By this time of the year I have no doubt many, both amateur and
professional photographers, are either contemplating or are actually
at work making their stock of plates for the coming season, and it is
to be hoped that we shall have more favorable weather than we had last
year.
Some four or five years since I tried using bromide of zinc instead of
the ordinary salts, namely, bromide of ammonium or potassium. I only
made one batch of plates at the time, which possessed several
important features I considered an advantage, and I think well worth
while following out. I do not think it can be denied that ordinary
gelatine plates, if exposed in a weak light, fall very short of the
results obtained with wet collodion when compared side by side,
gelatine being almost useless under these conditions, and there is a
decided gain in the result in this respect if the emulsion be made
with zinc bromide.
In using bromide of zinc there is a slight difficulty to overcome, but
it _can_ be overcome, as I have succeeded in making a perfect
emulsion. It will, I have no doubt, be remembered that Mr. L. Warnerke
was the first to call attention to this salt in the days of collodion
emulsion; and I think he claimed for an emulsion prepared with it that
the image would stand more forcing without fogging to gain any amount
of intensity. This was said of a collodion emulsion, and I also find
that it is the same when used in a gelatine emulsion. I have heard a
great many say, when speaking about the intensity of gelatine plates,
that they can get any amount of intensity. I grant that in a studio
where the operator has full command over the lighting of his subject
by means of blinds, but it is not so in the field, especially when the
light is dull. I have seen thousands of negatives, and as a rule I
have found want of intensity has been the fault, and generally through
the light. Now if we can find a remedy for this, it will be a step in
advance.
What I claim for bromide of zinc is that a rapid plate can be made
with it, and any degree of intensity can be readily obtained with a
very small proportion of pyrogallic acid in the developer. The cry as
always is to use plenty of pyrogallic acid and you can get any amount
of intensity. I remember, in the early days of gelatine, as much as
six grains being recommended, and I have myself, under extraordinary
circumstances, used as much as ten grains to the ounce; but I think it
is now, to a certain extent, a thing of the past. With the plates to
which I refer, I found that I only required to use for a 7½ × 5 plate
one grain of pyrogallic acid in about three ounces of developer to get
full density without the slightest difficulty. If the ordinary
quantity were used far too much density was obtained, and the plate
ruined beyond recovery; but with so small a quantity of pyro. the
plate was not so much stained as with a larger quantity, and the
negative took far less time to develop on account of the intensity
being so readily obtained.
In making a gelatine emulsion with zinc it must be _decidedly acid_ or
it fogs. I prefer nitric acid for the purpose. I also found that some
samples of the bromide behaved in a very peculiar way. All went on
well until it came to the washing, when the bromide of silver washed
out slowly, rendering the washing water slightly milky; this continued
until the whole of the bromide of silver was discharged from the
gelatine, and the latter rendered perfectly transparent as in the
first instance. I remember a gentleman mentioning at one of the
meetings of the South London Photographic Society that he was troubled
in the same way as I was at that time. I think if a few experiments
were made in this direction with the zinc salt and worked out, it
would be a great advantage.--_Wm. Brooks, in Br. Jour. of Photo_.
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