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difference in the expense of the resulting plates of glass.

36. When the gauge is properly placed on the platinum, the
sides are raised perpendicularly : this produces four projecting
folded triangular corners, which being pressed close, are then
turned against the sides, and a square tray is finished, which
has no aperture or orifice below its upper edge. The folding
of these corners is a matter of much more consequence than
might be anticipated. The plate is seldom so regular that the
parts of two neighbouring sides which come together at a corner
are exactly of equal height ; neither is it desirable that it should
be so, and the unsymmetrical position of the gauge to the plate,
already recommended (35), is almost sure to prevent it. In
that case, of the two sides of the folded corner, one will be
higher than the other, and if the corner be so folded that its
lower side is towards the tray and beneath its edge, a kind of
siphon is formed which becomes charged with fluid by capillary
action, and continues to discharge glass from the tray during
the whole time of heating, notwithstanding that all the edges
are much above the level of the fluid within. This in a long
experiment is competent to occasion serious injury.

37. I have found, even when the edges of a corner have
been of equal height, but below the edge of the side against
which they are disposed, that still this capillary and siphon
action has gone on, and the reason is not difficult to com-
prehend ; the corners therefore have always been folded in
such a manner, that their highest edge has been inwards, and
both their edges above the level of the corresponding edge of
the tray. To effect this, the line of their lateral flexure is not




252 On the Manufacture of Optical Glass. [1829.

perpendicular to the bottom of the tray, but a little outwards
above, and the proper degree of inclination is easily given by
using a mould upon which to bend the corners. This should
be a thick square piece of wood having the four corners cut
with different degrees of obliquity : when the corners of the
tray are first imperfectly formed, it will be easy to ascertain by
trial, which corner of this mould
will give the obliquity and position
already described as necessary, ,
after which the folding may be
easily finished upon it. The ac-
companying sketch represents first
a good and then a bad folding.

38. All occasion for changes in the folds, especially at the
corners, should be avoided. The folds should be decided
upon as the work proceeds, so advantageously as to make
alterations unnecessary. The closer the corners are pressed,
the smaller is the quantity of glass contained in them, and the
less risk is there of the platinum being broken when the finished
glass is taken out ; but it is proper to avoid general contact
betw r een the corners and the sides against which they are dis-
posed, otherwise welding is likely to occur during the stirring,
and the platinum is injured for future experiments.

39. The tray being formed is again to be examined for holes,
first by a light as before (31), and then in the following
manner : Being laid upon a sheet of bibulous paper, alcohol
is to be carefully poured in until the fluid is within the fourth
or the sixth of an inch of the lowest edge of the tray, so as to
occasion no running over at the sides or corners. If a large
hole exist, it will be rendered visible immediately ; but if none
such appear, a large basin or some other cover is to be placed
over the tray to prevent evaporation, but without touching the
vessel or its contents ; and the whole is to remain undisturbed
for some hours. Being then examined, the wetting of the
paper will indicate a hole or a badly-folded corner, and will
point out the faulty place : the tray may easily be shifted from
one part of the paper to another for the discovery of any
moistened places beneath. Sometimes holes occur so small
that alcohol will not run in a sensible quantity through them.
Suspected places of this kind and suspicious corners also should



1 829.] On the Manufacture of Optical Glass. 253

be examined by a clean dry point of bibulous paper, wbich
soon shows* by its change of appearance, the transmission of
any fluid : but attention is required that no false indication be
produced by carelessly bringing the paper near the upper
edges of the platinum, especially in the folded places. These
minute holes do not occasion much harm in the furnace, but
no fault should be allowed to pass which care can correct.

40. When the tray is faulty, the alcohol must be removed
by a small siphon, the holes soldered in the manner before
described (32), and the tray again tried. When it proves
good, it is, after the removal of the alcohol, to be heated red-
hot in every part by the flame of a large spirit-lamp, and then
reserved with care in a clean place until required.

4-1. If the platinum has been used before, it should first be
ascertained that none of the glass from the former experiment
remain on it. If there be any portion, the plate must be
returned to the weak acid or pickle out of which it has been
taken. If free from glass, it should then be examined as to
any chemical injury it may have suffered. Any part which is
altered in appearance, or has been attacked by the acid, or
which tarnishes when heated to redness by the spirit-lamp, has
been thus affected ; and it will depend upon the extent of the
action whether the plate is unfit for further use. No chemical
injury is occasioned by the proper and successful performance
of an experiment.

4.2. An examination for holes by the candle or lamp must
next be made, especially in the folds at the corners and where
adhesion of the platinum from welding may have occurred, and
any that are discovered are to be marked as before (31). The
plate should then be flattened by being put between two sheets
of writing-paper upon a smooth table, and the edge of a folding
knife or some other smooth substance drawn over it ; but if
this be done whilst old glass adheres to the plate, it is almost
certain to produce injury. The holes are then to be soldered
and mended, the patches being applied upon the same side as
before. The gauge for the new tray is to be applied to the
plate, shifted, if there be occasion, from its old position, as
before intimated (35), and the folding of the tray, its comple-
tion and examination, to take place as before.

43. It is desirable never to cut the platinum smaller than can



254 On the Manufacture of Optical Glass. [1829.

be helped, but always to make the largest plate upon it for
which it is competent. Then, when operated with a second or
third time, smaller gauges may be used, and the folds will not
be repeated in the same place; and if injury occurs to the
metal, being generally at the sides of the tray, the middle part
will still be left for the preparation of smaller plates of glass.

If such large plates of platinum are required for trays as can
hardly be rolled at once, there is no difficulty in making a
folded joint and rendering it tight by soldering with gold.

44. A kind of furnace, unlike the former, is now required
for the completion of the glass, and its delivery in the state of
an annealed plate. This furnace shall be described accurately
in the Appendix. It may here be sufficient to state that it
consists of a fire-place in which coals are burnt; of a part
beyond, acting both as furnace and flue, in which coke is
used; and of a chamber above, to be heated by the fire,
though out of the course of both flame and smoke. It is in
this chamber that the glass is made ; so that, by the arrange-
ment adopted, at the same time the substances are fused and
access for stirring allowed, the essential condition of excluding
impurity or reducing matter is also fulfilled.

45. The fire-place itself is of the ordinary construction, and
fed with fuel by an aperture in front in the usual way. I have
found abundant reason to be satisfied that the passage of steam
beneath the bars of the grate is of considerable use ; for which
reason an iron trough charged with water occupies the lower
part of the ash-pit. The bars are by this arrangement pre-
served very cool and do not burn away ; they are easily kept
open and clear of clinkers ; the free passage of air to the fire
is permitted ; and the action of the furnace retained at a high
point for any number of hours together.

46. That part of the furnace beneath the chamber requires
peculiar and careful arrangement ; for at the same time that
such a heat as will soften the neighbouring materials is pro-
duced there, the bottom of the chamber in its softened state
and charged with several pounds of materials, has to be firmly
supported for many hours together without change of position.

47. The coke necessary in this part is introduced by two or
more holes in the side of the furnace, which, when necessary,
are stopped by bricks. The bottom of the chamber is sup-



1829.] On the Manufacture of Optical Glass. 255

ported on ledges at the sides, and upon the ends of fire-bricks
in the middle, firmly placed at intervals so as neither to stop
the passage for smoke and flame, nor the cross passages for
the introduction of coke.

48. The value of the coke arrangement in this as in the other
furnace is very great. The heat obtained by the united action
of the coke and the flame from the fire-place, is abundantly
sufficient, and, whilst obtained at the necessary point, does
not involve that degree of mechanical action required for
stoking and stirring, which is necessary with coals, and would
risk the destruction of the soft thin bottom of the glass
chamber. It further occasioned the perfect combustion of
the smoke produced in the coal fire, which at first was so
considerable in quantity that, had it continued unaltered, the
experiments must have been removed from the Royal Institu-
tion ; in which case they would probably have been discon-
tinued altogether.

The flue is the same as that connected with the former
furnace, and has a damper for regulating the heat, especially
useful during the annealing operation.

49. The chamber was at first of cast iron, that material
being selected as one which would bear a sufficient tempe-
rature without melting, would conduct and transmit the heat
freely to the substances within, and could be easily obtained
of the requisite form. The upper aperture was closed by
plate-iron covers, and in the first trials all appeared to answer
well; but when large experiments were made, and the heat
was continued for a long time, the bottom gave way and
became irregular : and upon endeavouring to rectify this, and
place the tray of glass level by means of sand, the transmission
of heat to the glass was prevented, the temperature of the iron
rose, and the bottom melted. Besides these injurious liabilities,
if the smallest portion of glass passed out of the tray, the
moment it touched the iron it was reduced, the lead im-
mediately caused fusion of the platinum, and in an instant the
tray was destroyed, the experiment stopped, the glass rendered
black and useless, and the bottom of the chamber covered with
lead and rendered unfit for another operation.

50. Finally, one very curious action of the iron was dis-
covered, which immediately caused its rejection, Plates of



256 On the Manufacture of Optical Glass. [1829.

glass, which seemed very good in other respects, were fre-
quently so discoloured by dark smoky clouds as to be useless.
These could not be referred to any impurity which had been
left in the materials or had entered accidentally, and, as the
platinum was in all such cases altered and injured, 'was at first
supposed to be occasioned by some particular action exerted
between it and glass at high temperatures. But upon every
fair trial to verify such chemical action, the proofs failed, how-
ever high the temperature used, or however minutely the
metal was divided. At last the cause was discovered. To
understand it, it must be known that the platinum tray, with the
glass in it, was either placed directly upon the bottom of the
iron pan, or, for greater security, with only a plate of platinum
intervening ; and that the whole was covered by an evaporating
basin turned upside down, forming a sort of inner chamber
within the large one. In this confined state the oxygen of the
portion of air present was soon abstracted by the heated metal,
an oxide of iron being formed in consequence, and at the same
time also a portion of carbonic oxide from the carbon in the
cast iron. At the high temperature to which the experiment
was raised, this carbonic oxide was competent to reduce a por-
tion of the oxide of lead in the glass to the metallic state, itself
becoming carbonic acid ; but as soon as the carbonic acid so pro-
duced came in contact with the heated iron, it was again con-
verted, according to the well-known condition of the chemical
affinities at these temperatures, into carbonic oxide, and went
back to the glass to repeat its evil operation and produce more
metallic lead. In this way it was that the glass became sullied by
smoky clouds consisting of metallic lead. It was the lead thus
evolved, also, that, by alloying with the platinum, had produced
the appearance of chemical action always visible in these cases ;
and now I knew how to account for the failure of many ex-
periments in consequence of the formation of holes in the trays
in a manner before quite inexplicable : for in the experiments
purposely made to investigate this point, sometimes the glass
was darkened only at the surface, the lower part being quite
clear and good ; and then, though the platinum tray was fre-
quently cut through as with a knife all round level with the
surface of the glass, it was quite unaltered below. At other
times the superficial stain was in a greater quantity, andjiad



1829.] On the Manufacture of Optical Glass. 257

collected together into little drops like fat upon hot water, and
upon examination each little globule was found to be soft
brilliant metallic lead. At other times a much larger globule
hung from the middle of the surface into the glass, barely
sustained there, and ready to sink by the least agitation when
in a heated state, and in some instances the bottom of the tray
was alloyed and perforated by globules of lead which had thus
been formed and deposited, and the glass just running out,
whilst another globule was in progress of formation at the sur-
face exactly over the place of the hole.

51. When iron was dismissed as the material of the chamber,
earthenware was resorted to. The sides were built up of
brick, and the bottom formed of tiles, which resting at the
sides upon ledges, and at the middle upon the fire-brick sup-
ports (47), could be replaced at pleasure. The same iron
covers were used for the upper aperture of the chamber as
before.

52. The use of earthenware as the material, made it far
more difficult to apply a sufficient heat to the contents of the
chamber than before, because of its inferiority to the iron as a
conductor of heat ; and a series of investigations were required
to discover that substance which, at the same time that it had
sufficient strength and exerted no injurious influence, was also
a sufficiently good conductor. Reigate fire-stone, recommended
by the builders, did not answer the purpose, and moreover in
thin plates was liable to fuse and slag. Slate, however care-
fully heated, shivered and split not only across but parallel
to its structure ; and then, as soon as air intervened, it trans-
mitted too little heat. It also softened, became curved, and
let in air and smoke, and at last gradually fused, becoming
unable to bear the weight of a large experiment. Yorkshire
stone, rubbed down into plates fthsof an inch thick, answered
moderately well, if the application of heat was carefully made
and gradually raised. It cracked in a few places, but did not
fall to pieces ; and it was more difficult of fusion than the
former substances. Fire-tiles of various kinds were tried ;
those made of Stourbridge clay answered the best, and, when
about f ths of an inch thick and carefully heated, might be
successfully used ; but that which we finally arrived at was
the use of plates made of the materials from which Cornish



258 On the Manufacture of Optical Glass. [1829.

crucibles are manufactured. These we obtained through the
intervention of our President; they were purposely manu-
factured for us by Mr. Michell of Caleneck in Cornwall, a
gentleman who has been ever willing and anxious to assist
us in our inquiries, by supplying us with vessels of any size
or form, or any other article which it was in his power to
produce.

53. The Cornish plates have not much cohesion, and feel
tender in the hand. They may be rubbed down to a flat sur-
face, and resist any heat which can be applied to them in these
or in much more powerful furnaces. They are therefore
readily brought to any thickness, and when of about f ths of
an inch, and supported in the furnace as before described (47),
have strength to bear any weight required to be placed upon
them. They do not crack, nor do they force themselves to
pieces by expansion ; but they are porous, as indeed are in a
greater or smaller degree all the materials of which the chamber
and its sides are now composed.

54. The porosity of these materials was of great importance ;
for it allowed of the passage of gaseous matter, and that even
of a reducing nature, from the fire into the chamber. I have
frequently had evidence that the sides and bottom might be
considered as a very sieve-like partition between the fire, the
flue, and the ' space called the chamber ; for when the upper
aperture has been closed, there has been a current through
the chamber in the direction of the flame, the gaseous matter
entering at the extremity nearest the fire, and passing out at
the end towards the flue. In one or two cases, oxide of lead
was actually reduced, and the glass thus rendered cloudy.

55. Hence it became necessary to use some certain means of
maintaining an oxygenating atmosphere about the glass ; to ob-
tain which, and also to prevent any other injurious vapours from
the fire entering the space beneath and within the earthenware
covers (51), the expedient was adopted of allowing a current
of fresh air to pass continually into that space and circulate
about the glass. To effect this, a clean earthenware tube, glazed
within, was inserted horizontally into the side of the furnace,
in such a manner that one extremity was flush with the inside
of the chamber, and of such height, that its lower edge corre-
sponded with the level of the bottom upon which the glass in



1S29.] On the Manufacture of Optical Glass.



259



its tray was to be placed, whilst the other end of the tube
reached to and was flush with the outside of the furnace. A
loose piece of tube, similar in kind but smaller in diameter,
being laid upon the bottom of the chamber, and applied at its
end to the orifice of the larger one, served as a continuation
of it until the inner extremity reached to and was under the
cover of the glass experiment. When the furnace was hot,
there was always a draft inwards through this tube; but
the quantity of air admitted was regulated by a valve (70).
The air, by first passing through the hot sides of the
furnace, and then through the shorter ignited tube serving for
connexion, was transmitted in a thoroughly heated state to
the place where its presence was required, without producing
any serious cooling effect ; it there maintained a continually oxy-
genating atmosphere, and, judging from the effects, prevented
the draught inwards of any vapours from the fire to the space
beneath the glass covers.

56. The next point of importance, in the preparation of the
glass, is the arrangement of the tray in the furnace just
described. To understand this, it will be necessary to say
that the glass-chamber is 25 inches long, 13 inches wide, and
8 inches deep, and that the fire being at one end, the flue is
at the other. Plates of glass 7 inches square have been made
in it ; but it would probably require a larger furnace to make
much larger pieces.

57. The bottom of the chamber being perfectly level and




clean, the gauge-board, on which the tray was formed (35),



260 On the Manufacture of Optical Glass. [1829.

should be placed on the middle of the half next the fire, and
then a piece of connecting air-tube taken, which being laid on
the bottom of the chamber, may extend from the fixed air-tube
by the side of the gauge as far as the middle, or even towards
the further side of the chamber. After this, pieces of Cornish
tile (53), or other clean earthenware which will not fly in the
fire, contain but little iron, and are free from glaze, are to be
prepared, of such size that they will fit in loosely round the
gauge, covering the rest of that half of the chamber bottom,
and serving to support the sides of the tray when in its place.
This support to the tray is highly needful ; for, otherwise, the
weight of the glass, and the action of stirring, would be more
than the thin and heated platinum could support. The thick-
ness of the pieces should be, for the plate in question, about
1 inch, and they should be all uniform in that respect. They
should never rise so high as the edge of the platinum, lest glass
should accidentally pass from the tray to them, or impurities
from them to the glass. An excellent guide to their thickness
is, to make it similar to that of the intended plate. When they
have been roughly arranged around the gauge, the latter
should be withdrawn, and the tray itself introduced, the
pieces being now finally adjusted about it. They should not
be so arranged as to press against its sides ; but the latter
should be at liberty, though only so much, that upon the least
tendency of the sides outwards, they should be supported by
the pieces. The assistance thus given should be directed
rather to the sides than the corners, and it is better that the
latter should not be in contact with these adjuncts, but be
allowed to sustain themselves, for they are strong enough for
the purpose, and the corners are always those places at which,
from one circumstance or another, the glass is most likely to
pass outwards.

58. The piece of earthenware which is fitted nearest the
mouth of the air-tube should have its angle taken off, or some
other provision made, as by making the orifice of the tube
oblique, that the passage of air may be uninterrupted ; and on
that side the tube itself may frequently form the support to
the tray. If it does, and is glazed on the exterior, a piece of
loose platina foil should be wrapped round it at the part where



1829.] On the Manufacture of Optical Glass. 261

it touches the tray, to prevent adhesion by the glaze when cold
The general disposition of the tray, the tube, and the packings,
may be seen in the sketch (57).

59. When the first set of packing pieces is properly ad-
justed, a second series is to be arranged over them; but these
are to be removed backward from the tray about the third or
half of an inch all round, that accidental contact with its edges
may be avoided. Their thickness should be sufficient to raise
them level with, or rather above, the edges of the tray. All
these adjusting pieces are to be rendered perfectly clean and
free from dust before they are applied. Their use is not only
to afford support and assistance to the platinum tray, but also
to sustain the glass covers, and likewise, by retaining the Heat
upon the bottom of the chamber, prevent much of the incon-
venience that would otherwise occur at the times of stirring
the glass.

60. The tray-covers have, up to this period, consisted of
inverted evaporating basins, suspended at pleasure, in the
manner before described, by platina wires (26). When the
platinum trays used have been sufficiently small to admit of the
arrangement in our present furnace, two, and even three covers
have been used simultaneously, each prepared with its own
platinum suspension ; but of such size, that the larger could be
placed over and enclose the smaller, without touching it. In
such cases the temperature of the glass, after being lowered
by stirring, or in any other way, rose very rapidly ; but with the
large plate of 7 inches square, the furnace would admit of but
one glass cover of sufficient size, and the only additional
assistance which could be obtained was that which was given
by putting a similar but smaller cover on the outside and



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