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Title: Glass Manufacture
Author: Rosenhain, Walter
Language: English
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Copyright Status: Not copyrighted in the United States. If you live elsewhere check the laws of your country before downloading this ebook. See comments about copyright issues at end of book.

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The “Westminster” Series


GLASS MANUFACTURE



  GLASS
  MANUFACTURE

  BY
  WALTER ROSENHAIN B.A. B.C.E.

  SUPERINTENDENT OF THE DEPARTMENT OF METALLURGY AND
  METALLURGICAL CHEMISTRY AT THE NATIONAL
  PHYSICAL LABORATORY

  [Illustration]

  NEW YORK
  D. VAN NOSTRAND COMPANY
  23 MURRAY AND 27 WARREN STREETS
  1908



  BRADBURY, AGNEW, & CO. LD., PRINTERS,
  LONDON AND TONBRIDGE.



PREFACE


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 avoiding
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
products 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 commercial 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
difficulties. 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
considerations of a commercial 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
manipulations, 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 means forgotten,
but the entire field is so large and beset with such great experimental
difficulties that even the labours of a list of investigators that
includes the names of Fraunhofer and Faraday, Stokes, Hopkinson, Abbé
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 the science of glass still in a very
elementary condition. To take two examples in illustration of this
fact we may mention the question of the connection between chemical
composition 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 Abbé 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 composition 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 busied 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 knowledge 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
glass are used for experimental purposes, and these influences and
others of the same nature vitally affect all the results of small-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 admitted that to undertake 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 still greater magnitude is furnished by the success of the
aniline dye works of Germany which are built up on purely scientific
achievements. The glass industry as a whole, supplying some of the
absolute necessaries 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 assistance in
the reading of proofs and various suggestions in connection with the
contents of this book.



TABLE OF CONTENTS


                                                                    PAGE
  PREFACE                                                              v


  CHAPTER I.

  THE PHYSICAL AND CHEMICAL PROPERTIES OF GLASS.

  Definition of the term “Glass”--Amorphous 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--Considerations 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             _p._ 1


  CHAPTER II.

  THE PHYSICAL PROPERTIES OF GLASS.

  Mechanical properties: tensile strength, crushing strength,
      elasticity, ductility, and hardness--Thermal properties
      of glass: thermal endurance, coefficient of expansion,
      thermal conductivity--Thermometer glass--Electrical
      properties of glass--Transparency and colour of glass      _p._ 18


  CHAPTER III.

  THE RAW MATERIALS OF GLASS MANUFACTURE.

  General considerations--Chemical purity, moisture, and
      physical condition, constancy of quality--Sources
      of silica, sand and sandstone--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           _p._ 35


  CHAPTER IV.

  CRUCIBLES AND FURNACES FOR THE FUSION OF GLASS.

  Fire-clay and silica-brick--Manufacture of glass-melting
      pots--Drying and first heating of pots--Blocks for tank
      and other furnaces--Uses of silica brick--Furnaces--
      Coal-fired and gas-fired furnaces--Gas producers--
      Regenerative furnaces, principles and construction of
      Siemens’ furnaces--Recuperative furnaces--General
      arrangements of modern tank furnaces--Relative
      advantages of tank and pot furnaces                        _p._ 54


  CHAPTER V.

  THE PROCESS OF FUSION.

  Mixing of raw materials by hand and by machinery--The
      charging operation--Chemical reactions during melting of
      carbonate mixtures, and of sulphate mixtures--Influence
      of carbon on the reactions--The fining process             _p._ 73


  CHAPTER VI.

  PROCESSES USED IN THE WORKING OF GLASS.

  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              _p._ 84


  CHAPTER VII.

  BOTTLE GLASS.

  Raw 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--The
      “Boucher” bottle-blowing machine--Annealing of bottles--
      Large bottles, carboys--Aids to the blower--Sievert’s
      process--Large shallow vessels, bath-tubs                  _p._ 95


  CHAPTER VIII.

  BLOWN AND PRESSED GLASS.

  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              _p._ 108


  CHAPTER IX.

  ROLLED OR PLATE GLASS.

  Rolled plate glass--Furnaces--Raw materials--Process of
      ladling--The rolling table--Annealing--Cutting and
      sorting--Patterns on 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--Difficulties and limitations--
      Advantages of wired glass                                 _p._ 122


  CHAPTER X.

  SHEET AND CROWN GLASS.

  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 cylinder, 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 limitations--Crown glass--
      The blowing process--Limitations                          _p._ 149


  CHAPTER XI.

  COLOURED GLASSES.

  Definition 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 modern glass--
      Technical uses of coloured glass, photography, railway
      and marine signals                                        _p._ 178


  CHAPTER XII.

  OPTICAL GLASS.

  Nature and properties of optical glass--Homogeneity--
      Formation and removal of striæ 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 ν--Specification of optical
      properties in terms of certain spectrum lines--Table 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 Abbé 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._ 205


  CHAPTER XIII.

  OPTICAL GLASS.

  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     _p._ 223


  CHAPTER XIV.

  MISCELLANEOUS PRODUCTS.

  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 produced
      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--Manufacture 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._ 238


  APPENDIX--Bibliography of Glass Manufacture                   _p._ 253



GLASS MANUFACTURE



CHAPTER I.

THE PHYSICAL AND CHEMICAL PROPERTIES OF GLASS.


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 translucent. Hardness and brittleness
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 structure
are generally visible to the unaided eye or can be revealed by the
microscope. Vitreous bodies on the other hand are characterised by
the entire absence of such a structure, and the mechanical, optical
and chemical behaviour of such bodies is consistent only with the
assumption that their molecules possess the same arrangement, 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 undergoing any
critical change or exhibiting any discontinuity of behaviour, such as
is exhibited 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 apparently 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 involving 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 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, all 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), and lead oxide. The manner 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 regards 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 properties, but
the requisite temperature cannot be attained in regenerative gas-fired
furnaces such as are ordinarily used by glass manufacturers. The
production of this glass has accordingly been carried on upon a small
scale only by means of laboratory furnaces heated by oxy-acetylene
flames, while latterly a less perfect variety of silica glass-ware
has been produced on a large scale by the aid of electric furnaces.
Such methods are, however, obviously limited to very special products
commanding special prices.

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 compounds, 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
fire-clay vessels. These are chosen, as regards their own chemical
composition, so as to offer to the molten glass a few 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 atmospheric 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 manufacturer
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 a 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 purpose by facilitating the
complicated manipulations to which the glass must be subjected in the
manufacturing 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 thermometer 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 thermometers--changes which vitiated the accuracy of most early
thermometers. A still more extensive adaptation of chemical composition
to the attainment of desired physical properties has been reached
primarily as a result of the labours of Schott and Abbé, 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
possibilities 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 of the German
investigators.

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 purposes,
as for instance when glass is called upon to resist exposure to hot,
damp climates, or is intended to contain corrosive liquids, the rules
which are an adequate guide to the glass-maker in meeting ordinary
requirements are 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
German Reichsanstalt (Imperial Physical Laboratory) at Charlottenburg.
Here also the labours of Schott and Abbé proved helpful, until at the
present time such glass as that used by the Jena firm in the production
of laboratory ware, and certain other special glasses of that kind, are
fitted to meet the most stringent requirements.

Leaving aside the inferior glasses, containing, generally, more than 15
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,
_i.e._, 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 off, and the whole surface will be dulled. As
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 flame 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 mechanically 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 even
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, exert a rapid
action on all kinds of glass. Only certain special glasses, containing
an excessive proportion of basic constituents and of such substances 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 connection with the action of acids upon glass, mention 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. It 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 carbonate 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 minerals--and glass is to be reckoned among these--are
subject to bacterial decomposition, a well-known example in another
direction being the “maturing” of clays by storage in the dark, the
change in the clay being accompanied by an evolution of ammonia gas. In
the case of glass it has been shown that specks of organic dust falling
upon a surface 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
establish a satisfactory test whereby this property of a given glass
may be ascertained without actually 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 porcelain 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
examined, and as a rule the less stable glasses show a dull, dimmed
surface as compared with the more stable ones. A more satisfactory
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 effect
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 regarded as measuring the stability of the
glass; the best kinds of glass remain practically free from coloured
film even on prolonged exposure. A test of a somewhat different kind
is one devised in its original form by Dr. Zschimmer, of the Jena
glass works; this depends upon the fact that the disintegrating action
of moist air can be very much accelerated if both the moisture and
the temperature of the air surrounding the glass be considerably
increased. For this purpose the samples of glass are exposed to a
current of air saturated with moisture at a temperature of about 80°
C. in a specially arranged incubator for one or more days, means being
provided for securing a constant stream of moist air during the whole
time. On examining the glass surfaces after this exposure--any wiping
or other cleaning of the surfaces being avoided--various qualities
of glass are found to show widely varying appearances. The best and
most stable glasses remain entirely unaffected; less stable kinds show
small specks, which merge into a generally dulled surface in unstable
kinds. There is no doubt that this test gives a sharp classification
of glasses, but it yet remains to be proved that this classification
agrees with their true relative durability in practice; the writer
is inclined to doubt whether this is really the case, since certain
glasses that have proved very satisfactory in this respect in practical
use all over the world were classed among the less stable kinds by this
test.

Before leaving the subject of the chemical behaviour of glass, a
reference should be made to the changes which glass undergoes 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 effects, 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 under 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 sunshine 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
found 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 this image is apt to re-appear as a “ghost” if the same glass be
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.



CHAPTER II.

THE PHYSICAL PROPERTIES OF GLASS.


_The Mechanical Properties of Glass_ 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 skylights
depend upon the strength of the material to a very considerable
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 affairs 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 approximations. The following data are only intended
to give an idea of the general order of strength to be looked for in
glass:--

  Tensile strength:
    From 1 to 4  tons per sq. in. (Trautwine).
     ”   ⅓ to 1¼  ”    ”     ”    (Henrivaux).
     ”   2 to 5½  ”    ”     ”    (Winkelmann and Schott).
     ”   5 to 6   ”    ”     ”    (Kowalski).

  Crushing strength:
    From 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 research it would probably be possible
to produce glasses of considerably greater mechanical 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
effects 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, and the great internal stress which always
exists in such glass is thereby relieved. All these peculiarities are,
of course, dependent as to their degree upon 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 processes 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 Ductility of Glass._--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. Measurable 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 permanent set has been ascribed
to incipient fissuring of the glass, and this explanation is probably
correct. On the other hand, it is in the writer’s opinion very probable
that 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 the view as yet lacks full
experimental confirmation.

_The Hardness_ of glass is a property of some importance in most of
the applications 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 found 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 capable of
decidedly scratching each other under suitable conditions. 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
chemical composition upon the hardness of glass. In general terms it
may be said that glasses rich in silica 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 at least as much upon the
nature of the processes which it has undergone as upon its chemical
composition.

_The Thermal Properties of Glass_, although not of such general
importance as the mechanical properties, are yet of considerable
interest in a large number of the practical uses to which glass is
constantly applied. Perhaps 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 chimneys employed in connection with incandescent
gas burners, 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
considerable 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 sufficiently
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,
while high tensile strength will tend to prevent it. The thermal
conductivity of the glass will also affect the result, because the
intensity of the tensile stress set up in the colder layers of glass
will depend upon the temperature gradient which exists in the glass;
thus if glass were a good conductor of heat it would never be possible
to set up a sufficient difference 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 temperature 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 will 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, Young’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 × 10^{-7} as the lower,
and 122 × 10^{-7} 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 × 10^{-7} and 648
× 10^{-7} respectively. 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. Thus 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 coefficients
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 production 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 under 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 much 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. The special
tubes made of two layers of glass above referred to are manufactured
by the Jena Glass Works for special purposes, among which boiler gauge
glasses are the most important. It should be also 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. More recently the use of certain varieties of
nickel steel has been patented for this purpose, since it is possible
to obtain nickel steel alloys of almost any desired coefficient of
expansion 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.
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 considerable difference of coefficient of expansion
will 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 manufacture.

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 graduations 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 thermometer has been exposed for
any length of time to a high temperature, 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 immediate past
history of the instrument; but, thanks to the Jena Works, 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 conductivity_ 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 statements apply, in a still more marked degree, 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 medium. 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 insulation 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
hygroscopic tendency. The softer glasses are much 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.
Quite recently a variety of glass has been brought forward which is
peculiar in having a comparatively low electrical resistance, so that
for certain purposes it can be used as an electric conductor. Although
interesting in itself, this glass is not very likely to prove useful
even for the limited number of applications that could be found for an
electrically conducting glass, since it is very rich in alkali, and is,
therefore, likely to be unstable chemically, even under the action of
atmospheric agencies alone.

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. XII.), 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 necessary 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 extensively put
into which the function of transmitting light does not very largely
enter. Almost the only such example of use is the modern 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 well over 95 per cent. of
the glass used in the world is employed for purposes where transmission
of light is essential to the attainment 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 colloidal organic bodies, such as gelatine
and celluloid, possess the property to a degree 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 perfectly
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 correspondingly weakened. In 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 absorption of light
quite unmistakably. In fact, not only does all glass absorb light,
but it does this to a different degree 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 light is slightly coloured.
Thus the purest and whitest of glasses, when examined in very thick
pieces, always show a decided 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 far 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 a 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 entering daylight should be avoided, and the use
of the most colourless glass obtainable will be desirable. Again, in
photographic 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
available. Although for 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 every-day 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. 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.



CHAPTER III.

THE RAW MATERIALS OF GLASS MANUFACTURE.


The choice of raw materials for all branches 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 filtration, 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 affects the resulting composition of the glass, and if
the sand cannot be obtained perfectly 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. In 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 trace 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 manufacture. 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
presence in the undissolved state in the finished glass, unless
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 places,
rapidly agglomerates into hard masses, at the same time absorbing
a certain percentage of moisture. Such properties are not always
to be avoided, salt-cake for example being at the present time 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 the 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 resulting 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, 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 Lippe in Germany, whence sand is delivered commercially with a
guaranteed silica content of 99·98 per cent. Sand of excellent quality,
although not quite so good as the above, is obtained at Hohenbocka in
Germany (Saxony) and at a few other places in Europe. In England no
deposit of sand of such purity is at present being exploited.

Next in order of value to these exceedingly pure sands, 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; thus in England the
sands of Leighton in Bedfordshire and of Lynn on the East Coast, are
largely used. Finally, for the manufacture of the cheapest class of
bottles, sands containing up to 2 per cent. of iron and a considerable
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 “sandstone.” 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 so as 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 the price of the materials is
prohibitive for glass-making purposes. The use of ground quartz and
flint is therefore confined to the ceramic industries in which these
substances serve as sources of silica for both bodies and glazes;
in former times, however, ground flint was extensively 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 here, since their high
silica-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 and of seaweed or “kelp”; in
both cases the alkali was obtained in the form of carbonate and was
ordinarily 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
potassium. 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 silica in the presence of water
vapour. Introduced into an ordinary glass furnace, therefore, these
salts would be driven off as vapour before they could combine with the
other ingredients in the desired form of double silicates.

Alkalies are, therefore, introduced into the glass mixture in less
volatile and more 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 Soda_, 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 process of alkali
manufacture--_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
connected 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 atmosphere
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 never 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 per cent. 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 should not exceed 0·5
per cent. in amount, and in the best salt-cake is generally under 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, containing salt-cake as one
ingredient, are stored. In practice, therefore, salt-cake can only be
ground as it is wanted, and its physical properties 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, frequent analytical determinations of the moisture
contents of the pearl-ash are necessary, and the composition of the
glass mixture requires adjustment in accordance with the results of
these determinations.

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 course, 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 are 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 highest requirements.

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, however, contain a
considerable proportion of alumina, while all but the purest varieties
also contain more or less considerable quantities of iron. Some
glass-makers regard alumina as an undesirable constituent, while others
take the opposite view, and upon this view their use of felspathic
minerals will depend. For the cheaper varieties of glass, however,
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 mineral 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 expensive.

(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 is required. The metallic oxides which are used for the
production of coloured glass are, of course, also basic bodies. These
will be treated in connection with coloured glasses, with the exception
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 either 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, followed 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 regards the greater part of its mass,
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 limestone 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, however, 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 off, 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 off; 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 lime into glass mixtures in the form of
gypsum or calcium sulphate, but the decomposition of this compound,
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 commercial value of
calcium sulphate for glass-making is probably slight.

_The Compounds of Barium_ may best be dealt with at this stage, since
they are chemically so closely allied to the compounds of lime just
described. Barium occurs in nature in considerable quantities 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 calcium sulphate discussed above,
except that the barium compound is much more easily reduced and
decomposed than the lime compound. 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 prepared salts. Of these the most important
is the carbonate, commercially described as “precipitated carbonate
of barium”; this precipitated compound, however, does not ordinarily
correspond to the chemically pure substance, but contains more or less
considerable 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 produced. Both the nitrate and the hydrate of barium
are commercially available, but they are very costly ingredients for
use in the production 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 the
carbonate.

_Magnesia_ is another glass-forming base that is closely related,
chemically, to calcium and barium. This element is usually introduced
into glass mixtures in the form of either the carbonate 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 is 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 special value; in ordinary lime glasses this element, as has
already been mentioned, is to be regarded 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 only
introduced into special optical glasses, a special “zinc crown” having
found some application. 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 introduced into
glass mixtures.

_Lead_ is one of the most widely-used ingredients of glass; the glasses
containing this substance in notable quantity are all characterised to
a greater or less degree 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, although the other oxides of
lead might be employed almost equally well. Red lead is a mixture
of two oxides of lead (PbO and Pb_{2}O_{3}) in approximately such
proportions as to correspond to the formula Pb_{3}O_{4}. It is prepared
by the roasting of metallic lead in suitable furnaces, where the molten
lead is exposed to currents of hot air. The product is obtainable in
considerable purity, very small proportions of silica, derived from the
furnace bed, and of iron derived from the tools with which the lead is
handled, being the principal foreign substances found in good red lead.
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 in 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 proportions 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 adopted by potters of first
“fritting” the lead and thus rendering it comparatively 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 proper 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 therefore 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 ingredients 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 degree. 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 contained, while in
some cases the element is intentionally introduced in small proportions
(about 2 per cent. to 3 per cent. of Al_{2}O_{3}) by the use of
felspar as an ingredient of the mixture. Where larger proportions of
alumina are required, the substance is introduced in the form of the
hydrate, which is obtainable commercially 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 compounds, 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 (MnO_{2}), although the
lower oxide (Mn_{3}O_{4}) can also be used. The material ordinarily
used is the natural manganese ore, mined chiefly in Russia; 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 gold.

_Arsenic_ is another substance frequently introduced into “white”
glass mixtures. This element is universally introduced in the form of
the white arsenic of commerce (_i.e._, arsenious acid, As_{2}O_{3})
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 form 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 excessively 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.



CHAPTER IV.

CRUCIBLES AND FURNACES FOR THE FUSION OF GLASS.


For the successful production of substances which are formed by a
process of fusion, the use of refractory materials of a proper kind is
of great importance. In the production of glass the double difficulty
has to be overcome of finding substances capable of being formed into
furnaces and crucibles which shall not only resist the softening and
melting action of the furnace heat for long periods of time, but
shall also resist the dissolving action of the molten glass itself.
The refractory materials employed in connection with glass-making
thus fall into two distinct groups, members of one group being those
which meet both of the above requirements and can therefore be used in
positions exposed to direct contact with molten glass, while members
of the second group are materials which resist the action of the heat
and flame gases but cannot resist the dissolving effect of the glass
itself; these, of course, can only be placed where molten glass is not
liable to touch them. We shall deal with the former group first.

Those portions of glass-melting plant which come into contact with
molten glass are almost universally made of some form of fire-clay.
To discuss in detail the composition and properties of the varieties
of fire-clay best suited to this purpose would exceed the entire
limits of this book, so that only a few leading principles can be
stated. Taking first the clays intended for the production of crucibles
or “pots,” we find that for the purposes of the production of such
objects the prepared clay must possess a certain degree of plasticity
while damp and a considerable degree of strength when dried. The
dried and burnt material must be so refractory as to resist the high
temperatures used in glass-melting without undergoing fusion or even
serious softening. Clays of various composition and physical nature
also differ very widely in their power of resisting the chemical
attack of molten glass; all clays are more or less dissolved under
these circumstances, but not only the rate, but also the manner, of
dissolution is of importance, so that frequently a clay which dissolves
rapidly but uniformly is preferred to one which dissolves more
slowly but in such an irregular manner as to throw off particles of
undissolved material which contaminate the glass in the form of opaque
enclosures or “stones.” It is also to be noted that the best results
in this direction can only be obtained by careful adaptation of the
clay employed to the particular kind of glass which is to be melted in
the crucibles in question. In England this question has not received
the amount of attention it deserves, but in Germany and America the
available fire-clays of the country have been systematically studied
and exploited. As a result the glass-maker has at his disposal a large
selection of materials of accurately known physical and chemical
properties. By carefully correlating these with the performance of his
“pots” in the furnaces, the manufacturer is able to select the most
suitable material, and is, moreover, in a position to know in what
direction to look for improvement or for replacement if the supply of a
satisfactory brand should cease.

We may now follow briefly the process of manufacture of a fire-clay
pot or crucible. The size and shape of the crucible will depend upon
the particular purpose for which it is intended. Crucibles varying in
capacity from 4 cwt. to 2½ tons of glass are used for various kinds
of glass, but the more usual sizes lie between 30 in. and 50 in. in
diameter. For many kinds of glass the shape of the pot is simply that
of an open basin, circular or oval in plan and larger in diameter at
the brim than at the base (Fig. 1), but for the production of flint
glass, and of other glasses which are to be protected from contact with
the flame and gases of the furnace, so-called “covered” pots are used.
In these the basin--here of a more nearly cylindrical shape--is covered
over by a dome, and access is allowed only by a relatively small hooded
opening (Fig. 2). Covered pots are built up on wooden moulds, which are
made collapsible, and are removed before the drying of the pot is begun.

[Illustration: FIG. 1.--Open “pot” or crucible for glass melting.]

[Illustration: FIG. 2.--Covered pot for glass melting, as used for
flint glass and optical glass.]

The material for pot-making is first prepared with great care. The
proper variety of clay having been selected, it is ground to a fine
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 burn specially-selected fire-clay separately for this
purpose. The quantity of such burnt material added to the mixture
depends upon the chemical nature and especially 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 clay is allowed to dry, and further when the dry mass is
subsequently burnt; the burnt material or “chamotte,” having already
undergone these shrinking processes, acts both as a neutral diluent
and also as a skeleton strengthening the whole mass and reducing the
tendency to form cracks.

The virgin clay and chamotte 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 stiff, plastic dough, but the full
toughness and plasticity of such clay mixtures can only be developed by
prolonged storage of the damp mass. In the next stage of the process,
the plastic clay 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 sufficiently 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 drying 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, _i.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 kilns placed near the melting
furnaces. In these the pot or pots are exposed to a very gradually
increasing temperature, until 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 useless. 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.

Fire-clay is also used in the manufacture of bricks and blocks of
various sizes required for the construction of glass-melting furnaces.
Here fire-clay is only used in positions where contact with molten
glass is expected, as in the walls of the basin or tank proper in
“tank” furnaces, or at a level below that of the pot or crucible in
pot furnaces; in the latter position leakage of glass from broken pots
or overflow being liable to result in an accumulation of molten glass
on the floors or walls of the furnace and passages. The fire-bricks
used in these latter positions are usually of a much poorer quality
of fire-clay than that used for the manufacture of pots, and this
is justified in so far as certain of the requirements that apply to
crucibles do not apply here--but on the other hand the use of more
refractory bricks would result in a longer life for the furnace. Such
bricks, it should be noted, are not laid in mortar when used for
furnace construction, but are set in a thin paste of fire-clay in
water, and these joints are kept as thin as possible. The part of the
furnace known as the “siege” (French “siège”), _i.e._, the floor of the
furnace upon which the pots are placed, is usually built of very large
blocks of fire-clay, made of coarse materials calculated to give great
strength. At or near the points where the flame enters the furnace,
these blocks rapidly wear away, partly by melting but chiefly by a
process of abrasion, for it seems that a rapidly moving flame has an
abrading action of a very marked kind.

The actual tanks or basins which contain the molten glass in tank
furnaces are also built of large blocks of fire-clay, but these are
made of the best procurable materials, and should receive at least as
much care in every respect as crucibles; it is true that their shape
and size gives them greater strength, but on the other hand these
blocks are expected to resist the contact of molten glass for very
much longer periods of time than the average crucible. To understand
the requirements for tank-blocks it is necessary to anticipate the
next section to the extent of stating that in tank furnaces the glass
is contained, during melting, refining and working, in a basin built
up of large blocks. These blocks are not cemented together in any way,
but are built up “dry” and are supported on the outside by a system of
iron bars and rods. The molten glass penetrates between the blocks to a
certain extent, but as the outside of all such blocks is intentionally
kept as cold as possible the glass rapidly stiffens as it penetrates
further into these interstices, and this stiffened glass effectually
binds the blocks together and prevents all leakage. It will thus be
seen that the blocks are exposed to the full heat of the furnace and
to the corroding action of the glass on the inner side, but are kept
cold on the outer side. As this state of affairs tends to produce
cracks, these blocks are necessarily made of rather coarse material.
On the other hand, the material of a block never gets so hot as the
wall of a crucible, which is heated from both sides, so that extreme
refractoriness is not so essential.

It is impossible, within the limits of this chapter, to go into the
details of the choice of materials for tank-blocks; it is a subject
upon which no finally satisfactory conclusion has yet been reached,
and what has been said above will suffice to show the nature of the
considerations upon which such choice must be based.

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 considerations,
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 discolouration. 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 (SiO_{2}). 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 and a little alumina 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 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.

We now turn to consider, very briefly, the general design and
arrangement of some typical 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 a 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
especially 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 chimney which stands over the
entire chamber and communicates with it by a number of small openings.
In a more refined furnace, the chamber itself is double, and the flame,
after playing around 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 chimney or cone. We need not give any greater attention
to these primitive furnaces, since they are practically 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 appliance 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 meets a current of air, also
heated by the aid of the waste heat of the furnace. Hot gas and hot
air burn 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
accomplished 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 excess of air, such as is required when solid fuels are to
be burnt completely. For this reason much higher temperatures 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 combustion 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, 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 (CH_{4}), 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 oxide 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 temperatures--a proceeding that would have been impossible
if it had been attempted to burn these fuels directly in the furnace.

[Illustration: FIG. 3.--Diagram of the arrangements of a regenerative
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 types 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 regenerative furnaces the hot products
of combustion, after leaving the furnace chamber proper, and before
reaching the chimney, 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 sufficiently 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
regenerator chambers before entering the furnace chamber 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 undergoing 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 interchange 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. 3.

In recuperative furnaces the same principle is utilised in a somewhat
different manner; the outgoing products of combustion pass through
tubular channels formed in fire-clay blocks, while the 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 relative merits of the two systems have been hotly contested;
the regenerative system has the advantage of avoiding all reliance on
the heat conductivity of fire-clay, while it also avoids the somewhat
complicated special tubular blocks required for the other system; on
the other hand, the recuperative system avoids the necessity for all
“reversing” valves and their regular periodical working, while it also
occupies somewhat less space. Temperatures sufficiently high for all
glass-melting purposes can be attained by both means.

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
being allowed to mix just before they enter the furnace chamber proper.
The economy and efficiency of the furnace depend to a very great
extent upon the manner in which this mixing is accomplished. Rapid and
complete mixing of air and gas results in an intensely hot, but short
and local flame, while slower mixing tends to lengthen the flame and
spread the heat through the entire furnace chamber; on the other hand
if the mixing of gas and air is too slow, combustion may not have been
completed in the short time occupied by the gases in passing through
the furnace, and combustion may either continue in the outflow flues
and regenerators, or it may be prevented by the narrowness of these
passages, and unburnt gases may pass to the chimney. When the openings
or “ports” are properly proportioned, and the draught of the chimney
is properly regulated, combustion should be just complete as the gases
leave the furnace chamber, and under these circumstances small tongues
of keen flame will escape from every opening in the furnace; large
smoky flames issuing from a gas-fired furnace indicate incomplete
combustion.

[Illustration: FIG. 4.--Sectional diagram of a regenerative pot furnace
working with covered pots.]

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 closed or “covered”
crucibles is shown in Fig. 4. 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 column, 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 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 uniformity of heat distribution
in a gas-fired 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 expansion
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 Fig. 5.

[Illustration: FIG. 5.--Diagram of a furnace with “horse-shoe” flame.]

In general arrangement, a tank furnace for glass melting resembles an
open-hearth steel furnace. The tank or basin, as already indicated,
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 level of the glass, the whole being covered by a vault
built of silica brick. Figs. 6 and 7 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 regenerative 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 regenerative
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 is a very
general similarity between various furnaces of this type.

[Illustration: FIG. 6.--Longitudinal sectional diagram of tank
furnace.]

[Illustration: FIG. 7.--Transverse sectional diagram of tank furnace,
showing regenerators and gas and air passages.]

It is beyond the scope of this book to discuss the relative merits
of tank and pot melting furnaces; wherever the former can be made to
produce glass of adequate quality for the purpose desired, the great
economy of the tank furnace inevitably carries all before it, so that
bottle glass, for example, is now made exclusively in tanks, and
the same applies also to rolled plate of the ordinary kind, and to
the great majority of sheet glass. On the other hand, where special
qualities of glass are required in relatively small quantities, or
where the requirements as to quality are very extreme, the pot furnace
remains indispensable. Optical glass and coloured glasses are examples
of this kind, although some tinted glasses are used in sufficient
quantity to justify the use of small tank furnaces for their
production. The causes of the greater economy of the tank furnace
are numerous, and complicated by the detailed requirements of each
particular manufacture, but the most important factors in the question
may be summed 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 surface 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 and
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 or the ladle.

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 to effectually melt together materials which, in the open
basin of a tank, could not be kept together long enough to combine.



CHAPTER V.

THE PROCESS OF FUSION.


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 depends 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 grinding.

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 thoroughly 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 attempted 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 counterbalanced 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 pot 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 defective crucibles, is, however,
unavoidable, 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 regenerators 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 decreasing 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, while 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 furnace. 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 difficult that it has not yet
been carried out at all fully. 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 carbonate
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 carbonate of lime much in the
same manner as, for instance, hydrochloric 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 degree
of mixing attained in the preparation of the raw materials. Although
in the long run 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 expulsion of
all carbonic acid from the compounds present (although it may remain
entangled in the glass in the form of bubbles) 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
carbonate 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 in
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
circumstances. The action of the nitrates is chiefly dependent upon
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 would result in the complete blackening
of the glass.

A much more complicated set of reactions occur when the alkali 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 amount of carbon in
the form of coke, charcoal or anthracite coal, is introduced 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 takes place. The rationale of the whole process lies, no
doubt, in the fact that sulphite of soda (Na_{2}SO_{3}) is much more
readily decomposed by the action of hot silica than the sulphate
(Na_{2}SO_{4}) 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 amount 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 reactions is
again a mixture of silicates, but a certain amount of undecomposed
sulphate is always found in such glasses, while gaseous oxides of
sulphur escape from these furnaces in considerable quantity. Under
exceptional circumstances the glass may even contain sulphides of soda
or of lime, and sometimes even suspended carbon, but these are abnormal
constituents and result in the serious discolouration 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 conducted
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 off 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 thoroughly, 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 requirements 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 regarded 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, however, is a point where
the melter’s experience 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 skin 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 furnaces the fine
glass flows into the working chamber of the tank which is always kept
at the working temperature.



CHAPTER VI.

PROCESSES USED IN THE WORKING OF GLASS.


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 purposes to be discussed in connection with the special product
in question.

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 of physical properties somewhat
resembling those of glass, we may take thick treacle and suppose it
confined 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 from the fact that a ladle cannot 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 contents of the ladle and the glass remaining in the furnace are
contaminated. 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 the ladle is very considerably chilled by contact
with the cold metal, and is thus too stiff 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.

The general process of rolling requires little treatment here. Two
essentially different processes are used; in one the glass is thrown
on a flat table and rolled out by a moving roller passing along the
table; in the other the glass passes between two moving rollers, 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 level or free from irregularities. It has been found that it is
quite impossible to prevent these irregularities, 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
be made _perfectly_ 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 used 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 or
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 removed 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 such great variations of
temperature. In 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 outflowing glass, such materials as fire-clay leading to
contamination from detached fragments, while chilled metal leads to
local chilling of the glass. Unless, therefore, the various processes
of drawing glass into sheets direct from the furnace undergo very
material improvement, the laborious process of gathering is likely to
retain its importance even in the production of such large objects as
sheets of window glass.

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 stiff 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 introduction 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 exposure to heat involved for the operator all tend to increase
the cost of such work. Mechanical aids have been invented, and some of
these are in actual use, but they are chiefly confined to mechanism
for relieving the operator of the great weight of the gathering in its
later stages.

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 mouthpiece 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. The first stage of all blowing processes
consists in the formation of a hollow sphere by blowing 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 tools, 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 mould from which the shape of the finished
bottle is thus directly derived.

The art of the blower further takes the fullest advantages 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 pressure 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 unaided 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 use of compressed air for blowing large or heavy
articles; the pressure available 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 under 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 accustomed to the working of the plant the results achieved
are decidedly 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 producing. 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, and 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 processes
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 generating
the requisite internal pressure. This use of the expansive force of
steam generated by the heat of the hot glass body has received great
development at the hands of Sievert in Germany, whose process is
described in Chapter VII.

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 involved 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 mould by means
of an internal plunger which is forced 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 characteristic
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 majority
of cheap and useful glass articles of domestic use have been made
available by the development of the pressing industry.

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 requirements of quality are not so high are
sometimes pressed direct from small gatherings taken from the molten
glass in the crucible.



CHAPTER VII.

BOTTLE GLASS.


Although bottles are in some respects the cheapest and crudest products
that are 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 ingredients
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 bottles shall
be strong enough to resist the internal pressure which may come upon
them when used for the storage of fermented or effervescent liquors as
well as the shock of ordinary use, while the glass itself must possess
sufficient chemical resistance to remain unattacked by the more or less
corrosive liquids which it is called upon to contain. Further, from
the point of view of the bottle manufacturer it is desirable that the
glass shall be readily fusible, easily worked, and easily annealed.
In other branches of glass manufacture 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 fusible, workable glass containing little alkali.
Such alkali as bottle glass does contain is frequently derived from
felspathic minerals, which generally also 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 for 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 the furnace 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 the furnace 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
what is probably a stage of transition. Up to the middle of last
century the processes in use were little better than those of the
middle ages; the first step of 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
inventions have been put forward with the aim of producing bottles by
entirely different and wholly mechanical processes with the object of
eliminating 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 manufacturing processes, the machine-made article
will ultimately supersede the hand-made product. Even now, mechanical
processes are largely in use both in America and Europe, and at some
recent exhibitions machine-made bottles have been shown which in every
point of quality were 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 to 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 breath, 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 stiff 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 gathered. 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 the 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 almost
universal. 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 thickening in an
accurate and rapid manner.

Important 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 affect the
economy of the process 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. At the present time, however,
there are a number of bottle-works actually at work producing bottles
by mechanical means; one of the most successful of these machines is
that devised by Boucher, of Cognac. The products of this machine,
exhibited in Paris at the exhibition of 1900, were equal, and possibly
superior, to the best hand-made bottles. The Boucher machine, although
by no means entirely automatic, requires no highly-skilled labour
beyond that of a workman whose duty it is to operate the various
levers of the machine at the right instant and in the proper order.

The details of the machine, as set forth in the patents and other
published descriptions, are somewhat complicated, and vary somewhat
in the different models; the general principle and mode of operation
is, however, the same in all varieties of the machine, and we shall
therefore give a brief account of it here.

In the Boucher process, the glass is first gathered from the furnace,
but as no blowing-pipes are required, the gathering is done on a light
iron rod, thus saving the gatherer much of the labour of carrying
the heavy pipes. The requisite quantity of the glass so gathered is
then dropped into the first or “measuring” mould of the machine, the
“thread” being cut by hand by the operator. From the measuring mould,
the glass is next caused to pass into the “neck” mould; the glass
flows into this mould, and is further pressed into it by the aid of
compressed air, applied above the free surface of the glass. At this
stage the still liquid glass has the external shape of the neck of
the bottle, but the mass of glass is solid, _i.e._, no cavity has yet
been produced in it. The formation of the cavity is next begun by the
action of a plunger which is driven into the “solid” mass of glass
filling the neck mould, this plunger thus punching out the passage
through the neck of the bottle. As soon as the plunger is withdrawn,
compressed air is admitted into the cavity so formed, and the mass
of glass is at the same time inverted, and that part occupying the
position of what is to be the shoulder of the bottle is allowed to
descend while being blown out by the compressed air. This process of
distension is limited, and the desired shape is imparted to the mass
by bringing towards it a third mould, by contact with which the glass
is considerably stiffened--a row of jets of compressed air, impinging
on the outside of the glass forming the shoulder of the bottle, being
further used to stiffen the glass, once the requisite extension has
been attained. The mass has now a shape very similar to that known
as a “parason” in hand bottle-blowing, and is by this time decidedly
stiff. It is now introduced into the finishing mould and is blown into
perfect contact with the mould by powerful air-pressure, thus attaining
the proper shape of barrel and base; the indentation of the base is,
however, sometimes produced on a separate machine or press. During
all these operations the neck of the bottle, which was the first part
to be formed, has remained firmly held in the neck mould, and all the
movements that have been described are performed by means of levers
actuating movements of this mould as a whole, which, of course, carry
the glass with them. The last movement of the levers, which releases
the bottle from the finishing mould, also opens the neck mould, and
thus leaves the bottle finished and entirely free.

It will be seen that the process adopted in this machine follows as
closely as possible the various stages of hand blowing, but that the
mechanical movements of the machine replace the laborious and difficult
technique of the blower. One such machine is capable of producing
as many as 120 bottles, each weighing 1¾ lbs., per hour, but this
is accomplished only by having some of the moulds in duplicate and
so arranged as to come into use alternately. The machine itself is
attended by one “moulder,” who operates the levers, and by a youth,
who carries the finished bottles to the annealing kiln, while, of
course, the services of a gatherer are also required. The appearance of
a bottle works equipped with these machines is in striking contrast to
that of a hand-blowing works, where the stages around the working-holes
are crowded with men doing arduous work under very severe conditions of
temperature and atmosphere. Finally, it must be pointed out that the
use of the Boucher machine is by no means confined to the production of
the cheapest kinds of bottles, but that it has shown itself especially
well suited to the production of champagne and other bottles that
are required to withstand a high internal pressure, the machine-made
bottles showing excellent results under pressure tests. The machine
is also used for the production of moulded glass-ware of white glass,
since it can be adapted to the production of any kind of glass vessel
that can be produced by blowing into a mould.

The annealing of bottles was formerly carried out in large chambers or
kilns of very simple construction, in which the bottles were stacked as
made, the kiln being previously heated to the requisite temperature:
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 unloaded 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 kiln 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
used 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 vapourised 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 mechanical 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.

A process of producing hollow glass vessels of very large size by
purely mechanical means has, however, been introduced during recent
years by P. Sievert, of Dresden. By the methods of this inventor,
glass vessels of quite unprecedented size--such as bath-tubs freely
accommodating 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 numerous small holes through which steam or compressed air may be
blown when desired. The slab of viscous glass, when properly spread
over this 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 against 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 upwards into the mould, ultimately forcing
the glass 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.



CHAPTER VIII.

BLOWN AND PRESSED GLASS.


In many ways very similar to the processes employed in the production
of bottles are those used in the manufacture of all hollow 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 immaterial, all kinds of fusible materials
can be utilised, loamy or ferruginous sands and refuse glass of all
kinds being employed. Where somewhat higher requirements 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.
In these high-class products two very distinct kinds of glass are
met with. One class, of which the Bohemian “crystal” is the highest
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
scarcely met with in the production of the better qualities of hollow
glass-ware; medicine bottles and other articles of moderate quality
might be produced in tanks, but the quantity of glass required for
such purposes is seldom large enough to justify such large plant. For
the best qualities of colourless glass-ware, however, the tank-furnace
could not be used on account of the fact that both as regards colour
and freedom from defects, the product of a tank-furnace is never equal
to the best product of pot-furnaces. For flint-glass, indeed, _covered_
pots or crucibles must be used in order to adequately protect the
molten glass from the reducing action of the furnace gases and from
contamination by dust. The materials of which the pots are constructed
are also chosen with a view to avoiding all risk of introducing
colouring or otherwise injurious impurities from that source.

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 end 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 used 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 the same
applies to medicine bottles, lamp chimneys, and the bulbs for electric
light; in connection with lamp-chimneys 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 subsequently cut off.

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 typical 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 off,
pressing in, and distending the glass as required, a flat board and
a stone or metal plat 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 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 bulb 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 shoulder 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 well above the shoulder and annealed in this shape.
Subsequently the glass is finished in a trimming room or workshop by
being cut off at the desired point and having the rough edge rounded
off by the aid of a blowpipe flame. The cutting-off 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 sometimes replaced by slightly grinding and polishing
the edges.

[Illustration: FIG. 8.--Sectional diagram of the evolution of a
tumbler.]

The evolution of an ordinary tumbler, as just described, and as
illustrated diagrammatically in Fig. 8, is typical of the whole process
of hollow-glass blowing, but of course the number of operations, 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 choice of colour and ornamentation 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.

In this connection mention should perhaps be made of the application 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 explained
in Chapter XI., 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 X. and this process lends itself to a
variety of manipulations resulting in the distribution of the coloured
layer of glass in almost any desired manner over the object in hand.
The principal objection to this process, however, lies in the fact that
pots of molten glass of all the colours 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
manipulations 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 condition. 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 by 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
assuming 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 tongs 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 skilful 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 they do not possess the extreme regularity
of size and shape which are associated with machine-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 variability 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
hand-work.

In the description given above of bottle-blowing by hand we have
already seen an example of the use of moulds in aiding 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 remains a closed vessel,
and is only opened and trimmed 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 mould
results in a certain crinkling or roughening of the glass, 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 into 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 for the purpose, 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 described.

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 mould 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
manufacture 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 could not
possibly be introduced when finished. The process of pressing being
a purely mechanical one and requiring no very elaborate plant and
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
mould 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 interstice, and the traces of these fins will always remain
visible on the finished article; the very perfect fit required to
entirely prevent the formation 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 are generally of the hand-lever type;
power presses could no doubt be used, but it is contended that the
hand-press has a very great advantage in allowing the operator to judge
by touch when sufficient pressure has been exerted, and this is an
important consideration, since an excessive pressure would either force
the glass out of the mould altogether or would be liable to burst or
injure the mould seriously. The actual presses consist of vertical
guides and levers for controlling the movement of the plunger and a
table for holding the moulds, and in some cases a system of cranks and
levers for opening and closing the moulds. The process of pressing is
exceedingly simple. The proper quantity of glass is gathered from the
pot on a solid rod and dropped into the mould. The thread of glass
which remains between the glass in the mould and that remaining on
the iron is cut off with a pair of shears, and then the plunger is
lowered into the mould and allowed to remain there until the glass
has stiffened sufficiently to retain its shape, when the plunger is
withdrawn. In this proceeding 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
sufficiently plastic to readily adapt itself to the configuration of
the mould. 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 difficulty with
which the glass-pressing process has to contend. It is overcome to
some extent by the aid of the reheating process described above; but
this is only a partial remedy, and in the majority of pressed glass
products the surface is “covered” as far as possible by the application
of relief decorations such as grooves, spirals, and ribbings. An
attempt is sometimes made to imitate the appearance of cut glass,
but the rounding of the angles during the reheating process destroys
the sharpness of the effect and allows of the ready detection of the
imitation, while the cheapness of the decoration when applied in the
mould has frequently led manufacturers to grossly over-decorate, and,
therefore, destroy all claim to beauty in their wares.



CHAPTER IX.

ROLLED OR PLATE-GLASS.


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
always 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 that can be produced by
processes that involve rolling, and have led to the somewhat curious
result 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 importance, 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
the glass during the rolling process; thus 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 effect.

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 it admits to the building
where the glass is used. On the ground of cheapness it will be obvious
from what we have said above (Chapter IV.), that such glass can
only be produced economically in large tank furnaces, and these are
universally 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 withdrawn 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
where 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 means of handling the raw materials and of
charging them into the furnace are therefore adopted wherever possible.

The glass is withdrawn from the furnace by means of large iron ladles.
These ladles are used of varying sizes in such a way as to contain the
proper amount of glass to roll to 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 overhead 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 a “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 overhead
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 flame generally issues from
the working aperture, whence it is drawn upward under the hood of the
furnace. 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
distinctly different methods are employed. In one, only the perfectly
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 the entire 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
preponderating advantage.

[Illustration: FIG. 9.--Rolling table for rolled plate-glass.]

The rolling table used in the manufacture of rolled plate is
essentially a cast-iron slab of sufficient size to accommodate the
largest sheet which is to be rolled; over this slab moves a massive
iron roller which may be actuated either by hand or by mechanical
power--the latter, however, being now almost universal. 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. 9. 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 under 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 uniformly down
the length of the tunnel. 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 into recesses cut to receive them. At regular
intervals 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 forward 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 by 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 lining, which introduces small lumps of partially melted
fire-clay into the glass. In a rolled plate tank furnace which is
properly constructed and worked, the percentage of sheets which have to
be cut up on account of such enclosures should be very small, at all
events until the furnace is old, when the linings naturally show an
increasing tendency to disintegrate.

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; this modification
consists in engraving upon the cast-iron plate of the rolling table in
intaglio any pattern that is to appear upon the glass 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
very considerable. Further, as these tables and their bed-plates are so
very heavy, they are 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 Fig. 10, which represents the sectional elevation of
the appliance. After leaving the rolling machine, the course of the
“figured rolled plate” produced 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.

[Illustration: FIG. 10.--Sectional diagram of machine for rolling
“figured rolled” plate-glass.]

Figured rolled glass, being essentially of an ornamental or decorative
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 manufacturers; 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 underlying the
production of coloured glass will be found in Chapter XI.

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
process.

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, limestone, 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 smaller “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 brunt 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 expenditure 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 or cradles. 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
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
diagrammatically in Fig. 11, which shows sectional views of the sheets
before and after grinding on an exaggerated scale.

[Illustration: FIG. 11.--Sectional diagram illustrating waste of glass
in grinding curved or irregular plate.]

While it is evident that careful 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, and 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 sufficiently 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 examination saves the laborious
polishing of much useless glass.

The process of grinding and polishing plate-glass consists 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 processes
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 account 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 machinery.

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 uneven 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 result.
The most perfect adhesion between glass and plaster is attained by
spreading the paste of plaster on the up-turned surface of the slab of
glass and lowering the iron bed-plate of the grinding table down upon
it, the bed-plate with the adhering slab of glass being afterwards
turned 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 consisting 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, however, 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 grinding and smoothing processes
are complete, the table with its attached glass is moved so as to come
beneath a superstructure carrying the polishing rubbers, and the whole
is then elevated so as to allow the rubbers to bear on the glass.

The earliest forms of grinding 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 on 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 mechanism. 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 smoothing
that has been attained; in favourable cases two or three hours are
sufficient to convert the “grey” surface 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 little or no 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 to gradually fill
up the pits and hollows. The part played by 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 understand 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 successful 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 great 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 patents will now be apparent. The
very large expansion of the use of plate-glass in modern building
construction, together with the steady reduction in the prices of
plate, are evidence of the success that has attended the efforts of
inventors and manufacturers in this direction.

At the present time, plate-glass is manufactured in very large sheets,
measuring up to 26 ft. in length by 14 ft. in width, and in thickness
varying from 3/16th of an inch up to 1½ 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 a
rigid process of 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 its
oblique rays reveal every minute imperfection of the glass; these
imperfections are marked with chalk, and the plate is subsequently cut
up so as to avoid the defects that have thus been detected.

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 substance,
but rather a mixture of a number of substances of different density
and viscosity. Wherever this mixture is not sufficiently intimate, the
presence of diverse constituents becomes apparent in the form of striæ,
arising from the refraction or bending of light-rays as they pass from
one medium into another of different density. Except in glass that has
undergone elaborate stirring processes, such striæ 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 inconspicuous as possible. In plate-glass
this is generally secured in a very perfect manner, and to ordinary
observation no striæ 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 striæ, generally running in fine lines parallel
with the polished surfaces of the glass. This uniform direction of
the striæ 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 manipulation
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.e._, 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 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 could 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 troubles are those actually
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
satisfactorily 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 over
shop-fronts, the counters and shelves of some shops, and even
tombstones are sometimes made of black or white polished plate. From
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 should 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, burglar 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 of these is the considerable difference 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 results 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 glass; but, as a matter of fact,
this similarity of thermal 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 introducing
wire netting into glass during the rolling 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 lessen the
adhesion between the wire and the glass. This difficulty, however, can
be overcome more readily 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. On the whole, however, wired glass is perhaps
still to be regarded as a product whose evolution is not yet complete,
and there can be no doubt that there are great possibilities open to
the material when its manufacture has been more fully developed.



CHAPTER X.

SHEET AND CROWN GLASS.


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
windows, 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 comment, the general public being,
apparently, remarkably nonobservant 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, however, made such strides that
it is now possible to obtain polished plate-glass thin enough and
light enough for almost every architectural 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 would be beyond the scope of the present
book to enter in detail into the construction of these various types
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 vault of the furnace chamber,
to the shape and position of the apertures by which the gas and air
are admitted into the furnace, and the resultant shape and disposition
of the flame, and finally to the position and arrangement of the
regenerative 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 disadvantages--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 consequently 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 contamination arising from
crystallisation or “devitrification” occurring 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 combustion, it is
now 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; the modern tendency, however, is
strongly 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 and a certain economy of fuel, 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 5
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 small combustion 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 arrangement 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 consumption.
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 type
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 underneath
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 types of regenerative
furnaces are due to the considerable dimensions of the regenerative
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 “recuperative.” In
these furnaces there is no alternation of flow, and the regenerator
chambers are replaced by the “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 gases pass out to the
chimney, while the incoming gas and air pass through the other set of
pipes. A transference of heat between the two currents of gas takes
place by the conductivity of the fire-brick, and thus the outgoing
gases are continuously cooled while the ingoing gases are heated--the
transference of heat being somewhat 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 regenerator chambers,
and to some extent it is found preferable in practice, but there
are certain disadvantages associated with the system which arise
principally from the peculiar nature of the material--fire-brick--of
which the recuperators must be 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 only effect will be to allow some
of the combustible gas to pass direct to the chimney, and at the same
time a dilution of the gases entering the furnace by an addition of
products of combustion from the waste-gas flues. This, of course, will
materially reduce the efficiency of the furnace and require a higher
fuel consumption if the temperature of the furnace is to be maintained
at its proper level. If, however, the leakage should become more
serious, a disastrous explosion might easily result, particularly if
the nature of the leakage were such as to allow the incoming gas and
air to mix in the flues. 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 regenerative furnace.

Tank furnaces for the production of sheet-glass in this country 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 difference 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 process of blowing sheet-glass in an English glassworks
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 round 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 in 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 off 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 stiff, 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
already 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 carried
out determine, to a large extent, the quality of the resulting sheet
of glass; 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 glass must be
protected from dust and dirt of all kinds, since minute 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 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 cylinder
into which the glass is to be blown. This is done by bringing the
glass into the successive shapes shown in Fig. 12, 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 final
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 already mentioned.

[Illustration: FIG. 12.--Early stages in the formation of cylinders for
sheet glass.]

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 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 movement 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, until finally the glass has assumed the form of a
cylinder of equal thickness all over, but closed with a rounded dome at
the lower end (Fig. 13). This rounded end is now opened. In the case
of fairly thin and light cylinders this is done by holding the thumb
over the mouthpiece 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
considerable 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 withdraws
the cylinder from the furnace, and holding it vertically downwards in
his pit, spins it rapidly about its longitudinal axis. The soft glass
at the lower end immediately opens out under the centrifugal 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 of
the cylinder. The final operation of straightening out the opened end
is carried out in the same way as described above for lighter cylinders.

[Illustration: FIG. 13.--Later stage in sheet glass blowing.]

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 to completely 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
for fresh use.

On the wooden rack the cylinder of glass is allowed to cool to a
certain extent, and then the remaining portion of the neck and shoulder
(see Fig. 13) 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 stiff, the thread of glass
is pushed off 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 uniform 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
coefficient 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 flattening
and annealing process. For this purpose the split cylinders are taken
to a special kiln, generally 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 slab placed in a chamber 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 kiln, which
communicates with the flattening chamber. This consists, similarly to
other continuous annealing kilns already described in connection with
other varieties of glass, of a long tunnel, 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 annealing kiln the sheets of glass are sometimes covered
with a white deposit arising from the products of combustion 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 materials 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 contamination 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 in the glass,
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” in the glass. 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 successive layers of the
gathering. “String” is a very common defect in all sheet-glass. To some
extent it may arise from want of homogeneity 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 striæ, due to want of homogeneity in the glass, are
much more common in flint glass than in the soda-lime glasses used for
sheet manufacture, but are not unknown in the latter. On the other
hand, even if the glass be as homogeneous as possible, the gatherer
can produce these striæ 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 thick viscous
layer of aluminiferous glass remains undisturbed. If, however, the
gatherer brings his pipe too near the side of the ring, the glass
will draw some of this different layer on to the gathering, and this
glass will form thick ridges and striæ 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 truly flat
sheet in the flattening kiln. Sometimes, in practice, the “cylinder” is
wider at one end than at the other, or, worse still, it is of uneven
diameter, showing expanded and contracted areas alternately. 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 are 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 throughout 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 manner as plate-glass. Here the great
thickness of glass required seems to render the blowing of long
cylinders very difficult, 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, involving the employment
of much skilled labour, and involving the production of a relatively
complicated form, viz., the closed cylinder, as a preliminary 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 ultimately
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 experimented with in
Europe, but their complete technical and commercial success has yet to
be proved; 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 solve the problem completely, in which case
they would, of course, rapidly 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 impossibility of
keeping such a narrow orifice open and at the same time regulating the
flow of glass made this proposal impracticable, although the use of
drawing orifices has been revived in one of the latest processes.

The American process, which is said to be at work under commercial
conditions, is not entirely satisfactory 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 this process an iron
ring is lowered into the bath of molten glass through an aperture
from above; the glass is allowed to adhere to the ring which is then
slowly raised by mechanical means, drawing a cylinder of glass with
it. If left to itself, such a cylinder, owing to the effects of
surface tension in the glass, would soon contract and break off, but
the American invention avoids this action by chilling each bit of the
cylinder as soon as it is formed. This is done by the aid of air blasts
delivered upon both sides of the glass as it emerges from the bath,
and it is claimed that by this means cylinders of any desired length
and diameter may be drawn direct from the bath. The obviously great
mechanical difficulties connected with these operations have probably
been overcome, but not without sacrificing much of the simplicity of
the arrangement, and the relative economy of this process as a whole,
compared with the hand process, has yet to assert itself.

The inventions of Fourcault, which are at present being developed on
the Continent by a syndicate of glass manufacturers, 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 immersed in the glass and
then slowly raised, but in this case the piece immersed is simply a
straight bar, and the aim is to draw out a flat sheet. In this case
the tendency, 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
subsidiary devices have been added. Those that have been patented
include a mechanism of linked metal rods so arranged as to be immersed
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 2½ to 3 millimetres (about 1/8 inch) being the least that is
practicable, on account of the large amount of breakage that occurs
with weaker sheets. This process, in its present stage of development,
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.

_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 manipulation
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 glass 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 “punty” (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 4 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 buildings, 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.



CHAPTER XI.

COLOURED GLASSES.


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 colourless as
possible under the circumstances. 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 yielding 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 colourless 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 sufficient 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 colour in a transparent
medium arises from the fact that the medium has different absorbing
powers for light of different colours. All transparent substances,
and certainly glass, are only _partially_ transparent: all light
waves passing through such a substance are gradually absorbed, and
the extent to which they are absorbed differs according to the length
of these waves. It always happens that for some special wave-lengths
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 colourless glasses this process, so
far as the waves of ordinary light are concerned, only goes on to a
negligibly 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._, all have a power of selective
absorption; but in the case of those which are nearly colourless in
the ordinary sense, this absorption 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 absorption 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 similar piece of 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 satisfactory answer
to this question is not, in the writer’s opinion, available 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 atoms or “ions” of these elements had
the specific power of absorbing 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
of iron compounds, for example, may be green or red according to the
state of combination 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 the broad statement that colouring substance in
glass may be roughly divided into two kinds or groups; the first
and probably the largest group are those bodies which occur in glass
in true solution, the element itself being present in the combined
state as a silicate or other such compound (borate, phosphate, 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 the same way as the colouring of water when a coloured
salt--such as copper sulphate--is dissolved in it. The second class of
colouring substances, however, behave in a different manner; they are
probably present in the glass in a state of extremely fine division,
and held not in true solution, but really in a sort of mechanical
suspension that approximates 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 colouring 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 producing
a different grouping of the minute particles, altering their size or
shape, or even obliterating them entirely by allowing 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 that the
lighter elements do not as a rule tend to the production of coloured
glasses, while the heavier elements, so far as they can be retained in
the glass in either solution or suspension, tend to produce an intense
colouring effect. 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 compounds, 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
affect 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 effects 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 to
slightly 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 understood,
the particles of suspended colouring-material--be it metallic copper or
cuprous oxide--grow in size and attain visible dimensions, appearing as
minute shimmering flakes, thus producing the beautiful substance known
as “aventurine.”

_Silver_ is never introduced into glass mixtures, the reason being that
it is so readily reduced to the metallic state from all its compounds
that it cannot be retained in the glass except in a finely-divided
form, causing the glass to assume a black, metallic appearance
resembling 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 purpose.

_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 effect 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 combination 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 effect, nor
does there seem to be any tendency for the separation of free boron.
The compounds of aluminium also possess no colouring effect, although
certain 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
fundamental 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 finely 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 Thallium_ 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 sometimes
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 facilitate 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
proportions. The sulphide of antimony, however, exerts a colouring
influence, although its volatile and unstable character renders the
effects uncertain.

_Vanadium_, 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
proportions. 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 affect the colour of glass in a variety of ways. The colouring
effects 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 conditions
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 anticipated; 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
proportionately 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 characteristic 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 mentioned 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 will produce depends
upon other factors besides the chemical composition 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 affect 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 substance, such as a piece of wood. It will thus be
seen that while manganese is a most useful element for the glass-maker,
its employment 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 colouring
ingredients for the production of what may be called “compound”
colours, the function of the manganese being to provide the “warm”
element, _i.e._, the pink or purple component, required. One of the
most important uses of manganese coming under this head is its use as
a “decolouriser.” By a “decolouriser” the glass-maker understands a
substance which can be used to improve the colour of a glass which,
from the nature of its raw materials and conditions of melting, would
have a greener colour 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 complementary colour which would, in itself, be 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 manganese 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 simultaneously
introduced into a glass mixture, the resulting colour being, 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 iron in measurable quantity, while the cheapest kinds of
glass contain considerable proportions of this element. The colouring
effects of iron have already been alluded 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 containing 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 corresponding 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, however,
much used as a colouring agent in practice, although it has been
advocated 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
approximately 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 variations 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 more 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 practically 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 indiscriminately
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 called upon to produce a large variety of
different tints, the glass-maker is also called 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
accomplished 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 is
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 coefficient 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
resulting 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 glass-ware 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 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 circumstances is, however, very limited
by the necessity of avoiding any great softening of the substratum 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
stability as compared with the glass itself.

The whole subject of painting on glass, even from the purely technical
as apart from the æsthetic 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, somewhat 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 practically
entirely free from such imperfections as air-bubbles, striæ, 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 produce the same “improvement” 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 regularities, 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 striæ, 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 desirable
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 internal
texture is rendered non-homogeneous by the deliberate introduction of
solid and gaseous impurities and by manipulations so arranged as to
leave the glass in layers of different density, which appear in the
finished glass as “striæ.” 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 art.

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 particular 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 photographers’ “dark” rooms, and for the
light-filters used for orthochromatic 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.



CHAPTER XII.

OPTICAL GLASS.


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 specifically
“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
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 familiar 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 characteristic 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
complete, 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 striæ 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 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 full of glass
is entirely free from veins or striæ. In many cases these defects
are very minute, and sometimes escape observation until 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 striæ can scarcely
escape detection. These methods depend upon the principle that a beam
of parallel light passing through a plate of glass will meet with no
disturbance so long as the glass is homogeneous, but if striæ are
present, they will cause the light to deviate from parallelism wherever
it falls upon them. Under such illumination, therefore, the striæ
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. 14.

[Illustration: FIG. 14.--Diagram of striæ-testing apparatus.

_L_, source of light; _S_, slit; _A_ and _B_, simple convex lenses;
_G_, glass under test; _E_, eye of observer. The arrows indicate the
paths of light-rays.]

_Transparency_ and _colour_ are obviously fundamentally important
properties of glass. In one sense homogeneity is essential to
transparency, 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 perfectly 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 to the glass 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 sacrificed 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 effect 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 the glass
to the velocity with which they travel through free space. 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 with a different velocity
from that with which the longer waves of red light are transmitted.
The consequence 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, to fully characterise the optical properties 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, the systematic use of the particular set of lines which is
now usually employed being due to the initiative of Abbé and Schott at
Jena, who initiated the system of specifying the optical properties of
glass in this way. The actual lines chosen are the line known as A′,
corresponding to 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 importance in the calculations for achromatic lenses is
obtained by dividing the mean dispersion into the refractive index for
the D line minus one (usually written (C-F)/(n_{D}-1)=ν). 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 ν. The following table
(taken from the Catalogue of the Optical Convention, 1905) gives a list
of optical glasses produced by Messrs. Chance, of Birmingham. This
list, although it is not nearly so long as that issued by the French
and German firms who manufacture optical glass, contains examples of
the most important types of optical glass 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.


TABLE OF OPTICAL PROPERTIES.

  --------------+------+----+------+-----------------------------------
                |      |    |      |            Partial and
                |      |    |Medium|   Relative Partial Dispersions.
                |      |    | Dis- +------+----+------+----+------+----
      Name.     |n_{D}.| ν. | per- |      |C-D |      |D-F |      |F-G′
                |      |    | sion.| C-D. |--- | D-F. |--- |F-G′. |---
                |      |    | C-F. |      |C-F.|      |C-F.|      |C-F.
  --------------+------+----+------+------+----+------+----+------+----
  Extra Hard    |      |    |      |      |    |      |    |      |
    Crown       |1·4959|64·4|·00770|·00228|·296|·00542|·704|·00431|·560
  Boro-silicate |      |    |      |      |    |      |    |      |
    Crown       |1·5096|63·3|·00803|·00236|·294|·00562|·700|·00446|·555
  Hard Crown    |1·5175|60·5|·00856|·00252|·294|·00604|·706|·00484|·554
 *Medium Barium |      |    |      |      |    |      |    |      |
    Crown       |1·5738|57·9|·00990|·00293|·296|·00697|·704|·00552|·557
 *Densest       |      |    |      |      |    |      |    |      |
    Barium      |      |    |      |      |    |      |    |      |
    Crown       |1·6065|57·9|·01046|·00308|·294|·00738|·705|·00589|·563
  Soft Crown    |1·5152|56·9|·00906|·00264|·291|·00642|·708|·00517|·570
 *Medium Barium |      |    |      |      |    |      |    |      |
    Crown       |1·5660|56·3|·01006|·00297|·295|·00709|·704|·00576|·572
  Barium Light  |      |    |      |      |    |      |    |      |
    Flint       |1·5452|53·5|·01020|·00298|·292|·00722|·701|·00582|·570
  Extra Light   |      |    |      |      |    |      |    |      |
    Flint       |1·5316|49·0|·01085|·00313|·288|·00772|·711|·00630|·580
  Extra Light   |      |    |      |      |    |      |    |      |
    Flint       |1·5333|48·5|·01099|·00322|·293|·00777|·707|·00640|·582
  Boro-silicate |      |    |      |      |    |      |    |      |
    Flint       |1·5623|47·4|·01187|·00343|·289|·00844|·711|·00693|·584
 *Barium Light  |      |    |      |      |    |      |    |      |
    Flint       |1·5833|46·6|·01251|·00362|·288|·00889|·711|·00721|·576
  Soda Flint    |1·5482|45·8|·01195|·00343|·287|·00852|·713|·00690|·577
  Light Flint   |1·5472|45·8|·01196|·00348|·291|·00848|·709|·00707|·591
  Light Flint   |1·5610|43·2|·01299|·00372|·287|·00927|·713|·00770|·593
  Light Flint   |1·5760|41·0|·01404|·00402|·286|·01002|·713|·00840|·598
  Light Flint   |1·5787|40·7|·01420|·00404|·284|·01016|·715|·00840|·591
  Dense Flint   |1·6118|36·9|·01657|·00470|·284|·01187|·716|·01004|·606
  Dense Flint   |1·6214|36·1|·01722|·00491|·285|·01231|·715|·01046|·608
  Dense Flint   |1·6225|36·0|·01729|·00493|·286|·01236|·715|·01054|·609
  Extra Dense   |      |    |      |      |    |      |    |      |
    Flint       |1·6469|33·7|·01917|·00541|·285|·01376|·720|·01170|·655
  Densest Flint |1·7129|29·9|·02384|·00670|·281|·01714|·789|·01661|·678
  --------------+------+----+------+------+----+------+----+------+----

In the table on p. 212 the first column contains the ordinary trade
names by which the various types of glass are 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 ν, while the word “crown,” originally applied
only to lime-silicate glasses, is now used for all glass having a high
value of ν. In the next column of the table are given the refractive
indices of the glasses, while the third column contains the values of
ν. It will be seen that 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 ν decreases as the
refractive index increases. The glasses which are an exception to this
rule are indicated by an *. As a matter of fact this rule applied
to all glasses that were known or were at all events commercially
available prior to the modern advances in optical glass manufacture
which were initiated by Abbé and Schott of Jena. It was Abbé’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_{D} and ν is not that of approximately simple inverse
proportionality 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 ν value of an ordinary crown glass. It would lead too far into
the subject of lens construction to explain in detail the possibility
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 telescope
objectives 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 manufactories 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 Abbé 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 to G′ give as already indicated the differences between
the refractive indices for the C, D, F and G′ lines respectively; the
smaller figures in the intermediate columns indicate the ratio of
each of these differences to the mean dispersion of the glass. 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 comparatively long-drawn out
at the red end, 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 to entirely 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
Abbé 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 ν 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. At the present time the only pair of really perfectly
achromatic glasses 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 ν values of the two glasses only differ
by 10, and at least one of these glasses is not readily obtainable in
really satisfactory optical quality. On the other hand, practically
perfectly achromatised lenses (generally known as “apochromatic”) have
been produced, especially by Zeiss of Jena, for microscopic purposes,
by the careful selection of glasses suited to each other in this
respect. Such a solution of the problem is further facilitated by the
fact that in these lenses more than two varieties of glass can be
used to neutralise one another, while a natural mineral (fluorite)
is also employed. From the glass-maker’s point of view, however, the
problem of producing a satisfactory pair of glasses capable of entirely
achromatising one another has yet to be solved.

  ---------+------+----+-------+------+------+------+------+------+-----
           |      |    |       |      | c-d  |      | d-f  |      | f-g′
    Name.  | n_{D}| ν  |  C-F. | C-D. | ---  | D-F. | ---  | F-G′.| ---
           |      |    |       |      | c-f. |      | c-f. |      | c-f.
  ---------+------+----+-------+------+------+------+------+------+-----
  Telescope|      |    |       |      |      |      |      |      |
    Crown  |1·5254|61·7|·00852 |·00250| ·292 |·00602| ·707 |·00484| ·568
  Telescope|      |    |       |      |      |      |      |      |
    Flint  |1·5211|51·8|·001007|·00297| ·294 |·00710| ·705 |·00577| ·573
  ---------+------+----+-------+------+------+------+------+------+-----

The table of optical glasses given above, although brief as compared
with the lists issued by French 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 1·49
and 1·71, and even if we admit as practical glasses such extreme
types--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 1·0, 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 conceivable.
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 1·81. The values of ν 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 physically possible to
obtain transparent substances having optical properties 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 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 composition of a glass is “forced” by the addition
of special substances intended to affect 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 them 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 sufficient size and
purity for optical purposes, and the author believes that in this
direction progress in optical materials is ultimately bound to lie[1].

    [1] See a Paper by the present author on “Possible Directions
        of Progress in Optical Glass”--Proceedings of the Optical
        Convention, London, 1905.]

In addition to possessing the requisite optical constants, a good
colour and perfect homogeneity, certain other properties are essential
in good optical glass. These are the general physical and chemical
qualities which are essential in all good glass, but especially
emphasised 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
above table 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
counterbalance 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, such fingermarks
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 affect 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 consequence. 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 dimming 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 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 degree of hardness and a low refractive index are found together.
This statement 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 subjected 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 difficult 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 mentioned 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 inadmissible, especially as it
is accompanied by small variations in the effective index of refraction
of various portions of 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 required 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.



CHAPTER XIII.

OPTICAL GLASS.


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_. From 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 introduction of pipes or
ladles would result in the contamination of the glass by striæ 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 production of
technically perfect glass. On the other hand, the manufacturer has a
certain amount of choice as to the precise form in which the various
chemical ingredients are to be introduced into the mixture, and he
makes his choice among oxides, carbonates, nitrates, 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.

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 melting
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 striæ 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 in different works. In some the old-fashioned conical
coal furnaces are still used, the disadvantages attached to their
employment being outweighed--in the opinion of the manufacturers--by
their simplicity and ease of regulation. In other works gas-fired
regenerative furnaces of the most recent type are installed, and in
these also optical glass of the highest quality can be produced. 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 pot or crucible 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 temperatures, and this it would be difficult, if not impossible,
to secure if more than one pot 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 single-pot melting furnace, a very important part of
the equipment of the optical glass works is formed by a number of kilns
or ovens 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.

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 illustrated
in Fig. 2. The optical glass pots, however, are made considerably
thinner in the wall, since they are not required to withstand 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, in fact, 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 aluminiferous 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 aluminiferous substances
is generally more viscous than 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 viscous
glass through the entire pot in the form of veins or striæ, which are
only removed during the stirring process. On the other hand, more of
this viscous 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 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 fire-clay would shrink rapidly and
the entire vessel would fall to pieces. Even under the best conditions
it is not possible to avoid the occasional 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.

As 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 accessible 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 until 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 temperature,
but before the raw materials are introduced, a small quantity of the
cullet, which has been reserved for this purpose, is thrown into 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 introduced 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 the 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 from 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 to
entirely 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 kinds 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 the lenses
and prisms made of these glasses. These bubbles are, however, very
minute and do not interfere with the optical performance of the lenses,
&c., 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 that completely outweigh 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 condition 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 adopted, 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.

The next steps in the process are those of stirring the molten glass
with a view to rendering it homogeneous and free from striæ. 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. 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. This operation is always laborious
and trying; the workman is necessarily exposed to the intense heat
radiated from the open mouth of the crucible, so that men have to
relieve each other at frequent intervals.

During the earlier stages of the stirring process the glass is very
hot and mobile, but the stirring is continued, with short intervals,
until the glass is so cold and stiff that the stirrer can scarcely be
moved in it at all, so that the work of moving the stirrer becomes
heavy towards the end of the operation. The actual amount of stirring
required varies according to the nature of the glass, and the size
of the pot or crucible in question. Some meltings are found to be
satisfactory after as little as four hours’ stirring, while for others
as much as 20 hours are required.

When the glass has stiffened 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 sometimes
withdrawn from the glass, but this is laborious, and entails dragging
a considerable quantity of glass out of the pot with 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 striæ 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
effected within the furnace itself is never very rapid, and the furnace
as well as the pot is chilled. Further when the glass has been chilled
down to a certain point this rapid rate of cooling must be arrested, as
otherwise the whole contents 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 until
the glass has cooled down approximately to the ordinary temperature, a
process that 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 bound 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 suitable 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. It will be seen
that this handling of a heavy mass of intensely hot material involves
much labour, while there is also a risk of losing the 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 more 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 treatment.

The next step of this treatment consists in moulding the rough broken
lump 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 suitable
kiln until it is soft enough to adapt itself to the shape of the
mould provided for the purpose. In 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 sufficient to allow it to cool down naturally in a
large kiln in the course of six or eight days. For 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 regulated 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 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 of
their faces or edges are ground and polished in such a way as to permit
of the examination of the glass for bubbles, striæ and other 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. In 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. In view of these
difficulties it is not surprising to find that the dimensions of our
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 metre in diameter.



CHAPTER XIV.

MISCELLANEOUS PRODUCTS.


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 to fully enumerate them all; there are, however, a certain
number of these products which, while of considerable importance in
themselves, yet do not fall readily under any of the headings of the
preceding chapters. A short space will therefore 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.

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 regulate 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 necessary 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 movement. 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 left to 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 flame is also required. For this purpose, also, glass containing
lead is not admissible, since this would blacken under the influence
of the blow-pipe flame. Soda-lime glasses rather rich in alkali are
most frequently used for these purposes; one consequence of their
chemical composition, however, is that such glass tends to undergo
decomposition when stored for any length 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 the 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 the dull, rough surface is left at the end of the
operation.

For purposes where the glass is to be exposed to high temperatures,
tubing made of so-called “hard glass” is employed. This is practically
a form of Bohemian crystal glass, the chemical composition 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 manufactured by Schott, of Jena. 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 showing 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 has now a formidable rival in the shape of vitrified
silica, which is now available as a satisfactory commercial 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 coefficient 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 care need be exercised in heating or
cooling articles 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 relatively 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 having undergone very great improvements quite recently.
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 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 substance as iron
or copper oxide is allowed to come into contact with heated vitrified
silica is, in fact, 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, undoubtedly possesses great advantages
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. Quite
recently, the Jena works have produced special glasses which are more
transparent to these ultra-violet rays than ordinary glass, but 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 employment 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 would interfere hopelessly with the performance of
the lenses. This is now overcome by the use of vitreous silica lenses,
in the case of the “ultra-violet microscope,” as made by Carl Zeiss,
of Jena. 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
vapourise and even to boil vigorously at temperatures not very far
above its melting point. Quite recently, however, 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, so that it may shortly be possible to
produce vitreous silica in large and perfectly clear blocks.

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 especially 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 manufacture 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 attenuated
form glass rod becomes glass thread or fibre; this is produced by
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, however,
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 example, in which several such dresses
were worn would soon be contaminated with innumerable fine, sharp
particles of glass which would produce an injurious effect 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 a nail or bolt to a small bead of
fused silica produced by the aid of an oxygen-fed blowpipe; 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 Gems._--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 purpose 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 XI. To a casual observer the difference in sparkle
and brilliance which arises from the difference 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 refractometer 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 scratch 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 applicable, 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 reproductions of natural gems, but which have been produced
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 understood, and the
chemistry of silicate minerals is developed, the artificial production
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 influence
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 interfere with
its use. It is, however, well known that surfaces 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 apparently
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 hammer, 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. Numerous inventors, among whom De la Bastie and Siemens
figure most conspicuously, have endeavoured to utilise these properties
of chilled glass, not exactly by endeavouring 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 these 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, including
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 purposes, 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 application 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 experience 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.

Water-glass, 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 production; 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 here.

In concluding this chapter, we wish to describe one more product of the
glassworks, 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 subsequently 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.

Lighthouse 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 purposes 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,
previously raised to the necessary degree 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 mirror 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.



APPENDIX

BIBLIOGRAPHY.


The existing literature of glass manufacture is so limited that a
complete bibliography could almost be given on a single page; in the
English language, in particular, there are exceedingly few books
and papers on the subject. 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 inaccessible works only to find that they
contain little that is of value for his purpose.


_English Books and Papers on Glass Manufacture._

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.

Glass. Articles in 9th Edition of Encyclopædia 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 Edition of Encyclopædia Britannica.
By Harry J. Powell. A brief summary of more recent developments.
Particularly valuable in reference to artistic English flint-glass.

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. Particularly interesting in connection with the subjects of
Chapters I., II., XII., and XIII. 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 properties 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 XII.).

Catalogue of the Optical Convention Exhibition, London, 1905. 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 modern optical glass
manufacture.

Old English Glasses. By Albert Hartshorne. 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.e._, lamp
work.


_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
interesting as being the work of the man who introduced optical glass
manufacture into England.

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

Le Verre et le Crystal. By J. Henrivaux. (P. Vicq Dunod et Cie.,
Paris.) A lengthy book profusely illustrated 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 æsthetical matter.

La Verrerie au XX^{ieme} Siècle. 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.


_German Books on Glass Manufacture._

Die Glasfabrikation. By R. Gerner. (A. Hartleben’s Verlag, Vienna
and Leipzig, 1897.) A concise and clear account of most of the more
important processes of glass manufacture. Very practical in character.
The information given appears to be reliable, although far from
complete.

Die Herstellung Grosser Glaskoerper and Die Bearbeitung Grosser
Glaskoerper. By C. Wetzel. (Hartleben’s Verlag, 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 specifications.

Glasfabriken 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 modern 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
throughout the technical and scientific publications of the world;
those that have proved of real interest and importance 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.



INDEX


  A.

  Abbé, 8, 10, 210, 218

  Absorption of light in glass, 32, 179

  Acid, action of, on glass, 11
    boric, action of, on glass, 11, 186
    carbonic, action of, on glass, 12
    hydrofluoric, action of, on glass, 12
    phosphoric, action of, on glass, 11

  Air, compressed, 91, 105, 117

  Alkali chlorides, use of, in glass manufacture, 41
    content of hygroscopic glass, 6
    metals, 184
    nitrates, 44, 78
    sources of, 40

  Alkaline liquids, action of, on glass, 11

  Aluminium, 51, 186

  Ammonia soda, 41

  Anastigmatic photographic lenses, 213

  Ancient windows, colours of, 16, 202

  Annealing bottles, 103
    kiln, 103
      for optical glass, 235
      for plate glass, 135
      for rolled plate glass, 127

  Anthracite coal, 42, 53, 79

  Antimony, 188

  Apochromatic objectives, 213

  Arsenic, 52, 78, 105, 117, 188

  Artificial gems, 246

  Auerbach, 22

  Aventurine, 185


  B.

  Bacteria, action of, on glass, 13

  Barium compounds, 47, 186
    crown glass, 212
    glass, 7

  Barytes, 48

  Bases other than alkalies, sources of, 45

  Beads, 244

  Behaviour, chemical, of glass, 6

  Bending plate glass, 144

  Bevelling, 145

  Black ash, 41

  Blisters in sheet glass, 160, 168

  Blocks, fire-clay, 58
    tank, 59

  Blower, sheet glass, 158

  Blower’s chair, 111

  Blowing glass, 89
    holes, 91, 161, 189
    sheet glass, 161

  Blown glass, decoration of, 114
    plate glass, 171

  Bohemian glass, 109, 240

  Boiling up, 81

  Bottles, annealing of, 103
    blowing, improvements in, 99
      machines, 100
    colour of, 96
    manufacture, furnace for, 97
    moulds for blowing, 98
    production of, by hand, 98
    raw materials for, 95
    strength of, 18

  Boric acid, 11

  Boron, 186

  Boro-silicate crown, 212

  Boucher’s bottle-blowing machine, 101

  Bricks, fire-clay, 58
    silica, 60

  Bubbles in optical glass, 230
    removal of, 81

  Burning, pots, 58


  C.

  Cadmium, 186

  Calcium carbonate, 46
    oxide, 45, 186
    sulphate, 47

  Carbon, 53, 79, 186

  Carbonate of soda, 41

  Carbonic acid, action of, on glass, 12

  Carboys, blowing of, 104

  Casting plate glass, 132

  Chair, glass-blower’s, 111

  Chalk, 46

  Chamotte, 57

  Chance, 211

  Charcoal, 42, 58, 79

  Charging furnaces, 75

  Chemical behaviour of glass, 6
    composition of glass, 5
      of optical glass, 217
    reactions during fusion, 76

  Chilled glass, 247

  Chimneys, gaslight, 23
    lamp, 238

  Chromium, colouring effect of, 190

  Cleaning of lenses, 220

  Coal, anthracite, 42, 53, 79

  Cobalt, colouring effect of, 197

  Coke, 42, 53, 79

  Colour of ancient windows, 16
    glass, 32
      theory of, 181
    optical glass, 208
    sheet glass, 167

  Coloured blown glass, 113
    glass, 178
      technical uses of, 203

  Combustion tubing, 7, 241

  Compressed air for glass blowing, 91, 105, 117

  Conductivity, electrical, of glass, 30
    thermal, of glass, 24, 29

  Copper, colouring effect of, 184
    ruby, 184, 188, 198

  Corrosion of glass, 11

  Covered pots, 56, 109

  Crown, boro-silicate, 219
    glass, 175, 211
    hard, 212, 219
    soft, 212
    telescope, 215

  Crowns, furnace, 60

  Crucibles, manufacture of, 56
    for glass melting, 54

  Crushing strength of glass, 19

  Cryolite, 52

  Crystallisation of glass, 3

  Crystals, mineral, 218

  Cullet, 74
    for optical glass, 224

  Cutting rolled plate glass, 128

  Cylinders, sheet glass, 161, 171


  D.

  Decolourisation of glass, 52, 188, 190, 193, 197

  Decoration of blown glass, 114

  Defects in rolled plate glass, 129
    sheet glass, 166

  Definition of glass, 1

  De la Bastie, 248

  Devitrification, 3, 11

  Diamond, refractive index of, 216

  Dimming of glass surfaces, 12

  Dinas bricks, 61

  Dipping of sheet glass, 166

  Dispersion of optical glass, 209
    partial, 214

  Double refraction in optical glass, 221
    rolling machine, 130

  Drawing tubes, 239

  Ductility of glass, 20

  Durability of glass, tests for, 14

  Dust, action of, on lenses, 220
    glass, 245


  E.

  Elasticity of glass, 20, 24

  Electrical properties of glass, 29

  Epinal, 39

  Etching of glass, 12

  Expansion, coefficient of thermal, 24, 25


  F.

  Felspar, 40, 44

  Fibres, glass, 245
    silica, 245

  Figured rolled plate glass, 87, 130
    cutting of, 131

  Finger-marks on lenses, 219

  Fining of glass, 81
    optical glass, 229

  Fire-clay, action of, on glass, 6
    for pots, 55
    wetting up, 57

  Fire-polish, 117

  Flashed glass, 25, 199

  Flint, 40
    boro-silicate, 212
    dense, 212, 246
    densest, 212
    extra dense, 212
    glass, 7, 49, 78, 108, 211
    light, 212
    soda, 212
    telescope, 215

  Fluorite, refractive index of, 216

  Fontainebleau, 38

  Founding of optical glass, 227

  Fourcault process, 174

  Fresnel, 251

  Furnace crowns, 60
    gas, 63

  Furnaces for bottle manufacture, 97
      glass melting, 54, 62
      optical glass, 225
      plate glass, 133
      rolled plate glass, 122
      sheet glass, 151, 170
    ports, 67
    recuperative, 66, 156
    regenerative, 66, 155
    tank, 59, 69
      economy of, 72

  Fusion, process of, 73
    temperature of glass, 5

  Freezing of glass, 2


  G.

  Gaslight, chimneys for, 23

  Gas producers, 62, 64

  Gatherer, 158

  Gathering of glass, 85, 88, 158

  Gauge tubes, 10, 18, 23, 26

  Gems, artificial, 246

  Ghosts, photographic, 16

  Glauber’s salt, 43

  Gold, colouring effect of, 185

  Grinding plate glass, 137

  Gypsum, 47


  H.

  Hardened glass, 20

  Hardness of glass, 21
    tests for, 22

  Heavy spar, 48

  Henrivaux, 19

  Hertz, 22

  Hock-bottle colour, 195

  Hohenbocka, 38

  Hollow glass-ware, 108

  Horse-shoe flame, 69

  Hydrofluoric acid, action of, on glass, 12

  Hygroscopic glass, alkali content of, 6


  I.

  Indentation modulus, 22

  Index, refractive, 216

  Insulating properties of glass, 29

  Iron, 96
    colouring effect of, 196
    oxidation of, in glass, 195

  Irregularities caused by rolling, 86


  J.

  Jena, 7, 10, 14, 26, 29, 203, 210, 213, 241


  K.

  Kelp, 40

  Kowalski, 19


  L.

  Laboratory ware, 10, 23

  Ladling glass, 85
    rolled plate glass, 124

  Lagre, 166

  Lamp-chimneys, 110, 238

  Lamp-work, 240, 244

  Large vessels, production of, 105

  Lead, 49, 183, 188

  Lear for rolled plate glass, 127
    sheet glass, 165

  Leighton, 39

  Lenses, cleaning of, 220
    finger-marks on, 220
    pressing small, 94

  Light, action of, on glass, 15

  Lighthouse glass, 178, 250

  Lime, slaked, 45

  Lime-stone, 46

  Limited range of vitreous bodies, 4

  Lippe, 38

  Lynn, 39


  M.

  Machines, bevelling, 145
    double rolling, 130
    grinding, 139
    polishing, 141

  Magnesia, 48, 186

  Manganese, 15, 52, 80

  Mangin mirrors, 252

  Marver, 111

  Massive glass, 249

  Mechanical properties of glass, 18

  Metal, attachment of, to glass, 26

  Minerals, crystalline, 217

  Mirrors, 145
    searchlight, 251

  Mixing of materials, 73

  Moulds for glass-blowing, 90, 110, 116
    pressed glass, 119

  Muffled glass, 172

  Muranese glass, 123


  N.

  Nickel, 96
    colouring effect of, 197
    steel, 27, 148

  Nitrates, alkali, 44, 78


  O.

  Objectives, apochromatic, 213
    telescope, 218

  Opal glass, 45, 52, 186

  Opaque plate glass, 146

  Open pots, 56

  Optical glass, annealing, 235
      chemical composition of, 217
      cooling of, 233
      cost of, 237
      fining, 229
      founding, 227
      furnaces for, 225
      hardness of, 220
      moulding, 235
      pressing, 93
      range of, 216
      raw materials for, 223
      sorting, 235
      stability of, 219
      strain in, 221
      stirring, 231
      yield of, 236
    properties of glass, 205


  P.

  Painting on glass, 201

  Parason, 102

  Patent plate glass, 171

  Paving stones, glass, 249

  Pearl ash, 43

  Phosphoric acid, 11

  Phosphorus, 188

  Photographic ghosts, 16
    lenses, anastigmatic, 213
      colour of, 209

  Pipe, glass-maker’s, 89
    sheet-blower’s, 158
    warmer, 158

  Plate glass, annealing kiln for, 135
    bending of, 144
    blown, 171
    casting, 132
    colour of, 33
    figured rolled, 87
    flatness of, 134
    furnaces for, 133
    grinding machines, 139
      of, 137
    mirrors, 145
    opaque, 146
    polishing machines, 141
    polishing of, 137
    raw materials for, 132
    rolled, 86, 123
    silvering, 146
    sizes of, 143
    strength of, 15
    striæ in, 143
    wired, 27, 147

  Platinum, 27

  Polishing, theory of, 141

  Pontil, 98, 176, 239

  Potash, 43

  Potato, use of, in glass melting, 81

  Ports, furnace, 67

  Pots, burning of, 58
    covered, 56
    drying of, 58
    for flint glass, 109
      optical glass, 226
    manufacture of, 56
    open, 56

  Pouring of glass, 85, 87

  Pressed glass, 92, 118
    composition of, 120

  Presses for glass, 119

  Proofs, 82, 231

  Purity of materials, 36


  Q.

  Quartz, 40


  R.

  Range, limited, of vitreous bodies, 4

  Recuperative furnaces, 66, 156

  Red lead, 49

  Refraction, double, in optical glass, 221
    of light in optical glass, 209

  Refractive index, 216

  Regenerative furnace, 66, 155

  Reichsanstalt, 10

  Resistance to crystallisation of glass, 4

  Rings for lighthouse lenses, 251

  Rod, glass, 245

  Rolled plate glass, 86, 123
    annealing, 127
    cutting, 128
    defects of, 129
    figured, 130
    furnaces, 123
    ladling, 124
    raw materials for, 124
    rolling, 126
    sorting, 129
    surface of, 122

  Rolling of glass, 86

  Rubies, artificial, 247

  Ruby, copper, 184, 188, 198
    flashed, 184
    gold, 185

  Rupert’s drops, 248


  S.

  Salt-cake, 37, 42, 79, 189

  Sand, 38

  Sandstone, 39

  Schott, 8, 19, 203, 213, 241

  Scratches on sheet glass, 169

  Searchlights, 250

  Seed in sheet glass, 167

  Selenium, colouring effect of, 190

  Sheet glass, 70
    blisters in, 160, 168
    blowing, 161
    colour of, 33, 167
    compared with plate, 149
    cylinders, 161, 171
    defects of, 166
    dipping, 166
    flattening, 165
    furnaces, 151, 170
    lear, 165
    mechanical production of, 173
    raw materials for, 150
    sorting, 166
    splitting, 164
    strength of, 18

  Siedentopf, 182

  Siege blocks, 59

  Siemens, 248

  Sievert, 92, 105, 117, 172
    processes, 105, 117

  Signal glasses, 203

  Silica bricks, 61
    glass, 5, 26, 241
    sources of, 37

  Silicon, colouring effect of, 187

  Silver, colouring effect of, 185

  Silvering plate glass, 146

  Sizes of plate glass, 142

  Soda ash, 41
    carbonate, 41
    sulphate, 37, 42, 79
    sulphide, 80
    sulphite, 79

  Solidification of glass, 1

  Solutions, analogy of, with glass, 206

  Sorting rolled plate glass, 129

  Specific heat of glass, 25, 29
    inductive capacity of glass, 29

  Stains, coloured, 200

  Stassfurth, 44

  Stones in rolled plate glass, 129
    sheet glass, 167

  Storage of materials, 37

  Strain in optical glass, 221

  Strength of glass, 19

  Striæ in coloured glass, 203
      optical glass, 206, 227
      plate glass, 143
    testing apparatus, 207

  String in sheet glass, 168

  Strontium, 86

  Structure of glass, 1

  Sulphur, colouring effect of, 189

  Surfaces, chemical behaviour of glass, 8, 10

  Szigmondi, 182


  T.

  Table, rolling, 126

  Tank blocks, 59
    furnaces, 59, 69
      economy of, 72
      for sheet glass, 152

  Telescope objectives, 213

  Temperature of fusion of glass, 5

  Tempered glass, 20, 248

  Tensile strength of glass, 19

  Thallium, 183, 188

  Theory of colours in glass, 181
    polishing, 141

  Thermal endurance of glass, 23
    properties of glass, 23

  Thermometer glass, 7, 8, 28

  Tin, colouring effect of, 187

  Tonnelot, 7

  Transparency of glass, 31
    optical glass, 208

  Trautwine, 19

  Tubing, 238
    combustion, 7
    drawing of, 239

  Tumblers, 111


  U.

  Ultra-violet microscope, 243


  V.

  Vanadium, colouring effect of, 189

  Veins in optical glass, 206, 227


  W.

  Water, action of, on glass, 10
   glass, 250

  Wetting up clay, 57

  Winkelmann, 19

  Wired plate glass, 27, 147

  Witherite, 48

  Wool, glass, 245


  Y.

  Young’s modulus, 20


  Z.

  Zaffre, 197

  Zeiss, 213, 244

  Zinc, colouring effect of, 49, 186

  Zschimmer, 14


BRADBURY, AGNEW, & CO. LD., PRINTERS, LONDON AND TONBRIDGE.



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_NOW READY._

  =INDIA-RUBBER AND ITS MANUFACTURE=, with Chapters on Gutta-Percha
      and Balata. By H. L. Terry, F.I.C., Assoc. Inst. M.M.

  =ELECTRIC POWER AND TRACTION.= By F. H. Davies, A.M.I.E.E.

  =LIQUID AND GASEOUS FUELS, AND THE PART THEY play in Modern Power
      Production.= By Professor Vivian B. Lewes, F.I.C., F.C.S.

  =COAL.= By James Tonge, M.I.M.E., F.G.S., etc. (Lecturer on Mining
      at Victoria University, Manchester).

  =THE BOOK: Its History and Development.= By Cyril Davenport, V.D.,
      F.S.A.

  =IRON AND STEEL.= By J. H. Stansbie, B.Sc., F.I.C.

  =TOWN GAS FOR LIGHTING AND HEATING.= By W. H. Y. Webber, C. E.

  =GLASS.= By Walter Rosenhain, Superintendent of the Department of
      Metallurgy in the National Physical Laboratory, late Scientific
      Adviser in the Glass Works of Messrs. Chance Bros. & Co.


_IN PREPARATION._

  =PATENTS. Trade Marks and Designs.= By Kenneth R. Swan (Oxon.), of
      the Inner Temple, Barrister-at-Law.

  =THE MANUFACTURE OF PAPER.= By R. W. Sindall, F.C.S.

  =WOOD PULP AND ITS APPLICATIONS.= By C. F. Cross, E. J. Bevan, and
      R. W. Sindall.

  =STEAM ENGINES.= By J. T. Rossiter, M.I.E.E., A.M.I.M.E.

  =PRECIOUS STONES. With a Chapter on Artificial Stones.= By W.
      Goodchild, M.B., B.Ch.

  =ELECTRIC LAMPS.= By Maurice Solomon, A.C.G.I., A.M.I.E.E.

  =STEAM LOCOMOTIVES.= By Vaughan Pendred, M.I.Mech.E.

  =GOLD AND PRECIOUS METALS.= By Thomas K. Rose, D.Sc., of the Royal
      Mint.

  =ELECTRO-METALLURGY.= By J. B. C. Kershaw, F.I.C.

  =PHOTOGRAPHY.= By Alfred Watkins, President-Elect of the
      Photographic Convention, 1907.

  =COMMERCIAL PAINTS AND PAINTING.= By A S. Jennings, Hon. Consulting
      Examiner, City and Guilds of London Institute.

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Transcriber’s Notes


Punctuation, hyphenation, and spelling were made consistent when a
predominant preference was found in this book; otherwise they were not
changed.

Simple typographical errors were corrected; occasional unbalanced
quotation marks retained.

Ambiguous hyphens at the ends of lines were retained.

The appearance of the Table of Contents has been altered slightly.

Index not checked for proper alphabetization or correct page references.

In the Index, it was unclear to the Transcriber whether some of the
indentations indicated sub-entries or implied ditto marks, so the
indentations have been treated here as multiple levels of sub-entries.





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