the furnace. In the case of tank furnaces this is a simple matter,
since in these the temperature is kept as nearly constant as possible,
and raw materials may, therefore, be introduced at almost any time,
the amount introduced being so regulated as to keep the level of the
molten glass or “metal” as nearly constant as possible. The actual
introduction is managed by means of a large opening or door at what is
known as the “melting end” of the furnace. Normally this opening is
covered by a large fire-brick block suspended by a chain running over
pulleys and counterbalanced by a counterpoise weight. When charging
is to begin, this block is raised and the opening is uncovered. The
raw materials are then introduced either by hand, by the aid of
long-handled shovels, or they are first filled into a long scoop
moved by mechanical means forward into the furnace, where it is given
a half-turn, which empties the contents out, and is then rapidly
withdrawn.
This charging process may be repeated every half-hour, or larger
quantities may be introduced once every four hours, according to the
practice that may be adopted at any particular furnace.
In the case of pot furnaces the charging process is not so simple.
Here the first charge of raw materials has to be introduced into a pot
which has been almost entirely emptied during the working-out process,
and the temperature of the furnace has also fallen very considerably
during this time. Before new material is introduced, the heat of the
furnace must first be adequately restored. If this is not done, the
fusion of the glass takes an abnormal course and very imperfect results
arise. Further, the quantity of material introduced at one time must
be carefully adjusted to the capacity of the pot. During the earlier
stages of fusion most glass mixtures form large masses of foam, and
if the crucible has been too heavily charged this foam overflows,
with the result that valuable material is lost and the floor and
passages of the furnace are clogged with glass. A certain amount of
overflow, as well as leakage from defective crucibles, is, however,
unavoidable, and for this purpose every pot furnace is provided with a
chamber so placed that the glass will flow into it and so be prevented
from finding its way into the regenerators or other parts where its
presence would hinder the working of the furnace. These receptacles or
“pockets” must, however, be periodically cleared of their contents
from outside, and this constitutes one of the most irksome operations
connected with glass manufacture. Owing to the occurrence of foaming
and to the fact that the raw materials occupy much more space than the
glass formed from them, it is necessary to fill the pot with fresh
batches of raw materials several times, the quantity which can be
introduced decreasing each time. The number of times that this must be
done depends upon the particular circumstances, but from four to eight
“fillings” are commonly used for various kinds of glass and size of
pot. The precise stage at which a fresh batch of raw materials should
be introduced is another matter requiring careful attention. For some
purposes it is necessary to wait until the previous batch is completely
melted, while in other cases raw material may be added whilst some of
the previous batch is still floating on the surface of the glass in the
pot.
We have now to consider the chemical reactions which take place in the
mixture of raw materials that are introduced into the hot furnace. The
exact course of these reactions is not known in very great detail, as
this could only be ascertained by an elaborate research on the nature
of the intermediate products that result under various circumstances.
A research of this kind would throw much light on the whole of the
melting processes but is in itself so difficult that it has not yet
been carried out at all fully. We can therefore only give an account of
the chemical changes from our knowledge of the end-results and of a few
intermediate products that are known. To take the simplest case, we may
consider a mixture consisting of sand, carbonate of lime and carbonate
of soda mixed in suitable proportions. In such a case we know that
the mere action of heat alone will produce two changes--the carbonate
of soda will melt and the carbonate of lime will lose its carbonic
acid and be “burnt” or converted into caustic lime. The first stage
of the fusion process thus probably results in a mass consisting of
sand grains and grains of carbonate of lime undergoing decomposition,
all cemented together by molten carbonate of soda. This mass will
be full of bubbles, some derived from the air enclosed between the
grains of the original mixture and thus trapped by the melting mass,
and others formed by the carbonic acid which is being driven off in
the form of gas by the decomposition of the carbonate of lime. At the
temperature of the furnace, however, silica has the properties of a
strong acid, and not only attacks the carbonate of lime much in the
same manner as, for instance, hydrochloric acid would do in the cold,
but the silica also attacks the carbonate of soda, which heat alone
can scarcely decompose. The exact order in which these reactions take
place will depend upon the temperature of the furnace and the degree
of mixing attained in the preparation of the raw materials. Although
in the long run the final result will probably be the same as regards
purely chemical constitution, much of the technical success of the
process must depend upon the exact sequence of the changes involved, as
this must govern the number and size of the bubbles that are formed in
the glass and the fluidity of the mass from which these bubbles have
to free themselves. In the present state of our knowledge, however,
we can only say that the final result is the complete expulsion of
all carbonic acid from the compounds present (although it may remain
entangled in the glass in the form of bubbles) and the formation of
silicates of both lime and soda which remain in the finished glass in a
state partly of mutual chemical combination, partly of mutual solution.
The description of the process of fusion just given applies, with
slight modifications, to the melting of ordinary flint-glass mixtures
as well as to lime glasses, with the one modification that the
carbonate of lime of the lime-soda glass is replaced by red-lead, and
the gas evolved by the decomposition of the red-lead is oxygen in place
of the carbonic acid evolved from the decomposition of the carbonate
of lime. In the case of both lime and flint glasses, however, certain
other substances besides those mentioned are usually introduced in
small quantities. Although these substances do not very materially
affect the end-products of the chemical reactions, they very materially
affect the intermediate stages, and thus serve the purpose for which
they are introduced by affecting the course of the chemical changes in
a favourable manner. The substances usually employed for this purpose
are arsenic and nitrate of either soda or potash. The manner in which
the arsenic acts is very obscure and cannot be discussed in detail
here; the chief factors in its action are, however, its volatility and
its power of either absorbing oxygen or parting with it according to
circumstances. The action of the nitrates is chiefly dependent upon
the oxygen which they yield on decomposition by heat. This oxygen is
in some cases stored up by other ingredients of the mixture and only
given off at a much later stage, when the evolution of this gas assists
in the removal of the last small bubbles of inert air or carbonic acid
gas still left in the glass. The oxidising action of the nitrates,
however, serves chiefly for the destruction of organic matter and
the full oxidation of any iron present; both processes which tend to
improve the colour of the glass, while in the case of flint glasses the
presence of these oxidising additions is necessary to avoid all risk of
reduction of lead, since this would result in the complete blackening
of the glass.
A much more complicated set of reactions occur when the alkali of a
soda-lime glass is introduced either partly or wholly in the form of
sulphate of soda (salt-cake). We have already pointed out that the
unaided action of heat and of silica is not sufficient to bring about
the rapid decomposition of sulphate of soda which is required for
successful glass manufacture, and that the intervention of reducing
agents is required. For this purpose a certain amount of carbon in
the form of coke, charcoal or anthracite coal, is introduced into all
salt-cake mixtures, but the reducing gases of the furnace atmosphere
also play an important part in the reactions that take place. Here
again it is not possible to give anything but an incomplete account
of what takes place. The rationale of the whole process lies, no
doubt, in the fact that sulphite of soda (Na_{2}SO_{3}) is much more
readily decomposed by the action of hot silica than the sulphate
(Na_{2}SO_{4}) itself, so that the essential action of the reducing
agents consists in robbing the sulphate of part of its oxygen, thus
reducing it to the condition of sulphite and rendering it accessible
to the attack of silicic acid. But if we attempt to express such a
reaction in the usual manner by a chemical equation from which the
quantity of carbon required to effect the reduction in question can be
calculated, we find that the amount of carbon required in practice
is very considerably less than that given by this theory; it follows
therefore that either this very large amount of reducing action must
be ascribed to the furnace gases, or that the actual reactions are not
strictly of the kind we have described. Both explanations are probably
partly correct, and in practice the amount of carbon to be used in a
given mixture and furnace can only be found by actual trial, in which
the manufacturer is, of course, guided by the results obtained with
other furnaces of a similar type. The end-product of the reactions is
again a mixture of silicates, but a certain amount of undecomposed
sulphate is always found in such glasses, while gaseous oxides of
sulphur escape from these furnaces in considerable quantity. Under
exceptional circumstances the glass may even contain sulphides of soda
or of lime, and sometimes even suspended carbon, but these are abnormal
constituents and result in the serious discolouration of the glass.
It is obvious that to a mixture containing carbon as a reducing agent
such oxidising materials as nitrates cannot be added, but small
quantities of arsenic and of manganese dioxide are added because
their other properties are sufficiently valuable to outweigh their
disadvantages as oxidising agents.
Having now briefly considered the process of fusion proper, we pass
to the second stage in the melting of glass. In a properly conducted
glass-furnace, when the last trace of undecomposed raw materials has
disappeared, we find the glass as a transparent mass throughout which
gas bubbles are thickly disseminated. For the majority of purposes it
is necessary to free the glass as perfectly as possible from these
bubbles before it is worked into its final form. This freeing or
“fining” process is carried out by further and more intense heating
of the molten glass, which is thereby rendered more fluid and allows
the bubbles to disengage themselves by rising to the surface. This
occurs much more readily when the bubbles are large; very minute
bubbles, in fact, show no inclination to rise through the fluid mass.
The glass-maker accordingly compounds his mixtures of raw materials in
such a way as to yield large bubbles, or, failing that, he adds to the
molten mass some substance that evolves a great many large bubbles, and
these in their upward course through the glass sweep the small ones
away with them. The added substance may be an inorganic volatile body,
such as arsenic, or more frequently some vegetable substance containing
much moisture is introduced into the glass. The most usual method is
to place a potato in the crook of a forked iron rod and then to dip
the rod with the attached potato into the molten glass; the heat at
once begins to drive off the moisture and to decompose the potato, so
that there is a violent ebullition of the whole mass. This “boiling
up” process assists the fining considerably and also serves to mix the
whole contents of the pot very thoroughly, but it has some attendant
disadvantages, such as the introduction of oxide of iron into the glass
from the rod which is used in the operation, while the contaminated
material adhering to the walls of the pot itself is dragged off and
mixed with the rest of the glass by the violent stirring action that
takes place. It is, of course, further obvious that this process can
only be usefully applied to glass melted in pots, since the bulk of
the molten glass in a tank furnace could not be reached at all in
this manner. Mixtures that are to be melted in tanks must therefore be
capable of freeing themselves of their enclosed bubbles without such
outside aid. In a tank, in fact, the whole melting process proceeds on
somewhat different lines, since the temperature of the furnace is never
intentionally varied, while on the other hand the melting glass travels
down the furnace into regions whose temperature can be regulated to
favour the various stages of the process that take place in each part
of the furnace. On the whole, however, it is an undoubted fact that
while the running of a pot furnace can be varied, within wide limits,
to suit the requirements of whatever mixture it is desired to melt, in
the case of tank furnaces the mixture must be closely adjusted to the
requirements of the furnace, whose general “run” cannot be very readily
altered.
The completion of the “fining” process is generally determined by
taking samples of the glass out of the pot or tank and examining them
for enclosed bubbles. Such samples may be obtained in a variety of
ways, the most usual method being to dip a flat iron rod just below
the surface of the glass and to lift it out vertically upwards, thus
retaining on the flat surface of the rod some of the glass that lay
there at the moment when the rod was immersed. These test samples or
“proofs” are examined very carefully, and if no trace of bubbles can
be observed the glass is generally regarded as “fine,” but it is by
no means certain that the absence of bubbles from such a small sample
will prove that the whole mass is free; that, however, is a point where
the melter’s experience enables him to judge how far he may rely upon
the indications given by the “proofs.” When the glass is “fine” it
frequently happens that the surface of the molten mass is contaminated
by specks of foreign matter floating on the glass; for the purpose of
removing these, the surface of all glass is skimmed before work is
begun upon it. This is done by removing the surface skin of glass by
means of suitably shaped iron rods, upon which small masses of molten
glass are first “gathered.” Finally, it only remains to reduce the
temperature of the glass from that of the melting and fining process to
the much lower temperature at which the various methods of working the
glass are carried out. In pot furnaces this is accomplished by lowering
the temperature of the entire furnace, while in tank furnaces the fine
glass flows into the working chamber of the tank which is always kept
at the working temperature.
CHAPTER VI.
PROCESSES USED IN THE WORKING OF GLASS.
In the previous chapter we have followed in outline the process of
fusion and fining of glass, leaving the molten material ready for
working up into the final shape. Up to that point the process is very
similar in all kinds of glass, although the furnaces, pots and utensils
employed vary considerably, as do also the temperatures to which the
materials are heated at various stages. The working processes, however,
differ entirely from one class of product to another, as obviously the
process employed for the production of a sheet of plate-glass can have
little in common with that used in the manufacture of a wine-glass. On
the other hand, the modes of working hot glass are not so numerous as
the products that are produced, so that we find very similar appliances
and manipulation recurring in various branches of the industry. For
that reason we propose to deal here with the principal methods of
manipulating glass, leaving the details of each method as applied to
special purposes to be discussed in connection with the special product
in question.
The first stage in the working of all glass is the removal of a
suitable quantity of molten glass from the furnace. Practically only
three methods are available, viz., ladling, pouring and gathering.
If we think of a familiar substance of physical properties somewhat
resembling those of glass, we may take thick treacle and suppose it
confined in a jar or bottle; there are three obvious ways of extracting
it from the bottle: we may ladle it out with a spoon, or we may pour it
out by tilting the whole bottle, or we may dip a spoon or fork into the
thick liquid, slowly draw it out and turn it round as we do so, thus
bringing out on the spoon or fork a round adherent mass or “gathering”
of treacle. In the case of molten glass, the process of ladling is
by far the simplest, but it has certain very decided limitations
and disadvantages. These arise from the fact that a ladle cannot be
introduced into molten glass without contaminating the whole mass of
glass, at any rate with numerous air bubbles. The metal of the ladle
carries with it a considerable amount of closely adherent air which is
partially detached while in contact with the hot glass, so that both
the contents of the ladle and the glass remaining in the furnace are
contaminated. These bubbles might perhaps be avoided if hot ladles were
used, but in that case the glass would adhere to the surface of the
metal, and each ladle would require laborious cleaning after each time
that it was used. In practice, therefore, ladling is only used for the
production of those classes of glass where the presence of a certain
number of air-bells is not injurious, and the ladles are kept cold by
immersion in water after each time of use. The use of the cold ladle
has, however, the further disadvantage that a certain quantity of the
glass withdrawn in the ladle is very considerably chilled by contact
with the cold metal, and is thus too stiff to undergo the further
processes satisfactorily--this chilled glass has, therefore, to be
rejected from each ladleful; this not only involves loss of glass, but
also necessitates the separation of this spoilt glass from the rest.
The general process of rolling requires little treatment here. Two
essentially different processes are used; in one the glass is thrown
on a flat table and rolled out by a moving roller passing along the
table; in the other the glass passes between two moving rollers, and
the sheet so formed is received on a moving table or slab. The former
mode of rolling is used for the production of the ordinary rolled plate
glass; if the surface of both table and roller is smooth, the glass
also has a comparatively smooth surface, but the surface is far from
being level or free from irregularities. It has been found that it is
quite impossible to prevent these irregularities, which appear to arise
from the buckling of the glass against the iron surfaces with which
it comes into contact; when rolled, the glass is too stiff to recover
its true, smooth surface under the influence of surface tension, so
that it retains all the marks of roller and table--nor can the roller
be made _perfectly_ smooth, since in that case it appears to slip over
the glass and does not roll it out properly. All efforts, therefore,
to produce a glass having a true and smooth surface by direct rolling
have failed, and are likely to fail, so long as tables and rollers
are made of materials similar to those now in use. The process of
rolling on a stationary table is, however, used for the manufacture of
plate-glass; but here the slab as rolled has still the rough, uneven
surface similar to that of ordinary “rolled plate,” and this is removed
and replaced by a true polished surface by the mechanical processes
of grinding and polishing. The second mode of rolling, _i.e._, with
two or more “stationary” rollers and a moving table, is used for the
production of rolled plate having special surface features or patterns;
the variety of rolled glass known as “figured rolled plate,” having a
deeply imprinted pattern, is produced in this way. This method requires
much more complicated mechanical appliances, some of which are still
protected by patent rights.
Ladling being thus limited to the production of inferior kinds of
glass, the better varieties are dependent upon either gathering or
pouring. The former process is limited as regards the quantity of
glass that can be dealt with in one piece, although surprisingly large
quantities can be gathered upon a single pipe; the great masses of
glass, however, that are required for the production of modern polished
plate could not be handled in this way, and the method of pouring is
accordingly adopted. For this purpose either the pots in which the
glass has been originally melted, or others specially designed for
this purpose, and into which the molten glass has been transferred,
are removed bodily from the furnace by the aid of powerful mechanical
appliances; they are then carried by overhead cranes to the place where
the glass is to be rolled into the form of a plate, and there the pot
is tilted and the molten glass is allowed to run out and to form a
pool on the rolling table, the passage of the great roller ultimately
rolling the pool out into a sheet much as dough is rolled out with
a rolling-pin. This process is obviously only possible with pots or
crucibles of a suitable size, and is, moreover, very destructive
to these pots, since they are exposed to such great variations of
temperature. In the case of tank furnaces, numerous devices have been
patented for allowing the glass to flow out over a sill or weir of
suitable size, ready to be rolled or drawn into the form of sheets or
slabs; but none of these devices have, so far as the writer is aware,
found their way into practice; the reason for this probably lies in the
fact that it is not easy to find a material which will present a smooth
face to the outflowing glass, such materials as fire-clay leading to
contamination from detached fragments, while chilled metal leads to
local chilling of the glass. Unless, therefore, the various processes
of drawing glass into sheets direct from the furnace undergo very
material improvement, the laborious process of gathering is likely to
retain its importance even in the production of such large objects as
sheets of window glass.
In its essence the process of gathering consists in introducing into
the glass a heated iron rod or tube to which a small quantity of glass
is allowed to adhere; rod and glass are removed from the furnace
together, and the small adherent ball of glass is allowed to cool so
far as to become stiff enough to carry its own weight. The rod with its
adherent ball is then again dipped into the glass, where a fresh layer
of glass attaches itself to the ball already on the rod. The whole is
again withdrawn, allowed to cool down, and then dipped into the molten
glass again to gather a fresh quantity. This cycle of operations is
repeated until the desired quantity of glass is attached to the rod
or tube. These operations, particularly when weights of thirty or
forty pounds of glass have to be gathered, require the exercise of a
great deal of skill and care; the introduction of the gathering into
the molten glass is each time liable to produce air-bells which would
spoil the whole mass of glass or would contaminate the contents of the
crucible, while subsequently the mass of hot glass adhering to the rod
or pipe tends to run down and even to drop off entirely if not properly
checked by suitable rotation of the pipe. Further, the manual labour
and exposure to heat involved for the operator all tend to increase
the cost of such work. Mechanical aids have been invented, and some of
these are in actual use, but they are chiefly confined to mechanism
for relieving the operator of the great weight of the gathering in its
later stages.
Just as ladling is nearly always preliminary to rolling, so gathering
is usually the preliminary to some blowing process, although the
blowing is often combined with and sometimes replaced by the mechanical
pressing of the glass. Where the glass is to be blown, the gathering
is always made on a glass-maker’s pipe. This is an iron tube from 4
to 6 ft. long, provided at one end with a wooden casing to serve as a
handle, and with a suitably arranged mouthpiece for blowing. The shape
of the lower or “butt” end of the pipe depends upon the character and
size of the objects to be blown; for small articles the pipe must be
narrow and light, but for heavy sheet-glass the butt of the pipe is
extended into a conical mass whose base is from 2 to 3 in. in diameter.
The bore of the pipe at both ends also depends upon the class of work
for which it is intended. The first stage of all blowing processes
consists in the formation of a hollow sphere by blowing into the pipe,
the pressure of the breath being as a rule sufficient to cause the
gradual distension of the hot mass of glass. From this rudimentary
hollow sphere the various shapes of blown articles are then evolved by
a series of manipulations which vary very widely in different branches
of manufacture. They generally consist, however, in gradually changing
the shape of the mass of glass by the pressure either of hand tools or
of specially prepared moulds or blocks against which the glass is held
or turned, either with or without simultaneous blowing into the pipe.
The extent to which the aid of such moulds and blocks is invoked varies
continuously from the production of the hand-made vase or glass to
the moulded bottle; in the former, practically only hand tools, whose
shape bears no direct resemblance to that of the finished article, are
employed, while in the latter the elongated hollow mass of glass is
placed inside a mould, and internal air-pressure is used to press the
glass into contact with the mould from which the shape of the finished
bottle is thus directly derived.
The art of the blower further takes the fullest advantages of the
peculiar physical properties of glass while in the heated viscous
condition, the material being made to flow under the action of gravity
and centrifugal forces, as well as under the pressure of the breath,
the glass being held aloft, twirled or swung about to ensure the
production of the various shapes required. For the great majority of
such purposes the unaided manipulations of the operator are sufficient,
but various mechanical aids are used to facilitate the more laborious
stages of the work, while for the simpler forms that are required in
very great numbers, such as bottles, the whole of the operations are
now carried out by automatic machines. Of the more usual mechanical
aids at the disposal of the glass-blower, we have already mentioned
hand-tools, blocks, and moulds of various kinds. Next in importance
to these is the use of compressed air for blowing large or heavy
articles; the pressure available by the human breath is very limited,
and the volume of air that can be thus delivered is not very large,
while the constant use of the lungs for such a purpose is trying
for the workman. In many works, therefore, air under pressure is
supplied to the benches or stages where the blowing is done, and the
blowers’ pipes can be coupled to this air-supply by means of flexible
connections when required. The principal difficulty lies in the
correct regulation of the air-pressure for each special purpose; but
this difficulty has been overcome by the use of delicate valves under
the control of each blower, who can thus regulate the pressure to
his own exact requirements. Such a system, of course, requires some
little practice on the part of the men using it, but when they have
become accustomed to the working of the plant the results achieved
are decidedly better and more regular than those obtained by mouth
blowing. Besides the use of compressed air supplied in the way just
indicated, several other devices are in use to aid the blower in
producing the requisite pressure in the interior of the hollow bodies
he is producing. The simplest of all these consists in utilising the
expansive force of the air enclosed in the hollow body when that body
is exposed to heat. Thus, for instance, in blowing a cylinder of
sheet-glass, if the blower holds his thumb over the aperture of his
pipe, and brings the closed end of the cylinder near the hot “blowing
hole,” the heat which softens that end of the glass will also act upon
the enclosed air, and will very rapidly produce such an expansive
effect as to burst open the softened end of the cylinder, and this
means of opening the closed ends of the cylinder is frequently employed
in practice. It is, of course, obvious that any other expansive fluid
might be employed in a similar manner, and in some blowing processes
it has long been the practice to introduce a small quantity of water
into the interior of the hollow body, when the rapid expansion of
the steam produced thereby is utilised for the purpose of generating
the requisite internal pressure. This use of the expansive force of
steam generated by the heat of the hot glass body has received great
development at the hands of Sievert in Germany, whose process is
described in Chapter VII.
Whatever mechanical aids are employed to facilitate the various stages
of the process, all glass blowing involves a series of operations
requiring considerable skill, while the whole manner of dealing with
the glass is essentially extravagant of material, except perhaps in
the production of bottles or flasks having narrow mouths. The reason
for this latter statement lies in the fact that by blowing it is
only possible to produce closed or nearly closed hollow bodies or
vessels; thus a blown wine-glass or tumbler is formed with a hood or
dome closing in the open top of the glass, and this hood or dome has
subsequently to be removed by subsidiary processes, such as cutting off
by the aid of strong local heat or by grinding, and the cut edge has to
be provided with a smooth finish. In the case of comparatively small
articles like glasses the loss involved from this cause is not so very
great, but were large flat bowls or dishes to be produced by blowing,
the loss in the dome or covering would be very serious. This difficulty
is entirely avoided by the process of pressing glass. We have already
indicated the manner in which moulds are used for the production of
the desired shape in the case of bottles, etc., but in these cases,
where the final object is to be a hollow vessel, the glass is readily
forced into contact with the mould by means of internal air--or
steam--pressure; in the process to which we are now referring, however,
the hot glass is forced into contact with the external mould by means
of an internal plunger which is forced downward with considerable
force. By this means, flat or shallow bodies can be produced without
the preliminary formation of a completely closed vessel, while it is
obvious that by the use of suitable moulds, complicated and elaborate
shapes can be produced. It is true, of course, that pressed articles do
not show the same smooth and brilliant surface which is characteristic
of the fire-polish of blown articles, while the facility with which
elaborate surface ornamentation can be applied by this process has
not tended to artistic refinement in design, but the great majority
of cheap and useful glass articles of domestic use have been made
available by the development of the pressing industry.
In the ordinary course, pressed glass is produced direct from the
molten material, which is introduced into the presses either by
gathering or by means of ladles, but for some special purposes glass is
brought into its final shape by mechanical pressure after having first
been allowed to solidify and having then been specially re-heated to
undergo the pressing or moulding process. This is principally done in
the case of the best kinds of optical glass, where the molten glass
is first allowed to cool in the actual crucible and is then broken up
into lumps of a suitable size, from which the more defective portions
can be rejected, the more perfect portions only being heated up again
in special kilns and then forced to take the desired shape by being
pressed--sometimes with hand tools only and sometimes by the aid of
powerful presses--into moulds of the required shape. Small lenses,
however, for which the requirements of quality are not so high are
sometimes pressed direct from small gatherings taken from the molten
glass in the crucible.
CHAPTER VII.
BOTTLE GLASS.
Although bottles are in some respects the cheapest and crudest products
that are manufactured of glass, their uses are so innumerable and their
numbers so enormous that their production constitutes a most important
branch of the industry.
In the choice of raw materials for the production of ordinary bottles
cheapness is necessarily the first consideration. Natural minerals,
bye-products of other industries, and the crudest chemicals are
utilised so long as it is possible by compounding these ingredients
in suitable proportions to obtain a glass whose composition meets
the somewhat crude requirements which bottles are expected to meet.
The most essential of these requirements are that the bottles shall
be strong enough to resist the internal pressure which may come upon
them when used for the storage of fermented or effervescent liquors as
well as the shock of ordinary use, while the glass itself must possess
sufficient chemical resistance to remain unattacked by the more or less
corrosive liquids which it is called upon to contain. Further, from
the point of view of the bottle manufacturer it is desirable that the
glass shall be readily fusible, easily worked, and easily annealed.
In other branches of glass manufacture increased fusibility is often
attained by increasing the alkali contents of the glass, but in bottle
making this is inadmissible, both on account of the prohibitive cost
of alkali and because an increased alkali content renders the glass
more liable to chemical attack. On the other hand, in many varieties
of bottle the _colour_ of the glass is nearly, or quite, immaterial
so that the introduction of relatively large proportions of iron
oxide is permissible. This substance acts as a flux and assists in
the production of a fusible, workable glass containing little alkali.
Such alkali as bottle glass does contain is frequently derived from
felspathic minerals, which generally also contain considerable
proportions of iron. The use of these minerals also introduces
notable proportions of alumina into the glass. In certain classes
of bottles, notably those used for special wines, certain shades of
colour are required--the well-known “Hock bottle” colour being an
example. The presence of iron in the glass tends to the production of
a green or greenish-yellow colour deepening to a black opacity if the
quantity of iron be high. The lighter shades of this green tint may
be “neutralised” by the introduction of manganese into the glass, the
resulting colours ranging from light amber to purple; nickel oxide is
also sometimes used as a colouring material in these glasses.
In the production of ordinary bottles the continuous tank furnace has
now entirely superseded the old pot furnaces, the character of the
product being in this case particularly suited to this process of
production. The modern bottle-glass tank is generally an oblong basin
having one semi-circular end. The flame is often of the “horse-shoe”
type, the gases both entering and leaving the furnace at the flat
or charging end of the furnace. The raw materials are thrown into
the furnace at the square end of the tank, and the glass flows
uninterruptedly down the furnace to the colder semi-circular end where
the working holes are situated. At these points fire-clay rings are
kept floating on the glass, and from within these the gatherer takes
his gathering, the rings serving to retain the grosser impurities
carried down by the glass. The producing power of such a furnace, even
when the bottles are blown by hand, is very considerable; a furnace
having ten working holes and containing normally about 85 tons of
molten glass will yield some four million bottles per annum, and
furnaces of considerably larger capacity are in use.
The methods of bottle making are at the present time passing through
what is probably a stage of transition. Up to the middle of last
century the processes in use were little better than those of the
middle ages; the first step of a more modern development of the
industry took the direction of improved tools and implements for
carrying out the old operations. More recently a whole series of
inventions have been put forward with the aim of producing bottles by
entirely different and wholly mechanical processes with the object of
eliminating the uncertain element of skilled labour entirely. While it
must be admitted that some of the earlier of these inventions proved
to be brilliantly ingenious failures, there is little doubt that
here, as in other manufacturing processes, the machine-made article
will ultimately supersede the hand-made product. Even now, mechanical
processes are largely in use both in America and Europe, and at some
recent exhibitions machine-made bottles have been shown which in every
point of quality were superior to the best hand-made goods.
The first stage in the production of bottles by hand, and also for most
of the machine processes, is that of gathering the requisite quantity
of glass. The bottle-blower’s pipe is between 5 and 6 ft. long, and is
provided with a slightly enlarged end or “nose” upon which the glass is
gathered. Three gatherings are generally sufficient for the production
of ordinary bottles, but for extra large bottles, and especially for
carboys, heavier gatherings are necessary, and for these the gatherer
must go to the furnace four, five, or even six times. When the
requisite quantity of glass has been gathered on the pipe the gathering
is worked and rounded by rolling it either on a flat metal plate or
“marver,” or in a hollowed block made of wood or more rarely of metal;
by this process the glass is formed into a well-rounded, symmetrical
pear-shaped body. The blower now distends the mass gradually by the
pressure of his breath, at the same time swinging the pipe, the effect
of these movements being to draw the bulk of the glass downwards,
leaving a thinner and colder portion having the rudimentary shape of
the neck of the bottle next to the pipe. In the oldest form of the
process the next stage in the production of the bottle is accomplished
by the aid of a cylindrical mould of fire-clay, whose diameter is that
of the external size of the finished bottle. The pear-shaped bulb of
glass is for this purpose re-heated at the melting furnace, and is
then placed inside the fire-clay mould. By vigorous blowing, and a
rapid rotation of the pipe and glass, the bulb is forced to assume
the cylindrical shape of the mould, the glass forming the neck of the
bottle being at this stage of the process too cold and stiff to be
further deformed. The next step is the formation of the concavity found
in the base of wine and beer bottles; this is produced by pushing up
the hot plastic glass that forms the bottom of the bottle as it leaves
the clay mould. This is done by a second workman using an iron rod
known as the “pontil,” upon which a small mass of glass has previously
been gathered. This mass of glass remains attached to the bottom of
the bottle, which is thus for the moment fastened both to the “pontil”
and to the blower’s pipe. The blower, however, immediately detaches
the bottle from the pipe at the point where the neck of the bottle
is intended to end, effecting this by locally chilling the glass--a
process known by the descriptive term of “wetting off.” The unfinished
bottle is now attached to and handled by means of the “pontil.” The
neck is softened by re-heating it over the furnace, and is then moulded
into the desired shape by the aid of specially-shaped tongs. Finally
a thread of glass is wound round the end of the neck to produce the
thickening usually found at that point. The finished bottle, still
attached to the “pontil,” is now carried to the annealing kiln, where
it is placed in position and detached from the “pontil” by a sharp
blow, which severs the glass that had been gathered on the “pontil”
from the bottom of the bottle.
The process, in the form described above, has been obsolete for
many years, improvements, consisting of appliances for facilitating
the various operations, having been gradually introduced. The most
important of these is the substitution of metal moulds for the
fire-clay moulds of earlier times. These metallic moulds are made to
open and close at will by the action of a pedal, and are designed to
give the entire bottle its final shape, except for the indentation
of the bottom, although this is sometimes produced by a convex
piece placed on the bottom of the mould. In the formation of the
neck thickening, also, important mechanical aids have become almost
universal. These last consist of tongs provided with rollers and
arranged to rotate about an axis that terminates in a tapered spike
which enters the neck of the bottle; by pressing the tongs together so
as to bring the rollers against the outside of the neck and rotating
the whole, the rollers are made to form the neck thickening in an
accurate and rapid manner.
Important and valuable as these improvements of the ancient process
of bottle-blowing undoubtedly are, they do not touch the main
disadvantages of the process--disadvantages that seriously affect the
economy of the process and the well-being of the workers employed upon
it. It is consequently not surprising that a great number of inventors
have laboured at the problem of the purely mechanical production of
bottles. A large number of patents have accordingly been taken out in
connection with bottle-making machinery. The first of these to attain
any favour was that devised by Ashley, but although great claims were
made for it, its use has not extended. At the present time, however,
there are a number of bottle-works actually at work producing bottles
by mechanical means; one of the most successful of these machines is
that devised by Boucher, of Cognac. The products of this machine,
exhibited in Paris at the exhibition of 1900, were equal, and possibly
superior, to the best hand-made bottles. The Boucher machine, although
by no means entirely automatic, requires no highly-skilled labour
beyond that of a workman whose duty it is to operate the various
levers of the machine at the right instant and in the proper order.
The details of the machine, as set forth in the patents and other
published descriptions, are somewhat complicated, and vary somewhat
in the different models; the general principle and mode of operation
is, however, the same in all varieties of the machine, and we shall
therefore give a brief account of it here.
In the Boucher process, the glass is first gathered from the furnace,
but as no blowing-pipes are required, the gathering is done on a light
iron rod, thus saving the gatherer much of the labour of carrying
the heavy pipes. The requisite quantity of the glass so gathered is
then dropped into the first or “measuring” mould of the machine, the
“thread” being cut by hand by the operator. From the measuring mould,
the glass is next caused to pass into the “neck” mould; the glass
flows into this mould, and is further pressed into it by the aid of
compressed air, applied above the free surface of the glass. At this
stage the still liquid glass has the external shape of the neck of
the bottle, but the mass of glass is solid, _i.e._, no cavity has yet
been produced in it. The formation of the cavity is next begun by the
action of a plunger which is driven into the “solid” mass of glass
filling the neck mould, this plunger thus punching out the passage
through the neck of the bottle. As soon as the plunger is withdrawn,
compressed air is admitted into the cavity so formed, and the mass
of glass is at the same time inverted, and that part occupying the
position of what is to be the shoulder of the bottle is allowed to
descend while being blown out by the compressed air. This process of
distension is limited, and the desired shape is imparted to the mass
by bringing towards it a third mould, by contact with which the glass
is considerably stiffened--a row of jets of compressed air, impinging
on the outside of the glass forming the shoulder of the bottle, being
further used to stiffen the glass, once the requisite extension has
been attained. The mass has now a shape very similar to that known
as a “parason” in hand bottle-blowing, and is by this time decidedly
stiff. It is now introduced into the finishing mould and is blown into
perfect contact with the mould by powerful air-pressure, thus attaining
the proper shape of barrel and base; the indentation of the base is,
however, sometimes produced on a separate machine or press. During
all these operations the neck of the bottle, which was the first part
to be formed, has remained firmly held in the neck mould, and all the
movements that have been described are performed by means of levers
actuating movements of this mould as a whole, which, of course, carry
the glass with them. The last movement of the levers, which releases
the bottle from the finishing mould, also opens the neck mould, and
thus leaves the bottle finished and entirely free.
It will be seen that the process adopted in this machine follows as
closely as possible the various stages of hand blowing, but that the
mechanical movements of the machine replace the laborious and difficult
technique of the blower. One such machine is capable of producing
as many as 120 bottles, each weighing 1¾ lbs., per hour, but this
is accomplished only by having some of the moulds in duplicate and
so arranged as to come into use alternately. The machine itself is
attended by one “moulder,” who operates the levers, and by a youth,
who carries the finished bottles to the annealing kiln, while, of
course, the services of a gatherer are also required. The appearance of
a bottle works equipped with these machines is in striking contrast to
that of a hand-blowing works, where the stages around the working-holes
are crowded with men doing arduous work under very severe conditions of
temperature and atmosphere. Finally, it must be pointed out that the
use of the Boucher machine is by no means confined to the production of
the cheapest kinds of bottles, but that it has shown itself especially
well suited to the production of champagne and other bottles that
are required to withstand a high internal pressure, the machine-made
bottles showing excellent results under pressure tests. The machine
is also used for the production of moulded glass-ware of white glass,
since it can be adapted to the production of any kind of glass vessel
that can be produced by blowing into a mould.
The annealing of bottles was formerly carried out in large chambers or
kilns of very simple construction, in which the bottles were stacked as
made, the kiln being previously heated to the requisite temperature:
when full, the kiln was closed up in a rough temporary manner and
allowed to cool naturally, thus annealing the bottles stacked within
it. In this branch of glass-making also, however, the continuous
annealing kiln has superseded the older kinds, and continuous kilns are
now almost universal in bottle-making. In these kilns, which consist
of long tunnels, kept hot at one end and having a gradually decreasing
temperature as the other end is approached, the bottles are stacked on
trucks which are slowly drawn through the kiln from the hot to the cold
end. At the cold end the trucks are unloaded and are then returned, by
an outside route, to the charging end, but of course the bottles cannot
be stacked on the truck until it has actually entered the hot end of
the tunnel and acquired the temperature there prevailing. In a slightly
different form of kiln, the bottles are carried down the kiln on a
species of conveyer belt formed of iron plates, but the principle of
all these appliances is similar even when used for very different kinds
of glass.
In the account of bottle manufacture given above we have referred
almost exclusively to the mode of production of the ordinary bottles
used for the storage of such liquids as wine, beer, spirits, etc., and
we will now deal with some other branches of manufacture closely allied
to these.
An important branch of glass manufacture is the production of vessels
of large dimensions. Those most closely allied to ordinary bottles are
the vessels known as carboys, used for the storage and transportation
in bulk of chemical liquids, and especially of acids. Formerly these
were blown by hand in a manner closely resembling that used for
ordinary bottles, but the weight of the mass of glass to be handled by
gatherer and blower is very great, while the lung-power of a blower is
not sufficient to produce the great expansion required. Formerly the
only aid available to the blower was the device of injecting into the
hot, hollow glass body, at an early stage of the process, a quantity
of water or alcohol; this liquid was immediately vapourised by the
heat of the glass, and if the blower closed the mouthpiece end of his
pipe by placing his thumb over it, the expansive force of the vapour
so generated served to blow out the glass to the desired extent. More
recently mechanical aids to the production of these large vessels
have become available, first in the shape of mechanical arrangements
for relieving the workmen of the full weight of the glass and pipe
by providing suitable arms upon which the whole can be supported
without interfering with the blower’s freedom of manipulating the pipe
and glass in the desired way; further, a supply of compressed air,
which can be readily connected with the pipe at any desired moment,
facilitates the blowing process.
A process of producing hollow glass vessels of very large size by
purely mechanical means has, however, been introduced during recent
years by P. Sievert, of Dresden. By the methods of this inventor,
glass vessels of quite unprecedented size--such as bath-tubs freely
accommodating full-grown men--can be produced. For this purpose the
glass is spread out on the surface of a large cast-iron plate, provided
with numerous small holes through which steam or compressed air may be
blown when desired. The slab of viscous glass, when properly spread
over this plate, is clamped down against it all around the outside
edge by means of a suitably-shaped iron collar, which holds the glass
in air-tight contact against the plate beneath. The whole iron plate,
with the slab of glass clamped to it, is now turned over, so that the
glass hangs down under the plate. The glass immediately begins to sag
under its own weight, and is assisted in this tendency by a suitable
blowing of steam or air into the space between the plate and the glass.
In blowing bath-tubs in this way the glass is allowed to distend
downwards until the desired depth is attained, when further distension
is arrested by bringing a flat supporting plate under the glass, which
is pressed against this flat plate by the pressure of the air, thus
forming the flat bottom of the tub. In this process the outline of the
object is determined by the shape of the clamping bars or plate that
fix the edges of the hot glass against the iron plate described above,
and by this means almost any desired shape can be given to objects of
simple form.
It is obvious that this process can also be employed for blowing a
hollow body into contact with a mould of any desired form and forcing
the hot glass to take the exact shape of the mould; for smaller
bodies, however, the blowing in of separately generated steam is not
required, the heat of the molten glass itself being used to generate
the necessary steam. For this purpose the requisite quantity of glass
is dropped on the surface of a wet slab of asbestos. On this surface
the glass remains floating upon a layer of steam, which is constantly
renewed by the intense heating action of the hot glass on the water
contained in the asbestos below. The moulds used in this process are
provided with a sharp edge or lip, and as soon as the glass has spread
into a slab of sufficient size, the inverted mould is brought down
upon the glass and pressed against it. The sharp lip or edge of the
mould forces the glass into close contact with the asbestos under
it all around the edge of the mould, thereby enclosing the space
existing between the rest of the glass and the wet asbestos. The heat
of the glass continues to generate steam at a rapid rate, but now the
steam can no longer escape from under the glass around the edges, and
therefore blows the glass upwards into the mould, ultimately forcing
the glass into intimate contact with the surface of the mould; when
this is accomplished, the pressure of the steam rises rapidly, and
ultimately lifts the entire mould and glass sufficiently to allow
the excess steam to escape--and this is the sign that the blowing is
complete. The whole process takes only a very few seconds, and is very
successful when applied to suitable glass and used with moulds of
proper shape. It is, of course, obvious that ordinary narrow-mouthed
bottles could not be produced in this way, but wide-mouthed bottles and
jars are made in this manner, although the chief utility of the process
lies in the production of comparatively shallow articles, which are not
of a shape that lends itself to pressing.
CHAPTER VIII.
BLOWN AND PRESSED GLASS.
In many ways very similar to the processes employed in the production
of bottles are those used in the manufacture of all hollow glass
vessels that are produced by blowing, either with or without the aid
of moulds. Apart from the actual shapes of the articles themselves,
however, the principal difference between bottles and the better
classes of hollow glass-ware lies in the composition and quality of
the glass itself. In this respect all grades of manufacture are to
be met with, from the light-coloured greenish or bluish glass used
for medicine bottles to the most perfectly colourless and brilliant
“crystal” or flint glass. This gradation in the perfection of the
glass represents a corresponding gradation in the care bestowed upon
the choice of raw materials and the various manipulations of melting
the glass. As we have seen, for the commonest kinds of bottles, where
colour and quality are immaterial, all kinds of fusible materials
can be utilised, loamy or ferruginous sands and refuse glass of all
kinds being employed. Where somewhat higher requirements have to be
met, rather purer sands have to be used as sources of silica, while
lime and alkali must be introduced in purer forms, the alkali in the
shape of the cheapest qualities of salt-cake and the lime in that
of lime-stones reasonably free from iron and magnesia. Finally, for
the best qualities of glass the purest sand obtainable is used, being
often specially washed to remove all loamy matter, while the alkali
is introduced in the form of carbonate, a chemical product which in
its better qualities is practically free from injurious impurities.
In these high-class products two very distinct kinds of glass are
met with. One class, of which the Bohemian “crystal” is the highest
example, is chemically of the nature of an alkali-lime silicate, the
alkali in the case of the Bohemian glass being potash; the other
variety of glass contains no lime, its place being taken by lead,
typical of this class being English flint glass. In some varieties of
glass, lead is also replaced, partially or entirely, by barium, but
this material is chiefly used for the manufacture of pressed glass.
The higher grades of quality in glass, which thus require increased
refinement in the raw materials, also demand increased refinement
in the furnaces and appliances employed in their melting. The
tank-furnace, which holds the field in bottle manufacture, is
scarcely met with in the production of the better qualities of hollow
glass-ware; medicine bottles and other articles of moderate quality
might be produced in tanks, but the quantity of glass required for
such purposes is seldom large enough to justify such large plant. For
the best qualities of colourless glass-ware, however, the tank-furnace
could not be used on account of the fact that both as regards colour
and freedom from defects, the product of a tank-furnace is never equal
to the best product of pot-furnaces. For flint-glass, indeed, _covered_
pots or crucibles must be used in order to adequately protect the
molten glass from the reducing action of the furnace gases and from
contamination by dust. The materials of which the pots are constructed
are also chosen with a view to avoiding all risk of introducing
colouring or otherwise injurious impurities from that source.
In all processes for the production of hollow glass-ware, the glass or
“metal” is taken from the pot by the process of gathering which has
already been described; where blown articles are to be produced, as
distinct from pressed goods, the initial stage is always the formation
of a small hollow globe or bulb at the end of the glass-blower’s pipe.
The subsequent manipulations depend upon the nature of the article to
be produced. The article may either be made entirely by hand work, or
rather “chair” work, as it is usually called, or the manipulations
may be facilitated and the product cheapened--while its character is,
of course, also modified--by the aid of moulds, which are used to
bring the object to its proper shape and to impress upon it certain
decorative mouldings or markings. As we have already seen, ordinary
bottles are now always blown with the aid of moulds, and the same
applies to medicine bottles, lamp chimneys, and the bulbs for electric
light; in connection with lamp-chimneys it should be noted that they
are blown in moulds in the form of cylindrical bottles with a flat
bottom and a domed top, the ends being subsequently cut off.
Many of the cheaper varieties of tumblers and glasses are also blown in
moulds, but they can be, and sometimes are, produced by hand, and as
their manufacture is typical of that of all hand-blown hollow ware, we
shall now describe it in some detail as an example of this class of
work.
The implements used by the glass-blower and his assistants for this
work are few and simple. The largest item is the glass-blower’s bench
or chair, which is simply a rough wooden bench provided with two
projecting side-rails or arms. When finishing a piece of work the
blower sits on this bench, and the pipe lies across the two rails
in front of him in such a position that by rolling it backwards and
forwards along the rails he can readily keep the pipe in gentle
rotation. In addition to the ordinary blower’s pipe and a “pontil”
or rod for attaching small quantities of glass whereby the piece in
hand can be held, the only other tools used by the blower are a number
of shears and pincers of various shapes which serve for cutting off,
pressing in, and distending the glass as required, a flat board and
a stone or metal plat or “marver” being also used for the purpose of
moulding the glass.
As already indicated, the first step in the production of such an
object as a tumbler consists in gathering a suitable quantity of glass
on the pipe and blowing it into a small bulb. This bulb is blown out
to the proper size and is then elongated by gently swinging the pipe.
The next step is the flattening of the lower end of the bulb by gently
pressing it on the “marver” or flat plate provided for such purposes;
in this way the flat bottom of the glass is formed, and the bulb now
has the shape of the finished glass, but remains attached to the pipe
by a shoulder and neck. The earliest practice was to separate the
tumbler from the pipe at such a point as to leave the tumbler of the
correct length, the remaining operation consisting in holding the
glass, first fixed to a pontil for the purpose, into the furnace so
as to heat the broken edge; this edge was thereby rounded off, and the
brim of the glass could be widened or otherwise shaped by rotating
the glass or pressing it in or out by the aid of pieces of wood. In
modern practice, however, this is not usual, the glass being separated
from the pipe well above the shoulder and annealed in this shape.
Subsequently the glass is finished in a trimming room or workshop by
being cut off at the desired point and having the rough edge rounded
off by the aid of a blowpipe flame. The cutting-off operation is
carried out in a great variety of ways, the most usual being by the
action of heat applied locally and suddenly, either by the aid of
specially-shaped flat blowpipe flames or by an electrically-heated
wire. Machines for carrying out this operation, as well as the
subsequent rounding of the edge automatically, are in use, but the
latter process is sometimes replaced by slightly grinding and polishing
the edges.
[Illustration: FIG. 8.--Sectional diagram of the evolution of a
tumbler.]
The evolution of an ordinary tumbler, as just described, and as
illustrated diagrammatically in Fig. 8, is typical of the whole process
of hollow-glass blowing, but of course the number of operations, as
well as the care and skill involved in each step, increases rapidly as
the form of the vessel becomes more complex; in the highest class of
work a very considerable element of artistic taste and judgment on the
part of the operative also becomes essential, for, although the form
of the object as well as the choice of colour and ornamentation are
chosen by the designer, the blower has to translate the drawing of the
designer into glass, and although his skill enables him to attain a
considerable degree of fidelity in his rendering, many details remain
at his own option, and the proper management of these is no small
factor in the success of the whole work.
In this connection mention should perhaps be made of the application of
colour and other decorations to this kind of glass. A very considerable
range of effects of this kind is now available to the glass-worker.
In the first place the body of the glass used for the production of
the articles in question may be coloured by the addition of suitable
colouring materials to the molten glass or raw materials, as explained
in Chapter XI., but this procedure has very obvious limitations; where
the article is built up of glass from several gatherings--as, for
example, is the case in an ordinary wine-glass, where the bowl, leg and
foot are each made of separate gatherings--it is possible to use glass
of different colours for these different parts, and this is commonly
done in the production of wine glasses having ruby or green bowls and
white legs and feet. A further modification in the application of
colour is obtainable by taking up two or more gatherings on the same
pipe and superposing a large gathering of white glass on a smaller
one of coloured glass; this is analogous to the process of “flashing”
sheet glass, described in Chapter X. and this process lends itself to a
variety of manipulations resulting in the distribution of the coloured
layer of glass in almost any desired manner over the object in hand.
The principal objection to this process, however, lies in the fact that
pots of molten glass of all the colours desired must be kept available
to the blower at the same time, and this is not easily arranged for in
any reasonably economical manner. For this reason, and also because the
manipulations are simpler, coloured glass intended for application to
blown glass-ware is generally used in the form of short rods previously
prepared; these rods are suitably heated, and the coloured glass can
then be applied to the article in hand at any desired place and in
as small or large a quantity as required. If the two glasses thus
brought into contact are properly related to one another as regards
chemical composition and physical properties, they blend very readily
and perfectly, and the result is quite as good as could be obtained by
using the coloured glass in the molten condition. Other decorations,
such as gilding or other metallic lustres and also various kinds of
iridescence, are produced upon the finished glass. Metallic lustres
are obtained by placing upon the surface of the glass, and slightly
fusing into it a layer of particles of the actual metal. In some cases
this is done by rolling the glass vessel, while still hot, in a mass of
metallic foil of the kind desired, when a sufficient quantity readily
adheres; in other cases the metal is applied in the form of a flux or
glaze containing a large proportion of an easily-reduced compound of
the metal, and this is afterwards reduced to the metallic state by the
action of heat, sometimes aided by that of smoke or other reducing
gases. An iridescent surface is produced upon certain varieties of
glass by the corrosive action of acid vapours; in fact, in localities
where the atmosphere is tainted with sulphur fumes it is quite usual
to see an iridescent lustre on the surface of ordinary window glass.
There are, of course, numerous other means of decorating blown and
other glass, such as cutting, engraving, etching, silvering, etc., but
it would lie beyond the scope of the present volume to deal with these,
since they are outside the field of actual glass manufacture.
In the production of hollow glass-ware by hand, the glass-blower avails
himself to the full of the property so characteristic of glass of
assuming a pasty or viscous condition when suitably heated; by raising
or lowering the temperature of his material, the blower can at will
render it stiffer or more fluid; by blowing he can distend it, draw it
out by the aid of gravity or centrifugal action, or he can mould it
with the aid of rods and tongs of suitable shape, while at times he
allows it to fall or festoon under its own weight while held aloft.
With all these manipulations at his disposal, the skilful operative
is able to work the glass to his will and to fashion objects of great
variety and beauty, but it should be noted that objects produced by
hand in this way will bear the mark of the processes employed in their
production in the fact that they do not possess the extreme regularity
of size and shape which are associated with machine-made articles;
there is a certain natural variability in the exact shape of curves and
festoons that is foreign to the products of mechanical processes. For
some purposes this variability is a disadvantage, while to some minds
it appears as a defect, and methods have been devised for facilitating
the production of strictly uniform glass-ware by the use of moulds as
an aid to the work of the glass-blower. While undoubtedly reducing the
value and beauty of the ware from the purely artistic standpoint, these
aids to hand-work have rendered possible an immense expansion of the
entire industry, since, with the use of moulds, presentable glass-ware
can be produced by hands far less skilled than those required for pure
hand-work.
In the description given above of bottle-blowing by hand we have
already seen an example of the use of moulds in aiding the blower to
form his object to the desired size and shape. Much more complicated
and decorative objects can, however, be produced by the use of moulds.
Such objects as globes and shades for gas, oil and electric lamps,
when of a light substance and suitable shape, are usually produced by
blowing bulbs of glass into moulds, where they acquire the general
shape as well as the detailed decorated surface configuration which
they afterwards present. Here again the body remains a closed vessel,
and is only opened and trimmed to the final shape at the end of the
operation when all the blowing and moulding have been done. Articles
blown in this way very frequently show “mould marks,” since the
contact of the hot glass with the relatively cold surface of the mould
results in a certain crinkling or roughening of the glass, much as in
the process of rolling. This effect can be minimised by dressing the
interior surfaces of the moulds with suitable greasy dressings, whose
chief property should be that they do not stick to the hot glass and
leave little or no residue when gradually burnt away in the mould;
the proper care of the moulds and their maintenance is in fact the
first essential to successful manufacture in this as well as in the
pressed-glass industry. Even under the most favourable conditions,
however, the surface of glass blown into moulds is not so good as
that of hand-blown articles which have never come into contact with
cold materials, and therefore retain undiminished the natural “fire
polish” which glass possesses when allowed to cool freely from the
molten state. An effort at producing a similar brilliance of surface
on moulded and pressed articles is often made by exposing them, after
they have attained their final form, to the heat of a furnace to such
an extent as to soften the surfaces and allow the glass to re-solidify
under the undisturbed influence of surface-tension much as it would do
in solidifying freely in the first place. Unfortunately this process
cannot be carried out without more or less softening the entire
article, so that skilful manipulation is required to prevent serious
deformation of the object, while a certain amount of rounding off in
all sharp corners and angles cannot be avoided.
The air-pressure required to bring the whole of the surfaces of a large
and possibly complicated piece of glass into contact with the surfaces
of the mould is sometimes very considerable, and the lung-power of the
blower is often insufficient for the purpose; in many works, therefore,
compressed air is supplied for the purpose, arrangements being employed
whereby the operative can quickly connect the mouthpiece of his pipe
with the air-main, while he can accurately control the pressure by
means of a suitable valve. The Sievert process of moulding by the aid
of steam pressure has already been described.
Although the evolution of the industry scarcely followed this path,
it is not a large step to pass from a process in which air-pressure
is used to drive viscous glass into contact with a mould to a process
in which the pressure of the air is replaced by the pressure of a
suitably-shaped solid plunger, and this is essentially the widely-used
process of glass pressing. In the first instance this mode of
manufacture is obviously applicable to solid or flat and shallow
articles which could not be conveniently evolved from the spherical
bulb which stands as embryo of all blown glass; at first sight it
would seem in fact as though the process must be limited to articles
of such a shape that a plunger can readily enter and leave the concave
portions. By the ingenious device, however, of pressing two halves of
a closed or nearly closed vessel simultaneously in two adjacent moulds
and then pressing the two halves together while still hot enough to
unite, it has been made possible to produce by the press alone such
objects as water-jugs, for example, into which a plunger could not
possibly be introduced when finished. The process of pressing being
a purely mechanical one and requiring no very elaborate plant and
little skilled labour, has placed upon the market a host of cheap and
extremely useful articles, thus serving to widen very considerably
the useful applications of glass. On the other hand, the process has
been and is still used to some extent for the production of articles
intended to imitate the products of other processes such as hand-blown
and cut glass, with the result that a great deal of glass has been
produced which cannot possibly be classed as beautiful and much of
which can lay as little claim to utility.
The essential feature of the process of glass pressing consists, as
already indicated, in forcing a layer of glass into contact with a
mould by the pressure of a mechanically actuated plunger. For this
purpose a suitable mould and plunger as well as a press for holding the
former and actuating the latter are required. The moulds are generally
made of a special quality of close-grained cast-iron, and they are
kept trimmed and dressed in much the same manner as the moulds used
for blowing (except that the latter are sometimes made of wood). For
the purpose of facilitating the removal of the finished article, the
moulds are generally made in several pieces which fit into one another
and can be separated by means of hinges. A very important point about
these moulds is that the various pieces should fit accurately into
one another, since otherwise a minute “fin” of glass will be forced
into every interstice, and the traces of these fins will always remain
visible on the finished article; the very perfect fit required to
entirely prevent the formation of such fins is, of course, scarcely
attainable in practice except in the case of new moulds, so that the
traces of fins are generally to be found on all pressed articles, and
serve as a ready means of identifying these products when an attempt
is made to imitate better classes of glass-ware by their means. The
presses used in this process are generally of the hand-lever type;
power presses could no doubt be used, but it is contended that the
hand-press has a very great advantage in allowing the operator to judge
by touch when sufficient pressure has been exerted, and this is an
important consideration, since an excessive pressure would either force
the glass out of the mould altogether or would be liable to burst or
injure the mould seriously. The actual presses consist of vertical
guides and levers for controlling the movement of the plunger and a
table for holding the moulds, and in some cases a system of cranks and
levers for opening and closing the moulds. The process of pressing is
exceedingly simple. The proper quantity of glass is gathered from the
pot on a solid rod and dropped into the mould. The thread of glass
which remains between the glass in the mould and that remaining on
the iron is cut off with a pair of shears, and then the plunger is
lowered into the mould and allowed to remain there until the glass
has stiffened sufficiently to retain its shape, when the plunger is
withdrawn. In this proceeding it will be seen that the glass is forced
into intimate contact with the relatively cold surfaces of mould and
plunger, and while undergoing this treatment the glass must remain
sufficiently plastic to readily adapt itself to the configuration of
the mould. It is therefore not surprising to find that the pressing
process can only be used successfully with glass of a kind specially
adapted for it. Certain varieties of flint glass and some barium
glasses are used for this purpose, but the greater quantity of pressed
glass, particularly as produced on the Continent, is made of a
lime-alkali silicate containing considerable quantities of both soda
and potash and relatively little lime; while sufficiently resistant for
most purposes, this glass is particularly soft and adaptable while in
the viscous condition.
The deleterious effect produced upon glass surfaces when brought into
contact with relatively cold metal has already been referred to above,
and it only remains to add that this is the principal difficulty with
which the glass-pressing process has to contend. It is overcome to
some extent by the aid of the reheating process described above; but
this is only a partial remedy, and in the majority of pressed glass
products the surface is “covered” as far as possible by the application
of relief decorations such as grooves, spirals, and ribbings. An
attempt is sometimes made to imitate the appearance of cut glass,
but the rounding of the angles during the reheating process destroys
the sharpness of the effect and allows of the ready detection of the
imitation, while the cheapness of the decoration when applied in the
mould has frequently led manufacturers to grossly over-decorate, and,
therefore, destroy all claim to beauty in their wares.
CHAPTER IX.
ROLLED OR PLATE-GLASS.
In the present chapter we propose to deal with all those processes
of glass manufacture in which the first stage consists in converting
the glass into a slab or plate by some process of rolling. We have
already considered the general character of the rolling process, and
have seen that, although hot, viscous glass lends itself readily to
being rolled into sheets or slabs, these cannot be turned out with
a smooth, flat surface. In practice the surface of rolled glass is
always more or less dimmed by contact with the minute irregularities
of table or roller, and larger irregularities of the surface arise
from the buckling that occurs at a great many places in the sheet.
These limitations govern the varieties of glass that can be produced by
processes that involve rolling, and have led to the somewhat curious
result that both the cheapest and roughest, as well as the best and
most expensive kinds of flat glass, are produced by rolling processes.
Ordinary rough “rolled plate,” such as that used in the skylights of
workshops and of railway stations, is the extreme on the one hand,
while polished plate-glass represents the other end of the scale. The
apparent paradox is, however, solved when it is noted that in the
production of polished plate-glass the character of the surface of
the glass as it leaves the rollers is of very minor importance, since
it is entirely obliterated by the subsequent processes of grinding,
smoothing, and polishing. Intermediate between the rough “rolled” and
the “polished” plate-glass we have a variety of glasses in which the
appearance of the rolled surface is hidden or disguised to a greater or
lesser extent by the application of a pattern that is impressed upon
the glass during the rolling process; thus we have rolled plate having
a ribbed or lozenge-patterned surface, or the well-known variety of
“figured rolled” plate, sometimes known as “Muranese,” whose elaborate
and deeply-imprinted patterns give a very brilliant effect.
Rolled plate-glass being practically the roughest and cheapest form of
glazing, is principally employed where appearance is not considered,
and its chief requirement is, therefore, cheapness, although both the
colour and quality of the glass are of importance as affecting the
quantity and character of the light which it admits to the building
where the glass is used. On the ground of cheapness it will be obvious
from what we have said above (Chapter IV.), that such glass can
only be produced economically in large tank furnaces, and these are
universally used for this purpose. The requirements as regards freedom
from enclosed foreign bodies of small size and of enclosed air-bells
are not very high in such glass, and, therefore, tanks of very simple
form are generally used. No refinements for regulating the temperature
of various parts of the furnace in order to ensure perfect fining of
the glass are required, and the furnace generally consists simply of an
oblong chamber or tank, at one end of which the raw materials are fed
in, while the glass is withdrawn by means of ladles from one or two
suitable apertures at the other end. For economical working, however,
the furnace must be capable of working at a high temperature, because
a cheap glass mixture is necessarily somewhat infusible, at all events
where colour is considered. This will be obvious if we remember that
the fusibility of a glass depends upon its alkali contents, and alkali
is the most expensive constituent of such glasses.
The actual raw materials used in the production of rolled plate-glass
are sand, limestone and salt-cake, with the requisite addition of
carbon and of fluxing and purifying materials. The selection of these
materials is made with a view to the greatest purity and constancy
of composition which is available within the strictly-set limits of
price which the low value of the finished product entails. These
materials are handled in very large quantities, outputs of from 60 to
150 tons of finished glass per week from a single furnace being by no
means uncommon; mechanical means of handling the raw materials and of
charging them into the furnace are therefore adopted wherever possible.
The glass is withdrawn from the furnace by means of large iron ladles.
These ladles are used of varying sizes in such a way as to contain the
proper amount of glass to roll to the various sizes of sheets required.
The sizes used are sometimes very large, and ladles holding as much
as 180 to 200 lbs. of glass are used. These ladles, when filled with
glass, are not carried by hand, but are suspended from slings attached
to trolleys that run on an overhead rail. The ladler, whose body is
protected by a felt apron and his face by a mask having view-holes
glazed with green glass, takes the empty ladle from a water-trough, in
which it has been cooled, carries it to the slightly inclined gangway
that leads up to the opening in the front of the furnace, and there
introduces the ladle into the molten glass, giving it a half-turn so
as to fill it with a “solid” mass of glass. By giving the ladle two or
three rapid upward jerks, the operator then detaches the glass in the
ladle as far as possible from the sheets and threads of glass which
would otherwise follow its withdrawal; then the part of the handle of
the ladle near the bowl is placed in the hook attached to the overhead
trolley, and by bearing his weight on the other end of the handle, the
workman draws the whole ladle up from the molten bath in the furnace
and out through the working aperture. This operation only takes a
few seconds to perform, but during this time the ladler is exposed
to great heat, as a more or less intense flame generally issues from
the working aperture, whence it is drawn upward under the hood of the
furnace. From the furnace opening, the ladler, generally aided by a
boy, runs the full ladle to the rolling table and there empties the
ladle upon the table just in front of the roller. In doing this, two
distinctly different methods are employed. In one, only the perfectly
fluid portion of the glass is poured out of the ladle by gradually
tilting it, the chilled glass next to the walls of the ladle being
retained there and ultimately returned to the furnace while still hot.
In the other method, the chilling of the glass is minimised as far as
possible, and the entire contents of the ladle are emptied upon the
rolling table by the ladler, who turns the entire ladle over with a
rapid jerk which is so arranged as to throw the coldest part of the
glass well away from the rest. When the sheet is subsequently rolled
this chilled portion is readily recognised by its darker colour, and
since it lies entirely at one end of the sheet it is detached before
the sheet goes any further. Neither method appears to present any
preponderating advantage.
[Illustration: FIG. 9.--Rolling table for rolled plate-glass.]
The rolling table used in the manufacture of rolled plate is
essentially a cast-iron slab of sufficient size to accommodate the
largest sheet which is to be rolled; over this slab moves a massive
iron roller which may be actuated either by hand or by mechanical
power--the latter, however, being now almost universal. The thickness
of the sheet to be rolled is regulated by means of slips of iron placed
at the sides of the table in such a way as to prevent the roller from
descending any further towards the surface of the table: so long as
the layer of glass is thicker than these slips, the entire weight of
the roller comes upon the soft glass and presses it down, but as
soon as the required thickness is attained, the weight of the roller
is taken by the iron slips and the glass is not further reduced in
thickness. The width of the sheet is regulated by means of a pair of
iron guides, formed to fit the forward face of the roller and the
surface of the table, in the manner indicated in Fig. 9. The roller,
as it moves forward, pushes these guides before it, and the glass is
confined between them. When the roller has passed over the glass, the
sheet is left on the iron table in a red-hot, soft condition, and it
must be allowed to cool and harden to a certain extent before it can
be safely moved. In this interval, the chilled portion--if any--is
partially severed by an incision made in the sheet by means of a long
iron implement somewhat like a large knife, and then the sheet is
loosened from the bed of the table by passing under it, with a smooth
rapid stroke, a flat-bladed iron tool. The sheet is next removed to
the annealing kiln or “lear,” being first drawn on to a stone slab and
thence pushed into the mouth of the kiln. At this stage the chilled
portion of the sheet is completely severed by a blow which causes the
glass to break along the incision previously made.
The rolled-plate annealing kiln is essentially a long, low tunnel,
kept hot at one end, where the freshly-rolled sheets are introduced,
and cold at the other end, the temperature decreasing uniformly down
the length of the tunnel. The sheets pass down this tunnel at a
slow rate, and are thus gradually cooled and annealed sufficiently
to undergo the necessary operations of cutting, etc. Although thus
simple in principle, the proper design and working of these “lears” is
by no means simple or easy, since success depends upon the correct
adjustment of temperatures throughout the length of the tunnel and a
proper rate of movement of the sheets, while the manner of handling and
supporting the sheets is vital to their remaining flat and unbroken.
The actual movement of the sheets is effected by a system of moving
grids which run longitudinally down the tunnel. The sheets ordinarily
lie flat upon the stone slabs that form the floor of the tunnel, and
the grids are lowered into recesses cut to receive them. At regular
intervals the iron grid bars are raised just sufficiently to lift the
sheets from the bed of the kiln, and are then moved longitudinally a
short distance, carrying the sheets forward with them and immediately
afterwards again depositing them on the stone bed. The grids return to
their former position while lowered into their recesses below the level
of the kiln bed.
When they emerge from the annealing kiln or “lear” the sheets of rolled
plate-glass are carried to the cutting and sorting room. Here the
sheets are trimmed and cut to size. The edges of the sheets as they
leave the rolling table are somewhat irregular, and sometimes a little
“beaded,” while the ends are always very irregular. Ends and edges are
therefore cut square or “trimmed” by the aid of the cutting diamond.
For this purpose the sheet is laid upon a flat table, the smoothest
side of the sheet being placed upwards, and long cuts are taken with
a diamond--good diamonds of adequate size and skilful operators being
necessary to ensure good cutting on such thick glass over long lengths.
Strips of glass six or eight feet long and half an inch wide are
frequently detached in the course of this operation, and the final
separation is aided by slight tapping of the underside of the glass
just below the cut and--if necessary--by breaking the strip off by the
aid of suitable tongs.
No very elaborate “sorting” of rolled plate glass is required, except
perhaps that the shade of colour in the glass may vary slightly from
time to time, and it is generally preferable to keep to one shade of
glass in filling any particular order. Apart from this, the rolled
plate cutter has merely to cut out gross defects which would interfere
too seriously with the usefulness of the glass. As we have already
indicated, air-bells and minute enclosures of opaque matter are not
objectionable in this kind of glass, but large pieces of opaque
material must generally be cut out and rejected, not only because
they are too unsightly to pass even for rough glazing purposes, but
also because they entail a considerable risk of spontaneous cracking
of the glass--in fact, visible cracks are nearly always seen around
large “stones,” as these inclusions are called. These may arise from
various causes, such as incomplete melting of the raw materials, or the
contamination of the raw materials with infusible impurities, but the
most fruitful source of trouble in this direction lies in the crumbling
of the furnace lining, which introduces small lumps of partially melted
fire-clay into the glass. In a rolled plate tank furnace which is
properly constructed and worked, the percentage of sheets which have to
be cut up on account of such enclosures should be very small, at all
events until the furnace is old, when the linings naturally show an
increasing tendency to disintegrate.
Returning now to the rolling process, it is readily seen that a very
slight modification will result in the production of rolled plate-glass
having a pattern impressed upon one surface; this modification
consists in engraving upon the cast-iron plate of the rolling table in
intaglio any pattern that is to appear upon the glass in relief. As
a matter of fact only very simple patterns are produced in this way,
such as close parallel longitudinal ribbing and a lozenge-pattern, the
reason probably being that the cost of cutting an elaborate pattern
over the large area of the bed-plate of one of these tables would be
very considerable. Further, as these tables and their bed-plates are so
very heavy, they are not readily interchanged or left standing idle, so
that only patterns required in very great quantity could be profitably
produced in this way. These disadvantages are, however, largely
overcome by the double-rolling machine. In this machine, into whose
rather elaborate details we cannot enter here, the glass is rolled
out into a sheet of the desired size and thickness by being passed
between two rollers revolving about stationary axes, the finished sheet
emerging over another roller, and passing on to a stone slab that
moves forward at the same rate as the sheet is fed down upon it. In
this machine a pattern can be readily imprinted upon the soft sheet
as it passes over the last roller by means of a fourth roller, upon
which the pattern is engraved; this is pressed down upon the sheet,
and leaves upon it a clear, sharp and deep impress of its pattern.
The general arrangement of the rollers in this machine is shown in
the diagram of Fig. 10, which represents the sectional elevation of
the appliance. After leaving the rolling machine, the course of the
“figured rolled plate” produced in this manner is exactly similar to
that of ordinary rolled plate, except that as a somewhat softer kind of
glass is generally used for “figured,” the temperature of the annealing
kilns requires somewhat different adjustment. The cutting of the glass
also requires rather more care, and it should be noted that such glass
can only be cut with a diamond on the smooth side; the side upon which
the pattern has been impressed in relief cannot be materially affected
by a diamond. This is one reason why it is not feasible to produce such
glass with a pattern on both sides.
[Illustration: FIG. 10.--Sectional diagram of machine for rolling
“figured rolled” plate-glass.]
Figured rolled glass, being essentially of an ornamental or decorative
nature, is generally produced in either brilliantly white glass or in
special tints and colours, and the mixtures used for attaining these
are, of course, the trade property of the various manufacturers; the
whiteness of the glass, however, is only obtainable by the use of very
pure and, therefore, expensive materials. As regards the coloured
plate-glasses, a general account of the principles underlying the
production of coloured glass will be found in Chapter XI.
The manufacture of polished plate-glass really stands somewhat by
itself, almost the only feature which it has in common with the
branches of manufacture just described being the initial rolling
process.
The raw materials for the production of plate-glass are chosen with
the greatest possible care to ensure purity and regularity; owing to
the very considerable thickness of glass which is sometimes employed
in plate, and also to the linear dimensions of the sheets which allow
of numerous internal reflections, the colour of the glass would become
unpleasantly obtrusive if the shade were at all pronounced. The actual
raw materials used vary somewhat from one works to another; but, as
a rule, they consist of sand, limestone, and salt-cake, with some
soda-ash and the usual additions of fluxing and purifying material such
as arsenic, manganese, etc. The glass is generally melted in pots, and
extreme care is required to ensure perfect melting and fining, since
very minute defects are readily visible in this glass when finished,
and, of course, detract most seriously from its value.
The method of transferring the glass from the melting-pot to the
rolling table differs somewhat in different works. In many cases the
melting-pots themselves are taken bodily from the furnace and emptied
upon the bed-plate of the rolling machine, while in other cases the
glass is first transferred to smaller “casting” pots, where it has to
be heated again until it has freed itself from the bubbles enclosed
during the transference, and then these smaller pots are used for
pouring the glass upon the rolling slab. The advantage of the latter
more complicated method lies, no doubt, in the fact that the large
melting-pots, which have to bear the brunt of the heat and chemical
action during the early stages of melting, are not exposed to the great
additional strain of being taken from the hot furnace and exposed
for some time to the cold outside air. Apart from the mechanical
risks of fracture, this treatment exposes the pots to grave risks of
breakage from unequal expansion and contraction on account of the great
differences of temperature involved. Where smaller special casting-pots
are used, these are not exposed to such prolonged heat in the furnace,
and are never exposed to the chemical action of the raw materials, so
that these subsidiary pots may perhaps be made of a material better
adapted to withstand sudden changes of temperature than the high-class
fire-clay which must be used in the construction of melting pots. On
the other hand, the transference of the glass from the melting to
the casting-pots involves a laborious operation of ladling and the
refining of the glass, with its attendant expenditure of time and fuel.
Finally, the production of plate-glass in tank furnaces could only be
attempted by the aid of such casting-pots in which the glass would have
to undergo a second fining after being ladled from the tank, and this
would materially lessen the economy of the tank for this purpose, while
it is by no means an easy matter to produce in tank furnaces qualities
of glass equal as regards colour and purity to the best products of the
pot furnace.
The withdrawal of the pots containing the molten glass from the furnace
is now universally carried out by powerful machinery. The pots are
provided on their outer surface with projections by which they can be
held in suitably-shaped tongs or cradles. A part of the furnace wall,
which is constructed each time in a temporary manner, is broken down;
the pot is raised from the bed or “siege” of the furnace by the aid
of levers, and is then bodily lifted out by means of a powerful fork.
The pot is then lifted and carried by means of cranes until it is in
position above the rolling table; there the pot is tilted and the glass
poured out in a steady stream upon the table, care being taken to avoid
the inclusion of air-bells in the mass during the process of pouring.
When empty, the pot is returned to the furnace as rapidly as possible,
the glass being meanwhile rolled out into a slab by the machine.
Except for the greater size and weight of both table and roller, the
plate-glass rolling table is similar to that already described in
connection with rolled plate. Of course, since the glass is poured
direct from the pot, there is no chilled glass to be removed. Further,
owing to the large size of sheets frequently required, the bed of the
rolling table cannot be made of a single slab of cast-iron, a number of
carefully jointed plates being, in fact, preferable, as they are less
liable to warp under the action of the hot glass.
In arranging the whole of the rolling plant, the chief consideration
to be kept in mind is that it is necessary to produce a flat sheet of
glass of as nearly as possible equal thickness all over. The final
thickness of the whole slab when ground and polished into a sheet
of plate-glass must necessarily be slightly less than that of the
thinnest part of the rough rolled sheet. If, therefore, there are any
considerable variations of thickness, the result will be that in some
parts of the sheet a considerable thickness of glass will have to be
removed during the grinding process. This will arise to a still more
serious extent if the sheet as a whole should be bent or warped so
as to depart materially from flatness. The two cases are illustrated
diagrammatically in Fig. 11, which shows sectional views of the sheets
before and after grinding on an exaggerated scale.
[Illustration: FIG. 11.--Sectional diagram illustrating waste of glass
in grinding curved or irregular plate.]
While it is evident that careful design of the rolling table will avoid
all tendency to the formation of sheets of such undesirable form, it
is a much more difficult matter to avoid all distortion of the sheet
during the annealing process and while the sheet is being moved from
the rolling table to the annealing kiln. Owing to the great size of
the slabs of glass to be dealt with, and still more to the stringent
requirement of flatness, the continuous annealing kiln, in which the
glass travels slowly down a tunnel from the hot to the cold end, has
not been adopted for the annealing of plate-glass, and a form of
annealing kiln is still used for that glass which is similar in its
mode of operation to the old-fashioned kilns that were used for other
kinds of glass before the continuous kiln was introduced. These kilns
simply consist of chambers in which the hot glass is sealed up and
allowed to cool slowly and uniformly during a more or less protracted
period. In the case of plate-glass, the slabs are laid flat on the
stone bed of the kiln. This stone bed is built up of carefully dressed
stone, or blocks of fire-brick bedded in sand in such a way that they
can expand freely laterally without causing any tendency for the floor
to buckle upwards as it would do if the blocks were set firmly against
one another. The whole chamber is previously heated to the requisite
temperature at which the glass still shows a very slight plasticity.
The hot glass slabs from the rolling table are laid upon the bed
of this kiln, several being usually placed side by side in the one
chamber, and the slabs in the course of the first few hours settle down
to the contour of the bed of the kiln, from which shape and position
they are never disturbed until they are removed when quite cold. In
modern practice the cooling of a kiln is allowed to occupy from four
to five days; even this rate of cooling is only permissible if care
is taken to provide for the even cooling of all parts of the kiln,
and for this purpose special air-passages are built into the walls of
the chamber and beneath the bed upon which the glass rests, and air
circulation is admitted to these in such a way as to allow the whole of
the kiln to cool down at the same rate; in the absence of such special
arrangements, the upper parts of the kiln would probably cool much more
rapidly than the base, so that the glass would be much warmer on its
under than on its upper surface.
When the slabs of plate-glass are removed from the annealing kilns
they very closely resemble sheets of rolled plate in appearance,
and they are quite sufficiently transparent to allow of examination
and the rejection of the more grossly defective portions; the more
minute defects, of course, can only be detected after the sheets have
been polished, but this preliminary examination saves the laborious
polishing of much useless glass.
The process of grinding and polishing plate-glass consists of three
principal stages. In the first stage the surfaces of the glass are
ground so as to be as perfectly flat and parallel as possible; in order
to effect this object as rapidly as possible, a coarse abrasive is
used which leaves the glass with a rough grey surface. In the second
stage, that of smoothing, these rough grey surfaces are ground down
with several grades of successively finer abrasive until finally an
exceedingly smooth grey surface is left. In the third and final stage,
the smooth grey surface is converted into the brilliant polished
surface with which we are familiar by the action of a polishing medium.
Originally the various stages of the grinding and polishing processes
were carried out by hand, but a whole series of ingenious machines
has been produced for effecting the same purpose more rapidly and
more perfectly than hand-labour could ever do. We cannot hope to give
any detailed account of the various systems of grinding and polishing
machines which are even now in use, but must content ourselves with
a survey of some of the more important considerations governing the
design and construction of such machinery.
In the first place, before vigorous mechanical work can be applied to
the surface of a plate of glass, that plate must be firmly fixed in a
definite position relatively to the rest of the machinery, and such
firm fixing of a plate of glass is by no means readily attained, since
the plate must be supported over its whole area if local fracture is to
be avoided. While the surface of the plate is in the uneven condition
in which it leaves the rolling table, such a firm setting of the glass
can only be attained by bedding it in plaster, and this must be done in
such a manner as to avoid the formation of air-bubbles between plaster
and glass; if bubbles are allowed to form, they constitute places where
the glass is unsupported. During the grinding and polishing processes
these unsupported places yield to the heavy pressure that comes upon
them, and irregularities in the finished polished surfaces result.
The most perfect adhesion between glass and plaster is attained by
spreading the paste of plaster on the up-turned surface of the slab of
glass and lowering the iron bed-plate of the grinding table down upon
it, the bed-plate with the adhering slab of glass being afterwards
turned over and brought into position in the grinding machine. When one
side of the glass has been polished, it is generally found sufficient
to lay the slab down on a bed of damp cloth, to which it adheres very
firmly, although sliding is entirely prevented by a few blocks fixed to
the table in such a way as to abut against the edges of the sheet. In
many works, however, the glass is set in plaster for the grinding and
polishing of the second side as well as of the first.
The process of grinding and polishing is still regarded in many
plate-glass works as consisting of three distinct processes, known
as rough grinding, smoothing and polishing respectively. Formerly
these three stages of the process were carried out separately; at
first by hand, and later by three different machines. In the most
modern practice, however, the rough and smooth grinding are done on
the same machine, the only change required being the substitution of
a finer grade of abrasive at each step for the coarser grade used in
the previous stage. For the polishing process, however, the rubbing
implements themselves must be of a different kind, for while the
grinding and smoothing is generally done by means of cast-iron rubbers
moving over the glass, the polishing is done with felt pads. The table
of the machine, to which the glass under treatment is attached, is
therefore made movable, and when the grinding and smoothing processes
are complete, the table with its attached glass is moved so as to come
beneath a superstructure carrying the polishing rubbers, and the whole
is then elevated so as to allow the rubbers to bear on the glass.
The earliest forms of grinding machines gave a reciprocal motion to
the table which carries the glass, or the grinding rubbers were moved
backward and forward over the stationary table. Rotary machines,
however, were introduced and rapidly asserted their superiority, until,
at the present time, practically all plate-glass is ground on rotating
tables, some of these attaining a diameter of over 30 ft. The grinding
“rubbers” consist of heavy iron slabs, or of wood boxes shod with iron,
but of much smaller diameter than the grinding table. The rubbers
themselves are rotary, being caused to rotate either by the frictional
drive of the rotating table below them, or by the action of independent
driving mechanism, but the design of the motions must be so arranged
that the relative motion of rubber and glass shall be approximately the
same at all parts of the glass sheets, otherwise curved instead of
plane surfaces would be formed. This condition can be met by placing
the axes of the rubbers at suitable points on the diameter of the
table. The abrasive is fed on to the glass in the form of a thin paste,
and when each grade or “course” has done the work required of it, the
whole table is washed down thoroughly with water and then the next
finer grade is applied. The function of the first or coarsest grade is
simply to remove the surface irregularities and to form a rough but
plane surface. The abrasive ordinarily employed is sharp sand, but
only comparatively light pressure can be applied, especially at the
beginning of this stage, since at that period the weight of the rubber
is at times borne by relatively small areas of glass that project here
and there above the general level of the slab. As these are ground
away, the rubbers take a larger and more uniform bearing, and greater
pressure can be applied. The subsequent courses of finer abrasives
are only required to remove the coarse pittings left in the surface
by the action of the first rough grinding sand; the finer abrasive
replaces the deep pits of the former grade by shallower pits, and this
is carried on in a number of steps until a very smooth “grey” surface
is attained and the smoothing process is complete. The revolving table
or “platform” is now detached from the driving mechanism, and moved
along suitably placed rails on wheels provided for that purpose, until
it stands below the polishing mechanism. Here it is attached to a
fresh driving mechanism, and it is then either raised so as to bring
the glass into contact with the felt-covered polishing rubbers, or
the latter are lowered down upon the glass. The polishing rubbers are
large felt-covered slabs of wood or iron which are pressed against
the glass with considerable force; their movement is very similar to
that of the grinding rubbers, but in place of an abrasive they are
supplied with a thin paste of rouge and water. The time required for
the polishing process depends upon the perfection of the smoothing
that has been attained; in favourable cases two or three hours are
sufficient to convert the “grey” surface into a perfectly polished one;
where, however, somewhat deeper pits have been left in the glass, the
time required for polishing may be much longer, and the polish attained
will not be so perfect. The mode of action of a polishing medium such
as rouge is now recognised to be totally different in character from
that of even the finest abrasive; the grains of the abrasive act by
their hardness and the sharpness of their edges, chipping away tiny
particles of the glass, so that the glass steadily loses weight during
the grinding and smoothing processes. During the polishing process,
however, there is little or no further loss of weight, the glass
forming the hills or highest parts of the minutely pitted surface being
dragged or smeared over the surface in such a way as to gradually fill
up the pits and hollows. The part played by the polishing medium is
probably partly chemical and partly physical, but it results, together
with the pressure of the rubber, in giving to the surface molecules
of the glass a certain amount of freedom of movement, similar to that
of the molecules of a viscid liquid; the surface layers of glass
are thus enabled to “flow” under the action of the polisher and to
smooth out the surface to the beautiful level smoothness which is so
characteristic of the surfaces of liquids at rest. This explanation
of the polishing process enables us to understand why the proper
consistency of the polishing paste, as well as the proper adjustment
of the speed and pressure of the rubbers, plays such an important part
in successful polishing; it also serves to explain the well-known fact
that rapid polishing only takes place when the glass surface has begun
to be perceptibly heated by the friction spent upon it.
It has been estimated that, on the average, slabs of plate-glass lose
one-third of their original weight in the grinding and polishing
processes, and it is obvious that the erosion of this great weight of
glass must absorb a great amount of mechanical energy, while the cost
of the plant and upkeep is proportionately great. Every factor that
tends to diminish either the total weight of glass to be removed per
square yard of finished plate, or reduces the cost of removal, must
be of the utmost importance in this manufacture. The flatness of the
plates as they leave the annealing kiln has already been referred
to, and the reason why the processes of grinding and polishing have
formed the subject for innumerable patents will now be apparent. The
very large expansion of the use of plate-glass in modern building
construction, together with the steady reduction in the prices of
plate, are evidence of the success that has attended the efforts of
inventors and manufacturers in this direction.
At the present time, plate-glass is manufactured in very large sheets,
measuring up to 26 ft. in length by 14 ft. in width, and in thickness
varying from 3/16th of an inch up to 1½ in., or more, for special
purposes. At the same time the quality of the glass is far higher
to-day than it was at earlier times. This high quality chiefly results
from more careful choice of raw materials and greater freedom from
the defects arising during the melting and refining processes, while a
rigid process of inspection is applied to the glass as it comes from
the polishing machines. For this purpose the sheets are examined in
a darkened room by the aid of a lamp placed in such a way that its
oblique rays reveal every minute imperfection of the glass; these
imperfections are marked with chalk, and the plate is subsequently cut
up so as to avoid the defects that have thus been detected.
Perhaps the most remarkable fact about the quality of modern
plate-glass is its relatively high degree of homogeneity. Glass, as we
have seen in Chapter I., is not a chemically homogeneous substance,
but rather a mixture of a number of substances of different density
and viscosity. Wherever this mixture is not sufficiently intimate, the
presence of diverse constituents becomes apparent in the form of striæ,
arising from the refraction or bending of light-rays as they pass from
one medium into another of different density. Except in glass that has
undergone elaborate stirring processes, such striæ are never absent,
but the skill of the glass-maker consists in making them as few and as
minute as possible, and causing them to assume directions and positions
in which they shall be as inconspicuous as possible. In plate-glass
this is generally secured in a very perfect manner, and to ordinary
observation no striæ are visible when a piece of plate-glass is looked
at in the ordinary way, _i.e._, through its smallest thickness; if the
same piece of glass be looked at transversely, the edges having first
been polished in such a way as to render this possible, the glass will
be seen to be full of striæ, generally running in fine lines parallel
with the polished surfaces of the glass. This uniform direction of
the striæ is partly derived from the fact that the glass has been
caused to flow in this direction by the action of the roller when first
formed into a slab, but this process would not obliterate any serious
inequalities of density which might exist in the glass as it leaves the
pot, so that successful results are only attainable if great care is
taken to secure the greatest possible homogeneity in the glass during
the melting process.
At the present time probably the greater bulk of plate-glass is used
for the purpose of glazing windows of various kinds, principally the
show windows of shops, etc. As used for this purpose the glass is
finished when polished and cut to size. The only further manipulation
that is sometimes required is that of bending the glass to some
desired curvature, examples of bent plate-glass window-panes being
very frequently seen. This bending is carried out on the finished
glass, _i.e._, after it has been polished; the glass is carefully
heated in a special furnace until softened, and is then gently made to
lie against a stone or metal mould which has been provided with the
desired curvature. It is obvious that during this operation there are
great risks of spoiling the glass; roughening of the surface by contact
with irregular surfaces on either the mould, the floor of the kiln, or
the implements used in handling the glass, can only be avoided by the
exercise of much skill and care, while all dust must also be excluded
since any particles settling on the surface of the hot glass would be
“burnt in,” and could not afterwards be detached. Small defects can,
of course, be subsequently removed by local hand-polishing, and this
operation is nearly always resorted to where polished glass has to
undergo fire-treatment for the purpose of bending.
In addition to its use for glazing in the ordinary sense, plate-glass
is employed for a number of purposes; the most important and frequent
of these is in the construction of the better varieties of mirrors.
For this purpose the glass is frequently bevelled at the edges, and
sometimes a certain amount of cutting is also introduced on the face of
the mirror. Bevelling is carried out on special grinding and polishing
machines, and a great variety of these are in use at the present time.
The process consists in grinding off the corners of the sheet of glass
and replacing the rough perpendicular edge left by the cutting diamond
by a smooth polished slope running down from the front surface to the
lower edge at an angle of from 45 to 60 degrees. Since only relatively
small quantities of glass have to be removed, small grinding rubbers
only are used, and in some of the latest machines these take the form
of rapidly-revolving emery or carborundum wheels. These grinding wheels
have proved so successful in grinding even the hardest metals that it
is surprising to find their use in the glass industry almost entirely
restricted to the “cutting” of the better kinds of flint and “crystal”
glass for table ware or other ornamental purposes. The reason probably
lies in the fact that the use of such grinding wheels results in the
generation of a very considerable amount of local heat, this effect
being intensified on account of the low heat-conducting power of glass.
If a piece of glass be held even lightly against a rapidly-revolving
emery wheel it will be seen that the part in contact with the wheel
is visibly red-hot. This local heating is liable to lead to chipping
and cracking of the glass, and these troubles are those actually
experienced when emery or carborundum grinding is attempted on larger
pieces of glass. In the case of at least one modern bevel-grinding
machine, however, it is claimed that the injurious effects of local
heating are avoided by carrying out the entire operation under water.
For the purpose of use in mirrors, plate-glass is frequently silvered,
and this process is carried on so extensively that it has come to
constitute an entire industry which has no essential connection
with glass manufacture itself; for that reason we do not propose to
enter on the subject here, only adding that the nature and quality
of the glass itself considerably affects the ease and success of the
various silvering processes. Ordinary plate-glass, of course, takes
the various silvering coatings very easily and uniformly, but there
are numerous kinds of glass to which this does not apply, although
there are probably few varieties of glass which are sufficiently
stable for practical use, and to which a silvering coating cannot be
satisfactorily applied, provided that the most suitable process be
chosen in each case.
While there is little if any use for coloured glass in the form of
polished plate, entirely opaque plate-glass, coloured both black
and white, is used for certain purposes. Thus, glass fascias over
shop-fronts, the counters and shelves of some shops, and even
tombstones are sometimes made of black or white polished plate. From
the point of view of glass manufacture, however, these varieties only
differ from ordinary plate-glass in respect of certain additions to
the raw materials, resulting in the production of the white or black
opacity. The subsequent treatment of the glass is identical with that
of ordinary plate-glass, except that these opaque varieties are rarely
required to be polished on both sides, so that the operations are
simplified to that extent.
Certain limitations to the use of all kinds of plate-glass, whether
rough-rolled, figured or polished, were formerly set by the fact
that under the influence of fire, partitions of glass were liable to
crack, splinter and fall to pieces, thus causing damage beyond their
own destruction and leaving a free passage for the propagation of
the fire. To overcome these disadvantages, glass manufacturers have
been led to introduce a network or meshing of wire into the body of
such glass. Provided that the glass and wire can be made so as to
unite properly, then the properties of such reinforced or “wired”
glass should be extremely valuable. In the event of breakage from any
cause, such as fire or a violent blow, while the glass would still
crack, the fragments would be held together by the wire network,
and the plates of glass as a whole would remain in place, neither
causing destruction through flying fragments nor allowing fire or,
for the matter of that, burglar a free passage. The utility of such
a material has been readily recognised, but the difficulty lies in
its production. These difficulties arise from two causes. The most
serious of these is the considerable difference between the thermal
expansion of the glass and of the wire to be embedded in it. The wire
is necessarily introduced into red-hot glass while the latter is being
rolled or cast, and therefore glass and wire have to cool down from a
red heat together. During this cooling process the wire contracts much
more than the glass, and breakage either results immediately, or the
glass is left in a condition of severe strain and is liable to crack
spontaneously afterwards. An attempt has been made to overcome this
difficulty by using wire made of a nickel steel alloy, whose thermal
expansion is very similar to that of glass; but, as a matter of fact,
this similarity of thermal expansion is only known to hold for a short
range of moderate temperatures, and probably does not hold when the
steel alloy is heated to redness. In another direction, greater success
is to be attained by the use of wire of a very ductile metal which
should yield to the stress that comes upon it during cooling; probably
copper wire would answer the purpose, but the great cost of copper is a
deterrent from its use. A second difficulty is met with in introducing
wire netting into glass during the rolling operation, and this lies in
effecting a clean join between glass and wire. Most metals when heated
give off a considerable quantity of gas, and when this gas is evolved
after the wire has been embedded in glass, numerous bubbles are formed,
and these not only render the glass very unsightly but also lessen the
adhesion between the wire and the glass. This difficulty, however, can
be overcome more readily than the first, since the surface of the metal
can be kept clean and the gas expelled from the interior of the wire
by preliminary heating. On the whole, however, wired glass is perhaps
still to be regarded as a product whose evolution is not yet complete,
and there can be no doubt that there are great possibilities open to
the material when its manufacture has been more fully developed.
CHAPTER X.
SHEET AND CROWN GLASS.
In the preceding chapter we have dealt with the processes of
manufacture employed in the production of both the crudest and the
most perfect forms of flat glass as used for such purposes as the
glazing of window openings. The products now to be dealt with are
of an intermediate character, sheet-glass possessing many of the
properties of polished plate, but lacking some very important ones;
thus sheet-glass is sufficiently transparent to allow an observer to
see through it with little or no disturbance--in the best varieties
of sheet-glass the optical distortion caused by its irregularities
is so small that the glass appears nearly as perfect as polished
plate--but in the cheap glass that is used for the glazing of ordinary
windows, sheets are often employed which produce the most disturbing,
and sometimes the most ludicrous, distortions of objects seen through
them. It is a curious fact that even in good houses the use of such
inferior glass is tolerated without comment, the general public being,
apparently, remarkably nonobservant in this respect. In another
direction sheet-glass has the great advantage over plate-glass that
it is very much lighter, or can at least be produced of much smaller
weight and thickness, although this advantage entails the consequent
disadvantage that sheet-glass is usually much weaker than plate, and
can only be used in much smaller sizes. In recent times the production
of relatively thin plate-glass has, however, made such strides that
it is now possible to obtain polished plate-glass thin enough and
light enough for almost every architectural purpose. Finally, the most
important advantage of sheet-glass, and the one which alone secures its
use in a great number of cases in preference to plate-glass, is its
cheapness, the price of ordinary sheet-glass being about one-fourth
that of plate-glass of the same size.