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above the principal one.

61. The first and important cover is to be selected of such
dimensions, that when in its place and resting by its edges
upon the packing pieces, it shall fully enclose the platinum tray
and its charge, not only for the purpose of accumulating heat
and confining an oxygenating atmosphere within, but also
sheltering the glass, and preventing any oxide of iron from the
chamber covers, or dirt from other sources, falling into it.
These covers, when hot, are raised and removed by means of
clean iron rods, which being sufficiently thick to have abundant



262 On the Manufacture of Optical Glass. [1829.

strength, and no injurious degree of elasticity, are made taper
at one extremity, and slightly curved there. This end is
easily introduced beneath the platinum suspension wire, and as
easily withdrawn when the cover is removed.

62. All these matters being preliminarily arranged, the final
disposition of the tray and its charge is made. The air-tube
is carefully wiped, and its external aperture closed by a clean
loose plug of dry sponge. The tray is for the last time freed
from dust by inversion and blowing upon it, and is put into its
place. The quantity of rough glass necessary for the required
plate, about 8lbs in the present instance (30), is carefully
weighed out, and then introduced by an evaporating basin, or
some other means which shall not allow of the admission of
any reducing or colouring matter, or permit any portion of
glass to pass beyond the edges of the tray. The tray-covers
are then to be arranged in their places ; the iron covers of the
chamber likewise adjusted, and over all are to be placed a set
of thick earthenware tiles, which have been fitted together so
as to constitute a general covering to the whole, well calculated
to retain heat.

63. The ensuing part of the process is one in which the
precise order and most advantageous proceedings have not
yet been ascertained. Variations have been made up to the
very last experiment, and it is only by still more extensive
experience that the arrangement will ultimately be settled.

64. A fire being lighted in the furnace, and some coke put
beneath the glass chamber, the temperature is gradually raised.
In about an hour the bottom of the chamber begins to appear
ignited, and in four hours the top iron covers are usually dull
red-hot. These appearances are useful as indications of the
progress of the operation. When the furnace has been heated
for the first half hour, then every care is taken that the tem-
perature may be fully sustained to the end of the experiment ;
and besides the ordinary kind of attention to the fire, par-
ticular care is taken that coke be supplied, by the lateral
holes, to the part beneath the chamber ; for, if the fuel there
be allowed to burn out, the heat soon falls, notwithstanding
the flame from the coals. Although the fire may seem quickly
to have attained its best condition, yet the temperature con-
tinues to rise in the chamber long afterwards ; for, from the



1829.] On the Manufacture of Optical Glass. 263

quantity of lateral brick-work to be beated, it is usually many
bours before tbe sides of tbe chamber are so hot, that the tray
and its contents have attained their highest temperature. At
the same time it must be understood that the heat of the glass is
very much governed, especially at the early part of an experi-
ment, by the number of tray-covers over it, and rises far more
rapidly, and much higher, with two or three covers than with
one.

65. Perhaps the glass may with propriety be examined once,
early in the experiment, for the purpose of ascertaining that
the tray and its contents are safe ; but usually it is left for six
or eight, or a greater number of hours, that the whole may
fuse, the temperature rise, and the bubbles escape. When the
glass is to be examined, the tile and iron covers are to be re-
moved from over that half of the chamber containing it, by
which, consequently, the tray-covers are exposed ; these are
next to be carefully raised, one by one, using the iron instru-
ment before described, for the purpose (61), and, as they are
removed, are to be carefully put into the further part of the
chamber, which still remains covered, where they will be re-
tained in a heated state. This prevents their cracking arid
falling to pieces, as they would do if brought into the open air.
If the experiment, and consequently the covers, are upon so
large a scale that the latter cannot all be placed in this situa-
tion, then the exterior ones may be placed upon the top of the
heated covers and tiles ; but the particular cover, which imme-
diately encloses the glass, being of great importance, must be
put into the further safe part of the furnace, that it may be
carefully preserved from injury, and ready to be replaced over
the glass with the least possible disturbance.

66. The moment the last cover is removed, the glass is ex-
posed to any falling substance from the iron plates, or tiles, or
other sources, so that extreme attention is required at such
times to keep the place free from dust, and to perform every
requisite operation as quietly as possible. The current of hot
air which rises from the chamber, ascending and striking against
the ceiling, frequently causes, by change of temperature and
mechanical agitation, the separation of small particles of mat-
ter, which, descending, cause risk of injury to the glass; for
which reason it may sometimes be needful to have a temporary



264t On the Manufacture of Optical Glass. [1829.

shelter fixed over the furnace, either of tin plate, clean boards,
or some other material which shall not throw oft* scales or im-
purities of any kind.

67. If, by any unfortunate accident, a fragment of matter
does fall into the glass, it should be instantly removed. It
certainly will not sink, because of the great density of the glass,
and may be taken out, usually with facility, by touching it, and
the glass in its neighbourhood, with the platinum stirrer (28),
or the bottom of the platinum ladle (28). In carrying it and the
adhering glass away, great attention should be given, that none
of the latter fall over the sides of the tray ; since such portion
might be a means of introducing impurity hereafter, or of
cementing the tray and the earthenware together in a very in-
convenient and injurious manner.

68. If, also, it should be observed at this time, that there is
a superabundance of glass in the tray, and not sufficient distance
between its surface and the edges of the platinum, the excess
should be ladled out (28), an operation easily performed, but
which must be done with care.

69. When the glass is ascertained to be in a proper condition,
and that there is no appearance of any portion of it outside the
tray, the covers are to be replaced, the chamber closed, and the
heat continued. If the tray-covers be glazed, some precaution is
required in their arrangement ; for on putting the second cover
over the first, if they are left in contact by a portion of glazed
surface, they will be found, upon their next removal, to adhere
at that place. They should never be put in contact therefore
with each other, or, if that cannot be avoided, a piece of old
platina foil should be laid upon the place where the contact is
necessary (58).

70. Whilst the glass is covered and subjected to a high tem-
perature, there is, as before stated, an inward current of fresh
air passing continually to and about it through the air-tube,
during the whole time of the experiment (55).

It was necessary to apply a valve to the external orifice of
this tube to regulate the supply; for the draught was so con-
siderable, that the glass was cooled by it, and much dust car-
ried in. Finding reason to believe that even when very much
diminished, the quantity of soots and dust in a London atmo-
sphere, and especially in that portion of it taken from an expe-



1829.] On the Manufacture of Optical Glass. 265

rimental room in which a powerful furnace was at work, were
competent to do much harm in eighteen or twenty-four hours,
by giving colour and forming striae, experiments were made on
the means of cleansing the entering air. It was found easy to
effect this, by the assistance of two or three Woulfe's bottles,
or two or three jars, inverted one within another, using at the
same time portions of diluted sulphuric acid, or such solutions
of salts in the vessels as would not supply any moisture to the
air, but rather take water with the dust from it. In these cases
the air did not bubble through the liquid, but only passed close
to its surface, and had time to deposit its dust during its pass-
age through the enclosed spaces above the fluid ; but finally
a still simpler arrangement was used, consisting merely of a
plug of clean dry sponge fitted into the end of the tube, which,
at the same time that it allowed sufficient air to pass, seemed,
from the appearance of the tube afterwards, to have excluded
every impurity.

71. There are two conditions of the finished glass, each of
great importance, which it is the object of the process to secure
in this state of the substance. One, and the most essential, is
the absence of all striae and irregularities of composition ; the
other, the absence of even the most minute bubbles. The first
is obtained by agitation and perfect mixture of the whole ; the
latter, principally by a state of repose : so that the means re-
quired to be successful on both points are directly opposed to
each other. Were the glass absolutely incapable of change by
the long-continued action of heat, it would be easy first to ren-
der it uniform by stirring, and then to leave it in a quiescent
state, until the bubbles had disappeared ; but I am not yet fully
assured of the fact which is necessary to this order of proceed-
ings. That the glass, as far as proportions are concerned, if
changed at all, is altered only in an extremely minute and inap-
preciable degree, is shown by some experiments, in which, after
a portion had been prepared and heated for many hours, and
also stirred well, the resulting piece was divided into smaller
portions, and these heated at different temperatures, in platinum
trays, for sixteen hours. Three portions were heated as power-
fully as the furnace would admit of; three only to redness, which
may be considered as a very low heat ; and three to an inter-
mediate degree : all were cooled slowly and annealed for an



266 On the Manufacture of Optical Glass.

equal time. The specific gravities of each after the experi-
ments were as follows :

Highest heat . 5-4206 5-4211 5-4203 Mean sp. gr. 5*42066
Intermediate heat 5-4253 5-4242 5-4255 5-42500
Least heat . . 5-4258 5'4262 5'4235 5-42516
Original glass . 5 -4247 5-4261 . .7.: 5-42540

72. Here, notwithstanding the irregularities between the
similar experiments, there seems, from the comparison of the
mean specific gravities, to be a gradual though minute diminu-
tion of density, as the glasses have been more powerfully heated ;
and I found also, that when glass was so well stirred as to leave
no doubt that it was thoroughly well mixed, yet being left in
the furnace at a high temperature for eight or nine hours, it
contained striae.

73. On the other hand, first to render the glass perfectly
free from bubbles and clear, and then to stir out the irregula-
rities of composition, I have not found to be a practicable pro-
cess ; because the stirring, in the manner in which I have yet
performed it, tends to introduce bubbles into the glass ; and
though these are small, still they are objectionable. Hence
a mixed process has been adopted, which, as I have before
stated, is subject to correction from future experiments. To
render the process as far as it has been carried sufficiently in-
telligible to others, I will first describe the circumstances con-
nected with stirring, and their influence upon striae ; and after-
wards, the plans adopted for the dispersion of bubbles.

74. It is not a small degree of stirring and agitation which
is sufficient to make a fluid of mixed materials homogeneous ;
especially when the mixture is not exceedingly fluid, but
has, like tar or syrup, a considerable degree of tenacity.
An idea of the extent to which it must be carried, and of the
general nature of striae in fluids, may be gained by taking a
glass full of clear saturated syrup, made from white sugar,
putting a few drops of water into it, and stirring the whole to-
gether. It may then be remarked how slow the striae are in
disappearing ; and when they are apparently destroyed, if the
whole be left for some hours, it will frequently happen that a
separation will take place into a lower heavy, and a superincum-
bent light portion, which, when stirred together again, produce
striae. In the glass, the stirring must be in the utmost degree



1829.] On the Manufacture of Optical Glass. 267

perfect ; for if there be the least difference in different parts, it
is liable to form striae : nor are the different portions allowed by
the process to arrange themselves according to their specific
gravities, in which case one part might perhaps be removed
from another, after the glass was finished and cold ; but the
ascending and descending currents which inevitably take place
in the fluid matter, are certain to arrange the irregularities in
such a manner as to produce the strongest possible bad effect.

75. The instrument used for stirring has hitherto consisted of
a piece of plate platinum, which for the seven-inch glass (taken
as illustrating the process) is 6^ inches in length and f ths of
an inch in breadth. It is perforated with various irregular
holes, that, when drawn through the glass like a rake, it may
effectually mix the parts. A piece of thick platinum wire, about
13 inches long, is riveted to it, and the extremity of this
screwed into the end of a clean iron rod which answers the
purpose of a handle. No small or cellular apertures should be
allowed in this stirrer ; for they will frequently retain air or
moisture, which may cause bubbles in the heated matter and do
much harm. A little gold, therefore, should be applied to the
part where the stem is attached, and fused, so that all hollows
may be filled up. Stirrers of different dimensions are to be
provided for different-sized plates of glass. Before being used,
they should be steeped in dilute nitric acid, and also heated to
redness in the spirit-lamp, just previous to their immersion in
the glass for the first time in each experiment.

76. When a stirring is to be performed, the tiles and iron
covers are removed from the first part of the chamber (44. 49.
65), the tray-covers also taken off and put into the back part
of the chamber (61. 65), the glass quickly examined, to give
assurance that all is in good condition, and then the stirring
commenced. The stirrer should be put in gently, that no air
may be carried down with it, and then drawn through the glass
quickly but steadily, so as to mingle effectually, but not to en-
danger forcing the substance over the edges of the tray or to
run the risk of involving air-bubbles. The chamber and its
contents are cooled by the necessary exposure to the atmo-
sphere, and therefore, when the agitation has been continued
until the glass is so much lowered in temperature as to become
thick, it should be discontinued, the stirrer carefully removed,



268 On the Manufacture of Optical Glass. [1829.

the tray-covers replaced, the chamber covers restored to their
situation, and the temperature allowed to rise for fifteen or
twenty minutes, when the operation may be renewed.

77. All the precautions against loose particles, dust, and
soot, that were before spoken of (66), should be adopted in
this operation. In the act of stirring, the instrument should
not be struck carelessly against the bottom or sides of the tray ;
for the platinum in this highly heated state is very soft, and a
hole would readily be forced through it ; nor should it be
brought forcibly against the corners, for the metal is in such a
favourable condition for welding, that the least blow upon a
double part causes adhesion. By merely allowing the stirrer,
when ignited, to sink upon the bottom of the tray rather more
hastily than usual, it has adhered to the place ; and when, for
safety, an underlying plate of platinum was used (50), it was
always found welded to the tray at the places which the stirrer
had touched a little more forcibly than the adjacent parts, and
could not afterwards be separated without leaving holes in the
metal. This circumstance was the principal occasion of the
advantages afforded by the use of the underlying plate being
given up.

78. The heat which has to be borne during the operation of
stirring is very considerable, especially upon the hands ; but
at such a moment no retreat from the work, because of mere
personal inconvenience, can be allowed. But the circumstance
renders the use of a cover for the stirring hand very advan-
tageous. I have found a loose linen bag, into which the hand
could go freely, more convenient for this purpose than a glove ;
for being in contact with the skin at distant parts only, the
hand is preserved at a much lower temperature. Two small
holes in it, one at the front and the other at the top, allow the
handle of the stirrer to pass obliquely through, by which
arrangement it is easily held with firmness, and the bag itself
prevented from slipping towards the glass. It should not be
larger than to cover the wrist, or it will embarrass the move-
ments ; and it should be very stiffly starched and ironed, that
no fibrous particles may fly from it to the glass during the
stirring.

79. The glass which, adhering to, is brought away with the
stirrer, indicates, by its appearance, the general character



1829.] On the Manufacture of Optical Glass. 269

and state of that in the tray; but during its examination, the
experimenter must carefully refrain from touching it ; for if
the finger, or any other organic substance, come into contact
with it, the next time the instrument is immersed in the ignited
glass, the part touched will produce bubbles. It is therefore
of importance that the stirrer be preserved perfectly clean from
one stirring to another, for which purpose it may be deposited
so that the platinum shall be received in an evaporating basin,
the mouth of which is afterwards covered over.

80. In entering upon the considerations relative to the bub-
bles, it will be evident, from the nature of the materials and the
quantity of elastic matter originally present, that these air cavi-
ties are at first very numerous. The larger ones soon ascend to
the surface, and, breaking, are dissipated without inconvenience ;
but the smaller ones rise with far less readiness, and the smallest
have so little power of elevation, that the general currents in
the liquid appear sufficient to carry them downwards, or in any
other direction, and thus retain them for any period within the
mass. A useful idea of the length of time required for very
minute bubbles to ascend through a fluid having some tenacity,
may be gained by the person who will take a glassful of clear
concentrated white sugar syrup, and beat it up with a little
air, until a portion of the latter is in extremely minute bubbles.
If these are allowed to remain undisturbed, it will be observed,
that though the larger bubbles rise quickly, and the smaller
soon after, the smallest will continue for many hours under the
surface, destroying the pellucidness of the fluid ; and this will
be the case although there are none of those descending cur-
rents, resulting from difference of temperature, which in the
glass assist in retaining the bubbles beneath the surface.

81. From the great length of time which it required to libe-
rate the bubbles even from small pieces of glass, and when no
stirring was practised, I was induced to conclude that the evo-
lution of gaseous or vaporous matter had not ceased upon the
first fusion of the materials, but that the glass itself when highly
heated continued to evolve small portions for some time. It
occurred to me also, that in that case its formation might be
hastened and the final separation advanced by mixing some
extraneous and insoluble substance with the glass, to act as a
nucleus, just as pieces of wood, or paper, or grains of sand



270 On the Manufacture of Optical Glass. [1829.

operate when introduced into soda water or sparkling cham-
pagne ; in which cases they cause the gas, which has a tendency
to separate from the fluid, to leave it far more quickly and per-
fectly than if they had not heen present.

82. The substance I resorted to for this purpose was platinum
in the spongy state. It was chosen as being a body solid at
high temperatures, uninfluenced by the glass, easily reduced
to powder, and likely to retain its finely divided condition during
the operation : its preparation is described in the Appendix.
In experiments made expressly to ascertain its action, it was
found to assist powerfully in the evolution and separation of
the bubbles, and afterwards to sink so completely to the bot-
tom, that not a particle remained suspended in the mass. Even
stirring does not render it injurious ; for the particles, by that
action, are welded to the bottom, and the glass ultimately as free
from mixture with them as if they had never been present.

83. The spongy metal should be perfectly pure. It is easily
reduced to powder by rubbing it with a clean finger on clean
paper. No attrition with a hard substance should be allowed,
as that burnishes the metal, and takes away the roughness,
which is highly advantageous in assisting the evolution of the
bubbles. When reduced to powder, it should be again heated
upon a piece of platinum foil in the flame of a spirit-lamp.

84-. The quantity of powdered platinum which I have usually
employed has been about 7 or 8 grains for every pound weight
of glass. But in order to effect its more general and perfect
diffusion, I have usually mixed it with ten or twelve times its
bulk of pulverized glass. For this purpose, some of the rough
glass, the same in composition with that to be perfected, has
been crushed small in a clean agate mortar, and the finer parts
separated from the coarser on an inclined and shaken sheet of
paper. The former have been then mixed little by little with
the platinum, and rubbed slightly with the finger, to effect per-
fect separation of the metal ; and then the coarser parts have
been added, to increase the bulk. In this state it was ready
for use.

85. The time of introducing this prepared platinum is, like
the times of stirring, as yet under investigation. It has usually
been sprinkled from the platinum ladle (28) over the surface of
the well-fused and highly-heated glass, at the period of the first



1829.] On the Manufacture of Optical Glass. 271

stirring. This method has the advantage of bringing the as-
sisting substance into contact with the glass when the latter is
highly disposed to throw off its adhering gaseous matter, and
also allows of thorough mixture ; but it also causes the addi-
tion of fresh glass after the concoction of the materials has
been proceeding for many hours ; and it likewise occasions the
introduction of many bubbles formed by the air in the inter
stices of the powder.

86. On other occasions the prepared mixture of platinum and
glass has been introduced into the tray at the period when it
was charged with the due quantity of rough glass, and before
the application of fire. Particular attention was then paid to
its general diffusion throughout the charge, and on these occa-
sions its action commenced the moment the glass in contact
with it was fluid. I am inclined to believe the latter will ulti-
mately prove the better method of proceeding, both for the
greater length of time during which the platinum can act, and
for the facility and convenience of its introduction.

87. In either mode of appliance the platinum has been found
highly serviceable ; and in every case since its use, where stirring
has not been necessary, the resulting glass has proved to be
perfectly free from bubbles.

88. As already mentioned, the best periods for stirring and
repose have not been finally determined. Stirring introduces
bubbles, and therefore should, if possible, be avoided towards
the conclusion of the experiment. Rest, or at least that con-
dition in which there is no other motion than what is due to
the currents produced by slight differences of temperature,



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