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Printed in Great Britain 


The present volume on Glass Manufacture has been written chiefly 
for the benefit of those who are users of glass, and therefore, makes 
no claim to be an adequate guide or help to those engaged in glass 
manufacture itself. For this reason the account of manufacturing 
processes has been kept as non-technical as possible ; no detailed 
drawings of plant or appliances have been given, and only a few 
illustrative diagrams have been introduced for the purpose of avoid- 
ing lengthy verbal descriptions. In describing each process the 
object in view has been to give an insight into the rationale of each 
step, so far as it is known or understood, and thus to indicate the 
possibilities and limitations of the process and of its resulting pro- 
ducts rather than to provide a detailed guide to the technique of the 
various operations. The practical aim of the book has further been 
safeguarded by the fact that the processes described in these pages 
are, with the exception of those described as obsolete to the author's 
definite knowledge, in conmiercial use at the present time. For 
this reason many apparently ingenious and beautiful processes 
described in earlier books on glass have not been mentioned here, 
since the author could find no trace of their employment beyond the 
records of the various patents involved. On the other hand the 
reader must be warned to bear in mind that the peculiar conditions 
of the glass manufacturing industry have led to the practice on the 
part of manufacturers of keeping their processes as secret as possible, 
so that the task of the author who would give an accurate account 
of the best modern processes used in any given department of the 
industry is beset with great difiSculties. The author has endeavoured 



to steer the best course open to him under these circumstances, and 
he would appeal to the paucity of glass literature in the English 
language as evidence of the difficulty to which he refers. 

In addition to these difficulties, which arise largely from considera- 
tions of a conamercial nature, the writer of a book on glass is further 
confronted with technical difficulties of no inconsiderable order. As 
already indicated, the aim of the present author has been to describe 
processes from the point of view of principles and methods rather 
than as mere rule-of -thumb descriptions of manufacturing manipula- 
tions, but in doing this he is met at every turn by the fact that from 
the scientific side the greater part of the field of glass manufacture 
is a "terra incognita." In making this statement the labours of 
many eminent scientific workers are by no naeans forgotten, but the 
entire field is so large and beset with such great experimental diffi- 
culties that even the labours of a list of investigators that includes the 
names of Eraunhofer and Faraday, Stokes, Hopkinson, Abb6 and 
Schott, have resulted in little more than an accumulation of empirical 
data which, while they have been productive of great direct practical 
results, have left tfie science of glass still in a very elementary condi- 
tion. To take two examples in illustration of this fact we may 
mention the question of the connection between chemical composi- 
tion and any of the physical properties of glass, such as refraction and 
dispersion of light, and on the more mechanical side the question why 
all processes, such as rolling or moulding, which involve the contact 
of hot glass with metal result in a roughening of the glass surface. 
The former question has been studied by several of the investigators 
named above, Schott and Abb6 having particularly devoted an 
enormous amount of labour and money to the study of the question, 
with results which have proved disappointing from the scientific 
point of view. By prolonged experimenting and the employment 
of a costly system of trial and error an important series of novel and 
useful glasses has been produced by these workers, but no law by 
whose aid the optical properties of a glass of given chemical composi- 
tion could be predicted has yet been discovered, and as a summary 


of the known facts only the vaguest general principles are available 
for the guidance of those who wish to produce glasses of definite 
properties. The same applies in a similar degree to most of the other 
properties of glass, with the exception, perhaps, of density and 
thermal expansion ; attempts to generalise from the known data of 
a limited number of glasses generally meet with unqualified failure. 
The conclusion which one is forced to admit is that the fundamental 
principles underlying the nature and constitution of glasses have 
yet to be discovered. A study of the other question mentioned above 
as an example of the limitations of our knowledge leads to the same 
conclusion ; an almost endless succession of inventors have busier! 
themselves with devices for overcoming the roughening action of 
rollers and moulds upon glass, but without any real success. A long 
list of other examples of the same kind could be given, our know- 
ledge of the physical and chemical principles underlying many of the 
phenomena met^with in glass manufacture being deplorably deficient. 
It will thus be seen that to write a truly scientific account of glass 
manufacture is at the present time impossible, and the reader is 
asked to bear this in mind if he should find the chemical or physical 
explanations given in this book less frequent or less adequate than 
could be desired. 

Having dwelt somewhat emphatically on the limitations of our 
present scientific knowledge as applied to glass manufacture, it is 
perhaps scarcely necessary at the present time to emphasise the fact 
that this state of affairs should act as the strongest incentive to 
further investigation of the whole subject. The difficulty, however, 
lies in the fact that such investigation can scarcely be carried on by 
voluntary workers in ordinary laboratories, but must be undertaken 
with the active help of glass manufacturers at their works. Glass 
is essentially a substance that cannot be satisfactorily handled in 
small quantities, particularly so far as all the phenomena connected 
with its production and manipulation while hot are concerned ; the 
influences of containing vessels, of furnace gases and of rapid cooling 
are all enormously exaggerated if ounces instead of hundredweights 


or tons of gLxss are used for experimental purposes, and these 
influences and others of the same nature vitaUy affect all the results 
of smaU-scale laboratory operations. The progress of our scientific 
knowledge of glass — and the consequent development of the glass 
industry from its present state where rule-of -thumb and " practical 
experience" still hold excessive sway — lies in the hands of those 
concerned in the industry itself. It must be jidmitted that to under- 
take such work involves the expenditure of much time and money 
on the part of a manufacturer, while the field is so large and the 
problems so complicated that any adequate return cannot be 
promised for the immediate future ; on the other hand the very size 
of the field and the difficulty of the problems offers the promise of the 
greatest ultimate reward ; a really important scientific discovery in 
connection with glass would be certain to bring in its train industrial 
developments whose limits it is impossible to foresee. The industrial 
Success of the glass-works of Schott in Jena is often quoted as a 
brilliant example of commercial success resulting from purely 
scientific investigations in this actual field ; an example of stiU 
greater magnitude is furnished by the success of the aniline dye 
works of Germany, which are built up on purely scientific achieve- 
ments. The glass industry as a whole, supplying some of the abso- 
lute necessaiies of modern life, should be capable of offering the 
greatest rewards to success, and the example of other industries has 
shown that ultimate success is bound to reward properly -conducted 
and perseverant scientific research. Nowhere is this more urgently 
needed than in the whole field of glass manufacture. 

The author is indebted to Mr. W. C. Hancock for valuable assist- 
ance in the reading of proofs and various suggestions in connection 
with the contents of this book. 



In the eleven years which have elapsed since the first edition of 
this book was written, the Glass Industry has undergone very great 
development ; so far as the British industry is concerned, most of 
this development has — ^under the stimulus of war — been crowded 
into the period of the four years 1916 to 1919 inclusive, and much of 
this represents a supreme effort made under adverse conditions, an 
effort of which the glass manufacturers of Great Britain have good 
reason to be proud. In endeavouring to revise the present work in 
such a manner as to bring it into reasonable conformity with the 
present position of the industry while not too greatly exceeding the 
original framework, it has not been possible to deal with the history 
and development of each process. All that has been attempted is 
to introduce brief accounts of modern developments — of which the 
automatic blowing-machine of the Owens tjrpe may be taken as an 
example — and to modify expressions of views and 'anticipations 
where these have not been borne out by the course of events. 

Perhaps the most important modification introduced into the book 
is the chapter (Chapter IV.) dealing with Refractories. This subject 
has assumed a position of such fundamental importance in relation 
to glass manufacture that treatment in considerably greater detail 
appeared to be justified. 

Some typical analyses of various kinds of glassware are given in an 
Appendix ; the author has preferred to group them together in this 


manner, which affords ready comparison, rather than to scatter them 
through the various chapters. 

The author wishes to express his indebtedness to two of his 
colleagues at the National Physical Laboratory — Mr. W. H. Withey 
and Mr. E. A. Coad-Pryor — ^for valuable help in the preparation of 
this edition, the former in regard to the chemical testing and analysis 
of glass and the latter in connection with refractories. 


December ISth, 1918. 



Preface .......... v 



Definition of the term " Glass " — ^Ajnorphous structure the common 
feature of all vitreous bodies — Glass a congealed fluid — Glasses 
not definite chemical compounds but complex solutions — Range 
of chemical composition available for glass-making — Considera- 
tions governing chemical composition — Influence of composition 
on physical properties — Chemical stability of glass — ^Permanence 
of glass surfaces — ^Action of water, acids, and alkalies on glass — 
Action of light on glass . . . . . . . ^. 1 



Mechanical properties : tensile strength, crushing strength, elasticity, 
ductility, and hardness — ^Thermal properties of glass : thermal 
endurance, coefficient of expansion, thermal conductivity — ^Ther- 
mometer glass — Electrical properties of glass — ^Transparency and 
colour of glass . . . . . . . . ^. 16 



General considerations — Chemical purity, moisture, and physical 
condition, constancy of quality — Sources of silica, sand and sand- 
stone — ^Felspar — Sources of alkali : Soda ash (carbonate of soda), 
salt cake (sulphate of soda), pearl ash (carbonate of potash) — 
Alkali nitrates — ^Natural minerals containing alkalies — Sources 
of other bases : Lime, chalk, limestone, slaked lime — Gypsum 
(sulphate of lime) — ^Barium compounds — ^Magnesia and zinc — 
Lead oxide, red lead — ^Aluminium, manganese, arsenic — Carbon — 
Coke, charcoal, anthracite coal-^Fluorine — ^Boron — Zirconium p, 32 




Importance of refractories — Classification of refractories — Silica briclc 
and fire-clay — Qualities required in refractories — Effect of furnace 
temperature and duration of exposure — ** Melting " of refrac- 
tories — ^Firing, sintering, and fusion of fire-clays — ^Testing refrac- 
toriness — ^Tests under load — Silica and alumina — ^The equilibrium 
diagram — ^Typical analyses of refractories — Shrinkage of fire- 
clays and expansion of silica bricks — Standard tests — Refrac- 
tories in contact with glass — Resistance to solution — ^Action of con- 
centration currents — ^Plasticity and moulding processes — Shapes 
and sizes of glass melting pots — ^Process of pot-making — ^Dryin^ 
and firing of pots — Production of pots by slip-casting process — 
Casting of slips containing coarse grog — Silica brick : properties 
and treatment . . . . . . . . p. 50 



Coal-fired and gas-fired furnaces — Gas producers — Regenerative furnaces, 
principles and construction of Siemens' furnaces — Recui)erative 
furnaces — Relative merits of regenerative and recui)erative fur- 
naces — General arrangements of modem tank furnaces — Relative 
advantages of tank and pot furnaces . . . . . ^. 67 



Mixing of raw materials by hand and by machinery — ^The charging 
operation — Chemical reactions during melting of carbonate mix- 
tures, and of sulphate mixtures — Influence of carbon on the 
reactions — ^The fining process . . . . . j>. 79 



Ladling, gathering, and casting — Limitations of ladling — ^Ladling used 
for rolled glass, gathering for blown glass — Rolling of glass — 
Blowing processes and operations — ^Use of moulds — ^Pressing — 
Moulding — ^Annealing . . . . . . . jp. 88 





Baw materials — ^Furnaces — ^Predominance of tank -furnaces — ^Process of 
blowing bottles by hand — Gathering, marvering, blowing — ^Use 
of fire-clay and metal moulds — ^Formation of neck — Improved 
appliances, moulds and tools — ^Manufacture of bottles by machinery 
— ^Modern bottle-blowing machine — " Press and blow " machines — 
The Owens automatic machine — ^Annealing of bottles — ^Large 
bottles, carboys — ^Aids to the blower — Sievert's process — ^Large 
shallow vessels, bath-tubs ..... p, 105 



Raw materials — Bohemian glass and flint glass — Gathering and blowing 
— Chair work — Hand work — ^Production of tumblers by hand — 
Application of coloured glass to blown articles — ^Use of moulds 
as aids to blowing — Roughening effect of moulds — ^Fire-polishing 
by reheating — Use of compressed air — ^Pressed glass — ^Moulds and 
presses Capacity and limitations of pressing process . ^.116 



Rolled plate-glass — ^Furnaces — Raw materials — Process of ladling — 
The rolling table — ^Annealing — Cutting and sorting — Patterns en 
rolled plate — " Figured '* rolled plate — ^Machine used for double- 
rolling — ^Polished plate — Raw materials — Casting from melting 
pots — Special casting pots — The rolling table — Importance of 
flatness — ^Annealing kilns — Grinding and polishing processes — 
Machines used for grinding and polishing — ^Method of holding the 
glass — ^Abrasives and polishing materials — ^Theory of the polishing 
process — ^Limiting sizes of polished plate — Homogeneity of polished 
plate — ^Uses of plate -glass — Bent polished plate — ^Mirrors — 
Bevelling, process and machines — Wired plate glass, rolled and 
polished — ^Difl&culties and limitations — ^Advantages of wired 
glass .......... |>. 129 



Comparison of sheet with polished plate — Raw materials for sheet — 
Furnaces : various forms of tank -furnaces — Blowing process — 


Gathering, forming the gathering on blocks, forming the shoulder 
of the cylinder, blowing the cylinder, opening the end of the cylin- 
der, detaching cylinder from pipe — Cutting off the " cap " — 
Splitting the cylinder — Flattening and annealing — Cutting and 
sorting sheet-glass — ^Defects of sheet-glass — ^Variations of the 
process — ^Attempts to produce *' sheet " glass by rolling — Sievert's 
process — ^Direct drawing processes — ^The American process for 
drawing cylinders — ^Fourcault's processes — ^Difficulties and limi- 
tations — Crown glass — ^The blowing process — ^Limitations . p. 162 



Dciflnition of coloured glass — ^Physical causes of colour — Colouring 
substances : copper, silver, gold, carbon, tin, arsenic, sulphur, 
chromium, uranium, fluorine, manganese, iron, nickel, cobalt — 
Range and depths of tints available — Intensely coloured glasses 
— ^The process of " flashing " — Character of " flashed " glass — 
Colours produced on glass by painting : use of coloured " glazes '* 
as paints — ^Ancient stained glass and modem glass — ^Technical 
uses of coloured glass, photography, railway and marine signals 

i>. 176 



Nature and properties of optical glass — Homogeneity — ^Formation and 
removal of striae in solutions and in glass — ^Transparency and 
colour — ^Absorption of light in " decolourised " glasses — Refraction 
and dispersion — ^Definitions — Refractive index, dispersion, medium 
dispersion, the quantity » — Specification of optical properties in 
terms of certain spectrum lines — ^Tables of typical optical glasses 
and their optical constants — Crown and flint glasses — Relation 
between refraction and dispersion in the older and newer glasses — 
Work of Abb6 and Schott — ^Applications of the new glasses — 
Non-proportionality of dispersion in different types of glass — 
Resulting imperfections of achromatism — ^The relative partial 
dispersions of glasses — ^Pairs of glasses giving perfect achromatism 
not yet fully available — Constants of Schott's telescope crown and 
flint — Narrow range of optical glasses, consequent limitations in 
lens design — Causes of these narrow limits — Possible directions 
of extension — Chemical stability of optical glasses — Double 
refraction in optical glass arising from imperfect annealing p, 197 




The manufacture of optical glass — Raw materials — ^Mixing — ^Furnaces 
and crucibles — Kilns for heating pots — Transfer of pots from kiln 
to melting furnace — Introduction of cullet and raw materials — 
The fining process, difficulties and limitations — ^The stirring 
process — ^The final cooling of the glass — Rough sorting of the 
glass fragments — ^Moulding and final annealing of the moulded 
glass — Grinding and polishing of plates and discs for examination ; 
smallness of yield obtained — ^Difficulty of obtaining large blocks 
of perfect glass ....... .^.214 



Glass tubing — Gathering and drawing of ordinary tubes — Special 
varieties of tube — Combustion tubes — Tubes of vitreous silica — 
Varieties of vitreous silica — ^Transparent, glass-like silica ware — 
Great cost of production — ^Translucent " milky '* silica ware pro- 
duced electrically — Great thermal endurance of vitreous silica — 
Sensitiveness to chemical action of all basic substances at high 
temperatures — Glass rod and fibre — Glass wool — Quartz fibres — 
Glass beads — ^Artificial gems — ^Use of very dense flint glass coloured 
to imitate precious stones — ^Means of distinguishing imitations — 
Precious stones produced by artificial means — Chilled glass — Great 
strength and fragility of chilled glass — Rupert's drops — ^Manu- 
facture of ** tempered " glass by Siemens — ^De la Bastie's process 
— ^Massive glass, used for house construction and paving blocks — 
Water-glass (silicate of soda or potash), manufacture in tank- 
furnaces — Glass for lighthouse lenses and searchlight mirrors — 
Production by casting glass in iron moulds — Sizes and types of 
lenses and prisms produced . . . . . .p. 229 

Appendix I. — ^The composition of some typical glasses as given by 
chemical analysis . . . . . . .p. 241 

Appendix II. — Bibliography of glass manufacture . . , p. 24:4: 

Index .p. 247 




Although the term " glass " denotes a group of bodies which 
possess in common a number of well-defined and characteristic 
properties, it is difficult to frame a satisfactory definition of the 
term itself. Thus while the property of transparency is at once 
suggested by the word " glass," there are a number of true glasses 
which are not transparent, and some of which are not even trans- 
lucent. Hardness and brittbness also are properties more or less 
characteristic of glasses, yet very wide differences are to be found 
in this respect also, and bodies, both harder and more fragile than 
glass, are to be found among minerals and metals. Perhaps the only 
really universal property of glasses is that of possessing an amorphous 
structure, so that vitreous bodies as a whole may be regarded as 
typical of " structureless " solids. All bodies, whether liquid or 
solid, must possess an ultimate structure, be it atomic, molecular 
or electronic in character, but the structure here referred to is not 


that of individual molecules but rather the manner of grouping or 
aggregation of molecules. 

In the great majority of mineral or inorganic bodies the molecules 
in the solid phase are arranged in a definite grouping and the body 
is said to have a crystalline structure ; evidences of this strudnire 
are generally visible to the unaided eye or can be revealed by the 
microscope. Vitreous bodies on the other hand are characterised 

O.M. B 


by the entire absence of such a structure, and the mechanioal, 
optical and chemical behaviour of such bodies is consistent only 
with the assumption that their molecules possess the same arrang^e- 
ment, or rather lack of arrangement, that is found in liquids. 

The intimate resemblance between vitreous bodies and true 
liquids is further emphasised when it is realised that true liquids 
can in many instances pass into the vitreous state without under- 
going any critical change or exhibiting any discontinuity of behaviour 
such as occurs during the freezing of a crystalline body. In 
the latter class of substances the passage from the liquid to the 
crystalline state takes place at one definite temperature, and the 
change is accompanied by a considerable evolution of heat, so 
that the cooling of the mass is temporarily arrested. In the case of 
glasses, on the other hand, the passage from the liquid to the appa- 
rently solid condition is gradual and perfectly continuous, no 
evolution of heat or retardation of cooling being observed even by 
the aid of the most delicate instruments. We are thus justified in 
speaking of glasses as *' congealed liquids " ; the process of congealing 
in this case involves no change of structure, no re-arrangement 
of the molecules, but simply implies a gradual stiffening of the 
liquid until the viscosity becomes so great that the body behaves 
like a solid. It is, however, just this power of becoming exceedingly 
stiff or viscous when cooled down to ordinary temperatures that 
renders the existence of vitreous bodies possible. All glasses are 
capable of undergoing the change to the crystalline state when 
kept for a sufficient time at a suitable temperature. The process 
which then takes place is known as " devitrification," and sometimes 
gives rise to serious manufacturing difficulties. 

Molten glass may be regarded as a mutual solution of a number 
of chemical substances — ^usually silicates and borates. When cooled 
in the ordinary way these bodies remain mutually dissolved, and 
ordinary glass is thus simply a congealed solution. The dissolved 
substances have, however, natural freezing-points of their own, 
and if the molten mass be kept for any length of time at a temperature } 


a little below one of these freezing-points, that particular substance 
will begin to solidify separately in the form of crystals. The facility 
with which this will occur depends upon the properties of the 
ingredients and upon the proportions in which they are present in 
the glass. In some cases this devitrification sets in so readily that 
it can scarcely be prevented at all, while in other cases the glass 
must be maintained at the proper temperature for hours before 
crystallisation can be induced to set in. In either of these cases, 
provided that the glass is cooled sufficiently rapidly to prevent 
crystallisation, the sequence of events during the subsequent 
cooling of the mass is this : as the temperature falls further and 
further below the natural freezing-point of one or other of the 
dissolved bodies, the tendency of that body to crystallise out at 
first rapidly increases ; as the temperature falls, however, the 
resistance which the liquid presents to the motion of the molecules 
increases at a still greater rate, so that two opposing forces are at 
work, one of them an increasing tendency towards crystallisation, 
the other a still more rapidly increasing resistance to any change. 
There is thus for every glass a certain critical range of temperature 
during which the greatest tendency exists for the crystallising 
forces to overcome the internal resistance ; through this range the 
glass must be cooled at a relatively rapid rate if devitrification is 
to be avoided ; at lower temperatures the crystallising forces 
require increasingly longer periods of time to produce any sensible 
effect, until, as the ordinary temperature is approached, the forces 
of internal resistance entirely prevent all tendency to crystallisation. 

The phenomena just described in reality constitute the natural 
limit to the range of bodies which can be obtained in the vitreous 
state : as we approach this limit the glass requires more and more 
rapid cooling through the critical range of temperature, and is 
thus more and more liable to devitrify during the manufacturing 
processes, until finally the limit is set when no industrially feasible 
rapidity of cooling suffices to retain the mass in the vitreous state. 

While the range of bodies that can be obtained in the vitreous 

B 2 


state is very large, only a comparatively small number of substances 
are ordinarily incorporated in industrial glasses. With the exception 
of certain special glasses used for scientific purposes^ such as the 
construction of optical lenses, thermometers and vessels intended 
to resist unusual treatment, aU industrial glasses are of the nature 
of mixed silicates of a few bases, viz., the alkalies, sodium and 
potassium, the alkaline earths, calcium, magnesium, strontium, 
and barium, the oxides of iron and aluminium (generally present in 
minor quantities), zinc oxide, and lead oxide. The mannw in 
which these various elements enter into combination and solution 
with one another has been much investigated, and the more general 
conclusions have been anticipated in what has been said above. 
It is abundantly evident that glasses are not definite chemical 
compounds, but rather solutions, in varying proportions, of a 
series of definite compounds in one another. In many cases the 
actual constitution of industrial glasses is so complex as, for the 
present at all events, to baffle adequate chemical expression. 

One of the factors that limit the range of possible compositions 
of glasses has already been indicated, and two others must now 
be discussed. For industrial purposes, the cost and rarity of the 
ingredients becomes a vital bar at a certain stage ; thus the use of 
such elements as lithium, thallium, etc., is prohibitively costly. In 
another direction the glass-maker is very effectively restrained by 
the limitations of his furnaces as r^ards temperature. The presence 
of excessive proportions of silica, lime, alumina, etc., tends to 
raise the temperature required for the free fusion of the glass, and 
when this temperature seriously exceeds 1600° C, the manufacture 
of the glass in ordinary furnaces becomes impossible. Thus pure 
silica can be converted into a glass possessing very valuable pro- 
perties, but the requisite temperature cannot be attained in regenera- 
tive gas-fired furnaces such as are ordinarily used by glass manu- 
facturers. This limitation has, however, been overcome to a con- 
siderable extent in the manufacture of pure silica glass, sometimes 
known as vitreous silica. The high temperatures required in this 


case are obtained — on a relatively small scale, it is true— by means 
of electric or oxy-acetylene furnaces. None the less, vitreous silica 
has become an important commercial product, whether in the 
translucient and relatively cheap form produced by the fusion of 
sand in ah. electric furnace, or in the more perfect variety obtainable 
in smaller sizes either in the form of articles for laboratory use or 
even in small blocks suitable for the production of small lenses. 
The limitation of temperature, therefore, can no longer be regarded 
as insuperable, but the special methods required when the range 
of 1600° C. is exceeded involve special prices for the objects pro- 
duced as compared with more ordinary varieties of glass. 

A further limitation in the choice of chemical components is 
placed upon the manufacturer by the actual chemical behaviour 
of the glass both during manufacture and in use. As regards, chemical 
behaviour during manufacture, it must be borne in mind that, 
although glasses are of the nature of solutions rather than of com- 
pounds, yet these solutions tend towards a state of saturation ; 
thus a glass rich in silica and deficient in bases will readily dissolve 
any basic materials with which it may come in contact^ while, on 
the other hand, a glass rich in bases and poor in acid constituents 
such as silica, boric acid or alumina, will readily absorb acid bodies 
from its surroundings. During the process of melting, glass is 
universally contained in fireclay vessels. These are chosen, as 
regards their own chemical composition, so as to ojSer to the molten 
glass as few as possible of those materials in which the glass itself 
is deficient ; yet a limit arises in this respect also, since glasses 
very rich in bases, such as the very dense lead and barium glass 
made for optical purposes, rapidly attack any fire-clay with which 
they may come in contact. The finished glass also betrays its 
chemical composition by its chemical behaviour towards the atmo- 
spheric agents, such as moisture and carbonic acid, with which it 
comes in contact ; glasses containing an excessive proportion of 
alkali, for example, are found to be seriously hygroscopic and to 
undergo rapid decomposition, especially in a damp atmosphere. 


Within the limits set by these considerations, the glass manu- 
facturer chooses the chemical composition of his glass according to 
the purpose for which it is intended ; for most industrial products 
the cheapest and most accessible raw materials that will yield st 
glass of the requisite appearance are employed, while for special 
purposes the dependence of physical properties upon chemical 
composition is utilised, as far as possible, in order to attain a glass 
specially suited to the particular requirements in question. Thus 
the flint and barium, glasses used for table and ornamental ware 
derive from the dense and strongly refracting oxides of lead and 
barium their properties of brilliancy and weight. The fusibility 
and softness imparted to the glass by the presence of these bases 
further adapts it to its piu*pose by facilitating the complicated 
manipulations to which the glass must be subjected in the manu- 
facturing processes. 

Taking our next example at almost the opposite extreme, the 
hardest " combustion tubing," which is intended to resist a red heat 
without appreciable softening, is manufactured by reducing the 
basic contents of the glass to the lowest possible degree, especially 
minimising the alkali content, and using the most refractory bases 
available, such as lime, magnesia, and alumina in the highest 
possible proportions. Such glass is, of course, difficult to melt, 
and special furnaces are required for its production, but on the 
other hand this material meets requirements which ordinary soda- 
lime or flint glass tubing could never approach. Another instance 
of these refractory glasses is to be found in the Jena special ther- 
mometer glasses and in the French (Tonnelot) " Verre dur " ; 
the best of these glasses show little or no plasticity at temperatures 
approaching 500° C, and have thus rendered possible a considerable 
extension of the range of the mercury thermometer. Further 
modification of chemical composition has resulted in the production 
of glasses which are far less subject to those gradual changes which 
occur in ordinary glass when used for the manufacture of ther- 
mometers — changes which vitiated the accuracy of most early' 


thermometers. A still more extensive adaptation of chemical com- 
position to the attainment of desired physical properties has been 
reached primarily as a result of the labours of Schott and Abb6, 
in the case of optical glasses. The work of these men, and the 
developments which have followed from it, both at the works 
•founded by them at Jena and elsewhere, have so profoundly modified 
our knowledge of the range of possibiUties embraced by the class 
of vitreous bodies, that it is not at all easy at the present time to 
realise the former narrow and restricted meaning of the term " glass." 
The subject of the dependence of the optical properties of glass 
upon chemical composition will be referred to in detail in Chapter XII. 
on '' Optical Glass," but the outline of the influence of composition 
on properties here given could not be closed without some reference 
to this pioneer work. 

The chemical behaviour of glass surfaces, to which we have 
already referred, is of the utmost importance to all users of glass. 
The relatively neutral chemical behaviour of glass is, in fact, one 
of its most useful properties, and, next to its transparency, most 
frequently the governing factor in its employment for various 
purposes. Thus the entire use of glass for table-ware depends 
primarily upon the fact that it does not appreciably affect the 
composition and flavour of edible solids or liquids with which it 
is brought into contact — a property which is only very partially 
shared even by the noble metals. Again, the use of glass windows 
in places exposed to the weather would not be feasible if window- 
glass were appreciably attacked by the action of water or of the 
gases of the atmosphere. For these general purposes, it is true, 
most ordinary glasses are adequately resistant, but this degree 
of perfection in this respect is only the outcome of the centuries 
of experience which the practical glass-maker has behind him in 
the manufacture and behaviour of such glass. When, however, 
a higher degree of chemical resistance is required for special pur- 
poses, as for instance when glass is called upon to resist exposure 
'to hot, damp climates, or is intended to contain corrosive liquids. 


the ruleB whioh are an adequate guide to the glass-maker in meeting 
ordinary requirements aie no longer sufficient, particularly when 
the glass is expected to meet other stringent requirements as well. 
It has, in fact, frequently happened that a glass-maker, in striving 
to improve the colour or quality of his glass, as regards freedom 
from defects, brilliancy of surface, etc., has spoilt the chemical' 
durability of his products. The reason lies in the fact, long known 
in general terms, that an increased alkali content reduces the 
chemical resistance of glass, while at the same time such an increase 
of alkali is the readiest means whereby the glass-maker can improve 
his glass in other respects by making it more fusible and easier to 
work in every way. 

This subject of the chemical stability of glass surfaces attracted 
much attention during the later part of last century, and careful 
investigations on the subject were carried out, particularly at the 
Beichsanstalt (Imperial Physical Laboratory) at Charlottenburg. 
More recently (since 1914) this subject has also received much 
attention in England, where researches have been carried out, 
notably by Sir Herbert Jackson at King's College, London, and 
at the National Physical Laboratory. As a result glasses of British 
manufacture are now available which answer the most stringent 
requirements. Similar efforts have also been made, with successful 
results, in France and America. 

Leaving aside the inferior glasses, containing, generally, more 
than 16 per cent, of alkali, the behaviour of glass surfaces to the 
principal chemical agents may be summed up in the following 
statements. Pure water attacks all glass to a greater or lesser 
extent ; in the best glasses the prolonged action of cold water merely 
extracts a minute trace of alkalies, but in less perfect kinds the 
extraction of alkali is considerable on prolonged exposure even in 
the cold, and becomes rapidly more serious if the temperature is 
raised. Superheated water, f.6., water under steam pressure, 
becomes an active corroding agent, and the best glasses can only 
resist its action for a limited time. For the gauge-glass tubes of 


steam boilers working at the high pressures which are customary 
at the present time, specially durable glasses are required and 
can be obtained, although many of the gauge-tubes ordinarily. sold 
are quite unfit for the purpose, both from the present point of view 
and from that of strength and " thermal endurance." 

In certain classes of glass, the action of water, especially when 
hot, is not entirely confined to the surface, some water penetrating 
into the mass of the glass to an appreciable depth. The exact 
mechanism of this action is not known, but the writer inclines to 
the view that it arises from a partial hydration of some of the silica 
or silicates present in the glass. If such glasses be dried in the 
ordinary way and subsequently heated, the surface will be riddled 
with minute cracks, some glass may even flake ofi, and the whole 
surface will be dulled. Als such penetrating action sometimes takes 
place — ^in the poorer kinds of glass — ^by the action of atmospheric 
moisture when the glass is merely stored in a damp place, it is often 
mistaken for " devitrification." This latter action, however, is not 
known to occur at the ordinary temperature, although glass when 
heated in a fliame frequently shows the phenomenon ; it is, however, 
entirely distinct from the surface " corrosion " just described. 

Water containing alkaline substances in solution acts upon all 
glasses in a relatively rapid manner ; it acts by first abstracting 
silica from the glass, the alkali and lime being dissolved or mechani- 
cally removed at a later stage. Water containing acid bodies in 
solution — i.e., dilute acid — on the other hand acts upon most 
varieties of glass decidedly less energetically than pure water, 
and much less vigorously than alkaline solutions ; this peculiar 
behaviour probably depends upon the tendency of acids to prevent 
the hydration of silica, this substance being thereby enabled to 
act as a barrier to the solvent action of the water upon the alkaline 
constituents of the glass. The better varieties of glass are also 
practically impervious to the action of strong acids, although 
certain of these, such as phosphoric and hydrofluoric, ezert a rapid 
action on all kinds of glass. Only certain special glasses, containing 


an excessive proportion of basic constituents and of such siil>- 
stances as boric or phosphoric acid, are capable of being completely 
decomposed by the action of strong acids, such as hydrochloric or 
nitric, the bases entering into combination with the acids, while 
the silicic and other acids are liberated. 

In iDonnection with the action of acids upon glass, m«ition should 
be made of certain special actions that are of practical importance. 
The dissolving action of hydrofluoric acid upon glass is, of course, 
well known. This acid is used in practice both in the liquid and 
gaseous form, and also in that of compounds from which it is readily 
liberated (such as ammonium or sodium fluoride), for the purpose 
of '' etching " glass, and also in decomposing glass for purposes of 
chemical analysis. Next in importance ranks the action of carbonic 
acid gas upon glass, especially in the presence of moisture. The 
action in question is probably indirect in character ; the moisture 
of the air, condensing upon the surface of the glass, first exerts its 
dissolving action, and thus draws from the glass a certain quantity 
of alkali, which almost certainly at first goes into solution as alkali 
hydrate (potassium or sodium hydroxide) ; this alkaline solution, 
however, rapidly absorbs carbonic acid from the air, and the car- 
bonate of the alkali is formed. If the glass dries, this carbonate 
forms a coating of minute crystals on the surface of the glass, 
giving it a dull, dimmed appearance ; this, however, only occurs 
ordinarily with soda glasses, since the carbonate of potassium is 
too hygroscopic to remain in the dry solid state in any ordinary 
atmosphere. Potash glasses are, as such, no more stable chemically 
than soda glasses, but they are for the reason just given less liable 
to exhibit a dim surface. If the dimming process, in the case of a 
soda glass, has not gone too far, the brightness of the surface of the 
glass may be practically restored by washing it with water, in 
which the minute crystals of carbonate of soda readily dissolve, 
while separated silica is removed mechanically. An attempt made 
to clean the same dimmed surface by dry wiping would only result 
in finally ruining the surface, since the small sharp crystals of 


carbonate of soda would be rubbed about over the surface, scratching 
it in all directions. 

The dimming process in the case of the less resistant glasses is 
not only confined to the formation of alkaline carbonates ; the 
films of alkaline solution which are formed on the surface of glass 
form a ready breeding-ground for certain forms of bacteria and 
fungi, whose growth occurs partly at the expense of the glass itself ; 
the precise nature of these actions has not been fully studied, but 
there can be little doubt that silicate miaerals — and glass is to be 
reckoned among these — ^are subject to bacterial decomposition, 
a well-known example in* another direction being the " maturiag " 
of clays by storage in the dark, the change in the clay being accom- 
panied by an evolution of ammonia gas. In the case of glass it 
has been shown that specks of organic dust falling upon a surface 
may give rise to local decomposition. In this connection it is 
interesting to note the effect of the presence of a small proportion 
of boric acid in some glasses. The presence of this ingredient in 
small proportions is known to render the glass more resistant to 
atmospheric agencies, and more especially to render it less sensitive 
to the effects of organic dust particles lying upon the surface. It 
has been suggested — ^probably rightly — ^that the boric acid, entering 
into solution in the film of surface moisture, exerts its well-known 
antiseptic properties, thus protecting the glass from bacterial and 
fungoid activity. 

The durability of glass under the action of atmospheric agents 
is a matter of such importance that numerous efforts have been 
made to e^tablish a satisfactory test whereby this property of a 
given glass may be ascertained without awaiting the results of 
experience obtained by actual use under unfavourable conditions. 
One of the earliest of the tests proposed consisted in exposing 
surfaces of the glass to the vapour of hydrochloric acid. For this 
purpose some strong hydrochloric acid is placed in a glass or porce- 
lain basin, and strips of the glass to be tested are placed across the 
top of the basin, the whole being covered with a bell-jar. After 


several days the glass is ezamined» and as a rule the less stable 
glasses show a dull, dimmed surface as compared with the more 
stable ones. A more elaborate form of test depends upon the fact 
that aqueous ether solutions react readily with the less stable kinds 
of glass ; if a suitable dye, such as iod-eosin, be dissolved in the 
water-ether solution, then the efEect upon the less stable glasses 
when immersed in the solution is the formation of a strongly adherent 
pink film. The density or depth of colour of this film may be re- 
garded as measuring the stability of the glass ; the best kinds of 
glass remain practically free from coloured film. In its application 
to optical glass the test is made on a treshly fractured surface 
which, after careful brushing with a clean, dry brush, is first exposed 
to a moist atmosphere for a week in a closed vessel. The glass is 
then dipped in an aqueous ether solution of iod-eosin (terta- 
iodo-fluorescein) where the pink film is formed ; it is then washed 
by dipping in ether, which removes the excess of iod-eosin. The 
pink film is then dissolved ofE in water and the colour of the rtoulting 
solution is matched against a standard solution containing a known 
amount of sodium-iod-eosin. In this way a quantitative estimation 
of the amount of alkali liberated from the glass surface is obtained. 
Optical glasses have been classified in five categories according to 
the results given under this test, as follows : — 

Glass ** Hi " gives to 5 milligrammes of iod-eosin per sq. metre. 

Class "H« " „ 5 to 10 

aass"Ha " „ 10 to 20 

aass"H, " „ 20to40 

Class " Hg " „ 40 to 80 

In its application to glass-ware for use in chemical laboratories 
the test is used in a slightly difierent form. The vessels are first 
given a preliminary cleaning by standing with water at 20° C. for 
one week. They are then emptied and refilled with fresh water 
and allowed to stand for another week, again at 20° C. A measured 
volume of this water, now containing the alkalies dissolved from 
the glass, is then taken and shaken with a measured volume of the 

>> >> >> >> 

yy 99 99. 99 

99 99 99 99 


ether solution of iod-eosin. The water then becomes coloured 
by the formation of sodium iod-eosin a*nd some free iod-eosin is 
also dissolved in it. The latter is removed by shaking with ether 
saturated with water. The colour of the remaining water solution 
is then estimated by comparison with a standard solution as before. 

There is no doubt that this test, if the working conditions are 
very carefully observed, gives a sharp classification of glasses, 
but there is good reason to doubt whether this classification agrees 
with their true durability in practice, since certain glasses which 
have proved very satisfactory in this respect in practical use all 
over the world were classed among the less stable kinds by this 
test. On the other hand certain dense lead and barium glasses 
which are not very stable in practice, yet receive a high classification 
under the iod-eosin test, as this only takes note of alkali liberated. 

Recently a method of testing glass for durability has been 
developed in which the glass is exposed to superheated water in 
an autoclave under a steam pressure of 4 to 5 atmospheres. At 
the correspondingly high temperature, water attacks glass with 
comparative rapidity, and the test is based on the assumption that 
the resisting power of glass against the action of water in the auto- 
clave is proportional to its power of resistance under more ordinary 
conditioBS. This assumption has yet to be finally established, but 
the test furnishes a means of quantitative comparison between 
different kinds of glass which — ^in its present form — is only applicable 
to hollow-ware such as laboratory vessels. It might, however, be 
applied to optical and other glass if its trustworthiness is demon- 
strated. The extent to which the glass has been attacked is measured 
both by determining the alkali content of the water which has 
stood in the vessel during the test and also by evaporating this 
solution and weighing the total solid residue, and this residue will 
include, besides the alkali, any silica, lead, barium, etc., which 
may have been extracted from the glass by the superheated water. 

Before leaving the subject of the chemical behaviour of glass, 
a reference should be made to the changes which glass imdergoes 


when acted upon by light and other radiations. Under the influence 
of prolonged exposure to strong light, particularly to sunlight, 
and still more so to ultra-violet light, or the light of the sun at 
high altitudes, practically all kinds of glass undergo changes which 
generally take the form of changes of colour. Glasses containing 
manganese especially are apt to assume a purple or brown tinge 
under such circumstances, although the powerful action of radium 
radiations is capable of producing similar discoloration in glasses 
free from manganese. Apart from these latter efiects, of which 
very little is known as yet, there can be no doubt that the action 
of light brings about chemical changes within the glass, but it is by 
no means easy to ascertain the true nature of these changes, although 
they most probably consist in a transfer of oxygen from one to 
another of the oxides present in the glass. Although it has not 
been definitely proved, it seems very unlikely that the glass either 
loses or gains in any constituent during these changes. Good 
examples of the changes undergone by glass imder the action of 
sunlight are frequently found in skylights, where the oldest panes 
sometimes show a decided purple tint which they did not possess 
when first put in place. The glass spheres of the instruments used 
for obtaining records of the duration of simshine at meteorological 
stations also show signs of the changes due to light — ^the glass of 
these spheres when new has a light greenish tint, but after prolonged 
use the colour changes to a decided yellow. The coloured glass in 
stained-glass windows also shows signs of having undergone changes 
of tint in consequence of prolonged exposure to light ; glass removed 
from ancient windows usually shows a deeper tint in those portions 
which have been protected from the direct action of light by the 
leading in which the glass was set, and it is at least an open question 
whether the beauty of ancient glass may not be, in part, due to the 
mellowing effect of light upon some of the tints of the design. This 
photo-sensitiveness of glass is also of some importance in connection 
with the manufacture of photographic plates. It has been foimd 
that if the glass plate of a strongly-developed negative be cleaned, 


a decided trace of the former image is retained by the glass, and 
thisimage is apt to reappear as a '^ ghost" if the same glass is again 
coated with sensitive emulsion and again exposed and developed. 
The best makers of plates recognise this fact and do not re-coat glass 
that has once been used for the production of a negative. 



The Mechanical Properties of CHas$ are of considerable importance 
in many directions. Although glass is rarely used in such a manner 
that it is directly called upon to sustain serious mechanical stresses, 
the ordinary uses of glass in the glazing of large windows and sky- 
lights depend upon the strength of the material to a very con- 
siderable extent. Thus in the handling of plate-glass in the largest 
sheets, the mechanical strength of the plates must be relied upon 
to a considerable extent, and it is this factor which really limits 
the size of plate that can be safely handled and installed. The 
same limitation applies to sheet-glass also, for, although its lighter 
weight renders it less liable to break under its own weight, its 
thinner section renders it much more liable to accidental fracture. 
In special cases, also, the mechanical strength of glass must be 
relied upon to a considerable extent. Gauge tubes of high-pressure 
boilers, port-hole glasses in ships, the glass prisms inserted in 
pavement lights, and the glass bricks which have found some use 
in France, as well as champagne bottles and mineral water bottles 
and syphons, are all examples of uses in which glass is exposed to 
direct stresses. It is, therefore, a little surprising that while the 
mechanical properties of metals, timbers, and all manner of other 
materials have been studied in the fullest possible manner, those 
of glass have received very little attention, at all events so far as 
published data go. One reason for this state of afEairs is probably 
to be found in the fact that it is by no means easy to determine the 
strength of so brittle and hard a body as glass. As a consequence 
even the scanty data available can only be regarded as first approxi- 



mations. The following data are only intended to give an idea of 
the general order of strength to be looked for in glass: — 

Tensile strength : 
Erom 1 to 4 tons per sq. in. (Trautwine). 

1 to 1^ „ „ „ (Henrivaux). 

2 to 5| „ „ „ (Winkelmann and Schott). 
5 to 6 „ „ „ (Eowalski). 

Crushing strength : 

Prom 9 to 16 tons per sq. in. (Trautwine). 

3 to 8 „ „ „ (Winkelmann and Schott). 
20 to 27 „ „ „ (Kowalski). 

Of the above figures the experiments of Winkelmann and Schott 
are probably by far the most reliable, but these refer to a series 
of special Jena glasses, selected with a view to determining the 
influence of chemical composition on mechanical properties, and, 
unfortunately, this series does not include glasses at all closely 
resembling those ordinarily used for practical purposes. The 
attempt to connect tensile and crushing strength with chemical 
composition was also only very partially successful ; but the 
results serve to show that the chemical composition has a profound 
influence on the mechanical strength of glass, so that by systematic 
researxih it would probably be possible to produce glasses of con- 
siderably greater mechaniccd strength than those at present known. 
It must be noted in this connection that the mechanical properties 
of glass depend to a very considerable extent upon the rate of 
cooling which the specimen in question has undergone. It is well 
known that by rapid cooling, or quenching, the hardness of glass 
can be considerably increased ; such treatment also increases the 
strength both as against tension and compression, and numerous 
processes have been put forward for the purpose of utilising these 
efiects in practice. Unfortunately the " hardened " glass thus 
obtained is extremely sensitive to minute scratches, and flies to 
pieces as soon as the surface is broken, when the great internal 



stress which always exists in such glass is thereby relieved. All 
these peculiarities are, of course, dependent as to their d^ee up<»i 
the rapidity with which the glass has been cooled, and the aim of 
inventors in this field has been to devise a rapid cooling process 
which should strike the happy mean between the increased strength 
and the induced brittleness resulting from quenching. Thus pro- 
cesses for '* tempering " glass by cooling it in a blast of steam or 
in a bath of hot oil or grease have been brought forward ; but, 
although some such glass is manufactured, no very extensive 
practical application has resulted. 

Elasticity and DuctUUy of Olass. — ^In a series of glasses investigated 
by Winkelmann and Schott, the modulus of elasticity (Young's 
Modulus) varied from 3,500 to 5,100 tons per sq. in., the value being 
largely dependent upon the chemical composition of the glass. 
MeasTurable ductility has not been observed in glass under ordinary 
conditions except in the case of champagne bottles under test by 
internal hydraulic pressure ; in these tests it was found that a 
permanent increase of volume of a few tenths of a cubic centimetre 
could be obtained by the application of an internal pressure just 
short of that required to burst the bottle — ^pressure of the order 
of 18 to 30 atmospheres being involved. This small pemxan^it 
set has been ascribed to incipient Assuring of the glass, and this 
explanation is probably correct. On the other hand, glass is capable 
of decided flow under the prolonged action of relatively small forces ; 
the behaviour of large discs of worked optical glass suggests some 
such action, but a more familiar and well-marked example is found 
in the behaviour of ordinary glass tubing. If this is stored in a 
slightly inclined position — as in leaning against the wall in a corner — 
it gradually sags to a very marked extent, so that glass tubing which 
is required to remain straight must be kept lying flat on a shelf. 
In this respect glass behaves in a manner which recalls the behaviour 
of pitch or of sealing-wax, and like these materials it is essentially 
a liquid of very high viscosity. 

Hardness is a property of some importance in most of the applica- 


tions of glass. The durability of glass objects which are exposed 
to handling or to periodical cleaning must largely depend upon 
the power of the glass to resist scratching ; this applies to such 
objects as plate-glass windows and mirrors, spectacle and other 
lenses, and in a minor degree to table-ware. On the other hand, 
the exact definition and means of measuring hardness are not yet 
satisfactorily settled. Experimenters have foimd it very difficult 
to measure the direct resistance to scratching, since it is found, 
for example, that two glasses of very different hardness are yet 
oapable of decidedly scratching each other under suitable con- 
ditions. Resort has therefore been had to other methods of measuring 
hardness ; the method which, from the experimental point of view, 
is, perhaps, the most satisfactory, depends upon principles laid 
down by Hertz and elaborated experimentally by Auerbach. This 
depends upon measuring the size of the circular area of contact 
produced when a spherical lens is pressed against a flat plate of the 
same glass with a known pressure. Auerbach himself found some 
difficulty in deciding the exact connection between the " indentation 
modulus " thus determined and the actual hardness of the glass. 
This method is, therefore, of theoretical interest rather than of 
use in testing glasses for hardness. A test of a more practical kind 
consists in exposing specimens of the glasses to be tested. to abrasion 
against a revolving disc of cast-iron fed with emery or other abrasive, 
and to measure the loss of weight which results from a given amount 
of abrading action under a known contact pressure. If a number 
of specimens of different glasses are exposed to this test at one time, 
a very good comparison of their power of resisting abrasion can be 
obtained. It is not quite certain that this test measures the actual 
"" hardness " of the glass, but it affords some information as to its 
power of resisting abrasion, and for many purposes this power is 
the important factor. 

Hardness being, as indicated above, a somewhat indefinite term, 
it is not possible to give any precise statement as to the influence 
of chemiccd composition upon the hardness of glass. In general 

c 2 


t€rmB it maj be said that glasseB rich in ailica and lime will be 
found to be hard, while glasses rich in alkali, lead or barium are 
likely to be soft. It must, however, be borne in mind that rapid 
cooling, or even the lack of careful annealing, will produce a very 
great increase of hardness in even the softest glasses. The actual 
behaviour of a given specimen of glass will, therefore, depend 
upon the nature of the processes which it has undergone as well 
as upon its chemical composition. 

The Thermal Properties <^ GUuSy although not of such general 
importance as the mechanical propertiecf, are yet of considerable 
interest in a large number of the practical uses to which glass is 
constantly applied. Peihaps the most important of these properties 
is that known as thermal endurance, which measures the amount 
of sudden heating or cooling to which glass may be exposed without 
risk of fracture ; the chinmeys employed in connection with incan- 
descent gas burners, miner's lamp chimneys, boiler gauge glasses, 
laboratory vessels, and even table and domestic utensils are all 
exposed at times to sudden changes of temperature, and in many 
cases the value of the glass in question depends principally upon 
its power of undergoing such treatment without breakage. The 
property of " thermal endurance " itself depends upon a consider- 
able number of more or less independent factors, and their influence 
will be readily understood if we follow the manner in which sudden 
change of temperature produces stress and, sometimes, fracture in 
glass objects. If we suppose a hot liquid to be poured into a cold 
vessel, the first effect upon the material of the vessel will be to 
raise the temperature of the inner surface. Under the influence of 
this rise of temperature the material of this inner layer expands, 
or endeavours to expand, being restrained by the resistance of the 
central and outer layers of material which are still cold ; the result 
of this contest is, that while the inner layer is thrown into a state 
of compression, the outer and central layers are thrown into a 
state of tension. Accordingly, if the tension so produced is suffi- 
ciently great, the outer layers fracture under tension and the whole 


vessel is shattered by the propagation of the crack thus initiated. 
From this description of the process it will be seen that a high 
coefficient of expansion and a low modulus of elasticity will both 
favour fracture, whUe high tensile strength will tend to prevent it. 
The thermal conductivity of the glass will also afiect the result, 
because the intensity of the tensile stress set up in the colder layers 
of glass will depend upon the temperature gradient which ejrists in 
the glass ; thus if glass were a good conductor of heat it would 
never be possible to set up a sufficient dijSerence of temperature 
between adjacent layers to produce fracture ; for the same reason, 
vessels of very thin glass are less apt to break under temperature 
changes than those having thick walls, since the greatest difference 
of temperature that can be set up between the inner and outer 
layers of a thin-walled vessel can never be very considerable. It 
also follows from these considerations that if a cold glass vessel 
be simultaneously heated or cooled from both sides, it can be safely 
exposed to a much more sudden change of temperature than it 
could withstand if heated from one side alone ; on the other hand, 
when very thick masses of glass have to be heated, this must be 
done very gradually, as a considerable time will necessarily elapse 
before an increment of tempeorature applied to the outside will 
penetrate to the centre of the mass. It should also be noted here 
that in addition to the thermal conductivity of the glass, its heat 
capacity or specific heat also enters into this question, since heat 
will obviously penetrate more slowly through a glass whose own 
rise of temperature absorbs a greater quantity of heat. It wiU 


thus be seen that '' thermal endurance " is a somewhat complicated 
property, depending upon the factors named above, viz. : coefficient 
of expansion, thermal conductivity, specific heat, Yoimg's modulus 
of elasticity, and tensile strength. 

The coefficient of thermal expansion varies considerably in 
different glasses, and we can here only state the limiting values 
between which these coefficients usually lie ; these are 37 X 10^ 
as the lower, and 122 X 10""^ as the upper limit. These figures 


express the cubical expansion of the glass per degree Centigrade, 
the corresponding figures for steel and brass respectively being 
about 360 X 10"'' and 648 X 10 ~' respectively. It is interesting to 
compare with these figures the corresponding value for pure vitreous 
silica, which, between 0° and 100° C. is 15 X 10"'. It should be 
noted that vitreous bodies of extremely low expansibility are 
obtainable by the suitable choice of ingredients, but in some cases 
these *' glasses " are white opaque bodies, and in all cases they 
present great difficulty in manufacture, owing to the fact that alkalies 
and lime must be avoided in their composition. 

Quite apart from the question of thermal endurance, the expansive 
properties of glass are of some importance. Thud, when several 
kinds of glass have to be united, as, for example, in the process of 
producing ** flashed " coloured glass, it is essential that their co- 
efficients of expansion should be as nearly as possible the same ; 
otherwise considerable stresses will be set up when the glasses, • 
which have been joined at a red heat, are allowed to cool. On the 
other hand, this mutual stressing of two glasses owing to differences 
in their thermal expansion has been utilised for the producti(Hi of 
tubes and other glass objects possessing special strength. If a tube 
be drawn out of glass consisting of two layers, one considerably 
more expansible than the other, and the cooling process be rightly 
conducted, it is possible to produce a tube in which both the inner 
and outer layers of glass are imder a considerable compressive 
stress. Not only is glass, as we have seen above, enormously stronger 
as against compression than it is against tension, but glass under 
compressive stress behaves as though it were a mu<)h tougher 
material, being less liable to injury by scratches or blows. Moreover, 
if a tube in this condition be heated and then exposed to sudden 
cooling, the first effect of the application of cold will be a contraction 
of the surface layers, resulting in a relief of the initial condition of 
compression. These tubes are, therefore, remarkably indifferent 
to sudden cooling, although they are naturally more sensitive to 
sudden heating. In this respect they differ entirely from ordinary 


glass, which is considerably more sensitive to sudden cooling than 
to sudden heating, particularly when the heat or cold is applied to 
all the surfaces of the object at the same time. Special tubes made 
of two layers of glass in this manner have been manufactured for 
special purposes, among which boiler gauge glasses are the most 
important. It should also be mentioned here that the remarkable 
thermal endurance of vitrified silica, which can be raised to a red 
heat and then immersed in cold water without risk of breakage, is 
chiefly due to its very low coefficient of expansion. 

In another direction the expansive properties of glass are of 
importance wherever glass is rigidly attached to metal. At the 
present time this is done in several industrial products, such as 
incandescent electric lamps and " wired " plate glass. In certain 
varieties of incandescent lamps, metallic wires are sealed into the 
glass bulbs, and the only metal available for this purpose, at all 
events until recently, has been platinum, whose coefficient of 
expansion is low as compared with most metals, and whose freedom 
from oxidation when heated to the necessary temperature makes 
it easy to produce a clean joint between glass and metal. The 
use of certain varieties of nickel steel has been patentid for this 
purpose, since it is possible to obtain nickel steel alloys of almost 
any desired coefficient of e3q)ansion from that of the alloy known 
as "invar," having a negligibly small expansion compared with 
that of ordinary steel. By choosing a suitable member of this 
series a metal could be obtained whose coefficient of expansion 
corresponds exactly with that of the glass to which it is to be united. 
The oxidation of the nickel steel when heated to the temperature 
necessary for effecting its union with the glass presented serious 
difficulties to the production of a tight joint, and several devices 
for avoiding this oxidation have been patented. The most effective 
probably consists in coating the nickel steel with a thin sheath of 
platinum. In the incandescent electric lamp, although the joint 
between glass and metal is required to be perfectly air-tight, the 
two bodies are only attached to one another over a very short 


length. In wired plate glass, however, an entire layer of wire 
netting is interposed between two layers of glass, the wire being 
inserted during the process of rolling. Here a certain amount of 
oxidation of the wire is not of any serious importance, as it only 
appears to give rise to a few bubbles, whose presence does not 
interfere with the strength and usefulness of the glass ; but any 
consid^able difference of coefficient of expansion wiU produce the 
most serious results on account of the great lengths of glass and 
metal that are attached to each other. This factor has been neglected 
by some manufacturers, with the result that much of the wired glass 
of commerce is liable to crack spontaneously some time after it has 
left the manufacturer's hands, while there is also much loss by 
breakage during the process of manufaoture. 

Thermal expansion is a vital factor in yet another of the uses 
of glass. Our ordinary instrument for measuring temperature — 
the mercury thermometer — ^is very considerably affected by the 
expansive behaviour of glass. When a mercury thermometer is 
warmed the mercury column rises in the stem because the mercury 
expands upon warming to a greater extent than the glass vessel, 
bulb and stem, in which it is contained. The subject of the gradua- 
tions and corrections of the mercury glass thermometer is a very 
large one and somewhat outside the scope of the present volume ; 
but attention should be drawn in this place to the peculiarities of 
the behaviour of glass that have been discovered in this connection. 
One of these is that when first blown the bulb of a thermometer 
takes a very considerable time to acquire its final volume, the 
result being, that if a freshly made thermometer is graduated, 
after some time the zero of the instrument will be found considerably 
changed, generally in a direction which indicates that the volume 
of the bulb has slightly increased. By a special annealing or 
" ageing " process this change can be completed in a comparatively 
short time before the instrument is graduated. There is, however, 
a further peculiarity which is prominent in some thermometers, 
although very greatly reduced in the best modern glasses. This 


becomes apparent in a decided change of zero whenever the ther- 
mometer has been escposed for any length of time to a high tem- 
perature, the zero gradually returning more or less to its original 
position in the course of time. With thermometers made of glasses 
liable to these aberrations, the reading for a given temperature 
depended largely upon the inmiediate past history of the instru- 
ment ; but thermometer glasses are now available which are almost 
entirely free from this defect. In this connection the curious fact 
has been observed that glass containing both the alkalies (potash 
and soda) shows these thermal effects much more markedly than 
a glass containing one of the alkalies only. 

The thermal oovductivUy of glass, except in so far as it affects 
the thermal endurance, is not a matter of any great direct practical 
importance, although the fact that glass is always a comparatively 
poor conductor of heat is utilised in many of its applications, as, 
for example, the construction of conservatories and hot-houses, 
although even in that case the opacity of glass to thermal radiations 
of long wave lengths is of more importance than its low thermal 
conductivity. Similar stat^ents apply, in a still more marked 
d^ee, to the subject of the specific heat of glass. 

The Electrical Properties of glass are of much greater practical 

importance, glass being frequently used in electrical appliances as 

an insulating mediimi. The insulating properties of glass, as well 

as the property known as the specific inductive capacity, vary 

greatly according to the chemical composition of the material. 

Generally speaking, the harder glasses, i,e,, those richest in silica 

and lime, are the best insulators, while soft glasses, rich in lead or 

alkali, are much poorer in this respect. In practice, particularly 

when the glass insulator is exposed to even a moderately damp 

atmosphere, the nature of the glass affects the resulting insidation 

or absence of insulation, in another way. Almost all varieties of 

glass have the property of condensing upon their surfaces a decided 

film or layer of moisture from the atmosphere, and, as we have 

seen above, glasses differ very considerably in the degree to which 


they display this hygrosoopio tendency. The softer glasses are 
muoh more hygroscopic than the hard ones, and the resulting film 
of surface moisture serves to lessen or even to break down the 
insulating power of the glass, the electricity leaking away along the 
film of moisture. In the case of appliances for static electricity, 
where very high voltages have to be dealt with, an endeavour is 
sometimes made to avoid this leakage by varnishing the surface 
of the glass with shellac or other similar substance, and this proves 
a satisfactory remedy up to a certain point. 

Although glass at the ordinary temperature is rightly r^arded 
as one of the best insulators, yet at high temperatures, when the 
glass is in a molten or — ^niore properly— in a mobile condition, it 
becomes a relatively good conductor of electricity. Its conducting 
powers when in the mobile liquid state are due to the fact that it 
acts as an electrolyte, and — ^if matters are suitably arranged — ^it 
can be electrolysed in such a manner as to cause an actual separation 
of the metallic elements present at the cathode. This property of 
becoming a fairly good electrical conductor when molten makes 
it possible to heat glass electrically by the passage of a sufficiently 
heavy current, and this process is employed practically in certain 
types of electric furnace. It is, however, surprising to find that 
glass can begin to act as an electrolyte and can undergo appreciable 
electrolysis at a temperature as low as 200° C. This has been 
experimentally proved by Warburg and subsequently by Roberts- 
Austen, who describes his ei2q)eriments in the Third Report to the 
Alloys Research Committee of the Institution of Mechanical 
Engineers in 1895. The results are so remarkable that a part of the 
description may be quoted : — " Thick bulbs were blown from baro- 
meter tube (of soda glass) ; and in most of the experiments the glass 
was electrolysed, using mercury and an amalgam of some metal as 
cathode and anode respectively. The temperature was from 250° C. 
to 350° C. ; the electromotive force employed was 100 volts ; and 
the current in the case of thd sodium experiments averaged about 
one thousandth of an ampere and was sometimes as hi^h as on^ 


fiftieth of an ampere. . . . Li the experiments in which sodium 
amalgam had been placed in the bulb and pure mercury outside, 
sodium passed into the mercury to the extent of 0*03 grammes. . . . 
The passage of the mercury follows the ordinary laws of electrolysis. 
... It is doubtful whether the sodium from the amalgam actually 
penetrated right through the glass, but there can be no question 
that it replaced a considerable proportion of the sodium which the 
glass contained. An attempt to pass potasdium through the same 
glass failed. Gold was then used, both in the form of amalgam 
and dissolved in metallic lead, but in the latter case the temperature 
employed was of course higher. No gold was found to have been 
transmitted through the glass ; but the glass employed became 
coloured by gold, and minute spangles of the metal were found 
embedded in it." This work of Roberts-Austen confirmed the 
results of Warburg, who had found that while lithium could be 
caused to pass electrolytically through a soda glass, potash could 
not — ^the diJSerence being ascribed to the fact that lithium has 
a lower atomic volimie than sodium, while potassiimi has a higher 
atomic volume. Even in the case of lithiimi, however, the glass 
used for the electrolysis became opaque and brittle. Perhaps the 
most important significance of these facts lies in the obvious con- 
clusion that ordinary glass at so moderate a temperature as 200^ C. 
is sufficiently fluid to act as an electrolyte — ^a. fact which serves 
to strengthen the view — ^now almost universally accepted — ^that 
glass is essentially a viscous liquid and not a " solid " in the strict 

The most valuable and in many ways the most interesting of 
the properties of glass — ^its transparency — ^has not been dealt with 
as yet, and all mention of this subject has been postponed to the 
end of the present chapter, because the whole subject of the optical 
properties of glass will be dealt with more fully in the chapter on 
optical glass (Chap. XIII.), so that a very brief reference only need 
be made to the matter here. 

There can be no doubt that, in most of its practical applications, 


transparency is the fundamental and essential property which 
leads to the employment of glass in the place of either stronger or 
cheaper materials. By transparency, in this sense, we wish to 
include mere translucence also, since very frequently it is as neces- 
sary to avoid undisturbed visibility as it is to secure the admission 
of light. It is indeed hard to find any use to which glass is exten- 
sively put into which the function of transmitting light does not 
very largely enter. Almost the only such example of use is the 
modem application of opal glass to the covering of walls, and the 
use — ^not as yet widely extended — of pressed glass blocks as bricks 
and paving stones ; in these cases it is the hardness and smoothness 
of surface that gives to the vitreous body its superiority over other 
materials, but apart from these special cases, the fact remains 
that weU over 95 per cent, of the glass used in the world is employed 
for purposes where transmission of light is essential to the attain- 
ment of the desired result, either from the point of view of utility 
or from that of beauty. It is interesting to note that the power of 
transmitting light is not shared by many solid bodies. Some col- 
loidal organic bodies, such as gelatine and celluloid, possess the 
property to a d^ee comparable with glass, while certain mineral 
crystals, such as quartz and fluor-spar, may even surpass the finest 
glass in this respect ; while some of the other optical properties of 
glass are greatly exceeded by such natural substances as the diamond 
and the ruby. But the very brevity of this list is in itself striking, 
because it must be borne in mind that transparency by no means 
constitutes the only common characteristic of vitreous bodies. 

Although the transparency of glass is so valuable and indeed so 
essential a property of that substance, it must be remembered that 
no kind of glass is perfectly transparent. Quite apart from the 
fact that of the light that falls upon a glass surface, however per- 
fectly polished, a considerable proportion is turned back by reflection 
at the surface of entry and again by reflection at the surface of exit 
from the glass, a certain proportion of light is absorbed during 
its passage through the glass itself, and the transmitted beam is 


oorrespondingly weakened. Li the purest and best glasses this 
absorption is so small that in any moderate thickness very delicate 
instruments are required to show that there has been any loss of 
light at all ; but even the best glass, when examined through a 
thickness of 20 in. or more, always shows the effects of the absorp- 
tion of light quite unmistakably. In fact, not only does all glass 
absorb light, but it does this to a different d^ee according to the 
colour of the light, so that in passing through the glass a beam of 
white light becomes weakened in one of its constituent colours 
more than in the others, with the result that the emergent Ught is 
slightly coloured. Thus the purest and whitest of glasses, when 
examined iq very thick pieces, always show a de<5ided blue or green 
tint, although this tint is quite invisible on looking through a few 
inches of the glass. The ordinary glass of commerce, however, is 
|ar removed from even this approach to perfect transparency. The 
best plate glass shows a slight greenish-blue tint, which is just 
perceptible to the trained eye when a single sheet of moderate 
thickness is laid down upon a piece of white paper. When u sheet 
of this glass is viewed edgewise, in such a way that the light reaching 
the eye has traversed a considerable thickness, the greenish-blue 
tint of the glass becomes more apparent. By holding strips of 
various kinds of glass, cut to an equal length, close together and 
comparing the colour exhibited by their ends, a means of comparing 
the colours of apparently " white " glasses is readily obtained. It 
will be found that different specimens of glass differ most markedly 
in this respect. Sheet glass is, as a rule, decidedly deeper in colour 
than polished plate, but rolled plate is as a rule much greener — ^the 
colour of this glass can, in fact, in most cases be seen quite plainly 
in looking through or at the sheets in the ordinary way. 

The question of how far the colour of glass affects the value of 
the light which it transmits depends for its answer upon the purpose 
to which the lighted space is to be put. Where delicate comparisons 
of colour are to be made, or other delicate work involving the use 
of the colour sense is to be carried on, it is essential that all colouration 


of the altering daylight ahodd be avoided, and the use of the 
most coloorleBS glass obtainable will be desirable. Again , in photo- 
graphic studios it is important to secure a glass which shall absorb 
as small a proportion of the chemically active rays contained in 
daylight as possible, and special glasses for this purpose are avail- 
able. Although f<nr the present the price of these special glasses 
may prove prohibitive for the glazing of studio lights, their use is 
found highly advantageous where artificial light is to be used to 
the best advantage. On the other hand, for everyday purposes, 
the slight tinge of colour introduced into the light by the colour 
of ordinary sheet and plate glass, or even of greenish rolled plate 
glass, has no deleterious effect whatever, the majority of persons 
being entirely unconscious of its presence. 

Glass which has been intentionally rendered absorbent for light 
of certain wave-lengths is employed for special purposes. The 
whole range of coloured glasses will, of course, fall under this descrip- 
tion, but apart from these a whole series of glasses has been developed 
by the researches of CSrookes, the object being to produce a material 
for spectacle lenses which will protect the eye of the wearer from 
the harmful effects of very short (ultra-violet) and very long (infra- 
red) light waves. This is especially important for furnace workers, 
such as those employed in glass manufacture, whose eyesight 
suffers from continued exposure to the radiation of molten glass. 
Crookes has obtained remarkable results by the introduction of 
the oxides of the rare-earth elements, notably ceria, into his glasses, 
some of which, while so slightly tinted as to be scarcely notable, 
yet afford very considerable protection to the eye. For other 
purposes, such as observers at sea or in aeroplanes, who are obliged 
to face the glare of the sun and of its reflection in the water, deeply 
tinted glasses are employed. 

Another purpose for which glass intentionally rendered absorbent 
for ultra-violet light is sometimes employed with great advantage 
is the protection of valuable objects from fading or other deteriora- 
tion resulting from prolonged exposure to strong light. Such 


exposure of valuable objects occurs both in the rooms of private 
houses and in museums and picture galleries. The glazing of such 
places with glass which — ^without being unpleasantly tinted — ^yet 
absorbs the greater part of the ultra-violet rays would undoubtedly 
effect a great reduction in the rate of deterioration of exposed 
objects, since it has been shown that ultra-violet light, although not 
the sole cause of this kind of fading, is certainly one of the most 
important and active factors in the process. 

Further consideration of the subject of the transmission of light 
by glass, its absorption, refraction, dispersion, etc., are, however, 
best grouped together as the ^' optical '' properties of glass, and 
under that heading they will receive a fuller treatment in connection 
with the subject of the manufacture of glass for optical purposes. 



The ohoice of raw materials for all branohes of glass manufacture 
is a matter of vital importance. As a rule all ^' fixed " bodies that 
are once introduced into the glass-melting pot or furnace appear 
in the finished glass, while volatile or combustible bodies are more 
or less completely eliminated during the process of fusion. Thus, 
while the chemical manufacturer can purify his products by filtra- 
tion, crystallisation, or some other process of separation, the glass- 
maker must eliminate all undesirable ingredients before they are 
permitted to enter the furnace, and the stringency of this condition 
is increased by the fact that the transparency of glass makes the 
detection of defects of colour or quality exceedingly easy. For 
the production of the best varieties of glass, therefore, an exacting 
standard of purity is applied to the substances used as raw materials. 
As the quality of the product decreases, so also do the demands 
upon the purity of raw materials, until finally for the manufacture 
of common green bottles, even such very heterogeneous substances 
as basaltic rock and the miscellaneous residues of broken, defective 
and half-melted glass forming the refuse of other glassworks may 
be utilised more or less satisfactorily. 

For the best kinds of glass the most desirable quality in raw 
materials is thus as near an approach to purity as possible under 
commercial conditions, and next to that, as great a constancy of 
composition as possible. For instance, the quantity of moisture 
contained in a ton of sand appreciably afiects the resulting com- 
position of the glass, and if the sand cannot be obtained peri^ctly 
dry, it should at least contain a constant proportion of moisture, 
otherwise it becomes necessary to determine, by chemical tests. 


the percentage of moisture in the sand that is used from day to day, 
and to adjust the quantity used in accordance with the results of 
these tests, a proceeding which, of course, materially complicates 
the whole process. Li other cases, variable composition is not so 
readily allowed for, and uncontrollable variations in the composition 
of the glass result — at times the quality falls off unaccountably, or 
the glass refuses to melt freely at the usual temperature. The 
systematic employment of chemical analysis in the supervision of 
both the raw materials and of various products will frequently 
enable the manufacturer to ti:ace the causes of such undesirable 
occurrences ; but however necessary such control undoubtedly is, 
it cannot entirely compensate for the use of raw materials liable 
to too great a variation in composition or physical character. For 
not only the chemical composition but also the physical condition 
and properties of the material are of importance in glass manu- 
facture. Thus it is essential that materials to be used for glass- 
melting should be obtainable in a reasonably fine state of division, 
and in this connection it must be remembered that both exceedingly 
hard bodies and soft plastic substances can only be ground with 
very great difficulty. Further, where a substance occurs naturally 
as a powder, this powder should be of uniform and not too fine a 
grain, more especially if it belongs to the class of refractory rather 
than of fluxing ingredients. In that case the presence of coarser 
grains will result in their remaining in the undissolved state in the 
finished glass, xmless excessive heat and duration of " founding " 
be employed to permit of their dissolution. This applies chiefly 
to siliceous and calcareous ingredients, but hardened nodules of 
salt-cake may behave in a similar manner. 

A further consideration in the choice of raw materials is facility 
of storage. Thus limestone in the shape of large lumps of stone 
which are only ground to powder as required, is readily stored, and 
undergoes no deleterious change even if exposed to the weather ; 
on the other hand, sulphate of soda (salt-cake), if stored even in 
moderately dry plap^, rapidly agglomerates into hard masses, 



at the. same time absorbing a certaia perceatage of moisture. Such 
properties are not always to be avoided, salt-cake for example being 
an indispensable ingredient in many kinds of glass-making, but 
the value of a substance is in some cases materially lessened by 
such causes. 

The raw materials ordinarily employed in glass-making may be 
grouped into the following classes : — 

(1) Sources of silica. 

(2) Sources of alkalies. 

(3) Sources of bases other than alkalies. 

(1) Sources of Silica. — The principal source of silica is sand. This 
substance occurs in nature in geological deposits, often of very 
considerable area and depth. These deposits of sand have always 
been formed by the disintegration of a siliceous rock, and the 
fragments so formed have been sifted and transported by the 
agency of water, being finally deposited by a river either in the sea 
(marine deposits) or in lakes (lacustrine deposits), while the action 
of water, 'either during transport or after deposition, has frequently 
worn the individual particles into the shape of rounded grains. 

In consequence of this origin the chemical composition of sand 
varies very greatly with the nature of the rock whose denudation 
gave rise to the deposit. Where rocks very rich in silica, or even 
consisting of nearly pure silica, have been thus denuded, the result- 
ing sand is often very pure, deposits containing up to 99*9 per cent, 
silica being known. More frequently, however, the sand contains 
fragments of more or less decomposed felspar, and smaller quan- 
tities of many other minerals, which introduce alumina, iron and 
alkalies into its composition. Finally, " sands " of all ranges of 
composition from the pure varieties just referred to down to the 
clay marls, very rich in iron and alumina, are known. 

For the best varieties of glass, viz., optical glass, flint glass and 
the whitest sheet-glass, as well as for the best " Bohemian " glass, 
a very pure variety of sand is required, preferably containing less 


than 0*05 per cent, of iron, and not more than 0*05 per cent of other 
impurities such as alumina, lime or alkali. As a matter of fact, 
sands containing so little iron rarely contain any other impurity 
except alumina in measurable quantities. The best-known deposit 
of such sand in Europe is that at Fontainebleau, near Paris, but 
equally good sand is found at various places in Germany and Austria. 
The study of British resources in glass-making sands has been 
taken up with great energy during recent years, and an exhaustive 
memoir on the subject has been prepared by Professor P. 6. H. 
Boswell. Prom the accoimt there given it appears that there is 
at least one very promising British source of pure silica situated 
at Muckish Mountain, Co. Donegal, but this is rather of the nature 
of a crushed friable rock than a true sand. This material, however, 
appears to be sufficiently pure — ^if the quality can be maintained 
when exploitation on the large scale is attempted — ^to be used for 
the highest grades of glass. Samples and correspondingly good 
analyses can also be obtained from many other British sources 
which suggest satisfactory possibilities, and experiments to utilise 
such sands for good qualities of glass have been made, in some 
instances with the promise of success. The real test, however, is 
that of continued uniformity of satisfactory quality over long 
periods of commercial exploitation. Li many cases careful washing 
or even treatment with dilute acid would efiEect very material 
improvement, but it has been rightly pointed out that the cost of 
sand for most purposes must be kept so low that little margin 
remains for the careful treatment of the material at the pit or during 
transport. It follows from this consideration that a sand deposit 
which is naturally pure and uniform will — even if under the dis- 
advantage of a greater distance — ^render competition by less- 
favoured deposits very difficult. The real importance of finding 
a home source of supply of sand for high-quality glass has, however, 
been realised, and thanks to the work of Boswell progress in that 
direction may be confidently expected. A detailed account of 
the various sand deposits and their characteristics cannot 

D 2 


be given here, and the reader is referred to Boswell's original 

Next in order of value to these exceedingly pure sandls, come 
the glass-making sands of Belgium, notably of Epinal. These 
usually contain from 0*2 to 0-3 per cent, of iron and rather more 
alumina,, but they are used very largely for the manufacture of 
sheet and plate-glass. When the standard of quality is further 
relaxed, a large number of sand deposits become available, and the 
manufacturers of each district avail themselves of more or less 
local supplies. Finally, for the manufacture of the cheapest class 
of bottles, sands containing up to 2 per cent, of iron and a con- 
siderable proportion of other substances are employed. 

Silica, in various states of purity, occurs in nature in a number 
of other forms than that of sand. By far the commonest of these 
is that of more or less compact sedimentary rock, known as '" sand- 
stone." As far as chemical composition is concerned, some of these 
stones are admirably suited for making the best kinds of glass, 
although as a rule a stone is not so homogeneous as the material 
of a good sand-bed. The stone has the further disadvantage that 
it requires to be crushed to powder before it can be used for glass- 
making, and the crushed product is generally a mixture of grains 
of all sizes ranging from a fine dust to the largest size of grain 
passed by the sieves attached to the crushing machine. The presence 
of the very fine particles is a distinct objection from the glass-maker's 
point of view, so that it would probably be necessary to wash the 
sand to remove this dust — a process that in itself adds to the 
cost of the crushed stone and at the same time leads to the loss of 
a serious percentage of the material. Objections of the same kind 
apply, but with still greater force, to the use of powdered quartz 
or flint as sources of silica for the glass-maker ; further, these 
materials are exceedingly hard and therefore difficult to crush, 
so that their price is prohibitive for glass-making purposes. The 
use of ground quartz and flint is therefore confined to the ceramic 
industaries in which these substances serve as sources of silica for 


both bodies and glazes; in former times, however, ground flint 
was ext^isively used in the manufacture of the best kinds of glass, 
as the still surviving name of " flint glass " testifies. 

Minerals of the felspar class, consisting essentially of silicates of 
alumina and one or more of the alkalies^ are extensively used in 
glass-making and should be mentioned hercj since their high sHica- 
content (up to 70 per cent.) constitutes an effective source of silica: 
As a source of this substance, however, most felspars would be 
far too expensive, and their use is due to their content of alumina 
and alkali. 

(2) Sources of Alkali, — Originally the alkaline constituents of 
glass were derived from the ashes of plants aftd of seaweed or " kelp " ; 
in both cases the alkali was obtained in the form of carbonate and 
was otdinarily used in a very impure form; at the present time, 
however, the original source of alkali for industrial purposes is 
found in the natural deposits and other sources of the chlorides of 
sodium and potassiimi. At the present time it is not yet industrially 
possible to introduce the alkalies into glass mixtures in the natural 
form of chlorides. The principal difficulty in doing this arises from 
the fact that the chlorides are volatile at the temperature of glass- 
melting furnaces and are only acted upon by hot sili(» in the presence 
of water vapour. Introduced into an ordinary glass furnace, there- 
fore, these salts would be driven off as vapour before they could 
combine with the other ingredients in the desired form of double 

Alkalies are, therefore, introduced into the glass mixture in less 
volatile and m(nre readily attackable forms. Of these the carbonate 
is historically the earlier, while the sulphate is at the present time 
industrially by far the more important. The Carbonate of Sodai 
or soda ash, which is used in the production of some special glasses, 
and is an ingredient of English flint glasses, is produced by either 
of two well-known chemical processes. One of these is the " black 
ash," or " Le Blanc " process, in which the chloride is first converted 
into sulphate by the direct action of sulphuric acid, and the sulphate 


thus formed is converted into the carbonate by calcination with 
a mixture of calcium carbonate and coal. The sodium carbonate 
thus formed is separated by solution and subsequent evaporation. 
A purer form of sodium carbonate can be obtained with great 
regularity by the '* ammonia soda " process, in which a solution 
of sodium chloride is acted upon by ammonia and carbonic acid 
under pressure. Soda ash produced by this process is now supplied 
regularly for glass-making purposes in a state of great purity and 
constancy of composition. It is upon these qualities that the 
great advantages of this substance depend, since its relatively high 
cost precludes its use except for special kinds of glass, and for these 
purposes the qualities named are of great value. 

For most purposes of glass-making, such as the production of 
sheet and plate-glass of all kinds, the alkali is introduced in the 
form of salt-cake — i.e., sulphate of soda. This product is obtained 
as the result of the first step of the Le Blanc piocess of alkali manu- 
facture — i.e., by the action of sulphuric acid on sodium chloride ; 
salt-cake is thus a relatively crude product, and its use is due to 
the fact that it is by far the cheapest source of alkali available for 
glass-making. There are, however, certain disadvantages con- 
nected with its use. The chief of these is the fact that silica cannot 
decompose salt-cake without the aid of a reducing agent ; such 
a reducing agent is partly supplied by the flame-gases in the atmo- 
sphere of the furnace, but in addition to these a certain proportion 
of carbon, in the form of coke, charcoal or anthracite coal, must be 
added to all glass mixtures containing salt-cake. The use of a 
slightly incorrect quantity of carbon for this purpose leads to 
disastrous results, while even under the best conditions it is not 
easy to remove all traces of sulphur compounds from glass made 
in this way. A further risk of trouble arises in connection with 
salt-cake from the fact that it is nev^er entirely free from more or 
less deleterious impurities. According to the exact manner in which 
it has been prepared, the substance always contains a small excess 
either of undecomposed sodium chloride or of free sulphuric acid, 


or the latter may be present in the form of sulphate of lime. A 
good salt-cake, however, should contain at least 97 per cent, of 
anhydrous sodium sulphate, and not more than 1-0 percent, of 
either sodium chloride or sulphuric acid. While pure sodium 
sulphate is readily soluble in water, ordinary salt-cake always leaves 
an insoluble residue, consisting frequently of minute particles of 
clay or other material derived from the lining of the furnace in 
which it was prepared, or from the tools with which it was handled ; 
and these impurities are liable to become deleterious to the glass 
if present in any quantity. The insoluble residue shoidd not exceed 
0-6 per cent, in amount, and in the best salt-cake is generally imder 
0-2 per cent. 

Salt-cake possesses certain other properties that make it somewhat 
troublesome to deal with as a glass-making material. Thus, on 
prolonged exposure, particularly to moist air, the powdered salt-cake 
absorbs moisture from the atmosphere and undergoes partial 
conversion into the crystalline form of " Glauber's Salt," a process 
which results in the formation of exceedingly hard masses. Ground 
salt-cake, therefore, cannot be stored for any length of time without 
incurring the necessity of regrinding, and this accretive action even 
comes into play when mixtures of glass-making materials, con- 
taining salt-cake as one ingredient, are stored. In practice, therefore, 
salt-cake can only be ground as it is wanted, and its physical pro- 
perties make it difficult to grind it at all fine, while the dust arising 
from this process is peculiarly irritating, although not seriously 
injurious to health. 

Potash is utilised in glass-making almost entirely in the form of 
carbonate, generally called "pearl-ash." Originally derived from 
the ashes of wood and other land plants, this substance is now 
manufactured by processes similar to those described in the case of 
soda, the raw material being potassium chloride derived from 
natural deposits such as those at Stassfurth. The pearl-ash thus 
commercially obtainable is a fairly pure substance, but its use is 
complicated by the fact that it is strongly hygroscopic and rapidly 


absorbs water from the atmosphere. Where it is desired to produce 
potash glasses of constant composition, ftequent analjrtical deter- 
minations of the moisture contents of the pearl-ash are necessary, 
and the composition of the glass miztmre requires adjustment in 
accordance with the results of these determinations. 

As a result of the war it has become necessary to find other 
sources of potash than the Stassfurth deposits. This has been 
successfully done by the utilisation of the dust from blast-furnace 
gases and flues. More recently Spanish deposits of potash have 
developed a new source of supply. On the other hand, the tem- 
porary difficulty of obtaining supplies of potash has raised the 
question whether for glass-making purposes the use of potash is 
really essential and how far the potash glasses aie really different 
fronn those in which the corresponding amount of soda is used. It 
must be admitted that theie is a slight difference in favour of the 
potash glasses for some purposes ; certain varieties of optical 
glass, for instance, do not allow of the replacement of potash by 
soda, while for certain decorative purposes there is a slight difierence 
of appearance — ^but in many cases the demand for potash is due 
to little more than a prejudice on the part of a manufacturer or his 

The alkalies are also introduced into glass in the form of nitrates 
(potassium nitrate, or saltpetre, and sodium nitrate, or nitre), 
but although these substances act as sources of alkali in the glass, 
they are employed essentially for the sake of their oxygen contents. 
Such oxidising agents are not, of comse, added to glass mixtures 
containing sulphates and carbon, but are employed to purify the 
mixtures containing alkali carbonates, and more especially to oxidise 
the flint glasses. Since these substances aie only introduced into 
glass in small quantities, their extreme purity is not of such great 
importance to the glass-maker, and the ordinary " refined " qualities 
of both nitrates are found amply pure enough to answer the hig|hest 

A certain number of natural minerals which contain an appreciable • 


quantity of alkali are sometimes utilised as raw materials for glass 
manufacture. The most important of these are the minerals of the 
felspar. class already referred to. These, howevei, contain a con- 
siderable proportion of alumina, while all but the purest varieties 
also contain more or less considerable quantities of iron. While 
some glass-makers formerly regarded alumina as undesirable, it is 
now generally accepted that it is for most purposes, and to a limited 
extent, a valuable constituent, and upon this view the use of fels- 
pathic minerals is based. For the cheaper varieties of glass, how- 
ever, such as bottle glass, felspathic minerals and rocks, such as 
granite and basalt, are freely used as raw materials. Another 
mineral in which both alkali and alumina are found is cryolite. 
This is a double fluoride of soda and alumina, whose properties are 
particularly valuable in the production of opal and opalescent 
glasses. As a mere source of alkali, however, cryolite is much too 

(3) Sources of Bases other than Alkalies. — The most important of 
these are lime and lead oxide, the former being required for the 
production of all varieties of plate and sheet-glass, as well as for 
bottles and a large proportion of pressed and blown glass, while 
lead is an essential ingredient of all flint glass. The only other base 
having any considerable commercial importance in connection with 
glass-making is barium oxide, while oxide of zinc, magnesia, and 
a few other substances are used in the manufacture of special glasses 
for scientific, optical or technical purposes, where glass of special 
properties \a required. The metallic oxides which are used for the 
production of coloured glass are, of course, also basic bodies. These 
wiU be treated in connection with coloured glasses, with the excep- 
tion of manganese dioxide, which is used in large quantities in the 
manufacture of many ordinary " white " glasses. 

Calcium Oxide (lime) is generally introduced into glass mixtures 
in the form of eith» the carbonate or the hydrated oxide (slaked 
lime). The carbonate may be derived either from natural sources, 
or it may be of chemical origin, while the hydrate is always obtained 



by the calcination of the carbonate, foUowed by " slaking " the 
lime thus produced. Natural calcium carbonate occurs in great 
quantities in the form of chalk and limestone rocks. Both varieties 
are used for glass-making. Chalk is a soft friable material which is 
apt to clog during the grinding operations, particularly as the 
natural product is generally somewhat moist. As r^ards the 
greater part of its bulk, chalk is often found in a state of great 
purity, but it is frequently contaminated by the presence of scattered 
masses of flint. Chemically this impurity is not very objectionable 
to the glass-maker, since it merely introduces a small proportion 
of silica whose presence need scarcely be allowed for in laying down 
the mixture. On the other hand, if any fragments of flint remain 
in the mixture when put into the furnace, they prove very refractory, 
and are apt to be found as opaque enclosures in the finished glass. 

Natural limestone can also be obtained in great purity in many 
parts of the world. It is generally a hard and rather brittle rock 
that can be readily ground to powder of the requisite degree of 
fineness. Flint concretions are not so frequently found in this 
material, but, on the other hand, it is often contaminated with 
magnesia and iron. The former ingredient, when present in small 
quantities, tends to make the glass hard and viscous, so that lime- 
stone of the lowest possible magnesia content should be used, 
especially for the harder kinds of glass, such as plate and sheet- 
glass, etc. The iron contents of the limestone used must also be 
low where a white glass is required ; but since a smaller quantity 
of limestone is used for a given weight of glass produced than the 
quantity of sand used for the same purpose, the presence of a 
somewhat higher percentage of iron is permissible in the limestone 
as compared with the sand ; for the better varieties of glass, how- 
ever, the iron should not exceed 0*3 per cent, of the limestone. 

Slaked lime is sometimes used as the source of lime for special 
glasses where the process of manufacture renders it desirable to 
avoid the evolution of carbonic acid gas which takes place when 
the carbonate is heated and attacked by silica. When slaked lime 


is used only the water vapour of the hydrate is driven oft, and this 
occurs at a much lower temperature. For the production of slaked 
lime, an adequately pure form of limestone, preferably in the form 
of large lumps, is burnt in a kiln until the carbonic acid is entirely 
driven ofi ; after cooling, the lime so formed is slaked by hand. 
The product so obtained is, however, apt to vary both as regards 
contents of moisture and carbonic acid, which latter is readily 
absorbed from the atmosphere ; the use of this material, therefore, 
requires frequent analytical determinations of the lime contents 
and corresponding adjustments of the mixture if constant results 
are required. 

It is possible to introduce Ume into glass mixtures in the form 
of gypsum or calcium sulphate, but the decomposition of this com- 
pound, like that of sodium sulphate, requires the intervention of 
a reducing agent such as carbon, and the difficulties arising from 
this source in connection with the use of salt-cake are still further 
increased in the case of the calcium compound. Since limestones 
of considerable purity are more or less plentiful in many districts, 
the conunercial value of calcium sulphate for glass-making is probably 

The Compounds of Barium may best be dealt with at this stage, 
since they are chemically very closely allied to the compounds of 
lime just described. Barium occurs in nature in considerable quan- 
tities in the minerals known as barytes (heavy spar) and witherite 
respectively. The former is essentially sulphate of barium, while 
the latter is a carbonate of barium. The use of the sulphate meets 
with the same objection here as in the case of calciuna sulphate 
discussed above, except that the barium compoimd is much more 
easily reduced and decomposed than the lime compoimd. The 
natural mineral witherite is used to a considerable extent in the 
production of barium glasses, and these have been found capable 
of replacing lead glasses for certain purposes. On the other hand, 
for the best kinds of barium glasses, viz., those required for optical 
purposes, the element is introduced in the form of artificially pre- 


pared salts. Of these the most important is the carbonate, coin-- 
mercially described as '* precipitated carbonate of barium " ; this 
precipitated compound, however, does not ordinarily correspond 
to the chemically pure substance, but contains more or- less con- 
siderable quantities of sulphur compounds. The question whether 
these impurities are or are not objectionable can only be determined 
for each particular case, since much depends upon the special 
character of the glass to be product* Both the nitrate and the 
hydrate of barium are commercially available, but they are 
very costly ingredients . for use in the producti(»i of even the 
most expensive kinds of glass; these substances are, however, 
obtainable in a state of considerable purity, although the hydrate 
has the inconvenient property- of rapidly absorbing carbonic 
acid from the atmosphere, thus becoming converted into th« 

Magnesia is another glass-forming base that is closely related, 
chemically, to calcium and barium. This element is usuaUy intro- 
duced into glass mixtures in the form of either the caarbonate or 
the oxide. The carbonate occurs in nature in a more or less pure 
state in the form of magnesite, and by calcination the oxide id- 
obtained. The natural mineral and its product are, of course, 
by far the cheapest sources of magnesia, but as the element is only 
used in comparatively small quantities, the artificial precipitated 
* carbonate or calcined magnesia are frequently preferred. Magnesia 
is only introduced intentionally in notable quantities -in special 
glasses where the properties it confers are of particular >value ; in 
ordinary lime glasses this element, as has already been mentioned^ 
is to be r^arded as an undesirable impurity. 

Zinc, oxide lies, chemically, between the bases already discussed 
on the one hand, and lead oxide on the other. This element is 
introduced into certain optical glasses, a special "' ^c crown '' 
having found some application. Certain kinds of laboratory glass- 
ware also contain zinc. Chemically prepared zinc oxide is almost 
the only form in which the element is used, but the very volatile 


character of this substance must be borne in mind when it is intro- 
duced into glass mixtures. 

Lead is one of the most widely-used ingredients of glass ; the 
glasses containing this substance in notable quantity are all charac- 
terised to a greater or less d^ee by similar properties, such as 
considerable density and high refractive power, and are classed 
together under the name "flint glasses." Lead is now almost 
universally introduced into glass mixtures in the form of red lead, 
€dthough the other oxides of lead might be employed almost equally 
well. Bed lead is a mixture of two oxides of lead (PbO and PbaOs) 
in approximately such proportions as to correspond to the formula 
Pb304. It is prepared by the roasting of metaUic lead in suitable 
furnaces, where the molten lead is exposed to currents of hot air. 
The product is obtainable in considerable purity, very small pro- 
portions of silica, derived from the furnace bed, and of iron derived 
from the tools with which the lead is handled, being the principal 
fOTeign substances foimd in good red lead. Metallic lead is, how- 
ever, sometimes present, and this impurity is difficult to detect by 
chemical means. Silver would be an objectionable impurity, but 
owing to the modern perfect methods of de-silvering lead, that 
element is rarely found in lead products. Analytical control of 
red lead as used m the glass mixtures, and consequent adjustments 
of the mixture, are, however, necessary where exact constancy in 
the glass produced is desired. The reason for this necessity lies in 
the fact that the oxygen content, and therefore the lead-oxide (PbO) 
content, varies decidedly from batch to batch, while the material 
as actually delivered and used frequently contains notable pro- 
portions of moisture. 

A word should perhaps be said here as to methods of handling 
red lead on account of the injurious effects which the inhalation 
of lead dust produces upon the workmen exposed to it. For glass* 
making purposes it is not feasible to adopt the method employed 
by potters of first " fritting " the lead and thus rendering it com- 
paratively insoluble and innocuous ; even if this were done, tHe 


difficulty would only be moved one step further back, and would 
have to be overcome by those who undertook the preparation of 
the frit. The right solution of the problem, in the writer's opinion, 
is to be found in properly preventing the formation of lead dust, 
or at all events in protecting the workmen from the risk of inhaling 
it. Where only small quantities of lead glass are made, and there- 
fore only small quantities of lead are handled and mixed at 
a time, it is no doubt sufficient to provide the workmen engaged 
on this task with some efficient form of respirator to be worn during 
the whole of the time that they are engaged on such work, and to 
take, the further precautions necessary — ^by way of cleanliness and 
the provision of proper mess-rooms — ^to avoid any risk of lead 
dust either directly or indirectly contaminating their food. Where, 
however, large quantities of flint-glass are made every day, it is 
possible and proper to make more perfect arrangements for the 
mechanical handling and mixing of the lead with the other ingre- 
dients by the provision of suitable mixing and transporting machinery 
so arranged as to be dust-tight. It is only fair to state, however, 
that partly under their own initiative, partly under pressure from 
the authorities, glass-makers in this country are complying with 
these requirements in an adequate manner. 

Aluminium. — ^There are several varieties of glass into which 
alumina enters in notable quantities, the principal examples being 
certain optical and many opal glasses, while most ordinary glasses 
contain this substance in greater or less d^ee. In the latter the 
alumina is derived, by the inevitable processes of solution, from 
the fire-clay vessels or walls within which the molten glass is con 
tained, while in some cases the elem^it is intentionally introduced 
in small proportions (about 2 per cent, to 3 per cent, of AI2O3) by 
the use of felspar as an ingredient of the mixture. The introduction 
of alumina in the form of china clay (a relatively pure silicate of 
alumina) has recently been reconmiended. This can only be done 
successfully in very hot furnaces and provided that the china clay 
is very finely divided and intimately mixed with the batch — other- 


wise it is apt to agglomerate and to form insoluble " stones " in 
the glass. 

Where larger proportions of alumina are required, the substance 
is introduced in the form of the hydrate, which is obtainable com- 
mercially in a state of almost chemical purity, but of course at a 
correspondingly, high cost. In opal glasses alumina is derived 
partly or wholly from felspars, or in some cases from the use of the 
mineral cryolite. This is a double fluoride of aluminium and sodium 
which is found in great natural masses, chiefly in Greenland. Owing 
to the high price of this mineral, however, artificial substitutes of 
nearly identical composition and properties have been introduced 
and are used successfully in the glass and enamelling industries. 

Manganese, — ^Although the oxides of this element really belong 
to the class of colouring compoimds, they are so widely used in the 
manufacture of ordinary " white " glasses that it is desirable to 
deal with them here. The element manganese is most usually 
introduced into glass mixtures in the form of the per -oxide (Mn02), 
although the lower oxide (Mn304) can also be used. The material 
ordinarily used is the natural manganese ore, mined chiefly in 
Kussia ; the purest forms of this ore consist almost entirely of the 
per -oxide, but " brown " ores, containing more or less of the lower 
oxide, are also used with success. These ores always contain small 
amounts of iron and silica, but provided the iron is not present. 
in any considerable quantity, the value of the ore is measured by 
the percentage of manganese which it contains. The colouring and 
" decolourising " action of manganese will be discussed in a later 
chapter. Certain other substances, which have been suggested as 
either substitutes for, or improvements upon, manganese for this 
purpose need only be mentioned here, viz., nickel, selenium and 

Arsenic is another substance frequently introduced into " white " 
glass mixtures. This element is imiversally used in the form of 
the white arsenic of conamerce (i.e., arsenious acid, AsgOg) which 
is obtained in a pure form by a process of sublimation. Owing to 


the very poisonous nature of this material, special precautions 
must be taken in its use for glass making purposes to avoid all risk 
of poisoning. 

Carbon. — ^As has already been indicated, an admixture of carbon 
in some suitable f (Mrm is essential in the case of certain glass mixtures. 
The carbon for this purpose may be used in the form of either 
charcoal, coke, or anthracite coal. Of these, charcoal is undoubtedly 
the purest form of carbon, but it is expensive in this country. Coke 
varies very much in quality according to the coal from which it 
has been produced, but it always contains notable proportions of 
ash rich in iron, and also some sulphur. Anthracite coal can be 
obtained in a very pure form, containing considerably less ash 
than that found in most kinds of coke, and this is therefore probably 
the most convenient form of carbon for this purpose. 

Fluorine, — ^This element is employed in the form of fluorides, 
usually either calcium fluoride, or the double fluoride of sodium 
and aluminium already described (cryolite). Artificial compounds, 
of the nature of fluo-silicates, are also employed. Except for certain 
optical glasses of very recent introduction, fluorides are solely used 
for the purpose of rendering glass opaline. They have the dis- 
advantage that their presence in the batch usually causes violent 
attack upon the fire-clay of the pot in which the melting is carried 

Boron. — ^This element is introduced into glass either as borax 
(sodium bi-borate) or as boric acid ; it is present in the glass in the 
form of compounds or solutions of the oxide (B2O3). Its principal 
use is in optical glasses, where the acid is generally used in the 
batch. Borax is, however, sometimes used as a softening agent, 
particularly in coloured glasses in which the colouring oxide has 
a tendency to render the glass imdesirably hard. 

lAroonium, — ^Zirconium oxide or zirconia (Zr02) is a very heavy 
white substance which is extremely insoluble in most fluxes ; in 
the massive state resulting from fusion it ofiers very considerable 
resistance to attack by molten glass. In the finely-divided state 


resulting from precipitation, however, it can be incorporated in 
glass mixtures to a considerable extent. The resulting glass may, 
according to composition and circumstances, be either clear or 
opal. In the enamelling industry Zirconia is frequently used for 
the production of opaque white enamels, and is said to oSer 
advantages over tin oxide. Its use for the production of opal 
glass is not so well established. 




Thb furnaces and aruoibles in which the melting of glass is carried 
out are required to resist prolonged oacposure to very high tem- 
peratures and must, therefore, be constructed of materials capable 
of resisting the destructive effects of great heat. Such materials 
are usually described as ** Refractories," although a very large 
variety of dijSerent materials may be comprised under that generic 
term. The importance of such material to successful glass manu- 
facture, however, can scarcely be over-estimated. In the first 
place the life of the furnaces and pots plays a most important part 
in the economy of a glass-works, not only because of the cost in 
materials and labour entailed by each renewal, but also because 
of the very serious loss of output which the stoppage of a furnace 
implies. There is the further foctoi that during the process of 
deterioration which precedes the shutting-down of a furnace for 
repairs there is a constantly increasing loss of efficiency and fre- 
quently a deterioration in the quality of glass turned out. Where 
glass is melted in pots or crucibles the quality of the refractories 
used in the construction of the pots is of primary importance also 
from the point of view of the very strong and direct influence which 
the quality of the pot exerts on the quality of the glass. 

The subject of refractories is, howevei, a very large and complex 
one, and its technical details axe of interest rather to the manu- 
facturer than to the user of glass. The treatment of this subject 
in the present volume must, therefore, necessarily be confined to 
somewhat general considerations. The subject, however, is receiving 
much fresh attention since war conditions have led glass manu- 
facturers and manufacturers of refractories to realise that careful 


attention to the soientific study of refractories is essential to the 
prosperity of their industries. Some consideration of the subject 
is therefore necessary in this place. 

Broadly speaking the refractories used in glass manufacture may 
be divided into two classes, according as they are or are not exposed 
to direct contact with molten glass. Those parts of a furnace 
which are not in contact with glass, such as the arched '^ crown " 
or roof and the upper parts of the walls of furnaces, are almost 
invariably constructed of the material known as " silica brick," 
while those parts which come into contact with glass, such as pots, 
tank-blocks, and the lower parts of pot-furnaces, are almost invari- 
ably constructed of some kind of "fire-clay." The difference 
between these two classes of refractories is this, that while " silica 
brick " consistl^ almost entirely of silica (SiOg) held together by a 
very small proportion of other substances, chiefly lime, acting as 
binders, " fire-clay " consists essentially of a silicate of alumina 
(At203,2Si02), which is associated, in some cases, with considerable 
quantities of additional silica and with other substances in smaller 
proportions. The reason for this sharp differentiation lies in the 
fact that silica brick is rapidly attacked and dissolved by molten 
glass, whose contents of alkali or of lead enable it to enter into 
chemical combination with the silica. Fire-clay, on the other hand, 
is capable of developitig a very considerable degree of resistance 
to the dissolving action of molten glass, and although very gradual 
solution constantly takes place, articles made of suitable fire-clay 
often resist the action of glass for many weeks and — ^in the case 
of tank-blocks — even for years. It must not be supposed, however, 
that the silica brick, even in the crown of the furnace, is immune 
from chemical attack, since the dust carried in the flame, as well 
as vapours arising from the molten glass, attack the silica bricks 
very powerfully, so that their chemical composition after a pro- 
longed run in the furnace differs very widely from pure silica. The 
character of this chemical attack is, however, entirely different from 
that which occurs where there is direct contact with molten glass. 

X 2 


The qualities reqtiired in the varioas types of refractories may 
now be briefly oonsidered. Of these the first essential is the power 
of withstanding prolonged exposure to high temperatures. The 
actual temperatures attained in glass-melting furnaces are rarely, 
if ever, as high as those which axe regularly employed in such a 
process as steel-melting ; as a rule, glass-melting furnaces rarely 
pass beyond a temperature of 1500^ G. or at most 1600° C, while 
in many tank-furnaces the more usual maximum temperature 
does not exceed 1400° G. With this may be contrasted the tem- 
peratures well up to and above 1700° G., which have been observed 
in steel-melting, and the still higher range of temperature habitually 
employed in the dectric steel furnace. It would thus seem that 
from the purely temperature point of view the conditions ruling 
in glass-melting furnaces are not extremely stringent. On the 
other hand, it must be borne in mind that in this matter time is 
an important element, and in this respect the glass furnace makes 
a much more severe demand on its refractories than does a steel 
furnace. In a continuous tank furnace the full heat is steadily 
maintained, with only a very short uitermission for flue-deaning at 
week-ends, for many weeks in succession ; such furnaces have been 
known to work continuously for close on two years. In contrast 
with such a length of run, the work of a steel furnace is intermittent 
and relatively short, the maximum life rarely exceeding two months. 

At jQrst sight it would seem that in order to secure that a refractory 
should satisfactorily resist a certain maximum temperature, it 
should be suflSicient if its meltuig-point lay well above that maximum. 
Were refractories simple, pure substances in a crystalline state this 
condition would no doubt be sufiELcient, but the materials actually 
used are very far from being pure, simple crystalline solids, and 
consequently we find that they do not possess any real, definite 
" meltuig point " at all, and their passage from a hard, solid body 
capable of bearing a load to a glassy molten mass flowing imder its 
own weight is gradual and complex. Confining our attention for 
the moment entirely to fire-clays, the changes which occur during 


gradual heating to a very high temperature may be broadly described 
as follows : In its initial condition the fire-clay consists of a quantity 
of very infusible substances, viz. : clay substance proper and silica, 
accompanied by small quantities of much more fusible substances, 
such as felspar, iron oxide, etc. As soon as the temperature reaches 
the melting or softening point of the most fusible of these binding 
or fluxing substances, its particles, disseminated throughout the 
clay, become liquid and begin to exert a solvent action upon their 
surroundings. This action makes itself felt at first only upon the 
other more fusible substanees, but the clay substance and silica 
are also attacked and slowly dissolved. The result of this action, 
however, is to render the liquid which has already been formed 
extremely " thick " or viscous, so that at each temperature a limit 
is reached beyond which the action will not go — or will only go 
extremely slowly — ^until the temperature is raised. If a sample of 
fire-clay which has been heated only to the earlier stages of this 
process is allowed to cool and is examined by means of a fracture 
or a cut section, it is found to be still quite porous and there is very 
little visible sign — except under the microscope — of the formation 
of the minute amount of glassy material representing that portion 
which had been liquid while hot. But as the clay is exposed to 
still higher temperatures the solvent action of the liquid portion 
increases — ^the clay shrinks by becoming less porous and more 
dense as the process continues. A cooled specimen now shows an 
increasingly dense or " vitrified " fracture and the porosity is 
much diminished or — ^in the latest stages — ^has disappeared entirely, 
when the clay is said to be fully shrunk. Finally, if the process is 
continued far enough, by raising the temperature very high indeed, 
the proportion of liquid to still undissolved solid becomes so high 
that the whole mass behaves like a fluid and " runs " or, as it is 
usually termed, "melts." But it will be seen that the melting 
process has in reality been extremely gradual and that, long before 
a small piece of the clay would " run " imder its own weight, a 
brick of the same material would have " squatted " if any serious 



load had been imposed upon it. The point at which a given material 
of this kind will ** squat " must depend both upon the quantity of 
liquid preset at that temperature and upon how 'Hhick" or 
visoous that liquid is. 

It follows from these considerations, which can be very fully 
substantiated by a microscopic examination of samples of fire-days 
heated to various temperatures, that any attempt to determine 
the " melting poiut " of a refractory is entirely vain. Even the 
form of test frequently used, in which a trial cone of the clay under 
test is heated together with a small series of standard '^ Seger " 
cones and a comparison made as to their relative refractoriness, 
is not satisfactory because the small cones do not allow of any 
satisfactory condition of loadiag under which the material flows 
when " squatting " occurs, while the effects of viscosity are largely 
disr^arded as a result of very rapid heating. 

For practical purposes the really important test of any refractory 
is its power to bear a definite load at a high temperature. This 
test requires somewhat more elaborate appliances than the '' cone " 
test, but its indications are extremely valuable in compariug refrac- 
tories for any particular purpose. The best method of usiug such 
a test would be to determine the temperatures at which bricks or 
other pieces of standard size failed under a series of increasing 
loads, " failure " beiug defined as yielding at an appreciable rate. 
In a less elaborate form two loads are arbitrarily chosen and the 
temperatures determined at which the material fails under them. 
The test adopted by the Bureau of Standards, U.8.A., for this 
purpose requires that fire-bricks of grade " lA " shall withstand 
a load of 50 lbs. per sq. in. at a temperature of 1350° C, while for 
grade "IB" the load is reduced to 30 lbs. per sq. in. at the same 
temperature. At first sight it might perhaps appear that this 
" refractoriness under load " test should only be applied to the 
material intended for use as bricks or blocks in furnace construction, 
but even in material intended for pots this property is vitally 
important, since the hot clay must not only bear the load due to 



its own weight, but also the very considerable pressure of the 
molten glass. 

From what has been said already it will be evident that the 
refractoriness of a given material must depend very much upon 
its chemical composition, and. this chemical composition may 
itself be r^arded from two points of view : we have, first, the 
composition in regard to the fundamental refractories themselves, 
which, in connection with glass-melting, are almost exclusively 
silica and its combinations 
with alumina, and, second, 
the fluxing impurities 
present. The relations of 
silica and alumina are best 
recognised from the con- 
stitutional diagram of the 
silica-alumina system, which 
is reproduced in Eig. 1. 
Here, as we are dealing 
with two simple, crystalline 
solids, we have a curve 
of real melting or freezing 
— the upper line of the diagram. This falls from well above 
2000° C. at the melting-point of pure alumina down to a 
temperature just above 1600° C. for a mixture consisting of about 
87 per cent, of Si02 and 13 per cent, of AlgOg. There is an inter- 
vening break in the curve with a maximum corresponding to the 
formation of the mineral siUimanite — a mineral which is found 
in highly " vitrified " fire-clays and in hard porcelain. On the 
whole, however, the fusibility of the refractory portion of a fire- 
clay increases steadily with increasing silica-content up to 86 
per cent. It may be mentioned that so-called " clay substance " 
or " Kaolinite " — ^AlaOg 2Si02 — ^finds no place on this diagram ; 
this means that no such compound separates from fusions of silica- 
alunaina mixtures and^ indeed, that compound is known to undergo 






i SiUimanite 

Si/tifnsnit & 


Tridymite Si 




Fig. 1. 



decomposition on heatin^r — ^probably at a temperature between 
500° and 600° C, where clay is known to undergo an endothermic 
reaction on heating. Apart from this, therefore, the refractory 
value of a fire-clay, disregarding fluxing impurities, might be 
expressed by its alumina-silica ratio. 

In practice, however, fluxing impurities can by no means be 
disregarded, since it would be useless to have a highly refractory 
clay basis if it were accompanied by fluxes which would dissolve 
it at a comparatively low temperature. In this connection it may 
be noted that alumina is found to be much less readily dissolved 
in the fluxes ordinarily present than is silica. The fluxes themselves 
generally exist in the form of felspar (double silicates of alumina 
and an alkali) and of n^ica, while iron oxide is always present to 
some extttit. Many attempts have been made to arrive at formulsB 
by which the refractory value of a clay might be estimated from 
its analysis in terms of the ratios of silica to alumina and to the 
sum of the fluxing oxides present, but none of these formulae possess 
any great practical value. The only satisfactory way of forming 
an opinion as to the value of a clay as a refractory is by means of 
the tests under load at high temperatures. As a matter of interest, 
however, the chemical analyses of a few typical kinds of refractories 
used in glass furnaces are here given : — 

Ttptgal Analyses of Refbactobieb. 





Lime and 



Loss on 


Water, etc. 

Silica Brick ... 






/ Gross Almerode 









St. Loupe 









Stourbridge 1 






China Clay ... 






The power of resisting an adequate pressure at high temperatures, 
although of fundamental importance, is not the only essential 
property of refractories intended for use in glass-melting. Besides 


resisting the tendency to flow under load, the material must also 
resist other causes of destruction. One of these takes the form of 
cracking or flaking, usually termed " spalling," whereby a fire-brick 
or block is gradually disintegrated. As a rule this form of failure 
arises from the unequal shrinkage or expansion of different parts 
of the brick. In the case of silica brick this is liable to be a very 
serious cause of trouble ; in this material it arises from a volume 
change which occurs when the silica in the brick undergoes the 
transformation of quartz into tridymite. If the brick has previously 
been sufficiently severely fixed, i.e., if it has been heated long enough 
and at a sufficiently high temperature during manufacture, this trans- 
formation will have been sufficiently completed to avoid risk of 
cracking so long as the furnace in which the brick is used is not 
heated — or cooled — ^unduly quickly. In the case of fire-clay also, 
if the material is insufficiently burnt, so-called " after-shrinkage " 
is certain to occur and — ^if of sufficient magnitude — ^with disastrous 
results to the brick and the furnace. Fortunately the condition of 
a given sample of brick in this respect can be easily tested by expos- 
ing it in a suitable experimental furnace to a very high temperature 
and ascertaining whether it has undergone an undue amount of 
further shrinkage during this treatment. For tests of this nature 
the following standards have been suggested : a properly-fired 
silica brick should not change in volume by more than 1 or 2 per cent, 
after firing to cone 18, while an adequately fired fire-brick of first- 
rate material should not shrink by more than the same amount when 
fired to cone 16. Commercial products unfortunately frequently 
fail to pass such tests, as there is a considerable tendency on the 
part of manufacturers to avoid the cost and difficulty of firing their 
bricks at a sufficiently high temperature. The absolute necessity 
of doing this must, however, be recognised if the best results are 
to be obtained. Another test by which the degree of firing to which 
a given article has been exposed can be approximately ascertained 
is the measurement of the porosity of the fired materials. This is 
sometimes measured by weighing the water which the material can 



absorb in its pores, or by measuring, in an indirect manner, the 
amoont of air whioh can be forced into or drawn out of these pores 
by a given change of pressure. From the brief account given above 
of the process of vitrification when a fire-day is heated, it will be 
seen that diminishing porosity accompanies increasing solution of 
the day substance and silica in the fluxes, the full shrinkage resulting . 
from complete vitrification being accompanied by the complete 
disappearance of porosity. Porosity measurements, however, afiord 
comparable results only on materials of precisdy similar composition 
and cannot be accepted as a general test of quality. 

Where refractories are directly e3q>osed to contact with molten 
glass, the power of resisting solution by the glass is of primary 
importance. This consideration renders the sdection of materials 
intended for pot-making and for tank-blocks a matter requiring 
very great care. In the case of tank-blocks the conditions are not 
so severe as in pots, owing to the fact that the blocks are air-cooled 
on one side while the other is exposed to the glass. This circum- 
stance lowers the temperature of the fire-clay even where it is in 
contact with the glass and assists its resistance to solution. In 
the pot, on the other hand, and more particularly in the closed 
or covered pot (see bdow), the heat of the furnace is transmitted 
to the mdting glass through the walls of the pot, so that the fire- 
clay is, if anything, rather hotter than the glass with which it is in 
contact. It may be said at once that all ordinary fire-clays are 
distinctly soluble in molten glass, so that the problem of selection 
is reduced to finding a clay which shall dissolve as slowly as possible, 
and as uniformly as possible. Uniformity is particularly important 
both on account of the quality of the resulting glass — ^which is apt 
to be contaminated by particles of day detached from a pot under- 
going irregular attack, and on account of the durability of the 
pot, which is rendered useless if it is anywhere pierced by a single 
deep pit, even if it should have remained practically unattacked 
everywhere else. It is not definitdy known upon whftt factors of 
composition or constitution uniformity of attack depends, so that 


the glass manufacturer can, at present, be guided only by actual 
trials and experiences of various clays. Generally speaking, how- 
ever, the clays richest in alumina are also the most resistant to 
solution. The precise action, however, varies with the nature of 
the glass. Experiments recently carried out at the National Physical 
Laboratory indicate very clearly that one of the most important 
factors in the attack of molten glass on fire-clay lies in the currents 
which are set up in the glass as a result of the changes in density 
which the glass undergoes when either silica or alumina derived 
from the pot are dissolved in it. Where the density of the glass is 
lowered by the addition of silica and alumina, the glass close to 
the sides of the pot flows upwards and fresh glass, not yet laden 
with dissolved clay material, flows inwards towards the sides of the 
pot near the bottom and also impinges on the bottom of the pot. 
The result is relatively rapid attack on the bottom and on the 
sides near the bottom. At the surface, too, currents are set up — 
probably by the change in the surface tension of the molten glass 
resulting from dissolution of clay. If these changes are such as to 
induce a current of glass to flow towards the walls of the pot from 
the centre a rapid form of attack occurs where the surface of the 
glass meets the sides of the pot and a deep groove is formed around 
the pot at the glass level or "water-mark." The experimental 
study of these various factors has only been begun as yet, but they 
open up the possibility of controlling the mutual action of glass 
and clay, by such means as the use of special linings and other 
local protecting devices, in such a manner as to keep the glass < 
much freer from clay contamination and also to prolong the life 
of pots to a very considerable extent. 

Another class of properties is required of refractory materials in 
connection with the various processes which they are required to 
undergo in the course of manufacture into bricks, blocks and pots. 
For bricks and pots, perhaps, the principal requirement is that 
the material^ shall be capable of being formed, by moulding or 
pressing, or by the casting process referred to below, into the desired 


shapes and that, when dry, it shall be strong enough to be safely 
handled for the purpose of being placed in the kilns for firing. 
Practically aU fire-clays, and many other refractory materials, can 
be made to answer these simple requirements — ^in some cases by 
the addition of some substance which acts as a temporary binder 
although it disappears or is destroyed during firing. But for the 
production of pots, and particularly of covered pots, the conditions 
are not so simple, and imtil comparatively recently a considerable 
degree of plasticity was regarded as absolutely essential in pot- 
clays. For the casting process, however, plasticity is not required 
and is, indeed, a decided disadvantage, so that this limitation to 

Fig. 2.— Open " pot " or crucible ^^^' 3.— Covered pot for glass- 

f or glass-melting. melting, as used for flmt 

° ° glass and optica] glass. 

a considerable extent disappears. We will, however, first consider 
the older and still very widely practised processes of pot manu- 
facture out of plastic clays by hand, either with or without the 
aid of moulds. 

The pots used in glass manufacture are of two kinds, known as 
open and closed or covered respectively. The open pot is simply 
a vessel, circular or oval in plan and sUghtly larger at the top than 
at the base, as shown in the sketch. Fig. 2 ; they vary in size, in 
practice, from a capacity of 2 cwt. up to 3 tons, the larger ones 
measuring about 5 ft. in diameter at the top. Such open pots have 
the great advantage of simplicity of construction and are more 
efficient from the point of view of melting, since the heat of the 
furnace reaches the glass directly through the open top. On the 
other hand, they expose the glass to reaCtit>ii With the furnace gases 


and also do not protect it in any way from contamination by dust 
carried in the flame or by droppings from the roof of the furnace. 
Formerly, for instance, it was not thought possible to melt " flint " 
glasses, containing considerable proportions of lead oxide, in open 
pots owing to the reducing action of the furnace gases on the lead 
oxide in the glass. It has, however, been demonstrated that this 
difficulty can be successfully overcome by a suitable control of the 
flame both as to composition and direction, and good flint glass 
is now regularly melted in open pots. On the other hand, most 
optical glass and a good deal of the best flint and other '' extra 
white " glass is still regularly melted in " covered " pots. These 
covered pots have the shape shown in Fig. 3. Here the simple open 
pot is covered over with a hood or dome, provided with a snout or 
opening which is so placed as to communicate with the working 
opening of the furnace, and through this opening the pot is filled 
and all manipulations are carried out. The construction of such 
a covered pot is necessarily a matter of much greater difficulty than 
in the case of an open pot. 

The material for pot-making is first prepared with great care. 
The proper variety of clay having been selected, it is ground to a 
Sue powder in suitable mills and carefully sieved ; with this fine 
clay powder is mixed, in accurately determined proportions, a 
quantity of crushed burnt fire-clay. In some works this burnt 
material is obtained by simply grinding up fragments of old used 
pots, but the better practice is to bum specially selected fire-clay 
separately for this purpose. The quantity of such burnt material 
added to the mixture depends upon the chemical nature and especi- 
ally on the plasticity of the virgin clay employed ; with so-called 
" fat " or very plastic, clays up to 50 per cent, of burnt material is 
added, but with the leaner clays, such as those of the Stourbridge 
district in England, very much smaller proportions are used. The 
object of this addition of burnt material is to facilitate the safe 
drying of the finished pots and to diminish^— by dilution — ^the 
total amount of contraction which takes place both when plastic 


day is allowed to dry, and farther when the dry mass is sdbse- 
quently burnt; the burnt material or ''grog," having already 
undergone these shrinlring processes, acts both as a neutral diluent 
and also as a skeleton strengthemng the whole mass and reducmg 
the tendency to form cradcs. 

The virgin day and grog having been intimately mixed, the 
whole mass is " wet up " by the addition of a proper proportion of 
water and prolonged and vigorous kneading, usually in a suitable 
pug mill. The mass leaves this mill as a fairly stifi, plastic dough, 
but the full toughness and plastidty of such day mixtures can only 
be devdoped by prolonged storage of the damp mass. In the 
next stage of the process, the plastic day is passed to the " pot 
maker " in the form of thick rolls, and with these he gradually 
builds up the pots or crucibles from day to day, allowing the lowest 
parts to dry suffidently to enable them to bear the weight of the 
upper parts without giving way. The building of large pots in 
this way occupies several weeks, and during this time the premature 
diying of any part of the pot must be carefully avoided. After the 
completion of the pot, drying is allowed to take place, slowly at 
first, but more vigorously after a time when the risk of cracking 
is smaller ; when it is taken into use the pot is usually many months 
old and is thoroughly air-dry. The clay, however, is still hydrated, 
$.e., contains chemically combined water, and this is only expelled 
during the early stages of the burning process. This process is 
carried out in smaller furnaces or Idlns placed near the mdting 
furnaces. In these the pot or pots are exposed to a very gradually 
increasing temperature imtil a bright red heat is finally attained. 
This is a delicate process in which great care is required to secure 
gradual and uniform heating, especially during the earlier stages, 
otherwise the pots are apt to crack and become usdess. Finally, 
when a bright red heat has been maintained for at least a day, 
the pots are ready to be placed in the furnace, and this is ordinarily 
done while both pots and furnace are at a red heat, the pots never 
being allowed to cool down again once they have been burnt. 


Recent practice, recognising tbe great advantage of harder firing 
of the pots in reducing porosity and by the formation of silli- 
manite in the material rendering the pot less liable to attack, tends 
strongly in the direction of pre-heating pots to a very much higher 
temperature than the bright red heat formerly used. The kilns 
or **pot arches" are accordingly constructed so as to allow of 
higher temperatures being attained, while — after the pot has been 
placed in the furnace or " set " — ^the furnace is frequently run 
to its maTrimnm temperature for a number of hours for the 
purpose of fully firing the pot before any glass or batch is 

Reference must now be made to the process of producing pots 
and other refractory objects by means of slip-casting. This process 
has long been in use for the production of a great variety of objects 
in the ceramic industries. It consists essentially in preparing the 
material which is to form the body not as a stiff, tough plastic mass 
but as a thin mud or sludge, known as a '^ slip." This slip is poured 
into a mould made of plaster-6f-paris, which is highly porous and 
sucks up the water of the slip while leaving the suspended solid 
matter behind as a species of mud-deposit on the inner surface of 
the mould. In its simplest form, such a mould constitutes a vessel 
which is filled with the liquid slip ; when enough material has been 
deposited on the walls of the mould as the result of the suction of 
the plaster, the remaining slip is poured out. The material which 
is left adhering to the plaster walls of the mould then quickly 
begins to dry, the water it contains being rapidly sucked away by 
the plaster. After a short time this drying results in a small 
shrinkage and, if the shape of the mould is suitable, the clay object 
— still very soft and tender, but sufficiently coherent — shrinks away 
from the plaster mould and may be removed and allowed to dry 
slowly in a suitable place. The mould, if dried at intervals, may 
be used a large number of times. It will be seen that for this purpose 
the material to be employed does not require the high degree of 
plasticity needed for hand-moulding ; indeed, high plasticity is a 


distinct difficulty, since it causes too great and too rapid a shrinkage 
during the drying process. On the other hand, a little plasticity, 
accompanied by a small amount of drying shrinkage, is necessary 
in order to enable the material to free itself from the mould in 
which it has been cast. Clays which by themselves are imsuited 
for slip-casting can, however, be rendered satisfactory by the 
addition to the slip of certain substances. Thus additions of soda 
or of silicate of soda render a heavy, thick slip sufficiently fluid to 
be easily cast. These additions, were they to remain in the clay, 
would diminish its refractoriness very seriously ; fortunately, 
however, these soluble substances are carried away into the plaster 
mould and ultimately appear as a fringe of crystals, sometimes 
called *' whiskers," on the outside of the mould. 

This slip-casting process, as already indicated, has long found 
wide application in ceramic industries, but in those uses the whole 
of the body — ^which may consist of mixtures of such substances as 
china-clay, ball clay, finely-ground felspar, and finely-ground quartz 
— consists of material in an extremely fine state of division, with 
the result that the '^ slip " is a smooth-flowing liquid in which there 
is no serious tendency for the various constituents to separate during 
the casting process. For the production of refractories, however, 
an admixture of relatively coarse " grog " is essential — as has 
already been pointed out — ^in order to reduce drying and firing 
shrinkage and to render the final product more resistant to changes 
of temperature, and the initial difficulty in using the slip-casting 
process for refractories lay in avoiding the separation of the coarser 
grog particles, particularly while the slip is entering and standing 
in the mould. If any such concentKition of grog does occur, it 
leads to unequal contraction either during drying or in fixing, and 
the object cracks. This difficulty has, however, been overcome 
by a variety of devices, the imderlying principle being to accelerate 
the whole operation to such an extent that settling cannot occur 
and to introduce the slip into the plaster mould in such a manner 
as to avoid currents and eddies which would lead to local separation 


of the grog. Both tank-blocks and pots are now being commercially 
produced by this process, particularly in America, and extremely 
satisfactory results have been obtained, especially in regard to 
uniformity in the quality of the resulting pots. There is the further 
advantage that the process admits of the use of much non-plastic 
material. Thus china-clay — ^the purest and most refractory of all 
clays — is rendered available for pot manufacture, and the use 
of other non-plastic or only slightly plastic materials is rendered 
possible. In the production of pots with special linings, also, this 
process is likely to prove very important. 

We now turn to the second class of refractory materials used in 
the construction of glass-melting furnaces, viz., those which are 
so placed as not to come into contact with molten glass. Here 
mechanical strength and refractoriness are almost the only con- 
siderations, but in the roof-vaults or " crowns " of tank furnaces 
and also of furnaces in which glass is melted in open pots, there 
is the further consideration that the material of the bricks used 
shall not contain notable quantities of any colouring oxide, since 
small flakes, etc., are apt to drop down into the molten glass, and 
would thus be liable to cause serious discoloration. Such a material 
as chrome-ore brick is therefore excluded. As a matter of fact, 
some form of " silica brick " is in universal use. Bricks of this 
material, otherwise known as " Dinas bricks " from the place of 
their first origin, in Wales, consist of about 98 per cent, of silica 
(Si02). Pure silica cannot be baked or burnt into coherent bricks 
entirely by itself, since it possesses neither plasticity when wet nor 
any binding power when burnt, but an admixture of about 2 per cent, 
of lime makes it possible first to mould the bricks when wet and then 
to burn them so as to form fairly strong, coherent blocks. These 
are of amply adequate refractoriness for the highest temperatures 
that can be attained in industrial gas-fired furnaces, and their 
mechanical strength is sufficient to make it possible to build vaults 
of considerable span, but on the other hand this material requires 
very gradual heating and constant watching while the temperature 

O.H. ' 


is rising or falling to any considerable extent ; the reason for this 
difficulty lies in the fact that silica bricks swell very markedly 
during heating, so that unless a vault built of this material is given 
room to spread somewhat, it will rise seriously and may even break 
up completely. This risk is avoided by gradually slackening the 
tie-bolts that hold the vault together, and correspondingly " taking 
up the slack " as the vault cools when the furnace is let out. Sudden 
local heat also has a disastrous effect on this material, producing 
serious flaking. For positions where intense heat is to be borne, 
and at the same time mechanical strength is required, silica brick 
is a most valuable material, but owing to its chemical composition 
it is rapidly attacked by molten glass or by any material containing 
a notable proportion of basic constituents, so that the silica bricks 
can only be employed out of contact with glass. 




HAViNa discussed the materials required for their construction, 
we may now consider, very briefly, the general design and arrange- 
ment of some t3rpical glass-melting furnaces. The oldest and 
simplest form of furnace is, in effect, simply a box built of fire-brick, 
in the centre of which stands the crucible, while q* fire of wood or 
coal is placed upon either side. To attain any great degree of heat 
by such means, however, the size of the box or chamber and especi- 
ally of the grates in which the fires are maintained must be properly 
proportioned both to the dimensions of the crucible and to each 
other. The grates are generally wide and deep, while draught is 
provided by means of a tall conical chinmey which stands over 
the entire chamber and communicates with it by a number of 
small openings. Jn a more refined furnace the chamber itself is 
double, and the flame, after playing round the crucible in the 
inside of the chamber, is made to pass through the space between 
the Outer and inner chamber before passing to the chinmey or cone. 
We need not give any greater attention to these primitive furnaces, 
since they are obsolete at the present time. In modern furnaces 
the process of combustion is carried on in two distinct stages ; the 
first stage takes place in a subsidiary appUance known as a '^ gas- 
producer," where part of the heat which the fuel is capable of 
generating is utilised for the production of a combustible gas ; this 
gas passes into the furnace proper, either direct, while it is still hot 
from the producer, or after being conveyed some distance, when 
it is again heated up by the waste heat of the furnace. In either 
case the gas is hot when it enters the furnace proper, and there it 

r 2 


meets a current of air, also heated by the aid of the waste heat of 
the fomace. Hot gas and hot air bum rapidly and completely, 
and if properly proportioned yield exceedingly high temperatures. 
Seeing that in this process a part of the heat of combustion yielded 
by the fuel is generated in a subsidiary appliance and is thus lost 
to the furnace, it appears at first sight somewhat surprising that 
this system of firing is very considerably more efficient than the 
old " direct " system where the whole of the fuel is burnt in the 
furnace itself. But the advantage arises from the fact that in the 
newer system the fuel is handled in the gaseous form. This has the 
advantage, first and most important, that the heat escaping from 
the furnace in the hot products of combustion (chimney gases) 
can be transferred to the incoming unburnt gas and air and can 
thus be returned to the furnace. The manner in which this is accom- 
plished will be considered below, but it may be noted here that in 
some furnaces the escaping products of combustion are so thoroughly 
cooled that they are unable to produce an effective draught in the 
chimney of the furnace. Another advantage of the use of gaseous 
fuel is the fact that complete combustion can be obtained without 
the use of so great an excess of air as is required when solid fuels 
are to be burnt completely. For this reason much higher tem- 
peratures can be readily obtained with gaseous fuel, while the 
pre-heating of both gas and air also facilitates the attainment of 
high temperatures ; further, the great facility with which the flow 
of either gas or air can be regulated by means of suitable valves, 
makes it possible to secure much greater regularity in the working 
of the furnaces. Finally, in modern gas-producers, the amount of 
sensible heat generated and therefore lost to the furnace, is kept 
very low, the greater part of the heat set free by the partial com- 
bustion of coal in the producer being absorbed by the decomposition 
of a corresponding quantity of steam into hydrogen and carbonic 
oxide gas. The gas as it leaves one of these producers is not very- 
hot, and the percentage of heat lost in this way is therefore much 
smaller than in the older forms of gas-producer. 



It is again impossible, within the limits of this chapter, to enter 
into the details of construction and working of gas-producers. We 
must content ourselves with saying that most modern producers 
are of the form of a tower in which a thick bed of fuel is partially 
burnt and partly gasified under the action of a blast of air mixed 
with steam. The chemical actions that take place are complicated. 




^ y FURNACE ^ ^ 





5 «s 



Fig. 4. — ^Diagram of the arrangements of a regenerative furnace. 

but the final result is the production of a gas containing from 2 to 
8 or 10 per cent, of carbonic acid, 10 to 20 per cent, of hydrogen, 
8 to 25 per cent, of carbonic oxide (CO), 1 to 3 per cent, methane 
(CH4), and 45 to 60 per cent, of nitrogen, with varying quantities 
of moisture, tarry matter, and ammonia. In good producer gas the 
combustible constituents (hydrogen, carbonic o2dde and methane) 
should total from 30 to 48 per cent, of the whole by volume, but 
the exact composition to be expected depends very much on the 
type of producer and the class of fuel used. Some producers are 


capable of dealing with exceedingly low-grade fuels, and the gas 
which they yield can still be utilised for obtaining the highest tem- 
peratures — a proceeding that would have been impossible if it had 
been attempted to burn these fuels directly in the furnace. 

The gas on leaving the producer passes along fire-brick flues or 
passages to the furnace proper ; the path which it is now caused 
to take varies somewhat according to the arrangement of the 
furnace in question. Modern gas-fired furnaces usually belong to 
one of two distinct tjrpes according to the manner in which the 
heat of the escaping products of combustion is utilised for heating 
the incoming gas and air ; these two types are known as the 
" regenerative " and the " recuperative *' respectively. In r^ene- 
rative furnaces the hot products of combustion, after leaving the 
furnace diamber proper, and before reaching the chinmey, pass 
through chambers which are loosely stacked with fire-bricks ; these 
chambers absorb the heat of the escaping gases, and thus rapidly 
become hot. As soon as a sufiBlciently high temperature is attained 
in these chambers or "' regenerators," the path of the gas-currents 
is altered ; the escaping products of combustion are made to pass 
through, and thus to heat, a second set of regenerating chambers, 
while the incoming gas and air are drawn through the heated regene- 
rator chambers before entering the furnace proper. The incoming 
gas and air are thus heated, absorbing in turn the heat stored in 
the brickwork of the regenerators. It is evident that two sets of 
such regenerators are sufficient, the one set imdergoing the heating 
process at the hands of the escaping products of combustion, while 
the other set is giving up its heat to the incoming gas and air ; 
when this process has gone far enough, it is only necessary to inter- 
change the two sets of chambers, by the operation of suitable 
valves, and this series of alternations may be continued indefinitely. 
The arrangement is shown diagrammatically in Fig. 4. 

In recuperative furnaces the same principle is utilised in a some- 
what different manner ; the outgoing products of combustion pass 
through tubular channels formed in fire-clay blocks, while the 










^ V 



-*■ to Chimney 


ingoing gas and air pass around the outside of these same blocks ; 
the heat of the outgoing gases is thus transferred to the incoming 
gases by the process of conduction through the fire-clay walls of 
the recuperator tubes. The arrangement is shown diagrammatically 
in Fig. 5. 

The relative merits of the two systems cannot be definitely 
stated. While the recuperator as a rule occupies less space and 
avoids the need for " reversing " valves 
and their regular attendance, it is more 
complicated in construction and more 
liable to get out of order as the result 
of deterioration in the recuperator 
blocks or tubes. Defects developing 
in these parts allow the incoming gas 
or air to pass direct to the chimney 
instead of passing through the furnace, 
so that the efficiency of the furnace 
becomes seriously impaired. For 
a considerable period of time the 
regenerative furnace almost completely 
monopolised the field, particularly in 
England, but in recent years a simpler form of recuperator has 
been introduced and in Sweden and in England a number of suc- 
cessful recuperative furnaces have been installed. Some experi- 
mental furnaces at the National I^ysical Laboratory have been 
designed on the recuperative principle and have yielded excellent 
results. Here, however, the recuperator has been made of actual 
tubes which have been enabled to perform their functions very 
efficiently by the introduction of silicon carbide (sometimes known 
as " carborundum ") into their composition. It seems probable 
that large furnaces of this type may prove eminently successful in 
spite of the somewhat high cost of the material. 

In both systems of furnace, heated gas and heated air are admitted 
to the furnace by separate fire-brick flues or passages, air and gas 


Fig. 6 — ^Diagram illustrating 
the principle of the re- 
cuperative furnace. 



being allowed to mix just befcce the^ enter the fnmace chambei 
proper. The economy and efficioicjr of the furnace depend to a 
veiy great eocteot upon the mauner in which this mixing is accom- 
plished. Bapid and complete mixing of aii and gas resulta in an 
inteneelj hot, but short and local fiame, while slow^ mixing tends 
to lengthm the flame and spread the heat through the oitire furnace 
chamber ; on the otiier hand, if the mixing of gas and air is too 
slow, combustion may not have beeo completed in the short time 

'iQ. 6. — Sectional diagram of a regeneratiTe pot fmnace 
working with covered pots. 

i by the gases in passing through the furnace, and c<an- 
may either continue in the outflow flues and regenerators, 
ay be prevwted by the narrowness of these passages, and 
; gases may pass to the chimneiy. When the openings or 
" are proporly proportioned, and the draught of the chimney 
iriy regulated, combustion should be just complete as the 
Ave the furnace cbambea', and under theee circumstances 
>ngues of keen flame will escape from every opening in t^e 
; large smoky flames issuing from a gas-flred furnace 
I incomplete combustion. 



As has already been indicated, glass is melted either in pots or 
crucibles of various shapes and sizes, or in open tank furnaces. 
The general arrangement of a pot furnace working with dosed or 
" covered " crucibles is shown in Fig. 6. In this particular furnace 
the " ports " or apertures by which the gas and air enter the furnace 
chamber are placed in the floor of the chamber, but these apertures 
are often placed in the side or end walls, or even in a central colunm, 
the object being in all cases to heat all the pots as uniformly as 
possible and to avoid any intense local heating, which would merely 
endanger the particular crucible exposed to it, without greatly 
aiding the real work of the furnace. In pot furnaces, however, in 


Fig. 7. — ^Diagram of a furnace with "horse-shoe" flame. 

which the more refractory kinds of glass are to be melted, it is 
generally considered desirable that the flame should be made to 
play about the pots in such a way as to heat the lower parts of the 
pots most strongly. In connection with the question of the uni- 
formity of heat distribution in a gas-flred furnace it must further 
be borne in mind that in the case of regenerative furnaces the 
direction of the flame is reversed every time the valves are thrown 
over, and in practice this is done about once every half-hour ; this 
proceeding, of course, tends very much to equalise the temperature 
of the two sides of the furnace. In recuperative furnaces, on the 
other hand, the direction of the flame is not changed, and for that 
reason a flame returning upon itself, usually called a horse-shoe 
flame, is often employed ; this is obtained by placing the entry 
and exit ports side by side at one end of the furnace ; the impetus 



of the flame gases and their rapid eaqjansion during combustion 
carry the flame out across the furnace, while the chimney draught 
ultimately sucks it back to the exit ports, the shape of the flame 
being shown in Kg. 7. 

In general arrangement a tank furnace for glass-melting resembles 
an open-hearth steel furnace. The tank or basin is built up of a 
number of large fire-clay blocks, forming a bath varying in depth 
from 20 in. to 42 in. according to the design of the furnace and the 
kind of glass to be melted in it. The ports for entry of gas and 
air and for exit of the products of combustion are in most modern 
furnaces placed in the side walls of the furnace just above the 









FiQ. 8. — Longitudinal sectional diagram of tank furnace. 

level of the glass, the whole being covered by a vault built of silica 
brick. Figs. 8 and 9 show the general arrangement of a simple 
form of tank furnace such as that used in the manufacture of rolled 
plate glass. The furnace indicated in the diagram is intended for 
r^enerative working with alternating directions of flame; in 
recuperative furnaces the horse-shoe flame is always used in tanks, 
while this arrangement of ports is sometimes adopted for reg^iera- 
tive tanks also, particularly in the manufacture of bottles. For 
the production of sheet glass, tank furnaces are generally sub- 
divided into two compartments and are also provided with various 
constrictions intended to arrest impurities and to allow only clear 
glass to pass, but as regards the arrangement of flues and ports 


there ia a very general similarity between various furnaces of this 

Practice has, however, varied very widely in regard to the height 
of the open space or " flame space " which should be left in a tank 
furnace above the molten glass. The earliest furnaces were built 
with a very low " crown " or roof, so that the porta were actually 
openings in the roof. As a result the flame t^ded to play directly 

Fig. 9. — Transverae eectional diagram of tank furnace, 
showing regenerators and gas and air pasaa^H. 

upon the surface of the glass. Although useful in supplying reducing 
agents where salt-cake batches were being melted, this arrangement 
of flame proved unsatisfactory, principally owing to direct con- 
tamination of the glass by the flame. A reaction then set in and 
furnaces with very high roofs were tried, in which the flame played 
' at some height above the gUss and the heating of the glass took 
place largely by radiation from the roof of the furnace. This system 
proved decidedly inefficient, which is not surprising in view of the 
fact that the roof is only a relatively thin vault of silica brick which 
is air-cooled on the outside. Were the furnace built of a non- 


conductor of heat — or evoii of a very poor conductor — ^its dimensions 
would be immaterial witliin wide limits, and the larger internal 
spjace would allow of more complete combustion of the gaseous 
fuel. In practice, however, every square foot of external surface 
radiates a large amount of heat supplied by thermal conductivity 
of the walls from the heat of the flame, and it becomes seriously 
important to reduce external dimensions as far as possible. Modern 
practice, therefore, both in glass and steel fm*naces, tends to keep 
the crown as low as possible consistent with allowing the flame to 
sweep just over the surface of the bath and not smothering the 
flame between furnace roof and bath. 

The importance of the point just mentioned, viz., the effect of 
external surface area of a furnace on its thermal efficiency, affects 
another important factor of furnace design. This is the thickness 
of the walls. It would seem, at first sight, that increased wall 
thickness must lead to increased thermal efficiency by interposing 
greater resistance to the passage of heat from the interior to the 
exterior of the furnace. The increase of thickness, however, at the 
same time increases the exterior surface, and thus — ^if carried 
beyond a certain point — actually tends to increase the heat lost 
from the furnace to its surroundings. The thickness of furnace 
walls is, however, frequently governed by an entirely difierent 
consideration, which is diametrically opposed to the principle of 
heat conservation by reducing external losses. This consideration 
arises from the fact that in many cases the refractories used in 
furnace construction are not good enough to resist the temperature 
which they would attain if fully exposed to the interior heat ; 
in order to prevent the furnace from collapsing, therefore, it becomes 
necessary to keep the temperature of the walls from rising too 
much and for that purpose they are intentionally air-cooled by 
making them relatively very thin. The efficiency of the furnace 
from the point of view of fuel consumption is thus deliberately 
sacrificed on account of the weakness, at high temperatures, of 
the materials used in the construction of the furnace. The extent 


to which this occurs in practice can be readily seen by means of 
a simple experiment. Points can be found on the exterior of most 
glass-works furnaces where the temperature is low enough to allow 
the hand to be held against them. If at such a point the surface 
is covered for a few hours with some bad conductor of heat, such 
as a brick made of zirconia or of diatomite, it will be found that 
the wall of the furnace under this covering becomes red hot. This 
aspect of furnace construction serves to emphasise the importance 
of a thorough study of refractories and of methods of testing them 
in order that furnaces may be constructed of materials which do 
not require such vigorous and wasteful cooling in order to enable 
them to stand up. 

The relative merits of tank and pot furnaces depend entirely 
upon the character of the glass which the furnace is designed to 
produce. Wherever the tank can be made to produce glass of 
adequate quality its great economy inevitably carries all before 
it, so that bottle glass, for example, is made exclusively in tanks, 
and the same applies to rolled plate of the ordinary kind and also 
to the majority of sheet glass. Recently even better grades of 
glass have begun to be produced in tank furnaces, and special 
small tanks have come into use of varieties of glass where the 
output required is not sufficient to justify the continued use of one 
of the larger types. In some cases, so-called " day " tanks have 
come into use, in which the furnace runs hot for melting during 
the night and is then worked out during the day. This mode of 
working avoids the difficulty of keeping two parts of a small furnace 
at different temperatures, as would be required if melting and 
working were to be carried on simultaneously in the way that is 
done in large tank furnaces. 

On the other hand, where special qualities of glass are required 
in relatively small quantities, or where the requirements as to 
quality are very stringent, the pot furnace remains indispensable. 
Optical glass and most coloured glasses are examples of this kind. 

The causes of the greater economy of the tank furnace are nume- 


rous, and complicated by the detailed requirements of each par- 
ticular manufacture, but the most important factors in the question 
may be sununed up thus : — 

(1) The tank furnace utilises the heat of the flame more efficiently, 
as the glass is exposed to the heat in a basin whose surfaces covers 
the entire area of the furnace, while in a pot furnace there is much 
vacant, unused space. 

(2) The tank furnace permits of continuous working, the raw 
materials being introduced at one end while the glass is being 
withdrawn an(l worked at the other end. There are thus no idle 
periods, and each part of the furnace remains at or near the same 
temperature during the whole time that a furnace is alight. For 
a given size of plant, therefore, a tank furnace yields a much larger 
output, with a relatively smaller fuel consumption. 

(3) The tank furnace obviates the need for pots or crucibles, 
which are not only costly and troublesome to produce, but are 
liable to premature failure and require periodical renewal, which 
involves a serious loss of time for the furnace. 

(4) Finally, the molten glass in a tank furnace can be always 
maintained at or near one constant level and is, therefore, always 
convenient for withdrawal by means of the gatherer's pipe, the 
ladle, or the blowing machine. 

In pot furnaces, on the other hand, the composition of the glass 
can be more accurately regulated, and the molten glass itself 
can be more effectively protected from contamination either by 
matter dropping into it or by the action of the furnace gases, while 
in pots it is also possible effectually to melt together materials 
which, in the open basin of a tank, could not be kept together long 
enough to combine. 



It has already been indicated that, for glass-making purposes, 
the raw materials are required in a state of reasonably fine division. 
The exact degree of fineness required deperds very much upon, 
the nature of the ingredient in question, the general rule being 
that the more refractory and chemically resistant materials require 
to be most finely ground, while substances which melt and react 
readily, such as soda ash and salt-cake, do not require very fine 

Assuming that the materials are available in a suitable state of 
fineness, the first step in the process of glass melting consists in 
securing their admixture in the proper proportions. This may be 
done by hand entirely, by hand aided by some machinery, or entirely 
automatically. The process of hand mixing is only available for 
relatively small quantities of material and requires very careful 
supervision if inadequate mixing is to be avoided. In most cases 
the actual weighing out is done by hand, while the mixing is done 
by machinery. In this process the separate ingredients are weighed 
out from barrows or skips and are tipped into a large hopper whence 
each batch, as soon as it is completed, passes into the mixing chamber 
of the mixing machine. This may consist of nothing more than 
a cylindrical chamber in which steel arms revolve and stir up the 
contents, but more modern appliances take the form of rotating 
barrels or cylinders, set up on an inclined axis and provided with 
suitable shelves and baffles ; in these the materials are very tho- 
roughly shaken over and mixed. Where hand mixing is adopted, 
the various ingredients of each batch are thrown into a large bin 
and are there turned over several times with shovels, the entire 


material being ultimately sieved through a wire sieve of suitable 
mesh. In all cases the resulting mixture should be perfectly uniform 
in colour and texture, and analyses of different samples should show 
only small variations. With the mixture thus prepared the " cullet " 
or broken glass which is to be re-melted is now incorporated ; ideally 
this should also be uniformly distributed, but this is rarely atteno^pted 
in practice on the large scale. 

The next step in the process is the introduction of the mixture 
into the furnace. In the case of tank furnaces this is a simple 
matter, since in these the temperature is kept as nearly constant 
as possible, and raw materials may therefore be introduced at 
almost any time, the amount introduced being so regulated as to 
keep the level of the molten glass or " metal " as nearly constant 
as possible. The actual introduction is managed by means of a 
large opening or door at what is known as the " melting end " of 
the furnace. Normally this opening is covered by a large fire-brick 
block suspended by a chain running over pulleys and counter- 
balanced by a counterpoise weight. When charging is to begin, 
this block is raised and the opening is uncovered. The raw materials 
are then introduced either by hand, by the aid of long-handled 
shovels, or they are first filled into a long scoop moved by mechanical 
means forward into the furnace, where it is given a half-turn, which 
empties the contents out, and is then rapidly withdrawn. 

This charging process may be repeated every half-hour, or larger 
quantities may be introduced once every four hours, according to 
the practice that may be adopted at any particular furnace. 

In the case of ^ot furnaces the charging process is not so simple. 
Here the first charge of raw materials has to be introduced into a 
pot which has been almost entirely emptied during the working-out 
process, and the temperature of the furnace has also fallen very 
considerably during this time. Before new material is introduced 
the heat of the furnace must first be adequately restored. If this 
is not done, the fusion of the glass takes an abnormal course and 
very imperfect results arise. Further, the quantity of material 


introduced at one time must be carefully adjusted to the capacity 
of the pot. During the earlier stages of fusion most glass mixtures 
form large masses of foam, and if the crucible has been too heavily 
charged this foam overflows, with the result that valuable material 
is lost and the floor and passages of the furnace are clogged with 
glass. A certain amount of overflow, as well as leakage from defec- 
tive crucibles, is, however, imavoidable, and for this purpose every 
pot furnace is provided with a chamber so placed that the glass will 
flow into it and so be prevented from finding its way into the regene- 
rators or other parts where its presence would hinder the working 
of the furnace. These receptacles or " pockets " must, however, 
be periodically cleared of their contents from outside, and this 
constitutes one of the most irksome operations connected with 
glass manufacture. Owing to the occurrence of foaming and to the 
fact that the raw materials occupy much more space than the 
glass formed from them, it is necessary to fill the pot with fresh 
batches of raw materials several times, the quantity which can be 
introduced decriBasing each time. The number of times that this 
must be done depends upon the particular circumstances, but from 
four to eight " fillings " are commonly used for various kinds of 
glass and size of pot. The precise stage at which a fresh 
batch of raw materials should be introduced is another 
matter requiring careful attention. For some purposes it is 
necessary to wait until the previous batch is completely melted, 
but in other cases raw material may be added whilst some of 
the previous batch is still floating on the surface of the glass in 
the pot. 

We have now to consider the chemical reactions which take 
place in the mixture of raw materials that are introduced into the 
hot fumiBtce. The exact course of these reactions is not known in 
very great detail, as this could only be ascertained by an elaborate 
research on the nature of the intermediate products that result 
under various circumstances. A research of this kind would throw 
much light on the whole of the melting processes, but is in itself so 

G.M. O 


difficult that it has not yet been carried out at all fuUy. We can 
therefore only give an account of the chemical changes from our 
knowledge of the end-results and of a few intermediate products 
that are known. To take the simplest case, we may consider a 
mixture consisting of sand, carbonate of lime and carbonate of 
soda mixed in suitable proportions. In such a case we know that 
the mere action of heat alone will produce two changes — ^the car- 
bonate of soda will melt and the carbonate of lime will lose its 
carbonic acid and be " burnt " or converted into caustic lime. 
The first stage of the fusion process thus probably results in a mass 
consisting of sand grains and grains of carbonate of lime undergoing 
decomposition, all cemented together by molten carbonate of soda. 
This mass will be full of bubbles, some derived from the air enclosed 
between the grains of the original mixture and thus trapped by 
the melting mass, and others formed by the carbonic acid which is 
being driven off in the form of gas by the decomposition of the 
carbonate of lime. At the temperature of the furnace, however, 
silica has the properties of a strong acid, and not only attacks the 
carboi^te of lime much in the same manner as, for instance, hydro- 
chloric acid would do in the cold, but the silica also attacks the 
carbonate of soda, which heat alone can scarcely decompose. The 
exact order in which these reactions take place will depend upon 
the temperature of the furnace and the d^ee of mixing attained 
in the preparation of the raw materials. Although in the long inn 
the final result will probably be the same as regards purely chemical 
constitution, much of the technical success of the process must 
depend upon the exact sequence of the changes involved, as this 
must govern the number and size of the bubbles that are formed 
in the glass and the fluidity of the mass from which these bubbles 
have to free themselves. In the present state of our knowledge, 
however, we can only say that the final result is the complete 
eacptdsion of all carbonic acid from the compounds present (although 
it may remain entangled in the glass in the form of bubUes) and 
the formation of silicates of both lime and soda which remain in 


the finished glass in a state partly of mutual chemical combination, 
partly of mutual solution. 

The description of the process of fusion just given applies, with 
slight modifications, to the melting of ordinary flint-glass mixtures 
as well as to lime glasses, with the one modification that the car- 
bonate of lime of the lime-soda glass is replaced by red-lead, and 
the gas evolved by the decomposition of the red-lead is oxygen 
in place of the carbonic acid evolved from the decomposition of 
the carbonate of lime. In the case of both lime and flint glasses, 
however, certain other substances besides those mentioned are 
usually introduced in small quantities. Although these substances 
do not very materially affect the end-products of the chemical 
reactions, they very materially affect the intermediate stages, and 
thus serve the purpose for which they are introduced by affecting 
the course of the chemical changes m a favourable manner. The 
substances usually employed for this purpose are arsenic and 
nitrate of either soda or potash. The manner in which the arsenic 
acts is very obscure and cannot be discussed in detail here ; the 
chief factors in its action are, however, its volatility and its power 
of either absorbing oxygen or parting with it according to circund- 
stances. The action of the nitrates is chiefly dependent up6n the 
oxygen which they yield on decomposition by heat. This oxygen 
is in some cases stored up by other ingredients of the mixture and 
only given off at a much later stage, when the evolution of this 
gas assists in the removal of the last small bubbles of inert air or 
carbonic acid gas still left in the glass. The oxidising action of the 
nitrates, however, serves chiefly for the destruction of organic 
matter and the full oxidation of any iron present, both processes 
which tend to improve the colour of the glass, while in the case of 
flint glasses the presence of these oxidising additions is necessary 
to avoid all risk of reduction of lead, since this might result in the 
complete blackening of the glass, or the formation of metallic lead 
at the bottom of the pot. 

A much more cono^plicated set of reactions occur when the alkali 

a 2 


of a soda-lime glass is introduced either partly or wholly in the 
form of sulphate of soda (salt-cake). We have already pointed out 
that the unaided action of heat and of silica is not sufficient to 
bring about the rapid decomposition of sulphate of soda which is 
required for successful glass manufacture, and that the intervention 
of reducing agents is required. For this purpose a certain amoimt 
of carbon in the form of coke, charcoal or anthracite coal, is intro- 
duced into all salt-cake mixtures, but the reducing gases of the 
furnace atmosphere also play an important part in the reactions 
that take place. Here again it is not possible to give anything 
but an incomplete account of what occurs. The rationale of the 
whole process lies, no doubt, in the fact that sulphite of soda (NajSOj) 
is much more readily decomposed by the action of hot' silica than 
the sulphate (Naj^^OJ itself, so that the essential action of the 
reducing agents consists in robbing the sulphate of part of its 
oxygen, thus reducing it to the condition of sulphite and rendering 
it accessible to the attack of silicic acid. But if we attempt to 
express such a reaction in the usual manner by a chemical equation 
from which the quantity of carbon required to effect the reduction 
in question can be calculated, we find that the amount of carbon 
required in practice is very considerably less than that given by 
this theory ; it follows therefore that either this very large amount 
of reducing action must be ascribed to the furnace gases, or that 
the actual reactions are not strictly of the kind we have described. 
Both explanations are probably partly correct, and in practice the 
amoimt of carbon to be used in a given mixture and furnace can 
only be found by actual trial, in which the manufacturer is, of 
course, guided by the results obtained with other furnaces of a 
similar type. The end-product of the reaction is again a mixture 
of silicates, while gaseous oxides of sulphur escape from the chinmeys 
of these furnaces in considerable quantities. Some undecomposed 
sulphate, however, passes into the glass and its presence can 
always be detected analytically in glass made from salt-cake mix- 
tures. In normal working this sidphate remains in solution in the 


glass, but under certain conditions it may separate out in the form 
of white specks which constitute serious defects in the glass (" sul- 
phate stones "). Analytical control of the sulphate content of the 
finished glass constitutes one of the most useful ways of controlling 
the behaviour of salt-cake mixtures. 

On the other hand, if too great an amount of carbon is used in 
the batch or if the furnace gases are excessively reducing, the glass 
may be discoloured by the presence of alkali or lime sulphide, while 
in more extreme cases carbon-may be present in colloidal suspension, 
giving rise to colour in the glass ranging from light amber, through 
deep brown, to black. This colour, however, " burns out " on 
prolonged melting of the glass. 

It is obvious that to a mixture containing carbon as a reducing 
agent such oxidising materials as nitrates cannot be added, but 
small quantities of arsenic and of manganese dioxide are added 
because their other properties are sufficiently valuable to outweigh 
their disadvantages as oxidising agents. 

Having now briefly considered the process of fusion proper, we 
pass to the second stage in the melting of glass. In a properly con- 
ducted glass furnace, when the last trace of undecomposed raw 
materials has disappeared, we find the glass as a transparent mass 
throughout which gas bubbles are thickly disseminated. For the 
majority of purposes it is necessary to free the glass as perfectly as 
possible from these bubbles before it is worked into its final form. 
This freeing or " fining " process is carried out by further and more 
intense heating of the molten glass, which is thereby rendered more 
fluid and allows the bubbles to disengage themselves by rising to 
the surface. This occurs much more readily when the bubbles are 
large ; very minute bubbles, in fact, show no inclination to rise 
through the fluid mass. The glass-maker accordingly compounds 
his mixtures of raw materials in such a way as to yield large bubbles, 
or, failing that, he adds to the molten mass some substance that 
evolves a great many large bubbles, and these in their upward 
course through the glass sweep the small ones away with them. The 


added substance may be an inorganic volatile body, such as arsenic, 
or more frequently some vegetable substance containing mucH 
.moisture is introduced into the glass. The most usual method is to 
place a potato in the crook of a forked iron rod and then to dip the 
rod with the attached potato into the molten glass ; the heat at 
once begins to drive ofi the moisture and to decompose the potato, 
so that there is a violent ebullition of the whole mass. This ^^ boiling 
up " process assists the fining considerably and also serves to mix 
the whole contents of the pot very.thoroi^hly, but it has some 
attendant disadvantages, such as the introduction of oxide of iron 
into the glass from the rod which is used in the operation, while 
the contaminated material adhering to the walls of the pot itself 
is dragged off and mixed with the rest of the glass by the violent 
stirring action that takes place. It is, of course, further obvious 
that this process can only be usefully applied to glass melted in 
pots, since the bulk of the molten glass in a tank furnace could 
not be reached at all in this manner. Mixtures that are to be melted 
in tanks must therefore be capable of freeing themselves of their 
enclosed bubbles without such outside aid. In a tank, in fact, the 
whole melting process proceeds on somewhat different lines, since 
the temperature of the furnace is never intentionally varied, while 
on the other hand the melting glass travels down the furnace into 
regions whose temperature can be regulated to favour the various 
stages of the process that take place in each part of the furnace. On 
the whole, however, it is an undoubted fact that while the running 
of a pot furnace can be varied, within wide limits, to suit the requiro- 
ments of whatever mixture it is desired to melt, in the case of tank 
furnaces the mixture must be closely adjusted to the requirements 
of the furnace, whose general " run " cannot be very readily altered. 
The completion of the " fining " process is generally determined 
by taking samples of the glass out of the pot or tank and examining 
them for enclosed bubbles. Such samples may be obtained in a 
variety of ways, the most usual method being to dip a flat iron rod 
just below the surface of the glass and to lift it out vertically upwards. 


thus retaining on the flat surface of the rod some of the glass that 
lay there at the moment when the rod was immersed. These test 
samples or "proofs" are examined very carefully, and if no trace 
of bubbles can be observed the glass is generally r^arded as " fine," 
but it is by no means certain that the absence of bubbles from such 
a small sample will prove that the whole mass is free ; that, how- 
ever, is a point where the melter's escperience enables him to judge 
how far he may rely upon the indications given by the " proofs." 
When the glass is " fine " it frequently happens that the surface 
of the molten mass is contaminated by specks of foreign matter 
floating on the glass ; for the purpose of removing these, the surface 
of all glass is skimmed before work is begun upon it. This is done 
by removing the surface layer of glass by means of suitably shaped 
iron rods, upon which small masses of molten glass are first 
" gathered." Finally, it only remains to reduce the temperature 
of the glass from that of the melting and fining process to the much 
lower temperature at which the various methods of working the 
glass are carried out. In pot furnaces this is accomplished by 
lowering the temperature of the entire furnace, while in tank fur- 
naces the fine glass flows into the working chamber of the tank 
which is always kept at the working temperature. 



In the previous chapter we have followed in outline the process 
of fusion and fining of glass, leaving the molten material ready for 
working up into the final shape. Up to that point the process is 
very similar in all kinds of glass, although the furnaces, pots and 
utensils employed vary considerably, as do also the temperatures to 
which the materials are heated at various stages. The working 
processes, however, differ entirely from one class of product to 
another, as obviously the process employed for the production of 
a sheet of plate glass can have little in common with that used in 
the manufacture of a wine-glass. On the other hand, the modes of 
working hot glass are not so numerous as the products that are 
produced, so that we find very similar appliances and manipulation 
recurring in various branches of the industry. For that reason we 
propose to deal here with the principal methods of manipulating 
glass, leaving the details of each method as applied to special pur- 
poses to be discussed in connection with the special product in 

. The first stage in the working of all glass is the removal of a 
suitable quantity of molten glass from the furnace. Practically only 
three methods are available, viz., ladling, pouring and gathering. 
If we think of a familiar substance having physical properties 
somewhat resembling those of glass, we may take thick treacle and 
suppose it contained in a jar or bottle ; there are three obvious ways 
of extracting it from the bottle : we may ladle it out with a spoon, 
or we may pour it out by tilting the whole bottle, or we may dip 
a spoon or fork into the thick liquid, slowly draw it out and turn 


it round as we do so, thus bringing out on the spoon or fork a round 
adherent mass or " gathering " of treacle. In the case of molten 
glass the process of ladling is by far the simplest, but it has certain 
very decided limitations and disadvantages. These arise froni the 
fact that a ladle cannot generally be introduced into molten glass 
without contaminating the whole mass of glass, at any rate with 
numerous air bubbles. The metal of the ladle carries with it a 
considerable amount of closely adherent air which is partially 
detached while in contact with the hot glass, so that both the con- 
tents of the ladle and the glass remaining in the furnace are con- 
taminated. These bubbles might perhaps be avoided if hot ladles 
were used, but in that case the glass would adhere to the surface 
of the metal, and each ladle would require laborious cleaning after 
each time that it was used. In practice, therefore, ladling is only 
used for the production of those classes of glass where the presence 
of a certain number of air-bells is not injurious, and the ladles are 
kept cold by immersion in water after each time of use. The use 
of the cold ladle has, however, the further disadvantage that a 
certain quantity of the glass withdrawn in it is very considerably 
chilled by contact with the cold metal, and is thus too stiflE to undergo 
the further processes satisfactorily — ^this chilled glass has, therefore, 
to be rejected from each ladleful ; this not only involves loss of 
glass, but also necessitates the separation of this spoilt glass from 
the rest. Where a heavier ladle, made of thicker iron, is used and 
is allowed to become hotter, a coating or " skull " of glass remains 
adherent to the ladle after each time of use. This glass, which is 
liable to be contaminated with oxide derived from the ladle, has 
to be removed from the ladle and constitutes a loss, although it can 
be re-melted. 

In connection with certain types of glass-blowing machines 
ladling is used as the means of feeding the glass into the machines. 
The difficulty arising from the contamination of the glass with 
air bubbles still remains, but in some cases it has been got over by 
passing the glass from the ladle into a heated receptacle which 


acts as a settling or refining pot in which the glass can rid itself of 
any bubbles which have been introduced into it. As such bubbles 
are generally large they rise to the surface quickly, provided that 
the glass is kept fairly fluid. In this connection it is well to note 
that glass can be ladled in a much hotter and more fluid condition 
than that which is necessary for gathering. 

The general process of rolling requires little treatment here. 
Two essentially different processes are used; in one the glass is 
thrown on a fiat table and rolled out by a moving roller passing 
along the table : in the other the glass passes between two rollers re- 
volving on fixed axes, and the sheet so formed is received on a moving 
table or slab. The former mode of rolling is used for the production 
of the ordinary rolled plate glass ; if the surface of both table and 
roller is smooth, the glass also has a comparatively smooth surface, 
but the surface is far from being lev^l or free from irr^ularities. It 
has been found that it is quite impossible to prevent these irregu- 
larities, which appear to arise from the buckling of the glass against 
the iron surfaces with which it comes into contact ; when rolled, 
the glass is too stiff to recover its true, smooth surface under the 
influence of surface tension, so that it retains all the marks of roller 
and table — ^nor can the roller btf made perfecdy smooth, since in 
that case it appears to slip over the glass and does not roll it out 
properly. All efforts, therefore, to produce a glass having a true 
and smooth surface by direct rolling have failed, and are likely to 
fail, so long as tables and rollers are made of materials similar to 
those now in use. The process of rolling on a stationary table is, 
however, used for the manufacture of plate-glass; but here the 
slab as rolled has still the rough, uneven surface similar to that of 
ordinary "rolled plate," and this is removed and replaced by a 
true polished surface by the mechanical processes of grinding and 
polishing. The second mode of rolling, i.e., with two or more 
" stationary " rollers and a moving table, is uBed for the production 
of rolled plate having special surface features or patterns ; the 
variety of rolled glass known as " figured rolled plate," having a 


deeply imprinted pattern, is produced in this way. This method 
requires much more complicated mechanical appliances, some of 
which are still protected by patent rights. 

Ladling being thus limited to the production of inferior kinds of 
glass, the better varieties are dependent upon either gathering og 
pouring. The former process is limited as regards the quantity of 
glass that can be dealt with in one piece, although surprisingly 
large quantities can be gathered upon a single pipe; the great 
masses of glass, however, that are required for the production of 
modern polished plate could not be handled in this way, and the 
method of pouring is accordingly adopted. For this purpose either 
the pots in which the glass has been originally melted, or others 
specially designed for this purpose, and into which the molten glass 
has been transferred, are rempved bodily from the furnace by the 
aid of powerful mechanical appliances ; they are then carried by 
overhead cranes to the place where the glass is to be rolled into 
the form of a plate, and there the pot is tilted and the molten glass 
is allowed to run out and to form a pool on the rolling table, the 
passage of the great roller ultimately rolling the pool out into a 
sheet much as dough is rolled out with a rolling-pin. This process 
is obviously only possible with pots or crucibles of a suitable size^ 
and is, moreover, very destructive to these pots, since they are 
exposed to great variations of temperature. Li the case of tank 
furnaces, numerous devices have been patented for allowing the 
glass to flow out over a sill or weir of suitable size, ready to be 
rolled or drawn into the form of sheets or slabs ; but none of these 
devices have, so far as the writer is aware, found their way into 
practice ; the reason for this probably lies in the fact that it is not 
easy to find a material which will present a smooth face to the out- 
flowing glass, such materials as fire-clay leading to contamination 
from detached fragments, while chilled metal leads to local chilling 
of the glass. Although the various processes of drawing glass into 
sheets direct from the furnace have undergone very material improve- 
ment, the laborious process of gathering yet retains its importance 


even in the production of such large objects as sheets of window 

In its essence the process of gathering consists in introducing 
into the glass a heated iron rod or tube to which a small quantity 
of glass is allowed to adhere ; rod and glass are removed from the 
furnace together, and the small adherent ball of glass is allowed 
to cool so far as to become stifi enough to carry its own weight. 
The rod with its adherent ball is then again dipped into the glass, 
where a fresh layer of glass attaches itself to the ball already on 
the rod. The whole is again withdrawn, allowed to cool down, and 
then dipped into the molten glass again to gather a fresh quantity. 
This cycle of operations is repeated until the desired quantity of 
glass is attached to the rod or tube. These operations, particularly 
when weights of thirty or forty pounds of glass have to be gathered, 
require the exercise of a great deal of skill and care ; the introduc- 
tion of the gathering into the molten glass is each time liable to 
produce air bells which would spoil the whole mass of glass or would 
contaminate the contents of the crucible, while subsequently the 
mass of hot glass adhering to the rod or pipe tends to run down 
and even to drop off entirely if not properly checked by suitable 
rotation of the pipe. Further, the manual labour and eacposure to 
heat involved for the operator all tend to increase the cost of such 
work. Mechanical aids to gathering were at first confined to simple 
devices for relieving the operator of the great weight of the heavier 
gatherings, but more recently wholly mechanical devices for gather- 
ing have come into successful use, particularly in bottle-making. 
These depend upon suction, the molten glass being drawn up into 
an inverted cup whose edge is immersed in the glass contained in 
the furnace. These devices, however, require a special tjrpe of 
furnace or receptacle in which the molten glass is kept at a constant 

Just as ladling is nearly always preliminary to rolling, so gathering 
is usually the preliminary to some blowing process, although the 
blowing is often combined with and sometimes replaced by the 


mechanical pressing of the glass. Where the glass is to be blown, 
the gathering is always made on a glass-maker's pipe. This is an 
iron tube from 4 to 6 ft. long, provided at one end with a wooden 
casing to serve as a handle, and with a suitably arranged mouth- 
piece for blowing. The shape of the lower or " butt " end of the 
pipe depends upon the character and size of the oBjects to be blown ; 
for small articles the pipe must be narrow and light, but for heavy 
sheet glass the butt of the pipe is extended into a conical mass 
whose base is from 2 to 3 in. in diameter. The bore of the pipe at 
both ends also depends upon the class of work for which it is intended. 
At the end of the blowing operation, when the blown article has 
been detached or " cracked off " from the pipe, a fairly thick mass 
of glass remains attached to the butt of the pipe. This usually 
breaks off as it cools, or is readily knocked off, but its value as cullet 
for re-melting is greatly impaired by the fact that the glass is gene- 
rally contaminated by adhering layers of iron scale derived from 
the pipe. For this and other reasons the butts of pipes and of 
gathering irons are sometimes made of fire-clay, which is formed 
over the end of the iron tube. In order to secure better adhesion 
between the clay and the iron, the lower end of the pipe is provided 
with several pins. 

The first stage of all blowing processes consists in the formation 
of a hollow sphere by blowiog into the pipe, the pressure of the 
breath being as a rule sufficient to cause the gradual distension of 
the hot mass of glass. From this rudimentary hollow sphere the 
various shapes of blown articles are then evolved by a series of 
manipulations which vary very widely in different branches of 
manufacture. They generally consist, however, in gradually 
changing the shape of the mass of glass by the pressure either of 
hand-tools or of specially prepared moulds or blocks against which 
the glass is held or turned, either with or without simultaneous 
blowing into the pipe. The extent to which the aid of such moulds 
and blocks is invoked varies continuously from the production of 
the hand-made vase or glass to the moulded bottle ; in the former, 


practically only hand^ools, whose shape bears no direct resemblance 
to that of the finished article, are employed, while in the latter 
the elongated hollow mass of glass is placed inside a mould, and 
internal air pressure is used to press the glass into contact with the 
moidd from which the shape of the finished bottle is thus directly 

The art of the blower further takes the fullest advantage of the 
peculiar physical properties of glass while in the heated viscous 
condition, the material being made to flow under the action of 
gravity and centrifugal forces, as well as under the pres^sure of the 
breath, the glass being held aloft, twirled or swung about to ensure 
the production of the various shapes required. For the great 
majority of such purposes the imaided manipulations of the operator 
are sufficient, but various mechanical aids are used to facilitate 
the more laborious stages of the work, while for the simpler forms 
that are required in very great numbers, such as bottles, the 'whole 
of the operations are now carried out by automatic machines. Of 
the more usual mechanical aids at the disposal of the glass-blower, 
we have already mentioned hand-tools, blocks, and moulds of 
various kinds. Next in importance to these is the ubc of com- 
pressed air for blowing large or heavy articles ; the pressure avail- 
able by the human breath is very limited, and the volume of air 
that can be thus delivered is not very large, while the constant 
use of the lungs for such a purpose is trying for the workman. In 
many works, therefore, air imder pressure is supplied to the benches 
or stages where the blowing is done, and the blowers' pipes can be 
coupled to this air supply by means of flexible connections when 
required. The principal difficulty lies in the correct regulation of 
the air pressure for each special purpose ; but this difficulty has 
been overcome by the use of delicate valves under the control of 
each blower, who can thus regulate the pressure to his own exact 
requirements. Such a system, of course, requires some little practice 
on the part of the men using it, but when they have become accus- 
tomed to the working of the plant the results achieved ^e decided^ 


better and more regular than those obtained by mouth blowing. 
Besides the use of compressed air supplied in the way just indicated, 
several other devices are in use to aid the blower in producing the 
requisite pressure in the interior of the hollow bodies he is pro- 
ducing. The simplest of all these consists in utilising the expansive 
force of the air enclosed in the hollow body when that body is exposed 
to heat. Thus, for instance, in blowing a cylinder of sheet glass, 
if the blower holds his thumb over the aperture of his pipe, and 
brings the closed end of the cylinder near the hot " blowing hole," 
the heat which softens that end of the glass will also act upon the 
enclosed air, and will very rapidly produce such an expansive effect 
as to burst open the softened end of the cylinder. This means of 
opening the closed ends of the cylinder is frequently employed -in 
practice. It is, of course, obvious that any other expansive fluid 
might be employed in a similar manner, and in some blowing pro- 
cesses it has long been the practice to introduce a small quantity of 
water into the interior of the hollow body, when the rapid expansion 
of the steam produced thereby is utilised for the purpose of gene- 
rating the requisite internal pressure. This use of the expansive 
force of steam generated by the heat of the hot glass body has been 
utilised by Sievert, whose process is described in Chapter VIII. 

Whatever mechanical aids are employed to facilitate the various 
stages of the process, all glass blowing involves a series of operations 
requiring considerable skill, while the whole manner of dealing with 
the glass is essentially extravagant of material, except perhaps in 
the production of bottles or flasks having narrow mouths. The 
reason for this latter statement lies in the fact that by blowing it 
is only possible to produce closed or nearly closed hollow bodies or 
vessels ; thus a blown wine glass or tumbler is formed with a hood 
or dome closing in the open top of the glass, and this hood or dome 
has subsequently to be removed by subsidiary processes, such as 
cutting off by the aid of strong local heat or by grinding, and the 
cut edge has to be provided with a smooth finish. In the case of 
comparatively small articles like glasses the loss mvolved from 


this cause is not so very great, but were large flat bowls or dishes 
to be produced by blowing, the loss in the dome or covering would 
be very serious. This difficulty is entirely avoided by the process 
of pressing glass. We have already indicated the manner in which 
moulds are used for the production of the desired shape in the case 
of bottles, etc., but in these cases, where the final object is to be 
a hollow vessel, the glass is readily forced into contact with the 
mould by means of internal air — or steam — ^pressure ; in the 
process to which we are now referring, however, the hot glass is 
forced into contact with the external moidd by means of an internal 
plui^er which is pushed downward with considerable force. By 
this means flat or shallow bodies can be produced without the 
preliminary formation of a completely closed vessel, while it is 
obvious that by the use of suitable moulds, complicated and elaborate 
shapes can be produced. It is true, of course, that pressed articles 
do not show the same smooth and brilliant surface which is charac- 
teristic of the fire polish of blown articles, while the facility with 
which elaborate surface ornamentation can be applied by this 
process has not tended to artistic refinement in design, but the 
great nvajority of cheap and useful glass articles of domestic use 
have been made available by the development of the pressing 

In the ordinary course, pressed glass is produced direct from the 
molten material, which is introduced into the presses either by 
gathering or. by means of ladles, but for some special purposes 
glass is brought into its final shape by mechanical pressure after 
having first been allowed to solidify and having then been specially 
re-heated to undergo the pressing or moulding process. This is 
principally done in the case of the best kinds of optical glass, where 
the molten glass is first allowed to cool in the actual crucible and 
is then broken up into lumps of a suitable size, from which the 
more defective portions can be rejected, the more perfect portions 
only being heated up again in special kilns and then forced to take 
the desired shape by being pressed — sometimes with hand-tools 


only and sometimes by the aid of powerful presses — ^into moulds 
of the required shape. Small lenses, however, for which the require- 
ments of quality are not so high are sometimes pressed direct from 
small gatherings taken from the molten glass in the crucible. 

In almost every process of glass manufacture the final operation 
is that of Anmaling, and although the exact manner in which it is 
carried out must vary considerably according to the nature of the 
particular product in question, it may be well in this place to con- 
sider the general principles which underlie the operation. 

It is, of course, a well-known fact that when glass is either heated 
or cooled too rapidly it will " fly " or crack. If it is cooled rather 
more slowly, but still rather fast, it may not crack at the time, but 
it retains a large amount of internal strain and either cracks spon- 
taneously at a later period or is liable to sudden fracture through 
small accidental shocks. There is no doubt that much of the apparent 
great fragility of ordinary cheap glass ware is due to the presence 
of such internal^ stresses arising from inadequate annealing. The 
object of annealing, then, must be the removal — ^to as complete a 
degree as possible — of the internal stresses which arise from rapid 
cooling. To understand how annealing can effect this object we 
must consider how the internal stresses arise. 

Consider, for the sake of simplicity, a spherical mass of glass 
cooling down from a bright red heat, i.e., from a temperature at 
which the glass is quite soft. As cooling takes place, the outer 
layers of the glass will cool down, at first, much more rapidly than 
the interior. Glass is not a very good conductor of heat, and the 
outer layers lose their heat so rapidly by radiation and by trans- 
mission to the surrounding air that, for a considerable time, the 
flow of heat from the interior layers of glass is not able to balance 
the loss. But the flow of heat from the interior increases as the 
outer layers get colder, so that, after a time, a sort of equilibrium 
is set up. The temperature difference between inside and outside 
has then become large enough for the flow of heat outwards nearly 
(but not quite) to balance the loss of heat from the exterior surface. 

O.M. H 


During tlie cooling of any thick piece of glass, then, we must have 
a dijGEerence of temperature set up between the interior and the 
exterior, and this difference will be larger the faiSter the rate of 
cooling and the thicker the piece of glass. 

During the cooling process, however, a stage will be reached at 
which the outer layers have become sufficiently cold to be quite 
stiff and incapable of adjusting themselves to any forces that may 
act upon them. At this stage, then, we have a hard " set " outer 
shell surrounding a much hotter and softer interior mass of glass. 
Now glass, like the majority of known materials, contracts as it 
cools, and the greater the range of cooling, the greater also the 
amount of contraction which takes place. Suppose now that the 
interior of our sphere of cooling glass is, on the average, 200° C. 
hotter than the exterior, so that perhaps at one stage in the cooling 
process the exterior is 200° C. hotter than the surroimding air, 
while the interior is 400° C. hotter than the air. Ultimately, when 
the whole mass has cooled down, both exterior and interior must 
reach the same temperature. In cooling thus, however, the natural 
contraction of the interior would be something like (although not 
exactly) double as much as that of the exterior, since it had to cool 
down through twice the range of temperature. But since the outer 
layers are hard and stiff, and the interior mass is firnUy attached 
to the outer layers by the natural cohesion of the glass, the interior 
portions are hindered from undergoing their natural contraction. 
The result is that a state of tension is set up in the inner layers and 
a corresponding state of compression in the outer ones. One may 
think of the inner portion as having first been allowed to contract 
to the full luihindered extent and then to have been stretched back 
again to the size of the outer layers by the application of very large 
tensile forces, and when thus stretched to have been fixed or anchored 
to the outer layers. Now the actual distance through which the 
glass has to be stretched thus to undo — or to keep undone — a 
portion of its natural contraction, is only a very minute one, but it 
requires the application of exceedingly powerful forces to stretch 


solid glass even to so minute an extent. Consequently, if the 
difierence in temperature between outside and inside at the moment 
when the outer layers become hard has been large, the forces or 
stresses set up become so large that the glass breaks under them 
and cracks or " flies." If the difference has been rather smaller, 
the glass will not actually crack, but remain in a state of severe 
internal tension. Remembering that the difference of temperature 
which exists at the critical stage just mentioned depends upon the 
rate of cooling, we see at once that for the avoidance of internal 
stresses slow cooling must be adopted, and, further, that the thicker 
the pieces of glass to b^ dealt with, the slower must be the rate of 
cooling employed. It follows, also, that it is the rate of cooling at 
the critical stage when the outer layers begin to become stiff while 
the interior is still soft, that really determines the final state of 
the glass, so that very slow cooling is principally necessary through 
that critical range. 
. The simple case we have considered, of a cooling spherical mass 
of glass, serves to illustrate the principle, but the consideration of 
another class of case is also instructive. As an example we may take 
a flat-bottom,ed tumbler of the type in which the thickness of glass 
is nearly uniform throughout sides and bottom. Here the actual 
thickness of glass is nowhere very large and the danger of setting 
up large differences of temperature between outside and inside 
layers is not very great imless the glass is cooled very violently 
indeed. But there is another way in which differences of temperature 
may arise in such a case, with the resultant setting up of stresses 
and possible cracking. Suppose the tumbler finished by the blower 
and at a uniform temperature throughout. If it were now stood 
upright on a cold slab in such a way as to cool the bottom much 
faster than the sides, what would result ? If at first the sides were 
stm fairly soft the rapid contraction of the bottom, as it cooled 
down quickly, would draw the sides inwards near the bottom; 
then, with the bottom perhaps 200** or 300° colder than the sides, 
the whole would steadily cool. After a time the sides would have 

> < H 2 


become " set '' and stiff, but would still be much hotter than the 
bottom. Finally, when the bottom was quite cold and would no 
longer undergo thermal contraction, the sides would still be hot, 
and during the last stages of their cooling they in turn would contract 
still further if free to do so. The stiffness of the botton^ will prevent 
this and the ring of glass forming that part of the side wall nearest 
the bottom would have to remain stretched out to a larger diameter 
than it would naturally assume after cooling down. This again 
means that the glass is under a tensile force equal to that which 
would be required to stretch it mechanically to the size forced 
upon it by the stiffness of the previously cooled bottom. We would 
thus have a system of internal stresses very similar in amount, 
although very different in distribution from those which arise in 
a thick mass of glass. 

The general inference from these examples is that internal stresses 
will arise in any piece of glass in which widely different temperatures 
have been allowed to occur during the critical stage of cooling in 
which one portion is soft while another is already hard. At first 
sight it would seem that the only way to avoid such stresses would 
be to secure the most uniform possible cooling of the glass from the 
moment when its manufacture is finished. As a rule, however, this 
is not possible ; frequently the very success of the manipulation 
employed in manufacture depends on chilling one part of an article 
while another is still being moulded. The result is that severe 
internal stresses are set up in most kinds 6f glass during manu- 
facture, and it is the object of the annealing operation to remove 
these as completely as may be necessary for any particular purpose. 

Fortunately this can be done, provided only that the stresses 
set up have not been sufficient to cause actual cracking. If the 
strained glass is heated up to a temperature at which the whole of 
the glass is soft enough to yield a very little, and very gradually, 
to the sev^e internal forces, then these relieve themselves, and — 
provided that the subsequent cooling is sufficiently slow and uniform 
to avoid the occurrence of s^ious temperature differences within 


the glass, the article will come out of the operation relieved of 
internal stresses or — as it is usually called, "fully annealed." 
There are two points about the operation, however, which require 
careful consideration. The first is the temperature to which the 
glass must be raised in order to allow the strains to be relieved, 
and the second is the maximum rate of cooling which may be 
adopted for a given object and a given kind of glass without intro- 
ducing undue internal stresses. Both can be determined by com- 
paratively simple experiment. 

The temperature to which heating should be carried must be high 
enough to soften the glass to a certain extent but not high enough 
to soften the glass sufficiently to allow it to become distorted under 
its own weight. The temperature at which the glass becomes so 
soft that distortion will occur can be easily determined by placing 
a rod or tube made of the glass in question in a horizontal tube 
furnace heated by means of an electric current. The tube or rod 
is best supported at one end only. As soon as it becomes slightly 
soft the free end will begin to bend down. This temperature is 
easily determined by placing a thermo-couple in the furnace, gradu- 
ally raising its temperature and carefully watching the glass. As 
soon as the rod or tube begins to bend, the temperature is read, 
and this reading gives a temperature which is an upper limit which 
must not be approached during the annealing operation. Generally 
a temperature at least 50° C. lower is sufficiently high to remove 
all internal stresses in a few minutes, but the lower limit of tem- 
perature can also be determined experimentally, although rather 
more elaborate appliances are needed. 

For this experiment, and also for the testing of finished glass 
in r^ard to satisfactory annealing, we depend upon the fact 
that if polarised light is passed through glass it is not at all 
affected by its passage so long as there is no stress acting on 
the glass. If, however, there is internal stress, then the polarised 
light is affected in a very striking manner. Thus if we look through 
a tube with a Nicol prism fitted at each end, and the two prisms 


are turned into what is known as the *' crossed " position, no light 
passes and we see a black field of view. If glass free from stress is 
interposed between the two Nicols there is no change, but if strained 
glass is interposed light is at once seen in the field of view, and 
this may assume certain well-defined patterns if the glass happens 
to be strained in a symmetrical manner, and if the stresses are 
severe we may not only see black and white figures, but vividly 
coloured patterns make their appearance. It is thus an easy matter 
to ascertain whether a finished article is or is not fully annealed 
by examining it between crossed Nicols, or in the similar manner 
which is described in Chapter XIII. in connection with optical glass. 
To apply the test by polarised light to the determination of the 
annealing temperature it is necessary to place in a little electric 
tube furnace a block of the glass to be tested, but this block must 
have its two opposite ends nicely polished. The crossed Nicols 
are then so placed that the light in passing from the first Nicol (or 
polariser) to the second Nicol (or analyser) passes though the 
block of glass under test lying in the furnace tube — ^in fact, the 
furnace with the block <d glass in it must be placed between the 
crossed Nicols. When first put into the furnace the block of glass 
will show patterns— rprobably coloured — as the result of the presence 
of internal stresses. Then the temperature of the furnace is very 
gradually raised and the glass is watched through the crossed 
Nicols. Gradually, as the annealing temperature is approached, 
the patterns seen in the glass will begin to change. If — as is fre- 
quently the case — ^these take the form of rings, they will gradually 
widen out until finally there is perhaps only the trace of one ring 
left visible near the edges of the block of glass. A very slight further 
rise of temperature will then remove the last trace of strain, and 
the full annealing temperature has been reached. Actually, the 
temperature found will depend upon the rate of heating, but if 
a reasonably slow rate of heating is adopted the temperature found 
in the way indicated will be sufficiently accurate as a guide to 


With regard to the rate of cooling which can safely be employed, 
that must depend upon the size and shape of the object, the nature 
of the glass, and the degree of freedom from strain demanded in 
the finished article. The requisite rate can best be determined by 
a series of experiments in which specimen objects are heated to 
the previously ascertained annealing temperature and cooled at 
known rates. By examining them in polarised light afterwards 
it is possible to determine the maximum permissible rate of cooling. 
It is not, however, essential to carry this slow rate of coolin^ight 
down to the ordinary temperature; once the whole of the glass 
is " set " or stiff, no further internal stresses of a pernianent kind 
can be set up and more rapid cooling can safely be adopted below 
that temperature. The only limit imposed arises from the risk 
that the glass may crack as a result of temporary differences of 
temperature and consequent temporary internal stresses which 
might be set up by unduly rapid cooling. These temporary stresses 
arise when any piece of glass is suddenly heated or cooled, owing 
to the unequal expansions or contractions set up by differences of 
temperature between different parts. But, provided that no part 
of the glass is soft when this occurs, all the stresses die out so soon 
as the glass has attained a imif orm temperature. So long, therefore, 
as the glass is not actually cracked no harm is done by rapid 
cooling below the point at which the whole of the glass is quite 

The naethods of arriving at a correct annealing process for any 
given glass object which have just been outlined may appear com- 
plicated to the practical glass-worker who is accustomed to put 
his products into a " lear " and expects them to come out at the 
other end properly annealed. But a scientific treatment of the 
annealing problem is of much greater importance than is sometimes 
realised. Endless difficulties arise in the practical utilisation of 
glass products, particularly for technical purposes, which are 
idtimately traceable to unsatisfactory annealing. This is par- 
ticularly the case in technical products^ but makes itself felt in 


domestic usage also. An examination of any large number of glass 
objects in polarised hg^t reveals a very unsatisfactory state of 
practice in this respect. Closer attention to tbe wbole matter is 
therefore of great importance to the progress of the glass 



Although bottles are in some respects the cheapest and crudest 
products manufactured of glass, their uses are so innumerable and 
their numbers so enormous that their production constitutes a most 
important branch of the industry. 

In the choice of raw materials for the production of ordinary 
bottles cheapness is necessarily the first consideration. Natural 
minerals, bye-products of other industries, and the crudest chemicals 
are utilised so long as it is possible by compounding these mgredients 
in suitable proportions to obtain a glass whose composition meets 
the somewhat crude requirements which bottles are expected to 
meet. The most essential of these requirements are that the bottle s 
shall be strong enough to resist the internal pressure which may 
come upon them when used for the storage of fermented or effer- 
vescent liquors as well as the shock of ordinary use, while the glass 
itself must possess sufficient chemical resistance to remain un- 
attacked by the more or less corrosive liquids which it is called upon 
to contain. Further, from the point of view of the bottle manu- 
facturer it is desirable that the glass shall be readily fusible, easily 
worked, and easily annealed. In other branches of glass manu- 
facture increased fusibility is often attained by increasing the alkali 
contents of the glass, but in bottle-making this is inadmissible, 
both on account of the prohibitive cost of alkali and because an 
increased alkali content renders the glass more liable to chemical 
attack. On the other hand, in many varieties of bottle the colour 
of the glass is nearly, or quite, immaterial, so that the introduction 
of relatively large proportions of iron oxide is permissible. This 


substance acts as a flux and assists in the production of a foedble, 
workable glass containing little alkali. Such alkali as bottle glass 
does contain is frequently derived from felspathic minerals, whicli 
generally ^00 contain considerable proportions of iron. The use of 
these minerals also introduces notable proportions of alumina into 
the glass. In certain classes of bottles, notably those used ioc 
special wines, certain shades of colour are required — ^the well-known 
" Hock bottle " colour being an example. The presence of iron in 
the glass tends to the production of a green or greenish-yellow colour 
deepening to a black opacity if the quantity of iron be high. The 
lighter shades of this green tint may be "neutralised" by the 
introduction of manganese into the glass, the resulting colours 
ranging from light amber to purple ; nickel oxide is also sometimes 
used as a colouring material in these glasses. 

In the production of ordinary bottles the continuous tank furnace 
has now entirely superseded the old pot furnaces, the character of 
the product being in this case particularly suited to this process of 
production. The modern bottle glass tank is generally an oblong 
basin having one semi-circular end. The flame is often of the 
" horse-shoe " type, the gases both entering and leaving at the 
flat or charging end of the furnace. The raw materials are thrown 
into the furnace at the square end of the tank, and the glass flows 
uninterruptedly down to the colder semi-circular end where the 
working holes are situated. At these points fire-clay rings are kept 
floating on the glass, and from within these the gatherer takes his 
gathering, the rings serving to retain the grosser impurities carried 
down by the glass. The producing power of such a furnace, even 
when the bottles are blown by hand, is very considerable ; a furnace 
having ten working holes and containing normally about 85 tons 
of molten glass will yield some four million bottles per annum, and 
furnaces of considerably larger capacity are in use. 

The methods of bottle-making are at the present time passing 
through the later stages of a transition. Up to the middle of last 
century the processes in use were little better than those of the 


middle ages ; the first step in a more modern development of the 
industry took the direction of improved tools and implements for 
carrying out the old operations. More recently a whole series of 
inv«ntieiifi have been put forward for producing bottles by entirely 
different and wholly mechanical processes, eliminatmg the uncertain 
element of skilled labour entirely. While it must be admitted that 
some of the earlier of these inventions proved to be brilliantly 
ingenious failures, there is little doubt that here, as in other manu- 
facturing processes, the machine-made article will ultimately 
entirely supersede the hand-made product. Mechanical processes 
are already in extensive use both in America and Europe, and 
machine-made bottles are produced which in every point of quality 
are superior to the best hand-made goods. 

The first stage in the production of bottles by hand, and also 
for most of the machine processes, is that of gathering the requisite 
quantity of glass. The bottle-blower's pipe is between 5 and 6 ft, 
long, and is provided with a slightly enlarged end or " nose " upon 
which the glass is gathered. Three gatherings are generally sufficient 
for the production of ordinary bottles, but for extra large bottles, 
and especially for carboys, heavier gatherings are necessary, and 
for these the gatherer must go the furnace four, five, or even six 
times. When the requisite quantity of glass has been gathered on 
the pipe the gathering is worked and rounded by rolling it either 
on a flat metal plate or " marver," or in a hollowed block made of 
wood or more rarely of metal ; by this process the glass is formed 
into a well-rounded, symmetrical pear-shaped body. The blower 
now distends the mass gradually by the pressure of his br^th, at 
the same time swinging the pipe, the effect of these movements 
being to draw the bulk of the glass downwards, leaving a thinner 
and colder portion having the rudimentary shape of the neck of 
the bottle next to the pipe. In the oldest form of the process the 
next stage in the production of the bottle is accomplished by the 
aid of a cylindrical mould of fire-clay, whose diameter is that of 
the external size of the finished bottle. The pear-shaped bulb of 


glass is for this purpose re-heated at the melting furnace, and is 
then placed inside the fire-clay mould. By vigorous blowing, and 
a rapid rotation of the pipe and glass, the bulb is forced to assume 
the cylindrical shape of the mould, the glass forming the neck of 
the bottle being at this stage of the process too cold and stifi to be 
further deformed. The next step is the formation of the concavity 
found in the base of wine and beer bottles ; this is produced by 
pushing up the hot plastic glass that forms the bottom of the bottle 
as it leaves the clay mould. This is done by a second workman 
using an iron rod known as the " pontil," upon which a small mass 
of glass has previously been gath^ed. This mass of glass remains 
attached to the bottom of the bottle, which is thus for the moment 
fastened both to the "pontil" and to the blower's pipe. The 
blower, however, immediately detaches the bottle from the pipe 
at the point where the neck of the bottle is intended to end, effecting 
this by locally chilling the glass — a process known by the descriptive 
term of " wetting off." The unfinished bottle is now attached to 
and handled by means of the " pontil." The neck is softened by 
re-heating it over the furnace, and is then moulded into the desired 
shape by the aid of specially-shaped tongs. Finally a thread of 
glass is wound round the end of the neck to produce the thickening 
usually found at that point. The finished bottle, still attached to 
the "pontil," is now carried to the annealing kiln, where it is 
placed in position and detached from the "pontil" by a sharp 
blow, which severs the glass that had been gathered on the " pontil " 
from the bottom of the bottle. 

The process, in the form described above, has been obsolete for 
many years, improvements, consisting of appliances for facilitating 
the various operations, having been gradually introduced. The 
most important of these is tha substitution of metal moulds for the 
fire-clay moulds of earlier times. These metallic moulds are made 
to open and close at will by the action of a pedal, and are designed 
to give the entire bottle its final shape, except for the indentation 
of the bottom, although this is sometimes produced by a convex 


piece placed on the bottom of the mould. In the formation of the 
neck thickening, also, important mechanical aids have become 
alnxost imiversal. These last consist of tongs provided with rollers 
and arranged to rotate about an axis that terminates in a tapered 
spike which enters the neck of the bottle; by pressing the tongs 
together so as to bring the rollers against the outside of the neck 
and rotating the whole, the rollers are made to form the neck thicken- 
ing in an accurate and rapid manner. 

Lnportant and valuable as these improvements of the ancient 
process of bottle-blowing undoubtedly are, they do not touch the 
main disadvantages of the process — disadvantages that seriously 
afiect its economy and the well-being of the workers employed 
upon it. It is consequently not surprising that a great number of 
inventors have laboured at the problem of the purely mechanical 
production of bottles. A large number of patents have accordingly 
been taken out in connection with bottle-making machinery. The 
first of these to attain any favour was that devised by Ashley, but 
although great claims were made for it, its use has not extended. 
Recently, however, the mechanical blowing of bottles and similar 
articles has made very great strides, both in England and America. 
The machines used in England are still mostly of the semi-automatic 
type, requiring the service of gatherers and operators for various 
stages of the work. In America, on the other hand, development 
has reached the stage of the completely automatic machine iq 
which no hand-work is required, the machine taking the molten 
glass from the furnace and delivering the fully-annealed bottle 
at the mouth of the lear without manual intervention. Detailed 
descriptions of these machines cannot be given without elaborate 
drawings which would lie beyond the scope of the present book. 
A general account of the mode of operations of bottle-blowing 
machines is, therefore, all that can be given. 

The earlier machines, of which that invented and operated by 
Boucher, of Cognac, ia Erance, prior to 1900, is one of the most 
successful examples, closely imitated the process of bottle-blowing 


by hand, and this is still the case with many of the semi-automatic 
machines which operate on the *^ press and blow " principle. In 
these machines the glass is taken from the furnace by a gatherer 
on a gathering-iron (not a tube) and is dropped into the '' parison " 
mould. This mould serves in the first place to limit the amount 
of glass to the right quantity and also gives the glass its initial 
shape, i,e,y forms it into a '* parison." In the earlier machines the 
" thread " of the glass had to be cut ofi by hand, but in the newer 
tjrpes this is done by a mechanically-actuated shears or knife. In 
the '^ press and blow " machines the first step in the formation of 
the bottle consists in the piercing and shaping of the interior of the 
neck by a plunger, which is pressed into the soft glass lying in the 
neck of the parison mould. This plunger at the same time serves 
to chill and stiffen the glass of the neck, which then serves as a 
handle by which the machine holds and manipulates the bottle 
during the later stages. The only difficulty about this type of 
machine is that there is a downward limit to the size of plunger 
which will work satisfactorily for such a purpose, so that for small 
bottles, with very narrow necks, the plunger becomes too thin and 
weak. In that case the neck has to be formed by blowing alone, 
and the glass at and near the neck of the parison is chilled by air 
blown upon it from the outside. This type of machine is generally 
known as the " blow " type as distinct from the " press and blow " 
previously referred to. 

The subsequent stages in the formation of the bottle are very 
similar in all types of machines ; the parison is held by the neck 
and is automatically transferred to a finishing mould in which it 
is blown to the final shape by compressed air admitted through the 
previously formed neck. In the more usual semi-automatic machines 
the bottle is then carried by a boy from the machine to the lear for 

While machines of this type constitute a great advance in economy 
on the old hand process, they are still far behind the completely 
automatic machines, of which the Owens machine is probably the 


finest eocample. This macliine really comprises a complete special 
glass-miaking plant, in which the glass is melted in a special type 
of tank out of which it flows in a steady stream into a revolv- 
ing basin or tank in which the glass is maintained at a constant 
level and at the right working temperature. Close over the surface 
of the glass in this basin the parison moulds move on their arms 
and, at suitable intervals, dip their edges into the glass and — ^by 
means of suction — ^fill themselves with glass. When the mould is 
full it is automatically raised and the glass is cut off by a knife 
which passes imder the mould. The glass, while being sucked into 
the mould, is already formed into the neck and chilled, so that as 
the mould moves on, the glass is stifi enough to allow the mould to 
open, leaving the parison suspended by the neck. At this point 
compressed air is admitted into the neck opening and simultaneously 
the fijoishing mould closes around the parison, which is thus blown 
into its final shape. A little later the finishing mould opens and 
the neck of the bottle is released from the hold of the machine. The 
finished bottle then drops down a chute and passes — ^again auto- 
matically — ^to a continuous lear through which it is again carried 
automatically. The finished, annealed bottle is thus produced by 
entirely automatic means, without human handling at any stage. 
These machines, originally constructed with ^ix gathering arms, 
now have as many as fifteen, and a single machine deals with the 
entire output of a tank-furnace. 

It is, of course, obvious that the u]bility of suet machines is not 
confined to the production of bottles, but that any articles which 
can be blown in a simple mould can be produced in this way. Lamp- 
chimneys and electric light bulbs and many other articles come 
under this heading. For the production of electric light bulbs, 
however, special machines have been devised. In one of these the 
molten glass is ladled out of the melting furnace into a special small 
receptacle or tank in which the glass can be kept at a constant 
level, but this arrangement has serious disadvantages as compared 
with the Owens suction gathering device. On the other hand. 


dome very recent developments tend in the direction of using 
moulds which are automatically filled by molten glass flowing in 
a steady stream over a ledge or " weir " — ^but the details of these 
devices are not yet publicly known. The output capacity of these 
automatic machines is, of course, very large; a single machine 
making electric light bulbs, which are very light, has turned out 
as many as 18,000 bulbs pa: twenty-four hours. The automatic 
machines, however, are equally capable of dealing with very 
heavy masses of glass, and bottles varying in size from one 
otmce to one hundred ounces have been produced on the same 

The annealing of bottles was formerly carried out in large 
chambers or kilns of very simple construction, in which the bottles 
w^e stacked as made, the kiln being previously heated to the 
requisite temp^ature : when full, the kiln was closed up in a rough 
temporary manner and allowed to cool naturally, thus annealing 
the bottles stacked within it. In this branch of glass-making also, 
however, the continuous annealing kiln has superseded the older 
kinds, and continuous kilns are now almost universal in bottle- 
making. In these kilns, which consist of long tunnels, kept hot at 
one end and having a gradually decreasing temperature as the 
other end is approached, the bottles are stacked on trucks which 
are slowly drawn through the kiln from the hot to the cold end. 
At the cold end the trucks are imloaded and are then returned, by 
an outside route, to the charging end, but of course the bottles 
cannot be stacked on the truck until it has actually entered the 
hot end of the tunnel and acquired the temperature there prevailing. 
In a slightly different form of kiln the bottles are carried down the 
Tnln on a species of conveyer belt formed of iron plates, but the 
principle of all these appliances is similar even when used for very 
different kinds of glass. 

In the account of bottle manufacture given above we have referred 
almost exclusively to the mode of production of the ordinary bottles 
\iB&i for the storage of such liquids as wine, beer, spirits, etc., and 


we will now deal with some other branches of manufacture closely 
allied to these. 

An important branch of glass manufacture is the production of 
vessels of large dimensions. Those most closely allied to ordinary 
bottles are the vessels known as carboys, used for the storage and 
transportation in bulk of chemical liquids, and especially of acids. 
Formerly these were blown by hand in a manner closely resembling 
that used for ordinary bottles, but the weight of the mass of glass 
to be handled by gatherer and blower is very great, while the lung 
power of a blower is not sufficient to produce the great expansion 
required. Formerly the only aid available to the blower was the 
device of injecting into the hot, hollow glass body, at an early 
stage of the process, a quantity of water or alcohol ; this liquid was 
immediately vaporised by the heat of the glass, and if the blower 
closed the mouthpiece end of his pipe by placing his thumb over it, 
the expansive force of the vapour so generated served to blow out 
the glass to the desired extent. More recently aids to the production 
of these large vessels have become available, first in the shape of 
mechanical arrangements for relieving the workmen of the full 
weight of the glass and pipe by providing suitable arms upon which 
the whole can be supported without interfering with the blower's 
freedom of manipulating the pipe and glass in the desired way ; 
further, a supply of compressed air, which can be readily connected 
with the pipe at any desired moment, facilitates the blowing process. 
The newer bottle-blowing machines, however, can also deal with 

A process of producing hollow glass vessels of very large size by 
purely mechanical means has, however, been devised by P. Sievert. 
By the methods of this inventor glass vessels of quite unprecedented 
size — such as bath-tubs freely acconunodating full grown men — 
can be produced. For this purpose the glass is spread out on the 
surface of a large cast-iron plate, provided with niunerous small 
holes through which steam or compressed air may be blown when 
desired. The slab of viscous glass, when properly spread over this 

O.M. I 


plate, is clamped down against it all around the outside edge by 
means of a suitably shaped iron collar, which holds the glass in 
air-tight contact against the plate beneath. The whole iron plate, 
with the slab of glass clamped to it, is now turned over, so that the 
glass hangs down under the plate. The glass immediately begins 
to sag under its own weight, and is assisted in this tendency by a 
suitable blowing of steam or air into the space between the plate 
and the glass. In blowing bath tubs in this way the glass is allowed 
to distend downwards until the desired depth is attained, when 
further distension is arrested by bringing a flat supporting plate 
under the glass, which is pressed againfst this flat plate by the 
pressure of the air, thus forming the flat bottom of the tub. In 
this process the outline of the object is determined by the shape 
of the clamping bars or plate that fix the edges of the hot glass 
against the iron plate described above, and by this means almost 
any desired shape can be given to objects of simple form. 

It is obvious that this process can also be employed for blowing 
a hollow body into contact with a mould of any desired form and 
forcing the hot glass to take the exact shape of the mould ; for 
smaller bodies, however, the blowing in of separately generated 
steam is not required, the heat of the molten glass itself being used 
to generate the necessary steam. For this purpose the requisite 
quantity of glass is dropped on the surface of a wet slab of asbestos. 
On this surface the glass remains floating upon a layer of steam, 
which is constantly renewed by the intense heating action of the 
hot glass on the water contained in the asbestos below. The moulds 
used in this process are provided with a sharp edge or lip, and as 
soon as the glass has spread into a slab of sufficient size, the inverted 
mould is brought down upon the glass and pressed against it. The 
sharp lip or edge of the mould forces the glass into close contact 
with the asbestos under it all around the edge of the mould, thereby 
enclosing the space existing between the rest of the glass and the 
wet asbestos. The heat of the glass continues to generate steam 
at a rapid rate, but now the steam can no longer escape from under 


the glass around the edges, and therefore blows the glass up- 
wards into the mould, ultimately forcing it into intimate contact 
with the surface of the mould ; when this is accomplished the 
pressure of the steam rises rapidly, and ultimately lifts the entire 
mould and glass sufficiently to allow the excess steam to escape — 
and this is the sign that the blowing is complete. The whole process 
takes only a very few seconds, and is very successful when applied 
to suitable glass and used with moulds of proper shape. It is, of 
course, obvious that ordinary narrow-mouthed bottles could not 
be produced in this way, but wide-mouthed bottles and jars are 
made in this manner, although the chief utility of the process lies 
in the production of comparatively shallow articles, which are not 
of a shape that lends itself to pressing. 

I 2 



In many ways very similar to the processes employed in the 
production of bottles are those used in the manufacture of all hoUow 
glass vessels that are produced by blowing, either with or without 
the aid of moulds. Apart from the actual shapes of the articles 
themselves, however, the principal difference between bottles and 
the better classes of hollow glass ware lies in the composition and 
quality of the glass itself. In this respect all grades of manufacture 
are to be met with, from the light coloured greenish or bluish glass 
used for medicine bottles to the most perfectly colourless and 
brilliant " crystal " or flint glass. This gradation in the perfection 
of the glass represents a corresponding gradation in the care bestowed 
upon the choice of raw materials and the various manipulations of 
melting the glass. As we have seen, for the commonest kinds of 
bottles, where colour and quality are inmiaterial, all kinds of fusible 
materials can be utilised, loamy or ferruginous sands and refuse- 
glass of all kinds being employed. Where somewhat higher require- 
ments have to be met, rather purer sands have to be used as sources 
of silica, while lime and alkali must be introduced in purer forms, 
the alkali in the shape of the cheapest qualities of salt-cake and the 
lime in that of lime-stones reasonably free from iron and magnesia. 
Finally, for the best qualities of glass the purest sand obtainable 
is used, being often specially washed to remove all loamy matter, 
while the alkali is introduced in the form of carbonate, a chemical 
product which in its better qualities is practically free from injurious 
impurities. Tn these high class products two very distinct kinds 
of glass are met with. One class, of which the Bohemian " crystal " 


is a typical example, is chemically of the nature of an alkali-lime 
silicate, the alkali in the case of the Bohemian glass being potash ; 
the other variety of glass contains no lime, its place being taken 
by lead, typical of this class being English flint glass. In some 
varieties of glass, lead is also replaced, partially or entirely, by 
barium, but this material is chiefly used for the manufacture of 
pressed glass. 

The higher grades of quality in glass, which thus require increased 
refinement in the raw materials, also demand increased refinement 
in the furnaces and appliances employed in their melting. The 
tank-furnace, which holds the field in bottle-manufacture, is not 
so frequently met with in the production of the better qualities of 
glass-ware. In England the best grades of hollow glass-ware are 
inseparably associated with the highest quality of flint glass, which 
has hitherto been produced almost exclusively in covered pots, 
owing to the necessity of protecting the glass from all sources of 
possible contamination and from the reducing action of the furnace 
gases. Special tjrpes of pot-furnace have, however, been devised 
in which the presence of reducing gases near the surface of the 
glass in the pots is so well guarded against that even flint glass 
can be successfully melted in open pots. In the case of soda-lime 
or potash-lime glasses, such as those of Bohemia, however, no 
reducing gases need be feared and there is a strong tendency to 
introduce tank-furnaces for this purpose. The quantity required, 
however, is rarely sufficient to keep a large continuous tank at 
work, and efforts have therefore been made to evolve tank-furnaces 
suitable for much smaller outputs. A very small tank intended 
for continuous working would have the serious disadvantage that 
the glass would have neither space nor time to undergo fining — 
the melting and working ends of the tank would, in fact, be too 
close together. This has led to the evolution of the " day " tank, 
in which melting and fining goes on during the night, while towards 
morning the temperature is allowed to fall and the glass is " worked 
out " during the day, so that perhaps two-thirds of the contents of 


the furnace are taken out, the tank being refilled during the next 

In all processes for the production of hollow glass-ware the 
glass or " metal " is taken from the pot by the process of gathering 
which has already been described ; where blown articles are to be 
produced, as distinct from pressed goods, the initial stage is always 
the formation of a small hollow globe or bulb at the ^id of the 
glass-blower's pipe. The subsequent manipulations depend upon 
the nature of the article to be produced. The article may either be 
made entirely by hand-work, or rather " chair " work, as it is 
usually called, or the manipulations may be facilitated and the 
product cheapened — ^while its character is, of course, also modified — 
by the aid of moulds, which are uised to bring the object to its 
proper shape and to impress upon it certain decorative mouldings 
or markings. As we have already seen, ordinary bottles are now 
always blown with the aid of moulds, and frequently in machines, 
and the same applies to medicine bottles, lamp chimneys, and the 
bulbs for electric light ; in connection with lamp-chinmeys it should 
be noted that they are blown in moulds in the form of cylindrical 
bottles with a flat bottom and a domed top, the ends being subse- 
quently cut off. Each " bottle " is frequently arranged so as to 
cut up into a pair of chimneys. 

Many of the cheaper varieties of tumblers and glasses are also 
blown in moulds, but they can be, and sometimes are, produced by 
hand, and as their manufacture is tjrpical of that of all hand-blown 
hollow ware, we shall now describe it in some detail as an example 
of this class of work. 

The implements used by the glass-blower and his assistants for 
this work are few and simple. The largest item is the glass-blower's 
bench or chair, which is simply a rough wooden bench provided 
with two projecting side rails or arms. When finishing a piece of 
work the blower sits on this bench, and the pipe lies across the two 
rails in front of him in such a position that by rolling it backwards 
and forwards along the rails he can readily keep the pipe in gentle 



rotation. In addition to the ordinary blower's pipe and a " pontil " 
or rod for attaching small quantities of glass whereby the piece in 
hand can be held, the only other tools used by the blower are a 
number of shears and pincers of various shapes which serve for 
cutting oSt, pressing in, and distending the glass as required, a flat 
board and a stone or metal plate or " marver " being also used for 
the purpose of moulding the glass. 

As already indicated, the first step in the production of such an 
object as a tumbler consists in gathering a suitable quantity of 

Fig. 10. — Sectional diagram of the evolution of a tumbler. 

glass on the pipe and blowing it into a small bulb. This bulb is 
blown out to the proper size and is then elongated by gently swinging 
the pipe. The next step is the flattening of the lower end of the 
bidb by gently pressing it on the " marver " or flat plate provided 
for such purposes ; in this way the flat bottom of the glass is formed, 
and the bulb now has the shape of the finished glass, but remains 
attached to the pipe by a shoidder and neck. The earliest practice 
was to separate the tumbler from the pipe at such a point as to 
leave the tumbler of the correct length, the remaining operation 
consisting in holding the glass, first fixed to a pontil for the purpose, 
into the furnace so as to heat the broken edge; this edge was 
thereby rounded off, and the brim of the glass could be widened or 


otherwise shaped by rotating the glass or pressing it in or out by 
the aid of pieces of wood. In modern practice, however, this is 
not usual, the glass being separated from the pipe wel] above the 
shoulder and annealed in this shape. Subsequently the glass is 
finished in a trimming room or workshop by being cut o5 at the 
desired poiat and having the rough edge rounded oB. by the aid of 
a blowpipe flame. The cutting oB. operation is carried out in a 
great variety of ways, the most usual being by the action of heat 
applied locally and suddenly, either by the aid of specially shaped 
flat blowpipe flames or by an electrically heated wire. Machines 
for carrying out this operation, as well as the subsequent rounding 
of the edge automatically, are in use, but the latter process is some- 
times replaced by slightly grinding and polishing the edges. 

The evolution of an ordinary tumbler, as just described, and as 
illustrated diagranmiatically in Fig. 10, is tjrpical of the whole 
process of hollow-glass blowing, but of course the number of opera- 
tions, as well as the care and skill involved in each step, increases 
rapidly as the form of the vessel becomes more complex ; in the 
highest class of work a very considerable element of artistic taste 
and judgment on the part of the operative also becomes essential, 
for, although the form of the object as well as the colour and orna- 
mentation are chosen by the designer, the blower has to translate 
the drawing of the designer into glass, and although his skill enables 
him to attain a considerable degree of fidelity in his rendering, 
many details remain at his own option, and the proper management 
of these is no small factor in the success of the whole work. The 
brief description given above applies to the method of production 
of all simple articles such as a tumbler, which can be blown out 
of a single mass of glass. In many cases more complex shapes are 
required, which are better produced by uniting several separate 
pieces. A typical example of this kind is furnished by an ordinary 
wine glass, in which the bowl and the stem are produced separately 
and put together while the glass is still hot enough to unite com- 
pletely. The ordinary clear glass water jugs offer another example, 


the thick handles being made separately and attached in a similar 
manner. In all such cases the necessity for building up the finished 
article out of several separate pieces arises from the fact that a 
thin blown portion requires to be united with a thick, heavy portion, 
or at least with a solid portion. The union of two such different 
pieces, however, entails a special difficulty, arising from the unequal 
rates of cooling of the thick, solid portion and the thin, hollow 
parts. Such composite articles, unless very carefully annealed, 
are apt to crack near the junction of the thin and thick parts — ^a 
feature sometimes met with in the water jugs already mentioned. 

In this connection mention should perhaps be made of the appli- 
cation of colour and other decorations to this kind of glass. A very 
considerable range of effects of this kind is now available to the 
glass-worker. In the first place the body of the glass used for the 
production of the articles in question may be coloured by the 
addition of suitable colouring materials to the molten glass or raw 
materials, as eacplained in Chapter XII., but this procedure has 
very obvious limitations; where the article is built up of glass 
from several gatherings — as, for example, is the case in an ordinary 
wine glass, where the bowl, leg and foot are each made of separate 
gatherings — ^it is possible to use glass of different colours for these 
different parts, and this is commonly done in the production of 
wine glasses having ruby or green bowls and white legs and feet. 
A further modification in the application of colour is obtainable by 
taking up two or more gatherings on the same pipe and superposing. 
a large gathering of white glass on a smaller one of coloured glass ; 
this is analogous to the process of " flashing " sheet glass, described 
in Chapter XI., and this process lends itself to a variety of manipu- 
lations resulting in the distribution of the coloured layer of glass 
in almost any desired n^anner over the object in hand. The principal 
objection to this process, however, lies in the fact that pots of 
molten glass of all the coloiu:s desired must be kept available to 
the blower at the same time, and this is not easily arranged for in 
any reasonably economical manner. For this reason, and also 


because the maDipulations are simpler, coloured glass intended for 
application to blown glass ware is generally used in the form of 
short rods previously prepared ; these rods are suitably heated, 
and the coloured glass can then be applied to the article in hand 
at any desired place and in as small or large a quantity as required. 
If the two glasses thus brought into contact are properly related to 
one another as regards chemical composition and physical properties, 
they blend very readily and perfectly, and the result is quite as 
good as could be obtained by using the coloured glass in the molten 

Other decorations, such as gilding or other metallic lustres and 
also various kinds of iridescence, are produced upon the finished 
glass. Metallic lustres are obtained by placing upon the surface 
of the glass, and slightly fusing into it, a layer of particles of the 
actual metal. In some cases this is done by rolling the glass vessel, 
while still hot, in a mass of metallic foil of the kind desired, when 
a sufficient quantity readily adheres ; in other cases the metal is 
applied in the form of a flux or glaze containing a large proportion 
of an easily reduced compound of the metal, and this is afterwards 
reduced to the metallic state bv the action of heat, sometimes 
aided by that of smoke or other reducing gases. An iridescent 
surface is produced upon certain varieties of glass by the corrosive 
action of acid vapours : in fact, in localities where the atmosphere 
is tainted with sulphur fumes it is quite usual to see an iridescent 
lustre on the surface of ordinary window glass. There are, of course, 
numerous other means of decorating blown and other glass, such 
as cutting, engraving, etching, silvering, etc., but it would lie 
beyond the scope of the present volume to deal with these, since 
they are outside the field of actual glass manufacture. 

In the production of hollow glass-ware by hand, the glass-blower 
avails himself to the full of the property so characteristic of glass 
of assimiing a pasty or viscous condition when suitably heated ; 
by raising or lowering the temperature of his material, the blower 
can at will render it stiffer or more fluid ; by blowing he can distend 


it, draw it out by the aid of gravity or centrifugal action, or he 
can mould it with the aid of rods and tongi^ of suitable shape, while 
at times he allows it to fall or festoon under its own weight while 
held aloft. With all these manipulations at his disposal, the sldlful 
operative is able to work the glass to his will and to fashion objects 
of great variety and beauty, but it should be noted that objects 
produced by hand in this way will bear the mark of the processes 
employed in their production in the fact that thq^ do not possess 
the extreme regularity of size and shape which are associated with 
nxachine-made articles : there is a certain natural variability in 
the exact shape of curves and festoons that is foreign to the products 
of mechanical processes. For some purposes this v^xiability is a 
disadvantage, while to some minds it appears as a defect, and 
methods have been devised for facilitating the production of strictly 
uniform. glass-ware by the use of moulds as an aid to the work of 
the glass-blower. While undoubtedly reducing the value and beauty 
of the ware from the purely artistic standpoint, these aids to hand- 
work have rendered possible an immense expansion of the entire 
industry, since, with the use of moulds, presentable glass-ware can 
be produced by hands far less skilled than those required for pure 

In the description given above of bottle-blowing by hand we 
have already seen an example of the use of moulds in aidiug the 
blower to form his object to the desired size and shape. Much more 
complicated and decorative objects can, however, be produced by 
the use of moulds. Such objects as globes and shades for gas, oil 
and electric lamps, when of a light substance and suitable shape, 
are usually produced by blowing bulbs of glass into moulds, where 
they acquire the general shape as well as the detailed decorated 
surface configuration which they afterwards present. Here again 
the body ren^ains a closed vessel, and is only opened and trimimed 
to the final shape at the end of the operation when all the blowing 
and moulding have been done. Articles blown in this way very 
frequently show '' mould marks," since the contact of the hot glass 


with the relatively cold surface of the moidd results in a certain 
crinkling or roughening of the surface, much as in the process of 
rolling. This effect can be minimised by dressing the interior 
surfaces of the moulds with suitable greasy dressings, whose chief 
property should be that they do not stick to the hot glass and 
leave little or no residue when gradually burnt away in the mould ; 
the proper care of the moulds and their maintenance is in fact the 
first essential to successful manufacture in this as well as in the 
pressed-glass industry. Even under the most favourable conditions, 
however, the surface of glass blown into moulds is not so good as 
that of hand-blown articles which have never come into contact 
with cold materials, and therefore retain undiminished the natural 
" fire polish " which glass possesses when allowed to cool freely 
from the molten state. An effort at producing a similar brilliance 
of surface on moulded and pressed articles is often made by exposing 
them, after they have attained their final form, to the heat of a 
furnace to such an extent as to soften the surfaces and allow the 
glass to re-solidify under the undisturbed influence of surface- 
tension much as it would do in solidifying freely in' the first place. 
Unfortunately this process cannot be carried out without more or 
less softening the entire article, so that skilful manipulation is 
required to prevent serious deformation of the object, while a 
certain amount of rounding off in all sharp corners and angles 
cannot be avoided. 

The air-pressure required to bring the whole of the surfaces of a 
large and possibly complicated piece of glass iato contact with the 
surfaces of the mould is sometimes very considerable, and the 
lung-power of the blower is often insufficient for the purpose ; in 
many works, therefore, compressed air is supplied, arrangements 
being employed whereby the operative can quickly connect the 
mouthpiece of his pipe with the air main, while he can accurately 
control the pressure by means of a suitable valve. The Sievert 
process of moulding by the aid of steam pressure has already been 


Although the evolution of the industry scarcely followed this 
path, it is not a large step to pass from a process in which air pressure 
is used to drive viscous glass into contact with a naould to a process 
in which the pressure of the air is replaced by the pressure of a 
suitably-shaped solid plunger, and this is essentially the widely-used 
process of glass pressing. In the first instance this mode of manu- 
facture is obviously applicable to solid or flat and shallow articles 
which could not be conveniently evolved from the spherical bulb 
which stands as embryo of all blown glass ; at first sight it would 
seem in fact as though the process must be limited to articles of 
such a shape that a plunger can readily enter and leave the concave 
portions. By the ingenious device, however, of pressing two halves 
of a closed or nearly closed vessel simultaneously in two adjacent 
moulds and then pressing the two halves together while still hot 
enough to unite, it has been made possible to produce by the press 
alone such objects as water jugs, for example, into which a plunger 
coidd not possibly be introduced when finished. The process of 
pressing being a purely mechanical one and requiring little skilled 
labour, has placed upon the market a host of cheap and extremely 
useful articles, thus serving to widen very considerably the useful 
applications of glass. On the other hand, the process has been 
and is still used to some extent for the production of articles intended 
to imitate the products of other processes such as hand-blown and 
cut glass, with the result that a great deal of glass has been produced 
which cannot possibly be classed as beautiful and much of which 
can lay as little claim to utility. 

The essential feature of the process of glass pressing consists, as 
already indicated, in forcing a layer of glass into contact with a 
moidd by the pressure of a mechanically actuated plunger. For 
this purpose a suitable mould and plunger as well as a press for 
holding the former and actuating the latter are required. The 
moulds are generally made of a special quality of close-grained 
cast-iron, and they are kept trimmed and dressed in much the same 
manner as the moulds used for blowing (except that the latter are 


sometimes made of wood). For the purpose of facilitating the 
removal of the finished article the moulds are generally made in 
several pieces which fit into one another and can be separated by 
means of hinges. A very important point about these moulds is 
that the various pieces should fit accurately into one another, since 
otherwise a minute " fin " of glass will be forced into every inter- 
stice, and the traces of these fins will always remain visible on the 
finished article ; the very perfect fit required entirely to prevent 
the f ornaation of such fins is, of course, scarcely attainable in practice 
except in the case of new moulds, so that the traces of fins are 
generally to be found on all pressed articles, and serve as a ready 
means of identifying these products when an attempt is made to 
imitate better classes of glass-ware by their means. The presses 
used in this process were formerly hand-actuated, and some such 
machines are still in use in England ; they are, however, regarded 
as entirely obsolete in America, where they have been replaced by 
"one man" machines which require the attendance of only one 
skilled man — the gatherer. The advantages claimed for the old 
manual lever machines — ^that the operator could apply just the 
right amount of pressure to press the glass home without risk of 
causing it to overflow the mould or to produce an excessive pressure 
upon the mould — is met in the new power presses by devices which 
carefully regulate the quantity of glass admitted into the mould. 
It is found, too, that the length of time for which the plunger is 
allowed to remain in contact with the glass in the mould is of very 
great importance to the quality of the glass produced, and the 
modern automatic machines are provided with means for regulating 
this length of contact. In operating these machines the gatherer 
drops the molten glass into the parison mould and in doing so pulls 
a lever or trigger which sets the machine in motion. The first 
action is the movement of a knife which cuts off the glass from 
the thread attaching it to the gatherer's iron and at the same time 
delimits the amount of glass in the mould. The plunger of the 
press then descends, remains in contact with the glass for the 


desired time and then rises. The mould then moves to another 
position or " station," where it is opened and the glass withdrawn 
from it. In the older machines only one mould was used, but in 
order to save the time entailed by waiting for this mould to be 
freed of its glass, modern machines work with two or three moulds 
which are operated in succession. This not only increases the output 
of the machine, but also allows the moulds to maintain a better 
temperature and permits of their being carefully cleaned. 

The presses themselves necessarily consist of the guides, levers 
and operating cams required to produce the successive movements 
of the plunger and moulds. It is, however, extremely important 
that the various parts of the press should retain their exact relative 
position throughout the operations, so that a high degree of rigidity 
in the framework is essential. For this reason the earlier machines, 
in which the whole of the appliances were supported on a single 
column, have now been superseded by presses in which the plunger 
and moulds are supported between two columns. In the process 
of pressing it will be seen that the glass is forced into intimate 
contact with the relatively cold surfaces of mould and plunger, 
and while undergoing this treatment the glass must remain suffi- 
ciently plastic readily to adapt itself to the configuration of the 
moidd. It is therefore not surprising to find that the pressing 
process can only be used successfully with glass of a kind specially 
adapted for it. Certain varieties of flint glass and some barium 
glasses are used for this purpose, but the greater quantity of pressed 
glass, particularly as produced on the Continent, is made of a 
lime-alkali silicate containing considerable quantities of both soda 
and potash and relatively little lime ; while sufficiently resistant 
for most purposes, this glass is particularly soft and adaptable 
while in the viscous condition. 

The deleterious effect produced upon glass surfaces when brought 
into contact with relatively cold metal has already been referred 
to above, and it only remains to add that this is the principal diffi- 
culty with which the glass-pressing process has to contend. It is 


overcome to some extent by the re-heating or " fire polishing " 
process, to which reference has already been made in connection 
with glass blown into moulds. If this is applied to the outside of 
a pressed article, however, the outlines of the pattern tend to 
become rounded. This difficulty, in its turn, is sometimes overcome 
by the application of a relatively small amount of " cutting," i.c., of 
grinding and polishing in order to give a superior finish to the 
pressed article. If this is carried far enough the result is a much 
superior, but also a considerably more expensive article. The 
interior of pressed articles, however, cannot as a rule be covered 
with such a pattern of ridges, grooves, spirals or lozenges as would 
allow of finishing in this manner. For purposes of utility the interior 
surfaces of most articles require to be as smooth as possible. Here, 
therefore, the application of fire polishing finds a very useful field, 
but the difficulty that the general softening of the article may lead 
to its distortion or collapse becomes important. One method of 
avoiding this risk consists in carrying out the surface heating of 
the interior of the articles by means of jets of flame under pressure 
while the glass is still in the mould. This has the disadvantage 
that it keeps the mould occupied for too long a time and also 
raises the temperature of the mould to an undesirable extent. 
An improved method of operation, therefore, consists in removing 
the pressed article from the mould and keeping the outside stifi 
enough to resist deformation by means of a series of small jets of 
cold air playing upon the outside surface while the flames used for 
fire polishing play upon the inside. The strains set up by such 
drastic proceedings must, of course, be subsequently removed by 



In the present chapter we propose to deal with all those processes 
of glass manufacture in which the first stage consists in converting 
the glass into a slab or plate by some process of rolling. We have 
already considered the general character of the rolling process, 
and have seen that, although hot, viscous glass lends itself readily 
to being rolled into sheets or slabs, these cannot be turned out with 
a smooth, flat surface. In practice the surface of rolled glass is 
dlways more or less dimmed by contact with the minute irregularities 
of table or roller, and larger irregularities of the surface arise from 
the buckling that occurs at a great many places in the sheet. These 
limitations govern the varieties of glass which can be produced by 
processes that involve rolling, and have led to the somewhat curious 
residt that both the cheapest and roughest, as well as the best. and 
most expensive kinds of flat glass, are produced by rolling processes. 
Ordinary rough " rolled plate," such as that used in the skylights 
of workshops and of railway stations, is the extreme on the one 
hand, while polished plate-glass represents the other end of the 
scale. The apparent paradox is, however, solved when it is noted 
that in the production of polished plate-glass the character of the 
surface of the glass as it leaves the rollers is of very minor import- 
ance, since it is entirely obliterated by the subsequent processes of 
grinding, smoothing, and polishing. Intermediate between the 
rough " rolled " and the ** polished " plate-glass we have a variety 
of glasses in which the appearance of the rolled surface is hidden or 
disguised to a greater or lesser extent by the application of a pattern 
that is impressed upon tjie glass during the rolling process ; thu^ 

O.M, K 


we have rolled plate having a ribbed or lozenge patterned surface, 
or the well known variety of " figured rolled " plate, sometimes 
known as " Muranese," whose elaborate and deeply imprinted 
patterns give a very brilliant eflEect. 

Rolled plate-glass being practically the roughest and cheapest 
form of glazing, is principally employed where appearance is not 
considered, and its chief requirement is, therefore, cheapness, 
although both the colour and quality of the glass are of importance 
as affecting the quantity and character of the light which is admitted 
to the building where the glass is used. On the ground of cheapness 
it will be obvious from what we have said above (Chapter V.), that 
such glass can only be produced economically in large tank furnaces, 
and these are imiversally used for this purpose. The requirements 
as regards freedom from enclosed foreign bodies of small size and 
of enclosed air bells are not very high in such glass, and, therefore, 
tanks of very simple form are generally used. No refinements for 
regulating the temperature of various parts of the furnace in order 
to ensure perfect fining of the glass are required, and the furnace 
generally consists simply of an oblong chamber or tank, at one 
end of which the raw materials are fed in, while the glass is with- 
drawn by means of ladles from one or two suitable apertures at 
the other end. For economical working, however, the furnace 
must be capable of working at a high temperature, because a cheap 
glass mixture is necessarily somewhat infusible, at all events whe^e 
colour is considered. This will be obvious if we remember that 
the fusibility of a glass depends upon its alkali contents, and alkali 
is the most expensive constituent of such glasses. 

The actual raw materials used in the production of rolled plate- 
glass are sand, limestone and salt cake, with the requisite addition 
of carbon and of fluxing and purifying materials. The selection of 
these materials is made with a view to the greatest purity and 
constancy of composition which is available within the strictly set 
limits of price which the low value of the finished product entails. 
These materials are handled in very large quantities, outputs of 


from 60 to 150 tons of finished glass per week from a single furnace 
being by no means uncommon ; mechanical mean^ of handling the 
raw materials and of charging them into the furnace are therefore 
ado pted wherever possibl e — — 

The glass is withdrawn from the furnace by means of large iron 
ladles. These ladles are used of varying sizes in sxich a way as to 
contain the proper amount of glass for rolling into the various 
sizes of sheets required. The sizes used are sometimes very large, 
and ladles holding as much as 180 to 200 lbs. of glass are used. / 
These ladles, when filled with glass, are not carried by hand, but 
are suspended from slings attached to trolleys that run on an over- 
head rail. The ladler, -whose body is' protected by a felt apron and 
his face by a mask having view holes glazed with green glass, 
takes the empty ladle from a water trough, in which it has been 
cooled, carries it to the slightly inclined gangway that leads up to 
the opening in the front of the furnace, and there introduces the 
ladle into the molten glass, giving it a half turn so as to fill it with 
9 " solid " mass of glass. By giving the ladle two or three rapid 
upward jerks, the operator then detaches the glass in the ladle as 
far as possible from the sheets and threads of glass which would 
otherwise follow its withdrawal ; then the part of the handle of 
the ladle near the bowl is placed in the hook attached to the over- 
head trolley, and by bearing his weight on the other end of the 
handle the workman draws the whole ladle up from the molten 
bath in the furnace and out through the working aperture. This 
operation only takes a few seconds to perform, but during this 
time the ladler is exposed to great heat, as a more or less intense 
flamlJ generally issues from the working aperture, whence it is 
drawn upward under the hood of the furnace. Considerable 
advances have recently been made in protecting ladlers and others 
working close to furnaces at high temperatures from the incon- 
veniences and dangers attending such work. The devices adopted 
include the use of a screen made of loose hanging chains suspended 
before the furnace opening. Unlike the solid furnace door, this 

K 2 



screen need not be removed when the ladle is being used, as the 
chains part readily to let the ladle pass and at the same time the 
interstices of the links allow the worker to obtain an adequate view 
of the interior of the furnace. Another useful device consists in 
the provision of a screen of cold air which is forced out under pres- 
sure just in front of the furnace opening ; this cold air screen not 
only keeps the flame itself entirely away from the operator, but 
also cools the whole vicinity of the furnace in which he has to work. 
Its chilling effect on the contents of the ladle as it passes rapidly 
through the screen is negligible. Finally, the eyes of ladlers, and 
still more of gatherers and blowers, who are obliged to watch the 
molten glass for long periods of time, appear to require very special 
protection in order to avoid the risk of the eye disease known as 
" glass blowers' cataract." This disease has received much study, 
and the view is now held that it is due to the prolonged action of 
the invisible radiations (infra red and ultra violet, but principally 
the infra red) which molten glass sends out in very large quantities. 
Protection from these is best obtained by the wearing of spectacles 
made of special glasses which absorb these injurious rays while 
transmitting the necessary amount of light. Glasses for this pur- 
pose, in which some of the rare earth elements, notably cerium, 
are incorporated, have been developed as the result of a research 
by Sir William Crookes, and these promise to be of immense value 
to the glass workers. 

From the furnace opening the ladler, generally aided by a boy, 
runs the full ladle to the rolling table and there empties the ladle 
upon the table just in front of the roller. In doing this, two dis- 
tinctly different methods are employed. In. one, only the perfiotly 
fluid portion of the glass is poured out of the ladle by gradually 
tilting it, the chilled glass next to the walls of the ladle being retained 
there and ultimately returned to the furnace while still hot. In 
the other method the chilling of the glass is minimised as far as 
possible, and the entire contents of the ladle are emptied upon the 
rolling table by the ladler, who turns tfe^ ^tire ladle over with a^ 



rapid jerk which is so arranged as to throw the coldest part of the 
glass well away from the rest. When the sheet is subsequently 
rolled this chilled portion is readily recognised by its darker colour, 
and since it lies entirely at one end of the sheet it is detached before 
the sheet goes any further. Neither method appears to present 
any preponderatiag advantage. 

The rolling table used in the manufacture of rolled plate is essen- 
tially a cast-iron slab of suJOSicient size to accommodate the largest 
sheet which is to be rolled ; over this slab moves a massive iron 








\ AS 

I ' itTTTiiiiiimiinimi-j .T- rrr 






Fig. 11. — Rolling table for rolled plate-glass. 

roller which may be actuated either by hand or by mechanical 
power — ^the latter, however, being now almost imiversal. The 
thickness of the sheet to be rolled is regulated by means of slips 
of iron placed at the sides of the table in such a way as to prevent 
the roller from descending any further towards the surface of the 
table : so long as the layer of glass is thicker than these slips, the 
entire weight of the roller comes upon the soft glass and presses it 
down, but as soon as the required thickness is attained the weight 
of the roller is taken by the iron slips and the glass is not further 
reduced in thickness. The width of the sheet is regulated by means 
of a pair of iron guides, formed to fit the forward face of the roller 
and the surface of the table, in the manner indicated in Fig. 11. 


The roller, as it moves forward, pushes these guides before it, and 
the glass is confined between them. When the roller has passed 
over the glass, the sheet is left on the iron table in a red-hot, soft 
condition, and it must be allowed to cool and harden to a certain 
extent before it can be safely moved. In this interval the chilled 
portion — ^if any — is partially severed by an incision made in the 
sheet by means of a long iron implement somewhat like a large 
knife, and then the sheet is loosened from the bed of the table by 
passing imder it, with a smooth, rapid stroke, a flat bladed iron 
tool. The sheet is next removed to the annealing kiln or *' lear," 
being first drawn on to a stone slab and thence pushed into the 
mouth of the kiln. At this stage the chilled portion of the sheet is 
completely severed by a blow which causes the glass to break along 
the incision previously made. 

The rolled plate annealing kiln is essentially a long, low tunnel, 
kept hot at one end, where the freshly rolled sheets are introduced, 
and cold at the other end, the temperature decreasing imiformly 
down the length of the timnel. The sheets pass down this tunnel 
at a slow rate, and are thus gradually cooled and annealed sufficiently 
to undergo the necessary operations of cutting, etc. Although 
thus simple in principle, the proper design and working of these 
" lears " is by no means simple or easy, since success depends upon 
the correct adjustment of temperatures throughout the length of 
the tunnel and a proper rate of movement of the sheets, while the 
manner of handling and supporting the sheets is vital to their 
remaining flat and unbroken. The actual movement of the sheets 
is effected by a system of moving grids which run longitudinally 
down the tunnel. The sheets ordinarily lie flat upon the stone 
slabs that form the floor of the tunnel, and the grids are lowered 
intp recesses cut to receive them. At regular intervals of time the 
iron grid bars are raised just sufficiently to lift the sheets from the 
bed of the kiln, and are then moved longitudinally a short distance 
carrying the sheets fcH*ward with them and immediately afterwards 
again depositing them on the stone bed. The grids return to their 


former position while lowered into their recesses below the level 
of the kiln bed. 

When they emerge from the annealing kiln or " lear " the sheets 
of rolled plate-glass are carried to the cutting and sorting room. 
Here the sheets are trimmed and cut to size. The edges of the 
sheets as they leave the rolling table are somewhat irregular, and 
sometimes a little " beaded," while the ends are always very irregular. 
Ends and edges are therefore cut square or " trimmed " by the aid 
of the cutting diamond. For this purpose the sheet is laid upon a 
flat table, the smoothest side of the sheet being placed upwards, 
and long cuts are taken with a diamond — ^good diamonds of adequate 
size and skilful operators being necessary to ensure good cutting 
on such thick glass over long lengths. Strips of glass six or eight 
feet long and half an inch wide are frequently detached in the course 
of this operation, and the final separation is aided by slight tapping 
of the underside of the glass just below the cut and — ^if necessary — 
by breaking the strip off with the aid of suitable tongs. 

No very elaborate "sorting" of rolled plate-glass is required, 
except perhaps that the shade of colour in the glass may vary 
slightly from time to time, and it is generally preferable to keep 
to one shade of glass in filling any particular order. Apart from ' 
this, the rolled plate cutter has merely to cut out gross defects 
which would interfere too seriously with the usefulness of the glass. 
As we have already indicated, air bells and minute enclosures of 
opaque matter are not objectionable in this kind of glass, but large 
pieces of opaque material must generally be cut out and rejected, ' 
not only because they are too unsightly to pass even for rough 
glazing purposes, but also because they entail a considerable risk 
' of spontaneous cracking of the glass — in fact, visible cracks are 
nearly always seen around large " stones," as these inclusions are 
called. These may arise from various causes, such as incomplete 
melting of the raw materials, or the contamination of the raw 
materials with infusible impurities, but the most fruitful source 
of trouble in this direction lies in the crumbling of the furnace 


liniiig, which mtroduces small lumps of partially melt«d fire-clay 
into tlie glass. In a rolled plate taok-fumace whicli is properly 
constructed and worked, the percentage of sheets wMch have to 
be Cat up on accoont of such enclosuies should be very small, at 
all events until the furnace is old, when the linings naturally show 
an increasing tend^i^ to disint^ate. 

Returning now to the rolling process, it is readily seen that a 
very slight modification will result in the production of rolled 
plate-glass having a pattern impressed upon one surface ; ttiis 

Fio. 12, — Sectional diaeram of machine for loUing "figured 

rolled " plate-glasB. 

modification consists in engraving upon the cast iron plate of the 
rolling table in intaglio any pattern that is to appear upon the 
gUss in relief. As a matter of fact only very simple patterns are 
produced in this way, such as close parallel longitudinal ribbing 
and a lozenge pattern, the reason probably being that the cost of 
cutting an elaborate pattern over the large area of the bed plate of 
one of these tables would be v^y considerable. Further, as these 
tables and their bed plates are so very heavy, they Me not readily 
interchanged or left standing idle, so that only patterns required in 
very great quantity could be profitably produced in this way. 
These disadvantages are, however, largely overcome by the double- 
rolling machine. In this machine, into whose rather elaborate 


details we cannot enter here, the glass is rolled out into a sheet of 
the desired size and thickness by being passed between two rollers 
revolving about stationary axes, the finished sheet emerging over 
another roller, and passing on to a stone slab that moves forward 
at the same rate as the sheet is fed down upon it. In this machine 
a pattern can be readily imprinted upon the soft sheet as it passes 
over the last roller by means of a fourth roller, upon which the 
pattern is engraved ; this is pressed down upon the sheet, and 
leaves upon it a clear, sharp and deep impress of its pattern. The 
general arrangement of the rollers in this machine is shown in the 
diagram of Kg. 12, which represents the sectional elevation of the 
appliance. After leaving the rolling machine, the course of the 
" figured rolled plate " producend in this manner is exactly similar 
to that of ordinary rolled plate, except that as a somewhat softer 
kind of glass is generally used for " figured," the temperature of the 
annealing kilns requires somewhat different adjustment. The 
cutting of the glass also requires rather more care, and it should 
be noted that such glass can only be cut with a diamond on the 
smooth side ; the side upon which the pattern has been impressed 
in relief cannot be materially affected by a diamond. This is one 
reason why it is not feasible to produce such glass with a pattern , 
on both sides. 

Figured rolled glass, being essentially of an ornamental or decora- 
tive nature, is generally produced in either brilliantly white glass 
or in special tints and colours, and the mixtures used for attaining 
these are, of course, the trade property of the various manufac- 
turers ; the whiteness of the glass, however, is only obtainable by 
the use of very pure and, therefore, expensive materials. As regards 
the coloured plate-glasses, a general account of the principles under- 
lying the production of coloured glass will be found in Chapter XII. 

The manufacture of polished plate-glass really stands somewhat 
by itself, almost the only feature which it has in common with the 
branches of manufacture just described being the initial rolling 


The raw materials for the production of plate-glass are chosen 
with the greatest possible care to ensure purity and regularity ; 
owing to the very considerable thickness of glass which is sometimes 
employed in plate, and also to the linear dimensions of the sheets 
which allow of numerous internal reflections, the colour of the glass 
would become unpleasantly obtrusive if the shade were at all 
pronounced. The actual raw materials used vary somewhat from 
one works to another ; but, as a rule, they consist of sand, lime- 
stone, and salt-cake, with some soda ash and the usual additions 
of fluxing and purifying material such as arsenic, manganese, etc. 
The glass is generally melted in pots, and extreme care is required 
to ensure perfect melting and fining, since very minute defects are 
readily visible in this glass when finished, and, of course, detract 
most seriously from its value. 

The method of transferring the glass from the melting pot to 
the rolling table differs somewhat in different works. In many 
cases the melting pots themselves are taken bodily from the furnace 
and emptied upon the bed plate of the rolling machine, while in 
other cases the glass is first transferred to snaaller " casting " pots, 
where it has to be heated again until it has freed itself from the 
bubbles enclosed during the transference, and then these smaller 
pots are used for pouring the glass upon the rolling slab. The 
advantage of the latter more complicated method lies, no doubt, 
in the fact that the large melting pots, which have to bear the 
brimt of the heat and chemical action during the early stages of 
melting, are not exposed to the great additional strain of being 
taken from the hot furnace and exposed for some time to the cold 
outside air. Apart from the mechanical risks of fracture, this 
treatment exposes the pots to grave risks of breakage from unequal 
expansion and contraction on account of the great differences of 
temperature involved. Where smaller special casting-pots are 
used these are not exposed to such prolonged heat in the furnace, 
and are never exposed to the chemical action of the raw materials, 
so that these subsidiary pots may perhaps be made of a material 


better adapted to withstand sudden changes of temperature than 
the high-class fire-clay which must be used in the construction of 
melting pots. On the other hand, the transference of the glass 
from the melting to the casting pots involves a laborious operation 
of ladling and the refining of the glass, with its attendant expen- 
diture of time and fuel. Finally, the production of plate-glass in 
tank-furnaces could only be attempted by the aid of such casting 
pots in which the glass would have to undergo a second fining after 
being ladled from the tank, and this would materially lessen the 
economy of the tank for this purpose, while it is by no means an 
easy matter to produce in tank-furnaces qualities of glass equal as 
regards colour and purity to the best products of the pot furnace. 
The withdrawal of the pots containing the molten glass from 
the furnace is now universally carried out by powerful machinery. 
The pots are provided on their outer surface with projections by 
which they can be held in suitably shaped tongs ot ci^es. A part 
of the furnace wall, which is constructed each time in a temporary 
manner, is broken down ; the pot is raised from the bed or " siege " 
of the furnace by the aid of levers, and is then bodily lifted out by 
means of a powerful fork. The pot is then lifted and carried by 
means of cranes until it is in position above the rolling table ; there 
the pot is tilted and the glass poured out in a steady stream upon 
the table, care being taken to avoid the inclusion of air bells in the 
mass during the process of pouring. When empty the pot is returned 
to the furnace as rapidly as possible, the glass being meanwhile 
rolled out into a slab by the machine. Except for the greater size 
and weight of both table and roller, the plate-glass rolling table 
is similar to that already described in connection with rolled plate. 
Of course, since the glass is poured direct from the pot, there is no 
chilled glass to be removed. Further, owing to the large size of 
sheets frequently required, the bed of the rolling table cannot be 
made of a single slab of cast-iron, a number of carefully jointed 
plates being, in fact, preferable, as they are less liable to warp under 
the action of the hot glass. 



In arranging the whole of the rolling plant, the chief consideration 
to be kept in mind is that it is necessary to produce a flat sheet of 
glass of as nearly as possible equal thickness all over. The final 
thickness of the whole slab when ground and polished into a sheet 
of plate glass must necessarily be slightly less than that of the 
thinnest part of the rough rolled sheet. If, therefore, there are any 
considerable variations of thickness, the result will be that in some 
parts of the sheet a considerable thickness of glass will have to be 
removed during the grinding process. This will arise to a still more 





Fig 13. — Sectional diagram illustrating waste of glass in grinding 

curved or irregular plate. 

serious extent if the sheet as a whole should be bent or warped so 
as to depart materially from flatness. The two cases are illustrated 
diagranmiatically in Fig. 13, which shows sectional views of the 
sheets before and after grinding on an exaggerated scale. 

While it is evident that carefill design of the rolling table will 
avoid all tendency to the formation of sheets of such undesirable 
form, it is a much more difficult matter to avoid all distortion of 
the sheet during the annealing process and while the sheet is being 
moved from the rolling table to the annealing kiln. Owing to the 
great size of the slabs of glass to be dealt with, and still more to the 
stringent requirement of flatness, the continuous annealing kiln, 


in which the glass travels slowly down a tunnel from the hot to 
the cold end, has not been adopted for the annealing of plate-glass, 
and a form of annealing kiln is still used for that glass which is 
similar in its mode of operation to the old-fashioned kilns that 
were used for other kinds of glass before the continuous kiln was 
introduced. These kilns simply consist of chambers in which the 
hot glass is sealed up and allowed to cool slowly and uniformly 
during a more or less protracted period. In the case of plate-glass 
the slabs are laid flat on the stone bed of the kiln. This stone bed 
is built up of carefully dressed stone, or blocks of fire-brick bedded 
in sand in such a way that they can expand freely laterally without 
causing any tendency for the floor to buckle upwards as it would 
do if the blocks were set firmly against one another. The whole 
chamber is previously heated to the requisite temperature at which 
the glass still shows a very slight plasticity. The hot glass slabs 
from the rolling table are laid upon the bed of this kiln, several 
being usually placed side by side in the one chamber, an^ the slabs 
in the course of the first few hours settle down to the contour of 
the bed of the kiln, from which shape and position they are never 
disturbed until they are removed when quite cold. In modern 
practice the cooling of a kiln is allowed to occupy from four to five 
days ; even this rate of cooling is only permissible if care is taken 
to provide for the even cooling of all parts of the kiln, and for this 
purpose special air passages are built into the walls of the chamber 
and beneath the bed upon which the glass rests, and air circulation 
is admitted to these in such a way as to allow the whole of the kiln 
to cool down at the same rate ; in the absence of such special 
arrangements, the upper parts of the kiln would probably cool 
much more rapidly than the base, so that the glass would be much 
warmer on its under than on its upper surface. 

When the slabs of plate-glass are removed from the annealing 
kilns they very closely resemble sheets of rolled plate in appearance, 
and they are quite suJOSiciently transparent to allow of examination 
and the rejection of the more grossly defective portions ; the more 



minute defects, of course, can only be detected after the sheets 
have been polished, but this preliminary exahnm^ion saves the 
laborious polishing of much useless glass. 

The process of grinding and polishing plate-glass cornets of three 
principal stages. In the first stage the surfaces of the glass are 
ground so as to be as perfectly flat and parallel as possible ; in 
order to effect this object as rapidly aS possible, a coarse abrasive 
is used which leaves the glass with a rough grey surface. In the 
second stage, that of smoothing, these rough grey surfaces are 
ground down with several grades of successively finer abrasive 
until finally an exceedingly smooth grey surface is left. In the 
third and final stage the smooth grey surface is converted into the 
brilliant polished surface with which we are familiar by the action 
of a polishing medium. 

Originally the various stages of the grinding and polishing pro- 
cesses were carried out by hand, but a whole series of ingenious 
machines has been produced for effecting the same purpose more 
rapidly and more perfectly than hand labour could ever do. We 
cannot hope to give any detailed accoimt of the various systems 
of grinding and polishing machines which are even now in use, but 
must content ourselves with a survey of some of the more important 
considerations governing the design and construction of such 

In the first place, before vigorous mechanical work can be applied 
to the surface of a plate of glass, that plate must be firmly fixed 
in a definite position relatively to the rest of the machinery, and 
such firm fixing of a plate of glass is by no means readily attained, 
since the plate must be supported over its whole area if local fracture 
is to be avoided. While the surface of the plate is in the imeven 
condition in which it leaves the rolling table, such a firm setting of 
the glass can only be attained by bedding it in plaster, and this 
must be done in such a manner as to avoid the formation of air- 
bubbles between plaster and glass ; if bubbles are allowed to form, 
they constitute places where the glass is unsupported. During the 


grinding and polishing processes these unsupported places yield 
to the heavy pressure that comes upon them, and irregularities in 
the finished polished surfaces results The most perfect adhesion 
between glass and plaster is attained by spreading the paste of 
plaster on the upturned surface of the slab of glass and lowering 
the iron bed plate of the grindiQg table down upon it, the bed plate 
with the adhering slab of glass beiag afterwards tiurned over and 
brought into position in the grinding machine. When one side of 
the glass has been polished it is generally found sufficient to lay 
the slab down on a bed of damp cloth, to which it adheres very 
firmly, although sliding is entirely prevented by a few blocks fixed 
to the table in such a way as to abut against the edges of the sheet. 
In many works, however, the glass is set in plaster for the grinding 
and polishing of the second side as well as of the first. 

The process of grinding and polishing is still regarded in many 
plate-glass works as consistiag of three distinct processes, known 
as rough grinding, smoothing and polishing respectively. Formerly 
these three stages of the process were carried out separately ; at 
first by hand, and later by three different machines. In the most 
modern practice, however, the rough and smooth grinding are 
done on the same machine, the only change required being the 
substitution of a finer grade of abrasive at each step for the coarser 
grade used in the previous stage. For the polishing process, how- 
ever, the rubbing implements themselves must be of a different 
kind, for while the grinding and smoothing is generally done by 
means of cast-iron rubbers moving over the glass, the polishing is 
done with felt pads. The table of the machine, to which the glass 
under treatment is attached, is therefore made movable, and when 
the grindiQg and smoothing processes are complete, the table with 
its attached glass is moved so as to come beneath a superstructure 
carryiDg the polishing rubbers, and the whole is then elevated so 
as to allow the rubbers to bear on the glass. 

The earliest forms of grindiQg machines gave a reciprocal motion 
to the table which carries the glass, or the grinding rubbers were 


moved backward and forward over the stationary table. Rotary 
machines, however, were introduced and rapidly asserted their 
superiority, until, at the present time, practically all plate-glass is 
ground on rotating tables, some of these attaining a diameter of 
over 30 ft. The grinding " rubbers " consist of heavy iron slabs, 
or of wood boxes shod with iron, but of much smaller diameter than 
the grinding table. The rubbers themselves are rotary, being 
caused to rotate either by the frictional drive of the rotating table 
below them, or by the action of independent driving mechanism, 
but the design of the motions must be so arranged that the relative 
motion of rubber and glass shall be approximately the same at all 
parts of the glass sheets, otherwise curved instead of plane surfaces 
would be formed. This condition can be met by placing the axes 
of the rubbers at suitable points on the diameter of the table. The 
abrasive is fed on to the glass in the form of a thin paste, and when 
each grade or " course " has done the work required of it, the 
whole table is washed down thoroughly with water and then the 
next finer grade is applied. The function of the first or coarsest 
grade is simply to remove the surface irregularities and to form a 
rough but plane surface. The abrasive ordinarily employed is 
sharp sand, but only comparatively light pressure can be applied, 
especially at the beginning of this stage, since at that period the 
weight of the rubber is at times borne by relatively small areas of 
glass that project here and there above the general level of the 
slab. As these are ground away, the rubbers take a larger and 
more uniform bearing, and greater pressure can be applied. The 
subsequent courses of finer abrasives are only required to remove 
the coarse pittings left in the surface by the action of the first rough 
grinding sand; the finer abrasive replaces the deep pits of the 
former grade by shallower pits, and this is carried pn in a number 
of steps until a very smooth " grey " surface is attained and the 
smoothing process is complete. The revolving table or " platform " 
is now detached from the driving mechanism, and moved along 
suitably placed rails on wheels provided for that purpose, until it 


stands below the polishing machine. Here it is attached to a fresh 
driving mechanism, and it is then either raised so as to bring the 
glass into contact with the felt-covered polishing rubbers, or the 
latter are lowered down upon the glass. The polishing rubbers are 
large felt-covered slabs of wood or iron which are pressed against 
the glass with considerable force ; their movement is very similar 
to that of the grinding rubbers, but in place of an abrasive they 
are supplied with a thin paste of rouge and water. The time required 
for the polishing process depends upon the perfection of the smooth- 
ing that has been attained ; in favourable cases two or three hours 
are sufficient to convert the " grey " sur^ce into a perfectly polished 
one ; where, however, somewhat deeper pits have been left in the 
glass, the time required for polishing may be much longer, and the 
polish attained will not be so perfect. The mode of action of a 
polishing medium such as rouge is now recognised to be totally 
different in character from that of even the finest abrasive ; the 
grains of the abrasive act by their hardness and the sharpness of 
their edges, chipping away tiny particles of the glass, so that the 
glass steadily loses weight during the grinding and smoothing 
processes. During the polishing process, however, there is very 
little further loss of weight, the glass forming the hills or highest 
parts of the minutely pitted surface being dragged or smeared over 
the surface in such a way as gradually to fill up the pits and hollows. 
The action of the polishing medium is probably partly chemical 
and partly physical, but it results, together with the pressure of 
the rubber, in giving to the surface molecules of the glass a certain 
amount of freedom of movement, similar to that of the molecules 
of a viscid liquid ; the surface layers of glass are thus enabled to 
** flow " under the action of the polisher and to smooth out the 
surface to the beautiful level smoothness which is so characteristic 
of the surfaces of liquids at rest. This explanation of the polishing 
process enables us to imderstand why the proper consistency of the 
polishing paste, as well as the proper adjustment of the speed and 
pressure of the rubbers, plays such an important part in suc^cessful 

O.K. L 


polishing ; it also serves to explain the well-known fact that rapid 
polishing only takes place when the glass surface has begun to be 
perceptibly heated by the friction spent upon it. 

It has been estimated that, .on the average, slabs of plate-glass 
lose one-third of their original weight in the grinding and polishing 
processes, and it is obvious that the erosion of this great weight of 
glass must absorb a large amount of mechanical energy, while the 
cost of the plant and upkeep is proportionately great. Every 
factor that tends to diminish either the total weight of glass to be 
removed per square yard of finished plate, or reduces the cost of 
removal, must be of the utmost importance in this manufacture. 
The flatness of the plates as they leave the annealing kiln has 
already been referred to, and the reason why the processes of grinding 
and polishing have formed the subject for innumerable patentgjjUl 
now be apparent. /The very large expansion of the use of plate-glass 
in moderiTbiliMihg construction, together with the steady reduction 
in the prices of plate, are evidence of the success that has attended 
tibiQ efforts of inventors and manufacturers in this direction. j 

At the^resent time plate-glass is manulactuJ^dd in verylax^ 
sheets, measuring up to 26 ft. in length by 14 ft. in width, and in 
thickness varying from ^th of an inch up to 1^ in., or more, for 
special purposes. At the same time the quality of the glass is far 
higher to-day than it was at earlier times. This high quality chiefly 
results from more careful choice of raw materials and greater freedom 
from the defects arising during the melting and refining processes, 
while rigid, inspection is applied to the glass as it comes from the 
polishing machines. For this purpose the sheets are examined in 
a darkened room by the aid of a lamp placed in such a way that 
ts oblique rays reveal every minute imperfection of the glass ; 
these imperfections are marked with chalk, and the plate is subse- 
quently cut up so as to avoid the defects that have thus been 

Perhaps the most remarkable fact about the quality of modern 
plate-glass is its relatively high degree of homogeneity. Glass, as 


we have seen in Chapter I., is not a chemically homogeneous sub- 
stance, but rather a mixture of a number of substances of difterent 
density and viscosity. Wherever this mixture is not sufficiently 
intimate, the presence of diverse constituents becomes apparent in 
the form of striae, arising from the refraction or bending of light- 
rays as they pass from one medium into another of difterent density. 
Except in glass that has imdergone elaborate stirring processes, 
such strifis are never absent, but the skill of the glass-maker consists 
in making them as few and as minute as possible, and causing them 
to assume directions and positions in which they shall be as incon- 
spicuous as possible. Li plate-glass this is generallv secured in a 
very perfect manner, and to ordinary observation no 8tei« are 
visible when a piece of plate glass is looked at in the ordinary 
way, i.e., through its smallest thickness ; if the same piece of glass 
be looked at transversely, the edges having first been polished in 
such a way as to render this possible, the glass will be seen to be 
full of strisB, generally running in fine lines parallel with the polished 
surfaces of the glass. This uniform direction of the striae is partly 
derived from the fact that the glass has been caused to flow in this 
direction by the action of the roller when first formed into a slab, 
but this process would not obliterate any serious inequalities of density 
which might exist in the glass as it leaves the pot, so that successful 
results are only attainable if great care is taken to secure the greatest 
possible homogeneity in the glass during the melting process. 

At the present time probably the greater bulk of plate-glass is 
used for the purpose of glazing windows of various kinds, principally 
the show windows of shops, etc. As used for this purpose the glass 
is finished when polished and cut to size. The only further manipu- 
lation that is sometimes required is that of bending the glass to 
some desired curvature, examples of bent plate-glass window panes 
being very frequently seen. This bending is carried out on the' 
finished glass, i.6., after it has been polished ; the glass is carefully 
heated in a special furnace until softened, and is then gently made 
to lie against a stone or metal mould which has been provided with 

L 2 


the desired curvature. It is obvious that during this operation 
there are great risks of spoiling the glass ; roughening of the surface 
by contact with irregular surfaces on either the mould, the floor of 
the kiln, or the implements used in handling the glass, can only be 
avoided by the exercise of much skill and care, while all dust must 
also be excluded since any particles settling on the surface of the 
hot glass would be '' burnt in," and cotdd not afterwards be detached. 
Small defects can, of course, be subsequently removed by local 
hand polishing, and this operation is nearly always resorted to 
where polished glass has to undergo fire treatment for the purpose 
of bending. 

In addition to its use for glazing in the ordinary sense, plate-glass 
is employed for a number of purposes ; the most important and 
frequent of these is in the construction of the better varieties of 
mirrors. For this purpose the glass is frequently bevelled at the 
edges, and sometimes a certain amount of cutting is also introduced 
on the face of the mirror. Bevelling is carried out on special grinding 
and polishing machines, and a great variety of these are in use at 
the present time. The process consists in grinding off the corners 
of the sheet of glass and replacing the rough perpendicular edge 
left by the cutting diamond by a smooth polished slope running 
down from the front surface to the lower edge at an angle of from 
45 to 60 degrees. Since only relatively small quantities of glass have 
to be removed, small grinding rubbers only are used, and in some 
of the latest machines these take the form of rapidly-revolving 
emery or carborundum wheels. These grinding wheels have proved 
so successful in grinding even the hardest metals that it is surprising 
to find their use in the glass industry almost entirely restricted to 
the " cutting " of the better kinds of flint and " crystal " glass for 
table ware or other ornamental purposes. The reason probably 
lies in the fact that the use of such grinding wheels results in the 
generation of a very considerable amount of local heat, this effect 
being intensified on account of the low heat-conducting power of 
glass. If a piece of glass be held even lightly against a rapidly- 


revolving emery wheel it will be seen that the part in contact with 
the wheel is visibly red-hot. This local heating is liable to lead to 
chipping and cracking of the glass, and these are the troubles 
actuaUy experienced when emery or carborundum grinding is 
attempted on larger pieces of glass. In the case of at least one 
modern bevel-grinding machine, however, it is claimed that the 
injurious effects of local heating are avoided by carrying out the 
entire operation under water. 

For the purpose of use in mirrors, plate-glass is frequently silvered, 
and this process is carried on so extensively that it has come to 
constitute an entire industry which has no essential connection 
with glass manufacture itself ; for that reason we do not propose 
to enter on the subject here, only adding that the nature and quality 
of the glass itself considerably affects the ease and success of the 
various silvering processes. Ordinary plate-glass, of course, takes 
the various silvering coatings very easily and uniformly, but there 
are numerous kinds of glass to which this does not apply, although 
there are probably few varieties of glass which are sufficiently stable 
for practical use, and to which a silvering coating cannot be satis- 
factorily applied, provided that the most suitable process be chosen 
in each case. 

While there is little, if any, use for coloured glass in the form of 
polished plate, entirely opaque plate-glass, coloured both black 
and white, is used for certain purposes. Thus, glass fascias om^j 
shop-fronts, the counters and shelves of some shops, and even 
tombstones are sometimes made of black or white polished plate. 
IVom the point of view of glass manufacture, however, these varieties 
only differ from ordinary plate-glass in respect of certain additions 
to the raw materials, resulting in the production of the white or 
black opacity. The subsequent treatment of the glass is identical 
with that of ordinary plate-glass, except that these opaque varieties 
are rarely required to be polished on both sides, so that the operations 
are simplified to that extent. 

Certain limitations to the use of all kinds of plate-glass, whether 


rough-rolled, figured or polished, were formerly set by the fact 
that under the influence of fire, partitions of glass were liable to 
crack, splinter and fall to pieces, thus causing damage beyond their 
own destruction and leaving a free passage for the propagation of 
the fire. To overcome these disadvantages, glass manufacturers 
have been led to introduce a network or meshing of wire into the 
body of such glass. Provided that the glass and wire can be made 
so as to unite properly, then the properties of such reinforced or 
'' wired " glass shotdd be extremely valuable. In the event of 
breakage from any cause, such as fire or a violent blow, while the 
glass would still crack, the fragments would be held together by 
the wire network, and the plates of glass as a whole would remain 
in place, neither causing destruction through flying fragments nor 
allowing fire or, for the matter of that, burglars a free passage. The 
utility of such a material has been readily recognised, but the 
difficulty lies in its production. These difficulties arise from two 
causes. The most serious is the considerable diflerence between 
the thermal expansion of the glass and of the wire to be embedded 
in it. The wire is necessarily introduced into red-hot glass while 
the latter is being rolled or cast, and therefore glass and wire have 
to cool down from a red heat together. During this cooling process 
the wire contracts much more than the glass, and breakage either 
restdts immediately, or the glass is left in a condition of severe 
strain and is liable to crack spontaneously afterwards. An attempt 
has been made to overcome this difficulty by using wire made of 
a nickel steel alloy, whose thermal expansion is very similar to 
that of ghlss ; but, as a matter of fact, this similarity of thermit 
expansion is only known to hold for a short range of moderate 
temperatures, and probably does not hold when the steel alloy is 
heated to redness. In another direction, greater success is to be 
attained by the use of wire of a very ductile metal which should 
yield to the stress that comes upon it during cooling ; probably 
copper wire would answer the purpose, but the great cost of copper 
is a deterrent from its use. A second difficulty is met with in intro- 



dncing wire netting into glass during the roDing operation, and 
this lies in effecting a clean join between glass and wire. Most 
metals when heated give off a considerable quantity of gas, and 
when this gas is evolved after the wire has been embedded in glass, 
numerous bubbles are formed, and these not only render the glass 
very unsightly but also lessesn the adhesion between the wire and 
the glass. This difficulty, however, can be overcome more readil}' 
than the first, since the surface of the metal can be kept clean and 
the gas expelled from the interior of the wire by preliminary heating. 
In spite of these difficulties, however, wired plate-glass is now 
successfully manufactured and has attained a definite commercial 



In the preceding chapter we have dealt with the processes of 
manufacture employed in the production of both the crudest and 
the most perfect forms of flat glass as used for such purposes as the 
glazing of window openings. The products now to be dealt with 
are of an intermediate character, sheet-glass possessing many of 
the properties of polished plate, but lacking some very important 
ones ; thus sheet-glass is sufficiently transparent to allow an observer 
to see through it with little or no disturbance — ^in the best varieties 
of sheet-glass the optical distortion caused by its irregularities is 
so small that the glass appears nearly as perfect as polished plate — 
but in the cheap glass that is used for the glazing of ordinary win- 
dows, sheets are often employed which produce the most disturbing 
and sometimes the most ludicrous, distortions of objects seen through 
them. It is a curious fact that even in good houses the use of such 
inferior glass is tolerated without conmient, the general public 
being, apparently, remarkably non-observant in this respect. In 
another direction sheet-glass has the great advantage over plate- 
glass that it is very much lighter, or can at least be produced of 
much smaller weight and thickness, although this advantage entails 
the consequent disadvantage that sheet-glass is usually much 
weaker than plate, and can only be used in much smaller sizes. In 
recent times the production of relatively thin plate-glass has, how- 
ever, made such strides that it is now possible to obtain polished 
plate-glass thin enough and light enough for almost every archi- 
tectural purpose. Finally, the most important advantage of sheet- 
glass, and the one which alone secures its use in a great number of 


cases in preference to plate-glass, is its cheapness, the price of 
ordinary sheet-glass being about one-fourth that of plate-glass of 
the same size. 

The raw materials for the manufacture of sheet-glass are sand, 
limestone, salt-cake, and a few accessory substances, such as arsenic, 
oxide of manganese, anthracite coal or coke, which differ considerably 
according to the practice of each particular works. In a general 
way these materials have already been dealt with in Chapter III., 
and we need only add here that the sheet-glass manufacturer must 
keep in view two decidedly conflicting considerations. On the one 
hand the requirements made in the case of sheet-glass as regards 
colour and purity render a rigorous choice of raw material and the 
exclusion of anything at all doubtful very desirable ; but on the 
other hand the chief commercial consideration in connection with 
this product is its cheapness, and in order to maintain a low selling 
price at a profit to himself the manufacturer must rigorously exclude 
all expensive raw materials. For this reason sheet-glass works 
such as those of Belgium and some parts of Germany, which have 
large deposits of pure sand close at hand, possess a very considerable 
advantage over those in less favoured situations, since sand in 
particular forms so large a proportion of the glass, and the cost 
of carriage frequently exceeds, and in many cases very greatly 
exceeds, the actual price of the sand itself. The same considerations 
will apply, although in somewhat lesser degree, to the other bulky 
materials, such as limestone and salt-cake ; but both these are 
more generally obtainable at moderate prices than are glass-making 
sands of adequate quality for sheet manufacture. 

Ordinary " white " sheet-glass is now almost universally produced 
in tank -furnaces, and a very great variety of these furnaces are 
used or advocated for the purpose. It woiJd be beyond the scope 
of the present book to enter in detail into the construction of these 
various tjrpes of furnace or to discuss their relative merits at length. 
Only a brief outline of the chief characteristics of the most important 
forms of sheet tank-furnaces will therefore be given here. 


Sheet tanks differ from each other in several important respects ; 
these relate to the sub-division of the tank into one, two, or even 
three more or less separate chambers, to the depth of the bath of 
molten glass and the height of the '* crown " or vanlt of the furnace 
chamber, to the shape and position of the apertures by which the 
gas and ak are admitted into the furnace, and the resultant shape 
and disposition of the flame, and finally to the position and arrange- 
ment of the r^enerative appliances by which some of the heat of 
the waste gases is returned into the furnace. 

Taking these principal points in order, we find that in some 
sheet tank-furnaces the whole furnace constitutes a single large 
chamber. In this type of furnace the whole process of fusion and 
fining of the glass goes on in this single chamber, and an endeavour 
is made to graduate the temperature of the furnace in a suitable 
manner from the hot end where the raw materials have to be melted 
down to the colder end where the glass must be sufficiently viscous 
to be gathered on the pipes. It is obvious that this control of the 
temperature cannot be so perfect in a furnace of the single chamber 
type as in one that is sub-divided. Such sub-divided furnaces are, 
as a matter of fact, much more frequent in sheet-glass practice ; 
but this practice differs widely as to the manner and degree of the 
sub-division introduced. In the extreme form the glass practically 
passes through three independent furnaces merely connected with 
one another by suitable openings of relatively small area through 
which the glass flows from one to the other. If it were possible to 
build furnaces of materials that could resist the action of heat and 
of molten glass to an indefinite extent, it is probable that this 
extreme type would prove the best, since it gives the operator of 
the furnace the means of controlling the flow of glass in such a way 
that no unmelted material can leave the melting chamber and 
enter the fining chamber, and that no insufficiently fined glass can 
leave the fining chamber and find its way into the working chamber. 
But in practice the fact that this extreme sub-division introduces 
a great deal of extra furnace wall, exposed both to heat and to 


contact with the glass, involves very serious compensating dis- 
advantages — ^the cost of construction, maintenance and renewal 
of the furnace is greatly increased, while there is also an increased 
source of contamination of the glass from the erosion of the furnace 
walls. It is, therefore, in accordance with expectations to find 
that the most successful furnaces for the production of sheet-glass 
are intermediate in this respect between the simple open furnace 
and the completely sub-divided one. In some cases the working 
chamber is separated from the melting and fining chamber by a 
transverse wall above the level of the glass, while fire-clay blocks 
floating in the glass just below this cross wall serve to complete 
the separation and to retain any surface impurities that may float 
down the furnace. 

As regards the depth of glass in the tank, practice also varies 
very much. The advantages claimed for a deep bath are that the 
fire-clay bottom of the furnace is thereby kept colder and is conse- 
quently less attacked, so that this portion of the furnace will last 
for many years. On the other hand the existence of a great mass 
of glass at a moderate heat may easily prove the source of con- 
tamination arising from crystallisation or " devitrification " occur- 
ring there and spreading into the hotter glass above. Also, if for 
any reason it should become necessary to remove part or all of the 
contents of the tank, the greater mass of glass in those with deep 
baths becomes a formidable obstacle. On the whole, however, 
modern practice appears to favour the use of deeper baths, depths 
of 2 ft. 6 in. or even 3 ft. being very usual, while depths up to 4 ft. 
have been used. 

The question of the proper height of the " crown " or vault of 
the furnace is of considerable importance to the proper working of 
the tank. For the purpose of producing the most perfect com- 
bustion, it has been contended that a large free flame-space is 
required. The earlier glass-melting tanks, like the earlier steel 
furnaces, were built with very low crowns, forcing the flame into 
contact with the surface of the molten glass, the> object being to 


promote direct heating by immediate contact of flame and glass ; 
for a time there was a strong tendency in the direction of higher 
crowns, leaving the heating of the glass to be accomplished by 
radiation rather than direct conduction of heat. There can be 
little doubt that up to a certain point the enlargement of the flame* 
space tends towards greater cleanliness of working, but if the 
height of a furnace crown be excessive there is a decided loss of 
economy. Flame-spaces as high as 6 ft. from the level of the glass 
to the highest part of the crown have been used, but the more usual 
heights range from 2 ft. to 4 ft. 

The " ports " or apertures by which pre-heated gas and air 
enter the furnace chamber differ very widely in various furnaces. 
In some cases the gas and air are allowed to meet in a snaall com- 
bustion chamber just before entering the furnace itself, while in 
other cases the gas and air enter the furnace by entirely separate 
openings, only meeting in the furnace chamber. The latter arrange- 
ment tends to the formation of a highly reducing flame, which is 
advantageous for the reduction of salt-cake, but is by no means 
economical as regards fuel consiunption. On the other hand, by 
producing a perfect mixing of the entering gas and air in suitable 
proportions, the other type of ports can be made to give almost 
any kind of flame desired, although their tendency is to form a 
more oxidising atmosphere within the furnace. The latter tj^e 
of ports, although widely varied in detail, are now almost universally 
adopted in sheet tank furnaces. 

All modern tank furnaces work on the principle of the recovery of 
heat from the heated products of combustion as they leave the 
furnace, and the return of this heat to the furnace by utilising it 
to pre-heat the incoming gas and air ; but the means employed 
to effect the application of this " regenerative " principle differ 
considerably in various types of plant. Perhaps the most widely- 
used form of furnace is the direct descendant of the original Siemens 
regenerative furnace, in which four regenerator chambers are 
provided with means for reversing the flow of gas and air in such 


a way that each pair of chambers serves alternately to absorb the 
heat of the outgoing gases and subsequently to return this heat 
to the incoming air that passes through one, and the incoming gas 
that passes through the other of these chambers. In these furnaces 
the regenerator chambers themselves are generally placed under- 
neath the melting furnace, and they are built of fire-brick and filled 
with loosely-stacked fire-bricks, whose function it is to absorb or 
deliver the heat. In the most modern type of furnaces of this class the 
gas-regenerators are omitted entirely, the air only being pre-heated 
by means of regenerators, while the gas enters the furnace direct 
from the producer, thus carrying with it the heat generated in the 
producer during the gasification of the fuel. While this arrangement 
is undoubtedly economical, it has the serious disadvantage, especially 
in the manufacture of sheet-glass, that the gas, rushing direct from 
the producer into the furnace, carries with it a great deal of dust 
and ash, which it has no opportunity of depositing, as in the older 
types of furnace, in long flues. 

The most serious disadvantages of the ordinary tj^es of regene- 
rative furnaces are due to the considerable dimensions of the regene- 
rative apparatus, necessitating a costly form of construction and 
occupying a large space, while the necessity of periodically reversing 
the valves so as to secure the alternation in the flow of outgoing 
and incoming gases requires special attention on the part of the 
men engaged in operating the furnace, as well as the construction 
and maintenance of valves under conditions of heat and dirt that 
are not favourable to the life of mechanical appliances. It is claimed 
that all these disadvantages are overcome to a considerable extent 
in one or other of the various forms of furnace known as " recupera- 
tive." In these furnaces there is no alternation of flow, and the 
regenerator chambers are replaced by " recuperators." These 
consist of a large number of small flues or pipes passing through 
a built-up mass of fire-brick in two directions at right-angles to 
one another ; through the pipes running in one direction the waste 
ases pass out to the chimney, while the incoming gas and air pass 


throngli the other set of pipes. A transference of heat between 
the two coirents of gas takes place by the conductivity of the fire- 
brick, and thus the outgoing gases are continnoosly cooled while 
the ingomg gases are heated — ^the transference of heat being some- 
what similar to that which takes place in the surface condenser of 
a steam engine. Theoretically this is a much simpler arrangement 
than that of separate r^enerator chambers, and to some extent 
it is found preferable in practice, but there are certain disadvan- 
tages associated with the system which arise principally from 
the peculiar nature of the material— fire-brick — of which the recu- 
perators are generally constructed. In the first place, the heat- 
conductivity of fire-brick is not very high, so that, in order to 
secure efficiency, the recuperators must be large, and while the 
individual pipes must be of small diameter, their area as a whole 
must be large enough to allow the gases to pass through somewhat 
slowly. Next, owing to the tendency of fire-brick to warp, shrink 
and crack under the prolonged effects of high temperatures, it 
becomes difficult to prevent leakage of gases from one set of pipes 
into the other. If this occurs to a moderate extent its principal eftect 
will be to allow some of the combustible gas to pass direct to the 
chimney, at the same time causing a dilution of the gases entering 
the furnace by an addition of products of combustion from the waste- 
gas flues. This, of course, tends to reduce the efficiency of the furnace 
and requires a higher fuel consumption if the temperature is to be 
maintained at its proper level. Ultimately the leakage reaches a 
point where re-construction of the furnace becomes necessary. It 
follows from these considerations that, although the recuperative 
furnace is somewhat simpler and cheaper to construct, it requires, 
if anything, more careful maintenance than the older forms of 
r^enerative furnace. 

Tank-furnaces for the production of sheet-glass in this cotmtry 
are generally worked from early on Monday morning until late on 
Saturday night, glass-blowing operations being suspended during 
Sunday, although the heat of the furnace must be maintained. On 


the Continent, and especially in Belgium, the work in connection 
with these furnaces goes on without any intermission on Sunday — 
a diSerence which, however desirable the English practice may be, 
has the effect of handicapping the output of a British furnace of 
equal capacity by about 10 per cent, without materially lessening 
the working cost. 

The ordinary process of blowing sheet-glass in an English glass- 
works is generally carried out by groups of three workmen, viz., a 
"pipe-warmer," a "gatherer," and a "blower," although the 
precise division of the work varies according to circumstances. 
The pipe-warmer's work consists in the first place in fetching the 
blowing-pipe from a small subsidiary furnace in which he has 
previously placed it for the purpose of warming up the thick " nose " 
end upon which the glass is subsequently gathered. The sheet- 
blower's pipe itself is an iron tube about 4 ft. 6 in. long, provided 
at the one end with a wooden sleeve or handle, and a mouthpiece, 
while the other end is thickened up into a substantial cone, having 
a roimd end. Before introducing the pipe into the opening of the 
tank-furnace, the pipe-warmer must see that the hot end of the 
pipe is free from scale or dirt and must test, by blowing through 
it, whether the pipe is free from internal obstructions. He then 
places the butt of the pipe ia the opening of the furnace and allows 
it to acquire as nearly as possible the temperature of the molten 
glass. When this is the case the pipe is either handed on to the 
gatherer, or the pipe-warmer, who is usually only a youth, may 
take the process one step further before handing it on to the more 
highly skilled workman. This next step consists in taking up the 
first gathering of glass on the pipe. For this purpose the hot nose 
of the pipe is dipped into the molten glass, turned slowly round 
once or twice and then removed, the thread of viscous glass that 
comes up with the pipe being cut ofi against the fire-clay ring that 
floats in the glass in front of the working opening. A small quantity 
of glass is thus left adhering to the nose of the pipe, and this is 
now allowed to cool down until it is fairly stifl, the whole pipe being 


meanwhile rotated so as to keep this first gathering nicely rounded, 
while a slight application of air-pressure, by blowing down the 
pipe, forms a very small hollow space in the mass of glass and 
secures the freedom of the opening of the pipe. When the glass 
forming the first gathering has cooled sufficiently/ the gatherer 
proceeds to take up the second gathering upon it. The pipe is 
again introduced into the furnace and gradually dipped into the 
molten glass, but this must be done with great care so as to avoid 
the inclusion of air-bells between the glass aheady on the pipe and 
the new layer of hotter glass that is now taken up. This freedom 
from air-bells is secured by a skilful gatherer by a gradual rotation 
of the pipe as it is lowered into the glass, thus allowing the two 
layers of glass to come into contact with a sort of rolling motion 
that allows the air time to escape. When completely immersed, 
the pipe is rotated a few times and is then withdrawn and the | 
'' thread " again cut off. The mass of glass on the end of the pipe 
is now considerably larger than before and requires more careful 
manipulation to cause it to retain the proper, nearly spherical 
shape. During the cooling process which now follows the pipe is 
laid across an iron trough, kept brimful of water ; this serves to 
cool the pipe itself, and also allows the pipe to be readily rotated 
backwards and forwards by rolling it a little way along the trough. 
When the whole mass of glass has again cooled sufficiently to be 
manipulated without risk of rapid deformation, a third gathering 
of glass is taken up, in precisely the same manner as that already 
described for the second gathering, and if the quantity of glass 
required is large, or the glass itself is so hot and fluid that only a 
comparatively small weight adheres at each time of gathering, the 
process may be repeated a fourth or even a fifth time, but as the 
weight of pipe and adhering glass increases with each gathering, 
each step becomes more laborious, while the hot glass, being now 
held on a much larger sphere, tends to flow off more readily, so 
that greater skill is required to avoid " losing " the gathering. 
The care and skill with which these operations of gathering are 


earned out determine, to a large extent, the quality of the resulting 
sheet of glaaa ; any want of regularity in the shape of the gathering 
leads inevitably to variations of thickness in different parts of the 
sheet, while careless gathering will introduce bubbles or " blisters " 
and other markings. During the intermediate cooling stages the 
g'asa must, be protected from dust and dirt of all kinds, since small 
specks falling upon the hot glass give rise to an evolution of minute 
gas bubbles which become painfully evident in the sorting room. 
When the last gathering has been taken up and the mass cooled 
so far as to allow of its being carried about without fear of loss, 
the glass forms an approximately spherical mass, with the nose-end 


Fig. 14.— Early stages in the formation of cylinders far sheet- 

of the pipe at or near the centre of the sphere. The next stages 
of the process consist in the preliminary shaping of this mass in such 
a way as to bring the bulk of the glass beyond the end of the pipe, 
and then in forming just beyond the end of the pipe a widened 
shoulder of thinner and therefore colder glass, of the diameter 
required for the cyhnder into which the glass is to be blown. This 
is done by bringing the glass into the successive shapes shown in 
Fig. 14, the forming of the glass being effected by the aid of specially 
shaped blocks and other shaping instruments in which the glass 
is turned and blown. The &nal shape attained at this stage is a 
squat cylinder containing the bulk of the glass at its lower end, 
and connected to the pipe by the thinner and colder neck and 
shoulder aheady mentioned. 
At this point of the process the pipe with its adherent glass is 



handed over to the blower proper. This operator works on a special 
stage erected in front of small furnaces, called " blowing holes," 
although in some works these are dispensed with, and the stages 
are erected in front of the melting furnace itself. The sheet-blower's 
stage is simply a platform placed over or at the side of a suitable 
excavation which gives the blower the necessary space to swing 
the pipe and cylinder freely at arm's length. The blowing process 
itself involves very little actual blowing, but depends rather upon 

Fig. 15.— Later stage in sheet-glass blowing. 

the action of gravitation and on centrifugal effects for the formation 
of the large, elongated cylinder from the squat cylinder with which 
the blower commences. The process consists in holding the thick, 
lower end of the cylinder in the heating-furnace, and when sufficiently 
hot, withdrawing it and swinging the pipe with a pendulum move- 
ment in the blower's pit. The cylinder thus elongates itself under 
its own weight, and any tendency to collapse is counteracted by 
the application of air-pressure by the mouth, the pipe being also, 
at times, rotated rapidly about its own axis. The re-heating of the 
lower end of the cylinder is repeated several times, imtil finally 
the glass has assumed the form of a cylinder of equal thickliess all 


over, but closed with a rounded dome at the lower end (Fig. 15). 
This rounded end is now opened. In the case of fairly thin and 
light cylinders this is done by holding the thumb over the mouth- 
piece of the pipe in such a way as to make an air-tight seal, and then 
heating the end of the cylinder in the blowing-hole. The heat 
both softens the glass at the end and at the same time causes con- 
siderable expansion of the air enclosed in the cylinder, with the 
result that the end of the cylinder is burst open. After a little 
further heating, during which the glass at the end of the cylinder 
becomes very soft, and takes a wavy, curly shape, the blower with- 
draws the cylinder from the furnace, and holding it vertically down- 
wards in his pit, spins it rapidly about its longitudinal axis. The 
soft glass at the lower end inamediately opens out under the cen- 
trifugal action, and the blower increases the speed of rotation until 
the soft glass has opened out far enough to form a true continuation 
of the rest of the cylinder, and in this position it is allowed to solidify. 
With thick, heavy cylinders the first opening of the end is done 
in a different way. A small quantity of hot glass is taken up by 
an assistant on an iron rod, and is laid upon the centre of the closed 
end of the cylinder. The heat of this mass of hot glass softens the 
glass of the cylinder, and the operator, with the aid of a special 
pair of shears, cuts out a small circle of this softened glass, thus 
opening the end. The final operation of straightening out the 
opened end is carried out in the same way as described above for 
lighter cylinders. 

The completed cylinder, still attached to the pipe, is now carried 
away from the blowing-stage and laid upon a wooden rack ; then 
the blower takes up a piece of cold iron, and placing it against the 
neck of glass attaching the cylinder to the pipe, produces a crack ; 
a short jerk then serves completely to sever the pipe from the 
cylinder. A boy now takes the pipe to a stand where it is allowed 
to cool and where the adhering glass cracks off from it prior to 
passing it back to the pipe- warmer fpr fresh use. 

On the wooden rack the cylinder of glass is allowed to cool to 

M 2 


a certain extent, and then the remaining portion of the neck and 
shoulder (see Fig. 15) are removed. This is done by a boy who 
passes a thread of soft, hot glass around the cylinder at the point 
where it is to be cut off ; the thread of hot glass merely serves to 
produce intense local heating, for as soon as it has become stifi the 
thread of glass is pushed ofE and a cold or moist iron is applied to 
the cylinder at the point where it had been heated by the thread. 
As a rule a crack immediately runs completely round the cylinder 
along the line of the thread, and the " cap " is thus removed. The 
glass is now in the form of a imiform cylinder open at both ends^ 
but it must be opened out into a flat sheet before it can assume 
the familiar form of sheet-glass. 

The first stage in the opening-out process is that of splitting. 
For this purpose the cylinders are carried to a special stand, upon 
which they are laid in a horizontal position, and here a crack or 
cut is made along one of the generating lines of the cylinder. This 
may be done either by the application of a hot iron, followed, if 
necessary, by slight moistening, or by the aid of a cut from a heavy 
diamond drawn skilfully down the inside of the cylinder. It will 
be seen from the account of the process so far given that the glass 
has as yet undergone no real annealing, although the blower is 
expected to " anneal " his cylinder during the blowing process, 
as far as possible, by never allowing it to cool too suddenly, and 
this degree of annealing is usually sufficient to save the cylinder 
from breaking under its internal stresses when left to cool on the 
racks. The surface of the glass, however, is left in a decidedly 
hardened condition, especially on the outside, which has necessarily 
been most rapidly cooled. For this reason — among others — ^the 
splitting cut is always made on the inside of the cylinder. The 
difference between the rates of cooling of the outside and inside of 
the cylinder has a further effect, which becomes evident as soon 
as the cylinder is split. The outside having become hard while the 
inside was still relatively soft, the outer layers of glass are in a state 
of compression and the inner layers in a state of tension in the cold 


cylinder. As soon as the cylinder is split, however, these stresses 
are to some extent relieved, the inner layers being then free to 
contract and the outer layers to expand ; the result is an increase 
in the curvature of the cylinder, which slightly decreases in diameter, 
the cut edges overlapping. If the cylinder has been cooled rather 
too quickly, or if the glass itself has a high co-efficient of expansion, 
this release of internal stresses at the moment of splitting becomes 
very marked, and each cylinder splits with the sound of a small 
explosion, while if the internal stresses are still more severe, the 
cylinders may even fly to pieces as soon as they are cut. 

The next stage in the manufacture of a sheet of glass is the flatten- 
ing and annealing process. For this purpose the split cylinders are 
taken to a special kihi, generafly known as a " lear," or " lehr," 
where they are first of all raised to a dull red-heat ; they are then 
lifted, one at a time, on to a smooth stone or s^ab placed in a dhamber 
of the kiln where the heat is great enough to soften the glass. Here 
the cylinder is laid down with the split edges upwards, and by 
means of a wooden tool the glass is slowly spread out, being finally 
rubbed down into perfect contact with the slab or " lagre." From 
the flattening slab the sheet as it now is passes into the annealing 
kUn, which communicates with the flatteniag chamber. This 
consists, similarly to other continuous annealing kilns already 
described in connection with other varieties of glass, of a long 
timnel, heated to the temperature of the flattening kiln at one end 
and nearly cold at the other. The sheets are moved down this 
tunnel at a uniform slow rate by the action of a system of grids 
which, at intervals, lift the sheets from the bottom of the kiln, 
move them forward by a short distance, and again deposit them 
on the bottom, the grids themselves returning to their former 
position by a retrograde movement made below the level of the 
kiln-bottom, and therefore not affecting the glass. 

On leaving the annealiag kiln the sheets of glass are sometimes 
covered with a white deposit arising from the products of com- 
bustion in the kiln and their interaction with the glass itself. This 


deposit can be removed by simple mechanical rubbing, but it is 
usual to dip the glass into a weak acid bath, which dissolves the 
white film and leaves the glass clear and bright, ready for use. 

From the annealing kiln the finished sheets of glass are taken to 
the sorting-room, where they are examined in a good light against 
a black background, and are sorted according to their quality for 
different purposes. 

The defects which are found in sheet-glass are of a very varied 
nature, as would be anticipated from the long and complicated 
process of manufacture which the material undergoes in the course 
of its transformation from the raw material into the finished sheet 
of glass. A full enumeration of all possible defects, with their 
technical names, need not be given here, but a description of the 
more important and frequent ones will be useful. The defects may 
be conveniently grouped according to the stage of the process from 
which they originate. 

The first class of defects accordingly embraces those that arise 
from the condition of the glass as it exists in the working-end of 
the furnace. Chief of these are white opaque enclosures, known 
as " stones." These may arise from a variety of causes within the 
furnace, such as an admixture of infusible impurities with the raw 
materials, insufficient heat or duration of melting, leading to a 
residue of unmelted raw material in the finished glass, or from 
defective condition of the interior of the furnace, leading to con- 
tamination of the glass with small particles of fire-brick. Further, 
if any part of the furnace has been allowed to remain at too low 
a temperature, or if the composition of the glass is unsuitable, 
crystallisation may occur, and white patches of crystalline material 
may find their way into the finished sheets. Another defect that 
may arise from the condition of the glass in the furnace is the presence 
of numerous small bubbles, known as *' seed." By the blowing 
process these are drawn out into pointed ovals, and they are rarely 
quite absent from sheet-glass. They arise from either incomplete 
fining of the glass in the furnace or from allowing the glass to come 


into contact with minute particles of dust during the gathering 
process. Another possible defect to the glass itself may be found 
at times in too deep a colour. This is only seen readily when a 
sheet of some size is examined edgewise, as most varieties of ordinary 
sheet-glass are too free from colour to allow this to be judged by 
looking through the sheet in the ordinary way. It follows from 
this fact that for practical purposes, where the light always traverses 
one thickness of the glass only, a slight difference of colour should 
be regarded as a very minor consideration, at all events as compared 
with freedom from other defects. 

The gathering process in its turn is responsible for further defects 
of sheet-glass. Some of these, such as defects arising from the use 
of a dirty pipe, are never allowed to pass beyond the sorting-room, 
and are therefore of no interest to the user of glass. Of those whose 
traces are seen in the glass that passes into use, " blisters " and 
** string " are the most important. " Blisters " are somewhat 
larger, flat air-bells, arising from the inclusion of air between suc- 
cessive layers of the gathering. " String " is a very conmion defect 
in all sheet-glass. To some extent it may arise from want of homo- 
geneity in the glass itself. If this consists of layers of different 
densities and viscosities, the gatherer will take these up on his 
gathering, and ultimately they will form thickened ridges of glass 
running around the cylinders and across the sheets. Such striae, 
due to want of homogeneity in the glass, are much more common 
in flint glass than in the soda-lime glasses used for sheet manufac- 
ture, but are not unknown in the latter. On the other hana, even 
if the glass be as homogeneous as possible, the gatherer can produce 
these strisB if he takes up his glass from a place close to the side 
of the fire-clay ring that floats in the furnace in front of his working 
opening. Glass always acts chemically upon fire-clay, gradually 
forming a layer of glass next to the fire-clay that contains much 
more alumina than the rest of the contents of the furnace. Such 
a layer is formed on the surface of each ring in a sheet tank, 
but if the gathering is taken from the centre of the ring, this 



layer of aluminiferoos glass remains undisturbed. li, however 
the gatherer brings his pipe too near the side of the ring, the 
glass will draw some of this difierent layer on to the gathering, and 
this glass will form thick ridges and striie running across the sheet 
in all directions. Another defect for which the gatherer is generally 
responsible is that of variation of thickness within the same sheet. 
The blower, however, can also produce this defect. 

During the blowing proper a further series of defects may be 
introduced, principally by allowing particles of glass derived from 
certain stages of the process to fall upon the hot glass of the cylinder 
and there become attached permanently. More serious, and also 
more frequent, is the greater or less malformation of the cylinder. 
If the glass as it leaves the blower is of any shape other than that 
of a true cylinder, it becomes impossible to spread it into a tiuly 
flat sheet in the flattening kiln. Sometimes, in practice, the "' cylin- 
der " is wider at one end than at the other, or, worse still, it is of 
uneven diameter, showing expanded and contracted areas alter- 
nately. When such a cylinder comes to be spread out on the slab 
it cannot be flattened completely, and various hollows and hillocks 
are left, which mar the flatness of the sheet and interfere with the 
regular passage of light through it when in use. 

Finally, the process of flattening is apt to introduce defects of 
its own. The most common of these are scratches arising from 
marks left by the flattening tool ; indeed, in all sheet-glass it is 
quite possible to see, by careful examination of the surfaces, upon 
which side the flattening tool was used. Sheet-glass thus has one 
side decidedly brighter and better in surface than the other, the 
better side being that which rested upon the " lagre " during the 
flattening process. On the other hand, if the slab itself be not 
quite perfect, or if any foreign body be allowed to rest upon it, 
that side of the glass will be marked in a corresponding manner. 

In the account of the manufacture of sheet-glass given above, 
we have outlined one typical form of the process, but nearly every 
stage is subject to modifications according to the practice and 


particular circumstances of each works. We will now describe one 
or two special modifications that are of more general importance. 

First, as regards the melting process, although the tank-furnace 
has almost entirely superseded the pot-furnace for the production 
of ordinary sheet-glass, there are still some special circumstances 
under which the pot-furnace is capable of holding its own. Thus, 
where for special purposes it is desired to produce a variety of 
sheet-glass which, as regards all defects arising out of the glass 
itself, and especially as regards colour, is required to be as perfect 
as possible, melting in pots is found advantageous, and for some 
very special purposes even covered (hooded) pots tre used. For 
such special purposes, too, sulphate of soda is eliminated from the 
raw materials and carbonate of soda (soda ash) substituted. For 
the production of tinted glasses also, whether they are tinted through- 
out their mass, or merely covered with a thin layer of tinted glass 
(" flashed "), manufacture in pot- rather than tank-furnaces is 
generally adopted, the exact nature and composition of the glass 
being far better under control in the case of pots. 

The blowing process is also subject to wide variations of practice. 
The most important of these variations concerns the shape and 
dimensions of the cylinders. In English and Belgian works the 
dimensions of the cylinders are so chosen that the length of the 
cylinder constitutes the longest dimension of the finished sheet, 
the diameter of the cylinder forming the shorter dimension. In 
some parts of Germany, however, the practice is the reverse of 
this, the cylinders being blown shorter and much wider, so that 
the circumference of the cylinder constitutes the longest dimension 
of the finished sheet. It is, however, pretty generally recognised 
that the latter method has very serious disadvantages, although 
it is claimed that somewhat more perfect glass can be obtained by 
its means. For the production of a special variety of glass, known 
as " blown plate-glass," this method of blowing short wide cylinders 
is still adhered to. This is a very pure form of sheet-glass, blown into 
thick, small sheets which are subsequently ground and polished in 


the same maimer as plate-glass. Here the great thickness of glass 
required seems to render the blowing of long cylinders very difi&cult, 
and the other form is therefore adopted. On the other hand, English 
patent plate-glass, which is made by grinding and polishing the 
best quality of ordinary sheet-glass, is made from glass blown into 
long narrow cylinders in the manner described in detail above. 

The process of blowing described above is capable, with slight 
modifications, of yielding glass with surfaces other than the plain 
smooth face of ordinary sheet-glass. Thus fluted and ** muffled " 
glass are produced in a very similar manner to that described above 
for ordinary sheet, except that the fluting or the irregular surface 
markings which constitute the peculiarities of these two varieties 
of glass, are impressed upon the surface of the cylinder at an early 
stage in the process. 

From the outline description given above of the usual method 
of manufacture of sheet-glass, it will readily be seen that this is 
a long, complicated, and laborious process, requiring the employ- 
ment of much skilled labour, and involving the production of a 
relatively complicated form, viz., the closed cylinder, as a pre- 
liminary to the production of a very simple form, viz., the flat 
sheet. It is therefore by no means surprising to find that a great 
many inventors have worked and are still working at the problem 
of a direct mechanical method of producing flat glass possessing 
a natural " fire polish " at least equal to that of ordinary sheet- 
glass. The earlier inventors have almost uniformly endeavoured 
to attain this object by attempting to improve the process of rolling 
glass, with a view to obtaining rolled sheets having a satisfactory 
surface. We have already indicated why these efforts have never 
met with success and what reasons there are for believing that 
they are never likely to attain their object. A totally different 
line is that taken by Sievert, to whose inventions we have already 
referred in connection with the mechanical production of blown 
articles. This inventor has endeavoured to utilise his process for 
blowing large articles of glass for the direct production of sheets 


of flat glass. His method is to blow, by the steam process described 
in another chapter, a large cubical vessel, having flat sides, the 
flatness of these sides being ensured by blowing the vessel into or 
against a mould having flat sides. This flat-sided vessel is ulti- 
mately to be cut up into five large sheets. This process also appears 
to involve some of the main difficulties of rolling as regards the 
means of transferring the glass from the furnace to the plate of 
the blowing machine, and in practice the inventor has not yet 
succeeded in producing glass of sufficiently good surface for the 
purposes of sheet-glass. 

Another class of processes entirely avoid all means of transferring 
molten glass from the furnace to any machine by working on glass 
direct from the molten bath itself. Some of these processes are in 
actual use in America, and others are being tried in Europe ; there 
can, however, be little doubt that they have overcome the greatest 
of the many difficulties that stood in the way of the mechanical 
production of sheet-glass, and that they are therefore destined 
very shortly to supersede the hand process. 

One of the earliest of these direct processes proposed to allow 
the molten glass to flow out from the furnace, downward, through 
a narrow slit formed in the side or bottom of the tank. The impossi- 
bility of keeping such a narrow orifice open and at the same time 
regulating the flow of glass made this proposal impracticable, 
although th,e use of drawing orifices has been revived in one of the 
latest processes. 

The American process, which has now been at work under com- 
mercial conditions for a number of years, is not entirely satisfactoiy 
in this respect— that it is a mechanical process for the production 
of cylinders and not of flat sheets, so that the subsidiary processes 
of splitting and flattening still remain to be carried out as before. 

In one form of this process, which is known as the Frinck system, 
the glass is first transferred from the melting tank into a special 
furnace or basin in which a vertical fire-clay tube passes through 
the bath of molten glass from below upwards. The glass is allowed 


to fine or " settle " after ladling into this basin, and then the drawing 
operation is carried out. For this purpose an iron " bait " or cover, 
which has previously been electrically heated to the right tem- 
perature, is lowered into the molten glass, and is then steadily 
raised. The glass adheres to the " bait " and is drawn up with it. 
But if the cylinder thus formed were left unsupported, it would 
tend to contract and would soon be " drawn off " to a point. This 
tendency is avoided by blowing compressed air into the rising 
cylinder as it is formed, through the fire-clay tube already men- 
tioned. This use of internal pressure is found to be superior to 
the earlier device of chilling the cylinder as it emerged from the 
surface of the molten glass by means of jets of cold air. When the 
cylinder has been drawn to the desired length, the rate of raising 
is increased and the air-pressure adjusted in such a way that the 
cylinder " draws off." The whole long cylinder is then placed in 
a horizontal position, is detached from the " bait," and is then cut 
up into lengths before being split and flattened in the ordinary 

The inventions of Fourcault aim at a much more direct process. 
Here also the glass is drawn direct from the molten bath by the 
aid of a drawing-iron that is inmaersed in the glass and then slowly 
raised, but in this case the piece inamersed is simply a straight 
bar, and the aim is to draw out a flat sheet. In this case the ten- 
dency, under surface tension, is to contract the sheet into a thread, 
and apparently the simple device of chilling the emerging glass is 
not adequate to prevent this in a satisfactory manner, and sub- 
sidiary devices have been added. Those that have been patented 
include a mechanism of linked metal rods so arranged as to be 
inamersed and drawn out of the glass continuously with the emerging 
sheet, in such a manner as to support the vertical edges of the glass 
and so aid in resisting the tendency of the glass to contract laterally. 
Another device consists in the use of a slit or orifice formed in a 
large fire-brick that floats on the surface of the glass. Through this 
orifice the glass is drawn, of the desired thickness and width. The 


use of this orifice, however, interferes markedly with the perfection 
of the product, and in fact all the glass produced in this way shows 
quite plainly a set of longitudinal striations due to the inevitable 
irregularities in the lips of the drawing slot. Further, it appears 
to be impracticable to draw thin glass in this way, a thickness of 
from 2J to 3 millimetres (about ^ inch) being the least that is prac- 
ticable, on account of the large amount of breakage that occurs 
with weaker sheets. This process, in its present stage of develop- 
ment, however promising, does not appear to have solved the 
problem of mechanical manufacture of sheet-glass, since it is just 
in the thinner, lighter kinds of glass that the advantages of sheet 
are most pronounced. On the other hand, it is quite possible that 
this drawing process, or some development arising from it, may 
shortly supplant the casting process in the production of polished 
plate-glass, although for the largest sizes of this product also, the 
difficulty and danger of handling the weights involved may prove 
a serious obstacle. More recently, both these direct drawing devices 
and the closely allied " flow " devices in which the glass is allowed 
to flow in a thin stream or sheet over a suitably prepared ledge or 
" weir " have been much developed in America, notably by Owens 
and his collaborators, whose successful bottle-blowing machines 
have already been referred to. In Colburn's patented method, 
which is being developed by Owens, the glass is drawn direct from 
the furnace in a continuous sheet through the lear, the resulting 
sheets reaching a length up to 200 feet. Whether the product of 
this operation is of a quality capable of fulfilling the requirements 
of good sheet-glass is not yet certain. 

Crown Glass, — Although this is a branch of manufacture that is 
nearly obsolete it deserves brief notice here, partly because it is 
still used for the production of special articles, and also because 
it illustrates some interesting possibilities in the use and ixianipu- 
lation of glass. 

The process of blowing crown glass may be briefly described as 
that of first blowing an approximately spherical hollow ball, then 


opening this at one side and expanding the glas into a flat disc 
by the action of centrifugal forces produced by a rapid rotation of 
the glass in front of a large opening in a special heating furnace. 
The actual process involves, of course, the preliminary of gathering 
the proper quantity of glass, much in the manner already described 
in connection with sheet-glass manufacture. This gathering is then 
blown out into a hollow spherical vessel. This vessel is now attached 
to a subsidiary iron rod by means of a small gathering of hot glass, 
applied at the point opposite the pipe itself, the glass being thus, 
for a moment, attached to both the pipe and the " pontil " or 
" pmity " (as the rod is called). The pipe is, however, detached 
by cracking off the neck of the original glass, which now remains 
attached to the pontil in the shape of an open bowl. This bowl is 
now re-heated very strongly in front of a special furnace, the open 
side of the bowl being presented to the fire. The pontil is meanwhile 
held in a horizontal position and rotated. As the glass softens the 
rotation spreads it out, until finally the entire mass of glass is formed 
into a simple flat disc spinning rapidly before the mouth of the 
furnace. This flat disc or " table " of crown glass is allowed to 
cool somewhat, is detached from the pontil by a sharp jerk, and 
is then annealed in a simple kiln in which the glass is stacked, 
sealed up, and allowed to cool naturally. 

It is obvious that by this process no very large sheets of glass 
can be produced ; tables i ft. in diameter are already on the large 
side, and these can only be cut up into much smaller sheets on 
account of the lump of glass by which the table was originally 
attached to the pontil, and which remains fixed in the centre of the 
finished disc. For certain ornamental purposes, where an " antique " 
appearance is desired, these bullions are valued, but for practical 
purposes they interfere very seriously with the use of the glass. 
As a matter of fact, even several inches away from the central 
bullion itself, crown glass is generally marked with circular wavings, 
which render it readily recognisable in the windows of older build- 
ings, but which decidedly detract from the perfection of the glass. 


On the other hand, crown glass is still valued for certain purposes, 
such as microscope slides and cover glasses, where entire freedom 
from surface markings, such as those found in sheet -glass as a 
result of the flattening operations, is desirable. While, therefore, 
the process has merely an historical interest so far as ordinary 
sheet-glass purposes are concerned, it is still used in special cases. 



In various chapters throughout the foregoing portions of this 
book we have had occasion to refer to the colour of glass and the 
causes affecting it, but these references have chiefly been made 
from the point of view of the production of glasses as nearly colour- 
less as possible under the circiunstances. While it is obvious that 
for the great majority of the purposes for which it is used the absence 
of all visible coloration is desirable or even essential in the glass 
employed, there are numerous other uses where a definite coloration 
is required. Thus we have, as industrial and technical uses of 
coloured glass, the employment of ruby, green and purple glasses 
for signalling purposes, as in the signal lamps of our railways, the 
red tail-lights of motor-cars, or even the red or green sectors of 
certain harbour lights and lighthouses; again, coloured glasses, 
ruby, green, and yellow, are extensively employed in connection 
with photography. Rather less exacting in their demands upon 
the correctness of the colour employed are the architectural and 
ornamental uses to which coloured glass is so extensively put in 
both public and domestic buildings, while, finally, coloured glass 
is largely the foundation upon which the stained-glass worker builds 
up his artistic achievements; in another direction, coloured glass 
is also utilised in the production of ornamental articles and of some 
table-ware. While it must be admitted that in a great many cases 
the colour-resources of the glass-maker are hopelessly misapplied, 
yet in really artistic hands few other materials are capable of yield- 
ing results of equal beauty. 

By the " colour " of a glass is generally understood the tint or 


colour which is observed when it is viewed, in comparatively thin 
slices, by transmitted light ; the actual colour is thus a property, 
not so much of the kind or variety of glass as of each individual 
piece, since thick pieces out of the same melting will show a different 
tint from that seen in thinner pieces. As we have already pointed 
out, such glasses as sheet or plate, which appear practically colour- 
less when viewed in the ordinary way, show a very decided green 
colour when viewed through a considerable thickness. In the same 
way a very thin layer of the glass known as " flashing ruby " shows 
a brilliant red tint, but a thickness of one-sixteenth of an inch is 
sufiicient to render the glass practically opaque, giving it a black 
appearance by both transmitted and reflected light. Again, cobalt 
blue glass, when examined with a spectroscope in thin layers, is 
found to transmit a notable proportion of red rays, but thicker 
pieces entirely suppress these rays. These phenomena will be 
readily understood when we recollect that coloui* in a transparent 
medium arises from the fact that the medium has d ifferent absorbi ng 
powers for light of different^'colours. All transparent substances, 
and certamly glass, are only partially transparent : all light waves 
passing through such a substance are gradually aFsorbedT and the. 

extent to which they are absorbed differs according to the length 

of these wavesT It alwuyiii happens that for some special "Wave- 
lei^ths the^substance has the power of absorbing the energy of the 
entering waves and converting it into heat-vibrations of its own 
molecules or atoms. In the^ most transparent and colourles.s glasses 
this process, so far as the waves of ordinary light are concerned, 
only goes on to a ne^igibly slight extent ; if, however, we extend 
our view beyond the range of ordinary visible light, and consider 
the region of shorter waves that lies in the spectrum beyond the 
violet, we find that ordinary colourless glass becomes strongly 
absorbent ; thus to waves of about half the length of those which 
produce upon our eyes the impression of yellow light, ordinary 
glass is as opaque as is a piece of metal to white light. In this 
wider sense, then, we may fairly say that all glasses are coloured-— 





i,e.y all have a power of selective absorption ; but in the case of 
those which are nearly colourless in the ordinary sense, this absorp- 
tion takes place only for waves which are either decidedly shorter 
or decidedly longer than those to which our eyes are sensitive. 
Those glasses which appear coloured in the ordinary sense, on the 
other hand, owe this property to the fact that the power of absorp- 
tion for light-waves extends into the region of the visible spectrum ; 
thus a blue or violet glass is practically opaque to red rays, while 
a red glass is opaque to blue, green or violet rays. This statement 
may be verified in a striking manner by holding over one another 
a piece of deep blue or green glass and a piece of deep ruby glass — 
the combination will be found to be very nearly opaque even when 
each glass by itself is practically transparent. 

The question which now naturally presents itself to us is, what 
is the essential difference between, for instance, a piece of red glass 
and a piece of " white " glass that confers upon the former the 
power of absorbing blue light ? A perfectly complete and satis- 
factory answer to this question is not, in the writer's opinion, avail- 
able in the present state of our knowledge, but to a certain extent 
the difference between the two kinds of glass can be explained. 
The difference is produced, in the first instance, by introducing into 
the colourless glass some additional chemical element or elements, 
the substances in question being generally known as " colouring 
oxides," although they are by no means always introduced in the 
form of oxides, and are frequently present in the glass in entirely 
different forms. To a certain extent the colour of the glass may 
be ascribed to a definite " colouring " property of the chemical 
elements concerned ; thus most of the chemical compounds of such 
elements as nickel, cobalt, iron, manganese and copper are more 
or less deeply coloured substances, and it would seem*as if the 
tbtoms or " ions " of these elements had the specific power of absorb- 
ing certain varieties of light-waves while not materially affecting 
others. But this specific " colouring " property is not so easily 
explained when we recollect that the colours ot iron compounds, 


for example, may be green or red according to the state of com- 
bination in which that element is present, and that iron has also 
the power of imparting either a green or a yellow colour to glass 
according to circumstances. The detailed discussion of these 
questions, however, lies outside our present scope, and we must 
confine ourselves to th^iw?ead statement that colouring^^s«bstance 
in glass may bclfoughly divided into two kinds or groups^ the 
first and probkHy the largest group are those bodieswhiehoccur 
in glass in true solttt ion; the. dpment-iteelf-"b6ingpresent in the 
combined state as a silicate or other such compound (borate, phos- 
phate, etc.) which is soluble in the glass. In this class the colouring 
effect upon the glass is specifically that of the element introduced, 
and is brought about in tha-^same way as the colouring of water 
when a colour^^jsoK— such -as copper sulphate— is dissolved in it. 
The second/^tSlass^of colouring substances, however, behave iri^a 
different/manner ; they are probably present in the glass in a i 
state of Wtremely fine division, and held not in true solution^Jta^ 
really in a sort nf niipchaniofiil uuapunulun IhgTappToximates to the 
condition of what is known as a " colloidal solution." The point 
which is known beyond doubt, thanks to the researches of Siedentopf 
and Szigmondi on ultra-microscopical particles, is that in certain 
coloured glasses, of which ruby glass is the best example, the colour- 
ing substance, be it gold or cuprous oxide, is present in the form of 
minute but by no means atomic or molecular particles suspended 
in the glass. The presence of these particles has been made optically 
evident, although it can hardly be said that they have been rendered 
visible, and it is at all events probable that these suspended particles 
act each as a whole in absorbing the light-waves characteristic of 
the colour which they produce in glass. This being the case 
it is easy to understand how readily the colour of such glasses 
is altered or spoilt by manipulations which involve heating 
and cooling at different rates — ^too rapid a rate of cooling pro- 
ducing a different grouping of the minute particles, altering 
their size or shape, or even obliterating them entirely by allowing 

N 2 


the element in question to go into or to remain in solution in 
the glass. 

While it would be entirely foreign to the purpose of this volume 
to give in this place a series of recipes for the production of various 
kinds of coloured glass, it will be desirable to state in general terms 
the colours or range of colours which can be produced in various 
kinds of glass by the introduction of those chemical elements which 
are ordinarily used in this way. In general terms it may be said 

t the lighter e lements do not as a rule tend to the prod uction 
of coloured glasses, while the heavier p jfmP^t", ^ iii ^ can 
he retain^j ^ f]\^ gl ^^^ i^ ftithey y ^]^]f,j()n gp miwptnfgjii^nj tfnd to 
produ ce an int i nn n r rnlewing rffrrt The element lead appears 
to form a striking exception to this rule, but this is due to the 
fact that while the silicates of most of the other heavy elements 
are more or less unstable, the silicate of lead is very stable, and 
can only be decomposed by the action of reducing agents. When 
lead silicates are decomposed in this way, however, the resulting 
glass immediately receives an exceedingly deep colour, being 
turned a deep opaque black, although in very thin layers the colour 
is decidedly brown. On the other hand, glasses very rich in lead 
are always decidedly yellow in colour, and it has been shown that 
this coloration is due to the natural colour of lead silicates and not 
to the presence of impurities. What has just been said of lead 
applies, with only very slight modification, also to the rare metal 
thallium and its compoimds, which have been introduced into glass 
for special purposes. Leaving these two exceptional bodies on 
one side, we now pass to a consideration of the elements in the 
order of their chemical grouping. The rare elements will not be 
considered except in certain cases where their presence in traces is 
liable to afiect results attained in practice. 

The Alkali Metals, sodium, potassium, lithium, etc., and their 
compounds, have no specific colouring effect, although the presence 
of soda or of potash in a glass affects the colours produced by such 
substances as manganese, nickel, selenium, etc. 


Copper, as would be anticipated from the deep colour of most 
of its compounds, produces powerful colouring efEects on glass. 
Cupric silicates produce intense green, to greenish-blue tints. Copper, 
either as metal or oxide, added to glass in the ordinary way, always 
produces the green colour ; but when the full oxidation of the 
copper is prevented by the presence of a reducing body, and the 
glass is cooled slowly, or is exposed to repeated heating followed 
by slow cooling, an intense ruby coloration is produced., In practice 
this colour is produced by introducing tin as well as copper into 
the mixture, and so regulating the conditions of melting as to 
favour reduction rather than oxidation of the copper. Under these 
circumstances the copper is left in the glass in a finely divided and 
evenly suspended state ; if exactly the right state of division and 
suspension is arrived at, a beautiful red tint is the result, although 
the coloration of the glass is so intense that it can only be employed 
in very thin sheets, being " flashed " upon the surface of colourless 
glass to give it the necessary strength and thickness for practical 
use. It is further very easy slightly to alter the arrangement of 
the copper in the glass, with the result of producing an opaque, 
streaky substance resembling sealing-wax in colour and appearance, 
this product being, of course, useless from the glass-maker's point 
of view. Finally, by exceedingly slow cooling, and under other 
favouring conditions which are not really imderstood, the particles 
of suspended colouring-material — ^be it metallic copper or cuprous 
oxide— ^ow in size and attain visible dimensions, appearing as 
minute shimmering flakes, thus producing the beautiful substance 
known as ** aventurine." 

Silver is rarely introduced into glass mixtures, the reason 
being that it is so readily reduced to the metallic state from 
all its compounds that it cannot easily be retained in the 
glass except in a finely - divided form, causing the glass to 
assume a black, metallic appearance resiembUng the stains 
produced by the reduction of lead in flint glasses. On 
the other hand, silver yields a beautiful yellow colour when 


applied to glass as a surface stain, and it is widely used for that 

Gold is introduced into glass for the production of brilliant ruby 
tints ; its behaviour is very similar to that of copper, except that 
the noble metal has a great tendency to return to the metallic 
state without the aid of reducing agents. No addition of tin is 
therefore required, but the rate of cooling, etc., must be properly 
regulated, since rapidly cooled glass containing gold shows no 
special colour, the rich ruby tint being only developed when the 
glass is re-heated and cooled slowly. The colouring efiect of gold 
is undoubtedly more regular and uniform than that of copper, and 
it is accordingly possible to obtain much lighter shades of red with 
the aid of the noble metal. " Gold ruby " can therefore be obtained 
of a tint light enough to be used in sheets of ordinary thickness, 
and the process of " flashing " is not essential. 

The elements of the second group, such as magnesium, calcium, 
strontium, barium, zinc and cadmium, exert no strong specific 
colouring action on glass, with perhaps the exception of cadmium, 
and that element only does so to any considerable extent in com- 
bination with sulphur, sulphide of cadmium having the power of 
producing rich yellow colours in glass. The sulphur compounds of 
barium also readily produce deep green and yellow colours, and 
the formation of these tints is, indeed, very difficult to avoid in 
the case of glasses containing much barium. A colouring effect 
has sometimes been ascribed to zinc, but this is not in accordance 
with facts. 

Of the elements of the third group, only boron and aluminium 
are ever found in glass in any notable quantity. Boron is present 
in the form of boric acid or borates, and as such produces no colouring 
efiect, nor does there seem to be any tendency for the separation 
of free boron. The compounds of aluminiimi also possess no colour- 
ing effect, although some compounds of this element are utilised 
for imparting a white opacity to glass for certain purposes — such 
glass being known as " opal." 


The elements of the fourth group are of greater importance in 
connection with glass. Carbon is capable of exerting powerful 
colouring effects when introduced into glass. These effects are of 
two kinds, viz., indirect in consequence of the reducing action of 
carbon on other substances present, and direct from the presence 
of finely-divided carbon or carbides in the glass. The latter are 
similar in kind to those produced by the presence of other finely- 
divided elementary bodies (copper, gold, lead, etc.), except that the 
lightness of the carbon particles tends to the production of yellow 
and brown colours rather than of red and black, while the chemical 
nature of carbon renders the glass in which it is suspended indifferent 
to rapid cooling, so far as the carbon tint is concerned. The indirect 
effects of carbon, in reducing other substances that may be present 
in the glass, become evident with much smaller proportions of carbon 
than are required to produce visible direct effects. As we have 
seen above, carbon, in the form of coke, charcoal or anthracite coal, 
is regularly introduced, as a reducing medium, into glass mixtures 
containing sulphate of soda. If even a slight excess of carbon be 
used for this purpose, the formation of sulphides and poly-sulphides 
of sodium and of calcium results, and these bodies, like all sulphides, 
impart a greenish-yellow tint to the glass, at the same time bringing 
other undesirable results in their train. 

Silicon, in the form of silicic acid and its compounds, is a funda- 
mental constituent of all varieties of glass, and in this form is in 
no sense a colouring substance ; on the other hand, there is no 
doubt that under some conditions silicon may be reduced to the 
metallic state at temperatures which normally occur in glass- 
furnaces, and it is practically certain, that if present in glass in 
this condition, silicon would colour the glass. It is just possible 
that some of the colouring effects produced in ordinary glass by 
powerful reducing agents, such as carbon, either in the solid form 
or as a constituent of furnace gases, may be due to the reduction 
of silicon in the glass. 

Tin by itself does not appear to have any colouring effect upon 


glass, except that its oxide, in a findiy suspended state, produces 
opalescence and, in large quantities, white opacity. Tin, however, 
is used in conjunction with copper in the production of copper-ruby, 
to which reference has already been made. 

Lead and ThaUium have already been dealt with, and it only 
remains to add that their presence in the glass, although not in 
itself producing any intense colouring action, increases the colouring 
effects of other substances. This is probably merely a particular 
case of the fact that dense glasses, of high refractive index, are 
more sensitive to colouring agencies than the lighter glasses of low 
refractive index; this applies to barium as well as to lead and 
thallium glasses. 

Phosphorus occurs in some few glasses in the form of phosphoric 
acid, and this substance, as such, has no colouring effect. Calcium 
phosphate, however, is sometimes added to glasses for the purpose 
of producing opalescence. Its action in this respect is probably 
similar to that of tin oxide and aluminium fluoride, these substances 
all remaining undissolved in the glass in the form of minute particles 
in a finely divided and suspended state. 

Arsenic does not exert a colouring effect on glass, and owing to 
its volatile nature it can only be retained in glass in small quantities 
and under special conditions. A " decolourising " action is some- 
times ascribed to arsenic, but if this action really exists it can only 
be ascribed to the fact that arsenic compounds are capable of 
acting as carriers of oxygen, and their presence thus tends to facili- 
tate the oxidation of impurities contained in the glass. A further 
reference to this subject will be found below in reference to the 
compounds of manganese. 

Antimony, although frequently added to special glass mixtures, 
does not appear to produce any very powerful effects, except possibly 
in the direction of producing white opacity if present in large pro- 
portions. The sulphide of antimony, however, exerts a colouring 
influence, although its volatile and unstable character renders the 
effects uncertain. 


Vanadiumy owing to its rarity, is probably never added to glass 
mixtures for colouring purposes, although it is capable of producing 
vivid yellow and greenish tints when present even in minute pro- 
portions. On the other hand, vanadium occurs in small proportions 
in a number of fire-clays, including some of those of the Stourbridge 
district, and glass melted in pots containing this element is liable to 
have its colour spoilt by taking up the vanadium from the clay. 

Sulphur is an element whose presence in various forms is liable 
to afiect the colour of glass in a variety of ways. The colouring 
eflects of sodium-, calcium-, cadmium-, and antimony-sulphides 
have already been referred to. Sulphur probably never exists in 
glass in the uncombined state at all, but sulphur and its oxides, 
which are often contained in furnace gases, sometimes exert a very 
marked action upon hot glass. The presence of sulphur gases in 
the atmospheres of blowing-holes and annealing kilns is liable to 
produce in the glass a peculiar yellowish milkiness which penetrates 
for a considerable depth into the mass of the glass and cannot be 
removed by subsequent treatment. Glass vessels, particularly if 
made of glass produced from raw materials among which salt-cake 
has figured, are also affected by contact with fused sulphur or its 
vapour, the effect being a gradual disintegration of the glass. The 
precise mechanism of these actions is not known at present, but 
they probably consist in the formation of sulphur compounds 
within the glass, possibly giving rise to an evolution of minute 
bubbles of gas. 

Selenium, . which is chemically so closely related to sulphur, is 
a relatively rare element, which is, however, finding some use in 
glass-manufacture as a colouring and a decolouring agent. The 
introduction of selenium or of its compounds under suitable con- 
ditions into a glass mixture produces or tends to produce a peculiar 
yellowish-pink coloration, the intensity of the colour produced 
being dependent upon the chemical nature of the glass as a whole 
and, of course, upon the amount of selenium left in the glass at the 
end of the melting process, this latter in turn depending upon the 


duration and temperature of the process in question. The pink 
colour of selenium glass is best developed in those containing barium 
as a base, but it is also developed in lead glasses, while soda-lime 
glasses do not show the colour so well. As a " decolouriser " the 
action of selenium is entirely that of producing a complementary 
colour which is intended to " cover " the green or blue tint of the 
glass ; where the depth of the tint to be " covered " is small, selenium 
can be used very successfully in this way, although it is a relatively 
costly substance for such a purpose. No oxidising or " cleansing " 
action can be ascribed to selenium or its compounds. 

Chromium is one of the most intensely active colouring substances 
that are available for the glass-maker, and it is accordingly used 
very extensively. It has the advantage of relative cheapness, and 
can be conveniently obtained and introduced into glass in the form 
of pure compounds whose colouring effect can be accurately antici- 
pated; the colours produced by the aid of chromium have the 
further advantage of being very constant in character, being little 
affected by oxidising or reducing conditions, and only very slightly 
by the length or temperature of the melting process. The rate of 
cooling, in fact, appears to be the only factor that materially affects 
the colours produced by compounds of chromium. The colours 
produced by chromium alone are various depths of a bright green, 
the depth varying, of course, with the proportion of chromium 
that is present in the glass and with the purity of the glass itself. 
Very frequently chromium is used in conjunction with either iron 
or copper to produce various tints of " cold blue " and " celadon 
green " respectively. This element is most usually introduced 
into the glass mixture in the form, of potassium bichromate ; although 
other compounds might be employed, this substance presents 
several advantages to the glass-maker. In the first place, since 
the colouring effect of chromium is very intense, it must be used 
in very small quantities, and if chromic oxide itself were used, 
the weighing would have to be carried out with extreme care; 
potassium bichromate, however, contains a much smaller proportion 


of the effective colouring substance, so that much larger weights 
can be employed, and the accuracy of weighing required is pro- 
portionately reduced. A further consideration arises from the 
fact that chromic oxide is itself an extremely refractory body, 
and is therefore comparatively difficult to incorporate with glass, 
while its presence tends to make the glass itself more viscid and 
refractory ; the simultaneous introduction of the alkali, as provided 
by the use of the bichromate, is thus an advantage in restoring 
the fluidity and softness of the glass when finished, while also 
facilitating the solution of the chromium in the glass during the 
fusion process ; this process of solution, however, takes some 
time, chromium glasses being liable to appear patchy if insufficient 
time is given to the " founding." 

Uranium is one of the rarer and more costly elements, but is 
nevertheless used in glass-making for special purposes on account 
of the very beautiful fluorescent yellow colour which it imparts 
when added in small proportions. This yellow is quite charac- 
teristic and unmistakable, so that none of the other varieties of 
yellow glass can ever be used as a substitute for uranium glass, 
but the great cost of the latter prevents its extended use. Uranium 
is usually introduced into glass mixtures in the form of a chemical 
compound, such as uranyl-acetate or uranyl-nitrate, both these 
substances being obtainable in the form of small, intensely bright 
yellow crystals. 

Fluorine occurs in a number of glasses in the form of dissolved 
or suspended fluorides, principally fluoride of aluminium. The 
element is not essentially a colouring substance, and is only men- 
tioned here because the fluoride named is the most frequently used 
means of producing " opal " glass. The fluoride is most frequently 
introduced into the glass mixtures as calcium fluoride, used in 
conjunction with felspar, or as cryolite, a natural mineral which 
consists of a double fluoride of sodium and aluminium. 

Manganese is one of the most important colouring elements used 
by the glass-maker. When introduced into glass in the absence of 


other colouring ingredients, compounds of manganese produce a 
range of colours lying in the region of pinkish-purple to violet, 
according to the chemical nature of the glass. The exact colour 
produced varies according as the glass has lead, lime or barium as 
its base, and it also depends upon the presence of soda or potash 
as the alkaline constituent. The nature and intensity of the colour, 
however, which the addition of a given percentage of manganese 
win produce depends upon other factors besides the chemical com- 
position of the bases used in the mixture. The heat and duration 
of the " found " and the reducing or oxidising conditions of the 
furnace in which it has been carried on very materially afiect the 
result. Thus, a glass having a slight tinge of pink or purple derived 
from manganese can be rendered entirely colourless by the action 
of reducing gases or by introducing into the glass a reducing sub- 
stance, such as a piece of wood. It will thus be seen that while 
manganese is a most useful element for the glass-maker, its employ- 
ment requires much skill and care, and generally involves some 
troublesome manipulations before the desired result is attained. 

In practice, manganese is most frequently used with other colour- 
ing ingredients for the production of what may be called ** com- 
pound " colours, the function of the manganese being to provide 
the " warm " element, t.«., the pink or purple component, required. 
One of the most important uses of manganese coming under this 
head is its use as a " decolonriser/' By a " decolouris er " the 
glass-maker understands a substance which canbejised Jg improve 
the colour of a glass 'wEicE,"^om t£e nature of its raw materials 
and conditions of melting, would h ave a ^eener c olour than is 
thought desirable for the product in question. It may be said at 
once that the most perfect and satisfactory method of obtaining 
the better colour required is to adopt the use of purer raw materials 
and methods of melting less liable to lead to contamination of the 
glass. On the other hand, this radical course is often impossible on 
the ground of expense, and the less satisfactory course must be 
adopted of covering one undesirable colour by another comple- 


mentary colour which would, in itself, \)e equally undesirable. The 
rationale of this procedure depends upon the fact that a slight 
amount of absorption of light is not readily detected by the human 
eye if it be uniformly or nearly uniformly distributed over the 
whole range of the visible spectrum, i.e., if the colour of the resulting 
light is nearly neutral, while an equally slight absorption in one 
region of the spectrum, while actually allowing more light to pass 
through the glass, is at once detected by the eye owing to the colour 
of the transmitted light. Now it has been found that the colour 
produced in glass by the addition of very small proportions of 
manganese is approximately complementary to the greenish-blue 
tinge of the less pure varieties of ordinary glass ; the addition of 
manganese in suitable proportions to such glass therefore results 
in the production of a glass which transmits light of approximately 
neutral, usually slightly yellow, colour, the increased total absorption 
only becoming noticeable in large pieces. This " covering " of the 
greenish tinge is generally most completely successful in the case of 
soda-flint glasses, but the method is also used to a certain extent 
in the case of the soda-lime glasses used for sheet and plate-glass 
manufacture. Manganese added to glass for this purpose is generally 
introduced into the mixture in the form of the powdered black 
oxide (manganese dioxide), which is available as a natural ore in 
a condition of sufficient purity. Added in this form, the manganese 
compound exerts a double action, the decomposition of the dioxide 
resulting in the liberation of oxygen within the mass of melting 
glass, and this oxygen itself exerts a favourable influence on the 
resulting colour of the glass, since it removes organic materials 
whose subsequent reducing action would be deleterious, and it 
also converts all iron compounds present into the more highly- 
oxidised (ferric) state in which their colouring effects are less intense. 
The actual colouring effect of the manganese itself is, of course, 
afterwards developed, and produces the effects discussed above. 

The "covering" of the greenish tints due to iron and other 
compounds is only possible when these are present in very small 


proportions. When larger quantities of these substances have 
been introduced into the glass the addition of manganese modifies 
the resulting colour, but is no longer able to neutralise it. A very 
large range of colours can be obtained by using various proportions 
of iron and manganese, the best-known of these being the warm 
brown tint known as " hock-bottle," while all shades between this 
and the bright green of iron and the purple of manganese can be 
obtained by suitable mixtures. What has been said above as to 
the sensitiveness of manganese colours applies with even greater 
force to these mixed tints, since here both the iron and the manga- 
nese compounds are liable to undergo changes of oxidation . Copper- 
manganese and chromium-manganese colours are also used, as 
indeed almost any number of colouring ingredients may be simul- 
taneously introduced into a glass mixture, the resulting colour 
.b€irij[, as a rule, purely additive. 

Iron is so widely distributed among the materials of the earth's 
crust that it is exceedingly difficult to exclude it entirely from any 
kind of glass, although the purest varieties of glass contain the 
merest traces of this element. Cheaper varieties of glass, however, 

always contain iroi^4n measurable- qiiaiilit)' ,^wliilr-iJhe cheapest 
kinds of glass contain considerable proportions of this element. 
The colouring efiects of iron have already been aUuded to at various 
points in the earlier chapters as well as in the section on manganese 
just preceding. Little further remains to be said here. Just as 
the less highly oxidised compounds of iron — i.e., the " ferrous " 
compounds — always show a decided green tint, so glasses con- 
taining iron when melted under the usually prevalent reducing 
conditions of a glass-making furnace, show a decided green tint 
whose depth depends upon the amount of iron present, provided 
no manganese or other " decolouriser " has been introduced. 
" Ferrous " compounds are, however, readily converted into the 
more highly oxidised or " ferric " state by the action of oxidising 
agents, and this change can also be brought about in molten glass 
by the action of such substances as nitrates or other sources of 


oxygen. The ferric compounds, however, show characteristic yellow 
tints which are much less intense and vivid than the correspondmg 
green colours of the " ferrous " series, and a similar result is brought 
about by the oxidation of iron compounds contained in glass ; 
hence the " washing " or cleansing effects ascribed to oxidising 
agents introduced in the fusion of glass. It should, however, be 
borne in mind that the oxidation of other substances besides iron 
compounds, viz., organic matter, carbon and sulphur compounds, 
may, and probably does, play a most important part in this process 
in the case of most varieties of glass. 

Nickel exerts a powerful colouring influence on glass, in accordance 
with the fact that most of the other compounds of this element are 
also deeply coloured. The exact colour produced in glass depends 
upon the nature of the glass and on the condition of oxidation in 
which the nickel is present. The colours, however, are usually of 
a greenish-brown tint, although brighter colours can be produced 
by nickel under special conditions. This element is not much 
used as a colouring agent in practice, although it has been advo- 
cated as a " decolouriser." The writer is not, however, aware 
that it has ever been successfully used for this purpose, and, in fact, 
the colours to which it gives rise do not appear to be even approxi- 
mately complementary to the ordinary green and blue tints which 
" decolourisers " are intended to cover. 

Cobalt is one of the most powerful colouring agents in glass, and 
is very largely used in the production of all varieties of blue glass. 
The blue colour produced by cobalt is, in fact, probably the most 
" certain " of the colours available to the glass-maker, this tint 
being least affected by all those circumstances that lead to varia- 
tions in other tints. Almost the only difficulty involved in the use 
of cobalt is the great colouring power of this element, which requires 
that for most purposes only very small quantities may. be added 
to the glass mixture. Formerly cobalt was added to glass mixtures 
in the form of " zaffre," which was a very impure form of cobalt 
oxide. At the present time, however, the more expensive but much 


mpre satisfactory pure oxide of cobalt is in almost universal use. 
This substance shows a perfectly constant composition and, by 
means of accurate weighing, enables the glass-maker to introduce 
precisely the right amount of cobalt into his batch. 

The range of colours which are available to the modern glass 
manufacturer are, as will be seen from a consideration of the list of 
colouring elements given above, practically unlimited, particularly 
as these substances can be used in almost any combination to produce 
mixed or intermediate tints. This practicaUy infinite variety of 
possible tints, indeed, involves the principal difficulty encountered 
by the manufacturer of coloured glass, i,e., that of matching his 
tints, or of keeping the colour of any particular variety of glass so 
constant that pieces produced at various times can be used indis- 
criminately together. This ideal is, perhaps, never entirely realised, 
but in the case of glasses intended for special technical uses the ideal 
degree of constancy is very closely approached. 

In addition to being caUed upon to produce a large variety of 
different tints, the glass-maker is also caUed upon to produce various 
depths of the same tint. In many cases this can be readily done by 
the simple means of varying the amount of colouring material added 
to the glass. Where the colouring effect of small quantities of 
these substances is not excessively powerful there is no very great 
difficulty in doing this, but in certain cases this mode of regulating 
the intensity of the colour is not available. Thus copper-ruby glass 
cannot readily be made of so light a tint as to appear of reasonable 
depth when used in sheets of the thickness of ordinary sheet-glass. 
As has already been indicated, the desired tint is obtained by the 
process of " flashing," i.e., of placing a very thin layer of deep ruby- 
coloured glass upon the surface of a sheet of ordinary more or less 
colourless glass of the usual thickness. This is generally accom- 
plished by having a pot of molten ruby glass available close to a pot 
from which colourless glass is being gathered. A small gathering of 
ruby glass is first taken up on the pipe, and the remaining gatherings 
required for the production of the sheet are taken from the pot of 


colourless glass. When such a composite gathering is blown into a 
cylinder in the manner described in the previous chapter, the ruby 
glass lies as a thin layer over the inner face of the cylinder, but special 
care and skill on the part of the gatherer and blower are required to 
ensure that this layer shall be evenly distributed and of the right 
thickness to produce just the tint of ruby required. Since the whole 
layer of red glass is so thin, a very slight want of uniformity in its 
distribution leads to wide variations of tint, and in practice these 
are often seen in the less successful cylinders of such glass. 

The chemical composition of the ruby and the colourless glass 
which are to be employed for this purpose must also be properly 
adapted to one another in order to produce two glasses which shall 
have as nearly the same coefficient of thermal expansion as possible. 
If this requirement is not met, the resulting glass is subjected to 
internal strains which may lead to fracture, while, if the ruby glass 
has the higher co-efficient of expansion, the sheet after flattening 
tends to draw itself up on the " flashed " side and cannot be passed 
out of the annealing kiln in a properly flat condition. 

Although most usually applied to copper-ruby glass, the flashing 
process is often used with other colours also. Coloured glass of this 
kind is at once recognised when looked at through the edges. Thus 
examined the glass simply shows the greenish tint of ordinary sheet- 
glass which constitutes practically the entire thickness of the sheet. 
In the same way, if such " flashed " glass be cut or etched in such a 
way that the layer of coloured glass is removed in places, the result- 
ing pattern appears in white on the coloured ground — a feature which 
is utilised for certain decorative purposes. The flashing process just 
described, it should be noted, is applicable to any form of glassware 
which is blown from a gathering, and the coloured layer can be 
applied either upon the inside or outside of any object thus produced. 

In addition to the palette of colours which the glass-maker is able 
to supply, the artist in stained glass has a further range of colours at 
his disposal in the form of stains and transparent colours which can 
be applied to the surface of glass and developed and rendered more 

O.M. O 


or less permanent by being properly " fired." The colours produced 
in this way are also, in one sense, coloured glasses, or rather glazes, 
whose raw materials are put upon the glass by the brush of the 
painter, and only subsequently caused to combine and melt by 
suitable heating. The degree of heat applicable under these cir- 
cumstances is, however, very limited by the necessity of avoiding 
any great softening of the substratmn of glass, while many of the 
colours themselves are composed of materials which could not resist 
very high temperatures. The fluxes used in the composition of 
these colours must for this reason be of a very fusible kind, with the 
inevitable result of a greatly reduced chemical stabiUty as compared 
with the glass itself. 

The whole subject of painting on glass, even from the purely 
technical as apart from the aesthetic point of view, is a very wide 
one, and lies outside the scope of the present volume. Only one 
further technical point in connection with glass-painting and stained 
glass work will therefore be touched upon here. This is an example 
of the fact that the more technically " perfect " modern product is 
not always preferable for special purposes which have been well 
served by older and far less " perfect " products. The production 
of technically excellent coloured glass in modern times was, some- 
what surprisingly at first, accompanied by a very marked decline 
in the artistic beauty of stained glass windows produced with this 
modern material ; the ancient art of stained glass was, therefore, 
for a time regarded as a " lost art," and glass-makers were blamed 
for being unable to produce the brilliant and beautiful tints which 
had been formerly available. More careful study, however, revealed 
the fact that while the actual colour of modern glass was at least 
as brilliant and varied as that of ancient glass, the difference lay in 
the fact that the modern glass was practicaUy entirely free from such 
imperfections as air-bubbles, striaB, and other defects which improved 
appliances and methods had enabled the glass-maker to eliminate 
from his products. Finding the beauty of his wares greatly improved 
by this increased purity of the glass in the case of window glass and 


table ware, it was natural for the glass-maker to endeavour to pro- 
duce the same " unprovement " in the coloured glasses intended for 
artistic purposes and, indeed, it is more than likely that the stained- 
glass workers themselves pressed this line of improvement upon him 
by a demand for " better " glass. It turned out, however, on close 
examination, that this very perfection of modern glass rendered it 
less adapted for these artistic purposes. A perfect piece of glass, 
having smooth surfaces and no internal irregularities, allows the rays 
of light falling upon it to pass through undeflected in direction, and 
merely changed in colour, according to the tint of the glass in 
question. On looking at the glass, external objects can be quite 
clearly seen, and much of the interest and mystery of the glass itself 
is lost. On the other hand, when falling upon a piece of glass having 
an irregular surface, and containing all manner of irregularities such 
as strisB, air bells, and even pieces of enclosed solid matter, the light 
is scattered, refracted, and deflected into all manner of directions 
until it almost appears to emanate from the body of the glass itself, 
which thus appears almost to shine with an internal light of its own ; 
the eye can hardly perceive the presence of external objects, and the 
whole window appears as a brilliant self-luminous object. 

Once their attention had been drawn to these facts, modern glass- 
makers endeavoured, and with much success, to reproduce the desir- 
able qualities of the ancient glass, while still availing themselves of 
modern methods to produce more stable glasses and a wider range of 
colours. The irregular surface of the old glass is imitated by using 
rolled or " muffed " instead of ordinary blown glass, while the inter- 
nal texture is rendered non-homogeneous by the deliberate introduc- 
tion of solid and gaseous impurities and by manipulations so arranged 
as to leave the glass in layers of difEerent density, which appear in the 
finished glass as " striae." As a consequence, it is probably not too 
much to claim that the modern workers in coloured glass have 
materials at their disposal which are at least as suitable for the 
purpose as those that were available in the best days of the ancient 

o 2 


Some reference has already been made to the technical uses of 
coloured glass, but one or two further points in that connection 
remain to be discussed. For such technical purposes as railway 
and marine signals, the consensus of practical experience has decided 
in favour of certain colours of glass, such as red and green of par- 
ticular tints. On the other hand, for various purposes in connection 
with photography, the glass-maker does not appear to have been 
able to meet the new requirements, with the result that flimsy and 
otherwise unsatisfactory screens made of gelatine or celluloid 
stained with organic dyes are employed in place of coloured glass 
in such cases, for example, as the covering of lamps for use in photo- 
graphers' " dark rooms," and for the light-filters used for ortho- 
chromatic and tri-chromatic photography. In all these cases it is 
necessary to use a transparent coloured medium which transmits 
only light of a certain very definite range of wave-lengths, and 
there is no doubt that for the glass-maker, who is confined to the 
use of a number of elementary bodies for his colouring media, it is 
by no means easy to comply with these requirements of exact 
transmission and absorption. On the other hand, the field of 
available coloured glasses has not been fully explored from this 
point of view, the only extensive work on the subject having been 
done in connection with the Jena firm of Schott, who have put 
upon the market a series of coloured glasses of accurately-known 
absorbing power. There is, however, little doubt that a much 
greater extension of this field is possible, and that it will be opened 
up by a glass-maker who undertakes the exhaustive study of coloured 
glasses from this point of view, although it must be admitted that 
there is considerable doubt whether the results obtainable by the 
aid of aniline and other dyes as applied to gelatine can ever be 
equalled by coloured glasses. 



Optical glass differs so widely from all other varieties of glass 
that its manufacture may almost be regarded as a separate industry, 
to which, indeed, a separate volume could well be devoted. In the 
present chapter we propose to give an outline of the most important 
properties of optical glass, and in the next chapter to describe the 
more important features of the processes used in its production. 

The properties which affect the value of optical glass may roughly 
be divided into two groups. The first group comprises the specifi- 
cally " optical " properties — i.e., those directly influencing the 
behaviour of light in its passage through the glass, while the second 
group covers those properties of a more general nature, which are of 
special importance in glass that is to be used for optical purposes. 

Optical Properties of Glass, — The most essential property of glass u 
in this respect is homogeneity. We have already indicated that 
glass can never be regarded as a definite chemical substance or 
compound, but that it usually consists of mutual solutions of various 
complex silicates, borates, etc. Solutions being of the very nature 
of mixtures of two or more different substances, it follows that 
they can only become homogeneous when complete mixing has 
taken place. We have a faipiliar example of the formation of such 
a solution when sugar is dissolved in water. The water near the 
sugar becomes saturated with sugar and of different density from 
the remaining water ; if the liquid is slightly stirred a very charac- 
teristic phenomenon makes its appearance — the pure water and 
the dense sugar solution do not at once mix completely, the denser 
liquid remaining for a time disseminated throughout the whole 


fluid mass in the form of more or less fine lines, sheets, or eddies, 
and these are visible because the imperfectly mixed liquids have 
different effects on the light passing through them. In the case of 
sugar-water we are, however, dealing with a very mobile liquid, 
and a few turns of a tea-spoon suffice to render the mixture com- 
plete, and the liquid, which for a few moments had appeared turbid, 
becomes homogeneous and transparent. In the case of glass, when 
the raw materials are melted together, a mixture is formed of 
liquids of differing densities similar to that which was temporarily 
formed in the sugar- water solution. Molten glass, however, is never 
so mobile a liquid as ordinary water, nor is it in the ordinary course 
of manufacture subjected to any such thorough mixing action as 
that which is produced by a spoon in a glass of water. In glass as 
ordinarily manufactured, therefore, it is not surprising to find 
that the lack of homogeneity which originates during the melting 
persists to the end. Its effects can be traced whenever a thick piece 
of ordinary glass is carefully examined, when the threads or layers 
of differing densities can be recognised in the form of minute internal 
irregularities in the glass. These defects are known as striee or 
veins, and their presence in glass intended for the better kind of 
optical work renders the glass useless. As will be seen below, in 
the production of optical glass special means are adopted for the 
purpose of rendering it as homogeneous as possible ; in fact, the 
-vj early history of optical glass manufacture is simply the history of 
attempts to overcome this very defect. The problem is, however, 
beset by chemical and physical difficulties of no mean order, and 
even in the best modern practice only a small proportion of each 
melting or crucible fuil of glass is enti|:ely free from veins or strie. 
In m^ny cases these defects are very minute, and sometimes escape 
observation imtil the stage of the finished lens is reached. At that 
stage, however, their presence becomes painfully evident from the 
fact that they interfere seriously with the sharp definition of the 
images formed by the lens in question. It will be seen that in such 
a case time and money has been wasted by grinding and polishing 



what turns out to be a useless piece of glass. Methods are, therefore, 
used for examining the glass before it is worked, whereby the 
existence of the smallest strise can scarcely escape detection. These 
methods depend upon the principle that a beam of parallel Ught 
passing through a plate of glass will meet with no disturbance so 
long as the glass is homogeneous, but if striae are present, they will 
cause the light to deviate from parallelism wherever it falls upon 
them. Under such illumination, therefore, the striae will appear 
as either dark or bright lines, when they can be readily detected. 
One form of apparatus used for this purpose is illustrated in Fig. 16. 

Fig. 16. —Diagram of strise-test 

ng apparatus. 

L, source of light ; S. slit ; A and B^ simple convex lenses ; Q, glass 
under test; E, eye of observer. The arrows indicate the path of 
light -rays. 

Transparency and colour are obviously fundamentally important 
properties of glass. In one sense homogeneity is essential to trans- 
parency, but the aspect of the subject which we are now considering 
is that of the absorption of light in the course of regular transmission 
through glass. It may be said at once that no glass is either per- 
fectly transparent or, what comes to nearly the same thing, perfectly 
free from colour. In the case of the best optical glasses it is true 
that the absorption of light is very slight, but even these, when 
considerable thicknesses are viewed, show a greenish-yellow or 
bluish colouring. On the other hand, certain optical glasses which 
are used at the present time for many of our best lenses absorb 
light so strongly or are so deeply coloured that a thickness of a few 


inches is sufficient to reveal this defect. To some extent public 
taste or opinion which objects to the use of even a slightly greenish 
glass in optical instruments of good quality is to blame for the 
tint of these glasses. In many cases glass-makers could produce 
a very slightly greenish glass, but in order to overcome this colour 
they deliberately add to the glass a colouring oxide imparting a 
colour more or less complementary to the natural green tint. The 
result is a more or less neutral-tinted glass which, however, absorbs 
much more light than the naturally green glass would have done. 
Since such glass is frequently used for photographic lenses, it is 
interesting to note that the light rays whose transmission is sacri- 
ficed in order to avoid the green tint are those lying at or near the 
blue end of the spectrum, so that the photographic rapidity of the 
resulting lenses is decidedly reduced by the use of such glass. 

Refraction and Dispersion. — The quantitative properties of glass, 
governing its efiect upon incident and transmitted light, are, of 
course, of fundamental importance in all its optical uses. The 
fundamental optical constant of each variety of optical glass is 
known as its refractive index ; this number really represents the 
ratio of the velocity with which light waves are propagated through 
free space to the velocity with which they travel through the glass. 
Not only does this ratio vary with every change in the chemical 
composition and physical condition of the glass, but it also varies 
according to the length of the light waves themselves. In other 
words, the short waves of blue light are transmitted through glass 
more slowly than the longer waves of red light. The conse- 
quence is that when a beam of white light is passed through a 
prism it is split up and spread out into a number of beams 
representing all the colours of the spectrum in their proper order, 
the blue light suffering the greatest deflection from its original 
path, while the red light suffers least deflection. Both the actual 
and relative amount by which light rays of various colours are 
deflected under such circumstances depends upon the nature of the 
glass in question ; therefore, fully to characterise the optical pro- 


perties of a given kind of glass it is necessary to state not only its 
refractive index but to specify the refractive indices for a sufficient 
number of different wave-lengths of light, suitably distributed 
•through the spectrum. For this purpose a number of well-marked 
spectrum lines have been chosen. Praunhofer first used the dark 
lines in the solar spectrum (A to H) for this purpose, but Abb6 
substituted for these a series of lines which can be produced at 
will in the laboratory. The actual lines chosen are the line known 
as A', corresponding t/O a wave-length of 0'7677 micro-millimetres 
and the lines known as C, D, F, and G', whose wave-lengths, in the 
same units, are 0-6563, 0-5893, 0-4862, and 0-4341 respectively. 
The A' line, however, lies so near the extreme red end of the spectrum 
that the data concerning it are seldom required. 

As a matter of fact, the actual refractive index is only stated in 
most tables of optical glasses for sodium light (D line), the dispersive 
properties of the glass being indicated by tabulating the differences 
between the refractive indices for the various lines, the table thus 
containing columns marked C-D, D-F, F-G'. These figures are 
usually described as the " dispersion " of the glass from C to D, 
D to F, etc. In addition to these figures it is usual to tabulate what 
is called the *' mean dispersion " of the glass, which is simply the 
difference between the refractive indices for C and F lines ; this* 
interval is usually taken as representing that part of the spectrum 
which is of the greatest importance for visual purposes. A further 
constant which is of great inaportance in the calculations for achro- 
matic lenses is obtained by dividing the mean dispersion into the 

refractive index for the D line minus one (usually written =v). 

This term, for which no satisfactory name has yet been suggested, 
characterises the ratio of the dispersive power of the glass to its 
total refracting power. It is usually denoted by the Greek letter v. 
The following tables give lists of optical glasses produced by Messrs. 
Chance, of Birmingham. This list contains examples of all the 








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Light Flint 

Medium Barium Crown 

Barvta Light Flint . 

Medium Barium Crown 

Banrta Light Flint . 

Medium Barium Crown 

Light Flint 

Light Flint 

Baryta Light Flint . 

Light Flint 

Light Barium Flint . 

Medium Barium Crown 

Dense Barium Crown 

Dense Barium Crown 

Dense Barium Crown 

Dense Barium Crown 

Dense Flint 

Dense Barium Crown 

Dense Barium Crown 

Dense Barium Crown 

Dense Barium Crown 

Dense Flint 

Dense Barium Crown 

Dense Barium Crown 

Dense Flint 

Dense Flint 

Dense Barium Crown 

Dense Flint 

Dense Barium Crown 

Dense Barium Crown 

Dense Flint 

Dense Flint 

Dense Flint 

Extra Dense Flint 

Extra Dense Flin* . 

Double Extra Dense Flint. 

Double Extra Dense Flint . 


most important types of optical glads which are available at the 
present time. Those, however, who wish to use the data for the 
purpose of lens calculation are advised to consult the latest issues 
of the optical glass-makers' catalogues, since the range of types 
available, and even the actual figures for some of the glasses, are 
liable to variation from time to time. 

In the tables on pp. 202 — 205 the first column contains the ordinary 
trade name by which each type of glass is known. These names, 
while somewhat arbitrary, indicate in a rough way the chemical 
nature of the glass concerned. Thus the word " flint " always 
implies a glass containing lead and therefore having a comparatively 
high refractive index and low value of i;, while the word " crown," 
originally applied only to lime-silicate glasses, is now used for all 
glass having a high value of v. In the next column of the table 
ate given the refractive indices of the glasses, while the fourth column 
contains the values of v. It will be seen that in the first table the 
glasses are arranged in descending order of magnitude in respect 
of this constant. An inspection of the figures in these two columns 
will reveal the fact that for the majority of the glasses contained 
in this table the value of v decreases as the refractive index increases^ 
As a matter of fact this rule applied to practically all glasses that 
were known or were at all events commercially available prior to 
the modern advances in optical glass manufactiure which were 
initiated by Abb6 and Schott of Jena. It was Abb6's insight into 
the requirements of optical instrument design that led him to 
realise the importance of overcoming this limitation in the ratio 
between the dispersive and refractive powers of glass. With the 
collaboration of Schott he succeeded in producing a whole series 
of previously unknown varieties of optical glass in which the relation 

between n^^ and v is not that of approximately simple inverse propor- 
tionality which holds for the older crown and flint glasses. Most 
valuable and in many ways most typical of these new glasses are 
those known as the " barium crown " glasses, which combine the 


high refractive index of a light flint or even a dense flint glass with 
the high v value of an ordinary crown glass. The second table, in 
which the glasses are arranged in the order of their refractive indices, 
serves to show how far it has been possible to modify the dispersion 
for a given value of the refractive index. It would lead too far 
into the subject of lens construction to explain in detail the possi- 
bilities opened up to the optician by the use of these newer varieties 
of glass. We must content ourselves with pointing out that the 
great forward strides marked by the production of apochromatic 
microscope objectives, of anastigmatic photographic lenses, and the 
modern telescopes are all based upon the employment of these 
new optical media ; and although optical glasses of these newer 
types are at the present time produced in the optical glass manu- 
factories of France and England, in quality and quantity at least 
equal to the output of the Jena works themselves, these great optical 
achievements stand as a lasting monument to the pioneer work of 
Abbe and Schott in this field. 

The last six columns of the table of optical glasses given above 
contain figures which define the manner in which each of the glasses 
named distributes the various sections of the spectrum. The 
columns C-D, D-F, and F-G' give, as already indicated, the 
difEerences between the refractive indices for the C, D, F and G' lines 
respectively ; these differences, divided by the mean dispersion of 
the glass (C — F') give the quantities known as the relative partial 
dispersions. If all kinds of glass distributed the various portions 
of the spectrum in the same proportionate manner, merely differing 
in the total amount of dispersion produced, these figures would be 
identically the same for all glasses. In actual fact it will be seen 
that the figures differ very widely from one type of glass to another. 
A moment's consideration will show that when two glasses are used 
in a lens for the purpose of achromatising one another, i.e., when one 
is used to neutralise the dispersion of the other, such achromatisation 
can only be perfect if these ratios (the relative partial dispersions) 
are the same for both glasses. To put the same statement in more 



concrete terms, if the spectrum produced by one glass is com- 
paratively long drawn out at the red end, and relatively compressed 
at the blue end, while in the other glass the opposite relation holds 
between the two ends of the dispersion spectrum, it is evident that 
the two spectra can never be superposed in such a way as entirely to 
neutralise one another — ^the spectrum produced by the one glass 
will predominate and leave a residual colour at the blue end, while 
the other will predominate at the other end. In the case of lenses 
achromatised by the use of such glasses, there will always be a 
slight fringe of colour around the borders of the images which they 
produce. One of the aims which Abb6 and Schott set themselves 
in the production of new varieties of optical glass was to obtain 
one or more pairs of glasses in which the relative partial dispersions 
should be as nearly alike as possible while the actual values of r 
should differ as widely as possible. Some success in this direction 
was at first claimed by the Jena workers, but unfortunately some 
of the most promising glasses in this respect were found to be too 
unstable for practical use and had ultimately to be abandoned. 
The only pair of glasses approximating to perfect achromatism 
offered by the Jena firm is that tabulated below, and it will be seen 
that although the relative partial dispersions are very closely alike, 
the V values of the two glasses only differ by 10, and at least one of 











Telescope Crown 
Telescope Flint . 












these glasses is not readily obtainable in really satisfactory optical 
quality. On the other hand, by a suitable selection of three glasses, 
Cooke, of York, and other makers have been able to produce per- 
fectly achromatised telescope objectives, which are usually termed 
" apochroniatic." The problem has been solved for microscope 
objectives by Zeiss, of Jena, who have employed suitable glasses 


in conjunction with the natural mineral fluorite and other crystals. 
Prom the glass-maker's point of view, however, the problem of 
producing a satisfactory pair of glasses capable of entirely achro- 
matising one another has yet to be solved. 

The table of optical glasses given above, although brief as com- 
pared with the lists issued by Erench and German optical glass- 
makers, fairly covers the range of practically available glasses, and 
a rapid inspection will at once show how extremely limited this 
range really is. Thus the refractive index varies only between the 
limits 147 and 1*71, and even if we admit as practical glasses such 
extreme tyipes — offered by some makers — ^as would extend this 
range to 1*40 in one direction and to 1-80 in the other, this does not 
affect the present argument. Of course, a glass of a refractive 
index as low as l-O, or even 1*10, is not theoretically possible, since 
the mere density of any substance enters into the factors that 
affect its refractive index, and a glass having a density lower than 
that of water (whose refractive index is about 1*3) is scarcely con- 
ceivable. In the other direction, however, the limits met with in 
the case of glass are considerably exceeded by certain natural 
mineral substances. Thus the diamond has a refractive index of 
2-42, while the garnets show refractive indices from 1*75 to 1*8L 
The values of v found in the table of optical glasses are still more 
narrowly restricted, lying between 67 and 29, while such a mineral 
as fluorite shows a value of 95' 4. These facts show that it is physi- 
cally possible to obtain transparent substances having optical pro- 
perties lying far beyond the limited range covered by our present 
optical glasses, and it scarcely needs showing that if such an extended 
range of materials were available greatly increased possibilities 
would be opened up to the designer of optical instruments. It is 
consequently interesting to inquire as to the actual causes which 
limit the range of optical glasses at present available. It will be 
found that these limits are set by the properties of glass itself. While 
the more ordinary kinds of glass, having average optical properties 
and showing dispersive powers roughly conforming to the law of 

O.lf. P 


inverse proportionality with refractive index which governs the 
older varieties of optical glass, are chemically stable substances, 
showing little tendency to undergo either chemical changes or to 
crystallise during cooling, the more extreme glasses exhibit these 
undesirable features to an increasing extent the more nearly the 
limit of our present range is approached. As the chemical composi- 
tion of a glass is " forced " by the addition of special substances 
intended to afiect its optical properties in an abnormal direction, 
so the chemical and physical stability of the glass is rapidly lessened. 
The more extreme glasses, in fact, behave as active chemical agents 
readily entering into reaction or combination even with relatively 
inert substances in their environment — ^they act vigorously upon 
the fire-clay vessels in which they are melted, and they are readily 
attacked by acids, moisture or even warm air, when in the finished 
condition, while many of th^m can only be prevented from assuming 
the condition of a crystalline (and opaque) agglomerate by being 
rapidly cooled through certain critical ranges of temperature. A 
limit to the possibility of production is set by these tendencies when 
they exceed a certain amount — a point being reached where it 
ceases to be practicable to overcome the tendency of the glass to 
self-destruction. On the lines of our present glasses; therefore, it 
does not appear hopeful to look for any considerable extension of 
the range of our optical media. On the other hand, as the known 
optical properties of transparent crystalline minerals show, a much 
greater range of optical constants would become available if it 
were possible to manufacture artificial mineral crystals of suj£cient 
size and purity for optical purposes, and the author believes that 
in this direction progress in optical materials is ultimately bound 
to lie.^ 

In addition to possessing the requisite optical constants, a good 
colour and perfect homogeneity, certain other properties are essential 

^ See a Paper by the present author on "Possible Directions of 
Progress in Optical Glass" — Proceedings of the Optical Convention. 
London. 1905. 


in good optical glass. These are the general physical and chemical 
qualities which are essential in all good glass, but especially empha- 
sised by the fact that the requirements for optical glass are more 
stringent than for any other variety of the material. Thus chemical 
stability is of the greatest importance, for the best lenses would 
soon become useless if the action of atmospheric moisture were to 
affect them appreciably — ^the polished surfaces would rapidly 
become dull and the whole lens would soon be rendered useless. 
The conditions governing the chemical stability of glass and the 
methods of testing this quality have already been indicated 
(Chapters I. and II.). The harder varieties of optical glass, such 
as the glasses quoted in the tables pp. 202-205 under the names of 
"Hard Crown" and ''Boro-Silicate Crown," are probably among 
the most durable and chemically resistant of all varieties of glass, 
but as we have already indicated, when extreme optical properties 
are required, the necessary chemical composition of the glass always 
entails a sacrifice of this great chemical stability, until a limit is 
reached where valuable optical properties no longer counter- 
balance the serious disadvantage of a chemical composition which 
renders the glass liable to rapid disintegration. 

In certain special cases it is, perhaps, possible to protect lenses 
made of such unstable glass by covering them with cemented-on 
lenses of stable glass, but this device entails concomitant limitations 
in the design of the optical system and is, therefore, rarely used. In 
any case, however, it is well for the lens-designer to consider the 
relative stability of the glasses employed when arranging the order 
in which they are to be used, since it is obviously preferable to put a 
hard, durable glass on the outside of his system, where it is most 
directly exposed to atmospheric moisture, and is also subject to 
handling and " cleaning " by inexpert hands. This latter factor is 
a very important one for the life of any lens. In the first place, a 
glass surface is very seriously affected by the minute film of organic 
matter which is left upon it when it has been touched with even a 
clean finger ; unless the glass is of the best quality in this respect, 

p 2 


such finger-marks readily develop into iridescent spots and may 
even turn into black stains. Particles of dust allowed to settle on 
the surface of the glass will afiect it in the same way, so that the 
protection afforded by mere mechanical enclosure in the tube of an 
instrument is of decided value in preserving a glass surface. It 
should, however, be noted that in some instances the interior metal 
surfaces of optical instruments are varnished with substances that 
give off vapours for a long time after the instrument is completed, 
and in that case the inside lenses are apt to be tarnished in conse- 
quence. On the other hand, outside lenses are also exposed to direct 
mechanical injury from handling and " cleaning." As far as the 
latter operation is concerned, it frequently happens, particularly in 
glasses containing soda, that a slight surface dimming is formed on 
the glass when it has been left in a more or less damp place for a long 
time. This dinuning is chiefly due to the formation on the surface 
of a great number of very minute crystals of carbonate of soda, 
which are hard and sharp enough to scratch the glass itself if rubbed 
about over it. If such a lens be wiped with a dry cloth, however 
clean and soft, the effect is a permanent injury to the polished 
surface, which could readily be avoided by first washing the lens with 
clean water, or even by using a wet cloth instead of a dry one for the 
first wiping. The methods of testing glass for durability of surface 
have been described in Chapter I. 

The mechanical hardness of the glass is an important factor in 
determining its resistance to such injurious treatment or to the effects 
of accidental contact with hard, sharp bodies. The subject of the 
hardness of glass has already been discussed in a general way in 
Chapter II., and little remains to be added here. Broadly speaking, 
a high decree of hardness and a low refractive index are found to- 
gether. This sliatement is certainly true where any considerable 
difference of hardness is considered, as, for example, in comparing 
a hard crown glass with a dense flint ; but where the difference of 
refractive index or of density is small, it is not at all certain that the 
lighter glass will also be the harder. 


The properties involved in the quality known as " hardness " also 
affect in a very marked manner the behaviour of glass when sub- 
jected to the grinding and polishing processes. The ease with 
which a good polish can be obtained varies very much in different 
kinds of glass, both the hardest and the softest glasses showing 
themselves difilcult in this respect. The harder glasses are certainly 
less liable to accidental scratching during the polishing operations, 
and generally work in a cleaner manner ; but the time required to 
produce a satisfactory polish is much greater owing to the resistance 
to displacement offered by the molecules. Both the speed of working 
and the pressure exerted during the polishing operation have, in fact, 
to be carefully adapted to the quality of the glass in this respect 
if the best possible results are to be obtained. 

Another property which is essential in optical glass of the highest^ 
quality is that of freedom from internal strains. This subject will 
be again referred to later in connection with the annealing processes 
used in the manufacture of optical glass, and it need only be men- 
tioned here that the presence of internal strain is readily recognised 
in glass, by the aid of the polariscope. Perfectly annealed glass, 
entirely free from internal strains, produces no effect upon a beam 
of polarised light passing through it, while even slightly strained 
glass becomes markedly doubly-refracting. For many purposes of 
optics this double refraction becomes undesirable or even inadmis- 
sible, especially as it is accompanied by small variations in the effec- 
tive index of refraction of various portions ot the mass of glass. 
Further, if the amount of double refraction observed is at all serious 
it indicates a state of strain which may easily lead to the fracture of 
the whole piece, particularly when undergoing the earlier stages of 
the grinding process or if exposed to shocks of any sort. As will be 
seen below, perfectly annealed glass is obtainable, but very special 
means are r equir ed for its production, and the optician should for that 
reason avoid making unnecessarily extreme demands in this direction. 
The very small amount of double refraction frequently found in the 
better class of optical glass is entirely harmless for most purposes. 



The process of manufacturing the best qualities of optical glass 
may be briefly described as consisting in obtaining a crucible full 
of the purest and most homogeneous glass, and then allowing it to 
cool slowly and to solidify in situ. Prom the resulting mass of 
glass the best pieces are picked and moulded into the desired shape 
for optical use. It will be seen at once that in this process there is 
an essential difference from all others that have been described in 
this book — viz., that the glass is never removed from the melting- 
pot while molten, and that none of the operations of gathering, 
pouring, rolling, pressing, or blowing are applied to it. The reason 
for this apparently irrational mode of procedure lies in the fact that 
the perfect homogeneity essential for optical purposes can only be 
attained by laborious means, and can then only be retained if the 
glass is left to solidify undisturbed ; any movement by the intro- 
duction of pipes or ladles would result in the contamination of the 
glass by striae and other objectionable defects. 

The choice and proportion of raw materials used in the production 
of any given quality of optical glass is governed by the chemical 
composition which experiment has shown to be necessary to yield 
the desired optical properties. The composition of optical glass 
mixtures cannot therefore be varied to suit the conditions of the 
furnace or to facilitate ready melting and fining, so that many of the 
usual resources of the glass-maker cease to be available in the very 
case where their aid would be most welcome to facilitate the produc- 
tion of technically perfect glass. On the other hand, the manufac- 
turer has a certain amount of choice as to the precise form in which 


the various chemical ingredients are to be introduced into the mix- 
ture, and he makes his choice among oxides, carbonates, mtrates, and 
hydrates, according to the behaviour that it is desired to impart 
to the mass during the earlier stages of fusion. The state of purity 
in which the various substances are commercially obtainable also 
enters largely into the question, since the greatest possible degree of 
purity in the raw materials is essential to the production of glass of, 
good colour, or rather freedom from colour. The extreme import- 
ance of purity of raw materials for optical glass is now so fully 
recognised that some manufacturers go to the length of carrying out 
a special purification process of certain of the chemicals, although 
these have been purchased as " pure." 

Since homogeneity is so essential in the finished product, very 
thorough mixing of the raw materials is necessary in the case of 
optical glass, and the ingredients are for this purpose generally used 
in a state of finer division than is necessary with other varieties of 
glass. As a rule the quantities of mixture of any one kind that are 
required are not large enough to justify the use of mechanical 
appliances, and very careful hand-mixing is carried out. 

Although it is quite possible to obtain successful meltings from 
raw materials alone, it is preferable to mix with these a certain 
proportion of " cullet " or broken glass derived from a previous melt- 
ing of the same sort. The broken glass used for this purpose is first 
carefully picked over for the purpose of rejecting pieces that contain 
visible impurities, although pieces showing striae are not usually 
rejected. The greater part of this cullet is generally mixed as evenly 
as possible with the raw materials, but a certain proportion is reserved 
for another purpose, as explained below. 

The furnaces used for the production of optical glass vary very 
much in type. Until a few years ago some of the old conical coal 
furnaces were still in use. In these the pot stands on a block or 
" siege " raised between two very deep and wide grates ; the whole 
space is domed over by a fire-brick dome through which apertures 
are pierced at intervals so as to allow the products of combustion 


to pass out into the conical chimney which is built over the whole 
structure. These furnaces were very uneconomical, both becaufie 
they could only obtain the desired high temperature by the use of 
picked coal in large quantities and also because so large a proportion 
of the heat escaped in the waste gases. There was the further 
disadvantage that the grates required clearing at intervals, and at 
those times the temperatiure of the furnace was liable to drop dan- 
gerously. These furnaces were, however, so cheap and simple in 
construction and their use was so deeply rooted in tradition that they 
have — ^in some works — only recently been superseded by much more 
efficient regenerative gas-fired furnaces. As a rule, however, optical 
glass furnaces differ from other pot-furnaces found in glass works in 
this respect — ^that the former are usually constructed to receive one 
or two pots or crucibles only, while in other glass furnaces from four 
to twelve or even twenty pots are heated at the same time. The 
reason for this restriction in the capacity of the furnaces lies in the 
fact that since the mixtures used for optical glass cannot be adjusted 
to suit the furnace, the latter must be worked as far as possible in 
such a way as to suit the mixture to be melted in it, and this implies 
that every pot will require its own adjustment of times and tempera- 
tures, and this it would be difficult, if not impossible, to secure if 
more than one or two pots were heated in the same furnace. It is 
further to be remembered that the amount of care and attention 
required during the melting of a pot of optical glass is out of all 
proportion to that needed with other varieties, so that little would 
be gained by having a number of pots in one furnace, since several 
sets of men would be required to tend them. 

In addition to the melting furnace, a very important part of the 
equipment of the optical glass works is formed by a number of kilns 
which are used for the preliminary heating, and sometimes for the 
final cooling of the various crucibles or pots. Similar kilns are used 
in other branches of the industry, but in those cases the pots, once 
introduced into the furnace, are expected to last for a number of 
weeks, or even months. In optical glass manufacture, on the other 


hand, a pot is used once only, so that fresh pots are required for 
every new melting. The kilns in which these pots are heated up 
before being placed in the melting furnace are thus in very frequent 
use. As a rule they are simply fire-brick chambers provided with 
sufficient grate-room and flue-space to be gradually raised to a red 
heat in the course of four or five days, while for the purpose of gradual 
cooling they can be sealed up like the annealing kilns used for 
polished plate-glass. In the most modern practice, however, a 
more elaborate type of kiln or " pot arch " is employed, as it has been 
recognised that it is desirable, in order to confer greater resisting 
power upon the pot, to give it a much higher degree of preliminary 
firing. While the older, simple coal-fired pot arches could barely 
attain a temperature of 1,000° C. the newer gas-fired pre-heating 
furnaces are so designed as to allow of the firing of the pot being 
carried as high as 1,400° C. Even then it is sometimes thought desir- 
able, after the pre-heated pot has been " set " in the furnace, to 
carry the preliminary firing still further in the melting-furnace before 
any glass is charged into the pot. In that case the pre-fiiring is 
sometimes carried up to temperatures near 1,600° C, but in that case 
care is required to avoid attaining a temperature high enough to 
soften the pot and cause it to deform under its own weight. There 
can be no doubt, however, that such a preliminary hard firing of the 
pot is an imdoubted advantage especially if a particularly highly 
refractory clay is used, as such clay is too porous if not adequately 

The pots or crucibles in which optical glass is melted are usually 
of the same shape as the covered pots used for flint glass as illus- 
trated in Mg. 2. The optical glass pots, however, are made con- 
siderably thinner in the wall, since they are not required to with- 
stand the prolonged action of molten glass in the same way as pots 
used for flint glass manufacture. On the other hand, the fire-clays 
used for this purpose must be chosen with special care so as to avoid 
any contamination of the glass by iron or other impurities which 
might reach the glass from the pot. For the production of certain 


special glasses, pots made of special materials are required, since 
these glasses, when molten, produce a rapid chemical attack upon 
ordinary fire-clays. A certain amount of the aluminif erous material 
of the pot is, in fact, always introduced into the glass by the gradual 
dissolving action of glass on fire-clay which we have already described. 
The glass contaminated with these aluminif erous substances differs 
in density from the rest of the contents of the pot, and therefore 
ordinarily remains more or less adherent to the walls of the crucible, 
but the inevitable disturbances which accompany the processes of 
melting and fining lead to the dissemination of some of this glass 
through the entire pot in the form of veins or striae, which are only 
removed during the stirring process. On the other hand, more of 
this aluminiferous glass is constantly being formed so long as the 
glass remains molten, and if disturbances are not sufficiently avoided 
during the later stages of the process fresh veins may easily be formed. 
The actual operations of producing a melting of optical glass begin 
by the gradual heating-up of the pot in the kiln just described. 
When the pot has reached a full-red heat or, if fired to a higher 
temperature in a modern kiln, has been allowed to cool down again 
to a red heat, the doors of the kiln are opened and the pot drawn 
out by means of a long heavy iron fork running on wheels ; this 
implement is run into the mouth of the kiln and the tines of the fork 
are pushed under the pot, and the latter is then readily lifted up and 
withdrawn from the kiln. Meanwhile the temperature of the furnace 
has been regulated in such a manner as to be approximately equal to 
that attained by the heating kiln, so that the pot, when transferred 
as rapidly as possible from the kiln to the furnace, is not subjected 
to any very sudden heating ; were it attempted to place the new pot 
in a furnace at full melting heat the entire vessel would fall to pieces. 
Even under the best conditions it is not possible to avoid the occa- 
sional failure of a pot by cracking either at this or a slightly later 
stage of the process. The latter occurrence is apt to be particularly 
disastrous, as the pot may then be full of molten glass, which runs 
out and is lost. 


Aa soon as the empty pot has been put into place, the melting 
furnace is carefully sealed up by means of temporary work built of 
large fire-bricks, the whole being so arranged that the mouth of the 
hood of the pot is left accesssible by means of an aperture in the 
temporary furnace wall. This aperture can be closed by one or 
more slabs of fire-clay, and when these are removed an opening is 
left by which the raw materials are introduced, and through which 
the other manipulations are carried out. 

When this stage of the process is reached, the wagons containing 
the mixed raw materials are usually wheeled into place in front of 
the furnace, but the introduction of the materials themselves into 
the pot is not begun imtil several hours later, when the furnace has 
been vigorously heated and an approach to the melting heat has been 

[ attained. 

When the furnace and pot have attained the necessary tempera-^ . 
tuje, but before the raw materials are introduced, a small quantity \ 

^ of the cullet, which has been reserved for this purpose, is thrown into i 
the pot and allowed time to melt, and then only is the first charge of 
mixture put into the pot. The object of this proceeding is to coat 
the bottom and part of the walls of the pot with a layer of molten 
glass which serves to protect it from the chemical and physical 
attack of the raw materials during the violent action which takes 

, place when they are first exposed to the furnace heat. 

The gradual filling of the pot with molten glass is now carried 
out by the introduction of successive charges of raw material ; as 
the mixture not only occupies more space than the glass it forms, 
but also froths up a good deal during melting, the quantities intro- 
duced each time must be carefully adjusted so as to avoid an overflow 
of half-melted glass through the mouth of the pot. As the pot is 
more and more nearly filled, the space left for the raw materials is 
proportionately diminished, and the later charges are therefore 
much smaller than the first few. 

When, finally, sufficient material has been introduced to fill the 
pot completely, the next stage of the process commences. When 


the last charge of raw materials has melted, the glass in the pot is 
left in the state of a more or less viscous liquid full of bubbles of all 
sizes ; it is essential that these bubbles should escape and leave 
the glass pure and " fine," and this result can only be achieved by 
raising the temperature of the furnace and allowing the glass to 
become more fluid, while the rise of temperature also causes tbe 
bubbles to expand owing to the expansion of the gas contained in 
them. In both ways rise of temperature facilitates the escape of 
the bubbles, and the furnace is therefore heated to the full, and 
this extreme heat is maintained until the glass is free from bubbles. 
In the case of the more fusible glasses the temperature required for 
this purpose is not excessively high, and, indeed, in the case of 
these glasses care is taken to avoid too high a temperature, as it 
entails other disadvantages. In the case of the harder crown 
glasses, however, the difficulty lies in producing an adequately 
high temperature without at the same time endangering the life 
, of furnace and crucible. The difficulty of freeing the molten glass 
;-fr(5m bubbles constitutes one of the causes that limit the range 
^ ^of our optical glasses in one direction — still harder glasses could 
be melted, but it would not be feasible to maintain a temperature 
high enough to render them fluid enough to ** fine." 

In the case of other kinds of glass, again, it becomes impossible 
entirely to remove the bubbles from the molten mass even when 
very hot and very fluid. The exact cause is not known, but in 
some kinds of glass the bubbles formed are so minute that even 
when the glass is perfectly mobile the bubbles show no tendency 
to escape, while in other lands of glass there appears to be a steady 
evolution of minute bubbles as soon as the temperature is raised 
with a view to removing those already in the glass. As this property 
attaches to some of the most valuable of the newer varieties of 
optical glass, opticians and the public have learnt to put up with- 
the presence of minute bubbles in certain lenses and prisms. These 
bubbles are, however, very minute and do not interfere with the 
optical performance of the lenses, etc., except to the extent of 


arresting and scattering the very small proportion of light that 
falls upon them ; their presence is therefore to be regarded as a 
small but unavoidable drawback to the use of glasses which offer 
advantages which completely out- weigh this defect. 

Returning to the melting process, we find that the extreme 
heating required for the purpose of " fining " the glass is continued 
for a considerable period of time, as long as thirty hours in some 
cases, the glass being examined from time to time to test its con- 
dition as regards freedom from bubbles. This is done by taking 
a small sample of glass out of the pot and examining it to see if it 
still contains bubbles. In some works this test is made by taking 
up a very small gathering of glass on the end of a small pipe and 
blowing it into a spherical flask ; on looking at such a flask in a 
suitable light the presence of even minute bubbles is readily detected. 
In other works a simpler process is fidopted, a small quantity of 
glass being ladled out of the pot on the surface of a flat iron rod. 
It is allowed to cool on the rod, and when pushed off forms a small 
bar of glass some eight or ten inches long and about an inch wide ; 
in this also the presence of bubbles is easily detected. These test 
pieces are known among glass-makers as " proofs." 

^When proofs, taken as just described, have shown that the glass 
is free from bubbles, the extreme heat of the furnace is allowed to 
abate, and the fire-clay slabs in front of the mouth of the pot are 
removed. The next step is that of skimming the surface of the 
glass. Since most of the materials liable to contaminate the contents 
of a pot are specifically lighter than the molten glass, they will be 
found floating on the surface, and the surface glass is therefore 
removed with a view to ridding the glass of anything that may have 
been accidentally introduced and that has not melted and become 
incorporated with the molten mass. This skimming operation is 
much facilitated if the pot has been filled to such an extent that 
there is a slight tendency for the glass to overflow at the lip of the 
pot. If the pot is not quite full, skimming becomes a very difficult 


The next steps in the process are those of stirring the molten 
glass with a view to rendering it homogeneous and free from strisB. 
The stirrer used for this purpose is usually a cylinder of fire-clay, 
previously burnt and heated. This is provided with a deep square 
hole in one end, and it is held at first by means of a small iron bar 
passed into this hole. By this means the red-hot cylinder of fire-clay 
is introduced into the open mouth of the pot, and when it has 
attained approximately the temperature of the molten glass it is 
dipped into the glass itself, in which it ultimately floats. The 
operation of dipping the stirrer into the glass is carefully carried 
out in such a manner as to carry as little air as possible down into 
the glass ; at best, however, the pores of the clay stirrer are full 
of air, and this must be permitted to escape before stirring is begun, 
as otherwise minute air bells would be dispersed throughout the 
glass during the stirring operation. For this purpose the stirrer 
is left immersed in the glass for several hours while the pot is again 
closed up and the temperature raised. 

When stirring is to begin, the square, down-turned end of a long 
iron bar is introduced into the corresponding square hole in the 
upper end of the stirrer, and by this means the fire-clay cylinder 
is held in a vertical position in the glass and given the steady rotatory 
movement which constitutes the stirring process. For this purpose 
the long iron bar just mentioned is made to pass over a swivel-wheel, 
while a workman moves it steadily by the aid of a large wooden 
handle. The relative arrangement of the pot, the fire-clay stirrer 
immersed in the glass, and the bar or " crochet " by means of 
which it is moved, are shown in plan and vertical section in the 
sketches of Figs. 17 and 18. 

It was for a long time considered essential that the stirring opera- 
tion should be conducted by hand by specially skilled men who were 
supposed to know by the " feel " of the glass whether they were 
moviag the stirrer at the right rate for the condition of the glass 
at any given moment. The whole operation, however, is extremely 
trying, as the workman is exposed to the heat and glare from the 


foroace — particularly as lie has to watch the motion of the stirrer 
in the glass. In modern practice the movement of the stirring bar 
is actuated by electrically-driven mechanism, and although it is 
still held that the r^ults obtained are not as good as those achieved 
by the moat skilful hand-stirriug, there are very considerable 
advantages in the regularity and certainty of operation of the 
mechanical appliances. There can be 
no doubt, however, that the stirring 
process as at present conducted is a 
very imperfect and make-shift opera- 
tion and that here, perhaps, more 
than at any other point in the 
process, there is room for improve- 
ment based on careful scieaitific 

. PiQ. 17. 


Wtb the primitive type of stirrer still used in practice the actual 
amount of stirrii^ required is very considerable ; a certain number 
of hours' stirring is usually given, but there is no real certainty that 
this is adequate in any given case. The amount given varies accord- 
ing to the nature of the 
glass and the size of the 
pot. For some meltings 
only four hours are allotted, 
while for others as much 
as twenty hours are given. 
During the earlier stages 
F"^- 18. of the stirring process the 

glass is very hot and mobile, but the stirring is continued, with 
■ short intervals devoted to re-heating the gkss, until the glass is 
so cold and stiff that the stirrer can scarcely be moved in it at all. 
When the glass has stifEened to such an extent that it is no longer 
possible to continue the stirring, preparations are made for the 
final cooling-down of the pot of glass. The fire-clay stirrer is some- 
times withdrawn from the glass, but this is laborious, and entails 



dr^^u^ A considerable quantity of glass out of the pot witli the 
clay cylinder ; more usually, therefore, the stirrer is simply left 
embedded in the glass. 

The next object to be accomplished is that of cooling the glass as 
rapidly as safety will permit until it has become definitely '' set " — 
the purpose being to prevent the recrudescence of striae as a result 
^ of convection currents or other causes which might disturb the 
homogeneity of the glass. This rapid cooling is obtained in various 
ways ; in one mode of procedure the furnace is so arranged that by 
opening a number of apertures provided for the purpose cold air is 
drawn in and the pot and its contents chilled thereby without being 
moved. This method has the advantage that the pot containing 
the viscous glass is never moved or disturbed in any way, but on 
the other hand the cooling which can be efiEected within the furnace 
itself is never very rapid, and the furnace as well as the pot is chilled, 
a proceeding which tends towards rapid deterioration of the furnace. 
Further, when the glass has been chilled down to a certain point this 
rapid rate of cooling must be arrested, as otherwise the whole con- 
tents of the pot would crack and splinter into minute fragments. 
Where the pot has been left in the furnace this can only be done by 
sealing up the whole furnace with temporary brickwork and lutings 
of fire-clay, leaving it to act as an annealing kiln imtil the glass has 
cooled down approximately to the ordinary temperature, a process 
which occupies a period of from one to two weeks according to the 
size of the melting. Such enforced idleness of a melting furnace is 
of course very undesirable from an economical point of view, and it 
is generally avoided by adopting the alternative method of drawing 
the pot bodily out of the furnace as soon as. the stirring operation 
is ended. For this purpose the temporary brickwork forming the 
front of the furnace is broken down, and with the aid of a long crow- 
bar the bottom of the pot is levered up from the bed or siege of the 
furnace to which it adheres strongly, being boimd down by the sticky 
viscous mass of molten glass and half -molten fire-clay which always 
accumulates on the bed of the furnace. The pot being temporarily 


held up by the insertion of a piece of fire-brick, the tines of a long 
and heavy iron fork running on a massive iron truck are introduced 
beneath the pot ; an iron band provided with long handles is then 
passed around the pot, and the latter is then drawn forward by the 
aid of suitable pulley blocks. The tines of the fork are then raised, 
and the pot is wheeled out of the furnace and deposited upon a suit- 
able support. Here it is allowed to cool to the requisite extent, 
when it is again picked up on the tines of the fork and deposited 
in an annealing kiln which has been previously warmed to a suitable 
temperature. In the case of certain glasses, which have a strong 
tendency to devitrify during cooling, the degree of chilling which 
results from mere exposure to cold air outside the furnace is not 
sufficient and more rapid cooling is induced, either by a powerful 
air-blast from a fan or even by jets of cold water playing on the pot. 
It will be seen that this handling of a heavy mass of intensely 
hot niaterial involves much labour, while there is also a risk of 
losing th^ glass if the pot should break before the glass has set 
sufficiently. Every care is taken to prevent such an accident, 
the pot being wrapped round with chains or otherwise supported 
in such a way that a small crack could not readily develop into a 
large gap. 

When such a melting of glass has cooled sufficiently, either in the 
furnace or in the annealing kiln, to be safely handled, the whole pot 
is drawn out, and the fire-clay shell, which is generally found cracked 
into many pieces, is broken away by the aid of a hammer. Under 
favourable circumstances the whole of the glass may have cooled 
intact as one solid lump sometimes weighing over half a ton. Unless 
special care is taken, however, it is usual to find the glass 
more or less fissured, a number of large lumps being accompanied 
by a great mass of small fragments. These are now picked 
over, and all those which are free from visible imperfections 
or which can be readily detached from such imperfections by 
the aid of a chipping hammer are put upon one side for further 

O.M. Q 


The next step of this treatment consists in moulding the rough 
broken lumps into the shape of plates, blocks, or discs according to 
the purpose for which the glass may be required by the optician. 
The plant used for the moulding process varies widely, but in all 
cases the operation consists in gradually heating the glass in a suit- 
able kiln until it is soft enough to adapt itself to the shape of the 
mould provided for the purpose. Tn some cases these moulds are 
made of fire-clay, and the glass is simply allowed to settle into them 
by its own weight ; in other cases iron moulds are used, and the glass 
is worked into them by the aid of gentle pressure from wood or metal 
moulding tools. In yet other cases, particularly where the glass 
is required in the form of small thin discs or where it is to be formed 
into the approximate shape of concave or convex lenses, the aid of a 
press is sometimes invoked. 

In all cases the moulding process is followed by the final annealing, 
which consists in cooling the glass very gradually from the red heat 
at which it has been moulded, down to the ordinary temperature. 
The length of time occupied by such cooling depends very much upon 
the size of the object and also upon the degree of refinement to which 
it is necessary to carry the removal of small internal strains in the 
glass. For many purposes it is sufiicient to allow it to cool down j 
naturally in a large kiln in the course of six or eight days. For j 
special purposes, however, where perfect freedom from double 
refraction is demanded, much greater refinements are required, and ' 
special annealing kilns, whose temperature can be accurately regu- | 
lated and maintained, are employed. In these the annealing < 
operation can be carried out so gradually that a rate of cooling in ' 
which a fall of 1° C. occupies several hours can be maintained, so I 
that very perfectly annealed glass can be produced even in discs or ] 
blocks of large size. 

When removed from the annealing kiln the plates or discs of optical 
glass are taken to a grinding or polishing workshop, where certain 1 
of their faces or edges are ground and polished in such a way as to j 
permit of the examination of the glass for bubbles, striae and other j 


defects in the manner indicated in the previous chapter. As the 
amount of sorting that can be done while the glass is still in rough 
fragments is necessarily very limited, it follows that a considerable 
proportion of the glass which has been moulded and annealed must 
be rejected as useless when thus finally examined. A yield of perfect 
optical glass, amounting to 10 or at most 20 per cent, of the total 
contents of each pot, is therefore all that can be expected, and smaller 
yields are by no means infrequent — a consideration that will serve 
to explain the relatively high price of optical as compared with other 
varieties of glass. 

A consideration of the various factors that are involved in the 
production of a piece of perfect optical glass will make it apparent 
that the cost and difficulty of its production increases rapidly with 
the weight of the piece to be produced, so that it is not surprising 
to find that the price of very large discs of perfect optical glass such 
as those required for large astronomical telescopes, reaches figures 
which become prohibitive when very large sizes are considered. 
Thus, while it is quite possible to obtain say 100 pounds of good 
glass from a single melting if the glass is to be used in the form of 
pieces not weighing more than five or six pounds each, it is only 
rarely that a single block of perfect glass can be found weighing 
100 pounds. Li the former case the best pieces can be picked, the 
worst defects can be eliminated by chipping the rough fragments, 
and at a later stage other defective pieces can be cut off or ground 
away ; not so where a large single block is required. A single fine 
vein, perhaps too small to be visible to the unaided eye, may be 
found to run through a whole block in such a way that it cannot be 
removed without breaking or cutting up the whole piece, and it will 
be seen that the frequency with which this is liable to occur increases 
with the volume of the piece required. The difficulties of re-heating 
and moulding are also increased enormously with the size of the 
individual pieces of glass that have to be dealt with, and where very 
large pieces have to be heated and cooled accidental breakage 
becomes a serious risk. Li view of these difficulties it is not sur- 



prising to find that the dimensions of oar astronomical refractors 
appear to have approached their limit, but rather are we led to 
admiration of the skill and enterprise that has pushed this limit so 
far as to produce discs of optical glass measuring as much as one 
meixe in diameter. 



The field of glass manufacture is so wide and the number and 
variety of its products so great, that in the limited compass of this 
volume it is impossible fully to enumerate them all ; there are, 
however, a certain number of these products which, while of con- 
siderable importance in themselves, yet do not fall readily under any 
of the headings of the preceding chapters. A short space will there- 
fore be devoted to some of these in this place. 

Glass Tubing, — A widely -useful form of glass is that of tubes of all 
sizes and shapes, ranging from the fine capillary tubes used in the 
construction of thermometers to the heavy drawn or pressed pipes 
that have been employed for drainage and other purposes. The 
process of manufacture employed varies according to the size and 
nature of the tube that is required. Thus lamp-chimneys are really 
a variety of tube, used in short lengths and made of relatively wide 
diameter and thin walls. These are not, however, ordinarily made 
in the form of long tubes cut into short sections, but — as has already 
been mentioned — ^they are blown into moulds in the form of a thin- 
walled cylindrical bottle, whose neck and bottom are subsequently 
removed. By this process the various forms of chimneys for oil 
lamps, having contractions at certain parts of their length, can be 
readily produced. They are frequently blown in pairs. 

The articles more strictly described as glass tubes are, however, 
produced by a process in which actual blowing plays only a very 
minor part. A gathering of suitable size is taken up on a pipe, a very 
small interior hollow space is produced by blowing into the pipe, and 
then the gathering is elongated by swinging the pipe in a suitable 


manner. The end of the elongated gathering furthest from the pipe 
is then attached to a rod or '' pontil " held by a second workman, and 
the two men then proceed to move apart, drawing out the gathering 
of glass between them. According to the bore and thickness of wall 
required in the tube, the men r^ulate the speed at which they move 
apart ; the thinner the tube is to be the more rapidly they move, in 
order to draw the glass out to a sufficient extent before it hardens 
too much. The rate of drawing must, of course, also be adapted to 
the nature of the glass in question, and this will vary very widely. 
For the production of the smaller bored tubes the men find it neces- 
sary to separate at a smart trot, while heavy tubes such as are used 
for gauge-glasses, are drawn of hard glass by a very gradual move- 
ment. In some cases, the setting of the glass, when the tube has 
attained the desired thickness, is hastened by the aid of an air-blast, 
or — ^in more primitive fashion — ^by boys waving fans over the hot 
glass. In any case, suitable troughs are provided for receiving the 
tube when drawn, and from these the tube is taken to an annealing 
kiln to undergo this necessary operation. 

The glass used for the production of tubing varies very widely 
according to the purpose for which the product is intended. Almost 
any of the more usual varieties of glass can be readily drawn out into 
tubes, and the choice of the kind of glass to be employed is therefore 1 
based on other considerations. Tubing required for the use of the | 
lamp-worker, i.e., for the production of instruments or other articles - 
by the aid of the glass-blower's blow-pipe, must have the capacity ' 
of undergoing repeated cooling and heating without showing signs of 
crystallisation (devitrification), while reasonable softness in the | 
fl^,me is also required. For this purpose, also, glass containing lead 
is not admissible, since this is liable to blacken under the influence 
of the blow-pipe flame. Soda-lime glasses rather rich in alkali are I 
most frequently used for these purposes ; one consequence of their 
chemical composition, however, is that such glass tends to imdergo 
decomposition when stored for any l«igth of time, more especially 
in damp places. Frequently this decomposition only manifests itself ; 


on heating the glass in a flame, when it either flies to pieces or turns 
dull and rough on the surface. Such glass is sometimes said to have 
" devitrified," but this is not really the case ; what has actually 
happened is that atmospheric moisture has penetrated for some little 
distance into the thickness of the glass, probably hydrating some of 
the silica ; on heating, this moisture is driven off, with the result that 
either a few large cracks or innumerable fine ones, are formed. In 
the latter case these do not readily disappear when the glass is 
softened and a dull, rough surface is left at the end of the operation. 

For purposes where the glass is to be exposed to high tempera- 
tures, tubing made of so-called " hard glass " is employed. This is 
practically a form of Bohemian crystal glass, the chemical composi- 
tion being that of a potash-lime glass rather rich in lime. To some 
extent this Bohemian hard glass has been superseded by the special 
" combustion tube " glass introduced by Schott, of Jena, but now 
made by several British firms. This is a very refractory borosilicate 
glass containing some magnesia ; it certainly withstands higher 
temperatures than hard Bohemian glass, and is rather less sensitive to 
changes of temperature ; on the other hand, it has the inconvenient 
property of showiag a white opalescence when it has once been 
heated, and this, after a time, renders the glass completely opaque. 

For many purposes, where heat-resisting qualities are chiefly 
required, ordinary glass finds a formidable rival in the shape of 
vitrified silica, which has long been available as a satisfactory com- 
mercial product. This substance offers the great advantage that 
for most ordinary purposes it may be regarded as entirely infusible, 
since the intense heat of an oxygen-fed flame is required to soften 
or melt the silica. Further, vitreous silica has an extremely low 
coefllcient of expansion, and appears also to have a rather high 
coefficient of thermal conductivity. The result is that tubes and 
other articles made of this material possess an astonishing amount 
of thermal endurance (see Chapter II.). 

A white-hot tube or rod of this material can be plunged into cold 
water with impunity, and no special pare n^ed be exercised in 


heating or cooling objects made of this substance, unless articles 
of great size and thickness are involved, and even with these only 
little caution is needed. The only disadvantages which must be 
balanced against the great advantages just named lie in the rela- 
tively high cost of the articles and in their somewhat sensitive 
behaviour to certain chemical influences. As regards cost, vitreous 
silica is at present available in two different forms ; in the first form 
it resembles ordinary glass very closely in appearance, the shape 
and finish of the tubes and vessels of this kind being almost equal to 
that of good glass ware. This silica glass has, in fact, been worked 
from molten silica in a way more or less analogous to that in which 
ordinary glass is worked, the great extra cost of the silica ware being 
due, in part, at all events, to the extremely high temperature 
required for melting and working this material ; ordinarily, in the 
production of the class of silica ware now referred to, this heat is 
generated by the liberal — and therefore expensive — use of oxygen 
gas. In great contrast to this glass-like, transparent silica ware is 
the other form in which this material is available. This is a series 
of products obtained from the fusion of silica in special forms of 
electric furnace ; in this ware the minute bubbles so readily formed 
in the fusion of all forms of quartz are not even partially eliminated, 
and by their presence — often in the form of long-drawn-out, capillary 
hollows — ^they impart to this ware its very characteristic milky 
appearance. The price of this product, which is mostly used in the 
form of tubes, although such articles as basins, crucibles, and even 
muffles and other apparatus of considerable size are available, is 
much lower than that of the transparent variety, being in fact 
decidedly lower than that of the best porcelain ; on the other hand, 
even this price is considerably above that of the best glass tubing. 

Apart from the question of cost, the use of silica ware is further 
limited by its sensitiveness to all forms of basic materials. Thus 
alkaline solutions cannot be allowed to come into contact with this 
substance, since they attack it vigorously, especially when warm. 
At high temperatures all basic materials produce a rapid attack on 


silica ware, the silica, in fact, behaving as a strongly acid body at 
and above a red heat. The attack which occurs when such a sub- 
stance as iron or copper oxide is allowed to come into contact with 
heated vitrified silica is so rapid that a tube is completely destroyed 
in a few minutes, the formation of silicates resulting in the cracking 
and disintegration of the whole piece. While, therefore, silica ware, 
especially in its cheaper forms, imdoubtedly possesses great ad- 
vantages and possibilities, its use must be carried on with careful 
reference to its chemical nature. 

Vitreous silica, in addition to the uses and advantages just named, 
has also an interest from the optical point of view ; this arises from 
the fact that it is transparent to short (ultra-violet) light waves to 
which all ordinary varieties of glass are completely opaque. Although 
special glasses have been produced which are more transparent to 
ultra-violet rays than ordinary glass, even these fall far short of 
silica in this respect. This property of transparence to ultra-violet 
light is utilised in two widely different directions. One of these is 
in the production of ultra-violet light when required for medical 
or other special purposes ; a most energetic source of such rays is 
available by the use of tubes of .vitrified silica within which the 
mercury-vapour arc is produced. In another direction the employ- 
ment of quartz lenses makes it possible to take advantage of the 
optical properties of ultra-violet light in connection with microscopy ; 
for the purpose of constructing a perfect optical system, crystalline 
quartz would be useless, since its property of double refraction is apt 
to interfere with the performance of the lenses. This is now over- 
come by the use of vitreous silica lenses, in the case of the " ultra- 
violet microscope." So far, however, it has only been possible to 
produce quite small pieces of vitreous silica sufficiently free from 
bubbles to be used for optical purposes. The great difficulty lies not 
so much in merely melting the quartz down as in freeing it from the 
air bubbles enclosed within it ; the course usually adopted with 
glass, of raising the temperature and allowing the bubbles to rise to 
the surface becomes impossible in this case, because the silica itself 


begins to vaporise and even to boil vigorously at temperatures not 
very far above its melting point. Two American workers have 
claimed to be able to overcome this difficulty by the use of both 
vacuum and high pressure applied at the earlier and later stages of 
the fusion process respectively, but vitreous silica in larg« and 
perfectly clear blocks is not yet available. 

We have already indicated that glass tubing and rod form the 
basis upon which the glass-worker, with the aid of the blow-pipe or 
'* lamp," fashions his productions, which, of course, include a great 
number of scientific instruments and appliances used more espe- 
cially in the field of chemistry. In another direction also glass tubing 
serves as a basis for a branch of the glass industry ; this is the manu- 
facture of certain classes of glass beads, which are formed by cutting 
up a heated glass tube of suitable diameter and colour into short, 
more or less spherical sections. In some cases the colour of the 
beads is secured by using glass of the desired tint, but in other cases 
the beads are made of colourless glass, and a colouring substance is 
placed in the interior of the bead. 

Solid glass rods are also employed for a variety of purposes ; their 
mode of manufacture is exactly analogous to that of tubing, except 
that the gathering is drawn out without having first had a hollow 
space produced at its centre by the blower. In its most att^iuated | 
form glass rod becomes glass thread or fibre ; this is produced by j 
drawing hot glass very rapidly, the resulting thread being wound on 
a large wheel. At one time this material found considerable use, ^ 
since it was found possible to spin and weave the thinnest glass fibres 
into fabrics which could be used for dress purposes. It is not, how- 
ever, to be regretted that this fashion has neither extended nor 
survived, since it was certainly liable to produce serious injury to 
health. It is a well-known fact that there are few more injurious 
or even dangerous substances to be inhaled into the human throat 
and lungs than finely-divided glass ; glass fibre, moreover, when 
subjected to constant bending and wear, is bound to undergo frequent 
fracture, and the atmosphere of a ball-room, for e^ainple, in which 


several such dresses were worn would soon be contaminated with 
innumerable fine, sharp particles of glass which would produce an 
injurious efEect on those inhaling them. At the present time glass 
fibre is used for little else than the " glass wool " required for certain 
special purposes in chemical laboratories. 

Fused quartz or silica fibres, of extreme tenuity, but of relatively 
very great strength, are employed in many scientific instruments, 
where their extreme lightness and perfect elasticity and freedom 
from what is known as " elastic fatigue " renders them of very great 
value. These fibres are not drawn from a mass of molten silica, as 
is done with glass, but are produced by attaching q. nail or bolt to a 
small bead of fused silica produced by the aid of an oxygen-fed blow- 
pipe ; the nail or bolt is then suddenly shot away down a long passage 
or similar space by means of a cross-bow, drawing a very fine fibre 
of silica with it ; the most difficult part of this operation, however, 
consists in finding and handling the fibres thus produced. 

Artificial Oems. — The fact that pieces of suitably-coloured glass 
can be made to show a superficial, but sometimes more or less 
deceptive, resemblance to precious stones, has led to the manufacture 
of imitation jewels of all descriptions. The glass used for this pur- 
pose is usually a very dense flint glass whose high refractive index 
facilitates the imitation which is aimed at. The external shapes of 
gems are, of course, readily imitated by cutting and grinding the 
glass, while the requisite colours are attainable by means of the 
colouring materials described in Chapter XII. To a casual observer 
the difference in sparkle and brilliance which arises from the differ- 
ence between the refractive index of the heavy flint glass (about 1*8) 
and that of minerals (which ranges from 1*7 to 2'2) is not readily 
apparent, but closer examination will at once reveal the difference. 
The determination of the optical constants by means of a refracto- 
meter would at once reveal the true character of the imitation, but 
an even readier test is that of hardness. The dense flint-glass is 
naturally soft, and is readily scratched by most of the harder minerals 
while the precious stones, more particularly garnets, rubies and 


diamonds, are very hard. If an attempt is made to scratcli an 
ordinary sheet of window-glass, it will be found that most real 
precious stones will do so readily, while flint glass imitations will fail 
to make more than a slight mark, which is more smear than scratch. 
The test by determining the specific gravity is also obviously applic> 
able, since the flint glass will readily betray its presence by its high 
density (over 4). 

In quite a different class from the imitation gems made of cut 
flint glass are the artificial gems, which in nature and composition 
are exact reproduction of natural gems, but which have been pro- 
duced by artificial processes. As far as the writer is aware these are 
only found in any large numbers in the case of the ruby, but in that 
case, at all events, it is said that the production of the artificial 
crystals is at least as costly as the purchase of the natural stones. 
There can, however, be very little doubt that as the processes of 
fusion and crystallisation become better known and imderstood, and 
the chemistry of silicate minerals is developed, the artificial produc- 
tion of mineral crystals in, at all events, moderate sizes will become 
increasingly possible ; it is even to be hoped that their production 
will be so far perfected as to place their really valuable properties at 
the service of man. 

Chilled Glass. — In all the processes of glass manufacture described 
in the present book, annealing has always played an important part. 
The glass, after it has undergone its last treatment under the influ- 
ence of heat, is subjected to a gradual cooling process with the object 
of freeing it from the internal strains which it would otherwise 
retain, and which would, ordinarily, endanger its existence and int^- 
fere with its use. It is, however, well known that objects of glass 
subjected to such internal strains as result in a compressive stress on 
the glass near the surface, are less liable to injury, and are appa- 
rently stronger than when the glass is annealed and the stresses are 
removed. On the other hand, glass surfaces under tension are 
extremely delicate and fragile. In some respects, therefore, glass 
which has not been annealed may appear to be stronger than the 


annealed product. The well-known case of the Rupert's drop is an 
example of this kind. Rupert's drops are produced by dropping 
molten glass into water ; they generally take the form of a more or 
less spherical body having a long tail, tapering off into a thread, 
attached to it. Such a Rupert's drop may be struck with a heavy 
hanmier,«and will safely resist a blow that would splinter a similar 
body made of annealed glass. If, however, the surface be scratched, 
or the tip of the tail be broken off, the entire " drop " breaks up, 
sometimes with a violent explosion, into minute fragments. Nume- 
rous inventors, among whom De la Bastie and Siemens figure most 
conspicuously, have endeavoured to utilise these properties of chilled 
glass, not exactly by seeking to produce that extreme degree of 
internal strain which is characteristic of the Rupert's drop, but by 
producing what they describe as " tempered " glass, in which the 
internal strains have been reduced by less violent cooling to such 
an extent as to retain some of the advantages of the hardened, 
internally strained condition while approximating more or less to 
the safer state of annealed glass. At one time articles of this kind 
were frequently seen as curiosities, such as tumblers that could be 
dropped on the floor without breaking, etc., but those articles 
generally ended by receiving a slight scratch or chip and promptly 
falling into fragments. As a matter of fact, however, some tempered 
glass is actually manufactured by the firm of Siemens at the present 
time for special purposes. De la Bastie's process was tried in 
England, and some success was claimed for it ; but it is not in 
commercial operation at the present time, and never appears to 

have attained any great importance. 

Massive Glass. — Enthusiasts for the extension of the use of glass 
have endeavoured to apply it to a great variety of purposes, includ- 
ing the construction of buildings and the paving of streets. In the 
former case, which was exemplified at the Paris Exhibition of 1900, 
advantage was taken of the light-transmitting power of the material, 
but although the buildings erected with large blocks of cast glass 
were not displeasing in effect, this use has not found any considerable 


extension. For paving purpoBes, the hardness and durability of 
glass are the only useful qualities, and here also — although several 
trials have been made in France — ^no signs of any considerable appli- 
cation of the new products are as yet visible. What has been said 
above with reference to the injurious character of glass dust applies, 
further, to glass pavements, since their natural wear would result 
in the formation of considerable quantities of this dust. The 
advocates of glass paving, however, suggest that the hardness of 
glass would greatly reduce the actual amount of wear, and that 
consequently the dust would be reduced considerably. This is a 
matter which prolonged experi^ice alone can decide, but it does not 
seem obvious that glass blocks should wear more slowly than stone 
setts made of good granite, for example. On the other hand, the 
glass blocks could probably be produced more cheaply, since the 
labour of cutting to size would be obviated by casting the blocks to 
the desired dimensions. 

WcUer-glasSy or silicate of soda or potash, is perhaps scarcely to be 
classed under the heading of '^ Glass Manufacture " at all, but it 
bears a certain relationship to glass in several ways. Thus one of 
the modes of manufacturing water-glass is by the fusion of sand and 
alkali in tank-furnaces somewhat resembling those used for glass pro- 
duction ; the fused silicate, moreover, solidifies as a vitreous mass, 
in which respect it also resembles such substances as borax, etc. 
The uses of silicate of soda and potash are, however, so far removed 
from the field of glass-manufacture that we cannot enter into them 

In concluding this chapter, we wish to describe one more product 
of the glasswbrks, and this includes some of the most impressive and 
splendid examples of the glass-maker's art. These are the great 
mirrors and lenses by whose aid our lighthouses and searchlights 
send forth their powerful beams of light. Although these objects 
are called " mirrors " and " lenses," since they fulfil the functions 
of such optical organs, yet in their nature and mode of manufacture 
they are so far removed from the glass used for the production of 


other kinds of lenses that they could not be included under the 
heading of " optical glass."- 

The characteristic feature in the manufacture of optical glass is the 
manner in which each separate pot or melting is allowed to cool 
down and to break up into irregular fragments which are subse- 
quently moulded to the desired shape. Were it attempted to 
manufacture the large glass bodies required for lighthouse purposes 
in this manner, the cost would approximate to that of the large discs 
used for telescope objectives, and this would of course be entirely 
prohibitive. The requirements as regards colour, homogeneity and 
freedom from other defects, which must be met in lighthouse lenses, 
are further not nearly so stringent as those which are essential in 
ordinary optical work of good quality. The reason for this difference 
arises from the fact that lighthouse lenses and searchlight mirrors 
are used merely to impart a desired direction to a beam of light, And 
not for the purpose of producing sharply-defined images ; slight 
irregularities in the glass are therefore not of such serious importance. 

lAghthouse glass can therefore be produced by rather less elaborate 
means ; although every care is taken to make the glass as perfect as 
possible, it is brought into approximately the desired form by casting 
the molten glass in iron moulds of the proper shape. When removed 
from these moulds and annealed, the glass is fixed on large revolving 
tables and ground and polished to the final shape of lenses and 
annular lens-segments as required for the various types of Fresnel 
lighthouse lenses. In this way complete rings, forming annular 
lenses, are produced up to 48 inches diameter. Rings of larger size 
are usually built up of a number of segments, and these built-up 
rings sometimes have a radius as large as 7 feet. For the majority 
of lighthouse lenses, it should be added, a hard soda-lime glass 
having a refractive index of 1*50 to 1'52 is used, but for special pur- 
poses a dense flint glass having a refractive index of 1'63 is employed. 

Mirrors for searchlight purposes are of very varied forms and sizes, 
the shape depending largely upon the particular form of beam which 
they are designed to project. For many purposes a parabolic form 


is required, while in others, where a flat, fan-shaped beam is to be 
produced, a form having an elliptical section in a horizontal plane, 
and a parabolic section in the vertical plane is required. In most 
cases these mirrors are produced by bending plates of glass, pre- 
viously raised to the necessary d^ee of heat, over suitably shaped 
moulds, the surface being subsequently re-polished to remove any 
roughness resulting from the bending process. Another type of 
mirrors is that known as '' Mangin," which has two spherical surfaces 
placed eccentrically in such a way that the centre of the nxirror is 
considerably thinner than the periphery ; in this type of mirror the 
reflecting action of the back surface is modified by the refracting 
action of the front surface, but both are spherical, and can therefore 
be accurately ground and polished by the usual mechanical means. 
Such mirrors are manufactured of single pieces of glass up to 6 feet 
in diameter. 




Although the general type of composition of each of the more 
important varieties of glass has been indicated in the text of the 
various chapters, a table of the chemical composition of some typical 
examples as found by careful and expert chemical analysis is here 
added. It should be noted, however, that the chemical analysis of 
glass is a matter of great difficulty, requiring much skill and experience, 
particularly in the case of complex glasses which may contain boron 
and fluorine. Many published analyses must, for this reason, be regarded 
with caution. It should further be borne in mind that a " batch ** 
cannot be laid down, by simple calculation from an analysis, to produce 
a glass of the same chemical composition ; discrepancies, which may 
be considerable, arise from losses by volatilisation, and the composition 
of the glass is also affected by solution of material derived from the 
pot or other containing vessel. The composition of a " batch " which 
shall produce a glass of given analysis can thus, as a rule, be arrived 
at only after some experiments, although experience of similar cases 
may serve as a guide to some extent. 

O.M. K 



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The existing literature of glaas manufacture is still very limited ; 
in the English language, in particular, there are few books and papers 
on the subject. Recently, however, matters have been very much 
improved as a result of the activities of the Society of Glass Technology, 
whose Journal constitutes a most valuable source of information both 
in original papers and in abstracts from other publications. The French 
and German literature of the subject is a little more extensive. In 
giving a list of the works, and more particularly in referring to those 
which he has consulted in the preparation of the present volume, the 
author thinks it will be an advantage to indicate their scope, and, to 
some extent, what he believes to be their value, in order to save the 
student the trouble of seeking out comparatively inaccessibb works 
onl^ to find that they contain little that is of value for his purpose. 

English Books and Papers on GI418S Manufaeture, 

The Principles of Glass Making (George Bell & Sons). By Powell & 
Chance. An elementary book giving a clear and concise account of 
the older processes, more especially in connection with flint and plate- 

Glass and Glass Manufacture (Pitman & Sons). By P. Marson. 

Glass. Articles in 9th Edition of Encyclopsedia Britannica. A 
detailed account of processes, more or less covering the entire subject, 
but the processes described are mostly obsolete at the present time. 

Glass. Article in Supplement to 9th Editioil of Encyclopsedia 
Britannica. By Harry J. Powell. A brief summary of more recent 
developments. Particularly valuable in reference to artistic English 
flint glass. 

Glass. Articles by H. J. Powell and W. Rosenhain in 11th Edition, 
EncyclopflBdia Britannica. 

Jena Glass. By Hovestadt, translated by J. D., and A. Everett. 
Contains a full account of the scientific work on glass and its practical 
application, done in connection with the Jena Works of Schott. Par- 
ticularly interesting in connection with the subjects of Chapters I., II., 


XIII., and XIV. As the title indicates, the book is written from the 
Jena point of view, and scarcely does justice to work done elsewhere. 
The book has gained considerably at the hands of the translators. 

Some Properties of Glass. By W. Rosenhain. (Transactions of the 
Optical Society of London, 1903.) Gives a brief account of the pro- 
perties of glass as affecting its optical uses. 

Possible Directions of Progress in Optical Glass. By W. Rosenhain. 
^Proceedings of the Optical Convention, London, 1905.) Has been 
referred to in the text of this book (Chapter XIII.). 

Catalogue of the Optical Convention Exhibition, London, 1906. 
Contains historical and general notices of optical and lighthouse glass, 
glass- working machinery, etc. 

Glass for Optical Instruments. By R. T. Glazebrook. (Cantor 
Lectures to the Society of Arts.) Gives an account of modem optical 
glass manufacture. 

Optical Glass. By W. Rosenhain. (Cantor Lectures to the Royal 
Society of Arts.) Deals with the manufacture of optical glass and the 
problems which arise in connection with efforts to improve the process. 

Old English Glasses. By Albert Hartshome. Gives an account of 
the history of glass-making in England. 

The Methods of Glass Blowing. By W. Shenstone, Describes the 
manipulation of glass-blowing for experimental purposes, i.6., lamp 

French Books on Glass Manufacture. 

Guide du Verrier. By G. Bontemps. A classical work by one of 
the greatest experts of his day. Much of the contents of the book is, 
however, entirely out of date at the present time. The book is inter- 
esting as being the work of the man who introduced optical glass jnanu- 
facture into England. 

Verres et Emaux. By L. Coffignal. Chiefly of interest in connection 
with the subjects of Chapter IX. 

Le Verre et le Crystal. By J. Henrivaux. (P. Vicq Dunod et Cie., 
Paris. ) A lengthy book profusely illlustrated and giving a great wealth 
of detailed information. The writer was for some time the general 
manager of one of the largest plate-glass manufactories in Europe ; 
his account of plate-glass manufacture is, therefore, especially valuable. 
Much space in this book is devoted to historical and aesthetical matter. 

La Verrerie au XX^®™® Siecle. By J. Henrivaux. (Paris, R. Bernard 
et Cie., 1903.) Practically a supplement to the preceding; some of 
the processes and products described are, however, not of a practical 
nature. Chiefly valuable for recent developments in plate-glass and 
bottle-glass manufacture. 


Qerman Books on OUus MafMifaeture, 

Die GrlasfabrikatioQ. By R. Gemer. (A. Hartleben's Yerlag, Vienna 
and I>eipzig, 1897.) A concise and clear account of most of the more 
imi>ortant processes of glass manufacture. Very practical in character. 
The information given apx)ears to be reliable, although far from complete. 

Die Herstellung Grosser Glaskoerper and Die Bearbeitung Grosser 
Glaskoerper. By C. Wetzel. (Hartleben^s Yerlag, Vienna and Leipzig, 
1900 and 1901 respectively.) Describes numerous special processes and 
appliances devised for use in connection with large glass objects. Some 
of these descriptions, however, appear to be little more than transcripts 
from patent sx>ecifications. 

. Glasf abriken und Hohlglasfabrikation. By R. Dralle. (Leipzig, 
Baumgaertner, 1886.) Looked upon as a classic in Germany. Gives 
detailed plans and drawings of entire bottle works, including furnaces 
and all accessories. Deals principally with bottle manufacture. 

Die Glasfabrikation. By Dr. E. Tscheuschner. (Weimar, B. H. 
Voigt, 1888.) A full detailed account of all processes known at the 
time. The rapid progress of modem practice has, however, already 
rendered this book to some extent obsolete. 

Jenaer Glas. By Hovestadt. Already referred to in respect of the 
English translation. 

Der Sprechsaal. (Schmidt, Weimar.) A trade journal devoted to 
the discussion of technical matters relating to the glass and ceramic 
industries. Occasionally contains articles and abstracts of technical 
or scientific interest in connection with glass manufacture. 

In addition to the books and papers named in the above list, a great 
number of scientific papers, notes, etc., are to be found scattered through- 
out the technical and scientific publications of the world ; those that 
have proved of real interest and imjwrtance have, however, left their 
mark on the industry, and will be found described or referred to in 
connection with the various branches of manufacture described in the 
present volume or in the books named above. 

In addition to the Literature of Glass Manufacture proper, there is 
now an extensive literature dealing with BefrcKftories, much of which 
has an important bearing on Glass Manufacture. A full bibliography 
of this subject cannot be given here, but important references to it 
will be found in : — 

The Journal of the Society of Glass Technology (abstracts), which 
has already been mentioned. 

The Journal of the Ceramic Society (Refractories Section) ; and 

The Transactions of the Faraday Society ("General Discussion on 
Refractories "). 



Abbe, 7, 201, 206 

Ashley, 109 

Auerbach, 19 

Baudin, 242 

Bontemps, 245 

Boswell, 35 

Boucher, 109 

Chance, 201, 244 

Coffignal, 245 

Colbum, 173 

Cooke, 208 

Cookes, 132 

Crookes, 30 

De la Bastie, 237 

Dralle, 246 

Encyclopcedia Britanniea, 244 

Everett, 244 

Fourcault, 172 

Fraunhofer, 201 

Fresnel, 239 

Frinck, 171 

Gemer, 246 

Glazebrook, 245 

Hartshome, 245 

Henri vaux, 17, 245 

Hertz, 19 

Hovestadt, 244, 246 

Jackson, 8 

Jena, 6 

Kavalier, 242 

Eowalski, 17 

Mangin, 240 

Marson, 244 

National Physical Laboratory, 8, 

59, 71, 242, 243 
Nicol, 101 
Powell, 244 
Owens, 110, 173 
Roberts-Austen, 26 
Rosenhain, 244, 245 
Schott, 7, 17, 196, 206 
Seger, 54 
Shenstone, 245 
Siedentopf, 179 
Siemens, 156, 237 
Sievert, 95, 113, 170 
Sprechsaal, 246 
Szigmondi, 179 
Tonnelot, 6 
Trautwine, 17 
Tscheuschner, 246 
Walker, 242 
Warburg, 26 
Wetzel, 246 
Withey, 242, 243 
Winkelmann, 17 



Absorption of 

light, 29, 177, 189, 199 
ultra-violet light, 30 
Achromatisation, 207 
Acid, boric, 11, 48 

carbonic, 10, 82 
hydrofluoric, 10 
phosphoric, 10 
Acids, action of, 9 
After-shrinkage of refractories, 57 
Air, compressed, 124 

for blowing, 94 
cooling of furnaces, 76 
Alkali metals, 180 

sources of, 37 
Alkaline solutions, attack by, 9 
Alumina, 37, 242, 243 
in clay, 51 
hydrate, 47 
Alumina-silica diagram, 55 
Aluminium, 46, 182 
Amorphous structure, 1 
Analyses, chemical, 33, 241, 242, 
of refractories, 56 
Anastigmatic lenses, 207 
Ancient stained glass, 194 

windows, 14 
Annealing, 97 
bottles, 112 
optical glass, 226 
kiln for polished plate, 141 
sheet, 165 
temperature, 101 
water-jugs, 121 
rolled plate, 134 
Anthracite, 48 
Antimony, 184, 242, 243 
Apochromatic pair, 208 

I Apochromatio objectives, 207 
i Arsenic, 47, 83, 184, 242, 243 
I Artificial gems, 235 
Attachment to metal, 23 
Austrian lamp glass, 242 
Autoclave test, 13 
Automatic presses, 126 
Aventurine, 181 


Bacteria, action of, 11 
Barium, 117, 182 

carbonate, 43 

compounds, 43 

crown, 202, 203, 204, 206. 

flint, 203, 204, 205 

glass, 127 

hydrate, 44 

nitrate, 44 
Baryta light flint, 203, 205 
Barytes, 43 

Batch, composition of, 241 
Bath-tubs, blown, 113 
Beads, 234 
Beakers, 242 

English, 243 
Belgian sand, 36 
Bending polished plate, 147 
Bevelling, 148 
Bibliography, 244 
Bichromate, potassium, 186 
Blisters in sheet, 161, 167 
Blocks, paving and building, 237 
Blower (sheet), 159 
Blowing, 92, 93 

crown, 173 

mechanical aids, 94, 113 



Blowing, moulds for, 93 

sheet, 159 
Blown glass, 116 

plate glass, 169 
Blue, cobalt, 177 

cold, 186 
Bohemian beakers, 242 
crystal, 116 
hard glass, 231 
Boiling-up, 86 
Books on glass, 245, 246 
Borax, 48 
Boric acid, 11, 48 

anhydride, 242, 243 
Boro-silicate crown, 202, 204, 211 
Boron, 48, 182, 241 
Bottle-glass furnaces, 77 

making machines^ 109 

machine, Owen's, 
Bottles, 105 

annealing of, 1 12 
colour in, 105 
large, 112 

making by hand, 107 
mechanical production of, 109 
medicine, 116 
raw materials for, 105 
tank furnaces for, 106 
white, 243 
wide-mouthed, 116 
Brick, chrome-ore, 65 

silica, 51, 56, 57, 65 
British sand, 35 
Brown, carbon, 183 
Bubbles, caused by ladling, 89 
formation of, 82 
in vitreous silica, 233 
size of, 86 
Building blocks, 237 
Bulbs, electric light, 111, 118 
Bullions in crown, 174 


Cadmium-sulphide, colouring 

effect of, 182 
Calcium, 182 

fluoride in opal, 187 

phosphate in opal, 184 
Carbide, silicon, 71 
Carbon, 48 

and sulphates, 84 

colloidal, 85 

Carbon, colour due to, 85 

colouring effect of, 183 
Carbonate of barium, 43 

lime, 41 
soda, 37 
Carbonic acid, 10, 82 
Carborundum, 71 

wheels, 148 
Carboys, 113 
Casting lighthouse glass, 239 

pots (plate glass), 138 
Catalogue, optical convention, 245 
Cataract, glass-blower's, 132 
Celadon green, 186 
Ceramic Society, 246 
Cerium, 132 
Chain screens, 131 
Chair work, 118 
Chalk, 42 
Charcoal, 48 

Charging pot furnaces, 80 
tank furnace, 80 
Chemical agents, glasses as, 210 

analysis, 33, 241, 242, 

behaviour, 5 
composition and mecha- 
nical strength, 17 
composition and physi- 
cal properties, 6 
glassware, tests of, 12 
properties, 1 
stability, 211 
Chilled glass, 236 

(ladling), 132 
ChiUing by ladling, 89 
Chimney, gas, 242 

monopol, 243 
China clay, 46, 65 

analysis, 56 
Chlorides, alkali, 37 
Chrome-ore brick, 65 
Chromium, colouring effect of, 

Classification re durability, 12 
Clay, fire-, 51 

maturing of, 11 
vanadium in, 186 
Cleaning, 10 

lenses, 211 
Cobalt blue, 177 

colouring effect of, 191 
Cold air screen, 132 
blue, 186 



Coefficient of thennal expansion, 

Coke, 48 
Colloidal carbon, 86 

colouring matter, 179 
Colour, 199 

affected by rate of cooling, 

changes of, 14 
constancy of, 192 
in blown glass, 121 
of ancient glass, 194 
sheet, 167 
thick glass, 177 
** white " glass, 29 
Coloured glass, 176 
in rods, 122 
for light filters, 196 
Colouring effect of 

antimony sulphide, 184 
cadmium sulphide, 182 
carbon, 183 
chromium, 186 
cobalt, 191 
copper, 181 
gold, 182 
iron, 190 
lead, 180 
manganese, 187 
nickel, 191 
selenium, 185 
silver, 181 
tin, 183 
uranium, 187 
vanadium, 185 
Colouring matter, colloidal, 179 

dissolved, 179 
oxides, 178 
Colours, compound, 188 

stains, 193 
Combustion, 72 
tube, 231 
tubing, 6 
Composition of typical glasses, 

241, 242, 243 
Compressed air, 94, 124 
Compression, glass surfaces under, 

Conductivity, thermal, 26 
Congealed liquid, 2 
Cones, Seger, 64 
Constancy of colour, 192 
Constitution, 2 
Continuous annealing kilns, 1 12 

Convention, Optical, 246 
Cooling, criticid stage of, 99 
optical glass, 224 
rapid effect of, 97 
rate of, 101 
Copper-ruby, 192 
Copper, colouring effect of, 181 
Cost of optical glass, 227 
Covered pots, ^,117 
Crochet, 222 
Crown, 206 

barium, 206 

dense barium, 202, 203, 204, 

boro-sib'cate, 202, 204 

bullions in, 174 

furnace, 51 

glass, 173 

"tables," 174 

height of, 76 

optical, 202, 203, 204, 206 

telescope, 208 

zinc, 44 
Crowns, furnace, 66 

of sheet tanks, 155 
Crushing strength, 17 
CryoUte, 41, 47, 187 
Crystal, 116 
Crystals, 210 
Crystallisation, 3 
CuUet, 80 

for glazing pot, 219 
optical glass, 215 
Cutting, 128 

flint glass, 148 

off, electric, 120 

rolled plate, 135 
Cylinders, sheet, 161 

dimensions of, 169 


Dark-room lamps, coloured glass 

for, 196 
Day tanks, 77 
Decolourised glass, absorption of 

Ught by, 189 
Decolourisers, manganese, 188 

nickel, 191 
selenium, 186 
Decomposition of salt-cake, 38 
Decoration of blown glass, 121 
Defects in sheet, 166 
Definition of glass, 1 



Dense barium crown, 202, 203, 
204, 206 
flint, 203, 206 
Depth of bath in sheet tank, 166 
Determination of 

annealing temperature, 102 
safe rate of cooling, 103 
Devitrification, 2, 9 
of tubing, 230 
Diagram, constitutional, alumina- 
silica, 66 
Diamonds, 236 
Diamond cutting, 136 
Dimming, 10 
Dinas brick, 66 
Dispersion of light, 200 
mean, 201 
relative partial, 207 
Dissolved colouring matter, 179 
Distortion, temperature of, 101 
Double extra dense flint, 203, 206 
refraction, 213 
rolled, white, 243 
rolling machine, 137 
Drawing glass tubes, 230 

pots, optical glass, 224 
silica fibres, 236 
Drops, Rupert's, 237 
DuctiUty, 18 
Duplex tubes, 22 
Durability, tests of, 11 
Dust, prevention of, 46 


Economy of tank furnaces, 78 
Edges, finishing off, 119, 120 
Elasticity, 18 
Electric cutting off, 120 

light bulbs. 111, 118 
Electrical properties, 26 
Electrolysis, 26 
Emery wheels, 148 
English beakers, 243 

thermometer glass, 242 
Etching, 10 

Eyes, protection of, 132 
Expansion, coefficient of thermal, 

Extra dense flint, 203, 206 
Hght flint, 203, 204 


Fabrics, glass, 234 
Faraday Society, 246 

Fascias, glass, 149 
Felspar, 37, 41, 187 

in bottle glass, 106 
Ferric oxide, 242, 243 
Fibres, silica, 236 
Figured rolled plate, 90, 130, 137 
Fillings, 81 

for optical glass, 219 
Filling tank furnace, 80 
Finger marks on glass, 212 
** Fining," ^6 

of optical glass, 220 
Fire-clay, 61 

Gross Almerode, 66 

mould, 107 

plasticity of, 60 

St. Loupe, 66 

solution in glass, 69 

" squatting *' of, 63 

Stourbridge, 66 

vitrification of, 63 
FireipoKsh, 124, 128 

proof glass, 160 
Flame, horse-shoe, 106 

space in furnaces^ 76 
Flashed glass, 22, 121 

sheet, 169 
Flashing, 192 
Flattening sheet, 166 
FHnt, 36, 206 

glass, 37, 46, 61, 116, 127 
melting of, 83 

optical, 203, 204, 205 

telescope, 208 
Flow, 18 
"Flow" devices, 91, 112 

sheet, 173 
Fluor CrowD, 202, 204 
Fluorescent green, 187 
Fluorides, 10 

in opal, 187 
Fluorine, 48, 241, 242, 243 
Fluorite, 209 
Fluted sheet, 170 
Fluxes in refractories, 66 
Fontainebleau sand, 36 
French books on glass, 246 

thermometer glass, 242 
Fritting of lead, 46 
Furnace crown, 61, 66 
pockets, 81 
pot, 73, 117 
recuperative, 70 
tank, 74 



Furnaces, 50, 67 

air cooling of, 76 
flame space in, 75 
for optical glass, 215 
rolled plate, 130 
primitive, 67 
regenerative, 70 
wall thickness in, 76 
Fusion, process of, 79 
Fungi, action of, 11 


Garnets, 235 
Gas producer, 67, 69 
Gatherer (sheet), 159 
Gathering, 88, 92 

for bottles, 107 
machine, 111 
Gauge glass tubes, 8 
German books on glass, 246 
Gems, artificial, 235 
Glasses, 118 

Glass-blower's cataract, 132 
Glass, definition of, 1 
Glazing pot with cullet, 219 
Globes, Jena, 243 
Gold, 47 

colouring effect of, 182 
ruby, 182 
Green, Celadon, 186 

colour — copper, 181 
fluorescent uranium glass, 
Grinding lighthouse glass, 239 

plate glass, 142 
Grog, 62 

in slip-casting, 64 
Gross Almerode fire-clay, 56 
Guide du Verrier, 245 
Gypsum, 43 


Hard crown, 202, 204, 211 
Hard glass tubes, 231 
Hardened glass, 17 
Hardness, 18, 212 
Hock-bottle colour, 190 
Homogeneity, 197, 214 
Horseshoe flame, 106 
Hydration of surfaces, 9 
Hydrofluoric acid, 10 


Imitation gems, 235 
Impurities in salt cake, 39 
Incandescent gas chimney, 242 
Indentation modulus, 19 
Index, refractive, 200, 209 
Infra-red " light," 30 
Internal strain, 213 

stress, 97 
Invar, 23 
lod-eosin, 12 
Iridescence, 122 
Iron, 242, 243 

colouring effect of, 190 
oxide in bottle glass, 105 

Jars, 115 
Jena beakers, 242 
chimneys, 242 
glass (book on), 244 
heat-resisting globes, 243 
thermometer glass, 242 
Journal of the Ceramic Society, 

Journal of the Society of Glass 

Technology, 246 
Jugs, water, 121 


Kaolinite, 55 

Kiln, annealing, rolled plate, 134 

Kilns, continuous annealing, 112 

Laboratory glassware, 44 
Ladling, 88, 89 

for machines. 111 
rolled plate, 131 
Ladlers, protection of, 131 
Lagre, 165, 168 
Lamp chimneys, 20, 111, 118 
glass, Austrian, 242 
work, glass for, 230 
Lamps, incandescent, 23 
Large vessels, 113 
Lead, 45, 117, 184, 206, 242, 243 

colouring effect of, 180 
Lear, 103 
Lears, 134 



Leuses, achromatising, 207 
anastigmatic, 207 
lighthouse, 238 
polishing of, 213 
protection of, 211 
spectacle, 30 
Light, action of, 14 

absorption of, 29, 177, 189, 

barium crown, 202, 204 

flint, 203, 204, 206 
flint, 203, 204, 206 
fijters of coloured glass, 196 
polarised, 101 
refraction of, 200 
Lighthouse glass, 238 
Lime, 41, 242, 243 
Limestone, 42 
Limit of vitreous state, 3 
Limits of optical glass, 210 
Lines, spectrum, 201 
Losses by volatilisation, 241 
Lustres, metallic, 122 


Machine, bottle-making, 109 
double rolling, 136 
mixing, 79 
Owen's, 110 
Machines, grinding, for plate glass, 
ladling for, 89 
Magnesia, 44, 242, 243 
Magnesium, 182 

Manganese, 14, 47, 180, 242, 243 
colouring effect of, 187 
in bottle-glass, 106 
peroxide, oxidising action of, 
Marver, 107, 119 
Massive glass, 237 
Materials, raw, 31 

used in glass, 4 
Maturing of clay, 11 
Mean dispersion, 201 
Mechanical aids for blowing, 94 
mixing, 46 
production of sheet, 

properties, 16 
stirring, 223 
Medicine bottles, 116 

Medium barium crown, 202, 203, 

204, 206 
Melting in tanks, 86 

point of refractories, 62 
temperatures, 62 
Metal, attachment to, 23 
Metallic lustres, 122 
Microscope cover glass, 243 

objectives, apochro- 

matic, 207, 208 
slides, 176 
ultra-violet, 233 
Minerals, refractive, index of, 209 
Miners' lamp glass, 242 
Mirrors, 148 

lighthouse, 238 
Mangin, 240 
searchlight, 238 
Miscellaneous products, 228 
Mixing machine, 79 

mechanical, 46 
raw materials, 79 
Monopol chimney, 243 
Mould, fire-clay, 107 

marks, 123 
Moulding of pots, 60 

optical glass, 226 
Moulds, 118, 123 
for blowing, 93 

pressing, 125 
metal, 108 
Muffled glass, 196 
sheet, 170 
Muranese, 130 


Names, trade (optical glass), 206 
New glasses, 206 
Nickel, 47, 180 

colouring effect of, 191 

steel, 23 
Nicol prism, 101 
Nitrates, 83 

alkali, 40 


Objectives, apochromatic, 207, 

telescope, limit to 
size of, 227 
Opal, 182, 184 

fluorides in, 187 



Opal, pot, 243 
glaw, 47 
Opaque plate glass, 149 
Open pots, 60 
Optical Convention, 245 
Optical glass, 197 
annealing, 226 
cooling of, 224 
cost of, 227 
cnllet for, 215 
drawing pots, 224 
filling for, 219 
" finuig " of, 218 
for large telescopes, 227 
furnaces for, 215 
moulding, 226 
pot, arch for, 217 
attack by, 218 
setting for, 218 
pots for, 217 
raw materials for, 214 
skimming, 221 
stirring, 222 
taking proofs, 221 
yield of. 227 
Optical distortion in sheet glass, 
instruments, glass for, 245 
properties, 197 
Society, 245 
use of silica, 233 
Oxides, colouring, 178 
Oxidising action of manganese 
peroxide, 189 
agents, 83 


Painting on glass, 194 

Pair of apochromatic glasses, 208 

Parlson, 110 

Partial dispersion, 207 

Partitions in sheet tanks, 154 

Path of dispersion, 207 

Paving blocks, 237 

Pearl-ash, 39 

Phosphate, calcium, in opal, 184 

Phosphoric acid, 10 

Phosphorus, 184 

Photographic plates, 14 

Physical properties, 1, 16 

and chemical 
composition, 6 

Pipe, glass-makers', 93 

Pipe- warmer, 159 

Plasticity of fiie-clay, 60 
Plate (analysis of), 243 
figured rolled, 90 
glass, 129 

blown, 169 

grinding and polish- 
ing, 142 
poUuied, 138 
pouring, 138 
wired, 23, 150 
rolled, 90, 130 
Plates, photographic, 14 
Pockets, furnace, 81 
Polarised light, 101 
Polished plate, 129, 138 
annealing kiln for, 141 
bending, 147 
beveUing, 148 
dimensions of, 146 
for mirrors, 148 
opaque, 149 
raw materials, 138 
rolling table for, 139 
silvering, 149 
sorting, 146 
striffi in, 147 
Polishing lenses, 213 

lighthouse glass, 239 
plate glass, 142 
rubbers, 146 
Pontil, 108, 119, 174,230 
Porosity test on refractories, 57 
Ports, furnace, 72 

in sheet tanks, 156 
Pot arch, 217 
arches, 63 

attack by optical glass, 218 
Potash, 117, 180, 242, 243 

from blast furnaces, 40 
carbonate of, 39 
glasses, 10 
nitrate of, 83 
silicate of, 238 
Potassium bichromate, 186 
Potato, 86 
Pot furnace, 73, 78, 117 

charging, 80 
Pot-setting for optical glass, 218 
Pots, 50 

burning of, 62 
casting, for plate, 138 
covert, 117 
firing of, 217 
moulding of, 60 




Pots, open and covered, 60 
slip-casting of, 63 
for optical glass, 217 
sheet, 169 
Pouring, 88, 91 

plate glass, 138 
Press and blow machines, 110 
Pressed glass, 116 
Presses, 126 
Pressing, 96, 125 
Prevention of dust, 46 
Principles of annealing, 97 
Prism, Nicol's, 101 
Producer, gas, 67, 69 
Proofs in optical glass, 221 
Proof -taking, 86, 87 
Properties, electrical, 25 
of glass, 245 
optical, 197 
physical, 16 
thermal, 20 
Protection of eyes, 132 

ladlers, 131 
lenses, 211 

Quartz, 36 

Quartz fibres, 235 

Quartz-tridymite change, 57 


JRadia:?ion, infra-red, 132 

ultra-violet, 132 
Radium, action of, 14 
Range of optical glasses, 209 
Rate of cooling, effect on colour, 

Raw materials, 32 

for optical glass, 214 
polished plate, 138 
rolled plate, 130 
sheet, 153 
grain size, 35 
mixing, 79 
storage of, 33 
Reactions in melting, 81 
Recuperative furnaces, 70 

tankf umaces (sheet), 
Red lead, 45 
Refraction, double, 213 
of light, 200 

Refractive index, 200, 209 
Refractories, 50, 246 

after-shrinkage of, 57 
analyses of typical, 56 
attack on, 51 
fluxes in, 56 
" melting-point '* of, 52 
porosity test, 57 
spaUing of, 57 
tests of, 54 
Refractoriness and composition,55 

under load, 54 
Regenerative furnace, 70 

tank furnaces 
(sheet), 156 
Relative partial dispersion, 207 
Respirators for lead -mixing, 46 
Residual spectrum, 208 
Reversing valves, 71 
Rings, 106 
Rods, glass, 234 
RoUed plate, 90, 129, 130 
^analysis), 243 
annealing, 134 
cutting, 135 
figured, 130, 137 
furnaces, 77 
sorting, 135 
Rolling, 90 

table, 133 

for polished plate, 
Rouge for polishing plate glass, 

Ruby, copper, 181, 192 
flashed, 181 
gold, 182 
Rubies, 235 
Rupert's drops, 237 


Safe rate of cooling, determina- 
tion of, 103 
St. Loupe fire-clay, 56 
Salt-cake, 33, 38, 84 

*' setting " of, 39 
Saltpetre, 40 
Sampling, 86 
Sand, 34 

Belgian, 36 

Fontainebleau, 35 

for grinding plate, 144 

origin of, 34 



Sands, British, 35 

Sandstone, 36 

Screen, cold air, 132 

Screens, chain, 131 

Seed in sheet, 166 

Seger cones, 64 

Selective absorption of light, 178 

Selenium, 47, 180 

colouring effect of, 185 
Setting of salt-cake, 39 

pots for optical glass, 218 
Shears, 119 
Sheet, 162 

advantages of, 152 

(analysis), 243 

annealing, 165 

blisters in, 161, 167 

blowing, 159 

colour of, 167 

cylinders, cracking off, 163 
splitting, 164 

defects in, 166 

drawing cylinders, 171 
flat, 172 

flattening, 165 

flow-devices, 173 

fluted, 170 

Fourcault's system, 172 

Frinck system, 171 

made in pots, 169 

mechanical production of, 

muffled, 170 

opening cylinders, 163 

raw materials for, 153 

seed in, 166 

sizes of cylinders, 169 

sorting, 166 

stones in, 166 

string in, 167 

tank furnaces for, 153, 154, 

white deposit on, 166 
Siege, 215 

Silica-alumina diagram, 55 
SiUca, 242, 243 

brick, 61, 56, 57, 66 

bubbles in vitreous, 233 

flbres, 235 

glass, 4 

optical use of, 233 

sources of, 34 

thermal endurance of, 231 
expansion of, 22 

Silica, transparence to ultra-violet 
light, 233 
vitreous, 231 
Silica- ware, action of bases on, 

cost of, 232 
Silicate of potash, 238 

soda, 238 
Silicon, 183 
Silicon -carbide, 71 
Sillimanite, 55 
Silvering, 149 

Silver, colouring effect of, 191 
Size of telescope objectives, 227 

polished plate, 146 
Skimming, 87 

optical glass, 221 
Skulls in ladling, 89 
Slaked lime, 41, 42 
Slip-casting, pots, 63 
Society, Ceramic, 246 
Faraday, 246 
Optical, 245 

of Glass Technology, 244, 
Soda, 242, 243 
ash, 37 

in sheet, 169 
carbonate of, 37 
nitrate of, 83 
silicate of, 238 
sulphate of, 38 
sulphite, 84 
Sodium sulphides, 183 
Soft crown, 202, 204 
Solidification, 2 
Solution of clay by glass, 59 
Sorting polished plate, 146 
rolled plate, 135 
sheet, 166 
Sources of alkali, 37 

potash, 40 
Spalling of refractories, 57 
Spectacle lenses, 30 ' , 
Spectrum lines, 201 

residual, 208 
Splitting cylinders (sheet), 164 
" Squatting " of flre-olay, 53 
Stability, chemical, 211 
Stained glass, 194 
Steam, blowing by, 114 

use of, in blowing, 95 
Stirring, mechanical, 223 
optical glass, 222 



Stones in rolled plate, 135 
sheet, 166 
sulphate, 85 
Storage of raw materials, 33 
Stourbridge fire-clay, 66 
Strained glass, 102 
Strain, internal, 213 
Strength, 16 

crushing, 17 
tensile, 17 
Stress, internal, 97 
Stresses, temporary, 103 
Strise, 198, 218 

in coloured glass, 195 
in polished plate, 147 
testing for, 199 
String in sheet, 167 
Strontium, 182 , 

Structure, amorphous, 1 
Suction devices, 92, 111 
Sulphate in glass, 84 

of barium, 43 
lime, 43 
soda, 38 
stones, 85 
Sulphide of antimony, colouring 

^effect of, 184 
cadmium, colouring 
effect of, 182 
Sulphides of sodium, 183 
Sulphite of soda, 84 
Sulphur, 185 

compounds in glass, 38 
Sulphuric anhydride, 242, 243 
Surfaces, chemical behaviour of, 7 


Table, rolling, 132 

Table of optical glasses, 202, 203, 

204, 205 
Table-ware, 7 
" Tables," crown, 174 
Tank-blocks, 58 
Tank furnace, 52, 74 

filling, 80 
furnaces for bottles, 106 

sheet, 153, 154, 
economy of, 78 
Tanks, day, 77, 117 

melting process in, 86 
Technology, Society of Glass, 244, 

Telescope crown, 208 


Telescope, flint, 208 

objectives, limit to size 
of, 227 
Telescopes, optical glass for large, 

Temperature, annealing, 101 

of distortion, 101 
Temperatures in melting, 52 

limiting, 4 
Tempered glass, 237 
Tempering glass, 18 
Temporary stresses, 103 
Tensile strength, 17 
Tensions due to cooling, 98 
Test for striae, 199 
Tests, autoclave, 13 
(ft durability, 1 1 

lal^oratory glassware, 12 
refractories, 54 
Thallium, 180, 184 
Thermal conductivity, 25 
endurance, 9, 20 

of silica, 231 
properties, 20 
Thermometer, 24 
glass, 6, 242 
Thermometers, zero changes in, 24 
Thick glass, colour of, 177 
Thread, glass, 234 
Tin, colouring effect of, 183 
in ruby, 184 
on pressed glass, 126 
Tongs, glass-makers', 109 
Trade names (optical glass), 206 
Transactions of the Faraday 

Society, 246 
Translucence, 28 
Transparency, 1, 27, 199 
Triplets, apochoromatic, 208 
Tube, combustion, 231 
Tubes, devitrification in, 230 
duplex, 22 
hard glass, 231 
Tubing, 18 

combustion, 6 
glass, 228 
Tumbler, evolution of, 119 
Tumblers, 118 

Typical glasses, composition of, 
241, 242, 243 


Ultra-microscope, 179 
Ultra-violet light, absorption of, 30