For some years previous to the period of which I am writing, I was deeply interested in the question of "burning glasses," such as those of Buffon, Parker, and others; my aim being to construct an instrument of sufficient power to act on several ounces, instead of several grains, of the material, which was to be operated upon in crucibles, into which the focus of the lens was directed. In following up this idea, my attention was naturally turned to the enormous difficulty of producing perfectly homogeneous discs of optical glass of large diameters. Fraunhöfer's magnificent lenses of small size had for many years attracted universal admiration, and learned societies were intent on further investigation of the subject. Thus it was that Faraday commenced an enquiry which only ended in failure.
Fraunhöfer's system of manufacture was at that time a profound secret, and the small discs of glass which he sold at fabulous prices were the envy of all other optical glass makers. Faraday, whose scientific knowledge and attainments pointed him out as the most likely scientist to succeed in this new field of enquiry, was, I doubt not, led absolutely astray by the appearance of Fraunhöfer's small discs; had Faraday never seen one of them, and been left to his own resources, he would most probably have succeeded.
The small discs produced by Fraunhöfer, four or five inches in diameter and from half to three-quarters of an inch in thickness, showed what really appeared to be incontrovertible evidence that they were made in small open flat dishes, of the form shown in Fig. 21, page 102.
These little cakes of glass, a, had a flat shining upper surface,
evidently the natural, or fire, polish, as it is called, and were
rounded at the top edges as shown at b, the periphery of the flat cake
and its lower surface having the unmistakeable impress of the shallow
fireclay
dish shown in section at c. These apparently irresistible proofs that
the glass was made in small quantities, and was very fusible and very
fluid, no doubt deceived Faraday, and so misdirected his experiments as
to lead to failure; all of which became self-evident when the mode of
producing these little cakes was known. Glass made in large pots and at
the highest attainable temperature is only semi-fluid, and is found to
be of different densities in the upper and lower portions of the mass,
owing to the varying specific gravities of its constituents. A partial
admixture slowly going on in consequence of unequal expansion by heat
in so bad a conductor as glass, and the motion induced by air bubbles
slowly rising to the surface, have the effect of introducing veins, or
striæ, consisting of streaks of more or less dense portions carried
upwards by the rising air bubbles, running throughout the general mass,
and entirely spoiling it for optical purposes. Now Fraunhöfer, knowing
no means of preventing the formation of these veins or striæ,
proceeded on this simple but laborious mode of counteracting these
defects. He made a large potful of glass as perfect as he could by
simple fusion; he allowed it to get cold in the pot; he then sawed the
mass horizontally into slices, polished their surfaces, and thus
examined their internal structure; and wherever there was a line or
streak of more or less dense glass, the defective part was applied to a
glass-grinder's wheel and cut away, not as a deep narrow notch but by a
wide shallow indent; the surface was again polished for re-examination,
and this process was repeated until no more veins, or striæ, were
visible. The mutilated and indented disc of glass, sometimes cut nearly
half-way through, was then put into one of the shallow fireclay dishes
already described, gently heated at first, and finally made
sufficiently soft to sink down and acquire the form and dimensions of
the dish, the impress of whose surface it bore, while its upper surface
assumed the polished appearance of ordinary molten glass.
What I desired to achieve was the production, at a small cost, of large
and massive discs or lenses, which could not be produced by
Fraunhöfer's system. Among the several plans I proposed, I will
describe only two, each of which attacked the problem from an entirely
different standpoint. First, I may mention that I made a series of
laboratory experiments with viscid transparent fluids, contained in
glass vessels of various forms and under varied conditions. Venice
turpentine was first tried, but very viscid castor oil was the nearest
to glass in its indications of movement within itself. Small grains of
broken red sealing-wax, by their greater specific gravity, showed well
the tendency of the oxide of lead (used in flint glass) to subside; and
how, by rotating this vessel with one small fragment of sealing-wax,
its movement was restrained within a circle the diameter of which was
equal to the subsidence of the particle during a semi-rotation of the
vessel containing the oil. The effect of the gentle rotation or
rolling of the vessel was also experimented on in various ways. A small
portion of the viscid oil was poured out, and a very minute quantity of
blue powder ground up in it, just enough to give a faint blue colour.
This blue oil was then poured back again into the nearly
globular-shaped glass vessel, which must be considered as the glass
pot; a little movement of the vessel produced streaks of blue colour
like veins in marble, dispersed throughout the general mass of viscid
fluid. But by continuing to roll the glass globe slowly for about two
or three hours, not the slightest trace of veins or streaks of blue
remained visible, while a very slight tint of blue pervaded the whole
mass of oil, which was now perfectly homogeneous. It will be observed
that the motion so given to the whole mass did not divide it, as the
insertion of a stirrer would have done. I also demonstrated the fact
that stirring from the surface by a rod was wholly impossible without
the introduction of air in large quantities. So extraordinary is this
fact that I cannot refrain from putting it on record. Take a glass jar
or vessel, say ten inches deep and two inches in diameter, open at the
top and closed at the bottom, as shown in Figs. 22 and 23, on page 104.
Nearly fill it with clear, but viscid, castor oil, carefully removing
all traces of air from the fluid by exhaustion under the glass bell of
a common air-pump; place the jar on a table, take a polished metal, or,
preferably, a glass, rod about the
size of a blacklead pencil, and having a smooth, rounded end, wipe it,
and very slowly and steadily lower it some six inches into the oil, as
shown in Fig. 22; then as slowly and carefully withdraw it, occupying
quite a minute in doing so. There will remain no trace that anything
has entered the oil. Now place the jar again under the bell of the
air-pump, take a few strokes with it, and there will appear a line of
ill-defined mist, standing vertically upwards about six inches in
height in the centre of the jar; at each stroke of the pump it becomes
more visible, and enlarges in diameter. It soon assumes the appearance
of innumerable little globes, like the hard roe of a herring, as shown
at Fig. 23. A little more exhaustion, and these still further expand
and rush upwards by the thousand, until at last all the air adhering
to, and taken down by, the glass rod has been removed. What you may do,
and what you may not do, with molten glass was thus beautifully
illustrated by some of these preliminary experiments with viscid
fluids. You may move the glass about; you may rotate these viscid
fluids in a closed vessel; and you may even pour them, provided the
last part of your stream does not fall on the poured-out mass.
To return to actual glass, the subject divides itself into two main
systems of procedure, viz., you may make glass by the fusion of pure
silica, lime, and potash, or other alkali, with or without the addition
of a considerable quantity of oxide of lead, which is used where great
density and refractive power are required. Then it becomes more desirable that
an intimate mixture of the materials should take place, and throughout
the twelve to twenty hours required for fusion, no subsidence of the
heavy portions of the mixture, or flotation of the lighter ones, should
be suffered to take place, or a homogeneous mass will not be obtained.
It is manifestly easy to remove the heavy, sweet particles from the
lower part of a cup of tea by one or two gentle movements of a spoon,
and so get the whole cup of fluid equally sweet, but we have been
warned by our oil experiment that the fluid glass must not be stirred
by a rod or we introduce air; and if we wait long enough for the air
slowly to find its way again to the surface, we inevitably have an
interval in which the difference in specific gravity of the several
materials will assert itself, and we get subsidence, lose the
homogeneity of the mass, and all our stirring will have been in vain.
The outcome of these and other observed conditions was the proposal to
employ oscillating, semi-rotating, or continuous slow-rotating melting
crucibles, the latter being preferred. The crucibles and the proper
kind of furnace for this purpose may be largely varied; one of the
simple forms is given in Fig. 24, Plate IX.; its leading features are
representative of all the others.
The furnace there shown consists of a cylindrical casing of iron, A,
lined with firebrick, B; it is divided into two chambers, the lower
one, C, being provided with firebars on which the fuel rests; while the
upper chamber, D, is cylindrical in form with a curved roof, having an
opening at J formed in a circular piece of
moulded firebrick J*, which is removed when putting in the rotating
crucible H. Above the opening, J, is a suspended hood, E, which
communicates with a tall chimney; the lower part of the chamber, D, is
conical in form, having a small central opening, F, and four larger
cylindrical openings, G, surrounding it, each of which communicates
with the fire chamber, C, and allows the flame to ascend and play up
and around the crucible, H. This crucible is conical in form both
above and below its largest diameter, and terminates in a raised neck
or mouth at H*; the furnace, A, is suspended on axes, occupying the
position indicated by the letter M; these axes are fitted to a strong
iron ring or hoop, N, which surrounds the furnace, and is itself
supported on the axes, P, which rest on iron frames, O.
The axes, P, are placed at right angles to the axes, M, so as to allow
the crucible to roll or gyrate on its axis, its upper and lower ends
moving in a circular path. It will be observed that the crucible, H,
rests on one of its sides on the conical floor of the chamber, D, and
is kept in position by its lower spherical end, M, moving in the
cylindrical opening, E. Now if the furnace be moved quietly on its
axis, the crucible gravitating to the lowest inclined side of the
conical surface on which it rests, will roll round on its own axis so
long as the furnace is kept in motion. This motion of the furnace may
be easily effected by means of a short-throw half-crank on a vertical
axis passing upward through the floor in a line through the centre of
the furnace, the crank-pin having a spherical end fitting into the
cylindrical socket projecting downwards from the underside of the
ashpit. The motion of the furnace should be very slow, so as to give
about one revolution of the crucible in five or ten minutes, and thus
allow a constant movement of the whole of the material to take place
without dividing or breaking the continuity of the mass, preventing any
subsidence of the heavier particles, and securing the perfect
homogeneity of the whole. When the fusion and mixing is judged to be
complete, the crucible can be pushed with a rod into an upright
position, and, by drawing the fire, cooled as rapidly as possible by
the current of air flowing through the furnace.
Homogenous optical glass, free from those long "wreaths" or lines of
varying density, so common in ordinary glass, was also proposed to be
made in the following manner. A large potful of glass of the required
composition must be allowed to get cold, and then broken up, the
central portions only being selected for use. These pieces are to be
crushed, all the glass being reduced to absolute powder, and separated
by sifting; all pieces exceeding the size of a grain of rice should be
rejected. The very small and nearly equally-sized fragments that remain
are then to be carefully washed in distilled water and put into a
lenticular-shaped crucible, the exterior surface of which should be
glazed, so as to render it impervious and air-tight. The crucible
having been put into a suitable furnace and gradually heated, a small
platinum pipe communicating with the upper part of the crucible is also
connected with an exhaust pump, so as to remove every particle of air
from the crucible and from between
the granules of glass while these still retain their granular
condition. As soon as the glass becomes fluid, it forms a homogenous
mass, the law of diffusion equalising any minute differences in
composition of continuous grains, while wholly avoiding those long
"wreaths" or streaks so fatal in large masses of glass. On the strength
of these crude notions a number of various-shaped clay crucibles were
ordered to be made, with a view to carry on an elaborate series of
experiments on the lines indicated; but as these crucibles required at
least three months to dry, I had ample time to pursue some other
interesting investigations relative to the production of glass for
ordinary commercial purposes.
In going over a glass-works some years previously, I had noticed what
I, at the moment, thought was a great oversight in the mode of
proceeding. The materials employed, viz., sand, lime, and soda in
ascertained quantities, were laid in heaps upon the paved floor of the
glasshouse, and a labourer proceeded to shovel them into one large
heap, turning over the powdered materials, and mixing them together; a
certain quantity of oxide of manganese was added during the general
mixing operation, for the purpose of neutralising the green colour
given to glass by the small amount of oxide of iron contained in the
sand. The materials were then thrown into the large glass pots, which
were already red-hot inside the furnace. What appeared to me to be
wanting in this rough-and-ready operation was a far more intimate
blending of these dry materials. A grain of sand lying by itself is
infusible at the highest temperature attainable in a glass pot, and the
same may be said of a small lump of lime; but both are soluble in
alkali, if it be within their reach. These dry powders do not make
excursions in a glass pot and look about for each other, and if they
lie separated the time required for the whole to pass into a state of
solution will greatly depend on their mutual contact. In such matters I
always reason by analogy, and look for confirmation of my views to
other manufactures or processes with which I may happen to have become
more or less acquainted. I may here remark that I have always adopted a
different reading of the old proverb "A little knowledge is a
dangerous thing"; this may indeed be true, if your knowledge is equally
small on all subjects; but I have found a little knowledge on a great
many different things of infinite service to me. From my early youth I had a
strong desire to know something of any and all the varied manufactures
to which I have been able to gain access, and I have always felt a sort
of annoyance whenever any subject connected with manufacture was
mooted of which I knew absolutely nothing. The result of this feeling,
acting for a great many years on a powerful memory, has been that I
have really come to know this dangerous little of a very great many
industrial processes.
I have been led into this long digression because I meant to
illustrate my observations on the extreme slowness of the fusion of
glass by an analogy in the manufacture of gunpowder. I have shown how
impossible it is for the dry powdered materials employed in the
formation of glass to chemically react upon each other when they are
lying far apart. Now, if we take the three substances -- charcoal,
nitre, and sulphur, of which gunpowder is composed, and break them into
small fragments, then shake them loosely together, and put a pound or
two of this mixture on a stone floor and apply a match, the nitre will
fizzle a little briskly; the sulphur will burn fitfully or go out, and
the charcoal will last several minutes before it is consumed. If,
instead of this crude and imperfect mixture, we take the trouble to
grind these matters under edge-stones into a fine paste with water, and
then dry and granulate it, we have still the precise chemical elements
to deal with as we set fire to on the stone floor; but they now exist
in such close and intimate contact as to instantly act upon each other,
and a ton or two of these otherwise slow-burning materials will be
converted into gas in a fraction of a second. The inference I drew from
this analogy was simple enough, viz.: grind together the materials
required to form glass, and when the heat of the furnace arrives at the
point at which decomposition takes place, the whole will pass into the
state of fluid glass much more quickly, and will yield a more truly
homogenous glass than is obtained in the usual manner.
I was at this time engaged in constructing a large reverberatory
furnace for the fusion of glass on the open hearth, and I may forestall
what I have to say respecting this mode of founding glass, by stating
that when I employed a mixture of raw material merely shovelled into
the bath as practised in ordinary glass-making, it took from ten to
twelve hours to fuse half a ton of sand and lime in my new furnace; but
when I took precisely the same quantity and quality of materials which
had been reduced to a uniform powder, as fine as flour, by grinding the
mixed materials under edge stones, my glass, instead of requiring ten
or twelve hours for fusion, became beautifully fluid in four and a-half
to five hours. When I first tried this fine ground material in my
furnace, I patiently watched the whole process hour after hour; the
inert mass of dry white powder lay quietly under the rushing current of
flame passing over it, without showing any symptom of fusion. At last I
sought relief for my over-fatigued eyes by half an hour's turn up and
down the yard; and on my return into the glass-house, I was astonished
to hear a curious sound issuing from the furnace, closely resembling
the noise given out by a frying-pan when cooking fish; on the
application of my eye to the peep-hole of the furnace, I saw that the
level of the glass had risen an inch or two, and that a rapid boiling
was going on, caused by the disengagement of gas resulting from the
rapid reaction of soda on the silicic acid. I scarcely need say how
greatly I was pleased at witnessing in a first experiment so important
a result, and so distinct an example of the value of a little of this
so-called "dangerous knowledge."
Up to this period the fusion of glass in large crucibles was universal,
and the reverberatory furnace which I had erected at Baxter House for
this purpose was the first in which glass was made on an open hearth,
and the parent of all those bottle furnaces in which the fusion of
glass is carried on in open tanks. It was here also that the hollow box
roof was first used in reverberatory furnaces -- a form of roof
afterwards employed by me so economically in the reverberatory melting
furnaces used in the early days of the Bessemer steel manufacture. The
immense economy, in time, consumption of fuel, and cost of large
melting-pots, resulting from the fusion of glass on the open hearth of a
reverberatory furnace was accompanied by one great disadvantage, viz .,
the tendency of molten matter to fall from the roof of the furnace into
the bath, and thus spoil the glass. It was found that whenever the
underside of the furnace roof was exposed to an excessively high
temperature, the alkaline vapours from the bath beneath caused a fusion of the brickwork,
and tears, with their long tails, would fall slowly from above and
discolour the glass in the bath beneath. It was mainly to counteract
this injurious action that I invented the thin box roof which entirely
cured this defect, while the durability of the furnace arch was at
least four to one as compared to the ordinary solid form. How well I
still remember the trouble and anxiety these tears from the roof caused
me, and how I watched through the eye-holes of the furnace the effects
of the alkaline vapours on the hollow box roof when it was first under
trial. I looked ceaselessly into the fierce glare of the furnace, with
but a piece of thick glass between my eye and the bright molten mass,
only eighteen or twenty inches distant. When watching by the hour at a
time to see if a single tear was formed on the roof, the eye
accommodated itself to the intense light, and all within that glorious
mass of incandescent matter could be seen in its minutest details. I
remember one peculiar circumstance that stood out from all the rest;
while one of the hollow firebricks of the roof was in a condition of
plastic clay, the brickmaker had taken hold of it, and a hollow caused
by his thumb was beautifully delineated on the underside of this
particular brick; it happened to be opposite the eye-hole, and was an
excellent mark whereby any change in the state of the roof could, from
time to time, be observed. This must have been as far back as 1847, but
that thumb-mark is as indelibly impressed on my memory as it was on the
plastic clay. How many hours in succession I have watched that mark
through the fierce heat and blinding light of the incandescent furnace
I cannot now take upon myself to say, but my whole heart and mind were
so absorbed in the investigation that I never gave a thought to the
fearful risk I ran of destroying my sight. Now, when I recall these
facts vividly to memory, I can realise the folly I was guilty of, and
can, in all humility, thank Heaven that I am not at this moment a blind
old man.
It is now just forty-nine years since I succeeded in fusing the
materials used in the manufacture of glass on the open hearth of a
reverberatory furnace, in about one-third the time and with one-third
the fuel required for its fusion in the large and expensive glass pots
then in use. But there was still one great desideratum: the glass fused
in pots was usually blown into long round-ended cylinders or muffs, the
ends of which had to be opened while the glass was still hot and
plastic -- an operation requiring great skill and dexterity on the part
of the glass-blower. These open-ended cylinders, when cold, were slit
from end to end by a diamond, and again heated until sufficiently soft
to be spread out flat on the smooth stone bed of a furnace specially
constructed for that purpose; after which they had to be ground and
polished, if made into what is known as patent plate.
Now, what I proposed to do, instead of this slow, laborious, and
expensive series of operations, was simply to allow the semi-fluid
molten glass to escape by an opening extending along the whole length
of the bath, and about 1 1/2 in. in width, and to flow gently between a
pair of cold iron rollers, so as to determine its breadth and thickness
at a single operation. I aimed at converting the whole contents of the
furnace into one continuous sheet of glass in ten or fifteen minutes,
wholly without skilled manipulation of any kind, or the employment of
the other furnaces, which are necessary for opening and spreading the
blown cylinders before referred to. It will be obvious that the
continuous sheet as it passed from the rolls might be cut into any
desired lengths, and thus very much larger sheet glass could be made
than it was possible to obtain by blowing it into cylinders.
Having thus foreshadowed the design I had in view, I will briefly
explain the nature of the apparatus which I erected at Baxter House to
test the practicability of the scheme; and for this purpose I give the
engravings, Figs. 25 and 26, Plate X, by way of illustration.
Fig. 25
is a cross-section taken through the centre of the bath of the
reverberatory furnace, looking towards the fire-bridge A, over which
the flame passes. This flame is deflected downward on to the molten
glass B, occupying the hearth of the furnace C, which is a sort of
rectangular tank, having all along one side a slot or opening D,
against which an iron bar E is fixed, so as to close the slot and
prevent the escape of the semi-fluid glass. The arched roof, F, of the
furnace is formed of hollow boxes of firebrick, each box having a
round opening in each of four of its sides, while the upper side is
quite open and the lower one closed, and forms the underside of the
furnace roof.
In the front of the furnace is a cast-iron door frame D, lined with
firebrick. It extends the whole length of the tank in which the glass
is melted, and it is removed from its position when necessary, by
slings from a jib-crane attached to hooks H, which project from each
end of the frame. The flame, after passing over the glass materials in
the bath, travels downwards to an underground flue connected to a tall
chimney-shaft. There is also a narrow passage I, running downward into
the same underground flue, and extending upwards, as shown at I*, so
as to admit a current of flame, as indicated by arrows in front of the
opening D, and round the curved underlip C* of the cistern C, in order
to keep all the front part of the cistern in a highly-heated state.
In front of the furnace a rolling machine is fitted to a suitable
slide, so that it may be removed from the furnace a short distance, as
shown in Fig. 25, which is a side elevation of the apparatus. It
consists of a pair of smooth cast-iron hollow rollers M and N, into
which a current of cold water is allowed to flow through the pipes P
and Q, and from which the water escapes by similar pipes at the other
end of the rollers. A telescopic pipe R slides in and out of a fixed
pipe S, and thus keeps up an uninterrupted communication with the
water supply. The rollers are brought nearer together, or further
apart, by means of screws T in the usual manner, and thus regulate the
thickness of the sheet of glass.
Fig. 26 represents in section the furnace, from which the large
fire-door has been removed; the rolling machine has also been moved
along its slide, until its lower roller N is in almost close contact
with the lip C* of the melting cistern; this movement is effected by
turning the handle U, which actuates the wheel V, and the rack W, and
moves the whole rolling apparatus into position. When this has been
done, the rollers are thrown into gear with a shaft, not shown in the
drawings, and are caused to revolve at the desired speed. As soon as
the machine has been thrown into gear the iron bar E is withdrawn, when
a slowly-moving, white-hot, semi-fluid mass creeps out of the long
slot, and coming into contact with the lower revolving roll N, is moved
by it into the space between the rolls, and is compressed into a thin
continuous sheet from an eighth to a quarter of an inch in thickness,
as desired; a projecting V-shaped rib on the upper roller M, will cut the
glass into lengths equal to its circumference. The sheet of glass thus
severed from the general mass will rapidly slide down the smooth curved
surface X of the machine, and deposit itself on the flat stone bed at
the foot of the incline, from which it may be transferred into a
suitable annealing oven. It will be understood that, as soon as the
bath is empty, the rolling machine will be run back on its slide to the
position shown in Fig. 25. The bar E and the door G will then be
replaced, the bath charged with a fresh supply of raw material, and the
process be repeated as soon as the glass is in the proper condition.
From this general description of the process, and the simple mechanism
employed, it will be seen that a large quantity of glass could be
produced with a very small plant. Thus, suppose that the glass
materials are melted in five hours and that the time of casting is,
say, fifteen minutes, a cast would easily be made every six hours, or
four times per day. A bath only 4 ft. by 3 ft. in area, and 12 in.
deep, when making strong horticultural glass 1/10th in. thick, would
yield theoretically 5,760ft. per day (say 5,000), equal to, at least,
400 blown cylinders 4 ft. long by 1 ft. in diameter.
I was quietly pursuing my experiments with the apparatus described,
when I was unexpectedly called upon by an eminent glass-manufacturer.
He said that he had heard that I was doing something novel in the
production of sheet glass, and if my patent was secured, he should
much like to know what was the nature of the invention. I told him my
patent was secure, and that I should be happy to give him a general
outline of the scheme. He was greatly interested, and the shadows of
the evening had imperceptibly fallen upon us in my little private room
before my visitor rose to depart. He was very desirous to see the
experimental apparatus; and knowing that my guest, Mr. Lucas Chance,
was at the head of the largest glass-works in the kingdom, and worthy
of all confidence, I acquiesced in his strongly-expressed desire, and
said if he would call again the next day at noon, I would have a charge
of glass ready to roll into sheet in his presence.
The following morning, all was got in readiness for a cast. Mr. Chance
critically examined the rolling apparatus, and looked into the furnace
from time to time, just as a man would who thoroughly knew what he was
about; and when I said we had better now get to work, there were
myself, Mr. W. D. Allen, my eldest son Henry, a carpenter, and my
engine-driver present in the small room in which the furnace and
machine had been erected. As soon as the bar retaining the charge was
removed, and the tenacious semi-fluid glass touched the lower roll, the
thick round edge of the slowly-moving mass became engaged in the
narrow space, where the second roll took hold of it, and the bright
continuous sheet descended the inclined surface, darkening as it cooled
slightly. I had intentionally omitted the cutter in the roll so as to
make a continuous sheet; this had to be pulled away, for my little room
was not half long enough to accommodate it. The heat suddenly thrown
off from so large a white-hot surface threatened our garments if we
stood too near, and unfortunately some oily cotton waste took fire,
causing a momentary panic. Mr. Chance called out, "Cease the operation;
cease the operation!" We were all in a perspiration, and the long
adhesive sheet of glass, 70 ft. long by 2 1/2 ft. wide, was gathered up
before the door. The heat was very great, and throwing the rolls out
of gear, we all beat a hasty retreat. However, as far as the rapid
formation of thin sheet-glass was concerned, there could be no doubt
whatever, and I and my visitor sat down quietly to cool ourselves, and
think over what had taken place. Notwithstanding the mistake of not
putting in the cutter, and making the glass into small sheets, I had
the satisfaction of knowing that I had just made a sheet of glass more
than three times the length of the longest piece that had ever been
produced, and that Mr. Chance had seen, for the first time in his life,
a continuous sheet of glass flowing from a machine, wholly without any
skilled manipulation. "Well," said Mr. Chance, "you have gone a good
way, but you have much further to go yet before you touch the real
point -- the commercial point. Now it has struck me that we have so
many appliances, and so many skilled employes in all departments, that
perfecting such a novel process must be more easy and less expensive to
us than it can be to you. After all, should it not become so perfect
as to be a commercial success, or should some other way be found of
effecting the same result, you might have all your labour in vain; but
I freely admit that you have done enough to constitute an actual value as the
invention now stands. Just think it over, and determine whether you
will sell me your invention as it stands and make at once a profit on
what you have done, or whether you will spend more labour and money
with the chance of much greater remuneration, if you succeed
commercially, and no one else supersedes you? I am going down to
Birmingham this evening by the 9 P.M. train. Dine with me at seven
o'clock at the Euston Hotel, and tell me, yes or no, whether you are
disposed to sell your invention in its present state." With this he
took his leave, and I had still three hours to reflect over a most
unlooked-for proposition. It was very exciting, and I talked the matter
over with Mrs. Bessemer, and the general consensus of opinion was:
"Realise, by all means, if you can get an adequate amount, but don't
give it away." This decision, however, was a long way off a positive
fixed sum, to which a "yes" or "no" was to be uttered by both sides.
Time slipped on, and when the clock struck seven I found myself in a
snug private room at the Euston Hotel. We had a nice little dinner,
and, when the table was cleared, my host said: "Well, have you
decided?" I said that I had thought the matter over, and felt that any
sum must now be a sacrifice, but that I was prepared to sell on one
condition, viz., there was to be no discussion of price. I would name
what I had fixed on; would he give me a simple yes or no? To this he
agreed, and then I said, "The sum I have fixed on is six thousand
pounds." "Well," said Mr. Chance, "I will give you that sum for your
patent; I shall not go down to Birmingham to-night, and tomorrow at 11
A.M., if you will call at Messrs. Hooper and Co.'s, my solicitors, in
Sackville Street, we can settle the whole matter there and then." We
met as arranged, a short agreement was drawn up, Mr. Chance handed me a
cheque for £1,000 with a short-dated bill for £5,000, and we parted
very good friends, mutually pleased with our bargain.
As a fitting tribute to the memory of Mr. Chance's able solicitor, Mr.
Hooper, I may mention that I found him so shrewd and careful of his
client, and so just withal, that I from that day gave his firm all my
legal business as far as patents were concerned.
At this period I was deeply interested, from a scientific point of
view, in the plate-glass manufacture. I was, and ever shall be, a
great admirer of plate glass, which I hold to be one of the most
beautiful and most marvellous productions of all our varied
manufactures; and I must confess that, at the present day, I am
disgusted with that idiotic fashion which rejects this splendid
production for the small lead panes of a greenish bubbly glass, which,
with difficulty is now made bad enough to imitate the early and most
imperfect state of the glass manufacture; and which the bad taste -- or
rather the absence of taste of the present generation admires and
"tries to live up to."
It need not, therefore, be a matter of surprise that I felt a strong
desire to cheapen and facilitate the production of plate glass, a
manufacture which, in my enthusiasm, I attacked at all points,
beginning with the preparation, sorting, cleansing and blending of the
raw materials employed, followed by the novel device of a circular
reverberatory furnace, in which the founding pots were arranged in a
large circular chamber surmounted by a flatly-curved dome. There were
also similar furnaces designed for refining the glass, having a crane
revolving with the reverberatory dome of the furnace. The crane was, in
fact, a veritable automaton, that would remove the one small cover,
which, as the dome revolved, gave access in turn to a dozen large glass
pots placed in a circle. The three arms or grips of the crane descended
vertically into the furnace, and brought up the huge crucible, and when
emptied, replaced it in three or four minutes, within half an inch of
the exact spot whence it had been lifted. The casting table and all the
annealing ovens were arranged in a circle, all accessible from a
circular railway laid down in the great casting hall. I may also
mention that every detail of the grinding and polishing machinery had
undergone an entire change, rendering these operations more rapid and
more accurate.
I feel, however, that I dare not trouble my readers by entering into
further details. Suffice it to say that a revolution in the appliances
and mode of working pervaded the whole manufacture, to properly
describe which would fill an illustrated volume. Various portions of
the scheme were practically tested; I built a circular furnace for six
large pots and erected the automatic crane before referred to, which,
like a living thing, dived for a minute or two into the raging heat,
and brought forth
noiselessly the pot of molten glass (as easily as the human hand could
take a tumbler of water off the table), and returned it empty to the
same spot.
I was well satisfied with the whole scheme, and wished a few friends to
join me in the erection of a plate-glass works in London. My partner,
Mr. Robert Longsdon, with his usual architectural skill and good taste,
designed the necessary buildings for a complete works, embodying all
the novel modes of conducting each department of the manufacture. On
Plate XI., in Figs. 27 and 28, illustrations, showing an elevation and
section of his design are given, just to save the whole project from
oblivion. I have, at this moment, no sort of doubt that, had I
convinced others of one-fourth of the improvements embodied in this new
scheme, there would have been no difficulty in finding privately a few
friends who would have joined their capital to mine, and the works
would have been started. But it is difficult to impress one's ideas on
others, and I desired to have the personal and entire conviction of its
value on the part of all those I asked to join me in the enterprise;
without this I was resolved not to move further in the matter, and
failing to obtain it, the whole scheme was abandoned.
There is one point in connection with patented inventions upon which I
have always felt strongly. I have maintained that the public derive a
great advantage by useful inventions being patented, because the
invention so secured is valuable property, and the owner is
necessarily desirous of turning that property to the greatest
advantage; he either himself manufactures the patented article, or he
grants licenses to others to do so. In either ease the public reap the
advantage of being able to purchase a better or a cheaper article than
was before known to them, due to the inventor's perseverance in forcing
his property upon the market. But if a novel article or manufacture is
simply proposed by a writer, and published in the technical press or in
newspapers, as a rule (almost without a single exception) no
manufacturer will go to the trouble and expense of trying to work out
the proposed invention. He says to himself: "I shall not risk the
expense necessary to develop this new idea, for it may entirely fail;
or even if I succeed, its development will cost me much more than it
will cost other manufacturers, who
will immediately avail themselves of it if I succeed; no, let some one
else try it;" and so the invention is lost to the world in consequence
of having been given away. This loss to the public is equally the case
with patents that are not taken up; and one of the simplest and most
effective inventions which I ever made may be here cited as an example,
as it formed part of the novel system of plate-glass manufacture just
referred to. When a sheet of plate glass some 10 ft. or 12 ft. long and
6 ft. or 7 ft. wide has been ground perfectly flat on both sides, it
is still dull and grey, and has to be polished. For this purpose it is
usual to fix it firmly on to a large stone polishing table, so that the
powerful alternate pushing and pulling of the polishing rubbers over
its surface may not break or displace the sheet. To do this a large
quantity of plaster-of-Paris is mixed with water, and spread as quickly
as possible over the surface of a stone table much larger than a
billiard table. Then the sheet of glass is dexterously laid upon the
semi-fluid plaster, and carefully bedded by expert workmen so as to be
well supported at all parts of its extensive surface; the superfluous
plaster lying beyond the edges of the plate of glass is then scraped
away. The polishing machine must remain idle until the plaster is
sufficiently firm and hard to retain the glass safely in place. Care
must also be taken to thoroughly remove all smears of plaster around
the edges and any splashes on the surface, for the plaster is always
more or less gritty, and one or two particles of sharp grit will play
havoc with the polished surface, scratching it terribly. Let us suppose
that one side of the great glass sheet has been polished. It is then
necessary to unbed it, and this requires much skill. A man at each
corner inserts a thin blade of steel and gently prises the sheet up; he
must not spring it much, or the corner will snap off, and considerably
diminish the size of the sheet when squared up. With much risk and
trouble, the plate of glass is eventually released, and lifted off the
stone bed; then the workmen proceed to chip off the hard plaster which
firmly adheres to the table. This makes a great mess all round the
polishing machine by the flying about of chips of plaster. The stone
table having been chipped all over, and scraped quite clean, a fresh
lot of plaster is again mixed up, dexterously spread, and the sheet of
glass, with its unpolished
surface uppermost, is again bedded on the table, and all superfluous
surrounding plaster carefully cleared away. Again the powerful
polishing machine remains inactive, until the sheet of glass is firmly
stuck to the bed, and, after polishing, the same dangerous process of
springing the glass loose from the table has to be repeated. After its
removal, the bed has to be chipped all over, and the hard coating of
the plaster-of-paris removed, for the reception of another plate.
Such is the laborious, dirty, and risky process to which every sheet of
plate glass is subjected in the ordinary course of its manufacture. Now
let us see what was the simple mode which I patented of holding down a
sheet of plate glass securely during the polishing process. I employed
(see Figs. 29 to 32, page 120) a cast-iron ribbed plate of the size of
the polishing stone table, on the upper side of which a large slab of
slate (such as is used for billiard tables) was supported and bedded on
the ribs of the iron plate. This surface was then ground flat, in the
same manner as plate glass, the space beneath the slate and between the
iron ribs forming a shallow box. A number of round holes of about a
quarter of an inch in diameter were made through the slab of slate all
over its surface, at a distance of 4 in. or 5 in. apart, so that air
could enter the iron box freely from all parts of its surface; a pipe
of 1 in. in diameter led to a steam jet or other exhauster, so that air
could be withdrawn from the box or let into it by a small hand-tap, as
desired. This, then, was the whole apparatus which constituted the
invention; it was extremely inexpensive, and once made, almost
indestructible.
This device took the place of the stone table under the ordinary
polishing machine. Its operation may be described as follows:-- The
sheet of glass to be polished is gently slid upon the table, and covers
all the small holes in the slate bed; and if then the small tap which
connects the underside of the slate table with the steam jet or other
exhaust apparatus is turned on, a partial vacuum is formed beneath the
sheet of glass, and it becomes in an instant immovably fixed and
adherent to the slate bed on which it rests. There is no plaster
employed, and consequently none of the mess or labour attending its
mixture and chipping off; there is no delay in the use of the polishing
machine while the plaster
is becoming hard, or when it is being cleared away. The plate of glass
is an absolute fixture in less than a quarter of a minute after the
exhaust is turned on; and it is as rapidly released by reversing the
tap and readmitting the air to the box. The plate is then turned over,
and the tap being opened to the exhaust, it instantly becomes re-fixed
to the slate surface; there is no cost of plaster, there is no labour,
and no risk of snapping off the corner of the plate to release it. Take
a moderate sized plate of 6 ft. by 10 ft. (60 square feet), and take a
very low exhaust, say, 2 lb. per square inch, equal to 288 lb. per foot
or 17,280 lb. of atmospheric pressure, holding it immovably fixed.
Every schoolboy who has seen how powerfully the glass bell of an
air-pump is held in place by atmospheric pressure, must understand this
simple, effective, cleanly, inexpensive, safe and rapid way of holding
down a plate of glass.
This invention, which formed one item of the many improvements in the
plate-glass manufacture which I did not carry out, has been available
for the free and unrestricted use of the public for nearly fifty years,
and yet no plate-glass works in this, or any other, country has taken
advantage of it. The simple fact is that an invention must be nursed
and tended as a mother nurses her baby, or it inevitably perishes. Nor
is this almost incredible indifference to their interest the result of
the invention being unknown to the public; for I exhibited a polishing
table so constructed, among many other things, in the International
Exhibition of 1851, where it became one of the most attractive of my
exhibits. I well remember that on one occasion I was requested to be
present two hours prior to the opening of the Exhibition to the public,
and had the honour of showing and explaining the device to Her Most
Gracious Majesty, who was on that occasion accompanied by His Royal
Highness the Prince of Wales and other distinguished persons.
The slate table was about 4 ft. by 3 ft., and I used a plate of
polished glass a little less than a square yard, weighing about 36 lb.
The firm way in which it was held was most easily demonstrated by
placing one side of the plate about four or five inches on the edge of
the slate bed, and allowing the remainder to project. Not only did
the atmospheric pressure sustain the plate overhanging in this way, but
no one could lift it up or force it down. I was also able to illustrate
a fact but little known, viz., that a plate of perfectly flat ground
glass lying in absolute contact with a true plane surface cannot be
smashed by a blow from a wooden mallet with a curved face. I have
struck my yard-square sheet of glass at the Exhibition, when held down
by atmospheric pressure, dozens of times before the public, as hard as
I could strike it with a wooden mallet, and never broke a single sheet
in doing so. If it lay hollow and not in absolute contact with the
table, a child could fracture it in a dozen places, but when in contact
all over its surface, no amount of force less than that at which glass
crushes to powder will crack a properly supported sheet. From what I
have said I think I have shown that, however self-evident an invention
may be, or however advantageous it might be to a manufacturer, if it is
public property he will not touch it.
I have already so far trespassed on the patience of my readers in
reference to the manufacture of glass that I must bring these remarks
to a close. But there is just one little point that I may be excused
for mentioning; it has reference to the silvering of glass, which
everyone knows was effected by the amalgamation with mercury of a large
sheet of thin tinfoil, the amalgam adhering to the surface and
remaining on the side next the glass, a beautifully-polished and
highly-reflecting surface. But it had a bluish or leaden hue that was
most unfavourable to the fair sex, and spoiled the best complexion. I
thought much over this defect, and at last succeeded in greatly
improving the whiteness of the reflection. This I effected by the use
of pure silver powder. The sheet of tinfoil was employed as before,
and amalgamated with mercury, the greater part of which was drained off
the surface of the foil, and then pure silver in the form of an
impalpable powder (known as silver-bronze powder), was freely dusted
all over the amalgamated surface. The fine silver particles became
rapidly amalgamated or dissolved by the mercury, and when the sheet of
glass was slid on and pressure applied, an amalgam of pure silver
coated the glass, greatly improving the brilliancy and colour of the
mirror. This method seemed likely to have a great future, but before it
got into use, a process suggested by Liebig some years
before was developed and applied in a practical form by Professor Henry
Draper. By this method of working, which he used for the silvering of
glass mirrors for reflecting telescopes, Professor Draper entirely
dispensed with the tinfoil and mercury process, and deposited pure
silver direct on to the glass from its solution. This was a far more
perfect mode than my own of putting pure silver on to the glass, and
quite put an end to my process. At the present day all glass mirrors
are silvered by one or the other of several modified forms of Leibig's
admirable invention.
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