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.