Potassium ferridcyanide ... 10 per cent, solution.
Ammonium sulphocyanide 50 per cent, solution.
For the toning bath, to 1,000 parts of water add first 5 parts of A, then 2 parts of B, and 5 parts of C. The solution should be of a red colour, and is used similar to the above-mentioned uranium bath. The above physical uranium and iron toning is very easily carried out, and the desired tones may be obtained. In preparing the pictures, however, it must be remembered that these baths have an intensifying action. The prints before toning should have soft delicate half-tones, pure whites, and not too deep shadows; in no case should they be too brilliant, otherwise after toning they will be hard. Soft, harmonious, and somewhat thin negatives on soft-printing platinotype paper are the most suitable. If the toning process is a failure, or the desired tone is not obtained, the prints may be restored to their original condition by treatment with dilute ammonia. Any slight yellow stain may then be removed by dilute hydrochloric acid. The ferrocyanide compounds of iron and uranium with-stand the. action of acids, but are removed by alkalies; therefore the toning bath must be faintly acid, and the. washing of the toned prints must not be done by ordinary water containing lime or alkalies. It is in any case advisable to add a few drops of acetic acid to the washing waters to prevent the removal of the toning. Platinotypes may be generally tinted with solution of aloes, catechu, infusion of coffee or other dark-coloured organic liquids; and as the true platinum image is remarkably per-manent even against hypochlorous acid, these, or platinotypes which have become yellow by age, may be bleached and restored by the process described under Bleaching Prints or Engravings.
Mr. Silberer combines platinotype with gum-bichromate by first making a platinotype in the ordinary way, and then coating with a gum-bichromate mixture, and again printing for special effects, see Gum-Bichromate.
This is a process due to Mr. Alleyne Reynolds. No distinct formulae are given, but the operations are thus described: -