Ammonia bromide..........................................240 grains.
Gelatine..................................................24 grains
Water..................................................... 3 ounces.
Hydrochloric acid enough to slightly acidify the solution.
III.
Gelatine................................................. 20 grains.
Water.................................................... ½ ounce.
IV.
Hard gelatine (say Nelson's X opaque,
or Mr. A. L. Henderson's)................................240 grains.
Soft gelatine (Nelson's No.1)........................... 240 grains.
Water.....................................................24 ounces.
Nos. II., III., and IV. are allowed to stand until the gelatine is
softened. No. I is then warmed in a hock bottle until the gelatine is
just melted, when No. II. is poured into it, a little at a time, with
vigorous shaking, until the whole is emulsified. It is then transferred
to an ordinary jelly can, which is placed in a saucepan half full of
water over a ring Bunsen burner in the dark room, and boiled for half an
hour. It is then allowed to cool to about 100° Fahr., when No. III. is
added. The whole is then allowed to get quite cool, when it is poured,
with stirring, into about one pint of methylated spirit. If it be wished
the precipitate may now be filtered out and washed at once like an
ordinary filtrate, but I prefer to allow it to settle, which it will do
in about five minutes. The supernatant fluid is then gently poured off.
This fluid will have the appearance of still containing a considerable
amount of the silver bromide; but if it be kept and filtered it will be
seen that the quantity is really so small that it may be disregarded. We
all know what an alarming quantity of silver seems to be going down the
sink when we wash vessels to which a very small quantity of emulsion is
adhering. If filtering be resorted to the liquid which comes through
will be quite clear. This was somewhat unexpected by me, as, if an
emulsion containing the whole of the gelatine be precipitated into
alcohol in the usual way, the alcohol becomes milky with a substance
which could not, I imagine, be filtered from it.
Two or three ounces of methylated spirit are now added to the vessel
containing the silver bromide, and the latter well mixed with it. This
makes the precipitate "firmer"--if such an expression be allowable--and
this time it will sink to the bottom almost immediately after the
stirring has ceased, and the alcohol may be poured off.
I consider that the bromide in this state is practically free from
soluble salts, but it may be washed with one or two changes of water if
desired.
No. IV. is now gently heated till the gelatine is melted and the
precipitate mixed with it. It must be kept warm for some time, and
shaken vigorously until all granularity has disappeared, This is, of
course, ascertained by placing a drop of the emulsion on a piece of
glass, and examining it. If it be wished to keep the bromide of silver
for future use it may be placed on a piece of muslin stretched in the
drying-box, when it will dry in a very short time; and, although I
cannot speak from experience on this point, it will, I have no doubt,
keep for an indefinite time so long as light is kept from it.
If it be desired the ammonio-nitrate method may be used instead of the
boiling one, although in my hands it does not give such sensitiveness.
If it be desired to use this method, solution Nos. I, II., and IV. are
made up exactly as for the boiling method, except that No. II. is not
acidified. Liquid ammonia is then poured with stirring into the silver
solution, until it blackens and again clears. Emulsification is
performed exactly as described above, but instead of boiling, the
emulsion is kept at a temperature of about 100° Fahr. for half an hour,
when it is poured into the alcohol, no addition of gelatine being
previously made.
I think I may claim for the method which I have just described that it
is less troublesome and more certain than either the ordinary washing
method or the usual one of precipitating with alcohol, while it affords
an easy method of making sensitive silver bromide in such a form that it
can be more easily stored and afterwards manipulated than if it were in
the form of pellicle. The whole of the soluble salts are eliminated,
and also any gelatine which may have been destroyed in the cooking.
The amount of alcohol used is comparatively small; in fact, to prepare
silver bromide for a pint of emulsion very little more than a pint of
methylated spirit is required. Besides this I do not think that I would
be wrong in saying that the chance of green fog is reduced to a minimum.
Let me take this opportunity of thanking Captain Abney for his prompt
reply to my question about the connection between the proportion
of bromide to gelatine in emulsions, and the density of resulting
images.--_W. K. Burton, in British Journal of Photography_.
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[Illustration: Old Wrought Iron Gates, Guildhall.]
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