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rendered attention to very minute points essential ; for other-
wise striaB were inevitably formed, and even the destruction of
the apparatus involved. For this reason, after a certain num-
ber of trials upon composition had been made, one unvarying
set of proportions were adopted, and the attention given al-
together to the discovery and establishment of a process which
should yield constantly good results. This, as far as it has
been carried into effect and proved, it is now my object to
describe.

13. The glass with which I have principally worked is a sili-
cated borate of lead, consisting of single proportionals of silica,
boracic acid, and oxide of lead. The materials are first puri-
fied, then mixed, fused, and made into a rough glass, which is
afterwards finished and annealed in a platina tray.

14. Purification of Materials. Oxide of Lead. The oxide
of lead at first used was litharge ; but this source occasioned
frequent destruction of the platinum trays, in consequence of the
existence of particles of metallic lead, which alloying with the
platina, rendered it fusible. When red lead was substituted
for litharge, the same effect took place, due to the presence of
particles of carbonaceous and reducing matter. Both these
substances also contained so much iron and other impurities, as
to give a deep colour to the glass, far beyond what was expected



On the Manufacture of Optical Glass. [1829.

from the quantity of impurity present ; this was afterwards ex-
plained. Carbonate of lead was also found to be too impure.
Finally, all the oxide of lead necessary was purified, by being
converted into a nitrate, and crystallized once or twice, as oc-
casion might require.

15. For this purpose litharge is first washed, by which many
black carbonaceous and ferruginous particles are separated ; it
is then dissolved in diluted nitric acid, so as to form a hot sa-
turated solution, the operation being performed in clean earth-
enware vessels. Both the perfectly pure and the moderately
pure acid have been tried without any sensible difference in the
results : a little sulphuric acid does not seem injurious; and I
find that sulphate of lead will dissolve perfectly in the glass ;
but muriatic acid has been always avoided. As the acid, water,
and litharge are made to act on each other by heat, either pur-
posely applied or resulting from the chemical action going on,
it will be found that, when approaching towards neutrality, the
liquid will become very turbid. The hot saturated solution is
then to be poured from the remaining litharge and undissolved
nitrate of lead, and, after standing a few moments, again poured
from the sediment, and set aside to crystallize in a cool place.
Before it is left, however, it is to be examined as to its acidity :
if strongly acid to litmus paper, it is in a right state ; if not, a
little nitric acid should be added, for the crystals of nitrate
have always been compact and pure under such circumstances,
and more readily separable from insoluble matter.

16. After eighteen or twenty-four hours, the basins of cry-
stals are to be examined ; the clear mother-liquor carefully
poured off"; the crystals broken up in the basins, and then
repeatedly washed in fresh clear portions of the mother-liquor,
that any insoluble deposited matter may be removed. There
will generally be a portion of this deposit ; but if the process
has been well performed, the crystals will be quite free. If
they appear perfectly white or bluish white, they need not be
recrystallized ; but if yellow, they must be dissolved in water,
a little nitric acid added, and the crystallization repeated.
The nitrate in the mother-liquors and washings should be
purified by repeated processes.

17. The good crystals are to be washed in three or four
waters, to remove the last portion of deposit and adhering so-



1829.] Oft the Manufacture of Optical Glass. 243

luble impurities : but to prevent excessive solution of the nitrate,
the same portions of water may be used for several basins of
crystals washed at the same time, by making it pass from one
to another in succession. Being thus cleansed, they are to be
drained, put over the sand-bath, stirred and dried, and finally pre-
served in glass bottles. By this process much iron and sulphate
of lead are excluded ; and the purified nitrate is found to yield
a glass very far superior in colour to that prepared with the
ordinary oxides of lead, and to exert not the slightest action
on the platina: its use put an end to all the accidents and
failures which resulted from the presence of metallic lead in
the oxide. 166 parts by weight are to be considered as equi-
valent to one proportional or 112 parts of protoxide of lead.

18. Boracic acid. The boracic acid for these experiments
was obtained pure from the manufacturer, but before being
used was carefully examined. It was rejected unless it was in
white or bluish-white crystals, clean and entirely soluble in
water. Its solution was tested for iron by the ferro-prussiate
of potash and a drop of sulphuric acid, and also for other me-
tallic impurities by a little solution of sulphuretted hydrogen.
An ounce or two were heated and dissolved in a little water ;
and when cold, the soluble part separated and examined for
sulphuric acid, by a few drops of nitrate of baryta and a little
nitric acid. It was also examined for soda by dissolving three
or four ounces in hot water, adding ten or fifteen drops of sul-
phuric acid, and allowing the whole to cool and crystallize ;
expressing the mother-water from the crystals ; concentrating
it ; again crystallizing, and then acting upon the mother-liquor,
obtained at the second time, by strong alcohol ; continuing
to wash with the latter fluid until all was dissolved or an inso-
luble part left. If the latter circumstance occurred, the inso-
luble substance was examined for sulphate of soda, which, if
in any sensible quantity, occasioned the condemnation of the
boracic acid. This care respecting alkali in boracic acid was
taken in consequence of observing certain bad effects produced
in glasses, which appeared referable to its presence.

19. When the boracic acid was acknowledged as pure, 36
parts by weight of the crystals were considered as equivalent
to 24- parts or one proportional of the dry substance.

20. Silica. This material is in its most convenient state

R2



On the Manufacture of Optical Glass, [1829.

when it forms part of a combination consisting of two propor-
tions silica, and one oxide of lead. As yet, the silica I have
used has been the flint-glass-maker's sand, obtained from the
coast of Norfolk, well washed and calcined. The silicate has
been prepared by mixing two by weight of this sand with one
of litharge, or with such quantity of nitrate of lead as is equi-
valent to one of litharge (16) ; the mixture is put into a large
Hessian or Cornish crucible, which being covered over, has
been put into a furnace and raised to a bright red heat for
eighteen or twenty-four hours. On taking out the crucible,
the charge has been found diminished somewhat in bulk, and
of a porous structure and appearance like loaf-sugar. It
has been freed from the crucible, the outside portions removed,
and the pure parts carefully pulverized in a clean Wedgwood
mortar. The powder has then been washed over in water, so
as to obtain the whole in a fine state of division ; after which
it has been dried and preserved in bottles. No sieve should
be used in these comminuting operations, nor any reducing
or metallic matter brought in contact with the substance.
Every care should be taken to avoid contamination. 24* parts
by weight of the silicate are equivalent to 16 parts, or one pro-
portional of silica, and 8 parts of protoxide of lead.

21. The advantage of the silica in this combined state de-
pends upon the known composition of the substance, its com-
paratively easy pulverization, and ready fusion with the other
materials. That there is iron in the silica (and the litharge
when used) is objectionable ; and the trials for its removal have
only been delayed that the investigation of a more important
point, namely, a successful process, might proceed. From
some brief experiments, I am led to believe that an unexcep-
tionable source of silica will be obtained by acting upon this
silicate, in a state of fine division, by nitric acid and water, or
else by the use of rock crystal.

22. On some occasions I used pulverized flint glass as the
source of silica, conceiving that being already in a fusible state,
it must possess an advantage over other silica, in allowing rapid
mixture with the other materials. Allowance was made for the
oxide of lead present, and the alkali was permitted to pass, as
a substance that would probably do no harm. But a striking
effect took place, which at once showed the necessity of per-



1829.] On the Manufacture of Optical Glass. 245

fectly pure materials. The glass when finished and cold was
of a deep purple colour : this was immediately referred to the
manganese in the flint glass ; a supposition proved by repeat-
ing the experiment with other flint glass, and then with flint
glass of our own manufacture in which no manganese was used :
the latter glass gave no purple colour ; the former, a colour as
deep as that produced by the first flint glass.

23. Thus it appears that this very heavy glass, the silicated
borate of lead (and I find it to be the case with other heavy
glasses), has the power of developing the colour of mineral sub-
stances far beyond what flint glass possesses ; just as flint glass
surpasses in the same property plate and crown glass. In the
case in question, the manganese, which did not give a sensible
tint to the flint glass, produced a strong colour when diluted
eight or nine times by the heavy glass, for the proportion of
flint glass used was only ^jths of the whole. On making a few
experiments with iron, I find that the same strong development
of colour is produced with it in these heavy glasses ; so that
the utmost care is necessary to preserve all the materials during
their preparation, and the glass in every part of the process,
from metallic contamination.

24. The use of flint glass even without manganese was also
objectionable, because of the alkali in it, which, as before stated,
was found to produce bad effects, and rendered the glass con-
taining it very liable to tarnish.

25. Such are the materials from which the heavy optical glass
has been latterly manufactured. When the composition had
been determined upon, the proper proportions and quantities
of each have been weighed out in a clean balance and vessels.
Thus, for the silicated borate of lead glass, consisting of single
proportionals of each substance, 24 parts of the silicate were
taken, for they contained a proportional of silica equal to 16 parts,
and in addition 8 parts of protoxide of lead : the proportional
of oxide of lead has been taken as 112 parts ; but there being
8 in the silicate, the quantity of nitrate of lead equivalent to 104
parts only was required, and this is 154*14 parts: the equiva-
lent of dry boracic acid is 24, which being contained in 42 parts
of the crystals, that quantity was the one required. These pro-
portions when heated and submitted to mutual action leave only
152 parts of glass, or thereabout; for



246 On the Manufacture of Optical Glass. [1829.

154*14 nitrate of lead contain . . . 104 protoxide of lead.

24*00 silicate of lead contain . . . {

L 16 silica.

42'00 crystallized boracic acid contain 24 dry boracic acid.

152 glass.

Hence the materials for any quantity of glass can be easily
calculated; and if the above parts be ounces, about 91bs of
glass will result. The nitrate of lead is to be broken small
in a clean mortar, and then the other ingredients well mixed
with it in basins ; the use of metal or dirty implements being
carefully avoided,

26. The mixture is next melted, and made into rough glass.
This preparatory operation is necessary, because from the
quantity of vapourable matter which is disengaged in this part
of the process, the materials, if put at once into the finishing
vessel and furnace, might boil over and do injury ; and the acid
nature of the vapours themselves, if it did not occasion harm
by acting on neighbouring iron and other parts of the furnace,
would at least cause inconvenience. It is effected in a furnace
which will be particularly described in the Appendix to this
paper. It will be sufficient here to state, that being a close
furnace, the part immediately beyond the fire-place forms a
horizontal chamber, covered above by an iron plate having large
circular holes ; these allow crucibles to pass through them, and
to stand supported on the bottom of the chamber, whilst their
edges rise above the upper iron plate. In this way the fire is
applied very generally to the crucibles, whilst their mouths are
altogether exterior to the furnace, so that the introduction of
any reducing or colouring impurity from the fire is prevented,
and the greatest facility in introducing the mixture, of watching
its fusion, of stirring the glass, and finally of ladling it out, is
obtained. The holes through which these crucibles are inserted
are five or six in number ; they are never all in use at once, and
those out of use are covered by crucible covers. The heat is
not given altogether by flame ; but, whilst coal is used in the
fire-place, coke is applied between the crucibles, being intro-
duced for that purpose, and arranged, through the unoccupied
holes. The iron top of the furnace is covered by a second iron
plate, or, what is better, by earthenware plates, to retain the



1S29.] On the Manufacture of Optical Glass. 247

heat. The crucibles are of pure porcelain ware, and as thin
as they can be obtained. The covers for them are evaporating
dishes, considerably larger than the mouths of the crucibles :
being turned upside down, they rest, when in their places, upon
the neighbouring earthenware plate ; not touching the crucibles,
but preventing anything from falling into them, and preventing
the vapours from passing into the room. The latter are, by the
draught of the chimney, drawn through by the sides of the cru-
cible into the furnace, and carried away up the flue, so as to
occasion no annoyance to the operator. The covers are slung
by a piece of platinum wire, which, being passed across the mid-
dle on the outside, is bent at each end round the edges, so that
a rod of iron slightly curved at the extremity easily suffices to
remove them when the crucible is to be opened. Great care is
always taken to put them in clean situations, and that in their
removal nothing shall fall from them into the glass.

27. This furnace is found to be very effectual in its action ;
being connected with a high flue governed by a damper, great
command of the temperature is obtained. The crucibles be-
fore being used are examined as to soundness ; their tem-
perature is raised gradually, and should not be above a dull
red heat when the operation commences. The mixture already
described (25) is then introduced, and the crucible covered ;
decomposition of the nitrate of lead instantly commences ; the
boracic acid loses its water, all the fixed elements unite ; and it
is remarkable that though a considerable quantity of boracic
acid usually sublimes with the water when the latter is driven
off from its crystals unmixed with other substances, yet scarcely
a trace seems to evaporate in the present instance, in conse-
quence of the presence of the oxide of lead.

28. The heat should not be raised too high or the operation
hastened, and then the ebullition will proceed very gradually
and favourably, the rough materials being by degrees converted
into glass. Before the first charge is entirely melted a second
is put in, and when that is fused down, sometimes a third, ac-
cording to the quantity of glass present and the soundness of
the crucible. When all is fused, the temperature is allowed to
rise, but not too much, lest action upon the crucible to a serious
extent should occur ; the glass is then well agitated and mixed
by a platinum rake or stirrer, to be described hereafter. Finally,



248 On the Manufacture of Optical Glass. [1829.

the glass is either transferred by a platinum ladle into trays
roughly turned up out of old platina foil, or into a clean deep
white earthenware vessel containing much distilled water. In
the latter case it is obtained in a divided state, and when drained,
is dried on the sand-bath, and put up in clean bottles.

29. When a crucible has been emptied of its first portion of
glass, it will serve, if carefully used, for a second, third, fourth,
or for many operations; but it should be watched for cracks
and casualties, that the running of the glass into the furnace
may be prevented, and, if necessary, another vessel taken.

30. The rough glass thus prepared is in the next operation
to be converted into an annealed and finished plate. The size
must therefore be determined upon, and we will assume it as 7
inches square, and 8 tenths of an inch thick, that being the
dimension of the largest plate as yet made. For the purpose
of making a competent platinum vessel, a plate of that metal will
be required at least 10 inches square; but if larger, it should
not be cut, but either made into a tray with higher sides than
is absolutely needful, or else used first in the manufacture of
a larger plate of glass than the one to be described. It should
be of such thickness as to weigh at least 17'5 grains to the
square inch; and it is important that in its preparation a
good ingot, or the good part of an ingot, of platinum has been
selected, and that it has been rolled very gradually and care-
fully without the formation of any holes by the adhesion of
dirt or hard particles, or by the dragging of the metal in the
mills. The desired perfection is, I understand, best obtained
by rolling the platinum between two clean plates of good copper.

31. The plate, being laid upon clean paper or a cloth on a
smooth table, is to be cleansed with a cloth and a little water
or alcohol, and then to be ignited at every part by a large
spirit-lamp. It must next be carefully examined as to its state,
and the occurrence of places upon its surface where holes are
likely to exist. If the metal seems dragged in any place, an
effect indicated by a roughness upon the surface, or by short
lines parallel to each other but perpendicular to the course of
rolling, such place should be noted or marked, for which
purpose a dot of ink will be convenient. If a scale appears,
or a small portion is apparently folded over, it should also be
marked ; and if a black spot is visible (and they are sometimes



1829.] On the Manufacture of Optical Glass. 240

formed by the adhesion of a particle of dirt or grit), it should
be examined, and removed by the point of a knife, if necessary,
and its place also marked. All these places and the whole
surface of the plate should then be examined for holes by a
still stronger test, namely, by holding the sheet of metal before
and close to a bright light, as a candle .or lamp, in a dark
room, and every hole observed, marked. In making this ex-
amination, it must be done carefully and minutely, holding the
plate in different directions to the light (for sometimes the holes
are oblique), and being careful that no reflexion from illumined
objects, as the hands, on that side towards the face shall
give deceptive indications. In the marking, too, the indicating
spot should always be made at a certain distance from the hole,
as the fourth or the third of an inch, and on the same plate
constantly in the same direction or towards the same edge ; the
holes are then easily found again, and the mark remains during
the soldering to guide the operator.

32. The holes discovered by these examinations are to be
closed by little patches of platinum soldered with gold ; for gold,
like platinum, may be safely used in these experiments, when
reducing matter is absent. The gold has been used in the
finely divided state in which it is obtained by precipitation from
its solutions by means of sulphate of iron, but it must be washed
perfectly pure ; the patches are formed by cutting a piece of
clean new platinum foil into small square or rectangular plates :
a sufficient heat can usually be obtained by the use of the spirit-
lamp and mouth blowpipe. In the process of soldering, a little
of the powdered gold is heaped upon the hole and slightly
flattened by some clean instrument, the spirit-lamp is applied
underneath for a moment, which causes the gold to adhere
slightly, a selected patch of platinum is laid delicately upon the
gold, and then the heat of the spirit-lamp, urged by the blow-
pipe, is directed beneath against the place. Usually the gold
will melt and run instantly, the platinum patch will come into
close contact with the plate, and the operation will be completed.
If well done, the fused gold will appear all the way round in
the minute angle formed by the edge of the patch, and also
faintly at the hole on the opposite side of the plate.

33. Sometimes, when the patch is large, or in the middle of
a plate, the heat obtained as above is hardly sufficient to melt



250 On the Manufacture of Optical Glass. [1829.

the gold freely and cause perfect adhesion. In such cases, a
single or double piece of platinum foil loosely laid over the part,
prevents loss of heat from the upper surface, and frequently
causes such increased elevation of temperature as to render the
soldering perfect and effectual. In the few cases where this
expedient has not succeeded, I have resorted to the oxyalcohol
blowpipe, vising a small bladder of oxygen with a little attached
jet for the purpose. This has never failed to produce an
effectual heat, and 15 or 20 cubical inches of oxygen are suf-
ficient for many operations.

34. This application of patches and soldering is only secure
for small holes, i. e. such as a pin might pass through, and
smaller. The patches are always to be applied on that surface
of the plate which is to constitute the outside of the tray ; and
therefore, before the soldering begins, the two surfaces should
be examined, and the most polished and perfect selected as
that intended for the inside. The patches are valuable in
their use far beyond what the mere application of gold to the
hole would be ; for the heat afterwards applied to the tray,
when charged with glass, is abundantly sufficient to melt gold ;
in which case, if unsupported by the platinum patch, the weight
of glass and the action of stirring would probably force the
gold out of the hole and cause the tray to run ; whereas the
patch of platinum, although the gold holding it to the plate is
liquid, still adheres by so strong a capillary action as to be
sufficient to retain its place, and being outside is not disturbed
by the motion of the stirrer. Besides, after a long application
of heat, the gold and platinum combine so perfectly as to become
one piece of white alloy, infusible at the heat applied.

35. The plate is now to be folded into a tray, preparatory
to which, a piece of thin board is to be provided as a gauge,
which in the present instance must be 7 inches square.
This laid upon the plate and held tightly down, directs the
foldings of the sides, and would, if placed in the middle, leave
sufficient for edges one inch and a half high all round ; but as
the plate should serve for use several times, it is advantageous
to apply the gauge a little eccentric ; for then, when used for
a second and third operation, its place may be shifted, and
the folds not occurring where they did before, there is less
chance of holes being broken through the platinum. The folds



1829.] On the Manufacture of Optical Glass. 251

necessary at the corners of the tray are especially likely to
render the same parts unable to bear a second and third
bending ; but the necessity of having them in the same place
may be usefully obviated by placing the gauge oblique to the
sides in one direction and in another, on different occasions,
and moreover gives other advantages in finishing the folding
of the corners (36). These attentions, tending to the preser-
vation of the platinum for repeated service, are very needful, in
consequence of the great expense of the material : the value of
the plate in question is about 61. 10s., and when worn out, it
may be sold for about half that sum. Whether it be used
therefore once, twice, thrice, or four times, makes considerable



Online LibraryMichael FaradayExperimental researches in chemistry and physics → online text (page 24 of 49)