Since gelatine emulsion first came into use one of the greatest troubles
in connection with the manufacture of it has been that of washing.
According to the first methods the time taken for this part of the
process was, I believe, about twenty-four hours. It was very much
reduced and the ease of manufacture greatly facilitated by the methods
now most generally used, and which were, I believe, first communicated
by Messrs. Wratten and Wainright. I refer to those of precipitating with
alcohol and of straining the emulsion, when set, through canvas, so
as to divide it very finely. When the latter method is resorted to a
comparatively short time is sufficient to wash it. This method, although
a great improvement upon the older ones, yet leaves much to be desired,
especially for those who are not in the habit of making emulsion
regularly, but only an occasional batch. When the weather is at all warm
it takes a long time for the emulsion to set, unless ice be used, and
when once it is set the washing process is an exceedingly "messy" one
unless the water be cooled with ice; and the amount of water taken up
during washing is often so great that there is considerable difficulty
in getting the emulsion to set on the plates. In fact, even in cold
weather, it is not an easy process to conduct in the necessary near
approach to total darkness.
Considerable suspicion has of late been thrown upon the thoroughness of
the alcohol method, unless the emulsion has, previous to precipitation,
been freed of the greater part of the soluble salts by washing; that is
to say, it is doubtful whether the whole of the soluble salts can be
eliminated by the process, and, therefore, unless in exceptionally hot
weather, it would seem best not to trust to it, except as a further
security against soluble bromide and nitrate after washing. Besides
this, the consumption of alcohol is very large. Almost three times the
amount of the emulsion precipitated is required, and this, even when
methylated spirit is used, adds considerably to the expense. With a view
of doing away with the washing altogether, or, rather, of washing of
the silver bromide when not incorporated with the gelatine, several
processes have been invented. By these silver bromide is obtained in a
very fine state of division, ready to mix with gelatine and water in any
proportion.
The best known of them is Captain Abney's very ingenious glycerine
method, which seems to have been thoroughly successful in his hands,
although it has not been in every one's. The silver bromide obtained by
his process is not highly sensitive, and requires boiling with gelatine
before it is in a fit state to make a rapid plate.
We have lately had described in these columns a method of obtaining
bromide in a highly-sensitive state by means of the use of an acid,
whereby, after emulsifying and boiling, the viscosity of the gelatine
was destroyed, and the bromide in time deposited itself. During the late
hot weather, when washing became almost impossible, I was led to cast
about for some method of eliminating the soluble salts less tedious and
"sloppy" than that of washing, more certain and less expensive than that
of precipitating the whole of gelatine with alcohol, and which would
take less time than the method of obtaining the bromide in a pure form.
My first idea was to make up the solutions used in emulsifying in a very
concentrated form, and, after emulsifying, boiling, and allowing to
cool, to add to the thin emulsion thus obtained gelatine to the amount
of twenty grains to the ounce, and to precipitate this with alcohol,
the rest of the gelatine required to make up the bulk being afterwards
added, and the whole thoroughly incorporated by warming and shaking.
I was thus successful in reducing the amount of alcohol required to
one-third of what would be necessary if the whole of the emulsion were
precipitated; but still I found that, if a reliable emulsion were
required, the pellicle as formed had to be washed to free it from the
last trace of soluble salts.
It now struck me that it might be possible to precipitate the bromide of
silver direct from a very weak solution of gelatine, and obtain it in
such a form that it might be filtered, washed, and in every way treated
as an ordinary precipitate. I tried the following experiment. I took--
1. Silver nitrate....................... 200 grains
Water............................... 1½ ounce.
2. Ammonia bromide...................... 120 grains.
Water................................ 1½ ounce.
Gelatine............................. 12 grains.
I emulsified the two together in the usual way, allowed the whole
to cool, and then poured the thin emulsion into about ten ounces
of alcohol, stirring the while. As I had anticipated, a flocculent
precipitate was formed, which settled to the bottom of the vessel in a
few minutes. This was, in fact, sensitive bromide of silver mixed with
a very small quantity of gelatine (about five per cent.), and could, I
found, be treated in the same manner as a bromide precipitate from
an aqueous solution; it might be washed, either by decantation or by
filtration, easily dried, and doubtless could, when dry, be kept for an
indefinite time, and be at any time used by mixing with gelatine and
water in any proportion thought fit.
I found that a less amount of gelatine than four grains to the ounce was
sufficient to carry the bromide down, while five grains to the ounce
carried it down in something which I considered too near an approach to
a plastic mass.
It will be noticed that in the experiments which I have described the
emulsion had not been boiled, so that the sensitiveness of the bromide
was probably not great. As the experiment was done in daylight it was
of no practical use for making emulsion; but I have since made several
batches in this manner and have found them most satisfactory.
When sensitiveness is sought by boiling I rind it necessary to add a
small quantity of gelatine after boiling and before precipitating, as
that which has been kept for some time at a high temperature seems to
have lost the viscosity necessary to carry down the silver bromide in
such a form that it can he easily separated from the alcohol and water.
The practical manner of making an emulsion by this method may be as
follows. Make up the following mixtures:
I.
Silver nitrate...........................................400 grains.
Water..................................................... 3 ounces.