limestone, salt-cake, and a few accessory substances, such as arsenic,
oxide of manganese, anthracite coal or coke, which differ considerably
according to the practice of each particular works. In a general way
these materials have already been dealt with in Chapter III., and we
need only add here that the sheet-glass manufacturer must keep in
view two decidedly conflicting considerations. On the one hand the
requirements made in the case of sheet-glass as regards colour and
purity render a rigorous choice of raw material and the exclusion of
anything at all doubtful very desirable; but on the other hand the
chief commercial consideration in connection with this product is its
cheapness, and in order to maintain a low selling price at a profit
to himself the manufacturer must rigorously exclude all expensive raw
materials. For this reason sheet-glass, works such as those of Belgium
and some parts of Germany, which have large deposits of pure sand
close at hand, possess a very considerable advantage over those in
less favoured situations, since sand in particular forms so large a
proportion of the glass, and the cost of carriage frequently exceeds,
and in many cases very greatly exceeds, the actual price of the sand
itself. The same considerations will apply, although in somewhat lesser
degree, to the other bulky materials, such as limestone and salt-cake;
but both these are more generally obtainable at moderate prices than
are glass-making sands of adequate quality for sheet manufacture.
Ordinary “white” sheet-glass is now almost universally produced in
tank furnaces, and a very great variety of these furnaces are used or
advocated for the purpose. It would be beyond the scope of the present
book to enter in detail into the construction of these various types
of furnace or to discuss their relative merits at length. Only a brief
outline of the chief characteristics of the most important forms of
sheet-tank furnaces will therefore be given here.
Sheet tanks differ from each other in several important respects; these
relate to the sub-division of the tank into one, two, or even three
more or less separate chambers, to the depth of the bath of molten
glass and the height of the “crown” or vault of the furnace chamber,
to the shape and position of the apertures by which the gas and air
are admitted into the furnace, and the resultant shape and disposition
of the flame, and finally to the position and arrangement of the
regenerative appliances by which some of the heat of the waste gases is
returned into the furnace.
Taking these principal points in order, we find that in some sheet tank
furnaces the whole furnace constitutes a single large chamber. In this
type of furnace the whole process of fusion and fining of the glass
goes on in this single chamber, and an endeavour is made to graduate
the temperature of the furnace in a suitable manner from the hot end
where the raw materials have to be melted down to the colder end where
the glass must be sufficiently viscous to be gathered on the pipes. It
is obvious that this control of the temperature cannot be so perfect
in a furnace of the single chamber type as in one that is sub-divided.
Such sub-divided furnaces are, as a matter of fact, much more frequent
in sheet-glass practice; but this practice differs widely as to the
manner and degree of the sub-division introduced. In the extreme form
the glass practically passes through three independent furnaces merely
connected with one another by suitable openings of relatively small
area through which the glass flows from one to the other. If it were
possible to build furnaces of materials that could resist the action of
heat and of molten glass to an indefinite extent, it is probable that
this extreme type would prove the best, since it gives the operator
of the furnace the means of controlling the flow of glass in such a
way that no unmelted material can leave the melting chamber and enter
the fining chamber, and that no insufficiently fined glass can leave
the fining chamber and find its way into the working chamber. But in
practice the fact that this extreme sub-division introduces a great
deal of extra furnace wall, exposed both to heat and to contact with
the glass, involves very serious compensating disadvantages--the cost
of construction, maintenance and renewal of the furnace is greatly
increased, while there is also an increased source of contamination of
the glass from the erosion of the furnace walls. It is, therefore, in
accordance with expectations to find that the most successful furnaces
for the production of sheet-glass are intermediate in this respect
between the simple open furnace and the completely sub-divided one.
In some cases the working chamber is separated from the melting and
fining chamber by a transverse wall above the level of the glass, while
fire-clay blocks floating in the glass just below this cross wall serve
to complete the separation and to retain any surface impurities that
may float down the furnace.
As regards the depth of glass in the tank, practice also varies very
much. The advantages claimed for a deep bath are that the fire-clay
bottom of the furnace is thereby kept colder and is consequently less
attacked, so that this portion of the furnace will last for many
years. On the other hand the existence of a great mass of glass at a
moderate heat may easily prove the source of contamination arising from
crystallisation or “devitrification” occurring there and spreading
into the hotter glass above. Also, if for any reason it should become
necessary to remove part or all of the contents of the tank, the
greater mass of glass in those with deep baths becomes a formidable
obstacle. On the whole, however, modern practice appears to favour the
use of deeper baths, depths of 2 ft. 6 in. or even 3 ft. being very
usual, while depths up to 4 ft. have been used.
The question of the proper height of the “crown” or vault of the
furnace is of considerable importance to the proper working of the
tank. For the purpose of producing the most perfect combustion, it is
now contended that a large free flame-space is required. The earlier
glass-melting tanks, like the earlier steel furnaces, were built with
very low crowns, forcing the flame into contact with the surface
of the molten glass, the object being to promote direct heating by
immediate contact of flame and glass; the modern tendency, however, is
strongly in the direction of higher crowns, leaving the heating of the
glass to be accomplished by radiation rather than direct conduction
of heat. There can be little doubt that up to a certain point the
enlargement of the flame-space tends towards greater cleanliness of
working and a certain economy of fuel, but if the height of a furnace
crown be excessive there is a decided loss of economy. Flame-spaces as
high as 6 ft. from the level of the glass to the highest part of the
crown have been used, but the more usual heights range from 2 ft. to 5
ft.
The “ports” or apertures by which pre-heated gas and air enter the
furnace chamber differ very widely in various furnaces. In some cases
the gas and air are allowed to meet in a small combustion chamber just
before entering the furnace itself, while in other cases the gas and
air enter the furnace by entirely separate openings, only meeting in
the furnace chamber. The latter arrangement tends to the formation of
a highly reducing flame, which is advantageous for the reduction of
salt-cake, but is by no means economical as regards fuel consumption.
On the other hand, by producing a perfect mixing of the entering gas
and air in suitable proportions, the other type of ports can be made to
give almost any kind of flame desired, although their tendency is to
form a more oxidising atmosphere within the furnace. The latter type
of ports, although widely varied in detail, are now almost universally
adopted in sheet tank furnaces.
All modern tank furnaces work on the principle of the recovery of heat
from the heated products of combustion as they leave the furnace, and
the return of this heat to the furnace by utilising it to pre-heat the
incoming gas and air; but the means employed to effect the application
of this “regenerative” principle differ considerably in various types
of plant. Perhaps the most widely-used form of furnace is the direct
descendant of the original Siemens regenerative furnace, in which four
regenerator chambers are provided with means for reversing the flow of
gas and air in such a way that each pair of chambers serves alternately
to absorb the heat of the outgoing gases and subsequently to return
this heat to the incoming air that passes through one, and the incoming
gas that passes through the other of these chambers. In these furnaces,
the regenerator chambers themselves are generally placed underneath
the melting furnace, and they are built of fire-brick and filled with
loosely-stacked fire-bricks, whose function it is to absorb or deliver
the heat. In the most modern type of furnaces of this class, the
gas-regenerators are omitted entirely, the air only being pre-heated
by means of regenerators, while the gas enters the furnace direct
from the producer, thus carrying with it the heat generated in the
producer during the gasification of the fuel. While this arrangement
is undoubtedly economical, it has the serious disadvantage, especially
in the manufacture of sheet-glass, that the gas, rushing direct from
the producer into the furnace, carries with it a great deal of dust and
ash, which it has no opportunity of depositing, as in the older types
of furnace, in long flues.
The most serious disadvantages of the ordinary types of regenerative
furnaces are due to the considerable dimensions of the regenerative
apparatus, necessitating a costly form of construction and occupying
a large space, while the necessity of periodically reversing the
valves so as to secure the alternation in the flow of outgoing and
incoming gases requires special attention on the part of the men
engaged in operating the furnace, as well as the construction and
maintenance of valves under conditions of heat and dirt that are not
favourable to the life of mechanical appliances. It is claimed that
all these disadvantages are overcome to a considerable extent in one
or other of the various forms of furnace known as “recuperative.” In
these furnaces there is no alternation of flow, and the regenerator
chambers are replaced by the “recuperators.” These consist of a large
number of small flues or pipes passing through a built-up mass of
fire-brick in two directions at right-angles to one another; through
the pipes running in one direction the waste gases pass out to the
chimney, while the incoming gas and air pass through the other set of
pipes. A transference of heat between the two currents of gas takes
place by the conductivity of the fire-brick, and thus the outgoing
gases are continuously cooled while the ingoing gases are heated--the
transference of heat being somewhat similar to that which takes place
in the surface condenser of a steam engine. Theoretically this is a
much simpler arrangement than that of separate regenerator chambers,
and to some extent it is found preferable in practice, but there
are certain disadvantages associated with the system which arise
principally from the peculiar nature of the material--fire-brick--of
which the recuperators must be constructed. In the first place, the
heat-conductivity of fire-brick is not very high, so that, in order
to secure efficiency, the recuperators must be large, and while the
individual pipes must be of small diameter, their area as a whole must
be large enough to allow the gases to pass through somewhat slowly.
Next, owing to the tendency of fire-brick to warp, shrink and crack
under the prolonged effects of high temperatures, it becomes difficult
to prevent leakage of gases from one set of pipes into the other. If
this occurs to a moderate extent its only effect will be to allow some
of the combustible gas to pass direct to the chimney, and at the same
time a dilution of the gases entering the furnace by an addition of
products of combustion from the waste-gas flues. This, of course, will
materially reduce the efficiency of the furnace and require a higher
fuel consumption if the temperature of the furnace is to be maintained
at its proper level. If, however, the leakage should become more
serious, a disastrous explosion might easily result, particularly if
the nature of the leakage were such as to allow the incoming gas and
air to mix in the flues. It follows from these considerations that,
although the recuperative furnace is somewhat simpler and cheaper to
construct, it requires, if anything, more careful maintenance than the
older forms of regenerative furnace.
Tank furnaces for the production of sheet-glass in this country are
generally worked from early on Monday morning until late on Saturday
night, glass-blowing operations being suspended during Sunday, although
the heat of the furnace must be maintained. On the Continent, and
especially in Belgium, the work in connection with these furnaces goes
on without any intermission on Sunday--a difference which, however
desirable the English practice may be, has the effect of handicapping
the output of a British furnace of equal capacity by about 10 per
cent. without materially lessening the working cost.
The process of blowing sheet-glass in an English glassworks
is generally carried out by groups of three workmen, viz., a
“pipe-warmer,” a “gatherer” and a “blower,” although the precise
division of the work varies according to circumstances. The
pipe-warmer’s work consists in the first place in fetching the
blowing-pipe from a small subsidiary furnace in which he has previously
placed it for the purpose of warming up the thick “nose” end upon
which the glass is subsequently gathered. The sheet-blower’s pipe
itself is an iron tube about 4 ft. 6 in. long, provided at the one
end with a wooden sleeve or handle, and a mouthpiece, while the other
end is thickened up into a substantial cone, having a round end.
Before introducing the pipe into the opening of the tank furnace, the
pipe-warmer must see that the hot end of the pipe is free from scale
or dirt and must test, by blowing through it, whether the pipe is free
from internal obstructions. He then places the butt of the pipe in the
opening of the furnace and allows it to acquire as nearly as possible
the temperature of the molten glass. When this is the case the pipe is
either handed on to the gatherer, or the pipe-warmer, who is usually
only a youth, may take the process one step further before handing
it on to the more highly skilled workman. This next step consists in
taking up the first gathering of glass on the pipe. For this purpose
the hot nose of the pipe is dipped into the molten glass, turned slowly
round once or twice and then removed, the thread of viscous glass that
comes up with the pipe being cut off against the fire-clay ring that
floats in the glass in front of the working opening. A small quantity
of glass is thus left adhering to the nose of the pipe, and this is now
allowed to cool down until it is fairly stiff, the whole pipe being
meanwhile rotated so as to keep this first gathering nicely rounded,
while a slight application of air-pressure, by blowing down the pipe,
forms a very small hollow space in the mass of glass and secures the
freedom of the opening of the pipe. When the glass forming the first
gathering has cooled sufficiently, the gatherer proceeds to take up the
second gathering upon it. The pipe is again introduced into the furnace
and gradually dipped into the molten glass, but this must be done with
great care so as to avoid the inclusion of air-bells between the glass
already on the pipe and the new layer of hotter glass that is now taken
up. This freedom from air-bells is secured by a skilful gatherer by
a gradual rotation of the pipe as it is lowered into the glass, thus
allowing the two layers of glass to come into contact with a sort of
rolling motion that allows the air time to escape. When completely
immersed, the pipe is rotated a few times and is then withdrawn and the
“thread” again cut off. The mass of glass on the end of the pipe is now
considerably larger than before and requires more careful manipulation
to cause it to retain the proper, nearly spherical, shape. During the
cooling process which now follows the pipe is laid across an iron
trough, kept brimful of water; this serves to cool the pipe itself, and
also allows the pipe to be readily rotated backwards and forwards by
rolling it a little way along the trough. When the whole mass of glass
has again cooled sufficiently to be manipulated without risk of rapid
deformation, a third gathering of glass is taken up, in precisely the
same manner as that already described for the second gathering, and
if the quantity of glass required is large, or the glass itself is so
hot and fluid that only a comparatively small weight adheres at each
time of gathering, the process may be repeated a fourth or even a fifth
time, but as the weight of pipe and adhering glass increases with each
gathering, each step becomes more laborious, while the hot glass, being
now held on a much larger sphere, tends to flow off more readily, so
that greater skill is required to avoid “losing” the gathering.
The care and skill with which these operations of gathering are carried
out determine, to a large extent, the quality of the resulting sheet
of glass; any want of regularity in the shape of the gathering leads
inevitably to variations of thickness in different parts of the sheet,
while careless gathering will introduce bubbles or “blisters” and other
markings. During the intermediate cooling stages the glass must be
protected from dust and dirt of all kinds, since minute specks falling
upon the hot glass give rise to an evolution of minute gas bubbles
which become painfully evident in the sorting room.
When the last gathering has been taken up and the mass cooled so far
as to allow of its being carried about without fear of loss, the glass
forms an approximately spherical mass, with the nose-end of the pipe
at or near the centre of the sphere. The next stages of the process
consist in the preliminary shaping of this mass in such a way as to
bring the bulk of the glass beyond the end of the pipe, and then in
forming just beyond the end of the pipe a widened shoulder of thinner
and therefore colder glass, of the diameter required for the cylinder
into which the glass is to be blown. This is done by bringing the
glass into the successive shapes shown in Fig. 12, the forming of the
glass being effected by the aid of specially shaped blocks and other
shaping instruments in which the glass is turned and blown. The final
shape attained at this stage is a squat cylinder containing the bulk of
the glass at its lower end, and connected to the pipe by the thinner
and colder neck and shoulder already mentioned.
[Illustration: FIG. 12.--Early stages in the formation of cylinders for
sheet glass.]
At this point of the process the pipe with its adherent glass is handed
over to the blower proper. This operator works on a special stage
erected in front of small furnaces, called “blowing holes,” although
in some works these are dispensed with, and the stages are erected in
front of the melting furnace itself. The sheet-blower’s stage is simply
a platform placed over or at the side of a suitable excavation which
gives the blower the necessary space to swing the pipe and cylinder
freely at arm’s length. The blowing process itself involves very little
actual blowing, but depends rather upon the action of gravitation
and on centrifugal effects for the formation of the large, elongated
cylinder from the squat cylinder with which the blower commences. The
process consists in holding the thick, lower end of the cylinder in
the heating-furnace, and when sufficiently hot, withdrawing it and
swinging the pipe with a pendulum movement in the blower’s pit. The
cylinder thus elongates itself under its own weight, and any tendency
to collapse is counteracted by the application of air-pressure by the
mouth, the pipe being also, at times, rotated rapidly about its own
axis. The re-heating of the lower end of the cylinder is repeated
several times, until finally the glass has assumed the form of a
cylinder of equal thickness all over, but closed with a rounded dome at
the lower end (Fig. 13). This rounded end is now opened. In the case
of fairly thin and light cylinders this is done by holding the thumb
over the mouthpiece of the pipe in such a way as to make an air-tight
seal, and then heating the end of the cylinder in the blowing-hole. The
heat both softens the glass at the end and at the same time causes
considerable expansion of the air enclosed in the cylinder, with the
result that the end of the cylinder is burst open. After a little
further heating, during which the glass at the end of the cylinder
becomes very soft, and takes a wavy, curly shape, the blower withdraws
the cylinder from the furnace, and holding it vertically downwards in
his pit, spins it rapidly about its longitudinal axis. The soft glass
at the lower end immediately opens out under the centrifugal action,
and the blower increases the speed of rotation until the soft glass has
opened out far enough to form a true continuation of the rest of the
cylinder, and in this position it is allowed to solidify. With thick,
heavy cylinders, the first opening of the end is done in a different
way. A small quantity of hot glass is taken up by an assistant on
an iron rod, and is laid upon the centre of the closed end of the
cylinder. The heat of this mass of hot glass softens the glass of the
cylinder, and the operator, with the aid of a special pair of shears,
cuts out a small circle of this softened glass, thus opening the end of
the cylinder. The final operation of straightening out the opened end
is carried out in the same way as described above for lighter cylinders.
[Illustration: FIG. 13.--Later stage in sheet glass blowing.]
The completed cylinder, still attached to the pipe, is now carried away
from the blowing-stage and laid upon a wooden rack; then the blower
takes up a piece of cold iron, and placing it against the neck of glass
attaching the cylinder to the pipe, produces a crack; a short jerk then
serves to completely sever the pipe from the cylinder. A boy now takes
the pipe to a stand where it is allowed to cool and where the adhering
glass cracks off from it prior to passing it back to the pipe-warmer
for fresh use.
On the wooden rack the cylinder of glass is allowed to cool to a
certain extent, and then the remaining portion of the neck and shoulder
(see Fig. 13) are removed. This is done by a boy who passes a thread
of soft, hot glass around the cylinder at the point where it is to
be cut off; the thread of hot glass merely serves to produce intense
local heating, for as soon as it has become stiff, the thread of glass
is pushed off and a cold or moist iron is applied to the cylinder at
the point where it had been heated by the thread. As a rule a crack
immediately runs completely round the cylinder along the line of the
thread, and the “cap” is thus removed. The glass is now in the form of
a uniform cylinder open at both ends, but it must be opened out into a
flat sheet before it can assume the familiar form of sheet-glass.
The first stage in the opening-out process is that of splitting. For
this purpose the cylinders are carried to a special stand, upon which
they are laid in a horizontal position, and here a crack or cut is
made along one of the generating-lines of the cylinder. This may be
done either by the application of a hot iron, followed, if necessary,
by slight moistening, or by the aid of a cut from a heavy diamond
drawn skilfully down the inside of the cylinder. It will be seen
from the account of the process so far given, that the glass has as
yet undergone no real annealing, although the blower is expected to
“anneal” his cylinder during the blowing process, as far as possible,
by never allowing it to cool too suddenly, and this degree of annealing
is usually sufficient to save the cylinder from breaking under its
internal stresses when left to cool on the racks. The surface of the
glass, however, is left in a decidedly hardened condition, especially
on the outside, which has necessarily been most rapidly cooled. For
this reason--among others--the splitting cut is always made on the
inside of the cylinder. The difference between the rates of cooling
of the outside and inside of the cylinder has a further effect, which
becomes evident as soon as the cylinder is split. The outside having
become hard while the inside was still relatively soft, the outer
layers of glass are in a state of compression and the inner layers in
a state of tension in the cold cylinder. As soon as the cylinder is
split, however, these stresses are to some extent relieved, the inner
layers being then free to contract and the outer layers to expand; the
result is an increase in the curvature of the cylinder, which slightly
decreases in diameter, the cut edges overlapping. If the cylinder has
been cooled rather too quickly, or if the glass itself has a high
coefficient of expansion, this release of internal stresses at the
moment of splitting becomes very marked, and each cylinder splits with
the sound of a small explosion, while if the internal stresses are
still more severe, the cylinders may even fly to pieces as soon as they
are cut.
The next stage in the manufacture of a sheet of glass is the flattening
and annealing process. For this purpose the split cylinders are taken
to a special kiln, generally known as a “lear,” or “lehr,” where they
are first of all raised to a dull red-heat; they are then lifted, one
at a time, on to a smooth stone or slab placed in a chamber of the
kiln where the heat is great enough to soften the glass. Here the
cylinder is laid down with the split edges upwards, and by means of a
wooden tool the glass is slowly spread out, being finally rubbed down
into perfect contact with the slab or “lagre.” From the flattening
slab, the sheet as it now is passes into the annealing kiln, which
communicates with the flattening chamber. This consists, similarly to
other continuous annealing kilns already described in connection with
other varieties of glass, of a long tunnel, heated to the temperature
of the flattening kiln at one end and nearly cold at the other. The
sheets are moved down this tunnel at a uniform slow rate by the action
of a system of grids which, at intervals, lift the sheets from the
bottom of the kiln, move them forward by a short distance, and again
deposit them on the bottom, the grids themselves returning to their
former position by a retrograde movement made below the level of the
kiln-bottom, and therefore not affecting the glass.
On leaving the annealing kiln the sheets of glass are sometimes covered
with a white deposit arising from the products of combustion in the
kiln and their interaction with the glass itself. This deposit can be
removed by simple mechanical rubbing, but it is usual to dip the glass
into a weak acid bath, which dissolves the white film and leaves the
glass clear and bright, ready for use.
From the annealing kiln the finished sheets of glass are taken to the
sorting room, where they are examined in a good light against a black
background, and are sorted according to their quality for different
purposes.
The defects which are found in sheet-glass are of a very varied
nature, as would be anticipated from the long and complicated process
of manufacture which the material undergoes in the course of its
transformation from the raw materials into the finished sheet of glass.
A full enumeration of all possible defects, with their technical names,
need not be given here, but a description of the more important and
frequent ones will be useful. The defects may be conveniently grouped
according to the stage of the process from which they originate.
The first class of defects accordingly embraces those that arise from
the condition of the glass as it exists in the working-end of the
furnace. Chief of these are white opaque enclosures, known as “stones.”
These may arise from a variety of causes within the furnace, such as an
admixture of infusible impurities with the raw materials, insufficient
heat or duration of melting, leading to a residue of unmelted raw
material in the finished glass, or from defective condition of the
interior of the furnace, leading to contamination of the glass with
small particles of fire-brick. Further, if any part of the furnace has
been allowed to remain at too low a temperature, or if the composition
of the glass is unsuitable, crystallisation may occur in the glass,
and white patches of crystalline material may find their way into the
finished sheets. Another defect that may arise from the condition of
the glass in the furnace is the presence of numerous small bubbles,
known as “seed” in the glass. By the blowing process these are
drawn out into pointed ovals, and they are rarely quite absent from
sheet-glass. They arise from either incomplete fining of the glass in
the furnace or from allowing the glass to come into contact with minute
particles of dust during the gathering process. Another possible defect
to the glass itself may be found at times in too deep a colour. This
is only seen readily when a sheet of some size is examined edgewise,
as most varieties of ordinary sheet-glass are too free from colour to
allow this to be judged by looking through the sheet in the ordinary
way. It follows from this fact that for practical purposes, where
the light always traverses one thickness of the glass only, a slight
difference of colour should be regarded as a very minor consideration,
at all events as compared with freedom from other defects.
The gathering process in its turn is responsible for further defects of
sheet-glass. Some of these, such as defects arising from the use of a
dirty pipe, are never allowed to pass beyond the sorting-room, and are
therefore of no interest to the user of glass. Of those whose traces
are seen in the glass that passes into use, “blisters” and “string” are
the most important. “Blisters” are somewhat larger, flat air-bells,
arising from the inclusion of air between successive layers of the
gathering. “String” is a very common defect in all sheet-glass. To some
extent it may arise from want of homogeneity in the glass itself. If
this consists of layers of different densities and viscosities, the
gatherer will take these up on his gathering, and ultimately they will
form thickened ridges of glass running around the cylinders and across
the sheets. Such striæ, due to want of homogeneity in the glass, are
much more common in flint glass than in the soda-lime glasses used for
sheet manufacture, but are not unknown in the latter. On the other
hand, even if the glass be as homogeneous as possible, the gatherer
can produce these striæ if he takes up his glass from a place close
to the side of the fire-clay ring that floats in the furnace in front
of his working opening. Glass always acts chemically upon fire-clay,
gradually forming a layer of glass next to the fire-clay that contains
much more alumina than the rest of the contents of the furnace. Such
a layer is formed on the surface of each ring in a sheet tank, but if
the gathering is taken from the centre of the ring, this thick viscous
layer of aluminiferous glass remains undisturbed. If, however, the
gatherer brings his pipe too near the side of the ring, the glass
will draw some of this different layer on to the gathering, and this
glass will form thick ridges and striæ running across the sheet in
all directions. Another defect for which the gatherer is generally
responsible is that of variation of thickness within the same sheet.
The blower, however, can also produce this defect.
During the blowing proper, a further series of defects may be
introduced, principally by allowing particles of glass derived from
certain stages of the process to fall upon the hot glass of the
cylinder and there become attached permanently. More serious, and also
more frequent, is the greater or less malformation of the cylinder. If
the glass as it leaves the blower is of any shape other than that of
a true cylinder, it becomes impossible to spread it into a truly flat
sheet in the flattening kiln. Sometimes, in practice, the “cylinder” is
wider at one end than at the other, or, worse still, it is of uneven
diameter, showing expanded and contracted areas alternately. When such
a cylinder comes to be spread out on the slab it cannot be flattened
completely, and various hollows and hillocks are left, which mar the
flatness of the sheet and interfere with the regular passage of light
through it when in use.
Finally, the process of flattening is apt to introduce defects of its
own. The most common of these are scratches arising from marks left by
the flattening tool; indeed, in all sheet glass it is quite possible
to see, by careful examination of the surfaces, upon which side the
flattening tool was used. Sheet-glass thus has one side decidedly
brighter and better in surface than the other, the better side being
that which rested upon the “lagre” during the flattening process. On
the other hand, if the slab itself be not quite perfect, or if any
foreign body be allowed to rest upon it, that side of the glass will be
marked in a corresponding manner.
In the account of the manufacture of sheet-glass given above, we have
outlined one typical form of the process, but nearly every stage is
subject to modifications according to the practice and particular
circumstances of each works. We will now describe one or two special
modifications that are of more general importance.
First, as regards the melting process, although the tank-furnace
has almost entirely superseded the pot furnace for the production
of ordinary sheet-glass, there are still some special circumstances
under which the pot furnace is capable of holding its own. Thus,
where for special purposes it is desired to produce a variety of
sheet-glass which, as regards all defects arising out of the glass
itself, and especially as regards colour, is required to be as perfect
as possible, melting in pots is found advantageous, and for some very
special purposes even covered (hooded) pots are used. For such special
purposes, too, sulphate of soda is eliminated from the raw materials
and carbonate of soda (soda ash) substituted. For the production of
tinted glasses also, whether they are tinted throughout their mass,
or merely covered with a thin layer of tinted glass (“flashed”),
manufacture in pot rather than tank furnaces is generally adopted,
the exact nature and composition of the glass being far better under
control in the case of pots.
The blowing process is also subject to wide variations of practice. The
most important of these variations concerns the shape and dimensions
of the cylinders. In English and Belgian works the dimensions of the
cylinders are so chosen that the length of the cylinder constitutes the
longest dimension of the finished sheet, the diameter of the cylinder
forming the shorter dimension. In some parts of Germany, however, the
practice is the reverse of this, the cylinders being blown shorter
and much wider, so that the circumference of the cylinder constitutes
the longest dimension of the finished sheet. It is, however, pretty
generally recognised that the latter method has very serious
disadvantages, although it is claimed that somewhat more perfect
glass can be obtained by its means. For the production of a special
variety of glass, known as “blown plate glass,” this method of blowing
short wide cylinders is still adhered to. This is a very pure form of
sheet-glass, blown into thick, small sheets which are subsequently
ground and polished in the same manner as plate-glass. Here the great
thickness of glass required seems to render the blowing of long
cylinders very difficult, and the other form is therefore adopted. On
the other hand, English patent plate-glass, which is made by grinding
and polishing the best quality of ordinary sheet-glass, is made from
glass blown into long narrow cylinders in the manner described in
detail above.
The process of blowing described above is capable, with slight
modifications, of yielding glass with surfaces other than the plain
smooth face of ordinary sheet-glass. Thus fluted and “muffled” glass
are produced in a very similar manner to that described above for
ordinary sheet, except that the fluting or the irregular surface
markings which constitute the peculiarities of these two varieties of
glass, are impressed upon the surface of the cylinder at an early stage
in the process.
From the outline description given above of the usual method of
manufacture of sheet-glass, it will readily be seen that this is a
long, complicated, and laborious process, involving the employment
of much skilled labour, and involving the production of a relatively
complicated form, viz., the closed cylinder, as a preliminary to the
production of a very simple form, viz., the flat sheet. It is therefore
by no means surprising to find that a great many inventors have worked
and are still working at the problem of a direct mechanical method of
producing flat glass possessing a natural “fire polish” at least equal
to that of ordinary sheet-glass. The earlier inventors have almost
uniformly endeavoured to attain this object by attempting to improve
the process of rolling glass, with a view to obtaining rolled sheets
having a satisfactory surface. We have already indicated why these
efforts have never met with success and what reasons there are for
believing that they are never likely to attain their object. A totally
different line is that taken by Sievert, to whose inventions we have
already referred in connection with the mechanical production of blown
articles. This inventor has endeavoured to utilise his process for
blowing large articles of glass for the direct production of sheets
of flat glass. His method is to blow, by the steam process described
in another chapter, a large cubical vessel, having flat sides, the
flatness of these sides being ensured by blowing the vessel into or
against a mould having flat sides. This flat-sided vessel is ultimately
to be cut up into five large sheets. This process also appears to
involve some of the main difficulties of rolling as regards the
means of transferring the glass from the furnace to the plate of the
blowing machine, and in practice the inventor has not yet succeeded in
producing glass of sufficiently good surface for the purposes of sheet
glass.
Another class of processes entirely avoid all means of transferring
molten glass from the furnace to any machine, by working on glass
direct from the molten bath itself. Some of these processes are in
actual use in America, and others are being experimented with in
Europe, but their complete technical and commercial success has yet to
be proved; there can, however, be little doubt that they have overcome
the greatest of the many difficulties that stood in the way of the
mechanical production of sheet-glass, and that they are therefore
destined very shortly to solve the problem completely, in which case
they would, of course, rapidly supersede the hand process.
One of the earliest of these direct processes proposed to allow the
molten glass to flow out from the furnace, downward, through a narrow
slit formed in the side or bottom of the tank. The impossibility of
keeping such a narrow orifice open and at the same time regulating the
flow of glass made this proposal impracticable, although the use of
drawing orifices has been revived in one of the latest processes.
The American process, which is said to be at work under commercial
conditions, is not entirely satisfactory in this respect--that it is
a mechanical process for the production of cylinders and not of flat
sheets, so that the subsidiary processes of splitting and flattening
still remain to be carried out as before. In this process an iron
ring is lowered into the bath of molten glass through an aperture
from above; the glass is allowed to adhere to the ring which is then
slowly raised by mechanical means, drawing a cylinder of glass with
it. If left to itself, such a cylinder, owing to the effects of
surface tension in the glass, would soon contract and break off, but
the American invention avoids this action by chilling each bit of the
cylinder as soon as it is formed. This is done by the aid of air blasts
delivered upon both sides of the glass as it emerges from the bath,
and it is claimed that by this means cylinders of any desired length
and diameter may be drawn direct from the bath. The obviously great
mechanical difficulties connected with these operations have probably
been overcome, but not without sacrificing much of the simplicity of
the arrangement, and the relative economy of this process as a whole,
compared with the hand process, has yet to assert itself.
The inventions of Fourcault, which are at present being developed on
the Continent by a syndicate of glass manufacturers, aim at a much more
direct process. Here also the glass is drawn direct from the molten
bath by the aid of a drawing-iron that is immersed in the glass and
then slowly raised, but in this case the piece immersed is simply a
straight bar, and the aim is to draw out a flat sheet. In this case
the tendency, under surface tension, is to contract the sheet into
a thread, and apparently the simple device of chilling the emerging
glass is not adequate to prevent this in a satisfactory manner, and
subsidiary devices have been added. Those that have been patented
include a mechanism of linked metal rods so arranged as to be immersed
and drawn out of the glass continuously with the emerging sheet, in
such a manner as to support the vertical edges of the glass and so
aid in resisting the tendency of the glass to contract laterally.
Another device consists in the use of a slit or orifice formed in a
large fire-brick that floats on the surface of the glass. Through this
orifice the glass is drawn, of the desired thickness and width. The
use of this orifice, however, interferes markedly with the perfection
of the product, and in fact all the glass produced in this way shows
quite plainly a set of longitudinal striations due to the inevitable
irregularities in the lips of the drawing slot. Further, it appears
to be impracticable to draw _thin_ glass in this way, a thickness of
from 2½ to 3 millimetres (about 1/8 inch) being the least that is
practicable, on account of the large amount of breakage that occurs
with weaker sheets. This process, in its present stage of development,
however promising, does not appear to have solved the problem of
mechanical manufacture of sheet-glass, since it is just in the
thinner, lighter kinds of glass that the advantages of sheet are most
pronounced. On the other hand, it is quite possible that this drawing
process, or some development arising from it, may shortly supplant the
casting process in the production of polished plate-glass, although for
the largest sizes of this product also, the difficulty and danger of
handling the weights involved may prove a serious obstacle.
_Crown Glass._--Although this is a branch of manufacture that is
nearly obsolete, it deserves brief notice here, partly because it is
still used for the production of special articles, and also because it
illustrates some interesting possibilities in the use and manipulation
of glass.
The process of blowing crown glass may be briefly described as that of
first blowing an approximately spherical hollow ball, then opening this
at one side and expanding the glass into a flat disc by the action of
centrifugal forces produced by a rapid rotation of the glass in front
of a large opening in a special heating furnace. The actual process
involves, of course, the preliminary of gathering the proper quantity
of glass, much in the manner already described in connection with
sheet-glass manufacture. This gathering is then blown out into a hollow
spherical vessel. This vessel is now attached to a subsidiary iron
rod by means of a small gathering of hot glass, applied at the point
opposite the pipe itself, the glass being thus, for a moment, attached
to both the pipe and the “pontil” or “punty” (as the rod is called).
The pipe is, however, detached by cracking off the neck of the original
glass, which now remains attached to the pontil in the shape of an open
bowl. This bowl is now re-heated very strongly in front of a special
furnace, the open side of the bowl being presented to the fire. The
pontil is meanwhile held in a horizontal position and rotated. As the
glass softens the rotation spreads it out, until finally the entire
mass of glass is formed into a simple flat disc spinning rapidly before
the mouth of the furnace. This flat disc or “table” of crown glass is
allowed to cool somewhat, is detached from the pontil by a sharp jerk,
and is then annealed in a simple kiln in which the glass is stacked,
sealed up, and allowed to cool naturally.
It is obvious that by this process no very large sheets of glass can
be produced; tables 4 ft. in diameter are already on the large side,
and these can only be cut up into much smaller sheets on account of the
lump of glass by which the table was originally attached to the pontil,
and which remains fixed in the centre of the finished disc. For certain
ornamental purposes, where an “antique” appearance is desired, these
bullions are valued, but for practical purposes they interfere very
seriously with the use of the glass. As a matter of fact, even several
inches away from the central bullion itself, crown glass is generally
marked with circular wavings, which render it readily recognisable in
the windows of older buildings, but which decidedly detract from the
perfection of the glass. On the other hand, crown glass is still valued
for certain purposes, such as microscope slides and cover glasses,
where entire freedom from surface markings, such as those found in
sheet glass as a result of the flattening operations, is desirable.
While, therefore, the process has merely an historical interest so far
as ordinary sheet-glass purposes are concerned, it is still used in
special cases.
CHAPTER XI.
COLOURED GLASSES.
In various chapters throughout the foregoing portions of this book
we have had occasion to refer to the colour of glass and the causes
affecting it, but these references have chiefly been made from the
point of view of the production of glasses as nearly colourless as
possible under the circumstances. While it is obvious that for the
great majority of the purposes for which it is used the absence of
all visible coloration is desirable or even essential in the glass
employed, there are numerous other uses where a definite coloration is
required. Thus we have, as industrial and technical uses of coloured
glass, the employment of ruby, green and purple glasses for signalling
purposes, as in the signal lamps of our railways, the red tail-lights
of motor-cars, or even the red or green sectors of certain harbour
lights and lighthouses; again, coloured glasses, ruby, green, and
yellow, are extensively employed in connection with photography. Rather
less exacting in their demands upon the correctness of the colour
employed are the architectural and ornamental uses to which coloured
glass is so extensively put in both public and domestic buildings,
while, finally, coloured glass is largely the foundation upon which
the stained-glass worker builds up his artistic achievements; in
another direction, coloured glass is also utilised in the production of
ornamental articles and of some table-ware. While it must be admitted
that in a great many cases the colour-resources of the glass maker are
hopelessly misapplied, yet in really artistic hands few other materials
are capable of yielding results of equal beauty.
By the “colour” of a glass is generally understood the tint or colour
which is observed when it is viewed, in comparatively thin slices, by
transmitted light; the actual colour is thus a property, not so much of
the kind or variety of glass as of each individual piece, since thick
pieces out of the same melting will show a different tint from that
seen in thinner pieces. As we have already pointed out, such glasses as
sheet or plate, which appear practically colourless when viewed in the
ordinary way, show a very decided green colour when viewed through a
considerable thickness. In the same way, a very thin layer of the glass
known as “flashing ruby” shows a brilliant red tint, but a thickness of
one-sixteenth of an inch is sufficient to render the glass practically
opaque, giving it a black appearance by both transmitted and reflected
light. Again, cobalt blue glass, when examined with a spectroscope in
thin layers, is found to transmit a notable proportion of red rays,
but thicker pieces entirely suppress these rays. These phenomena will
be readily understood when we recollect that colour in a transparent
medium arises from the fact that the medium has different absorbing
powers for light of different colours. All transparent substances,
and certainly glass, are only _partially_ transparent: all light
waves passing through such a substance are gradually absorbed, and
the extent to which they are absorbed differs according to the length
of these waves. It always happens that for some special wave-lengths
the substance has the power of absorbing the energy of the entering
waves and converting it into heat-vibrations of its own molecules or
atoms. In the most transparent and colourless glasses this process, so
far as the waves of ordinary light are concerned, only goes on to a
negligibly slight extent; if, however, we extend our view beyond the
range of ordinary visible light, and consider the region of shorter
waves that lies in the spectrum beyond the violet, we find that
ordinary colourless glass becomes strongly absorbent; thus to waves
of about half the length of those which produce upon our eyes the
impression of yellow light, ordinary glass is as opaque as is a piece
of metal to white light. In this wider sense, then, we may fairly say
that all glasses are coloured--_i.e._, all have a power of selective
absorption; but in the case of those which are nearly colourless in
the ordinary sense, this absorption takes place only for waves which
are either decidedly shorter or decidedly longer than those to which
our eyes are sensitive. Those glasses which appear coloured in the
ordinary sense, on the other hand, owe this property to the fact that
the power of absorption for light-waves extends into the region of the
visible spectrum; thus a blue or violet glass is practically opaque to
red rays, while a red glass is opaque to blue, green or violet rays.
This statement may be verified in a striking manner by holding over one
another a piece of deep blue or green glass and a similar piece of ruby
glass--the combination will be found to be very nearly opaque even when
each glass by itself is practically transparent.
The question which now naturally presents itself to us is, what is
the essential difference between, for instance, a piece of red glass
and a piece of “white” glass that confers upon the former the power
of absorbing blue light? A perfectly complete and satisfactory answer
to this question is not, in the writer’s opinion, available in the
present state of our knowledge, but to a certain extent the difference
between the two kinds of glass can be explained. The difference is
_produced_, in the first instance by introducing into the colourless
glass some additional chemical element or elements, the substances
in question being generally known as “colouring oxides,” although
they are by no means always introduced in the form of oxides, and are
frequently present in the glass in entirely different forms. To a
certain extent the colour of the glass may be ascribed to a definite
“colouring” property of the chemical elements concerned; thus most
of the chemical compounds of such elements as nickel, cobalt, iron,
manganese and copper are more or less deeply coloured substances,
and it would seem as if the atoms or “ions” of these elements had
the specific power of absorbing certain varieties of light-waves
while not materially affecting others. But this specific “colouring”
property is not so easily explained when we recollect that the colours
of iron compounds, for example, may be green or red according to the
state of combination in which that element is present, and that iron
has also the power of imparting either a green or a yellow colour to
glass according to circumstances. The detailed discussion of these
questions, however, lies outside our present scope, and we must
confine ourselves to the broad statement that colouring substance in
glass may be roughly divided into two kinds or groups; the first
and probably the largest group are those bodies which occur in glass
in true solution, the element itself being present in the combined
state as a silicate or other such compound (borate, phosphate, etc.)
which is soluble in the glass. In this class, the colouring effect
upon the glass is specifically that of the element introduced, and is
brought about in the same way as the colouring of water when a coloured
salt--such as copper sulphate--is dissolved in it. The second class of
colouring substances, however, behave in a different manner; they are
probably present in the glass in a state of extremely fine division,
and held not in true solution, but really in a sort of mechanical
suspension that approximates to the condition of what is known as a
“colloidal solution.” The point which is known beyond doubt, thanks
to the researches of Siedentopf and Szigmondi on ultra-microscopical
particles, is that in certain coloured glasses, of which ruby glass is
the best example, the colouring substance, be it gold or cuprous oxide,
is present in the form of minute but by no means atomic or molecular
particles suspended in the glass. The presence of these particles has
been made optically evident, although it can hardly be said that they
have been rendered visible, and it is at all events probable that these
suspended particles act each as a whole in absorbing the light-waves
characteristic of the colour which they produce in glass. This being
the case, it is easy to understand how readily the colour of such
glasses is altered or spoilt by manipulations which involve heating
and cooling at different rates--too rapid a rate of cooling producing
a different grouping of the minute particles, altering their size or
shape, or even obliterating them entirely by allowing the element in
question to go into or to remain in solution in the glass.
While it would be entirely foreign to the purpose of this volume to
give in this place a series of recipes for the production of various
kinds of coloured glass, it will be desirable to state in general terms
the colours or range of colours which can be produced in various kinds
of glass by the introduction of those chemical elements which are
ordinarily used in this way. In general terms it may be said that the
lighter elements do not as a rule tend to the production of coloured
glasses, while the heavier elements, so far as they can be retained in
the glass in either solution or suspension, tend to produce an intense
colouring effect. The element lead appears to form a striking exception
to this rule, but this is due to the fact that while the silicates
of most of the other heavy elements are more or less unstable, the
silicate of lead is very stable, and can only be decomposed by the
action of reducing agents. When lead silicates are decomposed in this
way, however, the resulting glass immediately receives an exceedingly
deep colour, being turned a deep opaque black, although in very thin
layers the colour is decidedly brown. On the other hand, glasses
very rich in lead are always decidedly yellow in colour, and it has
been shown that this coloration is due to the natural colour of lead
silicates and not to the presence of impurities. What has just been
said of lead applies, with only very slight modification, also to the
rare metal thallium and its compounds, which have been introduced into
glass for special purposes. Leaving these two exceptional bodies on
one side, we now pass to a consideration of the elements in the order
of their chemical grouping. The rare elements will not be considered
except in certain cases where their presence in traces is liable to
affect results attained in practice.
The _Alkali Metals_, sodium, potassium, lithium, etc., and their
compounds, have no specific colouring effect, although the presence
of soda or of potash in a glass affects the colours produced by such
substances as manganese, nickel, selenium, etc.
_Copper_, as would be anticipated from the deep colour of most of
its compounds, produces powerful colouring effects on glass. Cupric
silicates produce intense green, to greenish-blue tints. Copper,
either as metal or oxide, added to glass in the ordinary way, always
produces the green colour; but when the full oxidation of the copper is
prevented by the presence of a reducing body, and the glass is cooled
slowly, or is exposed to repeated heating followed by slow cooling,
an intense ruby coloration is produced. In practice this colour is
produced by introducing tin as well as copper into the mixture, and
so regulating the conditions of melting as to favour reduction rather
than oxidation of the copper. Under these circumstances the copper is
left in the glass in a finely divided and evenly suspended state; if
exactly the right state of division and suspension is arrived at, a
beautiful red tint is the result, although the coloration of the glass
is so intense that it can only be employed in very thin sheets, being
“flashed” upon the surface of colourless glass to give it the necessary
strength and thickness for practical use. It is further very easy to
slightly alter the arrangement of the copper in the glass, with the
result of producing an opaque, streaky substance resembling sealing-wax
in colour and appearance, this product being, of course, useless from
the glass-maker’s point of view. Finally, by exceedingly slow cooling,
and under other favouring conditions which are not really understood,
the particles of suspended colouring-material--be it metallic copper or
cuprous oxide--grow in size and attain visible dimensions, appearing as
minute shimmering flakes, thus producing the beautiful substance known
as “aventurine.”
_Silver_ is never introduced into glass mixtures, the reason being that
it is so readily reduced to the metallic state from all its compounds
that it cannot be retained in the glass except in a finely-divided
form, causing the glass to assume a black, metallic appearance
resembling the stains produced by the reduction of lead in flint
glasses. On the other hand, silver yields a beautiful yellow colour
when applied to glass as a surface stain, and it is widely used for
that purpose.
_Gold_ is introduced into glass for the production of brilliant ruby
tints; its behaviour is very similar to that of copper, except that
the noble metal has a great tendency to return to the metallic state
without the aid of reducing agents. No addition of tin is therefore
required, but the rate of cooling, etc., must be properly regulated,
since rapidly cooled glass containing gold shows no special colour,
the rich ruby tint being only developed when the glass is re-heated
and cooled slowly. The colouring effect of gold is undoubtedly more
regular and uniform than that of copper, and it is accordingly possible
to obtain much lighter shades of red with the aid of the noble metal.
“Gold ruby” can therefore be obtained of a tint light enough to be used
in sheets of ordinary thickness, and the process of “flashing” is not
essential.
The elements of the second group, such as magnesium, calcium,
strontium, barium, zinc and cadmium, exert no strong specific colouring
action on glass, with perhaps the exception of cadmium, and that
element only does so to any considerable extent in combination with
sulphur, sulphide of cadmium having the power of producing rich yellow
colours in glass. The sulphur compounds of barium also readily produce
deep green and yellow colours, and the formation of these tints is,
indeed, very difficult to avoid in the case of glasses containing much
barium. A colouring effect has sometimes been ascribed to zinc, but
this is not in accordance with facts.
Of the elements of the third group, only boron and aluminium are ever
found in glass in any notable quantity. Boron is present in the form of
boric acid or borates, and as such produces no colouring effect, nor
does there seem to be any tendency for the separation of free boron.
The compounds of aluminium also possess no colouring effect, although
certain compounds of this element are utilised for imparting a white
opacity to glass for certain purposes--such glass being known as “opal.”
The elements of the fourth group are of greater importance in
connection with glass. Carbon is capable of exerting powerful colouring
effects when introduced into glass. These effects are of two kinds,
viz., indirect in consequence of the reducing action of carbon on other
substances present, and direct from the presence of finely-divided
carbon or carbides in the glass. The latter are similar in kind to
those produced by the presence of other finely-divided elementary
bodies (copper, gold, lead, etc.) except that the lightness of the
carbon particles tends to the production of yellow and brown colours
rather than of red and black, while the chemical nature of carbon
renders the glass in which it is suspended indifferent to rapid
cooling, so far as the carbon tint is concerned. The indirect effects
of carbon, in reducing other substances that may be present in the
glass, become evident with much smaller proportions of carbon than are
required to produce visible direct effects. As we have seen above,
carbon, in the form of coke, charcoal or anthracite coal, is regularly
introduced, as a reducing medium, into glass mixtures containing
sulphate of soda. If even a slight excess of carbon be used for this
purpose, the formation of sulphides and poly-sulphides of sodium and
of calcium results, and these bodies, like all sulphides, impart a
greenish-yellow tint to the glass, at the same time bringing other
undesirable results in their train.
_Silicon_, in the form of silicic acid and its compounds, is a
fundamental constituent of all varieties of glass, and in this form
is in no sense a colouring substance; on the other hand, there is no
doubt that under some conditions silicon may be reduced to the metallic
state at temperatures which normally occur in glass-furnaces, and it
is practically certain, that if present in glass in this condition,
silicon would colour the glass. It is just possible that some of the
colouring effects produced in ordinary glass by powerful reducing
agents, such as carbon, either in the solid form or as a constituent of
furnace gases, may be due to the reduction of silicon in the glass.
_Tin_ by itself does not appear to have any colouring effect upon
glass, except that its oxide, in a finely suspended state, produces
opalescence and, in large quantities, white opacity. Tin, however, is
used in conjunction with copper in the production of copper-ruby, to
which reference has already been made.
_Lead and Thallium_ have already been dealt with, and it only remains
to add that their presence in the glass, although not in itself
producing any intense colouring action, increases the colouring effects
of other substances. This is probably merely a particular case of the
fact that dense glasses, of high refractive index, are more sensitive
to colouring agencies than the lighter glasses of low refractive index;
this applies to barium as well as to lead and thallium glasses.
_Phosphorus_ occurs in some few glasses in the form of phosphoric
acid, and this substance, as such, has no colouring effect. Calcium
phosphate, however, is sometimes added to glasses for the purpose of
producing opalescence. Its action in this respect is probably similar
to that of tin oxide and aluminium fluoride, these substances all
remaining undissolved in the glass in the form of minute particles in a
finely divided and suspended state.
_Arsenic_ does not exert a colouring effect on glass, and owing to its
volatile nature it can only be retained in glass in small quantities
and under special conditions. A “decolourising” action is sometimes
ascribed to arsenic, but if this action really exists it can only be
ascribed to the fact that arsenic compounds are capable of acting as
carriers of oxygen, and their presence thus tends to facilitate the
oxidation of impurities contained in the glass. A further reference
to this subject will be found below in reference to the compounds of
manganese.
_Antimony_, although frequently added to special glass mixtures, does
not appear to produce any very powerful effects, except possibly
in the direction of producing white opacity if present in large
proportions. The sulphide of antimony, however, exerts a colouring
influence, although its volatile and unstable character renders the
effects uncertain.
_Vanadium_, owing to its rarity, is probably never added to glass
mixtures for colouring purposes, although it is capable of producing
vivid yellow and greenish tints when present even in minute
proportions. On the other hand, vanadium occurs in small proportions
in a number of fire-clays, including some of those of the Stourbridge
district, and glass melted in pots containing this element is liable to
have its colour spoilt by taking up the vanadium from the clay.
_Sulphur_ is an element whose presence in various forms is liable
to affect the colour of glass in a variety of ways. The colouring
effects of sodium-, calcium-, cadmium-, and antimony-sulphides have
already been referred to. Sulphur probably never exists in glass in
the uncombined state at all, but sulphur and its oxides, which are
often contained in furnace gases, sometimes exert a very marked action
upon hot glass. The presence of sulphur gases in the atmospheres of
blowing-holes and annealing kilns is liable to produce in the glass
a peculiar yellowish milkiness which penetrates for a considerable
depth into the mass of the glass and cannot be removed by subsequent
treatment. Glass vessels, particularly if made of glass produced from
raw materials among which salt-cake has figured, are also affected by
contact with fused sulphur or its vapour, the effect being a gradual
disintegration of the glass. The precise mechanism of these actions
is not known at present, but they probably consist in the formation
of sulphur compounds within the glass, possibly giving rise to an
evolution of minute bubbles of gas.
_Selenium_, which is chemically so closely related to sulphur, is
a relatively rare element, which is, however, finding some use
in glass-manufacture as a colouring and a decolouring agent. The
introduction of selenium or of its compounds under suitable conditions
into a glass mixture produces or tends to produce a peculiar
yellowish-pink coloration, the intensity of the colour produced being
dependent upon the chemical nature of the glass as a whole and, of
course, upon the amount of selenium left in the glass at the end of the
melting process, this latter in turn depending upon the duration and
temperature of the process in question. The pink colour of selenium
glass is best developed in those containing barium as a base, but it is
also developed in lead glasses, while soda-lime glasses do not show the
colour so well. As a “decolouriser” the action of selenium is entirely
that of producing a complementary colour which is intended to “cover”
the green or blue tint of the glass; where the depth of the tint to
be “covered” is small, selenium can be used very successfully in this
way, although it is a relatively costly substance for such a purpose.
No oxidising or “cleansing” action can be ascribed to selenium or its
compounds.
_Chromium_ is one of the most intensely active colouring substances
that are available for the glass-maker, and it is accordingly used
very extensively. It has the advantage of relative cheapness, and can
be conveniently obtained and introduced into glass in the form of pure
compounds whose colouring effect can be accurately anticipated; the
colours produced by the aid of chromium have the further advantage of
being very constant in character, being little affected by oxidising
or reducing conditions, and only very slightly by the length or
temperature of the melting process. The rate of cooling, in fact,
appears to be the only factor that materially affects the colours
produced by compounds of chromium. The colours produced by chromium
alone are various depths of a bright green, the depth varying, of
course, with the proportion of chromium that is present in the glass
and with the purity of the glass itself. Very frequently, chromium
is used in conjunction with either iron or copper to produce various
tints of “cold blue” and “celadon green” respectively. This element
is most usually introduced into the glass mixture in the form of
potassium bichromate; although other compounds might be employed,
this substance presents several advantages to the glass maker. In
the first place, since the colouring effect of chromium is very
intense, it must be used in very small quantities, and if chromic
oxide itself were used, the weighing would have to be carried out with
extreme care; potassium bichromate, however, contains a much smaller
proportion of the effective colouring substance, so that much larger
weights can be employed, and the accuracy of weighing required is
proportionately reduced. A further consideration arises from the fact
that chromic oxide is itself an extremely refractory body, and is
therefore comparatively difficult to incorporate with glass, while its
presence tends to make the glass itself more viscid and refractory; the
simultaneous introduction of the alkali, as provided by the use of the
bichromate, is thus an advantage in restoring the fluidity and softness
of the glass when finished, while also facilitating the solution of
the chromium in the glass during the fusion process; this process of
solution, however, takes some time, chromium glasses being liable to
appear patchy if insufficient time is given to the “founding.”
_Uranium_ is one of the rarer and more costly elements, but is
nevertheless used in glass-making for special purposes on account of
the very beautiful fluorescent yellow colour which it imparts when
added in small proportions. This yellow is quite characteristic and
unmistakable, so that none of the other varieties of yellow glass
can ever be used as a substitute for uranium glass, but the great
cost of the latter prevents its extended use. Uranium is usually
introduced into glass mixtures in the form of a chemical compound,
such as uranyl-acetate or uranyl-nitrate, both these substances being
obtainable in the form of small, intensely bright yellow crystals.
_Fluorine_ occurs in a number of glasses in the form of dissolved or
suspended fluorides, principally fluoride of aluminium. The element
is not essentially a colouring substance, and is only mentioned here
because the fluoride named is the most frequently used means of
producing “opal” glass. The fluoride is most frequently introduced
into the glass mixtures as calcium fluoride, used in conjunction with
felspar, or as cryolite, a natural mineral which consists of a double
fluoride of sodium and aluminium.
_Manganese_ is one of the most important colouring elements used by
the glass-maker. When introduced into glass in the absence of other
colouring ingredients, compounds of manganese produce a range of
colours lying in the region of pinkish-purple to violet, according
to the chemical nature of the glass. The exact colour produced varies
according as the glass has lead, lime or barium as its base, and it
also depends upon the presence of soda or potash as the alkaline
constituent. The nature and intensity of the colour, however, which
the addition of a given percentage of manganese will produce depends
upon other factors besides the chemical composition of the bases used
in the mixture. The heat and duration of the “found” and the reducing
or oxidising conditions of the furnace in which it has been carried
on very materially affect the result. Thus, a glass having a slight
tinge of pink or purple derived from manganese can be rendered entirely
colourless by the action of reducing gases or by introducing into the
glass a reducing substance, such as a piece of wood. It will thus be
seen that while manganese is a most useful element for the glass-maker,
its employment requires much skill and care, and generally involves
some troublesome manipulations before the desired result is attained.
In practice, manganese is most frequently used with other colouring
ingredients for the production of what may be called “compound”
colours, the function of the manganese being to provide the “warm”
element, _i.e._, the pink or purple component, required. One of the
most important uses of manganese coming under this head is its use as
a “decolouriser.” By a “decolouriser” the glass-maker understands a
substance which can be used to improve the colour of a glass which,
from the nature of its raw materials and conditions of melting, would
have a greener colour than is thought desirable for the product
in question. It may be said at once that the most perfect and
satisfactory method of obtaining the better colour required is to
adopt the use of purer raw materials and methods of melting less liable
to lead to contamination of the glass. On the other hand, this radical
course is often impossible on the ground of expense, and the less
satisfactory course must be adopted of covering one undesirable colour
by another complementary colour which would, in itself, be equally
undesirable. The rationale of this procedure depends upon the fact
that a slight amount of absorption of light is not readily detected by
the human eye if it be uniformly or nearly uniformly distributed over
the whole range of the visible spectrum, _i.e._, if the colour of the
resulting light is nearly neutral, while an equally slight absorption
in one region of the spectrum, while actually allowing more light
to pass through the glass, is at once detected by the eye owing to
the colour of the transmitted light. Now it has been found that the
colour produced in glass by the addition of very small proportions of
manganese is approximately complementary to the greenish-blue tinge of
the less pure varieties of ordinary glass; the addition of manganese in
suitable proportions to such glass therefore results in the production
of a glass which transmits light of approximately neutral, usually
slightly yellow, colour, the increased total absorption only becoming
noticeable in large pieces. This “covering” of the greenish tinge is
generally most completely successful in the case of soda-flint glasses,
but the method is also used to a certain extent in the case of the
soda-lime glasses used for sheet and plate-glass manufacture. Manganese
added to glass for this purpose is generally introduced into the
mixture in the form of the powdered black oxide (manganese dioxide),
which is available as a natural ore in a condition of sufficient
purity. Added in this form, the manganese compound exerts a double
action, the decomposition of the dioxide resulting in the liberation of
oxygen within the mass of melting glass, and this oxygen itself exerts
a favourable influence on the resulting colour of the glass, since it
removes organic materials whose subsequent reducing action would be
deleterious, and it also converts all iron compounds present into the
more highly-oxidised (ferric) state in which their colouring effects
are less intense. The actual colouring effect of the manganese itself
is, of course, afterwards developed, and produces the effects discussed
above.
The “covering” of the greenish tints due to iron and other compounds
is only possible when these are present in very small proportions.
When larger quantities of these substances have been introduced into
the glass the addition of manganese modifies the resulting colour, but
is no longer able to neutralise it. A very large range of colours can
be obtained by using various proportions of iron and manganese, the
best-known of these being the warm brown tint known as “hock-bottle,”
while all shades between this and the bright green of iron and the
purple of manganese can be obtained by suitable mixtures. What has been
said above as to the sensitiveness of manganese colours applies with
even greater force to these mixed tints, since here both the iron and
the manganese compounds are liable to undergo changes of oxidation.
Copper-manganese and chromium-manganese colours are also used, as
indeed almost any number of colouring ingredients may be simultaneously
introduced into a glass mixture, the resulting colour being, as a rule,
purely additive.
_Iron_ is so widely distributed among the materials of the earth’s
crust that it is exceedingly difficult to exclude it entirely from
any kind of glass, although the purest varieties of glass contain the
merest traces of this element. Cheaper varieties of glass, however,
always contain iron in measurable quantity, while the cheapest kinds of
glass contain considerable proportions of this element. The colouring
effects of iron have already been alluded to at various points in the
earlier chapters as well as in the section on manganese just preceding.
Little further remains to be said here. Just as the less highly
oxidised compounds of iron--_i.e._, the “ferrous” compounds--always
show a decided green tint, so glasses containing iron when melted under
the usually prevalent reducing conditions of a glass-making furnace,
show a decided green tint whose depth depends upon the amount of
iron present, provided no manganese or other “decolouriser” has been
introduced. “Ferrous” compounds are, however, readily converted into
the more highly oxidised or “ferric” state by the action of oxidising
agents, and this change can also be brought about in molten glass by
the action of such substances as nitrates or other sources of oxygen.
The ferric compounds, however, show characteristic yellow tints which
are much less intense and vivid than the corresponding green colours
of the “ferrous” series, and a similar result is brought about by the
oxidation of iron compounds contained in glass; hence the “washing” or
cleansing effects ascribed to oxidising agents introduced in the fusion
of glass. It should, however, be borne in mind that the oxidation of
other substances besides iron compounds, viz., organic matter, carbon
and sulphur compounds, may, and probably does, play a most important
part in this process in the case of most varieties of glass.
_Nickel_ exerts a powerful colouring influence on glass, in accordance
with the fact that most of the other compounds of this element are
also deeply coloured. The exact colour produced in glass depends
upon the nature of the glass and on the condition of oxidation in
which the nickel is present. The colours, however, are usually of
a greenish-brown tint, although brighter colours can be produced
by nickel under special conditions. This element is not, however,
much used as a colouring agent in practice, although it has been
advocated as a “decolouriser.” The writer is not, however, aware
that it has ever been successfully used for this purpose, and, in
fact, the colours to which it gives rise do not appear to be even
approximately complementary to the ordinary green and blue tints which
“decolourisers” are intended to cover.
_Cobalt_ is one of the most powerful colouring agents in glass, and is
very largely used in the production of all varieties of blue glass. The
blue colour produced by cobalt is, in fact, probably the most “certain”
of the colours available to the glass-maker, this tint being least
affected by all those circumstances that lead to variations in other
tints. Almost the only difficulty involved in the use of cobalt is the
great colouring power of this element, which requires that for most
purposes only very small quantities may be added to the glass mixture.
Formerly cobalt was added to glass mixtures in the form of “zaffre,”
which was a very impure form of cobalt oxide. At the present time,
however, the more expensive but much more satisfactory pure oxide of
cobalt is in almost universal use. This substance shows a perfectly
constant composition and, by means of accurate weighing, enables the
glass-maker to introduce precisely the right amount of cobalt into his
batch.
The range of colours which are available to the modern glass
manufacturer are, as will be seen from a consideration of the list of
colouring elements given above, practically unlimited, particularly as
these substances can be used in almost any combination to produce mixed
or intermediate tints. This practically infinite variety of possible
tints, indeed, involves the principal difficulty encountered by the
manufacturer of coloured glass, _i.e._, that of matching his tints, or
of keeping the colour of any particular variety of glass so constant
that pieces produced at various times can be used indiscriminately
together. This ideal is, perhaps, never entirely realised, but in the
case of glasses intended for special technical uses the ideal degree of
constancy is very closely approached.
In addition to being called upon to produce a large variety of
different tints, the glass-maker is also called upon to produce
various depths of the same tint. In many cases this can be readily
done by the simple means of varying the amount of colouring material
added to the glass. Where the colouring effect of small quantities of
these substances is not excessively powerful there is no very great
difficulty in doing this, but in certain cases this mode of regulating
the intensity of the colour is not available. Thus copper-ruby glass
cannot readily be made of so light a tint as to appear of reasonable
depth when used in sheets of the thickness of ordinary sheet-glass.
As has already been indicated, the desired tint is obtained by the
process of “flashing,” _i.e._, of placing a very thin layer of deep
ruby-coloured glass upon the surface of a sheet of ordinary more
or less colourless glass of the usual thickness. This is generally
accomplished by having a pot of molten ruby glass available close to a
pot from which colourless glass is being gathered. A small gathering
of ruby glass is first taken up on the pipe, and the remaining
gatherings required for the production of the sheet are taken from
the pot of colourless glass. When such a composite gathering is blown
into a cylinder in the manner described in the previous chapter, the
ruby glass lies as a thin layer over the inner face of the cylinder,
but special care and skill on the part of the gatherer and blower is
required to ensure that this layer shall be evenly distributed and
of the right thickness to produce just the tint of ruby required.
Since the whole layer of red glass is so thin, a very slight want of
uniformity in its distribution leads to wide variations of tint, and in
practice these are often seen in the less successful cylinders of such
glass.
The chemical composition of the ruby and the colourless glass which
are to be employed for this purpose must also be properly adapted
to one another in order to produce two glasses which shall have as
nearly the same coefficient of thermal expansion as possible. If this
requirement is not met, the resulting glass is subjected to internal
strains which may lead to fracture, while, if the ruby glass has the
higher coefficient of expansion, the sheet after flattening tends to
draw itself up on the “flashed” side and cannot be passed out of the
annealing kiln in a properly flat condition.
Although most usually applied to copper-ruby glass, the flashing
process is often used with other colours also. Coloured glass of
this kind is at once recognised when looked at through the edges.
Thus examined the glass simply shows the greenish tint of ordinary
sheet-glass which constitutes practically the entire thickness of the
sheet. In the same way, if such “flashed” glass be cut or etched in
such a way that the layer of coloured glass is removed in places, the
resulting pattern appears in white on the coloured ground--a feature
which is utilised for certain decorative purposes. The flashing
process just described, it should be noted, is applicable to any form
of glass-ware which is blown from a gathering, and the coloured layer
can be applied either upon the inside or outside of any object thus
produced.
In addition to the palette of colours which the glass-maker is able to
supply, the artist in stained glass has a further range of colours at
his disposal in the form of stains and transparent colours which can be
applied to the surface of glass and developed and rendered more or less
permanent by being properly “fired.” The colours produced in this way
are also, in one sense, coloured glasses, or rather glazes, whose raw
materials are put upon the glass by the brush of the painter, and only
subsequently caused to combine and melt by suitable heating. The degree
of heat applicable under these circumstances is, however, very limited
by the necessity of avoiding any great softening of the substratum of
glass, while many of the colours themselves are composed of materials
which could not resist very high temperatures. The fluxes used in
the composition of these colours must for this reason be of a very
fusible kind, with the inevitable result of a greatly reduced chemical
stability as compared with the glass itself.
The whole subject of painting on glass, even from the purely technical
as apart from the æsthetic point of view, is a very wide one, and lies
outside the scope of the present volume. Only one further technical
point in connection with glass-painting and stained glass work will
therefore be touched upon here. This is an example of the fact that
the more technically “perfect” modern product is not always preferable
for special purposes which have been well served by older and far
less “perfect” products. The production of technically excellent
coloured glass in modern times was, somewhat surprisingly at first,
accompanied by a very marked decline in the artistic beauty of stained
glass windows produced with this modern material; the ancient art of
stained glass was, therefore, for a time regarded as a “lost art,” and
glass-makers were blamed for being unable to produce the brilliant and
beautiful tints which had been formerly available. More careful study,
however, revealed the fact that while the actual colour of modern
glass was at least as brilliant and varied as that of ancient glass,
the difference lay in the fact that the modern glass was practically
entirely free from such imperfections as air-bubbles, striæ, and
other defects which improved appliances and methods had enabled the
glass-maker to eliminate from his products. Finding the beauty of his
wares greatly improved by this increased purity of the glass in the
case of window glass and table ware, it was natural for the glass-maker
to endeavour to produce the same “improvement” in the coloured glasses
intended for artistic purposes and, indeed, it is more than likely that
the stained-glass workers themselves pressed this line of improvement
upon him by a demand for “better” glass. It turned out, however, on
close examination, that this very perfection of modern glass rendered
it less adapted for these artistic purposes. A perfect piece of glass,
having smooth surfaces and no internal regularities, allows the rays
of light falling upon it to pass through undeflected in direction,
and merely changed in colour, according to the tint of the glass in
question. On looking at the glass, external objects can be quite
clearly seen, and much of the interest and mystery of the glass itself
is lost. On the other hand, when falling upon a piece of glass having
an irregular surface, and containing all manner of irregularities
such as striæ, air-bells, and even pieces of enclosed solid matter,
the light is scattered, refracted, and deflected into all manner of
directions until it almost appears to emanate from the body of the
glass itself, which thus appears almost to shine with an internal
light of its own; the eye can hardly perceive the presence of external
objects, and the whole window appears as a brilliant self-luminous
object.
Once their attention had been drawn to these facts, modern glass-makers
endeavoured, and with much success, to reproduce the desirable
qualities of the ancient glass, while still availing themselves of
modern methods to produce more stable glasses and a wider range of
colours. The irregular surface of the old glass is imitated by using
rolled or “muffed” instead of ordinary blown glass, while the internal
texture is rendered non-homogeneous by the deliberate introduction of
solid and gaseous impurities and by manipulations so arranged as to
leave the glass in layers of different density, which appear in the
finished glass as “striæ.” As a consequence, it is probably not too
much to claim that the modern workers in coloured glass have materials
at their disposal which are at least as suitable for the purpose as
those that were available in the best days of the ancient art.
Some reference has already been made to the technical uses of coloured
glass, but one or two further points in that connection remain to be
discussed. For such technical purposes as railway and marine signals,
the consensus of practical experience has decided in favour of certain
colours of glass, such as red and green of particular tints. On the
other hand, for various purposes in connection with photography,
the glass-maker does not appear to have been able to meet the new
requirements, with the result that flimsy and otherwise unsatisfactory
screens made of gelatine or celluloid stained with organic dyes are
employed in place of coloured glass in such cases, for example, as the
covering of lamps for use in photographers’ “dark” rooms, and for the
light-filters used for orthochromatic and tri-chromatic photography.
In all these cases it is necessary to use a transparent coloured
medium which transmits only light of a certain very definite range of
wave-lengths, and there is no doubt that for the glass-maker, who is
confined to the use of a number of elementary bodies for his colouring
media, it is by no means easy to comply with these requirements of
exact transmission and absorption. On the other hand, the field of
available coloured glasses has not been fully explored from this point
of view, the only extensive work on the subject having been done in
connection with the Jena firm of Schott, who have put upon the market
a series of coloured glasses of accurately-known absorbing power.
There is, however, little doubt that a much greater extension of this
field is possible, and that it will be opened up by a glass-maker who
undertakes the exhaustive study of coloured glasses from this point of
view, although it must be admitted that there is considerable doubt
whether the results obtainable by the aid of aniline and other dyes as
applied to gelatine can ever be equalled by coloured glasses.
CHAPTER XII.
OPTICAL GLASS.
Optical glass differs so widely from all other varieties of glass that
its manufacture may almost be regarded as a separate industry, to
which, indeed, a separate volume could well be devoted. In the present
chapter we propose to give an outline of the most important properties
of optical glass, and in the next chapter to describe the more
important features of the processes used in its production.
The properties which affect the value of optical glass may roughly be
divided into two groups. The first group comprises the specifically
“optical” properties--_i.e._, those directly influencing the behaviour
of light in its passage through the glass, while the second group
covers those properties of a more general nature, which are of special
importance in glass that is to be used for optical purposes.
_Optical Properties of Glass._--The most essential property of glass
in this respect is homogeneity. We have already indicated that glass
can never be regarded as a definite chemical substance or compound,
but that it usually consists of mutual solutions of various complex
silicates, borates, etc. Solutions being of the very nature of mixtures
of two or more different substances, it follows that they can only
become homogeneous when _complete_ mixing has taken place. We have
a familiar example of the formation of such a solution when sugar is
dissolved in water. The water near the sugar becomes saturated with
sugar and of different density from the remaining water; if the liquid
is _slightly_ stirred a very characteristic phenomenon makes its
appearance--the pure water and the dense sugar solution do not at once
mix completely, the denser liquid remaining for a time disseminated
throughout the whole fluid mass in the form of more or less fine lines,
sheets, or eddies, and these are visible because the imperfectly mixed
liquids have different effects on the light passing through them. In
the case of sugar-water we are, however, dealing with a very mobile
liquid, and a few turns of a tea-spoon suffice to render the mixture
complete, and the liquid, which for a few moments had appeared turbid,
becomes homogeneous and transparent. In the case of glass, when the
raw materials are melted together, a mixture is formed of liquids of
differing densities similar to that which was temporarily formed in
the sugar-water solution. Molten glass, however, is never so mobile
a liquid as ordinary water, nor is it in the ordinary course of
manufacture subjected to any such thorough mixing action as that which
is produced by a spoon in a glass of water. In glass as ordinarily
manufactured, therefore, it is not surprising to find that the lack of
homogeneity which originates during the melting persists to the end.
Its effects can be traced whenever a thick piece of ordinary glass is
carefully examined, when the threads or layers of differing densities
can be recognised in the form of minute internal irregularities in the
glass. These defects are known as striæ or veins, and their presence
in glass intended for the better kind of optical work renders the
glass useless. As will be seen below in the production of optical
glass, special means are adopted for the purpose of rendering it
as homogeneous as possible; in fact, the early history of optical
glass manufacture is simply the history of attempts to overcome this
very defect. The problem is, however, beset by chemical and physical
difficulties of no mean order, and even in the best modern practice
only a small proportion of each melting or crucible full of glass
is entirely free from veins or striæ. In many cases these defects
are very minute, and sometimes escape observation until the stage of
the finished lens is reached. At that stage, however, their presence
becomes painfully evident from the fact that they interfere seriously
with the sharp definition of the images formed by the lens in question.
It will be seen that in such a case time and money has been wasted
by grinding and polishing what turns out to be a useless piece of
glass. Methods are, therefore, used for examining the glass before it
is worked, whereby the existence of the smallest striæ can scarcely
escape detection. These methods depend upon the principle that a beam
of parallel light passing through a plate of glass will meet with no
disturbance so long as the glass is homogeneous, but if striæ are
present, they will cause the light to deviate from parallelism wherever
it falls upon them. Under such illumination, therefore, the striæ
will appear as either dark or bright lines, when they can be readily
detected. One form of apparatus used for this purpose is illustrated in
Fig. 14.
[Illustration: FIG. 14.--Diagram of striæ-testing apparatus.
_L_, source of light; _S_, slit; _A_ and _B_, simple convex lenses;
_G_, glass under test; _E_, eye of observer. The arrows indicate the
paths of light-rays.]
_Transparency_ and _colour_ are obviously fundamentally important
properties of glass. In one sense homogeneity is essential to
transparency, but the aspect of the subject which we are now
considering is that of the absorption of light in the course of regular
transmission through glass. It may be said at once that no glass is
either perfectly transparent or, what comes to nearly the same thing,
perfectly free from colour. In the case of the best optical glasses it
is true that the absorption of light is very slight, but even these,
when considerable thicknesses are viewed, show a greenish-yellow or
bluish colouring. On the other hand, certain optical glasses which are
used at the present time for many of our best lenses absorb light so
strongly or are so deeply coloured that a thickness of a few inches
is sufficient to reveal this defect. To some extent public taste or
opinion which objects to the use of even a slightly _greenish_ glass
in optical instruments of good quality is to blame for the tint of
these glasses. In many cases glass-makers could produce a very slightly
greenish glass, but in order to overcome this colour they deliberately
add to the glass a colouring oxide imparting to the glass a colour more
or less complementary to the natural green tint. The result is a more
or less neutral-tinted glass which, however, absorbs much more light
than the naturally green glass would have done. Since such glass is
frequently used for photographic lenses, it is interesting to note that
the light rays whose transmission is sacrificed in order to avoid the
green tint are those lying at or near the blue end of the spectrum, so
that the photographic rapidity of the resulting lenses is decidedly
reduced by the use of such glass.
_Refraction and Dispersion._--The quantitative properties of glass,
governing its effect upon incident and transmitted light, are, of
course, of fundamental importance in all its optical uses. The
fundamental optical constant of each variety of optical glass is known
as its refractive index; this number really represents the ratio of
the velocity with which light waves are propagated through the glass
to the velocity with which they travel through free space. Not only
does this ratio vary with every change in the chemical composition and
physical condition of the glass, but it also varies according to the
length of the light waves themselves. In other words, the short waves
of blue light are transmitted through glass with a different velocity
from that with which the longer waves of red light are transmitted.
The consequence is that when a beam of white light is passed through a
prism it is split up and spread out into a number of beams representing
all the colours of the spectrum in their proper order, the blue light
suffering the greatest deflection from its original path, while the
red light suffers least deflection. Both the actual and relative
amount by which light rays of various colours are deflected under
such circumstances depends upon the nature of the glass in question;
therefore, to fully characterise the optical properties of a given
kind of glass it is necessary to state not only its refractive index
but to specify the refractive indices for a sufficient number of
different wave-lengths of light, suitably distributed through the
spectrum. For this purpose a number of well-marked spectrum lines have
been chosen, the systematic use of the particular set of lines which is
now usually employed being due to the initiative of Abbé and Schott at
Jena, who initiated the system of specifying the optical properties of
glass in this way. The actual lines chosen are the line known as A′,
corresponding to a wave-length of 0·7677 micro-millimetres, and the
lines known as C, D, F, and G′, whose wave-lengths, in the same units,
are 0·6563, 0·5893, 0·4862, and 0·4341 respectively. The A′ line,
however, lies so near the extreme red end of the spectrum that the data
concerning it are seldom required.
As a matter of fact, the actual refractive index is only stated in most
tables of optical glasses for sodium light (D line), the dispersive
properties of the glass being indicated by tabulating the differences
between the refractive indices for the various lines, the table thus
containing columns marked C-D, D-F, F-G′. These figures are usually
described as the “dispersion” of the glass from C to D, D to F, etc.
In addition to these figures it is usual to tabulate what is called
the “mean dispersion” of the glass, which is simply the difference
between the refractive indices for C and F lines; this interval is
usually taken as representing that part of the spectrum which is of
the greatest importance for visual purposes. A further constant which
is of great importance in the calculations for achromatic lenses is
obtained by dividing the mean dispersion into the refractive index for
the D line minus one (usually written (C-F)/(n_{D}-1)=ν). This term,
for which no satisfactory name has yet been suggested, characterises
the ratio of the dispersive power of the glass to its total refracting
power. It is usually denoted by the Greek letter ν. The following table
(taken from the Catalogue of the Optical Convention, 1905) gives a list
of optical glasses produced by Messrs. Chance, of Birmingham. This
list, although it is not nearly so long as that issued by the French
and German firms who manufacture optical glass, contains examples of
the most important types of optical glass which are available at the
present time. Those, however, who wish to use the data for the purpose
of lens calculation are advised to consult the latest issues of the
optical glass-makers’ catalogues, since the range of types available,
and even the actual figures for some of the glasses, are liable to
variation from time to time.
TABLE OF OPTICAL PROPERTIES.
--------------+------+----+------+-----------------------------------
| | | | Partial and
| | |Medium| Relative Partial Dispersions.
| | | Dis- +------+----+------+----+------+----
Name. |n_{D}.| ν. | per- | |C-D | |D-F | |F-G′
| | | sion.| C-D. |--- | D-F. |--- |F-G′. |---
| | | C-F. | |C-F.| |C-F.| |C-F.
--------------+------+----+------+------+----+------+----+------+----
Extra Hard | | | | | | | | |
Crown |1·4959|64·4|·00770|·00228|·296|·00542|·704|·00431|·560
Boro-silicate | | | | | | | | |
Crown |1·5096|63·3|·00803|·00236|·294|·00562|·700|·00446|·555
Hard Crown |1·5175|60·5|·00856|·00252|·294|·00604|·706|·00484|·554
*Medium Barium | | | | | | | | |
Crown |1·5738|57·9|·00990|·00293|·296|·00697|·704|·00552|·557
*Densest | | | | | | | | |
Barium | | | | | | | | |
Crown |1·6065|57·9|·01046|·00308|·294|·00738|·705|·00589|·563
Soft Crown |1·5152|56·9|·00906|·00264|·291|·00642|·708|·00517|·570
*Medium Barium | | | | | | | | |
Crown |1·5660|56·3|·01006|·00297|·295|·00709|·704|·00576|·572
Barium Light | | | | | | | | |
Flint |1·5452|53·5|·01020|·00298|·292|·00722|·701|·00582|·570
Extra Light | | | | | | | | |
Flint |1·5316|49·0|·01085|·00313|·288|·00772|·711|·00630|·580
Extra Light | | | | | | | | |
Flint |1·5333|48·5|·01099|·00322|·293|·00777|·707|·00640|·582
Boro-silicate | | | | | | | | |
Flint |1·5623|47·4|·01187|·00343|·289|·00844|·711|·00693|·584
*Barium Light | | | | | | | | |
Flint |1·5833|46·6|·01251|·00362|·288|·00889|·711|·00721|·576
Soda Flint |1·5482|45·8|·01195|·00343|·287|·00852|·713|·00690|·577
Light Flint |1·5472|45·8|·01196|·00348|·291|·00848|·709|·00707|·591
Light Flint |1·5610|43·2|·01299|·00372|·287|·00927|·713|·00770|·593
Light Flint |1·5760|41·0|·01404|·00402|·286|·01002|·713|·00840|·598
Light Flint |1·5787|40·7|·01420|·00404|·284|·01016|·715|·00840|·591
Dense Flint |1·6118|36·9|·01657|·00470|·284|·01187|·716|·01004|·606
Dense Flint |1·6214|36·1|·01722|·00491|·285|·01231|·715|·01046|·608
Dense Flint |1·6225|36·0|·01729|·00493|·286|·01236|·715|·01054|·609
Extra Dense | | | | | | | | |
Flint |1·6469|33·7|·01917|·00541|·285|·01376|·720|·01170|·655
Densest Flint |1·7129|29·9|·02384|·00670|·281|·01714|·789|·01661|·678
--------------+------+----+------+------+----+------+----+------+----
In the table on p. 212 the first column contains the ordinary trade
names by which the various types of glass are known. These names,
while somewhat arbitrary, indicate in a rough way the chemical nature
of the glass concerned. Thus the word “flint” always implies a glass
containing lead and therefore having a comparatively high refractive
index and low value of ν, while the word “crown,” originally applied
only to lime-silicate glasses, is now used for all glass having a high
value of ν. In the next column of the table are given the refractive
indices of the glasses, while the third column contains the values of
ν. It will be seen that the glasses are arranged in descending order
of magnitude in respect of this constant. An inspection of the figures
in these two columns will reveal the fact that for the majority of
the glasses contained in this table the value of ν decreases as the
refractive index increases. The glasses which are an exception to this
rule are indicated by an *. As a matter of fact this rule applied
to all glasses that were known or were at all events commercially
available prior to the modern advances in optical glass manufacture
which were initiated by Abbé and Schott of Jena. It was Abbé’s insight
into the requirements of optical instrument design that led him to
realise the importance of overcoming this limitation in the ratio
between the dispersive and refractive powers of glass. With the
collaboration of Schott he succeeded in producing a whole series of
previously unknown varieties of optical glass in which the relation
between n_{D} and ν is not that of approximately simple inverse
proportionality which holds for the older crown and flint-glasses.
Most valuable and in many ways most typical of these new glasses are
those known as the “barium crown” glasses, which combine the high
refractive index of a light flint or even a dense flint-glass with the
high ν value of an ordinary crown glass. It would lead too far into
the subject of lens construction to explain in detail the possibility
opened up to the optician by the use of these newer varieties of glass.
We must content ourselves with pointing out that the great forward
strides marked by the production of apochromatic microscope objectives,
of anastigmatic photographic lenses, and the modern telescope
objectives are all based upon the employment of these new optical
media; and although optical glasses of these newer types are at the
present time produced in the optical glass manufactories of France and
England, in quality and quantity at least equal to the output of the
Jena works themselves, these great optical achievements stand as a
lasting monument to the pioneer work of Abbé and Schott in this field.
The last six columns of the table of optical glasses given above
contain figures which define the manner in which each of the glasses
named distributes the various sections of the spectrum. The columns
C-D, D-F, and F to G′ give as already indicated the differences between
the refractive indices for the C, D, F and G′ lines respectively; the
smaller figures in the intermediate columns indicate the ratio of
each of these differences to the mean dispersion of the glass. If all
kinds of glass distributed the various portions of the spectrum in the
same proportionate manner, merely differing in the total amount of
dispersion produced, these figures would be identically the same for
all glasses. In actual fact it will be seen that the figures differ
very widely from one type of glass to another. A moment’s consideration
will show that when two glasses are used in a lens for the purpose
of achromatising one another, _i.e._, when one is used to neutralise
the dispersion of the other, such achromatisation can only be perfect
if these ratios (the relative partial dispersions) are the same for
both glasses. To put the same statement in more concrete terms, if
the spectrum produced by one glass is comparatively long-drawn out
at the red end, relatively compressed at the blue end, while in the
other glass the opposite relation holds between the two ends of the
dispersion spectrum, it is evident that the two spectra can never be
superposed in such a way as to entirely neutralise one another--the
spectrum produced by the one glass will predominate and leave a
residual colour at the blue end, while the other will predominate
at the other end. In the case of lenses achromatised by the use of
such glasses, there will always be a slight fringe of colour around
the borders of the images which they produce. One of the aims which
Abbé and Schott set themselves in the production of new varieties of
optical glass was to obtain one or more pairs of glasses in which the
relative partial dispersions should be as nearly alike as possible
while the actual values of ν should differ as widely as possible. Some
success in this direction was at first claimed by the Jena workers,
but unfortunately some of the most promising glasses in this respect
were found to be too unstable for practical use and had ultimately to
be abandoned. At the present time the only pair of really perfectly
achromatic glasses offered by the Jena firm is that tabulated below,
and it will be seen that although the relative partial dispersions
are very closely alike, the ν values of the two glasses only differ
by 10, and at least one of these glasses is not readily obtainable in
really satisfactory optical quality. On the other hand, practically
perfectly achromatised lenses (generally known as “apochromatic”) have
been produced, especially by Zeiss of Jena, for microscopic purposes,
by the careful selection of glasses suited to each other in this
respect. Such a solution of the problem is further facilitated by the
fact that in these lenses more than two varieties of glass can be
used to neutralise one another, while a natural mineral (fluorite)
is also employed. From the glass-maker’s point of view, however, the
problem of producing a satisfactory pair of glasses capable of entirely
achromatising one another has yet to be solved.
---------+------+----+-------+------+------+------+------+------+-----
| | | | | c-d | | d-f | | f-g′
Name. | n_{D}| ν | C-F. | C-D. | --- | D-F. | --- | F-G′.| ---
| | | | | c-f. | | c-f. | | c-f.
---------+------+----+-------+------+------+------+------+------+-----
Telescope| | | | | | | | |
Crown |1·5254|61·7|·00852 |·00250| ·292 |·00602| ·707 |·00484| ·568
Telescope| | | | | | | | |
Flint |1·5211|51·8|·001007|·00297| ·294 |·00710| ·705 |·00577| ·573
---------+------+----+-------+------+------+------+------+------+-----
The table of optical glasses given above, although brief as compared
with the lists issued by French and German optical glass-makers,
fairly covers the range of practically available glasses, and a rapid
inspection will at once show how extremely limited this range really
is. Thus the refractive index varies only between the limits 1·49
and 1·71, and even if we admit as practical glasses such extreme
types--offered by some makers--as would extend this range to 1·40
in one direction and to 1·80 in the other, this does not affect the
present argument. Of course, a glass of a refractive index as low
as 1·0, or even 1·10, is not theoretically possible, since the mere
density of any substance enters into the factors that affect its
refractive index, and a glass having a density lower than that of
water (whose refractive index is about 1·3) is scarcely conceivable.
In the other direction, however, the limits met with in the case of
glass are considerably exceeded by certain natural mineral substances.
Thus the diamond has a refractive index of 2·42, while the garnets
show refractive indices from 1·75 to 1·81. The values of ν found in
the table of optical glasses are still more narrowly restricted,
lying between 67 and 29, while such a mineral as fluorite shows a
value of 95·4. These facts show that it is physically possible to
obtain transparent substances having optical properties lying far
beyond the limited range covered by our present optical glasses, and
it scarcely needs showing that if such an extended range of materials
were available greatly increased possibilities would be opened up to
the designer of optical instruments. It is consequently interesting
to inquire as to the actual causes which limit the range of optical
glasses at present available. It will be found that these limits are
set by the properties of glass itself. While the more ordinary kinds
of glass, having average optical properties and showing dispersive
powers roughly conforming to the law of inverse proportionality with
refractive index which governs the older varieties of optical glass,
are chemically stable substances, showing little tendency to undergo
either chemical changes or to crystallise during cooling, the more
extreme glasses exhibit these undesirable features to an increasing
extent the more nearly the limit of our present range is approached.
As the chemical composition of a glass is “forced” by the addition
of special substances intended to affect its optical properties in
an abnormal direction, so the chemical and physical stability of the
glass is rapidly lessened. The more extreme glasses, in fact, behave as
active chemical agents readily entering into reaction or combination
even with relatively inert substances in their environment--they act
vigorously upon the fire-clay vessels in which they are melted, and
they are readily attacked by acids, moisture or even warm air, when in
the finished condition, while many of them can only be prevented from
assuming the condition of a crystalline (and opaque) agglomerate by
being rapidly cooled through certain critical ranges of temperature.
A limit to the possibility of production is set by these tendencies
when they exceed a certain amount--a point being reached where it
ceases to be practicable to overcome the tendency of the glass to
self-destruction. On the lines of our present glasses, therefore, it
does not appear hopeful to look for any considerable extension of the
range of our optical media. On the other hand, as the known optical
properties of transparent crystalline minerals show, a much greater
range of optical constants would become available if it were possible
to manufacture artificial mineral _crystals_ of sufficient size and
purity for optical purposes, and the author believes that in this
direction progress in optical materials is ultimately bound to lie[1].
[1] See a Paper by the present author on “Possible Directions
of Progress in Optical Glass”--Proceedings of the Optical
Convention, London, 1905.]
In addition to possessing the requisite optical constants, a good
colour and perfect homogeneity, certain other properties are essential
in good optical glass. These are the general physical and chemical
qualities which are essential in all good glass, but especially
emphasised by the fact that the requirements for optical glass are more
stringent than for any other variety of the material. Thus chemical
stability is of the greatest importance, for the best lenses would
soon become useless if the action of atmospheric moisture were to
affect them appreciably--the polished surfaces would rapidly become
dull and the whole lens would soon be rendered useless. The conditions
governing the chemical stability of glass and the methods of testing
this quality have already been indicated (Chapters I. and II.). The
harder varieties of optical glass, such as the glasses quoted in the
above table under the names of “Hard Crown” and Boro-Silicate Crown,
are probably among the most durable and chemically resistant of all
varieties of glass, but as we have already indicated, when extreme
optical properties are required, the necessary chemical composition of
the glass always entails a sacrifice of this great chemical stability,
until a limit is reached where valuable optical properties no longer
counterbalance the serious disadvantage of a chemical composition
which renders the glass liable to rapid disintegration. In certain
special cases it is, perhaps, possible to protect lenses made of such
unstable glass by covering them with cemented-on lenses of stable
glass, but this device entails concomitant limitations in the design
of the optical system and is, therefore, rarely used. In any case,
however, it is well for the lens-designer to consider the relative
stability of the glasses employed when arranging the order in which
they are to be used, since it is obviously preferable to put a hard,
durable glass on the outside of his system, where it is most directly
exposed to atmospheric moisture, and is also subject to handling and
“cleaning” by inexpert hands. This latter factor is a very important
one for the life of any lens. In the first place, a glass surface is
very seriously affected by the minute film of organic matter which is
left upon it when it has been touched with even a clean finger; unless
the glass is of the best quality in this respect, such fingermarks
readily develop into iridescent spots and may even turn into black
stains. Particles of dust allowed to settle on the surface of the glass
will affect it in the same way, so that the protection afforded by mere
mechanical enclosure in the tube of an instrument is of decided value
in preserving a glass surface. It should, however, be noted that in
some instances the interior metal surfaces of optical instruments are
varnished with substances that give off vapours for a long time after
the instrument is completed, and in that case the inside lenses are apt
to be tarnished in consequence. On the other hand, outside lenses are
also exposed to direct mechanical injury from handling and “cleaning.”
As far as the latter operation is concerned, it frequently happens,
particularly in glasses containing soda, that a slight surface dimming
is formed on the glass when it has been left in a more or less damp
place for a long time. This dimming is chiefly due to the formation on
the surface of a great number of very minute crystals of carbonate of
soda, which are hard and sharp enough to scratch the glass itself if
rubbed about over it. If such a lens be wiped with a dry cloth, however
clean and soft, the effect is a permanent injury to the polished
surface, which could readily be avoided by first washing the lens with
clean water, or even by using a wet cloth instead of a dry one for the
first wiping.
The mechanical hardness of the glass is an important factor in
determining its resistance to such injurious treatment or to the
effects of accidental contact with hard, sharp bodies. The subject of
the hardness of glass has already been discussed in a general way in
Chapter II., and little remains to be added here. Broadly speaking, a
high degree of hardness and a low refractive index are found together.
This statement is certainly true where any considerable difference of
hardness is considered, as, for example, in comparing a hard crown
glass with a dense flint; but where the difference of refractive index
or of density is small, it is not at all certain that the lighter
glass will also be the harder.
The properties involved in the quality known as “hardness” also affect
in a very marked manner the behaviour of glass when subjected to the
grinding and polishing processes. The ease with which a good polish
can be obtained varies very much in different kinds of glass, both the
hardest and the softest glasses showing themselves difficult in this
respect. The harder glasses are certainly less liable to accidental
scratching during the polishing operations, and generally work in a
cleaner manner; but the time required to produce a satisfactory polish
is much greater owing to the resistance to displacement offered by the
molecules. Both the speed of working and the pressure exerted during
the polishing operation have, in fact, to be carefully adapted to the
quality of the glass in this respect if the best possible results are
to be obtained.
Another property which is essential in optical glass of the highest
quality is that of freedom from internal strains. This subject will be
again referred to later in connection with the annealing processes used
in the manufacture of optical glass, and it need only be mentioned here
that the presence of internal strain is readily recognised in glass,
by the aid of the polariscope. Perfectly annealed glass, entirely free
from internal strains, produces no effect upon a beam of polarised
light passing through it, while even slightly strained glass becomes
markedly doubly-refracting. For many purposes of optics this double
refraction becomes undesirable or even inadmissible, especially as it
is accompanied by small variations in the effective index of refraction
of various portions of the mass of glass. Further, if the amount of
double refraction observed is at all serious it indicates a state
of strain which may easily lead to the fracture of the whole piece,
particularly when undergoing the earlier stages of the grinding process
or if exposed to shocks of any sort. As will be seen below, perfectly
annealed glass is obtainable, but very special means are required for
its production, and the optician should for that reason avoid making
unnecessarily extreme demands in this direction. The very _small_
amount of double refraction frequently found in the better class of
optical glass is entirely harmless for most purposes.
CHAPTER XIII.
OPTICAL GLASS.