese silicate is readily affected by oxidising or reducing
conditions, the purple colour being present under oxidising influences
and a greenish-grey colour under reducing conditions. In using manganese
as a decolorizer, the glass maker may have added too much of it to his
glass, in which case it shows too prominent a purple colour. To destroy
this excess of colour he pushes either a little strip of green willow
wood or a clean potato to the bottom of the pot of metal. The reducing
action of the carbonaceous gas involved takes out the excess of purple
colour by partially reducing the manganese present to a colourless
state.
The colour of glass is gradually affected in course of time by sunlight.
This change in colour is often noticeable in old windows, the glass
having developed a yellowish green tint in course of time from the
action of the solar rays.
Glass which has been incompletely fused or not sufficiently melted to
give a complete solution of the materials present is in a weakened state
of cohesion and is liable to disintegration. The presence of
undecomposed sulphates, chlorides, or borates in the glass also tends to
early disintegration. A continual exudation and crystallisation of salt
takes place upon the surface until the glass wholly disintegrates away
to a white powdered salt.
Glass is a poor conductor of heat. When a piece of glass has been
expanded under the influence of heat, and is rapidly cooled, the
superficial outer portions become intensely strained and contracted upon
the interior portions, which retain the heat longer. Under these
conditions of cooling, glass is apt to “fly,” or collapse and fall to
pieces, owing to the outer portions giving way under the great strain.
These stresses or strains are relieved in the process of annealing,
under which they are gradually eased by a slow and regular cooling from
the heated condition. Certain glasses, the composition of which shows
considerable differences in the density of the respective bases present,
are more subject to this defect than those in which the bases are of
more even density and homogeneous in character. Such glasses should be
“de-graded” and re-melted in order more thoroughly to diffuse and
distribute the denser portions throughout the mass. In de-grading glass,
the hot glass is ladled out and quenched in cold water, dried, and
re-used as “cullet.”
------------------------------------------------------------------------
CHAPTER IV
THE COMPOSITION OF THE DIFFERENT KINDS OF GLASS
The composition of glasses may be simple, compound, or complex,
according to the number of bases or acids which may be present in the
mixture.
=The Simple types of glass= are exhibited in the soda silicate, potash
silicate, and lead silicate. The two former silicates are of most
industrial value.
=Soda Silicate= is made from a fusion of 100 parts of sand with 50 parts
of soda carbonate and 5 parts of charcoal. The charcoal is added to
facilitate the decomposition. The fused mass when cool is transparent
and of a pale, bluish, sea-green colour. Upon boiling it in water it
dissolves and gives a thick viscid solution called “Water Glass.” This
is extensively used in the various arts and manufactures. Textile fabric
and woodwork saturated with this solution and dried are rendered
fireproof. In the manufacture of artificial stone it forms, with lime
and other basic oxides, very stable cements. Mixed with silicious or
ganister it forms the well-known fire cements for repairing the cracks
in retorts, muffles, etc. Water glass is also used in soap, and colour
making, and for preserving eggs.
=Potash Silicate= is less used, being more expensive. It is produced
from a fusion of 100 parts sand, 60 parts potash carbonate, and 6 parts
charcoal.
=Lead Silicate= is composed of 100 parts sand and 66 parts of red lead
fused together. This silicate is mostly used in the manufacture of soft
enamels and artificial gems, and goes under the names of “Rocaili flux,”
“strass metal,” and “diamond paste.”
There is another form of soluble glass which is a combination of the
soda and potash silicates. This is really a double silicate and may be
produced by fusing sand 100 parts, soda carbonate 25 parts, potash
carbonate 30 parts, and 6 parts of charcoal. This silicate is used in
soap making. Soluble glass can also be formed by using sulphate of soda
as the alkali. In this case, a larger proportion of the alkaline salt
has to be used, also a larger amount of carbon, in order to complete the
decomposition of the sulphate. A mixture of sand 100 parts, saltcake 70
parts, and carbon 16 parts would produce sodium silicate. The boron
silicate and borate of alumina are two other forms of soluble glass used
in their simple states.
=The Compound Glasses= may be flint or crystal glass, soda-lime glass,
Bohemian glass, pressed glass, and sheet glass. These are the general
type of glasses used in the manufacture of domestic glasswares.
=Crystal Glass=, which is a silicate of lead and potash, is made from
best sand 100 parts, red lead 66 parts, potash carbonate 33 parts,
cullet 50 parts, to which a small proportion of potash nitre, arsenic,
and manganese dioxide is added. The bulk of English cut-glass table ware
and fancy goods are made from this type of glass. It gives very
brilliant and colourless results, more especially when cut and polished.
A second-rate quality of crystal glass for table ware may consist of a
silicate of lead and soda, as follows: sand 100 parts, red lead 66
parts, soda carbonate 25 parts, cullet 50 parts; with small proportions
of Chili nitre, arsenic, and manganese.
=Bohemian Glass= is made from sand 100 parts, potash carbonate 35 parts,
lime carbonate 15 parts, cullet 50 parts; with small proportions of
potash nitre, arsenic, and manganese dioxide. This type of glass is used
mostly by continental manufacturers for chemical ware, table and mirror
glass. It is a hard, brilliant, and stable glass, very suitable for
enamelled glassware. It is a silicate of potash and lime.
=Pressed Glass= consists of sand 100 parts, soda carbonate 50 parts,
barium carbonate 15 parts, cullet 50 parts; together with soda nitre,
arsenic, manganese, and cobalt. This is used by manufacturers of pressed
glass table ware or moulded ware. It is a silicate of soda and barium,
the barium having a direct influence in giving a good surface to the
pressed goods.
=Crown Glass= consists of a silicate of soda and lime; sand 100 parts,
soda carbonate 36 parts, lime carbonate 24 parts, soda sulphate 12
parts, cullet 50 parts; with traces of manganese and cobalt. This glass
is used for making sheet window glass by the crown, disc, and cylinder
methods.
=Plate Glass= is a silicate of soda and lime; sand 100 parts, soda
sulphate 55 parts, limestone 30 parts, coal or anthracite 5 parts; with
traces of nickel oxide, cobalt, or antimony oxide. This is used for cast
plate glass, rolled plate, cathedral glass, window and mirror glass.
=The Complex Glasses= may be described as those in which more than three
bases are introduced, and constitute such types of glasswares as
bottles, thermometer tubes, chemical ware, etc.
=Common Bottle Glass= may be described as an example of complex
formulae. Common bottle glass, or tank metal, is made from a silicate of
soda, alumina, lime, magnesia, and iron, as follows: Common sand,
containing iron and alumina, 100 parts; greenstone or basalt (a silicate
of alumina, iron, lime, magnesia, and potash), 25 parts; dolomite
limestone (magnesia and lime), 30 parts; sulphate of soda, 35 parts;
carbon, 5 parts. Felspathic granites may be also used in such glasses.
Bottle glasses require intense heat to melt, and are usually dark in
colour when made from igneous rocks, owing to the large amount of
colorific oxides present in such materials. These dark colours are not
objected to in bottles for stout, wine, and beer.
It will be noticed these formulae cover a long range, from the best
table glass to the commonest dark bottle glass. Besides these, opal,
opalescent, and fancy glasses are made, in which either arsenic, tin,
alumina, antimony, zinc or barium oxides or borates phosphates and
fluorides may enter into the compositions.
Glass makers’ recipes vary considerably in the proportions of the
various materials used, according to the locality and the type of
furnace used. Generally, it will be found that, where a gas-fired
furnace is in use, a larger proportion of sand can be used and a cheaper
metal produced.
The metals produced in covered pots are usually softer and contain more
lead and fluxes than those produced in open pots. In using open pots the
heat of the furnace has direct access to the surface of the metal
therein. In the case of covered pots, the heat has to be conducted
through the cover of the pot, which retards the heat to a certain
extent. On this account, softer mixtures are used in covered pots.
------------------------------------------------------------------------
CHAPTER V
COLOURED GLASS AND ARTIFICIAL GEMS
In colouring glass, either or several of the following colorific oxides
may be used. They are added to the batch before fusion. Varying
proportions are added, according to the depth of the colour desired.
Occasionally the colour is influenced by the nature and composition of
the rest of the batch. In some instances several colouring oxides are
used. In this way many delicate tints are obtained; in fact, there are
but few colours that cannot be produced in glass.
=For Green Glasses= the following oxides may be used: Chromium oxide, 2
to 6 per cent. of the batch; black oxide of copper, ·5 to 3 per cent.;
red iron oxide, ·5 to 1 per cent.; or a mixture of two or three of the
above oxides in less proportions. Salts of chromium, copper, or iron may
be used as the carbonates, sulphates, and chromates.
=For Blue Glasses=, cobalt oxide, ·1 to 1 per cent. of the batch; zaffre
blue or smalts, 1 to 3 per cent.; nickel oxide, 2 to 4 per cent.; iron
oxide, 1 to 2 per cent.; black oxide of copper, 2 per cent.
=For Violet and Purple=, manganese oxide, 2 to 4 per cent. of the batch.
=For Rubies=, red oxide of copper, gold chloride, purple of cassius,
antimony oxysulphide, selenium metal in small proportions.
=For Yellows=, uranium yellow, 4 to 6 per cent. of the batch; potassium
antimoniate, 10 per cent.; carbon, 6 per cent.; sulphur, 5 per cent.;
ferric oxide, 2 to 4 per cent. Silver nitrate and cadmium sulphide are
also used.
=Black Glass= is obtained from mixtures of cobalt oxide, nickel oxide,
iron oxide, platinum and iridium. Many very dark or black bottle glasses
are obtained by using basalt, iron ores, or greenstone in a powdered
form, added to the batch ingredients.
=White Glasses= or =Opal= are obtained by using phosphate of lime, talc,
cryolite, alumina, zinc oxide, calcium fluoride, either singly or in
double replacements of the bases present in the glass batches.
Many of the colouring oxides give distinctive colours to glass of
different compositions; also the resulting colours may vary with the
same colouring ingredient, according to reducing or oxidising meltings.
Thus, in a batch of reducing composition, red copper oxide gives ruby
glass, but in oxidising compositions the colour given is green or
bluish-green. Iron oxide in an oxidising batch gives a yellow. In the
reducing batch it gives bluish or green results. Manganese is similarly
affected.
Many colouring oxides give more brilliant tints with glasses made from
the silicates of potash and lime than if used in glasses composed from
silicates of lead and soda. For many colours the lead glasses are
preferred. In colouring the batches, the colouring oxides must be
intimately mixed with the batch materials before fusion, more especially
in the preparation of the pale tints, where only small quantities of
colouring are necessary. It is a well-known fact that careful mixings
give good meltings, for then the materials are more evenly distributed
and uniformly attacked during the melting. Careful and exact weighings
are necessary when using colorific oxides, and a pot is kept for each
respective colour melted, so that the different colours and crystal
glasses do not get contaminated with each other. When open pots are used
for colours, the colour pots should be kept together in one section of
the furnace, so that whilst melting, especially during the boiling up of
the batches, the colours do not splash over into the other pots
containing crystal metal.
As a rule, smaller pots are used for coloured glass; generally they are
only a third of the size of crystal melting pots. When this is so, they
are set together under one arch of the furnace, and the workman informed
which pots contain the respective colours. All colour cuttings and
scraps should be kept separate from other cullet for re-use. Coloured
glasses are expensive, and no waste of glass should be permitted.
=Artificial Gems.= In the manufacture of the glasses for imitation
“paste” jewels, every effort is made to procure pure materials and
colorific oxides. The base for making artificial gems is a very heavy
lead crystal glass termed “=Strass paste=,” which gives great brilliancy
and refraction. The composition of such a paste would be: Best white
sand 100 parts, pure red oxide of lead 150 parts, dry potash carbonate
30 parts. These should be thoroughly well melted until clear and free
from seed, and the molten mass ladled out of the pot and quenched in
cold water, or “de-graded.” This assists in making the paste
homogeneous. After repeated melting and de-grading, the paste or cullet
is collected, dried, and crushed for use in making the coloured pastes.
Usually, this strass metal is melted in small, white porcelain crucible
pots holding about 5 to 10 kilogrammes of the metal and heated in a
properly regulated gas and air injector furnace. The coloured paste is
kept in fusion for a whole day, after which it is slowly cooled and
annealed within the pot, and the gems cut from the lumps of glass thus
obtained. The following are some of the compositions used in the
preparation of the respective gems.
=Opal.= Powdered strass paste, 1,000 parts; white calcium phosphate, 200
parts; uranium yellow, 5 parts; pure manganese oxide, 3 parts; antimony
oxide, 8 parts.
=Ruby.= Powdered strass paste, 1,000 parts; purple of cassius, 1 part;
white oxide of tin, 5 parts; antimony oxide, 10 parts.
=Beryl.= Powdered strass, 1,000 parts; antimony oxysulphide, 10 parts;
cobalt oxide, ·25 parts.
=Amethyst.= Powdered strass glass, 1,000 parts; purest manganese oxide,
8 parts; pure cobalt oxide, 2 parts.
=Emerald.= Powdered strass glass, 1,000 parts; green chrome oxide, 1
part; black copper oxide, 8 parts.
=Sapphire.= Powdered strass glass, 1,000 parts; pure cobalt oxide, 15
parts.
=Topaz.= Powdered strass glass, 1,000 parts; antimony oxide, 50 parts;
uranium yellow, 10 parts.
=Garnet.= Powdered strass glass, 1,000 parts; antimony oxysulphide, 100
parts; gold chloride in solution, 1 part; pure manganese oxide, 4 parts.
=Turquoise.= Powdered strass glass, 1,000 parts; cobalt oxide, ·5 parts;
black copper oxide, 10 parts; white opal glass, made with tin oxide, 200
parts.
After suitable pieces of glass of the requisite tints are obtained, they
are cut and ground on a Lapidary’s wheel, then polished, engraved, and
set as gems.
=Artificial Pearls= are now cleverly made in glass. A tube of the
requisite size made of translucent or opal glass is cut into small
sections, which are heated on a tray to softening point whilst set in a
rotatory movement. As the heat increases they gradually melt in and seal
at the openings, when they are removed from the tray and sorted.
------------------------------------------------------------------------
CHAPTER VI
DECOLORIZERS
Decolorizers are the agents employed by the glass maker to neutralise or
subdue the objectionable tints given by the colouring action of small
traces of iron oxide, which exists as an impurity present in the
materials used or otherwise become accidentally admixed during the
process of the manufacture of glassware.
The small additions of manganese dioxide, arsenic, nitre, nickel oxide,
selenium, antimony, oxide, etc., to glass batches may be considered as
decolorizers. The most commonly used of these materials is manganese
dioxide, so the action of this material will be explained. Every glass
maker finds that one or other of the raw materials he uses may contain
impurities. It is seldom that glass makers’ sand can be obtained that
does not contain traces of iron oxide present as an impurity. Again, the
cullet collected from the glass house often contains iron scale or rust
from the blowing-irons, which firmly adheres to the glass and gets
admixed with the batch for re-melting. The presence of even very small
traces of iron in glass becomes evident as a pale sea-green tint when
viewed through any thickness of metal. The chemical action of the glass
upon the walls of the pot is continually dissolving a minute quantity of
iron from the and diffusing it throughout the metal, giving it a
tendency to the pale-green tint.
To subdue or neutralise this objectionable tint in the glass, the glass
maker uses certain metallic oxides which give delicate counter-tints.
Only those glasses which are made from the purest materials can be
decolorized to become sufficiently clear to use in making the best table
glassware. In optical glassware, where the use of manganese is not
permissible, the greatest care has to be taken in the selection and
testing of the materials to be used. If manganese oxide be used in
making optical glass, although the eye may not be sensitive enough to
observe the actual color absorption, glass is produced in which the
solar rays are obstructed, and much less light is transmitted by the
glass when used as an optical lens or prism. Therefore the optician
avails himself of those glasses which have not been decolorized as being
more satisfactory for his purpose, as more light is transmitted by such
glasses.
Apart from the pale sea-green tint given to glass by the presence of
small traces of iron, certain of the silicates themselves produce
natural colors. The soda silicate present in soda-lime metal tends to
give a pale bluish-green tint when viewed through any thickness of
glass. The lead silicate has a yellowish hue. Each of these influences
has to be counteracted if clear crystal glass is desired. The
decolorization of glass by manganese dioxide depends upon the purple
tint it gives to glass. This purple color, being complementary to the
pale green color given by the presence of iron, serves and acts as a
counter-tint, and by the absorption of the green light, a less
perceptible coloring is produced. In the case of the decolorization of
glass, we get the red and blue of the purple subduing the blue and
yellow or green tint given by the iron. But certain other factors are
necessary. The purple color from manganese oxide is given only to glass
in the presence of oxidizing agents; and the absence of sufficient
oxidising agents in the glass batch, the purple manganese colour is
unstable and its action as a counter-tint is lost. Therefore, the glass
maker uses strong oxidising agents in his glass mixtures for crystal
effects, usually in the form of potassium nitrate and red lead, which
liberate oxygen. Whilst undergoing decomposition in the glass melt, the
presence of this free oxygen keeps the manganese used in a higher state
of oxidation, and gives the necessary purple coloration. It is also
evident that, if the glass melting in the pot is kept at a high
temperature for any considerable length of time, this period of
oxidation cannot last, and, after all the free oxygen gas has been
evolved, any further heating tends to turn the glass greenish again or
of poor colour, by the conversion of the manganese into the lower state
of oxidation in which the purple colour is not evident. If by chance the
glass maker has added too much manganese to the glass, and the purple
colour becomes too evident, he resorts to the use of a small amount of
carbonaceous reducing agent, such as a piece of charred wood or potato,
which he plunges or pushes to the bottom of the pot by means of a forked
iron rod or pole, where it vaporises, giving off moisture and
carbonaceous gases which reduce the manganese purple colour to a lower
oxidised colourless state, and in a very short time the excess of purple
colour has disappeared and the glass appears colourless.
Much of the success of crystal glassmaking depends upon the proper
adjustment of the decolorizers used and obtaining the best colourless
effect. The quality of the manganese is important; only pure manganese
dioxide should be used. In many cases the mineral ore, pyrolusite, is
used on account of its cheapness. This is objectionable, as much iron
may be present in the ore, when its use as a remedy is worse than the
defect. The necessity of taking advantage of the services of a
consultant chemist here becomes apparent, for, if glass manufacturers
would only have their different consignments of materials examined and
tested from time to time, many of the disappointments and difficulties
experienced by them at present would be obviated. A considerable saving
in the cost of batch materials can be made by the judicious selection of
more suitable qualities in preference to inferior or adulterated
varieties. In many cases, a chemist can substitute for certain of the
expensive batch materials other cheaper materials introducing the same
elements at less expense, and still retain the same quality in the glass
produced.
------------------------------------------------------------------------
CHAPTER VII
THE REFRACTORY MATERIALS USED
Of the greatest importance to the glass manufacturer are the refractory
materials upon which the life of his furnace and pots depends. A few
notes giving a description of them and dealing with the manufacture of
the fire-resisting blocks used in building the furnaces will be of
interest.
The chief and most generally used of such materials are the goods. The
best known deposits of fire-clays in this country are those in the
Midlands, Stourbridge, Leeds, and Glasgow districts. In each of these
districts the mining of fire-clays and the manufacture of fire-resisting
goods for furnace work forms an important industry. The theoretical
composition of a true would be a double silicate of alumina, and in this
pure state it would be of a very refractory nature. But, naturally,
fire-clays show the presence of other bases, such as iron, lime,
magnesia, titanium, and alkalies, which, if present to any appreciable
extent, lower the degree of resistance to heat or refractoriness of the
clay. These other bases may be considered as impurities or natural
fluxing agents. The characteristics of a highly refractory clay suitable
for glass manufacturers’ requirements would be: (_a_) that such a clay
should show no signs of softening at the highest heat of the furnace;
(_b_) a squatting point not below Cone 31 or 1690° Centigrade; (_c_) a
high alumina content not below 30 per cent.; (_d_) the greatest freedom
from impurities; (_e_) a fine-grained texture; and (_f_) a high degree
of plasticity. These are the qualities most essential for glass house
work. The figures given by the chemical analyses of good fire-clays
would probably fall within the following limits—
Silica 49% to 65%
Alumina 48% to 31%
Ferric Oxide 0·5% to
1·5%
Titanium Oxide nil to
1·5%
Lime nil to
0·5%
Magnesia nil to ·2%
Total Potash and 0·5% to
Soda 1·8%
Clays of higher silica content than 70 per cent. would not be considered
suitable as pot-clays owing to the case in which glass attacks silicious
clays. It is important that chemical analyses of fire-clays should be
compared with results obtained from the analysis of fired or burnt
samples, or they should be recalculated to allow of such comparison, so
as to exclude the figures for the hygroscopic and chemically combined
water of the clays.
The writer gives the following particulars of a very suitable for glass
house pot-making. It is plastic and highly refractory, and is now being
considerably used by the trade. The clay is supplied by Mansfield Bros.,
Church Gresley. The figures are from a report made by Mr. J. W. Mellor,
D.Sc., of the County Laboratory, Stoke-on-Trent, and are as follows—
Raw Fire-clay Dried at 109° Cent.
Silica 46·45 per
cent.
Titanic Oxide 2·65 per cent.
Alumina 35·32 per
cent.
Ferric Oxide 1·31 per cent.
Manganese Oxide —
Magnesia 0·09 per cent.
Lime 0·41 per cent.
Potash 1·08 per cent.
Soda ·76 per cent.
Loss when 12·14 per
calcined over cent.
109° Cent
The melting point is given as equal to Seger Cone 33
or 1730° Centigrade.
The physical properties of fire-clays vary as well as their chemical
properties. The analysis alone of a is not always sufficient indication
as to its ultimate behavior when in use. Many physical tests have to be
carried out before a clay can be proved satisfactory for a particular
purpose, and much information can be gained by engaging the services of
a specialist upon refractory materials to carry out petrographic,
pyrochemical, and physical tests, and report upon the suitability of the
material for its specific purpose. Fire-clays should be plastic, and
this plasticity should be developed to its utmost to increase the
binding properties of the clay when used. To develop the plasticity,
fire-clays should be weathered or exposed in thin layers to the action
of atmospheric influences. The heat of the sun and the action of frosts
and rain have a direct influence in breaking up the clay and developing
its better properties. The use of new unweathered clay is the cause of
much trouble to the glass manufacturer who makes his own pots and
furnace goods, and on this account he should insist upon having his
clays weathered for some time before use, so as to have them thoroughly
matured. Before fire-clays are weathered or used for important work they
should undergo a process of selection and cleaning. When first raised
from the mines all foreign and inferior portions, carbonaceous matter,
vegetation, iron pyrites, and stones are removed. The best and cleanest
portions of the are sorted out and removed to the weathering beds, where
the lumps are broken down to small pieces about the size of an egg, and
left to mature and season by weathering.
This is then spread out in a layer about 2 ft. deep, and, after a period
of exposure to the action of the weather, the heap is turned by men
shoveling the clay from one side to the other. The clay, under the
continued action of the wind, frost, and rain, disintegrates and slacks
down until it is reduced to a mild, fine-grained mass, which condition
shows it to be well seasoned and ready for use. Fire-clays vary in this
respect: some clays season quickly in the course of a few months, others
take years to develop their proper nature. The former may be classed as
mild fire-clays, the latter as strong fire-clays.
After weathering, the clay is carted or conveyed to the clay-grinding
plant, where it is stored under cover until it is dry enough to be
ground on the clay-mill. Here the clay is fed into a revolving pan, and
crushed under heavy iron runners, and, after passing through
perforations in the bottom of the pan, it is elevated on to screens
which sieve the clay to a requisite degree of fineness. It is then
admixed with a large proportion of ground-burnt and the mixture is
tempered with water until it forms a plastic mass of dough, which is
conveyed to the workshops where the furnace blocks or pots are to be
made. These making and drying shops have false or double floors, under
which steam or heated air is passed using pipes or flues below the
floors, giving the steady and uniform heat which is necessary to dry the
goods as they are made. Heavy goods should on no account be hurried in
drying, lest trouble should occur through the goods cracking or warping.
In making the blocks for the furnaces the workman takes a portion of the
prepared clay and tramps the plastic mass into a wooden frame, or mold,
the shape and size of the block required, with due allowance made for
shrinkage. The blocks are made on the warm floor, which is of cement or
overlaid with quarries. When the mold is filled the surplus clay is cut
off and the wooden frame is lifted up, leaving the clay block on the
floor. The empty mold is then cleaned and refilled. The blocks are left
until they attain considerable stiffness from the evaporation of the
water present by the heat of the room. They are then dressed and cut to
the final shape desired, after which they are further dried until they
become quite hard and white. When thoroughly dry the blocks are removed
from the drying sheds to the kiln for burning.
In burning thick and heavy blocks much care and vigilance is required in
expelling the chemically combined water present in the clay, and, as the
temperature rises and approaches red heat, the rate of heating should be
retarded to allow proper oxidation to take place throughout the
structure of the blocks, and prevent black cores from being formed. In
all fire-clays, besides the mechanically admixed water used in preparing
the clay to a plastic mass, which is mostly driven off whilst in the
drying shed, there exists water in a chemically combined state. This the
combined water is not expelled below 250° Centigrade, and is tenaciously
held by many varieties of mild fire-clays. Due care has to be exercised
in dehydrating goods made from such clays; therefore the man in charge
of the burning regulates his fires, keeping the kiln at a moderate heat
for some time to allow this chemically combined water to be properly and
completely expelled. This dehydration stage in burning clay goods occurs
between the temperatures of 300° and 650° Centigrade.
After the dehydration stage of burning is completed, the fireman raises
the temperature within the kiln to a dull red heat, when the next stage
in the process of burning begins. This is the oxidation period, during
which any organic carbonaceous matter present in the clay is expelled.
During this stage in burning, goods require extended time, to allow for
the heated air to permeate and get to the interior portions of the
blocks and oxidize the cores; otherwise the blocks are badly burnt.
After the oxidation stage is completed, the fireman raises the heat
quickly until he obtains a high temperature, sufficient to eliminate and
complete the shrinkage of the goods. When this heat is sufficient to
complete the fire-shrinkage, the kiln is finished and is allowed to cool
down. The blocks, when cold, are then withdrawn and delivered to the
furnace builder.
For the erection of the furnaces several grades of blocks are used,
according to the conditions and nature of the heat they have to resist.
In the presence of reducing agents, fuel ash, or glass, goods vary
greatly as to their suitability. So the local conditions to which they
are to be subjected whilst under heat should be first ascertained, and
the mixtures for the blocks adapted accordingly. So many differences
exist in the pyrochemical and physical properties of clays that their
misuse is often apt to occur if the conditions under which they are to
be used are not properly understood and allowed for. A may show a the
high degree of refractoriness under a fusion test, and yet be less
suitable for a specific purpose than one of less refractoriness showing
better physical properties and of the more suitable chemical
constitution. The size of grain in both the burnt clay and raw clay used
in the mixtures for making glasshouse furnace blocks is of the greatest
importance. In many cases it is necessary to grade the ground-burnt
material used, so that the proportion of coarse grains to the fine flour
can be regulated to suit requirements. The burnt clay used in making the
furnace blocks should be hard and well burnt, to prevent any
after-shrinkage of the goods when they are used in the furnace.
Fire-clay goods for glass house furnaces should not be burnt at a lower
temperature than Cone 12, and in the construction of gas-fired furnaces
and tanks, burning the blocks at a higher temperature, Cone 14 would
give much better results.
On the Continent the glass manufacturers usually grind and mix their
fire-clays, with the result that they know exactly what they are using
in making their pots and furnace goods, and they are not then dependent
upon outside firms to carry out their wishes. English glass
manufacturers usually buy their clays ready mixed, and as often as not
have perforce to take the mixtures offered by the clay firms.
Unfortunately, in Great Britain many of the firms who supply the
refractory requirements of the glass trade are exceptionally backward in
applying technical knowledge to their trade; consequently, progress is
somewhat retarded in the glass trade as far as the refractory materials
are concerned. So obstinate is this ignorance of science that quite
recently one well-known firm replied to an inquiry for samples of
fire-clays to be sent for important research work then being undertaken
upon the resources of the country, stating “that, as their clay product
was perfect, and research work was quite unnecessary.” It often turns
out that their conservatism is simply a cloak to hide ignorance, as it
is quite evident to any technicist that there is ample scope for
improvement in the present goods on the market, and such an open
opportunity for a scientific investigation into the nature of their
fire-clays, however well known they may be, should be welcomed with
delight, and every facility and assistance offered for research chemists
to improve their material, and apply tests with the object of developing
the best properties of such refractories for special purposes.
------------------------------------------------------------------------
CHAPTER VIII
GLASS HOUSE FURNACES
The pots within which the raw materials are melted are set within a
strongly heated chamber called the glass furnace. The old circular type
of English furnace usually contains either six, ten, or twelve pots, and
will be described first. The pots stand in a circle upon a form of hob
called the “siege,” which constitutes the floor of the furnace. In the
centre of this chamber and below the level of the siege is the “eye” of
the furnace through which the flames come from the furnace fire below.
The burning fuel is contained in a circular or cylindrical-shaped
fire-box, about 4 ft. deep and 5 ft. in diameter, and is supported by a
number of strong iron bars across the bottom of the fire-box. Passing
under the fire-box, and across the whole width of the glass furnace,
there is an underground tunnel called the “cave,” each end of which is
exposed to the outside air, which is drawn in through the caves by the
draught of the chimney cone above the fires. These caves are of
sufficient height and width to allow the fireman, or “tizeur,” as he is
called, to attend to the stirring of the furnace fires from time to
time. Using a long hooked bar of iron, he rakes out the dead ashes and
clinkers, as they are formed, and stirs the fire through the bars by
prodding the fuel with a long poker. The coal is fed upon the furnace
fire through a narrow mouth situated in the glass house leading into a
chute which runs under the siege, from the glass house floor level
towards the fire-box of the furnace. The fuel is pushed down this chute
and falls into the fire-box and is fed at intervals of the half to
three-quarters of an hour, according to the heat desired and the draught
allowed.
[Illustration:
INTERIOR OF ENGLISH TYPE OF GLASS-MELTING FURNACE
]
Above the siege and over the pots is a covering called the crown of the
furnace, which is supported by fire-brick pillars. This is built of the
most refractory material possible to be obtained, as the hottest flames
from the furnace fires beat against this crown and are reverberated
downwards upon the surrounding pots. The flames, continuing their
course, pass between the pots into small openings or flues leading from
the siege floor and passing upwards through the pillars which are
situated between each pair of pots, they then escape from little
chimneys leading into the outer dome or conical-shaped structure so
familiar to outsiders. This outer truncated cone-shaped structure
constitutes the main chimney of the furnace. The furnace chamber
containing the pots is constructed entirely within this cone. The blocks
are carefully shaped, neatly fitted, and cemented together with a mortar
made of fine, plastic, raw ground mixed to thin paste with water. The
presence of any molten glass which escapes from a cracked pot, and the
fluxing action of the fuel ashes, cause severe corrosion of the blocks
forming the siege and fire-box, and these necessarily have to be made of
extra thickness in order to extend the life of the furnace. When the
furnace crown or siege becomes badly corroded away, the furnace has to
be put out for repair; so generally an auxiliary furnace is kept at
hand, in order that it may be started and the workmen transferred from
one furnace to the other whilst the repairs are being done.
[Illustration:
EXTERIOR VIEW OF ENGLISH GLASS-MELTING FURNACE
Pot Trolley in foreground
]
The action of the glass upon the siege of the furnace is very active,
and any leakage quickly destroys the blocks, leaving fissures which
gradually increase in size until the blocks are eaten right through.
Consequently, every care is taken to preserve the pots from losing
metal. If by chance any pot develops a crack through which the metal
leaks into the furnace, the glass working is ceased at that particular
pot, and every endeavour is made to ladle out what remains of the metal,
and so prevent any more running on to the siege and causing further
mischief. The metal is ladled out of the pot by means of thick, heavy,
iron spoons, with which the hot metal is scooped out of the pot and
dropped into a large cauldron containing water. This is very exhausting
work, but there is worse trouble still if the metal is allowed to
continue to run through the crack in the pot and over the siege into the
eye of the furnace, for it then fluxes with the ashes of the fuel,
causing them to form into a big mass of conglomerate, which, lying in
the fire, interferes with the draught and combustion of the fuel within
the furnace, and before the furnace can be got to work properly again
has to be cut away, piece by piece, through the firebars whilst hot,
until it is all removed. At the sign of any glass running down into the
fires and through the bars, the tizeur hurries up to give the word that
a pot is leaking in the furnace, and when the pot is isolated the work
of ladling the hot metal out into water begins in earnest. A pot which
has cracked and leaks is useless for any further work of melting glass,
and at a convenient time it has to be withdrawn from the furnace and a
new pot must be substituted. Glass-melting pots form a very expensive
item in the glass manufacturer’s costs; consequently, every care is
taken to prevent the pots within the furnace from getting chilled by
inadvertently allowing the fires to burn too low or allowing cold air to
rush through the bars, through unskilful clinkering and inattention to
the furnace fires. Sometimes these furnaces are fitted with a Frisbie
Feeder. This is a mechanical firing arrangement fitted underneath the
furnace bars, by which the fuel is fed upwards into the furnace box, so
that all smoke given off by the fuel baitings has to travel through the
hot fuel above, and thereby is more completely consumed, giving better
combustion than when the black fuel is thrown on the top of the hot bed
of fuel. A mechanically operated piston pushes up small charges of fuel
from within a cylindrical-shaped box, which works on a swivel backwards
and forwards as the fuel is fed into it.
In the old type of English furnace containing twelve pots, each 38 in.
diameter and holding about 15 cwts. of metal, the furnace would be
capable of melting 7 to 8 tons of glass a week, taking 40 tons of best
fuel. The more up-to-date glass-melting furnaces are constructed upon a
much better principle than the coal-fired old English type of furnace
just described. These are usually producer gas-fired and give more
economy and greater convenience in every way.
[Illustration:
FIG. A
SIEMENS SIEGBERT TYPE OF REGENERATIVE GLASS-MELTING FURNACE
]
In these better types of modern furnaces some form of regeneration or
recuperation of the waste heat is usually adopted. These furnaces are
much smaller and more compact; being gas-fired, they give much higher
temperatures, more complete combustion of the fuel, greater ease in
regulation, cleaner conditions, and far greater production than the
older types of English furnaces. Considering the reasonable initial cost
that the latest types of these modern furnaces can be built for, it
appears incredible that so many of the old out-of-date English furnaces
remain in use in this country.
[Illustration:
FIG. B
SIEMENS SIEGBERT TYPE OF REGENERATIVE GLASS-MELTING FURNACE
]
As examples of the types of regenerative and recuperative furnaces, a
description will be given of the Siemens Siegbert Gas-fired Regenerative
Furnace and the Hermansen Recuperative Furnace for glass-melting, which
are extensively used on the Continent and are giving remarkably good
results.
[Illustration:
FIG. C
SIEMENS SIEGBERT TYPE OF REGENERATIVE GLASS-MELTING FURNACE
]
In the Siemens Siegbert type, the furnace may be a rectangular or an
oval-shaped chamber, approximately 18 ft. by 9 ft., the crown of which
is about 4 ft. 6 in. high. No outer cone-shaped dome exists, and the
pots within the chamber are arranged much closer together and
practically touching each other round the furnace. The furnace chamber
is heated by a mixture of producer gas and heated air, the gas being
generated in an independent gas producer situated outside the glass
house and some little distance away from the furnace. At either end of
the furnace, beneath the floor of the siege, are two blocks of
regenerators. These are deep rectangular chambers containing an open
lateral arrangement of fire-brick chequers, through which the air or
products of combustion pass on their way to or from the furnace.
Port-holes are situated directly above these regenerators which lead the
gases through the floor or siege into the furnace chamber. The draught
is induced by a tall stack, which draws the gas from the gas producers
through a duplicate arrangement of flues to the port-holes at one end of
the furnace, where it is mixed with the air which has been drawn and
heated in its passage through the regenerator beneath. This gaseous
mixture, whilst in combustion, is drawn across the furnace chamber to
the other end of the furnace. The flames playing across the tops of the
pots on either side pass down through the port-holes and regenerator at
the opposite end. The hot gases or products of combustion, in passing
through the lateral channels of this regenerator, leave behind their
heat by the absorptive or conductive capacity of the fire-brick chequers
through which the hot gases have passed on their way to the stack. The
direction of the current is reversed at intervals of half an hour or
less by using an arrangement of valves situated in the gas and air
flues, so that the currents are made to travel on the contrary
direction, the air necessary for combustion then being drawn through the
hot block of regenerators which was previously heated by the exit gases.
On its way through these lateral channels the air becomes intensely
heated, and, when it is admixed with the coal gas at the porthole, this
pre-heated air accelerates the combustion and calorific intensity of the
gaseous mixture. The direction of the current is continually being
reversed at the interval of half an hour or less by the manipulation of
the valves, so long as the high temperature is desired.
In practice, however, the regenerators are only used whilst the batch
materials are being melted during the night, and by morning, when the
metal is melted and “plain,” the heat is brought back, or retarded, by
using the gas from the gas producers and cool atmospheric air under
natural draught, instead of the regenerated hot air. This cooler
mixture, naturally not being so active in combustion, maintains just
sufficient temperature for working the metal out during the day. Later
in the day, when the pots are emptied and refilled with batch, the
regenerators are re-connected and the founding proceeds again through
the night, and the metal is again got ready for the workmen coming in
next morning.
It will be seen that this method of melting and working out the metal
does away with night work, the furnace man alone remaining in charge
during the night. All firing is done outside the glass furnace room,
which is well lighted, clean, and free from coal dust, totally different
conditions from those existing in many English glass houses of to-day.
A Siemens Siegbert furnace taking ten open crucible pots, and filled
each day, turns out 15 to 18 tons of metal a week. The crucibles are
about 30 in. in diameter and have a capacity of 5-1/2 cwts. of metal
each. The amount of fuel consumed is about 18 tons a week. This type of
furnace costs about £1,600 to £2,000 to build. In the miter’s opinion, a
disadvantage of this furnace is that, during the reversing in the
direction of the fire gases, the greatest heat is suddenly brought to
bear on the cooler pots, resulting in a short life for the pots. The
temperature of the incoming air is not so constant as with the
recuperative type of furnace; however, with proper control, these
defects may be obviated to some extent.
[Illustration:
A MODERN GLASS HOUSE
The Hermansen Continuous Recuperative Glass-melting
Furnace in foreground (Twelve Covered Pot Type).
]
By the kindness of Messrs. Hermansen, the patentees, I am permitted to
illustrate their Recuperative Glass-melting Furnace, eight pot type.
[Illustration:
HERMANSEN GLASS HOUSE FURNACE (EIGHT POT TYPE)
]
[Illustration:
Sectional Elevation.
A
HERMANSEN’S CONTINUOUS RECUPERATIVE GLASS-MELTING FURNACE
_P._ Producer.
_B._ Burner.
_G.P._ Glass Pocket.
]
[Illustration:
B
HERMANSEN FURNACE
Cross Section through Gas Producer.
_P._ Gas Producer.
_R._ Recuperators.
]
The Hermansen furnace, like the Siemens furnace, is producer gas-fired.
The gas producer is built within the body of the furnace, (=P=) below
the glass house floor. On either side of this gas producer the
recuperators are situated. These are constructed by an arrangement of
tubes, designed to give two distinct continuous channels, the one
horizontal and the other vertical. The vertical channels are connected
with the atmosphere and supply the air necessary for combustion. The
horizontal channels (=R=) are the flues through which the hot waste
products of combustion are continually being drawn from the furnace by
the stack. It will be evident that, the horizontal channels being
intermediate to the vertical tubes, the waste heat is continually being
absorbed by the air travelling inwards. In other words, the air is
pre-heated by passing through flues which are surrounded by the hot
waste gases. Therefore, in this type of furnace there is no necessity
for reversing the currents to procure the necessary pre-heated air for
combustion, and the regulation of the furnace heat becomes a simple
matter of controlling the draught by means of the dampers provided in
the main flue. In this type of furnace the glass is melted nightly; open
or covered pots may be used, the capacity of which varies between 5 and
12 cwts., according to the class of glassware manufactured. The furnace
is designed in four, six, and eight pot types, and several are now
working in this country. These Hermansen furnaces are capable of
producing 20 tons of metal, with a fuel consumption of 16 tons.
[Illustration:
C
PLAN OF HERMANSEN’S FURNACE
(Eight Pot Type)
]
The Hermansen Continuous Recuperative Furnace is the most efficient
furnace known to the writer. It is easier to control than the
regenerative types. Being compact, it takes up little space and is easy
to repair, and its life well surpasses other types. The initial outlay
and cost of erection varies from £850 to £1,200. The combustion in this
type of furnace is so perfect that it is used with open crucible pots
for melting lead crystal glasses. On the Continent this furnace is in
general use for all types of glassware, and, from the amount of glass it
will melt, its efficiency is greater than the regenerative type.
=Tank Furnaces= are at present used for the melting of the commoner and
cheaper types of glass. They are so constructed as to contain a single
rectangular-shaped compartment, or tank, about 18 in. to 2 ft. deep, and
from 30 to 100 ft. long. The bed and retaining walls of this tank are
constructed of specially selected fireclay blocks; no pots are used.
Tank furnaces are simple and melt the glass economically, but the metal
produced is not nearly so good a quality as pot metal.
Tank furnaces are chiefly used for making the cheaper glasswares, such
as wine, stout, and beer bottles, gum bottles, ink-pots, sauce bottles,
and like goods, where a large production is essential. Improvements are
continually taking place in the design of this type of furnace, and much
finer and clearer metals are being produced. It is quite probable that
in the future tanks will be preferred for making cast plate and sheet
window glass, as a larger body of metal is held by them when compared
with pot furnaces. Like the Siemens and Hermansen furnaces, they are
gas-fired, but the port-holes by which the gas and air are introduced
and the products of combustion are withdrawn from the melting chamber,
are situated on either side, above the level of the metal, whilst the
glass blowers work at one end of the furnace. The melting and working of
the metal is continuous. The tank is divided by a shallow bridge, which
is partially submerged and situated midway between the two ends of the
furnace, dividing it into two sections, respectively the melting and
working compartments. This bridge keeps back all unmolten material and
allows only that portion which is melted to travel forward to the
working compartment. The tank is crowned or arched over, and at the
working end openings are provided to enable the glass workers to gather
the metal from within. Small rings, or syphons, are used, which,
floating on the metal, serves further to refine the glass as it is
gradually used. The batch mixture is filled through a convenient opening
near to the port-holes. Tank furnaces vary in capacity. Some have been
constructed to give an output of 300 tons of glass a week. This pace can
only be kept up with the aid of automatic bottle-making machinery; in
which case hand labor is practically eliminated.
Liquid fuel or oil-fired glass furnaces have not proved a success, being
very costly in repairs on account of the local heating effects of the
flames issuing from the burners vaporizing the oil.
Electric furnaces for glass-melting have been tried with partial
success. These are expensive in maintenance compared with their
efficiency in producing glass.
------------------------------------------------------------------------
CHAPTER IX
GLASS-MELTING POTS AND THEIR MANUFACTURE
Glass house pots are large hollow vessels made of refractory in which
the glass manufacturer melts the materials of which his glass is
composed, and which retain the molten metal whilst in a state of fusion
for the workmen’s use. In the case of the lead crystal glass, the
materials, whilst being melted, require protection from the flames,
smoke, and fuel ash present in the old English types of furnace
chambers, which would otherwise reduce the lead present to a metallic
state and spoil the glass; therefore, such glasses are melted in covered
or hooded pots and thus protected from the direct action of the flames.
Consideration has to be given to the extra amount of heat required from
the furnace to find its way through the hood of the pot. For crown plate
and chemical glassware, the metal is usually melted in open or uncovered
pots. In this case the fusion is facilitated by allowing the heat of the
furnace to come into direct contact with the materials within the pots.
Pots which are covered or hooded have an opening cut out in the front,
in a position just above the level of the molten metal. Through this
opening the workman gathers the hot metal. In the case of open pots, the
crucible is set in a similar position within the furnace, but the
working hole or mouth is built to form part of the construction of the
furnace in front of the crucible.
Good pots are of the greatest importance to the glass manufacturer, and
upon their life much of the success of glassmaking depends. They have
necessarily to resist the corrosive action of the raw materials and
molten glass within, and, at the same time, withstand the very intense
heat of the furnace without giving way under the great weight of the
glass within them. Should a pot of metal give way whilst in the furnace,
the loss is considerable and very serious, for not only has the metal
been wasted, but much of it has flooded the floor of the furnace and
siege, and, finding its way into the fire-box, attacks the furnace
walls, fusing and melting with the fuel ash, checking the draught, and
causing endless trouble.
Glass house pots are very difficult and expensive to manufacture, and
upon an average each pot has cost £10 by the time it is set within the
furnace; therefore every care is taken to extend their life by procuring
the best possible materials for their manufacture.
Only the best selected pot-clays available are used, and every endeavour
is made to keep them clean and free from foreign contamination. Only the
best portions of the seam are taken for this purpose, and a considerable
amount of diligence and stringent precaution is taken to procure the
best qualities. As the clay is raised from the mine, clay pickers look
over the lumps and select out the best portions. A foreman of long
experience is stationed at the head of the mine, and it is his duty to
supervise the clay pickers and see that every care is exercised to guard
against any unfortunate results which would naturally attend any
indiscriminate or indifferent selection. The best portions having been
selected and placed aside, the lumps are scraped on the surface to
remove any dirt, and broken into pieces about the size of an egg, which
are again carefully examined on all sides and cleaned from foreign
matter such as pyrites or bluish parts. If this is carefully done, and
the clays analysed and tested from time to time, a good pot-clay is
obtained.
The clay for burning is treated similarly and dried. It is then burnt to
a very high temperature and taken to the mill to be ground to the
necessary fineness of grain. All pot-clays are well seasoned and
weathered before use. They are first ground to a very fine flour and
then mixed with a ground-burnt clay, or “chamotte.” The proportion of
raw clay to burnt varies with most manufacturers but depends very much
upon the plasticity or binding property of the raw pot-clay used. The
burnt clay is preferable if ground to a size about 1 to 1-1/2 mm., being
sieved to take out any coarser particles. Some clays are more plastic
than others, so the proportions in the pot-clay mixtures may vary from
six parts of burnt clay to five of raw, down to one part of burnt clay
to three of raw clay. The proportions are reckoned by volume, not by
weight. The mixture is sieved into a trough and mixed with water to form
a stiff paste, and removed into a large tank, where it is allowed to
soak for some time. It is then well-tempered by treading with the bare
feet until the whole mass becomes plastic and tough. The clay mass is
turned and trodden several times, in order thoroughly to consolidate the
clay particles. Many efforts have been made to do this work
mechanically, but without success. The fact remains, and experience has
proved that, in the process of treading, the clay is more consolidated
than by any mechanical method of preparation. The tempered and toughened
clay is then allowed to sour and mature for a few weeks before use. It
is then ready for the pot maker to begin the work of building the pots.
The room in which the pots are to be made is kept evenly warm using a
series of hot water circulating pipes arranged around the outer walls.
Usually a temperature of between 60 to 70° Fahr. is maintained.
Double doors are provided at the entrance, with a porch, so as to
prevent sudden inrushes of cold air and prevent draughts in the
pot-making room. All unauthorised persons are prohibited entrance, and
only those who work therein are allowed free access. They are made
responsible for keeping the place clean, as well as looking after the
clay and taking care of the pots whilst they are being made.
The usual shape of a pot is of round section, 38 in. in diameter and 42
in. high, but many other shapes and sizes are used, according to the
class of goods being manufactured. Thus, for colours, a very much
smaller pot, less than one-third this size, is used, three of them,
taking the position of one large pot, being set within one arch. For
sheet and optical glass, a covered pot with a very large mouth or
working opening is used.
In some instances, as in the Hermansen furnace, the pots are oval or
egg-shaped. These are used on account of their larger capacity in
relation to the space occupied in the furnace. Other pots have an
interior division, which has a syphonic refining action upon the glass;
such pots permit of continuous melting and working, instead of the
intermittent process adopted when the regular or common shape is used.
For plate glass, open crucible or bowl-shaped pots are used.
In regard to the manner in which the pots are made, and their subsequent
treatment in annealing, the utmost care and control is necessary. In
making the pots, the pot maker begins by making the pot bottom first,
working the plastic clay paste into rolls about the size of a large
sausage. He takes these rolls and applies them one after another in a
circular form upon a round level board, the size of the bottom of the
pot. This board is supported on a low table. As he applies each roll, he
presses them together to exclude all air spaces between them, and
continuously work the rolls on the top of each other in circles, until
he gets a circular flat slab of clay in thickness about 4 in. and the
width of the pot bottom. He then has the necessary thickness and size of
the pot bottom formed as a clay slab, which is smoothed and leveled over
the face with a knife or straight piece of wood. The slab of clay is
then reversed upon another board, covered with a strong hurden cloth and
a layer of ground-burnt clay, which prevents the clay from sticking to
the board, and facilitates drying of the pots.
The first board is then removed, and the pot maker begins to build the
sides or walls of the pot upon the circular clay slab by working the
clay in rolls around the circumference of the slab to a thickness of 3
in., which gives the thickness of the pot walls. As he works and presses
on each roll with his right hand, he supports the inside of the curve
with his left hand, and presses roll after roll around the circumference
of the slab of clay, increasing the height of the walls until he attains
a height of about 6 in. The height of this wall is increased by about 6
in. every other day or so; these time intervals allow each section built
to stiffen a little before beginning upon the next section.
The workman passes from one pot bottom to another, building up these
sections until he builds each to a height of about 30 in., when he
places within each pot a clay ring about 18 in. in diameter, which he
has previously made.[3] After placing these rings within the pots, the
pot maker begins to form the hood or dome of the pot by working on the
clay rolls, and at the same time drawing the sides inwards towards the
middle, lessening the thickness of the walls and gradually diminishing
the open space until it is covered and sealed in. Whilst the clay is
still soft, the mouth or working opening is worked on and cut out of the
dome, and the whole finished and smoothed by means of wooden tools.
Footnote 3:
These rings, floating on the metal, are used by the glass makers to
keep back the scum of the glass away from the middle portion from
which he gathers.
The pots are now completed and are left to dry gradually at a moderate
heat, which is increased a little at the end of a few months in order to
thoroughly dry them. They are then removed from the boards and are ready
for the furnace.
Crucible pots are made in a similar way, except that at the height of
about 27 to 30 in. the pot maker finishes off the top edge of the walls
and leaves it in that form to be dried.
Many efforts have been made to manufacture pots by other methods. One
which has been tried with a fair amount of success is to cast the whole
pot or portions thereof by using a plaster case mould and pouring in
liquid clay slip. Another method which has been tried is to press the
form by means of a hydraulic press and mould. Other mechanical
contrivances have been used, but few of them have given such
satisfactory results as the hand-made pots.
MIXTURE FOR POT-CLAY
_By
volume._
(=Base=) Fine ground strong 5 parts
Fire-clay
(=Binder=) Fine ground mild Plastic 4 parts
Fire-clay
(=Grog=) Ground burnt Chamotte 2 parts
(=Grog=) Ground selected Potsherds 1/2 part
The fusion point of the mixture should not be less than Cone 32, or
1710° Centigrade.
Strong fire-clays are those coarser and harder grained, and are usually
more silicious and less plastic than the mild fire-clays. Mild
fire-clays are very fine-grained, plastic, and easily weathered clays.
They act as the binder portion in fixing the burnt grog used in
pot-clays.
The raw clays should be ground very fine and separately from the burnt
clays. The ground burnt should be crushed from hard and well-burnt
fire-clays, and should pass a sieve of ten meshes to the linear inch.
The mineralogical composition of the fire-clays for making pots is
important. The presence of pyrites renders fire-clays unsuitable as
pot-clays. Some indication as to the subsequent behavior of a can be
obtained by submitting it to a petrographic examination, and the usual
pyrochemical and physical tests carried out in testing refractory
materials. In this country, Stourbridge pot-clays are chiefly used for
pot-making, and so conservative are the majority of glass manufacturers
that they will not use other clays, although, in the writer’s opinion,
many better clays exist in Great Britain, and have now been introduced
and used successfully by some firms for pot-making.
Ground potsherds are selected pieces of old broken pots, cleaned from
any adhering glass. These selected pieces are crushed and ground in a
similar way to the burnt clay, and sieved to the same degree of fineness
before use.
=Plumbago= glass house pots are sometimes used. These are made from
mixtures of graphite, or plumbago, and raw . They are very refractory
and withstand the attack of very basic glasses, where such have to be
manufactured.
Pot rings are made by taking a long roll of clay about 3 in. in
thickness and shaping it around a circular frame. The two ends are
joined and finished smoothly, the frame took away, and the ring dried. A
ring is placed in each pot.
Stoppers are the lids used to close the mouth of covered pots whilst the
metal is being melted. These are made in plaster case molds by pressing
a bat of clay into the desired shape and releasing the outer case by
turning the whole upside down upon a board and lifting off the mold. An
indentation is made in the middle, forming a small hole. An iron rod can
there be inserted, by which the stopper can be lifted away from the pot
mouth whilst hot. Stoppers are burnt before use and are made in various
sizes to fit the mouths of different pots.
It is always advisable for the glass manufacturer to make his pots and
prepare his clay, as he then knows exactly what he is using, and he is
not dependent upon outside firms for his pots as he has them ready at
hand when needed. The conveyance of pots from one district to another by
rail or road is always accompanied by considerable risk, as the
vibrations are given they in such journeys often cause mischief. As they
are very heavy and fragile, their loading and unloading into the wagons
is often attended with a mishap. As often as not, latent strains are
caused, which only develop when the pot is put in the furnace.
=Annealing and Setting the Pots in the Furnace.= The pots, when made and
dried, being of raw clay have to be carefully annealed before they can
be introduced into the hot furnace. In doing this, the pot is removed
from the drying rooms and placed within a small auxiliary furnace called
a pot arch, which is constructed purposely to anneal them and get them
hot before placing them in the glassmaking furnace. The pot is moved by
picking it up on a long three-pronged iron trolley, made purposely to
lift and move them about. The pot is set within the pot arch, resting
upon two or three rows of fire-bricks, which allows the trolley to be
removed and brought away, leaving the pot in a raised position in the
pot arch. The doors of the pot arch are then closed and sealed with a
stiff clay paste or mortar, and slow fires started which gradually heat
the pot, until at the end of a week it is got to a white heat, and the
pot is ready to be removed and set within the furnace for melting the
glass.
At a convenient time, arrangements are made for setting the pot. All
other work about the glass house has to cease, as all hands are required
to help in the strenuous and arduous work. The old pot in the furnace,
which has done work for several months, has to be withdrawn from the
furnace and the new pot from the pot arch has to take its place. We see
gangs of men here and there. Some are pulling down the wall of bricks
from the front of the old pot, making an opening in readiness to remove
it. Another gang of men advance with long, heavy, strong iron crowbars,
sharpened at the points, with which by heavy blows and levering they
endeavour to loosen the old pot from the floor of the siege, to which it
has become firmly cemented by the heat and any leakage of glass which
may have taken place. Eventually, by their combined exertions, they
succeed in loosening the pot, and then, levering it up, they place the
low iron pot trolley under it and drag it out of the furnace, whence it
is taken away and thrown aside.
The old pot having been removed from the furnace, the glowing heat
radiates more intensely than ever into the faces of the men at work, who
endure it in relays whilst they work clearing away the old bricks and
preparing the siege for the new setting. When this is done, a gang of
men open the pot arch doors, and, placing the iron trolley under the new
pot, convey it to the opening in the glass furnace from which the old
pot has been removed. Facing the terrific heat, they struggle to push
the new pot into its place in the furnace, with the aid of crowbars, and
working in relays, in turn face the heat till at last it is got into
position. Naturally, everything has to be done in a hurry, so that the
new pot may not be chilled before it is got into the furnace by being
exposed too long to the outside air. The whole work proves very
exhausting to the men, as there is little protection from the heat.
After the pot is set in its place, the trolley is brought away and the
wall of bricks rebuilt up in front of the pot to protect it, clay being
daubed over the exterior of the brick wall to prevent any inrushes of
air, which would cause the pot to crack by finding a way through the
joints in the brickwork.
The furnace, during these operations, is driven and worked to its full
capacity, so as to allow for the very considerable loss of heat which
takes place whilst the opening is being made and the pots removed.
The above is a description of the usual method of pot setting. In more
modern and up-to-date works a travelling chain screen is used. This
screen is like a curtain of loose chains, which is adjusted to hang in
front of the open arch of the furnace and protects the workmen from the
fierce heat. At the same time it permits the workmen to see and carry
out the work of pot setting with greater ease and convenience. In using
this screen arrangement whilst setting, the pot is pushed through the
chain screen, which closes upon it after it has passed through. The
workmen are thus enabled to get closer to their work by manipulating the
crowbars through the screen as the heat is not radiated full upon them.
The newly set pot is allowed to stand empty in the furnace for a day or
two to regain heat before it is filled with batch. It is first glazed on
the inside by a workman taking a gathering of glass from another pot and
plastering or covering the inside all round with the hot metal, which
flows down and glazes the surface of the pot, giving it a certain amount
of protection from the attack of the raw batch materials which are to be
introduced later.
The founder, or glass melter, now takes charge of the pot, and he brings
up the mixture of batch and cullet and shovels it into the empty pot
until it is filled well above the mouth or level of the opening. The
heat of the furnace melts the batch, and after several hours it becomes
liquid and shrinks in volume so that probably only two-thirds of the
height or capacity of the pot is occupied. The pot is then again filled
with more batch materials until it is full of molten metal up to the
level of the mouth of the pot.
The furnace is kept going at its full heat until the founder, drawing a
small portion of the glass on the end of an iron rod, examines it and
finds that it is melted clear and free from seeds or bubbles of gas.
When clear, the metal is “plain,” and at this stage is in a very liquid,
fluid, and watery state, too liquid to be easily gathered. It is,
therefore, allowed to cool off by removing the stopper down and leaving
the mouth of the pot open, until the glass becomes more viscid, or of a
stiffer nature. The glass is then skimmed by dragging off any scum
present on the surface, which is due to undecomposed salts that may have
risen during the melting.
The metal is now ready for the glass blowers to begin work. Upon looking
into the pot, the ring will now be noticed floating on the surface of
the glass. This ring keeps back from its interior any further scum that
may arise whilst work is in progress. The glass blower always gathers
from within this ring, where the metal is cleanest; and from time to
time the metal within the ring is skimmed in order to keep that portion
in the best condition. When the greater part of the metal within the pot
has been gathered or worked out, the heat of the furnace is raised again
and fresh batch materials filled and the process repeated.
The time taken to melt the glass depends upon the heat of the furnace. A
gas-fired furnace will melt the batches in eight hours, but the old type
of English furnace takes much longer, usually two to three days.
------------------------------------------------------------------------
CHAPTER X
LEHRS AND ANNEALING
Owing to the peculiar structure of glass, and its liability to fly or
collapse when exposed to sudden changes of temperature, a process of
annealing becomes necessary in order to produce a more equal
distribution of the tensions throughout the structure of the glass;
otherwise, glassware of any thickness would be in such a state of
tension as to be extremely liable to fracture when passing through any
sudden change in the atmospheric temperature, especially in frosty
weather. In this state it is useless or dangerous for general purposes.
On this account most glasswares undergo a form of annealing at some time
during the process of their manufacture. And in the case of certain
goods, such as table glass, lamp glasses, optical glass, etc., special
care and time are devoted to this process of annealing. Often in the
case of improperly annealed glass, instances are known where its
unhomogeneous structure has suddenly given way as the result of
derangements set up by internal tension. Friction, or rough handling
whilst cleaning, at the ordinary temperature of the atmosphere, is
sufficient to cause a rupture. Therefore annealing cannot be too
carefully attended to.
For annealing the glass manufacturer uses a lehr, which is an arched
tunnel with a fully exposed opening at the exit end and partially closed
at the entrance end, where the goods are introduced. The lehr is heated
at the entrance end to a temperature of about 350° Cent., which
temperature is gradually diminished towards the exit end, which is quite
cool. The hot end, or entrance, should be constantly at a temperature
just short of the actual deformation or softening point of the glass
introduced; usually the entrance is in a position near, or convenient
to, the glass furnace around which the glass blowers make the goods.
In old-fashioned works coal-fired lehrs are used, but they are very
unsatisfactory and difficult to regulate. The heat of the lehrs in
modern works is maintained and regulated by a series of gas burners
situated under the floor of the tunnel or lehr. Along this floor are
placed iron trays linked up with each other to form a continually
travelling track, which gradually moves towards the cold end of the
lehr; these trays are operated by a mechanical jack and gears. As each
tray of goods comes out of the cooler end of the lehr, they are taken
off and conveyed to the warehouses for cleaning and packing, and the
empty tray is sent back to the entrance end to be linked up and refilled
again with fresh goods.
These tunnels, or lehrs, are about 40 ft. long, and as the glasswares
travel through on the trays they are subjected to the gradually
diminishing heat, until they are ultimately removed at the cooler end in
an annealed condition, in which state they are less liable to fracture
in use. The time occupied in travelling through the lehr is usually
about three days. But this period varies according to the nature of the
ware being manufactured. In special glasses, and in the annealing of
optical glass, the glass may undergo a process of annealing that takes
as long as ten days, and in other cases, where the glassware is made
very thin, no annealing at all is necessary. Usually the thicker and
heavier articles require the longest time in annealing. Table glass
which is made thick and heavy for cutting or decoration requires a
little more care and time in the lehr than ordinary plain glassware, as
the abrasive action of cutting quickly develops any latent strains and
causes fracture.
In some works, especially on the Continent, several small
externally-heated kilns are used for annealing, in which the hot
glassware, as it is made, is packed in tiers; when full, these kilns are
closed up and then allowed to cool of their own accord; after which they
are opened and the goods taken out to the warehouse. This is an
intermittent process of annealing, and is quite satisfactory for certain
classes of goods, such as lamp shades, which are usually of equal
thickness throughout their form.
The travelling or continuous form of lehr admits goods of more unequal
thickness in form and variety. Thus, wine-glasses, jugs, and bowls may
be annealed together with less risk of malformation in their shape than
would be present if they were annealed together in kilns. The
manufacturer can, by suitably arranging the temperature of the gas
burners, give more heat to one side of the lehr than to the other. He
then places the heavier goods on the hotter side and reserves the other
for lighter goods, such as wines, etc. They then travel down together
side by side under the most suitable conditions for the annealing of
each class.
Many physical changes take place in the glass passing through the lehr.
One remarkable effect is the slight change in colour which occurs in
glass decolorized with manganese. It is noticed that the glass becomes a
greener tint in passing through the lehr when the decolorization is just
on the margin of efficiency.
The state in which the structure of glass exists when quickly cooled and
the action of annealing might be explained. When glass is quickly
cooled, being a bad conductor of heat, insufficient time is allowed for
the middle or interior portions of the glasswares to settle down and
assume their normal state of solidification. The outer portion, or
crust, will first cool and contract with an enormous strain upon the hot
interior. This difference in the state of tension between the outer and
interior portions gives a want of uniformity, and stresses of tension
and thrust are developed, which cause the whole to collapse with the
slightest external scratch or heat change. In annealing, this strained
or forced condition in the structure of the glass is relieved by
subjecting the glass to a pre-heating, and gradually diminishing the
temperature, allowing a sufficient time for the different layers
mutually to adjust themselves to their comparative normal positions, and
thus relieve the strains within the mass. Much depends upon the
pre-heating temperature and the rate at which the diminution of the
temperature takes place. If this is properly provided for, the best
results are obtained in the stability of the resulting glass. The
presence of any stress can be determined by using a polariscope.
The average British glass manufacturer has little knowledge of the value
of a polariscope, or stress viewer, in ascertaining the physical state
of his glasswares, and until he adopts its use there is little prospect
of an improvement in his annealing methods. Much faulty annealed glass
is being turned out which the glass manufacturer is not aware of, and
which could be avoided by the intelligent use of such a simple
instrument, which detects badly annealed glass at once by the aid of
crossed nicols and a selenite plate.
Owing to the unequal densities of the various silicates present in the
heavy lead and barium glasses, they are more subject to striation and
require more careful annealing than the soda-lime glasses, in which the
silicates present are of more equal density. However, much depends upon
the proper “founding” and melting of such glasses. The use of a larger
proportion of cullet assists in breaking up striation. The presence of
striae or cords in glass disqualifies it for most purposes, as it is
usually found that, apart from their defective appearance, they tend to
produce stresses within the glass.
Transparency, brilliancy, stability, and homogeneity are important
factors in producing perfect glassware, and the proper development of
these distinguishing properties requires considerable skill on the part
of the glass manufacturer, alike from a technical, physical, and
practical standpoint.
------------------------------------------------------------------------
CHAPTER XI
THE MANIPULATION OF GLASS
GLASS MAKERS’ TOOLS AND MACHINES
The tools used by the glass blowers are few and simple. The greater part
of the crude form is produced by blowing out the hot glass into a
spherical or pear-shaped bulb and regulating the size and thickness by
gathering more or less material. The tools are mainly employed in
finishing and shaping this bulb into the desired form, such as shearing,
forming the neck spout, crimpling, and sticking on the handles to the
various shapes made.
According to the type of the goods manufactured, different manipulative
methods in forming the articles are adopted in various works.
The best English table glassware is mostly hand-made blown ware,
generally entirely executed by the handicraft of the workman without the
aid of moulds to form any part of the articles, and a considerable
amount of skill and practice is necessary before the workman is
competent enough to shape a number of articles exactly to the form of
his model. It is astonishing to notice the skill and precision with
which a workman produces wine-glasses one after another, so uniform that
one cannot trace any dissimilarity between them.
A second class, or cheaper form, of tableware is made by blowing the
sphere or bulb of hot glass within a mould, to give some part, or the
whole form, of the desired article. If only a portion of the intended
shape is thus formed by the mould, it is afterwards finished by hand
with tools. This is the general continental method of working, and has
only been partially adopted by this country for making tableware. Where
a number of articles of one shape have to be produced, this is by far
the most economical method. Glass tumblers, honey pots, and rose bowls
illustrate this class of ware.
Another class of tableware produced by a method of pressing the form is
known as “Pressed glassware.” The hot metal is gathered from the pot and
a portion cut off, and allowed to fall into an iron mould fixed within a
lever press, which carries a plunger fitting within the mould formed to
shape the interior and exterior, with the thickness of the glass as the
intermediate space between them. As the hot glass is introduced, the
workman brings down the lever arm and the plunger presses the hot metal
to shape. The plunger is then released and the mould reversed, turning
out the pressed form of glass, which is then carried away to be
fire-polished or further manipulated with tools before it goes to the
lehr. The case or mould portion is made in two halves, to facilitate the
removal of the hot glass after being pressed. Pressed glass tableware
can be recognised by the presence of seams, showing these divisions of
the mould. Many exquisite designs imitating cut-glass tableware are
executed in pressed glassware. The moulds are a very expensive item, as
there is much tool work in cutting the patterns and refacing them after
prolonged use. In making pressed goods, an oily, carbonaceous liquid is
used to give the moulds some protection and prevent the oxidation of the
iron. This liquid is from time to time applied, as the work of pressing
proceeds, by mopping the interior of the mould with a mop dipped in the
preparation.
Another process in glassmaking is that of bottle-making by automatic
machinery, in which the glass worker does little but gather the
requisite quantity of glass from the pot and place it into the revolving
clips of a bottle-making machine, which does the work of formation, by
the aid of compressed air delivered from a supply main. This is largely
of American introduction, and is the method adopted in making common
bottles. In some cases the bottle neck may be finished by a hand tool
after a mould has done its part of forming the bottle. Modern machines
have been perfected to do the whole work of gathering the metal, forming
the shape, and completing the bottle; a number of arms travelling round
a track carry the mould forms, which alternately dip into water to keep
them cool, open to receive the hot metal, close, deliver a requisite
pressure of air to extend the hot glass within the mould, and then
deliver the bottle on to a travelling belt, which takes them to be
annealed.
In the manufacture of bottles by machines, hand labour is practically
eliminated as far as the actual making of the bottle is concerned. The
bottle-making industry is undergoing great changes by the introduction
of such machinery. In some plants a ten-armed machine will produce
automatically 120 gross of 16 oz. bottles in twenty-four hours, at an
average cost of 1s. 6d. a gross.
Owen’s Bottle-making Machines are of this type. Such machines produce
700 bottles an hour, according to their size and the number of arms
fitted to the machine.
As an illustration of a less complicated bottle-making machine, “The
Harlington” may be described.
This machine consists principally of a table, on which is arranged on
the left-hand side a parrison mould, and on the right-hand side a column
with a revolving table carrying two finishing moulds.
[Illustration:
_By permission of_ _Melin & Co._
“THE HARLINGTON” BOTTLE-MAKING MACHINE
]
Below the table, near the parrison mould, is arranged an air cylinder,
through which a piston runs, operated by a hand lever. On the upper part
of the column, on which revolves the table with the two finishing
moulds, is also arranged an air cylinder operated by a hand lever.
The method of working is now as follows—
A gatherer puts the metal into the parrison mould into which it is
sucked by moving the left-hand lever. Through this operation the head of
the bottle is formed and finished. By reversing the lever, air enters
the parrison, thus blowing the same out to the height of the parrison
mould. The parrison mould is now opened and the parrison hanging in the
head-mould held by the tongues is placed under the blowing cylinder
above the open finishing mould. Now the latter is closed, and by moving
the lever, the bottle is blown and finished. Whilst this last operation
is being effected by a boy, the table is revolved and the previously
finished bottle is taken out and another parrison is made ready to be
handled in the described way. This machine produces 200 bottles per
hour.
=The Glass Blower’s Tools.= The glass maker’s chief tool is the
blow-iron. This is a tube of iron 1/2 to 1-1/4 in. wide and about 4 to 5
ft. long, one end of which is shaped or drawn in so as to be convenient
for holding to the lips, and the other end is slightly thickened into a
pear-shaped form, on which the hot metal is gathered.
In making crystal tableware the workman manipulates the glass he has
gathered on this blow-iron by marvering it on a marver. This is a heavy
slab of iron with a polished face about 1 ft. by 1 ft. 6 in., and 1 in.
thick, supported on a low table. Sometimes this marver may be a block of
wood with hollows of definite forms, in which the workman rotates the
hot glass he has gathered to regulate the form and thickness of the
metal to suit his work before beginning to blow it out into a hollow
bulb.
The pontil is a solid rod of iron of similar length and thickness to the
blow-iron. By gathering a little wad of hot glass on the pontil and
sticking it against the end of the bulb attached to the blow-iron, the
workman can detach the bulb from the blow-iron and hold it by the pontil
to which it has been transferred, and which enables him to work on the
other end or opening in the bulb which is exposed in detaching it from
the blow-iron.
[Illustration:
GLASS WORKER’S CHAIR
]
After re-heating the glass, he may shear it with his scissors or shears,
open it out with his pucellas, crimple it with his tongs, measure and
caliper it, or shape it to a template.
Whilst he is doing such operations he sits in a glass worker’s chair.
This chair has two long extending arms, which are slightly inclined, and
along which he rolls his blow-iron or pontil, with the glass article
attached, working upon the rotating form, turning the iron with one
hand, whilst he uses his tools with the other hand, to shape or cut the
glass to its requisite form whilst it is hot, soft, and malleable.
The shears are like an ordinary pair of scissors, and are used for
cutting the hot glass, or shearing off the tops of bowls and wines to
their proper height.
The pucellas is a steel, spring-handled tool in the form of tongs, which
the workman uses to widen, extend, or reduce the open forms of glass by
bringing pressure upon the grips of the tool whilst applying it to the
hot glass.
The glass maker also uses another form of spring tool in taking hold of
hot glass or pinching hot glass to form. These are the tongs.
The battledore, or palette, is a flat board of wood with a handle, used
for flattening and trueing the bottoms of jugs or decanters, etc.
The chest knife is a flat bar of iron, usually an old file, used for
knocking off the waste glass remaining on the blow-irons and pontils
after use. A chest or iron box is kept for collecting such waste glass
for further use. A pair of compasses, calipers, and a foot rule complete
the glass maker’s outfit of tools.
=Making a Wine-glass.= The manipulations in the manufacture of a
wine-glass will now be described. A common mule wine-glass is formed
from three distinct pieces of glass: (_a_) the bowl; (_b_) the leg;
(_c_) the foot.
A wine “shop,” or “chair,” consists of three men; a “workman,” whose
main work consists of finishing the wine-glass; a “servitor,” who forms
or shapes the bulb; a “foot maker,” who gathers and marvers the glass;
and a boy who carries away and cleans the blow-irons.
The “footmaker” of the “chair” gathers on the end of a blowing-iron
sufficient glass to form a bowl. This is then shaped on a marver until
the required shape is obtained. The footmaker then blows this out to a
hollow bulb similar in size to the pattern to which he is working. When
the bulb leaves the footmaker it is the shape of the bowl of the
wine-glass.
This is then handed over to the servitor, who drops a small piece of hot
glass on to the end of the bulb, and heats the whole by holding it in
the furnace. This serves to make the joint of the two pieces perfect.
The servitor next proceeds to draw out the leg from the small piece of
glass at the end of the bulb, leaving a button of glass at the end of
the leg. The servitor then dips the end of the leg into the molten glass
within the pot and gathers on sufficient glass to form a foot. He
spreads this portion of the glass out to the required shape and size
with a pair of wooden clappers, with which he squeezes the hot glass to
form the foot.
The servitor has now done his part of the work, and the glass is handed
to the workman. It is then cracked off, and the foot caught by a spring
clip arrangement attached to a pontil, called a “gadget.” The workman
now re-heats or melts the top edge of the glass by holding it within the
furnace, and when it is hot he cuts off the surplus glass with a pair of
shears. A line is chalked on at the correct distance from the foot, and
guides the workman in cutting the glass to the proper height. He then
melts the top again and opens it out with his spring tool to the
required shape, after which the glass is taken to the annealing lehr by
the boy, to be annealed.
Other forms of wine-glasses are made, and various methods are adopted,
according to the district and class of workmen.
For instance, the method of making the above common mule wine-glass
varies in different districts. Instead of gathering the metal for the
foot upon the leg of the glass, the workman may drop a piece of hot
glass, which has been gathered by the servitor, on to the button at the
end of the leg, and by means of a pair of wood clappers spread the hot
glass to form the foot.
In another method of making a wine-glass, the stem or leg is drawn out
from the body of the bulb by pinching down a knob at the end of the
glass. The servitor draws the leg out of this knob and knocks off the
extreme end. Meanwhile, the footmaker has been preparing a foot,
gathering a small portion of metal on a blow-iron and blowing it out and
shaping it into a double globule. The end globule forms the foot and the
second merely acts as a support. The footmaker takes these globules, and
the servitor sticks them on to the drawn stem of the wine whilst it is
hot; the blow-iron holding the globules is knocked away, leaving them
adhering to the leg of the wine-glass. The footmaker then knocks off the
second globule at the line between the two and, re-heating the bulb at
the foot of the glass, opens and widens the edges out. The glass then
goes to the workman to be finished in the same way as the common mule
wine-glass.
Many articles of glassware are formed with the aid of moulds. Take as an
illustration the manufacture of tumblers and honey pots. A quantity of
glass is gathered on the blow-iron, marvered, and blown out into an
elongated bulb, which is introduced into a mould divided in two halves,
which open or shut by hinges, a handle being fixed on either half to
facilitate the operation. The interior of the mould is made to the shape
of the article, and as the bulb of hot glass is introduced it is shut,
and the workman blows down his blow-iron and extends the glass until it
expands and fills the space within the mould, giving the complete form
of the article with a surplus of metal just where the blow-iron is
attached to the glass at the top. These tops are then cut off and
finished, either by the workman re-heating the article by attaching the
bottom to a pontil and shearing off the top edges, or the glass is
annealed in its unfinished state and the top surplus portion cut off by
an automatic machine specially constructed for cracking off such goods.
[Illustration:
GLASSWARE BLOWN IN MOULDS SHOWING PORTIONS CRACKED OFF
(_a_) Tumbler.
(_b_) Honey Pot.
]
Such machines consist of a set of revolving tables upon which the glass
articles are centred, and each in turn revolves in front of a thin,
pointed, hot jet of gas flame, which impinges on the glass at the height
at which the glass is to be cracked off. After one or two revolutions in
front of this hot pencil of flame, it is removed, and, by applying a
cold steel point so adjusted as to touch the part where the jet has
heated the glass, a chill is imparted which causes the upper portion of
the glass to crack away in a clear, sharp line round the glass. This top
portion of surplus glass is thrown aside and returned to the furnace for
re-melting as cullet.
The tumbler or honey pot is then conveyed to another machine which
fire-polishes the edges to a smooth finish.
This machine consists of a circular revolving frame carrying small
supports, which themselves rotate on their own centres. Upon each
support an article is placed to be fire-polished and the frame carries
them round, and they travel into another section of the machine, passing
under a hooded chamber, which is heated by a fierce jet of flame. The
jet of flame, which is localised on to the top edges of the tumblers or
other goods passing through the hood, gives just sufficient heat to melt
and round off the sharp edges of the glassware where they have been
cracked off by the previous machines. By using these machines in this
way labour is considerably economised, and as many as 300 or more
articles an hour can be cracked off and fire-polished with unskilled
labour.
These machines are extensively adopted in the manufacture of electric
light bulbs, shades, lamp chimneys, and tumblers.
Moulds are usually opened, shut, and dipped by boys, but in up-to-date
glass works an automatic machine called a “Mechanical Boy” is used. With
this machine, the mould is operated at the desire of the workman and not
at the desire of the boy. The output is considerably expedited by the
use of these automatic devices for opening and shutting the moulds.
It is obvious that whatever the shape of the mould, or whatever the
design within the case, the glass takes the impression and retains it in
after working. In this way, square sections, fluted indentations, or
raised bosses can be formed with facility and regularity.
[Illustration:
_By permission of_ _Melin & Co._
VERTICAL CRACKING-OFF MACHINE
]
The Glass Workers’ Union consider that the introduction of machinery
deprives men of their independence and right to work, but as yet the
glass blowers have been always fully occupied with useful work about the
factories in which such machines have been introduced, so it cannot be
said that they have been forced to be idle.
The advantages possessed by these automatic machines in their larger
output at so much less cost compared with hand labour is the great
factor in inducing their adoption; and in these days of progress and
competition such machines enable the glass manufacturers to cope with
the increasing demand and go far towards bringing a factory up to date
and making it well equipped.
Manufacturers should certainly turn their attention to these mechanical
methods, as their use is quite general on the Continent and in America,
and by their use the metal can be worked out of the pots or tanks much
more quickly, increasing considerably the turnout or capacity of the
furnace against the fuel consumption. Much of the glassware imported
into this country is composed of such articles as would have been
manipulated by machines, and, unless a similar method of manufacturing
them is adopted here, we cannot hope to compete with other countries in
supplying our own needs. In the writer’s opinion, it is mainly due to
the adoption of machinery for producing glassware that the continental
people have been enabled to undersell us in our own market, and English
manufacturers could produce at a much cheaper rate if they would only
adopt similar methods of manufacture and the gas-fired furnaces as used
abroad.
------------------------------------------------------------------------
CHAPTER XII
CROWN, SHEET, AND PLATE GLASS
The glass used in windows may be either crown, sheet, or plate.
=Crown Glass= is made in the form of circular flat discs about 4 ft. in
diameter. The workman, by repeated gatherings, collects sufficient glass
on the end of his blow-iron until he has a mass approximately 10 or 14
lb. in weight, which he marvers into a pear-shaped lump by rotating the
hot glass in the hollow of a wooden block. He then blows the glass into
a spherical bulb (_a_), which, by quick rotation, is widened and assumes
a mushroom shape (_b_). Another workman attaches a pontil to the outer
centre of this bulb by welding it on with a small portion of hot metal.
The blow-iron is then detached by wetting and chilling the glass near to
the blow pipe, which breaks away, leaving an opening in the bulb where
it has become detached (_c_).
This is then carried to an auxiliary heated furnace, which has a wide
opening emitting great heat, and by resting the pontil upon a convenient
support and rotating it quickly the action of centrifugal force and heat
causes the glass to spread out at the opening, which becomes larger and
larger until the glass finally opens out into a flat circular disc of
fairly even thickness throughout, with the pontil still at the centre,
forming a bullion point or slight swelling, due to the knob of glass
used in affixing it (_d_).
Next, the workman, keeping the disc in rotation, brings it away from the
furnace and allows the metal to stiffen and set by cooling, when it is
carried to the annealing oven and detached from the pontil. The discs
are then stacked up for annealing. When annealed, these are afterwards
cut across in sections or squares of convenient size by using a glass
cutter’s diamond.
[Illustration:
FOUR STAGES IN CROWN GLASS-MAKING
]
It is evident that the centre portion, containing the bullion point or
bull’s eye, is useless for plain window glazing, but occasionally these
are sought after by glass decorators for use in coloured leaded lights
for door panels, etc.
=Sheet Glass= is made in the form of thin, walled, hollow cylinders of
glass, which are split along their length and round the cap and then
opened out by heat and allowed to uncurl until each sheet lies out flat.
The workman gathers a sufficiency of glass upon his blow-iron by
repeated gatherings, and marvers it into a ball about as big as one’s
head. This is blown out (_a_) and widened by rotating the blow-iron
until he gets a mushroom shape (_b_), with a heavier bulk of glass at
the extremity than at the sides.
[Illustration:
SIX STAGES IN SHEET GLASS-MAKING
]
This extra thickness of glass at the extremity of the bulb tends to
lengthen the bulb of glass as he swings it in a pendulum fashion, and by
blowing and swinging it alternately he gets an extended form (_c_).
To permit the workman to swing the mass of glass out conveniently to the
full length of the intended cylinder, a long, narrow pit or trench is
provided below the floor level, and by standing alongside this trench
the workman is enabled to swing the glass within the trench at arm’s
length until the requisite length and width of cylinder are obtained.
This work requires a high degree of skill and strength. The shape of the
cylinder of glass is now as shown on page 91 (_d_).
The extremity of this cylinder is now re-heated and opened with the aid
of a spring tool with charred wooden prongs, until the opening is
enlarged and drawn out to the same diameter as it is throughout the
cylinder. It is now in the form of an open-ended cylinder (_e_).
The cap of the cylinder at the blow-iron end is now cracked off. A
thread of hot glass is wrapped round the shoulder near the cap, and the
line chilled by using a curved, hook-shaped rod of iron. Whilst the cap
is being cracked off, the cylinder is allowed to rest supported by a
wooden cradle.
The cylinder is now open at both ends (_f_) and is taken to the
flattening kiln or furnace. This kiln has a level, smooth floor, heated
from below, upon which the cylinders are flattened out. Placing the
cylinder on the floor in front of him, the workman places along the
inside length of the cylinder a long red-hot iron rod touching the
glass, and then chills the line with a touch from a cold iron rod. This
causes a split to take place along the whole length of the cylinder. As
these cylinders are split open, they are removed to a hotter zone within
the flattening kiln, where the heat causes the cylinder to uncurl and
gradually flatten out.
As the sheet becomes flat the workman levels it out with a flat block of
charred wood called a polisher. This is attached to a long handle, and
is rubbed over the face of the sheet of glass. The weight of the wooden
block is just sufficient to smooth out any creases and assists in
levelling out any irregularities of the surface. It is essential that
the floor upon which the glass is resting should be perfectly smooth and
level, and uniformly heated. As each sheet is levelled, it is removed to
the annealing oven and afterwards stacked up until cool, after which the
rectangular sheets are cut up to the various sizes required for window
panes.
It is evident that the crown glass method gives more waste in cutting
up, and does not provide such large sheets as the cylinder method. On
the other hand, cylinder glass always shows a certain amount of waviness
on the surface, and is not so brilliant as crown glass. The better
surface of crown glass no doubt is due to the fire-polishing it receives
when being expanded out into the disc. It appears to be somewhat
difficult to get a perfectly smooth level face to cylinder glass by
using the wooden polisher.
=Plate Glass= is used as mirror glass and in glazing shop windows and
showcases. It may vary between 1/4 and 3/4 in. in thickness, and is more
expensive to produce than crown or cylinder glass.
In the manufacture of best plate glass, the materials are melted in open
crucible-shaped pots of varying sizes; sometimes, in making large, heavy
plate, their capacity reaches 25 cwts. of metal. When the metal is plain
and clear from seeds it is either ladled out into smaller crucible pots
for casting, or, as in the case of casting large sheets, the whole
crucible of metal is lifted bodily out of the furnace by means of a
crane, and, after being skimmed, is conveyed by an overhead travelling
derrick to the casting table.
This table is a level iron bench the size of the plate to be cast, the
face of which consists of thick sheets of iron plate riveted together to
form a level top; along the whole length of each side of this table is a
raised flange of a height sufficient to give the thickness of the plate
of glass to be cast: resting on these two outer edges a long, heavy
metal roller runs, covering the full width of the table. The crucible of
hot metal is brought to a convenient position and the contents poured
out on the table in front of the metal rollers. These rollers then
travel along and squeeze or roll out the hot metal over the surface of
the table to the thickness regulated by the side pieces, which also
prevent the metal from flowing over the sides. The empty crucible is
then conveyed back to the furnace for refilling.
The cast plate of glass is then trimmed from any excess of glass at the
ends, and when set and stiff is lifted at one end slightly and pushed
forward into a conveniently situated annealing oven, where it is
re-heated and subjected to a gradually diminishing temperature to anneal
it. The plate of glass, as delivered from the annealing oven, shows
surfaces somewhat rough, wavy, and uneven, from the marks left by the
table and the roller, and it has to be ground and polished level and
smooth on both sides. This is done by fixing one face of the glass plate
in a plaster of Paris bedding and setting it within a mechanical
grinding machine.
This machine carries several revolving arms, to which are attached other
smaller plates of glass. These are used as the rubbers, a slurry or
paste of sharp sand and water, or abrasive powder, being interposed
between the two. The revolving circular motion of the arms causes a
grinding action between the two plates, which wears down any
irregularities and gives a more even face. After this, finer grades of
abrasive materials are employed, and, finally, polishing powder, until
the face of the glass plate is polished smooth and level. The large
plate of glass is then reversed and the process of grinding resumed on
the other side.
Much care is necessary in handling these large plates, and every
attention is necessary and devoted to get the largest pieces of plate
without defects. All portions showing defects have to be cut away, and,
consequently, reduce the size of the plate when finished.
In another method of making plate glass the molten metal is fed between
two or more parallel rollers, which are spaced apart to the thickness of
the glass required (about 1/4 in.). These rollers squeeze the glass out
to a uniform thickness. A roughly decorated surface is sometimes given
to this glass intentionally, by the metal rollers being indented with
some form of set star pattern. This glass is not ground or polished, and
is sold under the name of muffled or cathedral glass. It is mostly used
for roof lighting, where the transparency may be somewhat obscured.
Wired glass, or strengthened plate, is formed by embedding in the soft
glass, whilst being rolled, a network of metallic wire of special
composition to suit the temper of the glass. This wire is fed from a
separate roller into the space between the parallel rolls as the hot
metal is fed in from either side. It is necessary that the wire should
be made from a metallic alloy which is not easily oxidised. Another
method of strengthening plate glass consists in sealing together two
plates with an intersecting film of celluloid.
A decorated coloured rolled plate is made for use in leaded lights by
mixing portions of several differently coloured glasses together in a
small pot and slightly agitating the contents so as to intermix the
respective colours. When the glass is rolled out, a pretty agate or
marbled effect is obtained, due to the distributed coloured glasses
becoming intermixed. As a rule, these glasses are more or less
opalescent, and are only used for decorative purposes, church lights,
etc.
------------------------------------------------------------------------
CHAPTER XIII
TUBE, CANE, AND CHEMICAL GLASSWARE
Laboratory and chemical glassware consists of thin blown ware in the
form of flasks, beakers, test tubes, etc., used in chemical operations.
Most of these goods are blown in hinged moulds mechanically or
automatically operated by the worker. The lips and flanges of the necks
are neatly formed afterwards by re-heating and working the edge to a
form allowing them to pour cleanly, and prevent any fluid contained
therein from running down the sides of the flask or beaker whilst in
use. The heavier glassware, in the form of desiccators, measuring
cylinders, specimen jars, and three-necked bottles, are made by
handwork. Chemical apparatus has necessarily to be made from a permanent
stable highly refractory glass, so as to resist the solvent actions of
mineral acids, alkaline solutions, and boiling water, as well as sudden
changes in temperature.
The manufacture of tube and cane glass for various purposes forms a
large and extensive portion of the glass trade. Considerable quantities
of tube and cane glass in various sizes are used by lamp workers in the
manufacture of certain forms of chemical apparatus and filling electric
light bulbs. By re-heating glass tube and working before a blow-pipe
flame, the various forms of test tubes, pipettes, burettes, soda-lime
U-tubes, and condensers are made. Generally, for chemical apparatus two
classes of tube are made, one a soft soda tube, and the other hard
combustion tubing. Particular care has to be devoted to the grading and
sorting of the various sizes. The bore of the tube, the thickness of the
walls, and the outside width have all to be checked and the lengths
classed accordingly.
In the manufacture of tubing, unless the glass is of large size or great
thickness, it is not annealed, and shows a case-hardened condition which
materially increases the strength of the tube to resist internal
pressure, as is the case with boiler gauge tubing. In the manufacture of
apparatus from tube and cane, care must be taken that the various pieces
used in welding together the different portions of the apparatus should
be of the same temper and composition, and supplied from one source, so
that they may join and work perfectly together.
The lamp worker or glass blower should take care to get his supplies
from a reliable source, so that the glass pieces will be adapted to work
together. Trouble occurs when odd tubings from various makers are worked
together. The same applies to fancy glass working, where various
coloured canes are worked into ornaments. Reputable firms can always
supply from stock such colours and tubing properly adapted for their
specific purposes, and they take every precaution to see that the
various colours join and work together. Supplies of glass rod can be had
that will join on to platinum, nickel, iron, or copper wire with sound
joints.
In making cane glass, the workman gathers sufficient metal upon a
pontil: for thin cane he would gather less than for heavy thick cane.
After gathering, he marvers the metal into the form of a solid cylinder.
Meanwhile, an assistant gathers a little metal on a post or pontil with
a flattened end. The metal he has gathered has covered the flat end of
the post, and he holds this in readiness for the workman, who is now
re-heating the cylinder of glass at the pot mouth. As the cylinder of
glass becomes soft, he withdraws it and allows the end of the
cylindrical shaped mass of glass to fall gently upon the flat end of the
post, to which it adheres. They then carry the glass between them to a
wooden track or run-way, along which they walk at a smart pace in
opposite directions; stretching out the hot glass between them, it
gradually thins out and rests on the floor. The pace the men separate
apart from each other is regulated according to the thickness of the
cane desired: for very thin cane a smart trot is necessary, but for a
thick cane a slow walk is sufficient. As the glass is drawn out it is
allowed to rest on wooden supports, and when cool is cut up into
convenient lengths by scratching the glass with a steel file. These
lengths are collected and bundled up for sorting and classification. All
portions distorted or over-size are returned as cullet for re-melting
and re-use.
In tube making, instead of a solid cylinder as in cane making, the
workman, by gathering the glass on a blow-iron and blowing and marvering
it, obtains a thick-walled, hollow, cylindrical form. This is re-heated
and the end stuck to a post and drawn apart as before described in cane
making, forming a tube of a width proportional to the rate the two have
travelled apart in drawing it out, and to the quantity of metal
gathered. In this way the respective sizes and thicknesses are
regulated. A narrow cane or tube may be drawn out for 300 ft., but for a
thick or wide one probably only 30 ft. may be drawn. In making the
larger widths, some method of cooling, or fanning, is adopted, to ensure
uniform size by cooling the hot glass quickly as it is drawn out. It is
evident that, whatever shape is given to the original mass of glass
whilst being marvered, the tube will bear a similar shape in proportion,
either within or outside the glass. In this way, square, triangular, or
oval sections can be produced in both tube and cane.
The manufacture of white opal, coloured cane, and tube is carried out on
like methods to those used in ordinary cane and tube making.
We will now describe the manufacture of Filigree. This is rod or tube
containing opal or coloured threads, either straight, spiral, or
interlaced within a transparent glass; these threads follow the whole
length of the cane or tube.
This curious form of glasswork was originated by the Venetians, who are
exceptionally skilled in producing some elegant and ornamental filigree
decorated glassware.
The method of producing filigree cane consists of first taking a number
of short lengths of opal or coloured cane previously drawn and cut to
about 6 in. lengths. These are then placed in vertical positions around
the inner circumference of an iron cup mould, which may be about 5 in.
in diameter. The opal strips of cane are supported vertically in small
recesses provided in the rim of the mould at equidistant intervals. A
ball of hot crystal glass is gathered on a pontil and is lowered into
the inside of the mould, the hot metal coming in contact with the opal
strips of glass adheres to them, and upon withdrawing the glass it
brings the opal strips away with it arranged in sections round the
circumference of the ball of glass. This is now re-heated and marvered
until the canes or strips of opal are well embedded in the hot glass.
Then the workman gathers another coating of hot glass over the whole,
marvers it again into a cylindrical form, and then proceeds to draw it
out as described in cane making.
If a spiral form of lines is desired, the workmen, whilst drawing out
the cane, turn or twist the pontil and post in contrary directions.
These rotations cause the opal veins or threads to assume a spiral or
twisted form within the glass. Various coloured cane may be used in the
above process, and by placing them in alternate positions to the opal
strips within the cup mould some very pretty and curious filigree work
is obtained. These twisted filigree canes are used and manipulated over
again in the process of making the various Venetian goblets and wine
stems. Some fine effects in the application of filigree decoration can
be seen in the specimens of Venetian glassware exhibited in the British
Museum.
Millefiore work is produced by the workman, first spreading a layer of
an assortment of small coloured glass chips of varying sizes (between
1/8 and 1/4 in. cube) over the face of the marver, and then taking a
gathering of crystal metal on his blow-iron and rolling the ball of hot
glass into the coloured mixture on the marver. The hot glass collects up
a coating of the coloured chippings, and is then re-heated and again
marvered, another gathering of crystal metal is made, which incases the
whole. This is then blown out and worked into some form of ornament,
such as a paper weight, inkpot, or bowl, producing a curious result that
shows blotches of colours embedded within the glass, the effect of which
is increased if a backing of opal glass has been used in the first
gathering: this shows the coloured effect against a white background.
Spun Glass. Another curious form of glass is the spun glass which is
much employed in making fancy ornaments. Glass can be spun into a thread
so fine and flexible that it can be worked into a fabric like any
textile material. In this way, glass ties can be made by plaiting the
spun glass threads into the required form. Spun glass fibre is used in
making the brushes used for cleaning metals with acids. On account of
its greater resistance to acids than is shown by ordinary cloth, an
endeavour is being made to use spun glass cloth in certain industries as
a commercial application. Spun glass is used for making a form of filter
cloth which is being used successfully in filtering acid residues in
certain chemical processes, and, no doubt, when the elasticity and
strength of the glass threads can be more developed, the scope for its
use in other industrial processes will be increased.
The method of making spun glass thread consists in melting the end of a
plain or coloured glass rod (which may be square, round, or triangular
in section) in a blow-pipe flame and grasping the end which is melting
with a pair of pincers, drawing it out and affixing it to a wooden drum,
which is turned rapidly away from the glass being heated. The drum may
be 2 or 3 ft. in diameter, and as the glass is continually fed into the
heat it is drawn out into a very thin thread by the rapidly revolving
drum, and coiled up until a sufficient quantity has been obtained. The
thread is then cut across the drum, collected, and used for plaiting or
braiding into the fabric or cloth.
The iridescence and variety of colours yielded by the refraction of
light between the glass threads gives spun glass its peculiar effect,
very evident in the forms in which it is used in decorating small
ornaments such as forming the tails of glass birds.
Glass wool is made in a somewhat similar way, and is successfully used
as a non-conductive packing material for insulation from heat.
Glass frost or snow is made by blowing small gatherings of glass out to
a bursting point. These very thin shells are then crushed and the flakes
collected, and used for such purposes as surfacing sand paper or
decorating Christmas cards, being sieved to the requisite size and
affixed with a siccative to the paper.
Dolls’ eyes and artificial human eyes are made by well-trained operators
working before a blow-pipe flame and manipulating tube and cane of
delicately coloured tints to form the pupil and shell of the eye, the
veins being pencilled on with thin threads of red-coloured glass. A
considerable amount of skill and adaptation is necessary to do this
class of work, and much depends upon the matching of the coloured cane
glass used to give the natural effects. When properly made, so clever
and natural are these glass imitations of the human eye that it is with
difficulty that the ordinary observer can tell that they are not real. A
skilled worker will make the artificial eye to fit the muscles of the
socket and so move. In this way much ingenuity has been shown in fitting
the eye sockets damaged during the war.
Aventurine is a golden coloured glass containing minute yellowish
spangles or crystals reflecting upon each other and giving its peculiar
effect. This glass is obtained by the use of an excess of copper with
strong reducing agents in the glass, whereby the copper is partially
reduced within the glass, giving the pretty spangled effect. This glass
is often used in the form of jewel stones, being cut and polished and
fitted in ornaments. The process of making this glass was originated by
the Italians, and for some time it remained a secret with them, and even
now is styled “Italian aventurine.”
Chrome aventurine is another form, giving a green, spangled effect. This
is got by using an excess of chromium in the presence of reducing
agents.
The successful production of aventurine depends upon slowly cooling the
molten glass so as to assist crystallisation.
Mica schist, or flake mica, is used to give another curious effect in
glass. A gathering of some dark-coloured glass is rolled or marvered
upon a thin layer of flaked mica, and then a further gathering or
coating of clear crystal metal is made. The whole is then blown and
formed into some fancy ornament or vase. When finished, the glistening
mica flakes show through against the coloured background, giving a
curious silvery reflection.
------------------------------------------------------------------------
CHAPTER XIV
OPTICAL GLASS
The manufacture of optical glass forms a very important section of the
glass industry, and presents some of the most difficult problems the
glass maker has to deal with. It is in this section of the glass trade
that applied physical and chemical science becomes of the utmost
importance to the manufacturer. The production of optical glass is
impeded by any defects which become evident in the structure of glass
when examined under a polariscope. The presence of any striae, seeds, or
stresses within the structure of the glass disqualifies it for any
important optical work. It is a difficult matter to get pieces of
optical glass only a few inches in diameter of the right optical
constant and refractive index that are homogeneous enough to allow of
the light rays passing without some dispersion when set up for use. It
becomes necessary, therefore, to achromatise one glass with another in
the form of doublets to correct aberration. A high degree of
transparency and durability is necessary in all optical glasses.
The persistent evidence of stresses developed in the solidification of
the glass upon cooling, even when the glass is slowly and carefully
annealed, is a most difficult factor to deal with. In annealing optical
glass, the various temperatures and time periods have to be delicately
adjusted and controlled, or big losses result. Even then many efforts
may be made before a suitable piece of glass is obtained, and the costs
keep accumulating with each attempt, and some idea of the amount of
labour involved in the undertaking to produce optical glass at once
becomes evident. The use of decolorizers and impure materials is not
permissible, on account of the absorption and consequent resistance to
the passage of light rays. The annealing, instead of occupying one or
two days, is sometimes extended over a course of ten or fifteen days, in
order gradually to relieve any stress present. The pots in which the
glass is melted may only once be used, as the glass is usually allowed
to cool down gradually and undergo the process of annealing within the
pot.
The temperature of the furnace is controlled by regulating the draught
by means of dampers in the main flues, arranged to act so as to carry
out the annealing of the glass within the furnace. The regulation of the
temperature within the furnace is of the greatest importance; if too hot
the glass dissolves the clay of the pot, and if retarded too much it
gives difficulty in freeing the metal from seeds, and plaining or fining
the glass properly. Small furnaces containing one or two pots give the
best results. These furnaces are worked on an intermittent process of
first melting the glass and then gradually cooling to anneal the glass
within the pots in mass, the furnace being allowed to die out gradually.
When cool, the pots are broken away from the glass, which is then
cleaved into lumps. Each lump is carefully examined for any defects and
the best pieces selected for re-annealing. These are afterwards ground
to the desired shape in the form either of a lens or prism. The chances
are that not many pieces of perfect glass can be obtained from each pot
of metal, and probably out of a whole pot only a fifth would be suitable
for use after the process of selection and cleaving has taken place.
In the manufacture of optical glass, batch materials are chosen that do
not differ greatly in specific gravity. Every effort is devoted to
obtain the purest materials possible; the batches are finely ground and
well mixed before melting. The glass-melting pots should be made of the
purest and most refractory obtainable in order to prevent the solution
of any impurities into the glass whilst it is melting. In heating the
pots for melting optical glasses every endeavour is made to heat them
equally all round the top, bottom, and sides, so as to dissolve all
portions of the glass evenly and completely together. At times the
melted glass is stirred with a bent iron rod encased in a porcelain
tube, and the glass agitated in order thoroughly to mix the components
whilst fusing, and keep the composition of the glass as uniform as
possible. After the metal has melted and plained clear from all seed and
cords, the pot of metal is annealed, and when cooled the glass is
extracted in lumps and examined for any defective pieces, which are
rejected. The selected pieces are afterwards ground to the desired shape
and, if necessary, re-annealed. In this process the pots being used only
once, are expensive items, and they considerably increase the cost of
production.
Before the war the optical glass trade was confined to a few firms in
this country, who supplied only a fraction of our needs. We have been
dependent mostly upon continental supplies of optical glass, and it is
only quite recently that Government state assistance has been
forthcoming in giving scientific aid to manufacturers by investigating
and reorganising this section of the glass industry. It is to be hoped
that this state assistance will continue, and that the optical branch of
the glass trade will be perfected to such an extent that we may in
future be independent, and produce for ourselves all the optical glass
requirements of our navy and army. It is to be regretted that this
industry did not receive state assistance before the war. If it had, we
should certainly have been better prepared and equipped than was the
case at the start of the Great War.
------------------------------------------------------------------------
CHAPTER XV
DECORATED GLASSWARE
Certain methods of decorating glass are carried out whilst the glass is
being made by the workmen. Other methods consist in decorating the glass
after it has been made, such as cutting, fluting, etching, engraving,
and enamelling. In another form of decoration the method consists of a
combination of two or more of the above processes. The crystal glass may
be cased over with a thin covering of coloured glass by the glass
worker, and this outer coloured casing cut through by the glass cutters,
exposing and showing through the colourless crystal underneath with very
effective results.
A small portion of coloured glass, such as citron green, topaz, blue, or
ruby metal is gathered from the pot by an assistant, and the workman,
gathering a ball of crystal glass on his blow-iron, allows a portion of
the coloured metal held by the assistant to fall or drop upon the ball
of crystal. Upon blowing the whole out, the coloured metal is spread as
a thin casing upon the outside of the bulb of crystal. This bulb is then
worked into a wine-glass or other article, which, after annealing, is
sent to the glass cutter, who decorates the outer surface by cutting the
glass on his wheel. The colourless glass then shows through against the
coloured surface where it has been cut to the pattern, the colour
standing out in relief.
In another form of decoration, the workman allows small pear-shaped
tears or drops of coloured glass to fall upon the outer surface of a
bowl or vase, in equidistant positions round the circumference of the
article, By placing and working the coloured glass into position in this
way, some pretty artistic results are obtained, dependent upon the skill
and artistic taste of the workmen.
In another method of decoration, certain coloured glasses are used, the
composition of which causes them to turn opalescent upon re-heating the
glass to a dull red heat. The re-heating of the tops of crimpled flower
vases made from such glass gives pretty results, showing a gradual
fading opalescence, extending from the top edges to a few inches down
the vase, into a clear coloured glass at the foot of the stand. A
similar effect, without the opalescence, is obtained by the workman
gathering a small piece of coloured glass on the tip of his blow-iron,
and then taking a further gathering of clear crystal metal. The whole is
then blown out and worked into a vase or wine-glass, thus obtaining a
coloration denser at the top edges, where the vase or wine-glass has
been sheared off, and gradually fading away to a colourless glass a few
inches towards the foot, which is clear crystal.
There are also certain compositions which, when worked into a vase, and
re-heated on the edges, strike or turn to a colour such as pale blue or
ruby. These are self-coloured glasses, in which the colouring remains
latent until the glass is re-heated, like the opalescent glasses. In
these glasses the composition is the more essential factor.
=Glass cutting= is an effective way of decorating glassware. In using
this method, the crystal glassware is made fairly heavy and strong, so
as to permit of the deep cuttings which refract the light and show up
the prismatic patterns so brilliantly.
[Illustration:
MACHINE FOR SMOOTHING BOTTOMS OF TUMBLERS
]
In cutting glassware, the glass cutter works in front of a rotating disc
of iron carried in a frame. This wheel has a bevelled edge upon which a
fine jet of sand and water is allowed to drip from a tundish above. The
abrasive action of the sand cuts into the glass, and the workman, by
holding the glass dish or bowl against the wheel, follows the design or
pattern in diagonal lines across the article. These cuttings are
recrossed, and the intermediate diamond spaces filled in with lightly
cut set patterns, until the whole of the intended design is “roughed”
out over the surface of the glass, after which the glass is taken to
another frame carrying a stone wheel, which is of much finer abrasive
action. This stone wheel smooths the rough cuts done by the previous
wheel. After this the cuts are polished successively on a wood wheel and
brush with polishing powders, until a smooth and polished cut is
obtained.
As the value of the glass is greatly increased by cutting, only the best
and clearest articles of table glass are so treated. The work of cutting
becomes technical and expensive, according to the richness of the
cutting demanded. The crystal table glass made from lead gives the most
brilliancy in cutting. Soda-lime glasses are found to be hard to cut and
do not give such brilliant and prismatic effects as the glass made from
lead compositions.
An automatic machine for grinding, smoothing, and polishing the bottoms
of tumblers, etc., “bottoms” or grinds, smooths, and polishes tumblers
at the rate of 2,000 a day. Four vertical revolving wheels are fixed
within a frame, one iron, two stone, and one wood. Over each of these is
a rotating spindle carrying the tumbler so that the bottom of it is
automatically pressed against each vertical wheel in turn. The first
wheel does the roughing, the two next the smoothing, and the fourth the
polishing. These machines are simple and require only unskilled labour
to operate, and go far towards cheapening production.
=Glass engraving= and intaglio work is a much lighter and more artistic
method of decorating glass than the deep cutting before described. In
these processes the glass is cut or ground to a less extent, and a more
free treatment of design is possible. Floral ornamentation and natural
forms of applied designs can be carried out, and portions may be left
rough or polished, according to the effect of light and shade required.
The workman, whilst engraving, works before a small copper or metal
wheel rotating in a lathe, and uses fine grades of emery or carborundum
powders made into a paste with oil, as the abrasive medium. The frame
turning these wheels is like a lathe, and may be worked by a foot
treadle. The wheels are interchangeable, and an assortment of various
sizes, having different bevelled edges, is kept at hand in a case, from
which the engraver selects the one most suitable for the particular work
to be done.
Glassware for engraving and intaglio may be made much lighter than that
required for cutting.
=Etching= is a method of decorating glass by the chemical action of
hydrofluoric acid. This acid in its various combinations attacks glass,
decomposing its surface and giving a dull or semi-matt effect. Only
those portions of glass which constitute the design are exposed to the
acid paste or fumes. The other portions are protected by a covering of
beeswax, which is unaffected by the acid and protects any portions
covered by it.
[Illustration:
GLASS ENGRAVING
]
The process carried out is varied in many ways. In some cases pantograph
and etching machines are introduced to give the designs. A warm copper
plate, with the design or ornament engraved thereon, is covered with a
wax paste, and the surplus cleaned off with a palette knife or pad of
felt, leaving the paste in the recesses of the engraving; a piece of
thin tissue paper is laid over the engraved plate and takes an
impression of the design in wax. This tissue is then transferred to the
glass to be decorated, the wax design adheres to the glass, and the
paper is drawn away. A further resist or coating of wax is painted round
the design to protect the rest of the glass, and a paste composition
giving the action of hydrofluoric acid is applied, which after a short
time eats into the exposed portions of glass. After another short
interval, it is washed off, and the wax coating removed by washing the
glass in hot, soapy water. The design then appears in a matt state
against the clear, unattacked glass.
The mechanical method of etching the design is carried out by first
dipping the whole glass into a bath of hot liquid wax, allowing a thin
coating to set and cool upon the surface of the glass. The article is
then introduced into a machine which has a number of needles, worked by
sliding gears in an eccentric fashion. These needles are adjusted just
to scratch away the thin coating of the wax into a design, and expose
the glass in the form of a decorated scroll or band round the glass. The
glass is then dipped into a vat or bath of dilute hydrofluoric acid for
a few minutes, after which it is removed and washed, and the wax
recovered by heating the glass upon a perforated tray, when it melts and
runs off the glass, and is collected for further use. The article is
then washed and cleaned and shows the scroll or etched portions where
the needle has traced the design. Another effective result is obtained
by etching a design on the back of a plate glass panel. After cleaning
and silvering or gilding the back, the design appears in a matt silver
or gilt lustre upon viewing it from the front of the mirror.
Glass which has been sand-blasted has a similar appearance to etched
glass, but a rather coarser surface. The portions of the glass plate to
be decorated are exposed to the action of a blast of air, into which
fine, sharp-grained quartz sand is automatically fed. An abrasive
action, due to the force with which the particles of sand are blown
against the glass, takes place, rendering the surface opaque or matt.
This method is generally adopted in printing trade names or badges upon
bottles, etc. A stencil of parchment or lead foil is cut out to form,
and used to protect the glass and resist the abrasion where required.
Rubber gloves are worn by the operator. The work of sand-blasting is
executed within a small enclosed dust-proof chamber fitted with glass
panels. The operator manipulates the glass through openings in the sides
of the chamber. The air blast is supplied by a motor-driven air
compressor and is regulated by a foot pedal. The action is very sharp
and quick, and is a cheap and effective way of badging hotel glassware
and proprietary bottles.
Glassware may be decorated by being enamelled with coloured enamels. In
this method of decorating, soft, easily-fused, coloured enamels are
used, containing active fluxes such as borates of lime and lead, which
melt at low temperatures. These enamel colours are prepared by being
fused and then ground to fine powders, which are mixed with a siccative
or oil medium, and painted upon the glass. The painted ware is then
heated within a gas or wood-fired enamelling furnace or muffle, until
the painted designs are melted and fused well upon the glass. The glass
is re-annealed in cooling down the muffle. For this form of decoration,
a hard refractory glass is required that will not soften easily under
the heat of the muffle; otherwise the glassware becomes misshapen too
easily under the heat necessary to flux or fuse the enamels properly.
A form of staining glass is also practised which consists of applying
compositions containing silver salts to portions of the glass and firing
at a low heat. The silver stains the glass a deep yellow. The colour may
be varied by the use of copper salts, when a fine ruby stain is obtained
wherever applied.
=Iridescent= glassware is produced by several methods. Sometimes a small
proportion of silver and bismuth is added to a coloured glass batch, and
by manipulating the resulting glass in a carbonaceous flame the silver
is partially reduced within the glass, forming a pretty iridescent
reflection on the glassware. By a suitable adjustment of the oxygen
content in the composition of such glasses, the iridescence can be
regulated to such an extent that the slightest flash or reducing
influence gives a beautifully finished lustre over the ware.
Iridescence can also be formed by re-heating crystal glassware within a
chamber in which salts of tin, barium, aluminium, and strontium are
volatilised. This method produces a superficial iridescence which is not
quite so permanent as the previous process.
=Glass Silvering.= The silvering of mirrors is carried out by taking a
thoroughly cleaned plate of polished glass and floating one surface in a
solution of silver nitrate, to which a reducing agent is added. The
silver is thereby precipitated or deposited in a thin lustrous film upon
the glass, which causes reflection by the rays of light striking against
the silvered background.
After silvering, the back of the plate is coated with a protecting paint
or varnish, which dries and preserves the silver deposit and gives it
permanency.
In the manufacture of fancy ornaments, such as birds, hat pins, and
small animals, various coloured glass cane and tube are worked together
by the operator melting and welding the respective colours together
before a blow-pipe flame, the tails of the birds being formed by sealing
in a fan of spun glass into the body of the bird, which has been blown
out and formed from a piece of tube. Some very curious ornaments are
formed in this way. Glass buttons, pearl, and bead ornaments are formed
by working cane and tube of various coloured compositions before the
blow-pipe, sticking and shaping the various forms on to wire.
Mosaic glass decoration is used in jewellery in a mural or tessellated
form. In this method small cubical or other shaped cuttings of various
coloured opaque glass are inlaid in mastic cements or pastes to form the
design, the face being afterwards ground and polished smooth, and
mounted or set within the ornament.
Larger cuttings may be inlaid in cement for pavement or mural
decoration.
------------------------------------------------------------------------
CHAPTER XVI
ENGLISH AND FOREIGN METHODS OF GLASS MANUFACTURE COMPARED
The continental methods of glassmaking differ so much from the English
methods that a few remarks giving comparisons will be of interest. It is
noticeable that chemical and engineering science is more thoroughly
applied in the manufacture of glassware abroad. Their method of
specialising wherever possible, and the introduction of mechanical and
automatic machines have done much toward increasing their production and
efficiency.
The flourishing and extensive state of glassmaking abroad is shown by
the size and extent of the glass works, some of which work as many as
forty or fifty furnaces and employ 3,000 to 5,000 hands. Gas-fired
regenerative or recuperative furnaces are more generally used, which
permit higher temperatures, cheaper metal, and greater economy in fuel
and labour.
The present type of English furnace is very wasteful, and even with good
fuel it is difficult to maintain high temperatures and regularity in
working. Our method of firing, raking, and teasing is very exhausting to
the workmen in attendance.
In many English glass works, especially those in the Stourbridge
district, it is the practice to fill the pots on a Saturday morning and
take until the following Monday night to melt and plain the glass, no
glassware being made for three days of each week. Starting on Monday
night or Tuesday morning, the glass makers work in six hour shifts day
and night until Friday night or Saturday morning, when the pots are
again filled and the weekly course starts over again. Abroad the pots
are filled nightly and hold just sufficient metal to last out the work
during the day, and are built of a capacity to suit the articles being
made. The disadvantages of our method are obvious when a comparison is
made with the continental method of melting the glass nightly and
working it out daily, especially when the efficiency or output of the
furnaces as compared with their fuel consumption is taken into
consideration.
Abroad the furnaces are small and compact; they take up less floor
space, yet they are far greater in efficiency. As they are gas-fired,
the combustion is more complete, and by the use of regenerators or
recuperators greater heat is available for melting the glass quickly.
Larger proportions of sand are used in the glass mixtures, which, being
the cheaper component, cheapen the production of their glass wares.
Owing to the more perfect combustion which takes place within the
chambers of gas-fired furnaces abroad, lead glasses are successfully
melted within open crucible pots. When the heat comes into direct
contact with the batch materials being melted, it does its work quicker
and with less fuel consumption than is the case if it has first to be
conducted through the hood of covered pots which have necessarily to be
used in the old English type of furnace.
It is particularly noticeable that the glass workers abroad do not spend
so much time upon producing an article as is usual under the English
method of working. By the extensive use of moulds fitted to mechanical
contrivances operated by the foot, their work is expedited and made
simple and easy.
Technological education in the glass industry abroad is more thorough
and general. The glass workers, not having to work at night, have the
evenings free for recreation and education. It would do much towards
developing the English glass trade if night work for boys could be
abolished. The adoption of the continental system of melting the metal
during the night and working only during the day (by using gas-fired
furnaces) would do much in this direction. One cannot expect the youths
of the glass trade, who have to work during nights, to attend the
evening classes for educating themselves, without a severe strain upon
their constitutions. This fact partially accounts for the repeated
failure to establish technical classes and trade schools in the
glassmaking centres of this country. The conservatism and lack of
support from the glass manufacturers themselves account for much of the
slow progress and development of the trade. As a rule, it will be found
that the manufacturers have everything to gain by the better technical
education of their employees. It is with pleasure we notice that a few
at least are now taking this broader view and giving such schools their
hearty support and financial aid. In the glassmaking centres abroad
there are established state-aided technical and trade schools, where,
for a small nominal fee, the youths of the glass works are trained and
taught the principles of their industry. Apprenticeship in the factories
then becomes unnecessary.
The working hours abroad are usually sixty hours a week (ten hours a
day), compared with the English forty-four to fifty hours’ week (six
hour shifts).
The trade unions of the glass workers abroad are more progressive, and
their officials do not interfere with the manufacturers’ endeavours to
increase efficiency and cheapen production by introducing machinery. The
promotion of the workpeople goes by merit, and not by the dictation of
the trade union officials, as is too often the case in this country.
Here, very little sentiment or good-fellowship exists between the glass
workers’ union and the employers, and in its place the rank officialdom
of unionism has become so evident as to be a bar to the progress of the
industry. Instead of assisting the progress of the trade, and mediating
in cases of dispute, the union appears to exist as a buffer of
antagonism between the glass workers and their employers. Many a capable
youth in the glass trade here has been kept back from promotion to a
better position solely by the dictation of the union to which the men
belong. Cases are known where the union have restricted the workman’s
output when he may be working under piece rate. The best inducements may
have been offered him by the employer to increase his output, and,
although the workman may be willing to accept the master’s terms, we
find a union official stepping between them, and fixing the maximum
number of the articles that shall be made in his six hour shift.
Usually, this fixed quantity is got through in four hours, yet the
workman is not allowed to make more than the stipulated number fixed by
the union, or he is fined. Another incredible fact is that the employer
here, when in need of a workman, is not allowed to choose his own men.
He must apply to the union, and the man remaining longest on the
society’s unemployed book is then sent to him. Whatever his inefficiency
may be, the employer is bound to take him; if he employs anyone else, a
strike results. Such action is despotic and shows up the worst features
of trade unionism that can possibly be conceived. The English glass
industry has been repeatedly disorganised by this obstinate attitude of
the glass makers’ union, and a consequence is that the foreigner has
seized the opportunity to step in and increase his market, to the
detriment of our own trade; with this extended market, increased output,
and cheaper production, the foreigner undersells us in our own country.
It is to be hoped these adverse conditions will soon be remedied and the
English glass industry restored to a more flourishing state by the
prompt and united action of the men and masters, realising the gravity
of the position and acting accordingly.
------------------------------------------------------------------------
APPENDIX
JOURNALS AND BOOKS FOR REFERENCE
“American Pottery Gazette.” (New York, U.S.A.)
“Boswell’s Memoir on Sands Suitable for Glassmaking.” (Longmans, Green &
Co., London.)
“Pottery Gazette.” (Scott Greenwood, London.)
“Sprechsaal.” (Coburg, Germany.)
“Painting on Glass and Porcelain.” Hermann. (Scott Greenwood.)
“Decorated Glass Processes.” (Constable, London.)
“Jena Glass.” Hovestadt. (Macmillan & Co.)
“Glass Manufacture.” Rosenhain.
“Producer Gas-Fired Furnaces.” Ostwald.
“Glassmaking.” By A. Pellatt. (Bogue, London.)
“Gas and Coal Dust Firing.” Putsch. (Scott Greenwood.)
“The Collected Writings of H. Seger.” (Scott Greenwood.)
“Ceramic Industries.” Vol. I. By Mellor.
“Modern Brickmaking”; “British Clays, Sands, and Shales”; “Handbook of
Clay Working.” By A. B. Searle. (Griffin & Co.)
“Glass Blowing.” By Shenstone.
“Asch’s Silicates of Chemistry and Commerce.”
“Clays.” By A. B. Searle. (Pitman, London.)
“Fuel and Refractory Materials.” Sexton. (Mackie & Sons.)
“Furnaces and Refractories.” Harvard. (McGraw, New York)
SOCIETIES’ JOURNALS AND TRANSACTIONS
“The Society of Glass Technology.” (Sheffield.)
“The American Ceramic Society.” (Columbus, Ohio, U.S.A.)
“The English Ceramic Society.” (Stoke-on-Trent, Staffs.)
“Journal of the Society of Chemical Industry.” (Westminster, London.)
------------------------------------------------------------------------
INDEX
Aberration, 104
Acids, action of, on glass, 18, 19
Action of glass on , 45
Alkali, 23
Alumina, 9-11, 20
Amethyst, 31
Analysis of , 37
Ancient glass, 1
Annealing glass, 18
—— pots, 66
Arsenic, 31
Artificial eyes, 101
—— cements, 24
—— pearls, 31
Aventurine, 22-102
Barytes, 8-26
Basalt, 10
Bastie’s Process of hardening glass, 18
Batch, 11-13
Beads, 31-116
Black glass, 29
Blowing glass, 80, 82
Blow-iron, 80
Blue glass, 28
Bohemian glass, 25
Borates in glass, 7, 8, 9
Boric acid, 7
Bottle glass, 26, 27
Bottle-making, 77, 79
Bull’s eye, 90
Buttons, 116
Cane, 97
Capacity of pots, 51-52
—— of tank furnace, 56
Carbonate of soda, 6
Cements, 24
Chain screen, 68
Chair, Glassmakers’, 81
Chemical properties of glass, 4-15
Chemical Formulae, 12
Chimneys, Lamp, 16
Clays for pots and furnaces, 36
Coloured glasses, 28, 29
Colour of silicates, 11-22
Complex glass, 26
Composition of glass, 4-25
Compound glasses, 25
Conductivity of glass, 23
Continental glass, 3, 88, 118
Covered pots, 21-27
Cracking-off glass, 15, 17, 85
Crown glass, 26-89
Crucible pots, 21, 27, 64
Cullet, 10, 85
Cutting glass, 8, 10, 16
Decay in glass, 2
Decomposition, 2, 19
Decorated glass, 108
Decolorants, 32
Defects, 9, 23, 34
De-grading glass, 23
Density, 16
Devitrification, 3, 8, 20
Discovery of glass, 1
Doll’s eyes, 101
Education, Technical, 120
Electric furnaces, 58
Emerald, 31
Enamelling glass, 115
English type of furnace, 43
Engraving glass, 111
Etching, 19-112
Expansion, Thermal, 16
Eye of furnace, 43
Eyes, Artificial, 101
Fancy glass, 116
Filigree, 99
Fire-clay, 3, 11-36
—— analyses, 37
Fire-clay, blocks, 39, 45
——, Burnt, 39, 41, 61
—— crucibles, 64
——, Grinding of, 39
——, Melting point of, 64
——, Mild, 39, 65
—— pots, 62
——, Properties of, 36-38, 41
—— rings, 65
——, Selection of 38
—— stoppers, 66
—— Strong, 39, 64
——, Tempering, 39, 61
——, Weathering, 39, 61
Flint glass, 4
—— stones, 4
Fluorspar, 8
Foot maker, 82
Formulas, 12, 21
Frisbie’s Feeder, 47
Furnaces, 21, 41, 51, 57
Fusibility of glass, 9
Gadget, 28
Garnet, 31
Gas-fired furnaces, 47, 51, 55
Gathering, 76, 77
Glass, Afterworkings of, 86
——, Alkalies in, 23
——, Alumina in, 9, 11, 20
——, Ancient, 2
——, Annealing, 71
——, Cane, 97
——, Coloured, 28
—— cloth, 101
——, Cut, 109
——, Enamelled, 115
——, Founding of, 69, 74
—— furnaces, 21, 41, 51, 56
——, Gauge, 18
——, Grinding of, 94
——, Hardened, 95
——, Homogeneity of, 23
—— house pots, 62
——, Moulds for, 77
——, Melting of, 69
——, Plasticity of hot, 4-16
——, Polishing of, 92-94
——, Properties of, 15
——, Process of making, 15, 76
——, Sand-blasted, 114
——, Scum on, 69
——, Seeds in, 105
——, Silvered, 116
—— snow, 101
——, Stress in, 74
——, Strengthened, 95
——, Temperature of melting, 20
——, Tube, 96
——, Types of, 15, 25
——, Wired, 95
—— wool, 101
——, Yellow, 28
Grinding tumblers, 110
—— plate glass, 92, 94
Hardened, 18, 23
Hermansen’s Furnace, 52, 53
History, 1
Homogeneity, 23
Honey pot making, 85
Hydrofluoric acid, 19
Introduction of glassmaking in England, 2
Iridescence, 21-101
Iron in glass, 32
Italian Aventurine, 102
Laboratory glass, 25
Ladling glass, 45
Lamp glass chimneys, 16
Lead glass, 21
—— poisoning, 14
Lehr, 71
Light and glass, 33
Lime glass, 25, 26
Machines in glassmaking, 79, 111
Mechanical boy, 86
Millefiore, 100
Moulds, 85
Opalescent glass, 95-109
Opal glass, 29, 31
Optical glass, 5, 9, 33, 104
Oxidising agents, 7
Pearl ash, 6
Pearls, 31, 116
Phosphates in glass, 8
Polariscope, 74
Potash, 6
—— glass, 24
Pots, 8-13, 27-58
——, Annealing, 66
—— cracking, 45, 69
—— clays, 37, 64
——, Glazing, 69
——, Making, 62
——, Open, 21, 27, 64
——, Plumbago, 65
——, rings, stoppers, 63-65, 66
—— sherds, 65
——, Setting, 67
——, Trolley, 46
Plaining glass, 4, 51, 69
Plasticity, 11, 16
Plate glass, 26, 93
Plumbago, 65
Pressed glass, 26, 77
Pucellas, 82
Quartz glass, 19
Réaumur’s Porcelain, 17
Recipes for glass making, 25, 26
Recuperative furnaces, 52-54
Reduction in glass, 28, 31
Regenerative furnaces, 49
Rocaille flux, 25
Roman glass, 2
Ruby glass, 28, 31
Rupert drops, 18
Saltpetre, 7
Sands, 4
Sand-blast, 114
Scratching glass, 16
Screens for pot setting, 68
Seeds in glass, 4, 13, 105
Servitor, 83
Shearing glass, 81, 83
Sheet glass, 91
Siemens Furnace, 48
Siege of furnace, 43, 44
Silica, 4, 5
Silicates in glass, 24
Silvering glass, 116
Simple glasses, 24
Soda-lime glass, 21-26
Soft glass, 5
Soluble glass, 24
Spun glass, 15, 100
Stirring glass, 105
Strengthened glass, 95
Stress in glass, 18, 104
Striae, 9
Sulphates, 5, 25
Table glass, 25, 76, 77
Tank glass, 26, 57
Technical Education in glass manufacture, 120
Temperature of furnaces, 20, 105
Thermal expansion of glass, 16
Tin oxide in glass, 10
Tizeur, 43, 46
Tools, 76
Topaz, 31
Trades Unionism, 86
Tube, 26, 98
Tumblers, 85, 110
Turquoise, 31
Uranium, 28
Varieties of glass, 25, 102
Venetian glass, 2
Violet glass, 28
Waste glass, 30
Waterglass, 24
Wine-glass making, 82
Window glass, 1
Wired glass, 95
Working hours, 120
Zinc oxide, 9
_Printed by Sir Isaac Pitman & Sons, Ltd., Bath, England_
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● Transcriber’s Notes:
○ Missing or obscured punctuation was silently corrected.
○ Typographical errors were silently corrected.
○ Inconsistent spelling and hyphenation were made consistent only
when a predominant form was found in this book.
○ Text that was in italics is enclosed by underscores (_italics_);
text that was bold by “equal” signs (=bold=).
○ The use of a caret (^) before a letter, or letters, shows that the
following letter or letters was intended to be a superscript, as
in S^t Bartholomew or 10^{th} Century.
○ Superscripts are used to indicate numbers raised to a power. In
this plain text document, they are represented by characters like
this: “P^3” or “10^{18}”, _i.e._ P cubed or 10 to the 18th power.
○ Variables in formulae sometimes use subscripts, which look like
this: “A_{0}”. This would be read “A sub 0”.
*** End of this LibraryBlog Digital Book "Glass and Glass Manufacture" ***
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