....150 grains.
Liquor ammonia, 880................ 60 drops.
When all is thoroughly dissolved and of about 120 deg. temperature, add,
stirring all the time,
Nitrate silver..................... 60 grains,
Water.............................. 3/4 ounce.
Alcohol............................ 3/4 "
Then again add,
Nitrate silver.....................140 grains.
Water.............................. 1 ounce.
Alcohol............................ 1 "
Both solutions being warmed to about 120 deg..
My object is adding the silver in two quantities will be obvious to
many--viz., when the first portion of silver is mixed, nitrate of
ammonia is liberated (which is a powerful restrainer), and the bulk of
the solution being increased, the remainder of the silver may be added
in a much more concentrated state.
The alcohol, both in the gelatine and silver solutions, plays a most
important part: (1) It prevents decomposition of the gelatine. (2) It
allows the gelatine to be precipitated with a much smaller quantity of
alcohol (say about 10 ounces).
After letting the emulsion stand for a few minutes to ripen, I pour in
slowly about eight ounces of alcohol, stirring all the time, and keeping
the emulsion warm; the emulsion will adhere to the stirring-rod and the
bottom of the vessel in a soft mass, and all that is now required is to
pour away the alcohol, allow the emulsion to cool, tear it into small
pieces, wash in several changes of cold water, make up the quantity to
ten ounces, and strain; it is then ready for coating.
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