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370 HISTORY OF COLOR PHOTOGRAPHY
C. Manganous ferricyanide, saturated solution 200 parts Potassium bromide, 10% solution 10 parts
After bleaching the silver, the print is washed and treated with :
Potassium ferricyanide, 2 % solution 9 parts
Sodium hydroxide, 4% solution 10 parts
In this solution an image composed of manganese oxide is formed. This is finally treated with the color formers.
The catatype process of Ostwald and Gros (Jahrbuch., Vol. 17 (1903), p. 519) operated along a similar idea. A silver image is toned so that it becomes converted into one of platinum. If this be brought in contact with a paper that is sensitized with pyrogallic acid and potassium bromate, the pyro will be oxidized in contact with, and to an extent proportioned to, the image density, forming a stain. Copper sulphate is also useful as a catalyst. If an image of silver or platinum is flooded with peroxide, this substance will be decomposed in situ with the metal. The other portions will remain intact. By this means a peroxide negative image is left. The peroxide can then be made to oxidize amines or phenols to form dyes.
E. R. Bullock bleached the silver image with a mixture of ferricyanide and chromic acid or permanganate (U.S. P. 1279248). By this means there was formed an image of silver ferricyanide. The reduced manganous salts could be removed by washing with a five per cent oxalic acid solution. Great care must be taken that no halides are present as otherwise silver halide will be formed. After a thorough wash, the bleached image is treated with amines. Benzidine gave a strong blue, ortho-tolidine or dianisidine gave a green, and paraphenylenediamine or alpha naphthylamine, a purple. If the bleached image be treated with manganous bromide, silver bromide and manganous ferricyanide would be formed, making the subsequent oxidation of the amines easier to carry out. The silver can be fixed out with hypo to give greater brilliance.
A similar idea is utilized by Dr. Bela Caspar (U.S.P. 1956017; Eng. P. 379679). The esters, ester salts or ethers of the leuco bases of dyes are incorporated into an emulsion. These are not affected by the operations of exposure, development, fixation, or wash. The silver image is next converted into a substance that is capable of regenerating the dye. A photographic emulsion is prepared to contain diacetyl indigo white. After exposure, development, and fixation, the silver is converted by well known means into lead chromate. As long as no free chromic acid is formed, no action will take place with the leuco dye. To accomplish the formation of lead chromate, the image is first treated with lead salts plus potassium ferricyanide in the presence of ions like citrates, lactates, etc. The lead ferrocyanide is then treated with neutral dichromates or chromates. When an image composed of lead chromate is acidified, chromic acid is formed, which reacts with the leuco dye to yield the insoluble blue dye, indigo. The unreacted leuco dye as well as the lead can be removed by treatment with fixing agents and acetone.