PRINTING ON GELATINO-CHLORIDE EMULSION PAPER.
Mr. W. T. Wilkinson has recently brought forward the notion of using
gelatine instead of albumen as a medium for holding the silver chloride
in printing. He uses the following formula:--
Barium chloride 2,440 grains
Gelatine 2,000 "
Water 20 ounces
The gelatine is allowed to swell in the water, and, by the aid of heat,
is dissolved; the barium chloride is then added. Next he prepares--
Silver nitrate 1,700 grains
Water 5 ounces
and adds this to the former, little by little, in a large bottle with
much shaking, or pours it slowly into the former in a large jar,
stirring briskly the whole time. This makes an emulsion of silver
chloride, and is used without washing. When required for use, the
gelatine, which will have set when cold, is swelled by placing the jar
containing it in hot water, and is then transferred to a dish. The dish
should be kept warm by being placed, supported on small blocks, in a tin
tray (about two inches larger in dimensions every way than the dish)
filled with hot water, the temperature of which should be about 150° F.
to commence with. Saxe or Rive paper may be coated by rolling the sheet
face outwards, and placing the edge of the roll upon the gelatine. The
two corners of the paper in contact with the solution are then taken
hold of by the fingers, and raised. The paper will unroll of itself, and
take up a thin layer of the gelatine emulsion. The sheet of paper is
then suspended to dry. All these operations are, of course, conducted
in the dark room. The behaviour of the paper in the printing-frame is
precisely the same as albumenized paper, and the washing and toning are
conducted in the same way. For a fixing bath is used--
Sodium hyposulphite 2 ounces
Water 20 "
The washing after fixing is more rapid than with albumenized paper. It
is washed in ten or twelve changes of water for ten minutes, and then
placed for five minutes in an alum bath made as follows:--
Potash alum 5 ounces
Water 20 "
The print is washed in a few changes of water, and the prints are ready
for drying and mounting. The advantage of the alum bath is that the
hyposulphite is destroyed into harmless products, and the gelatine is
rendered insoluble by it. In the formula given there is large excess
of chloride, and we recommend that instead of using 2,440 grains of
barium chloride, 2,050 grains be used. (Mr. Wilkinson has used that
amount of the barium salt that would be required exactly to convert
1,700 grains of silver nitrate into silver chloride, if the formula for
barium chloride were BaCl_{3} instead of BaCl_{2}.) It will be seen
that whichever formula is used, there is no silver left to combine with
the gelatine, and hence the image will be entirely formed by metallic
silver, and not an organic salt of silver.