_This process gives black positive impressions on white ground from
positive clichés, and negative impressions from negative clichés._ It has
been attributed to Mr. Colas, but in reality it was invented by Mr.
Poitevin, who describes it as follows in his communication of May, 1860,
to the Société Francaise de Photographie:
“I make a solution containing—”
Iron perchloride, cryst 10 parts
Tartaric acid 3 parts
Water 100 parts
“I apply the paper on this mixture and let it dry spontaneously in the
dark, and at the moment of using it I completely desiccate it at a gentle
heat. Thus prepared the paper is of a deep yellow color. Light decolors
it rapidly, and ten or twelve minutes’ exposure through a positive cliché
suffices to well impress it, that is, to reduce in the whites the iron
perchloride to the state of protochloride.”
“To print, one is guided by the decoloration of the paper, and even for
more facility I add to the solution of iron perchloride and tartaric acid
a small quantity of a solution of potassium sulphocyanide for the purpose
of obtaining a red tint, which is more visible and disappears also under
the influence of light in proportion to the decomposition of the
perchloride. One obtains then after exposure a red design on the white
ground of the paper. This red color is not permanent. It even disappears
by keeping the proof in the dark.”
“To develop and then to fix the design thus obtained I wash rapidly the
paper in ordinary water, or better, in water holding chalk in suspension.
The red coloration disappears, a part of the iron perchloride is washed
out, and in the parts which have not been acted on by light the
perchloride is transformed into sesquioxide. I replace then the water by
solution of gallic acid or of tannin and the image progressively appears
in ink-black. When I judge the image to be sufficiently intense I wash
the proof in rain water, in preference to ordinary water, which might
cause the gallic acid and tannin to turn brown. I sponge between sheets of
blotting paper and let the proof dry spontaneously.”
“If in place of gallic acid I use a diluted solution of potassium
ferricyanide (red prussiate of potash), Prussian blue is formed in the
parts acted on by light. The preparation is even sensitive enough to
permit one to obtain an impression in the camera obscura in developing by
the ferricyanide.”
“As to the proofs in gallate (or tannate) of iron, they can be transformed
into Prussian blue in a solution of potassium ferrocyanide (yellow
prussiate of potash) slightly acidified by sulphuric acid.”
The paper most suitable for this process is that which has been previously
well sized with starch, as explained in a special paragraph of this
pamphlet. Paper prepared with a film of coagulated albumen gives also good
results. It may be prepared by brushing as well as by floating, but in
either case the paper should be wetted on the surface only and dried
rapidly at a temperature of about 115 deg. Fahr. (46 deg. C.) and kept in
a dry place. It does not keep for more than from ten to fifteen days,
owing to the hygroscopicity of the iron compound. Mr. Colas, who prepares
the paper for the Parisian market, I think, states that he avoids its
deterioration by keeping it wrapped in blotting paper, between two sheets
of India rubber, to exclude air and dampness. Silvered albumen and plain
paper, well desiccated, could be kept in that way for a certain period,
especially if the blotting paper is impregnated with sodium bicarbonate
and well dried.
Mr. A. Fisch advises to discard the preliminary washing and to develop
just on the removal of the proofs from the printing frame. In operating
in this manner the development is best made by floating, taking care that
the solution does not run off the back of the proof.