Bleaching, Photographic

Bleaching, Photographic


an intermediate stage in various treatment processes for color and black-and-white photographic materials, during which the metallic silver that forms the photographic image is oxidized by oxidizing agents that do not destroy the gelatin in the emulsion layer. As a result of photographic bleaching, metallic silver is converted into poorly soluble white salts or into soluble substances that are removed from the emulsion layer during subsequent processing. The most commonly used oxidizing agents are potassium ferricyanide, potassium dichromate and permanganate, ammonium persulfate, and mercuric chloride. The products of reduction of the oxidizing agents may be used to tan the gelatin in the emulsion layer. Tanning bleaching is used, for example, in the production of plates for imbibition color printing.

Treatment of monopack film with potassium ferricyanide leads to oxidation of all the metallic silver to silver ferricyanide, which in the process of photographic fixation forms a readily water-soluble compound with sodium thiosulfate that is washed out of the emulsion layer. Reversal of black-and-white photographic materials begins with the stage of photographic bleaching—the oxidation (by potassium dichromate or other oxidizing agents in the presence of sulfuric acid) of the metallic silver that forms the negative image into silver sulfate, which dissolves in water and is removed by washing. An increase in the optical density of photographic negatives and positives is achieved by intensification, which consists of two stages, photographic bleaching and blackening. During the first stage the metallic silver that forms the photographic image is oxidized by mercuric chloride or other oxidizing agents to give white salts of silver, which are blackened during the second stage by ammonia or a developer. Highly disperse mercury formed by the reduction of mercuric chloride is deposited on the blackened silver salts, thus increasing the intensity of the image.