If the paper be immediately placed in an absolutely dry room after exposure, the picture remains unchanged. In a moist atmosphere FeCl attracts moisture and, with a part of CuCl, is so decomposed that Fe2Cl3 is formed together with Cu2Cl.
After this action has commenced, if the proof be not immediately immersed in a solution of sulphocyanate of potassium, Cu2Cl passes over to a higher combination of chlorine, and the paper is again fit to be impressed anew by the action of light.
As long as FeCl or even Cu2Cl is present, if the print is immersed in the sulphocyanate solution, sulphocyanate of copper is immediately formed on the reduced parts, while on the others the sulphocyanide of copper, formed and dissolved by the sulphocyanide of potassium in excess, becomes decomposed with water in soluble sulphocyanide of copper and deposited as such on the parts already covered with the salt.
Frequently the prints appear yellow from formation of the double sulphocyanide of copper, but the color disappears by washing in water. Red coloration is due to decomposition into ferrocyanide of copper.
L. LIESEGANG'S PROCESS (1865).
Pour ammonia into a nitrate of uranium solution, wash the precipitate of uranate of ammonia in distilled water, then dissolve in citric acid.
Mix this solution of citrate of uranium and a little of a solution of chloride of gold with a paste prepared by dissolving tapioca in hot water. The quantity of chloride of gold must be small and the heat not too great, otherwise the gold would be reduced.
Spread the mixture with a sponge on the paper, which takes a brilliant yellow color, and expose when quite dry; the proofs have the delicacy and vigor of albumen prints.
The proofs come from the frame with a bluish-black color; they should not be toned, but merely fixed by washing until the yellow color of the paper has disappeared.
The color of the picture can be changed to a purple by a solution of chloride of tin.