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CHEMICAL TONING 309
loss of definition. This will take place if the silver is removed much faster than precipitation can take place.
After a thorough wash, the cadmium ferrocyanide is converted into the sulphide by treating with:
Sodium sulphide 5 parts
Sodium bicarbonate 7 parts
Water to 100 parts
A yellow image of cadmium sulphide is formed. Any yellow stains in nonimage portions that are not caused by deposition of cadmium sulphide, can be removed by treatment with a one and one-half per cent solution of hydrochloric acid. This is the yellow that is used in the Chromatone process, which is based upon the disclosures of Snyder and Rimbach. More will be said concerning this, below.
The magenta or red image can be prepared directly by toning with uranium. This substance was first used in 1859, when J. C. Burnett (Eder Handbuch, IV, 4, (i929)j P *59) prepared prints by exposing a sheet of paper sensitized with uranium salts, under a negative. The light reduced the uranium. The image was developed by treating this with ferricyanide, silver nitrate, gold thiocyanate, etc. When potassium ferricyanide was used as the developer, a reddish brown image resulted, that was much more suitable for two-color photography than for three.
A comprehensive study of uranium toning was published by Dr. Sedlaczek (Phot. Ind., (1924), p. 234; Am. Phot., (1925), p. 8). The tones obtained vary from a dark brown to a bright brick red. The brown tones are obtained from a solution containing a minimum of oxalate,
Uranyl nitrate 5 parts
Ammonium oxalate 10 parts
Hydrochloric acid, 10% 10 parts
Potassium ferricyanide 2 parts
Water to 1000 parts
By increasing the oxalate content to a maximum, such as the use of 100 parts of saturated oxalic acid, the tone becomes much brighter. He recommended especially the following formula:
Uranyl nitrate 5 parts
Potassium citrate 5 parts
Potassium ferricyanide 2 parts
Ammonia alum 10 parts
Hydrochloric acid, cone. 0.3 part
Water to 1000 parts
Since hydrochloric acid is used, the silver is converted into silver chloride. Hence fixation should follow the treatment. If cobalt salts be added to the hypo, colder tones are obtained, possibly by the formation of some cobaltous