By V. SCHUMANN.
In a recent paper in the _Wochenblatt_, says the _Photographic
News_, this investigator relates his experience of gelatine emulsion
containing chloride and iodide of silver. Gelatine films containing
pure chloride of silver can only be used in the camera in exceptional
cases; if, however, iodide be added, the resulting iodo-chloride
films answer most of the purposes of a gelatino-bromide plate. It may
be remarked that with gelatino-chloride emulsion an image is easily
developed with pyro or oxalate; but unfortunately, fogging is very
liable to set in. On strongly diluting the developing solution and
adding a large proportion of bromide, it is possible to obtain a
clear deposit, but the image is so thin that it is quite useless for
practical purposes.
Gelatino-iodide films possess totally different properties. The
development is extremely slow, without any tendency to fog; thus the
addition of a restrainer should be avoided. Iodo-chloride emulsion can
be prepared either by dissolving the chloride and iodide salts in the
gelatine solution, and then adding by degrees the silver nitrate; or
by making two separate emulsions of chloride and iodide of silver, and
then mixing the two after the washing process. It should be noted that
the properties of a compound or a mixture of the two haloids are very
different. A negative of the spectrum impressed on an iodo-chloride
film, prepared by mixing the two emulsions, shows two colored deposits.
The red end of the spectrum as far as the G line is reproduced in
the negative as a red tone, while that part of the spectrum from G
extending to the violet appears as a grayish violet deposit. When using
Stolze's potash developer, the difference of the two tones on the
negative appears even more marked.
Experiments were instituted to determine the most suitable proportion
of the silver haloids to be suspended in the emulsion. For this purpose
three emulsions were prepared according to the following formulæ:
No. 1.--IODO-CHLORIDE EMULSION.
A.--Ammonium chloride 0.64 gramme.
Potassium iodide 0.05 "
Gelatine 1.5 "
Water 15 c. c.
B.--Silver nitrate 1.55 gramme.
Water 15 c. c.
No. 2.--Same as No. 1. but with 0.15 potassium iodide instead of 0.05;
and 1.65 silver nitrate instead of 1.55.
No. 3.--Same as No. 1, but with 0.64 potassium iodide instead of 0.05;
and 2.14 silver nitrate instead of 1.55.
To prepare the emulsion, A and B were heated in a water bath and then
mixed slowly, with thorough shaking. The mixture, after an hour's
cooking, was allowed to stand over night; the emulsion was next washed
for seventy-two hours, and after slightly diluting, at once poured
over the plates. The emulsions prepared according to formulæ 1 and
2 transmit blue light, which, however, is much brighter than that
exhibited by gelatino-bromide emulsion. No. 3 emulsion transmits an
orange light.
Previous to cooking the emulsion, a small quantity from each sample
was spread on a glass plate, and, with the films prepared from the
fully digested emulsion, were placed in sunlight. The unripe emulsion
darkened much more quickly than that which had been digested. The
colors of the exposed films prepared according to Nos. 1 and 2 were
chocolate, and there was very little difference between the tones of
the ripe and unripe emulsion. With the plates made by No. 3 formula
there was, however, a great difference of color noticeable; thus, while
the unripe emulsion yielded a deposit not unlike that of Nos. 1 and
2, the films prepared from the ripe emulsion assumed a grayish green
color, which did not alter even after some weeks' exposure to daylight.