be briefly described as consisting in obtaining a crucible full of
the purest and most homogeneous glass, and then allowing it to cool
slowly and to solidify _in situ_. From the resulting mass of glass
the best pieces are picked and moulded into the desired shape for
optical use. It will be seen at once that in this process there is
an essential difference from all others that have been described in
this book--viz., that the glass is never removed from the melting-pot
while molten, and that none of the operations of gathering, pouring,
rolling, pressing, or blowing are applied to it. The reason for this
apparently irrational mode of procedure lies in the fact that the
perfect homogeneity essential for optical purposes can only be attained
by laborious means, and can then only be retained if the glass is left
to solidify undisturbed; any movement by the introduction of pipes or
ladles would result in the contamination of the glass by striæ and
other objectionable defects.
The choice and proportion of raw materials used in the production
of any given quality of optical glass is governed by the chemical
composition which experiment has shown to be necessary to yield the
desired optical properties. The composition of optical glass mixtures
cannot therefore be varied to suit the conditions of the furnace or
to facilitate ready melting and fining, so that many of the usual
resources of the glass-maker cease to be available in the very case
where their aid would be most welcome to facilitate the production of
technically perfect glass. On the other hand, the manufacturer has a
certain amount of choice as to the precise form in which the various
chemical ingredients are to be introduced into the mixture, and he
makes his choice among oxides, carbonates, nitrates, and hydrates,
according to the behaviour that it is desired to impart to the mass
during the earlier stages of fusion. The state of purity in which the
various substances are commercially obtainable also enters largely into
the question, since the greatest possible degree of purity in the raw
materials is essential to the production of glass of good colour, or
rather freedom from colour.
Since homogeneity is so essential in the finished product, very
thorough mixing of the raw materials is necessary in the case of
optical glass, and the ingredients are for this purpose generally used
in a state of finer division than is necessary with other varieties
of glass. As a rule the quantities of mixture of any one kind that
are required are not large enough to justify the use of mechanical
appliances, and very careful hand-mixing is carried out.
Although it is quite possible to obtain successful meltings from
raw materials alone, it is preferable to mix with these a certain
proportion of “cullet” or broken glass derived from a previous melting
of the same sort. The broken glass used for this purpose is first
carefully picked over for the purpose of rejecting pieces that contain
visible impurities, although pieces showing striæ are not usually
rejected. The greater part of this cullet is generally mixed as evenly
as possible with the raw materials, but a certain proportion is
reserved for another purpose, as explained below.
The furnaces used for the production of optical glass vary very
much in type in different works. In some the old-fashioned conical
coal furnaces are still used, the disadvantages attached to their
employment being outweighed--in the opinion of the manufacturers--by
their simplicity and ease of regulation. In other works gas-fired
regenerative furnaces of the most recent type are installed, and in
these also optical glass of the highest quality can be produced. As a
rule, however, optical glass furnaces differ from other pot-furnaces
found in glass-works in this respect--that the former are usually
constructed to receive one pot or crucible only, while in other glass
furnaces from four to twelve or even twenty pots are heated at the same
time. The reason for this restriction in the capacity of the furnaces
lies in the fact that since the mixtures used for optical glass cannot
be adjusted to suit the furnace, the latter must be worked as far as
possible in such a way as to suit the mixture to be melted in it, and
this implies that every pot will require its own adjustment of times
and temperatures, and this it would be difficult, if not impossible,
to secure if more than one pot were heated in the same furnace. It is
further to be remembered that the amount of care and attention required
during the melting of a pot of optical glass is out of all proportion
to that needed with other varieties, so that little would be gained
by having a number of pots in one furnace, since several sets of men
would be required to tend them.
In addition to the single-pot melting furnace, a very important part of
the equipment of the optical glass works is formed by a number of kilns
or ovens which are used for the preliminary heating, and sometimes for
the final cooling of the various crucibles or pots. Similar kilns are
used in other branches of the industry, but in those cases the pots,
once introduced into the furnace, are expected to last for a number of
weeks, or even months. In optical glass manufacture, on the other hand,
a pot is used once only, so that fresh pots are required for every
new melting. The kilns in which these pots are heated up before being
placed in the melting furnace are thus in very frequent use. As a rule
they are simply fire-brick chambers provided with sufficient grate-room
and flue-space to be gradually raised to a red heat in the course of
four or five days, while for the purpose of gradual cooling they can be
sealed up like the annealing kilns used for polished plate-glass.
The pots or crucibles in which optical glass is melted are usually of
the same shape as the covered pots used for flint-glass as illustrated
in Fig. 2. The optical glass pots, however, are made considerably
thinner in the wall, since they are not required to withstand the
prolonged action of molten glass in the same way as pots used for
flint-glass manufacture. On the other hand, the fire-clays used for
this purpose must be chosen with special care so as to avoid any
contamination of the glass by iron or other impurities which might
reach the glass from the pot. For the production of certain special
glasses, in fact, pots made of special materials are required, since
these glasses, when molten, produce a rapid chemical attack upon
ordinary fire-clays. A certain amount of the aluminiferous material
of the pot is, in fact, always introduced into the glass by the
gradual dissolving action of glass on fire-clay which we have already
described. The glass contaminated with these aluminiferous substances
is generally more viscous than the rest of the contents of the pot, and
therefore ordinarily remains more or less adherent to the walls of the
crucible, but the inevitable disturbances which accompany the processes
of melting and fining lead to the dissemination of some of this viscous
glass through the entire pot in the form of veins or striæ, which are
only removed during the stirring process. On the other hand, more of
this viscous glass is constantly being formed so long as the glass
remains molten, and if disturbances are not sufficiently avoided during
the later stages of the process fresh veins may easily be formed.
The actual operations of producing a melting of optical glass begin by
the gradual heating-up of the pot in the kiln just described. When the
pot has reached a full red heat the doors of the kiln are opened and
the pot drawn out by means of a long heavy iron fork running on wheels;
this implement is run into the mouth of the kiln and the tines of the
fork are pushed under the pot, and the latter is then readily lifted up
and withdrawn from the kiln. Meanwhile the temperature of the furnace
has been regulated in such a manner as to be approximately equal to
that attained by the heating kiln, so that the pot, when transferred
as rapidly as possible from the kiln to the furnace, is not subjected
to any very sudden heating; were it attempted to place the new pot in
a furnace at full melting heat the fire-clay would shrink rapidly and
the entire vessel would fall to pieces. Even under the best conditions
it is not possible to avoid the occasional failure of a pot by cracking
either at this or a slightly later stage of the process. The latter
occurrence is apt to be particularly disastrous, as the pot may then be
full of molten glass, which runs out and is lost.
As soon as the empty pot has been put into place, the melting furnace
is carefully sealed up by means of temporary work built of large
fire-bricks, the whole being so arranged that the mouth of the hood of
the pot is left accessible by means of an aperture in the temporary
furnace wall. This aperture can be closed by one or more slabs of
fire-clay, and when these are removed an opening is left by which the
raw materials are introduced, and through which the other manipulations
are carried out.
When this stage of the process is reached, the wagons containing the
mixed raw materials are usually wheeled into place in front of the
furnace, but the introduction of the materials themselves into the
pot is not begun until several hours later, when the furnace has been
vigorously heated and an approach to the melting heat has been attained.
When the furnace and pot have attained the necessary temperature,
but before the raw materials are introduced, a small quantity of the
cullet, which has been reserved for this purpose, is thrown into the
pot and allowed time to melt, and then only is the first charge of
mixture put into the pot. The object of this proceeding is to coat the
bottom and part of the walls of the pot with a layer of molten glass
which serves to protect it from the chemical and physical attack of
the raw materials during the violent action which takes place when they
are first exposed to the furnace heat.
The gradual filling of the pot with molten glass is now carried out by
the introduction of successive charges of raw material; as the mixture
not only occupies more space than the glass it forms, but also froths
up a good deal during melting, the quantities introduced each time
must be carefully adjusted so as to avoid an overflow of half-melted
glass through the mouth of the pot. As the pot is more and more nearly
filled, the space left for the raw materials is proportionately
diminished, and the later charges are therefore much smaller than the
first few.
When, finally, sufficient material has been introduced to fill the pot
completely, the next stage of the process commences. When the last
charge of raw materials has melted, the glass in the pot is left in the
state of a more or less viscous liquid full of bubbles of all sizes;
it is essential that these bubbles should escape and leave the glass
pure and “fine,” and this result can only be achieved by raising the
temperature of the furnace and allowing the glass to become more fluid,
while the rise of temperature also causes the bubbles to expand owing
to the expansion of the gas contained in them. In both ways, rise of
temperature facilitates the escape of the bubbles, and the furnace
is therefore heated to the full, and this extreme heat is maintained
until the glass is free from bubbles. In the case of the more fusible
glasses the temperature required for this purpose is not excessively
high, and, indeed, in the case of these glasses care is taken to avoid
too high a temperature, as it entails other disadvantages. In the case
of the harder crown glasses, however, the difficulty lies in producing
an adequately high temperature without at the same time endangering
the life of furnace and crucible. The difficulty of freeing the molten
glass from bubbles constitutes one of the causes that limit the range
of our optical glasses in one direction--still harder glasses could be
melted, but it would not be feasible to maintain a temperature high
enough to render them fluid enough to “fine.”
In the case of other kinds of glass, again, it becomes impossible to
entirely remove the bubbles from the molten mass even when very hot and
very fluid. The exact cause is not known, but in some kinds of glass
the bubbles formed are so minute that even when the glass is perfectly
mobile the bubbles show no tendency to escape, while in other kinds of
glass there appears to be a steady evolution of minute bubbles as soon
as the temperature is raised with a view to removing those already in
the glass. As this property attaches to some of the most valuable of
the newer varieties of optical glass, opticians and the public have
learnt to put up with the presence of minute bubbles in the lenses
and prisms made of these glasses. These bubbles are, however, very
minute and do not interfere with the optical performance of the lenses,
&c., except to the extent of arresting and scattering the very small
proportion of light that falls upon them; their presence is therefore
to be regarded as a small but unavoidable drawback to the use of
glasses which offer advantages that completely outweigh this defect.
Returning to the melting process, we find that the extreme heating
required for the purpose of “fining” the glass is continued for a
considerable period of time, as long as thirty hours in some cases,
the glass being examined from time to time to test its condition as
regards freedom from bubbles. This is done by taking a small sample
of glass out of the pot and examining it to see if it still contains
bubbles. In some works this test is made by taking up a very small
gathering of glass on the end of a small pipe and blowing it into a
spherical flask; on looking at such a flask in a suitable light the
presence of even minute bubbles is readily detected. In other works a
simpler process is adopted, a small quantity of glass being ladled out
of the pot on the surface of a flat iron rod. It is allowed to cool
on the rod, and when pushed off forms a small bar of glass some eight
or ten inches long and about an inch wide; in this also the presence
of bubbles is easily detected. These test pieces are known among
glass-makers as “proofs.”
When proofs, taken as just described, have shown that the glass is free
from bubbles, the extreme heat of the furnace is allowed to abate,
and the fire-clay slabs in front of the mouth of the pot are removed.
The next step is that of skimming the surface of the glass. Since
most of the materials liable to contaminate the contents of a pot are
specifically lighter than the molten glass, they will be found floating
on the surface, and the surface glass is therefore removed with a
view to ridding the glass of anything that may have been accidentally
introduced and that has not melted and become incorporated with the
molten mass.
The next steps in the process are those of stirring the molten glass
with a view to rendering it homogeneous and free from striæ. The
stirrer used for this purpose is usually a cylinder of fire-clay,
previously burnt and heated. This is provided with a deep square hole
in one end, and it is held at first by means of a small iron bar
passed into this hole. By this means the red-hot cylinder of fire-clay
is introduced into the open mouth of the pot, and when it has attained
approximately the temperature of the molten glass it is dipped into
the glass itself, in which it ultimately floats. When stirring is to
begin, the square, down-turned end of a long iron bar is introduced
into the corresponding square hole in the upper end of the stirrer, and
by this means the fire-clay cylinder is held in a vertical position in
the glass and given the steady rotatory movement which constitutes the
stirring process. For this purpose the long iron bar just mentioned is
made to pass over a swivel-wheel, while a workman moves it steadily by
the aid of a large wooden handle. This operation is always laborious
and trying; the workman is necessarily exposed to the intense heat
radiated from the open mouth of the crucible, so that men have to
relieve each other at frequent intervals.
During the earlier stages of the stirring process the glass is very
hot and mobile, but the stirring is continued, with short intervals,
until the glass is so cold and stiff that the stirrer can scarcely be
moved in it at all, so that the work of moving the stirrer becomes
heavy towards the end of the operation. The actual amount of stirring
required varies according to the nature of the glass, and the size
of the pot or crucible in question. Some meltings are found to be
satisfactory after as little as four hours’ stirring, while for others
as much as 20 hours are required.
When the glass has stiffened to such an extent that it is no longer
possible to continue the stirring, preparations are made for the final
cooling-down of the pot of glass. The fire-clay stirrer is sometimes
withdrawn from the glass, but this is laborious, and entails dragging
a considerable quantity of glass out of the pot with the clay cylinder;
more usually, therefore, the stirrer is simply left embedded in the
glass.
The next object to be accomplished is that of cooling the glass as
rapidly as safety will permit until it has become definitely “set”--the
purpose being to prevent the recrudescence of striæ as a result of
convection currents or other causes which might disturb the homogeneity
of the glass. This rapid cooling is obtained in various ways; in one
mode of procedure the furnace is so arranged that by opening a number
of apertures provided for the purpose cold air is drawn in and the pot
and its contents chilled thereby without being moved. This method has
the advantage that the pot containing the viscous glass is never moved
or disturbed in any way, but on the other hand the cooling which can be
effected within the furnace itself is never very rapid, and the furnace
as well as the pot is chilled. Further when the glass has been chilled
down to a certain point this rapid rate of cooling must be arrested, as
otherwise the whole contents of the pot would crack and splinter into
minute fragments. Where the pot has been left in the furnace this can
only be done by sealing up the whole furnace with temporary brickwork
and lutings of fire-clay, leaving it to act as an annealing kiln until
the glass has cooled down approximately to the ordinary temperature, a
process that occupies a period of from one to two weeks according to
the size of the melting. Such enforced idleness of a melting furnace
is of course very undesirable from an economical point of view, and it
is generally avoided by adopting the alternative method of drawing
the pot bodily out of the furnace as soon as the stirring operation is
ended. For this purpose the temporary brickwork forming the front of
the furnace is broken down, and with the aid of a long crow-bar the
bottom of the pot is levered up from the bed or siege of the furnace
to which it adheres strongly, being bound down by the sticky viscous
mass of molten glass and half-molten fire-clay which always accumulates
on the bed of the furnace. The pot being temporarily held up by the
insertion of a piece of fire-brick, the tines of a long and heavy
iron fork running on a massive iron truck are introduced beneath the
pot; an iron band provided with long handles is then passed around
the pot, and the latter is then drawn forward by the aid of suitable
pulley blocks. The tines of the fork are then raised, and the pot is
wheeled out of the furnace and deposited upon a suitable support. Here
it is allowed to cool to the requisite extent, when it is again picked
up on the tines of the fork and deposited in an annealing kiln which
has been previously warmed to a suitable temperature. It will be seen
that this handling of a heavy mass of intensely hot material involves
much labour, while there is also a risk of losing the glass if the pot
should break before the glass has set sufficiently. Every care is taken
to prevent such an accident, the pot being wrapped round with chains or
otherwise supported in such a way that a small crack could not readily
develop into a large gap.
When such a melting of glass has cooled sufficiently, either in the
furnace or in the annealing kiln, to be safely handled, the whole
pot is drawn out, and the fire-clay shell, which is generally found
cracked into many pieces, is broken away by the aid of a hammer. Under
favourable circumstances the whole of the glass may have cooled intact
as one solid lump sometimes weighing over half a ton. Unless special
care is taken, however, it is more usual to find the glass more or less
fissured, a number of large lumps being accompanied by a great mass of
small fragments. These are now picked over, and all those which are
free from visible imperfections or which can be readily detached from
such imperfections by the aid of a chipping hammer are put upon one
side for further treatment.
The next step of this treatment consists in moulding the rough broken
lump into the shape of plates, blocks, or discs according to the
purpose for which the glass may be required by the optician. The
plant used for the moulding process varies widely, but in all cases
the operation consists in gradually heating the glass in a suitable
kiln until it is soft enough to adapt itself to the shape of the
mould provided for the purpose. In some cases these moulds are made
of fire-clay, and the glass is simply allowed to settle into them by
its own weight; in other cases iron moulds are used, and the glass
is worked into them by the aid of gentle pressure from wood or metal
moulding tools. In yet other cases, particularly where the glass is
required in the form of small thin discs or where it is to be formed
into the approximate shape of concave or convex lenses, the aid of a
press is sometimes invoked.
In all cases the moulding process is followed by the final annealing,
which consists in cooling the glass very gradually from the red heat at
which it has been moulded, down to the ordinary temperature. The length
of time occupied by such cooling depends very much upon the size of the
object and also upon the degree of refinement to which it is necessary
to carry the removal of small internal strains in the glass. For many
purposes it is sufficient to allow it to cool down naturally in a
large kiln in the course of six or eight days. For special purposes,
however, where perfect freedom from double refraction is demanded, much
greater refinements are required, and special annealing kilns, whose
temperature can be accurately regulated and maintained, are employed.
In these the annealing operation can be carried out so gradually that a
rate of cooling in which a fall of 1° C. occupies several hours can be
maintained, so that very perfectly annealed glass can be produced even
in discs or blocks of large size.
When removed from the annealing kiln the plates or discs of optical
glass are taken to a grinding or polishing workshop, where certain of
their faces or edges are ground and polished in such a way as to permit
of the examination of the glass for bubbles, striæ and other defects
in the manner indicated in the previous chapter. As the amount of
sorting that can be done while the glass is still in rough fragments
is necessarily very limited, it follows that a considerable proportion
of the glass which has been moulded and annealed must be rejected as
useless when thus finally examined. A yield of perfect optical glass,
amounting to 10 or at most 20 per cent. of the total contents of each
pot, is therefore all that can be expected, and smaller yields are by
no means infrequent--a consideration that will serve to explain the
relatively high price of optical as compared with other varieties of
glass.
A consideration of the various factors that are involved in the
production of a piece of perfect optical glass will make it apparent
that the cost and difficulty of its production increases rapidly with
the weight of the piece to be produced, so that it is not surprising to
find that the price of very large discs of perfect optical glass such
as those required for large astronomical telescopes, reaches figures
which become prohibitive when very large sizes are considered. Thus,
while it is quite possible to obtain say 100 pounds of good glass from
a single melting if the glass is to be used in the form of pieces not
weighing more than five or six pounds each, it is only rarely that a
single block of perfect glass can be found weighing 100 pounds. In the
former case the best pieces can be picked, the worst defects can be
eliminated by chipping the rough fragments, and at a later stage other
defective pieces can be cut off or ground away; not so where a large
single block is required. A single fine vein, perhaps too small to be
visible to the unaided eye, may be found to run through a whole block
in such a way that it cannot be removed without breaking or cutting up
the whole piece, and it will be seen that the frequency with which this
is liable to occur increases with the volume of the piece required.
The difficulties of re-heating and moulding are also increased
enormously with the size of the individual pieces of glass that have
to be dealt with, and where very large pieces have to be heated and
cooled accidental breakage becomes a serious risk. In view of these
difficulties it is not surprising to find that the dimensions of our
astronomical refractors appear to have approached their limit, but
rather are we led to admiration of the skill and enterprise that has
pushed this limit so far as to produce discs of optical glass measuring
as much as one metre in diameter.
CHAPTER XIV.
MISCELLANEOUS PRODUCTS.
The field of glass-manufacture is so wide and the number and variety of
its products so great, that in the limited compass of this volume it is
impossible to fully enumerate them all; there are, however, a certain
number of these products which, while of considerable importance in
themselves, yet do not fall readily under any of the headings of the
preceding chapters. A short space will therefore be devoted to some of
these in this place.
_Glass Tubing._--A widely-useful form of glass is that of tubes of
all sizes and shapes, ranging from the fine capillary tubes used in
the construction of thermometers to the heavy drawn or pressed pipes
that have been employed for drainage and other purposes. The process
of manufacture employed varies according to the size and nature of
the tube that is required. Thus lamp-chimneys are really a variety of
tube, used in short lengths and made of relatively wide diameter and
thin walls. These are not, however, ordinarily made in the form of long
tubes cut into short sections, but--as has already been mentioned--they
are blown into moulds in the form of a thin-walled cylindrical bottle,
whose neck and bottom are subsequently removed. By this process the
various forms of chimneys for oil-lamps, having contractions at
certain parts of their length, can be readily produced.
The articles more strictly described as glass tubes are, however,
produced by a process in which actual blowing plays only a very minor
part. A gathering of suitable size is taken up on a pipe, a very small
interior hollow space is produced by blowing into the pipe, and then
the gathering is elongated by swinging the pipe in a suitable manner.
The end of the elongated gathering furthest from the pipe is then
attached to a rod or “pontil” held by a second workman, and the two men
then proceed to move apart, drawing out the gathering of glass between
them. According to the bore and thickness of wall required in the tube,
the men regulate the speed at which they move apart; the thinner the
tube is to be the more rapidly they move, in order to draw the glass
out to a sufficient extent before it hardens too much. The rate of
drawing must, of course, also be adapted to the nature of the glass in
question, and this will vary very widely. For the production of the
smaller bored tubes the men find it necessary to separate at a smart
trot, while heavy tubes such as are used for gauge-glasses, are drawn
of hard glass by a very gradual movement. In some cases, the setting
of the glass, when the tube has attained the desired thickness, is
hastened by the aid of an air-blast, or--in more primitive fashion--by
boys waving fans over the hot glass. In any case, suitable troughs are
provided for receiving the tube when drawn, and from these the tube is
taken to an annealing kiln to undergo this necessary operation.
The glass used for the production of tubing varies very widely
according to the purpose for which the product is intended. Almost
any of the more usual varieties of glass can be readily drawn out
into tubes, and the choice of the kind of glass to be employed is
therefore left to other considerations. Tubing required for the use of
the lamp-worker, _i.e._, for the production of instruments or other
articles by the aid of the glass-blower’s blow-pipe, must have the
capacity of undergoing repeated cooling and heating without showing
signs of crystallisation (devitrification), while reasonable softness
in the flame is also required. For this purpose, also, glass containing
lead is not admissible, since this would blacken under the influence
of the blow-pipe flame. Soda-lime glasses rather rich in alkali are
most frequently used for these purposes; one consequence of their
chemical composition, however, is that such glass tends to undergo
decomposition when stored for any length of time, more especially in
damp places. Frequently this decomposition only manifests itself on
heating the glass in a flame, when it either flies to pieces or turns
dull and rough on the surface. Such glass is sometimes said to have
“devitrified,” but this is not really the case; what has actually
happened is that the atmospheric moisture has penetrated for some
little distance into the thickness of the glass, probably hydrating
some of the silica; on heating, this moisture is driven off, with the
result that either a few large cracks, or innumerable fine ones, are
formed. In the latter case these do not readily disappear when the
glass is softened and the dull, rough surface is left at the end of the
operation.
For purposes where the glass is to be exposed to high temperatures,
tubing made of so-called “hard glass” is employed. This is practically
a form of Bohemian crystal glass, the chemical composition being
that of a potash-lime glass rather rich in lime. To some extent this
Bohemian hard glass has been superseded by the special “combustion
tube” glass manufactured by Schott, of Jena. This is a very refractory
borosilicate glass containing some magnesia; it certainly withstands
higher temperatures than hard Bohemian glass, and is rather less
sensitive to changes of temperature; on the other hand, it has the
inconvenient property of showing a white opalescence when it has once
been heated, and this, after a time, renders the glass completely
opaque.
For many purposes, where heat-resisting qualities are chiefly required,
ordinary glass has now a formidable rival in the shape of vitrified
silica, which is now available as a satisfactory commercial product.
This substance offers the great advantage that for most ordinary
purposes it may be regarded as entirely infusible, since the intense
heat of an oxygen-fed flame is required to soften or melt the silica.
Further, vitreous silica has an extremely low coefficient of expansion,
and appears also to have a rather high coefficient of thermal
conductivity. The result is that tubes and other articles made of
this material possess an astonishing amount of thermal endurance (see
Chapter II.).
A white-hot tube or rod of this material can be plunged into cold water
with impunity, and no special care need be exercised in heating or
cooling articles made of this substance, unless articles of great size
and thickness are involved, and even with these only little caution
is needed. The only disadvantages which must be balanced against the
great advantages just named lie in the relatively high cost of the
articles and in their somewhat sensitive behaviour to certain chemical
influences. As regards cost, vitreous silica is at present available in
two different forms; in the first form it resembles ordinary glass very
closely in appearance, the shape and finish of the tubes and vessels
of this kind having undergone very great improvements quite recently.
This silica glass has, in fact, been worked from molten silica in a way
more or less analogous to that in which ordinary glass is worked, the
great extra cost of the silica ware being due, in part, at all events,
to the extremely high temperature required for melting and working this
material; ordinarily, in the production of the class of silica ware
now referred to, this heat is generated by the liberal--and therefore
expensive--use of oxygen gas. In great contrast to this glass-like,
transparent silica ware is the other form in which this material is
available. This is a series of products obtained from the fusion of
silica in special forms of electric furnace; in this ware the minute
bubbles so readily formed in the fusion of all forms of quartz are not
even partially eliminated, and by their presence--often in the form of
long-drawn-out, capillary hollows--they impart to this ware its very
characteristic milky appearance. The price of this product, which is
mostly used in the form of tubes, although such articles as basins,
crucibles, and even muffles of considerable size are available, is much
lower than that of the transparent variety, being in fact decidedly
lower than that of the best porcelain; on the other hand, even this
price is considerably above that of the best glass tubing.
Apart from the question of cost, the use of silica ware is further
limited by its sensitiveness to all forms of basic materials. Thus
alkaline solutions cannot be allowed to come into contact with this
substance, since they attack it vigorously, especially when warm. At
high temperatures all basic materials produce a rapid attack on silica
ware, the silica, in fact, behaving as a strongly acid body at and
above a red heat. The attack which occurs when such a substance as iron
or copper oxide is allowed to come into contact with heated vitrified
silica is, in fact, so rapid that a tube is completely destroyed in
a few minutes, the formation of silicates resulting in the cracking
and disintegration of the whole piece. While, therefore, silica ware,
especially in its cheaper forms, undoubtedly possesses great advantages
and possibilities, its use must be carried on with careful reference to
its chemical nature.
Vitreous silica, in addition to the uses and advantages just named,
has also an interest from the optical point of view; this arises from
the fact that it is transparent to short (ultra-violet) light waves
to which all ordinary varieties of glass are completely opaque. Quite
recently, the Jena works have produced special glasses which are more
transparent to these ultra-violet rays than ordinary glass, but even
these fall far short of silica in this respect. This property of
transparence to ultra-violet light is utilised in two widely different
directions. One of these is in the production of ultra-violet light
when required for medical or other special purposes; a most energetic
source of such rays is available by the use of tubes of vitrified
silica within which the mercury-vapour arc is produced. In another
direction the employment of quartz lenses makes it possible to take
advantage of the optical properties of ultra-violet light in connection
with microscopy; for the purpose of constructing a perfect optical
system, crystalline quartz would be useless, since its property of
double refraction would interfere hopelessly with the performance of
the lenses. This is now overcome by the use of vitreous silica lenses,
in the case of the “ultra-violet microscope,” as made by Carl Zeiss,
of Jena. So far, however, it has only been possible to produce quite
small pieces of vitreous silica sufficiently free from bubbles to be
used for optical purposes. The great difficulty lies not so much in
merely melting the quartz down as in freeing it from the air-bubbles
enclosed within it; the course usually adopted with glass, of raising
the temperature and allowing the bubbles to rise to the surface,
becomes impossible in this case, because the silica itself begins to
vapourise and even to boil vigorously at temperatures not very far
above its melting point. Quite recently, however, two American workers
have claimed to be able to overcome this difficulty by the use of both
vacuum and high pressure applied at the earlier and later stages of
the fusion process respectively, so that it may shortly be possible to
produce vitreous silica in large and perfectly clear blocks.
We have already indicated that glass tubing and rod form the basis
upon which the glass-worker, with the aid of the blow-pipe or “lamp,”
fashions his productions, which, of course, include a great number of
scientific instruments and appliances used more especially in the field
of chemistry. In another direction also glass tubing serves as a basis
for a branch of the glass industry; this is the manufacture of certain
classes of glass beads, which are formed by cutting up a heated glass
tube of suitable diameter and colour into short, more or less spherical
sections. In some cases the colour of the beads is secured by using
glass of the desired tint, but in other cases the beads are made of
colourless glass, and a colouring substance is placed in the interior
of the bead.
Solid glass rods are also employed for a variety of purposes; their
mode of manufacture is exactly analogous to that of tubing, except
that the gathering is drawn out without having first had a hollow
space produced at its centre by the blower. In its most attenuated
form glass rod becomes glass thread or fibre; this is produced by
drawing hot glass very rapidly, the resulting thread being wound on a
large wheel. At one time this material found considerable use, since
it was found possible to spin and weave the thinnest glass fibres into
fabrics which could be used for dress purposes. It is not, however,
to be regretted that this fashion has neither extended nor survived,
since it was certainly liable to produce serious injury to health.
It is a well-known fact that there are few more injurious or even
dangerous substances to be inhaled into the human throat and lungs than
finely-divided glass; glass fibre, moreover, when subjected to constant
bending and wear, is bound to undergo frequent fracture, and the
atmosphere of a ball-room, for example, in which several such dresses
were worn would soon be contaminated with innumerable fine, sharp
particles of glass which would produce an injurious effect on those
inhaling them. At the present time glass fibre is used for little else
than the “glass wool” required for certain special purposes in chemical
laboratories.
Fused quartz or silica fibres, of extreme tenuity, but of relatively
very great strength, are employed in many scientific instruments, where
their extreme lightness and perfect elasticity and freedom from what
is known as “elastic fatigue” renders them of very great value. These
fibres are not drawn from a mass of molten silica, as is done with
glass, but are produced by attaching a nail or bolt to a small bead of
fused silica produced by the aid of an oxygen-fed blowpipe; the nail or
bolt is then suddenly shot away down a long passage or similar space by
means of a cross-bow, drawing a very fine fibre of silica with it; the
most difficult part of this operation, however, consists in finding and
handling the fibres thus produced.
_Artificial Gems._--The fact that pieces of suitably-coloured glass can
be made to show a superficial, but sometimes more or less deceptive,
resemblance to precious stones, has led to the manufacture of imitation
jewels of all descriptions. The glass used for this purpose is usually
a very dense flint-glass whose high refractive index facilitates the
imitation which is aimed at. The external shapes of gems are, of
course, readily imitated by cutting and grinding the glass, while the
requisite colours are attainable by means of the colouring materials
described in Chapter XI. To a casual observer the difference in sparkle
and brilliance which arises from the difference between the refractive
index of the heavy flint-glass (about 1·8) and that of minerals
(which ranges from 1·7 to 2·2) is not readily apparent, but closer
examination will at once reveal the difference. The determination of
the optical constants by means of a refractometer would at once reveal
the true character of the imitation, but an even readier test is that
of hardness. The dense flint-glass is naturally soft, and is readily
scratched by most of the harder minerals, while the precious stones,
more particularly garnets, rubies and diamonds, are very hard. If an
attempt is made to scratch an ordinary sheet of window-glass, it will
be found that most real precious stones will do so readily, while
flint-glass imitations will fail to make more than a slight mark,
which is more smear than scratch. The test by determining the specific
gravity is also obviously applicable, since the flint-glass will
readily betray its presence by its high density (over 4).
In quite a different class from the imitation gems made of cut
flint-glass are the artificial gems, which in nature and composition
are exact reproductions of natural gems, but which have been produced
by artificial processes. As far as the writer is aware these are
only found in any large numbers in the case of the ruby, but in that
case, at all events, it is said that the production of the artificial
crystals is at least as costly as the purchase of the natural stones.
There can, however, be very little doubt that as the processes of
fusion and crystallisation become better known and understood, and the
chemistry of silicate minerals is developed, the artificial production
of mineral crystals in, at all events, moderate sizes will become
increasingly possible; it is even to be hoped that their production
will be so far perfected as to place their really valuable properties
at the service of man.
_Chilled Glass._--In all the processes of glass manufacture described
in the present book, annealing has always played an important part. The
glass, after it has undergone its last treatment under the influence
of heat, is subjected to a gradual cooling process with the object of
freeing it from the internal strains which it would otherwise retain,
and which would, ordinarily, endanger its existence and interfere with
its use. It is, however, well known that surfaces of glass subjected
to such internal strains as result in a compressive stress on the
glass near the surface, are less liable to injury, and are apparently
stronger than when the glass is annealed and the stresses are removed.
On the other hand, glass surfaces under tension are extremely delicate
and fragile. In some respects, therefore, glass which has not been
annealed may appear to be stronger than the annealed product. The
well-known case of the Rupert’s drop is an example of this kind.
Rupert’s drops are produced by dropping molten glass into water; they
generally take the form of a more or less spherical body having a long
tail, tapering off into a thread, attached to it. Such a Rupert’s drop
may be struck with a heavy hammer, and will safely resist a blow that
would splinter a similar body made of annealed glass. If, however, the
surface be scratched, or the tip of the tail be broken off, the entire
“drop” breaks up, sometimes with a violent explosion, into minute
fragments. Numerous inventors, among whom De la Bastie and Siemens
figure most conspicuously, have endeavoured to utilise these properties
of chilled glass, not exactly by endeavouring to produce that extreme
degree of internal strain which is characteristic of the Rupert’s drop,
but by producing what they describe as “tempered” glass, in which the
internal strains have been reduced by less violent cooling to such an
extent as to retain some of the advantages of the hardened, internally
strained condition while approximating more or less to the safer state
of annealed glass. At one time articles of this kind were frequently
seen as curiosities, such as tumblers that could be dropped on the
floor without breaking, etc., but these articles generally ended by
receiving a slight scratch or chip and promptly falling into fragments.
As a matter of fact, however, some tempered glass is actually
manufactured by the firm of Siemens at the present time for special
purposes. De la Bastie’s process was tried in England, and some success
was claimed for it; but it is not in commercial operation at the
present time, and never appears to have attained any great importance.
_Massive Glass._--Enthusiasts for the extension of the use of glass
have endeavoured to apply it to a great variety of purposes, including
the construction of buildings and the paving of streets. In the
former case, which was exemplified at the Paris Exhibition of 1900,
advantage was taken of the light-transmitting power of the material,
but although the buildings erected with large blocks of cast glass were
not displeasing in effect, this use has not found any considerable
extension. For paving purposes, the hardness and durability of glass
are the only useful qualities, and here also--although several trials
have been made in France--no signs of any considerable application of
the new products are as yet visible. What has been said above with
reference to the injurious character of glass dust applies, further, to
glass pavements, since their natural wear would result in the formation
of considerable quantities of this dust. The advocates of glass paving,
however, suggest that the hardness of glass would greatly reduce the
actual amount of wear, and that consequently the dust would be reduced
considerably. This is a matter which prolonged experience alone can
decide, but it does not seem obvious that glass blocks should wear more
slowly than stone setts made of good granite, for example. On the other
hand, the glass blocks could probably be produced more cheaply, since
the labour of cutting to size would be obviated by casting the blocks
to the desired dimensions.
Water-glass, or silicate of soda or potash is perhaps scarcely to be
classed under the heading of “Glass Manufacture” at all, but it bears a
certain relationship to glass in several ways. Thus one of the modes of
manufacturing water-glass is by the fusion of sand and alkali in tank
furnaces somewhat resembling those used for glass production; the fused
silicate, moreover, solidifies as a vitreous mass, in which respect
it also resembles such substances as borax, etc. The uses of silicate
of soda and potash are, however, so far removed from the field of
glass-manufacture that we cannot enter into them here.
In concluding this chapter, we wish to describe one more product of the
glassworks, and this includes some of the most impressive and splendid
examples of the glass-maker’s art. These are the great mirrors and
lenses by whose aid our lighthouses and searchlights send forth their
powerful beams of light. Although these objects are called “mirrors”
and “lenses,” since they fulfil the functions of such optical organs,
yet in their nature and mode of manufacture they are so far removed
from the glass used for the production of other kinds of lenses that
they could not be included under the heading of “optical glass.”
The characteristic feature in the manufacture of optical glass is the
manner in which each separate pot or melting is allowed to cool down
and to break up into irregular fragments which are subsequently moulded
to the desired shape. Were it attempted to manufacture the large glass
bodies required for lighthouse purposes in this manner, the cost would
approximate to that of the large discs used for telescope objectives,
and this would of course be entirely prohibitive. The requirements as
regards colour, homogeneity and freedom from other defects, which must
be met in lighthouse lenses, are further not nearly so stringent as
those which are essential in ordinary optical work of good quality.
The reason for this difference arises from the fact that lighthouse
lenses and searchlight mirrors are used merely to impart a desired
direction to a beam of light, and not for the purpose of producing
sharply-defined images; slight irregularities in the glass are
therefore not of such serious importance.
Lighthouse glass can therefore be produced by rather less elaborate
means; although every care is taken to make the glass as perfect as
possible, it is brought into approximately the desired form by casting
the molten glass in iron moulds of the proper shape. When removed
from these moulds and annealed, the glass is fixed on large revolving
tables and ground and polished to the final shape of lenses and annular
lens-segments as required for the various types of Fresnel lighthouse
lenses. In this way complete rings, forming annular lenses, are
produced up to 48 inches diameter. Rings of larger size are usually
built up of a number of segments, and these built-up rings sometimes
have a radius as large as 7 feet. For the majority of lighthouse
lenses, it should be added, a hard soda-lime glass having a refractive
index of 1·50 to 1·52 is used, but for special purposes a dense
flint-glass having a refractive index of 1·63 is employed.
Mirrors for searchlight purposes are of very varied forms and sizes,
the shape depending largely upon the particular form of beam which
they are designed to project. For many purposes a parabolic form
is required, while in others, where a flat, fan-shaped beam is to
be produced, a form having an elliptical section in a horizontal
plane and a parabolic section in the vertical plane is required. In
most cases these mirrors are produced by bending plates of glass,
previously raised to the necessary degree of heat, over suitably
shaped moulds, the surface being subsequently re-polished to remove
any roughness resulting from the bending process. Another type of
mirrors is that known as “Mangin,” which has two spherical surfaces
placed eccentrically in such a way that the centre of the mirror is
considerably thinner than the periphery; in this type of mirror the
reflecting action of the back surface is modified by the refracting
action of the front surface, but both are spherical, and can therefore
be accurately ground and polished by the usual mechanical means. Such
mirrors are manufactured of single pieces of glass up to 6 feet in
diameter.
APPENDIX
BIBLIOGRAPHY.
The existing literature of glass manufacture is so limited that a
complete bibliography could almost be given on a single page; in the
English language, in particular, there are exceedingly few books
and papers on the subject. The French and German literature of the
subject is a little more extensive. In giving a list of the works,
and more particularly in referring to those which he has consulted in
the preparation of the present volume, the author thinks it will be
an advantage to indicate their scope, and, to some extent, what he
believes to be their value, in order to save the student the trouble
of seeking out comparatively inaccessible works only to find that they
contain little that is of value for his purpose.
_English Books and Papers on Glass Manufacture._
The Principles of Glass Making (George Bell & Sons). By Powell &
Chance. An elementary book giving a clear and concise account of
the older processes, more especially in connection with flint and
plate-glass.
Glass. Articles in 9th Edition of Encyclopædia Britannica. A detailed
account of processes, more or less covering the entire subject, but the
processes described are mostly obsolete at the present time.
Glass. Article in Supplement to 9th Edition of Encyclopædia Britannica.
By Harry J. Powell. A brief summary of more recent developments.
Particularly valuable in reference to artistic English flint-glass.
Jena Glass. By Hovestadt, translated by J. D. and A. Everett.
Contains a full account of the scientific work on glass and its
practical application, done in connection with the Jena Works of
Schott. Particularly interesting in connection with the subjects of
Chapters I., II., XII., and XIII. As the title indicates, the book is
written from the Jena point of view, and scarcely does justice to work
done elsewhere. The book has gained considerably at the hands of the
translators.
Some Properties of Glass. By W. Rosenhain. (Transactions of the Optical
Society of London, 1903.) Gives a brief account of the properties of
glass as affecting its optical uses.
Possible Directions of Progress in Optical Glass. By W. Rosenhain.
(Proceedings of the Optical Convention, London, 1905.) Has been
referred to in the text of this book (Chapter XII.).
Catalogue of the Optical Convention Exhibition, London, 1905. Contains
historical and general notices of optical and lighthouse glass,
glass-working machinery, etc.
Glass for Optical Instruments. By R. T. Glazebrook. (Cantor Lectures
to the Society of Arts.) Gives an account of modern optical glass
manufacture.
Old English Glasses. By Albert Hartshorne. Gives an account of the
history of glass-making in England.
The Methods of Glass Blowing. By W. Shenstone. Describes the
manipulation of glass-blowing for experimental purposes, _i.e._, lamp
work.
_French Books on Glass Manufacture._
Guide du Verrier. By G. Bontemps. A classical work by one of the
greatest experts of his day. Much of the contents of the book is,
however, entirely out of date at the present time. The book is
interesting as being the work of the man who introduced optical glass
manufacture into England.
Verres et Emaux. By L. Coffignal. Chiefly of interest in connection
with the subjects of Chapter VIII.
Le Verre et le Crystal. By J. Henrivaux. (P. Vicq Dunod et Cie.,
Paris.) A lengthy book profusely illustrated and giving a great wealth
of detailed information. The writer was for some time the general
manager of one of the largest plate-glass manufactories in Europe; his
account of plate-glass manufacture is, therefore, especially valuable.
Much space in this book is devoted to historical and æsthetical matter.
La Verrerie au XX^{ieme} Siècle. By J. Henrivaux. (Paris, R. Bernard
et Cie., 1903.) Practically a supplement to the preceding; some of
the processes and products described are, however, not of a practical
nature. Chiefly valuable for recent developments in plate-glass and
bottle-glass manufacture.
_German Books on Glass Manufacture._
Die Glasfabrikation. By R. Gerner. (A. Hartleben’s Verlag, Vienna
and Leipzig, 1897.) A concise and clear account of most of the more
important processes of glass manufacture. Very practical in character.
The information given appears to be reliable, although far from
complete.
Die Herstellung Grosser Glaskoerper and Die Bearbeitung Grosser
Glaskoerper. By C. Wetzel. (Hartleben’s Verlag, Vienna and Leipzig,
1900 and 1901 respectively.) Describes numerous special processes and
appliances devised for use in connection with large glass objects.
Some of these descriptions, however, appear to be little more than
transcripts from patent specifications.
Glasfabriken und Hohlglasfabrikation. By R. Dralle. (Leipzig,
Baumgaertner, 1886.) Looked upon as a classic in Germany. Gives
detailed plans and drawings of entire bottle-works, including furnaces
and all accessories. Deals principally with bottle manufacture.
Die Glasfabrikation. By Dr. E. Tscheuschner. (Weimar, B. H. Voigt,
1888.) A full detailed account of all processes known at the time. The
rapid progress of modern practice has, however, already rendered this
book to some extent obsolete.
Jenaer Glas. By Hovestadt. Already referred to in respect of the
English translation.
Der Sprechsaal. (Schmidt, Weimar.) A trade journal devoted to the
discussion of technical matters relating to the glass and ceramic
industries. Occasionally contains articles and abstracts of technical
or scientific interest in connection with glass manufacture.
* * * * *
In addition to the books and papers named in the above list, a great
number of scientific papers, notes, etc., are to be found scattered
throughout the technical and scientific publications of the world;
those that have proved of real interest and importance have, however,
left their mark on the industry, and will be found described or
referred to in connection with the various branches of manufacture
described in the present volume or in the books named above.
INDEX
A.
Abbé, 8, 10, 210, 218
Absorption of light in glass, 32, 179
Acid, action of, on glass, 11
boric, action of, on glass, 11, 186
carbonic, action of, on glass, 12
hydrofluoric, action of, on glass, 12
phosphoric, action of, on glass, 11
Air, compressed, 91, 105, 117
Alkali chlorides, use of, in glass manufacture, 41
content of hygroscopic glass, 6
metals, 184
nitrates, 44, 78
sources of, 40
Alkaline liquids, action of, on glass, 11
Aluminium, 51, 186
Ammonia soda, 41
Anastigmatic photographic lenses, 213
Ancient windows, colours of, 16, 202
Annealing bottles, 103
kiln, 103
for optical glass, 235
for plate glass, 135
for rolled plate glass, 127
Anthracite coal, 42, 53, 79
Antimony, 188
Apochromatic objectives, 213
Arsenic, 52, 78, 105, 117, 188
Artificial gems, 246
Auerbach, 22
Aventurine, 185
B.
Bacteria, action of, on glass, 13
Barium compounds, 47, 186
crown glass, 212
glass, 7
Barytes, 48
Bases other than alkalies, sources of, 45
Beads, 244
Behaviour, chemical, of glass, 6
Bending plate glass, 144
Bevelling, 145
Black ash, 41
Blisters in sheet glass, 160, 168
Blocks, fire-clay, 58
tank, 59
Blower, sheet glass, 158
Blower’s chair, 111
Blowing glass, 89
holes, 91, 161, 189
sheet glass, 161
Blown glass, decoration of, 114
plate glass, 171
Bohemian glass, 109, 240
Boiling up, 81
Bottles, annealing of, 103
blowing, improvements in, 99
machines, 100
colour of, 96
manufacture, furnace for, 97
moulds for blowing, 98
production of, by hand, 98
raw materials for, 95
strength of, 18
Boric acid, 11
Boron, 186
Boro-silicate crown, 212
Boucher’s bottle-blowing machine, 101
Bricks, fire-clay, 58
silica, 60
Bubbles in optical glass, 230
removal of, 81
Burning, pots, 58
C.
Cadmium, 186
Calcium carbonate, 46
oxide, 45, 186
sulphate, 47
Carbon, 53, 79, 186
Carbonate of soda, 41
Carbonic acid, action of, on glass, 12
Carboys, blowing of, 104
Casting plate glass, 132
Chair, glass-blower’s, 111
Chalk, 46
Chamotte, 57
Chance, 211
Charcoal, 42, 58, 79
Charging furnaces, 75
Chemical behaviour of glass, 6
composition of glass, 5
of optical glass, 217
reactions during fusion, 76
Chilled glass, 247
Chimneys, gaslight, 23
lamp, 238
Chromium, colouring effect of, 190
Cleaning of lenses, 220
Coal, anthracite, 42, 53, 79
Cobalt, colouring effect of, 197
Coke, 42, 53, 79
Colour of ancient windows, 16
glass, 32
theory of, 181
optical glass, 208
sheet glass, 167
Coloured blown glass, 113
glass, 178
technical uses of, 203
Combustion tubing, 7, 241
Compressed air for glass blowing, 91, 105, 117
Conductivity, electrical, of glass, 30
thermal, of glass, 24, 29
Copper, colouring effect of, 184
ruby, 184, 188, 198
Corrosion of glass, 11
Covered pots, 56, 109
Crown, boro-silicate, 219
glass, 175, 211
hard, 212, 219
soft, 212
telescope, 215
Crowns, furnace, 60
Crucibles, manufacture of, 56
for glass melting, 54
Crushing strength of glass, 19
Cryolite, 52
Crystallisation of glass, 3
Crystals, mineral, 218
Cullet, 74
for optical glass, 224
Cutting rolled plate glass, 128
Cylinders, sheet glass, 161, 171
D.
Decolourisation of glass, 52, 188, 190, 193, 197
Decoration of blown glass, 114
Defects in rolled plate glass, 129
sheet glass, 166
Definition of glass, 1
De la Bastie, 248
Devitrification, 3, 11
Diamond, refractive index of, 216
Dimming of glass surfaces, 12
Dinas bricks, 61
Dipping of sheet glass, 166
Dispersion of optical glass, 209
partial, 214
Double refraction in optical glass, 221
rolling machine, 130
Drawing tubes, 239
Ductility of glass, 20
Durability of glass, tests for, 14
Dust, action of, on lenses, 220
glass, 245
E.
Elasticity of glass, 20, 24
Electrical properties of glass, 29
Epinal, 39
Etching of glass, 12
Expansion, coefficient of thermal, 24, 25
F.
Felspar, 40, 44
Fibres, glass, 245
silica, 245
Figured rolled plate glass, 87, 130
cutting of, 131
Finger-marks on lenses, 219
Fining of glass, 81
optical glass, 229
Fire-clay, action of, on glass, 6
for pots, 55
wetting up, 57
Fire-polish, 117
Flashed glass, 25, 199
Flint, 40
boro-silicate, 212
dense, 212, 246
densest, 212
extra dense, 212
glass, 7, 49, 78, 108, 211
light, 212
soda, 212
telescope, 215
Fluorite, refractive index of, 216
Fontainebleau, 38
Founding of optical glass, 227
Fourcault process, 174
Fresnel, 251
Furnace crowns, 60
gas, 63
Furnaces for bottle manufacture, 97
glass melting, 54, 62
optical glass, 225
plate glass, 133
rolled plate glass, 122
sheet glass, 151, 170
ports, 67
recuperative, 66, 156
regenerative, 66, 155
tank, 59, 69
economy of, 72
Fusion, process of, 73
temperature of glass, 5
Freezing of glass, 2
G.
Gaslight, chimneys for, 23
Gas producers, 62, 64
Gatherer, 158
Gathering of glass, 85, 88, 158
Gauge tubes, 10, 18, 23, 26
Gems, artificial, 246
Ghosts, photographic, 16
Glauber’s salt, 43
Gold, colouring effect of, 185
Grinding plate glass, 137
Gypsum, 47
H.
Hardened glass, 20
Hardness of glass, 21
tests for, 22
Heavy spar, 48
Henrivaux, 19
Hertz, 22
Hock-bottle colour, 195
Hohenbocka, 38
Hollow glass-ware, 108
Horse-shoe flame, 69
Hydrofluoric acid, action of, on glass, 12
Hygroscopic glass, alkali content of, 6
I.
Indentation modulus, 22
Index, refractive, 216
Insulating properties of glass, 29
Iron, 96
colouring effect of, 196
oxidation of, in glass, 195
Irregularities caused by rolling, 86
J.
Jena, 7, 10, 14, 26, 29, 203, 210, 213, 241
K.
Kelp, 40
Kowalski, 19
L.
Laboratory ware, 10, 23
Ladling glass, 85
rolled plate glass, 124
Lagre, 166
Lamp-chimneys, 110, 238
Lamp-work, 240, 244
Large vessels, production of, 105
Lead, 49, 183, 188
Lear for rolled plate glass, 127
sheet glass, 165
Leighton, 39
Lenses, cleaning of, 220
finger-marks on, 220
pressing small, 94
Light, action of, on glass, 15
Lighthouse glass, 178, 250
Lime, slaked, 45
Lime-stone, 46
Limited range of vitreous bodies, 4
Lippe, 38
Lynn, 39
M.
Machines, bevelling, 145
double rolling, 130
grinding, 139
polishing, 141
Magnesia, 48, 186
Manganese, 15, 52, 80
Mangin mirrors, 252
Marver, 111
Massive glass, 249
Mechanical properties of glass, 18
Metal, attachment of, to glass, 26
Minerals, crystalline, 217
Mirrors, 145
searchlight, 251
Mixing of materials, 73
Moulds for glass-blowing, 90, 110, 116
pressed glass, 119
Muffled glass, 172
Muranese glass, 123
N.
Nickel, 96
colouring effect of, 197
steel, 27, 148
Nitrates, alkali, 44, 78
O.
Objectives, apochromatic, 213
telescope, 218
Opal glass, 45, 52, 186
Opaque plate glass, 146
Open pots, 56
Optical glass, annealing, 235
chemical composition of, 217
cooling of, 233
cost of, 237
fining, 229
founding, 227
furnaces for, 225
hardness of, 220
moulding, 235
pressing, 93
range of, 216
raw materials for, 223
sorting, 235
stability of, 219
strain in, 221
stirring, 231
yield of, 236
properties of glass, 205
P.
Painting on glass, 201
Parason, 102
Patent plate glass, 171
Paving stones, glass, 249
Pearl ash, 43
Phosphoric acid, 11
Phosphorus, 188
Photographic ghosts, 16
lenses, anastigmatic, 213
colour of, 209
Pipe, glass-maker’s, 89
sheet-blower’s, 158
warmer, 158
Plate glass, annealing kiln for, 135
bending of, 144
blown, 171
casting, 132
colour of, 33
figured rolled, 87
flatness of, 134
furnaces for, 133
grinding machines, 139
of, 137
mirrors, 145
opaque, 146
polishing machines, 141
polishing of, 137
raw materials for, 132
rolled, 86, 123
silvering, 146
sizes of, 143
strength of, 15
striæ in, 143
wired, 27, 147
Platinum, 27
Polishing, theory of, 141
Pontil, 98, 176, 239
Potash, 43
Potato, use of, in glass melting, 81
Ports, furnace, 67
Pots, burning of, 58
covered, 56
drying of, 58
for flint glass, 109
optical glass, 226
manufacture of, 56
open, 56
Pouring of glass, 85, 87
Pressed glass, 92, 118
composition of, 120
Presses for glass, 119
Proofs, 82, 231
Purity of materials, 36
Q.
Quartz, 40
R.
Range, limited, of vitreous bodies, 4
Recuperative furnaces, 66, 156
Red lead, 49
Refraction, double, in optical glass, 221
of light in optical glass, 209
Refractive index, 216
Regenerative furnace, 66, 155
Reichsanstalt, 10
Resistance to crystallisation of glass, 4
Rings for lighthouse lenses, 251
Rod, glass, 245
Rolled plate glass, 86, 123
annealing, 127
cutting, 128
defects of, 129
figured, 130
furnaces, 123
ladling, 124
raw materials for, 124
rolling, 126
sorting, 129
surface of, 122
Rolling of glass, 86
Rubies, artificial, 247
Ruby, copper, 184, 188, 198
flashed, 184
gold, 185
Rupert’s drops, 248
S.
Salt-cake, 37, 42, 79, 189
Sand, 38
Sandstone, 39
Schott, 8, 19, 203, 213, 241
Scratches on sheet glass, 169
Searchlights, 250
Seed in sheet glass, 167
Selenium, colouring effect of, 190
Sheet glass, 70
blisters in, 160, 168
blowing, 161
colour of, 33, 167
compared with plate, 149
cylinders, 161, 171
defects of, 166
dipping, 166
flattening, 165
furnaces, 151, 170
lear, 165
mechanical production of, 173
raw materials for, 150
sorting, 166
splitting, 164
strength of, 18
Siedentopf, 182
Siege blocks, 59
Siemens, 248
Sievert, 92, 105, 117, 172
processes, 105, 117
Signal glasses, 203
Silica bricks, 61
glass, 5, 26, 241
sources of, 37
Silicon, colouring effect of, 187
Silver, colouring effect of, 185
Silvering plate glass, 146
Sizes of plate glass, 142
Soda ash, 41
carbonate, 41
sulphate, 37, 42, 79
sulphide, 80
sulphite, 79
Solidification of glass, 1
Solutions, analogy of, with glass, 206
Sorting rolled plate glass, 129
Specific heat of glass, 25, 29
inductive capacity of glass, 29
Stains, coloured, 200
Stassfurth, 44
Stones in rolled plate glass, 129
sheet glass, 167
Storage of materials, 37
Strain in optical glass, 221
Strength of glass, 19
Striæ in coloured glass, 203
optical glass, 206, 227
plate glass, 143
testing apparatus, 207
String in sheet glass, 168
Strontium, 86
Structure of glass, 1
Sulphur, colouring effect of, 189
Surfaces, chemical behaviour of glass, 8, 10
Szigmondi, 182
T.
Table, rolling, 126
Tank blocks, 59
furnaces, 59, 69
economy of, 72
for sheet glass, 152
Telescope objectives, 213
Temperature of fusion of glass, 5
Tempered glass, 20, 248
Tensile strength of glass, 19
Thallium, 183, 188
Theory of colours in glass, 181
polishing, 141
Thermal endurance of glass, 23
properties of glass, 23
Thermometer glass, 7, 8, 28
Tin, colouring effect of, 187
Tonnelot, 7
Transparency of glass, 31
optical glass, 208
Trautwine, 19
Tubing, 238
combustion, 7
drawing of, 239
Tumblers, 111
U.
Ultra-violet microscope, 243
V.
Vanadium, colouring effect of, 189
Veins in optical glass, 206, 227
W.
Water, action of, on glass, 10
glass, 250
Wetting up clay, 57
Winkelmann, 19
Wired plate glass, 27, 147
Witherite, 48
Wool, glass, 245
Y.
Young’s modulus, 20
Z.
Zaffre, 197
Zeiss, 213, 244
Zinc, colouring effect of, 49, 186
Zschimmer, 14
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=INDIA-RUBBER AND ITS MANUFACTURE=, with Chapters on Gutta-Percha
and Balata. By H. L. Terry, F.I.C., Assoc. Inst. M.M.
=ELECTRIC POWER AND TRACTION.= By F. H. Davies, A.M.I.E.E.
=LIQUID AND GASEOUS FUELS, AND THE PART THEY play in Modern Power
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=IRON AND STEEL.= By J. H. Stansbie, B.Sc., F.I.C.
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R. W. Sindall.
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=PRECIOUS STONES. With a Chapter on Artificial Stones.= By W.
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Transcriber’s Notes
Punctuation, hyphenation, and spelling were made consistent when a
predominant preference was found in this book; otherwise they were not
changed.
Simple typographical errors were corrected; occasional unbalanced
quotation marks retained.
Ambiguous hyphens at the ends of lines were retained.
The appearance of the Table of Contents has been altered slightly.
Index not checked for proper alphabetization or correct page references.
In the Index, it was unclear to the Transcriber whether some of the
indentations indicated sub-entries or implied ditto marks, so the
indentations have been treated here as multiple levels of sub-entries.