Photographic fixer is a mix of chemicals used in the final step in the photographic processing of film or paper. The fixer stabilises the image, removing the unexposed silver halide remaining on the photographic film or photographic paper, leaving behind the reduced metallic silver that forms the image, making it insensitive to further action by light. Without fixing, the remaining silver halide would quickly darken and cause fogging of the image. The most common salts used are sodium thiosulfate—commonly called hypo—and ammonium thiosulfate—commonly used in modern rapid fixer formulae.
Fixer is used for processing all commonly used films, including black-and-white films, Kodachrome, and chromogenic films. In chromogenic films, the remaining silver must be removed by a chemical called a bleach fix, sometimes shortened to blix. This contains a mixture of ammonium thiosulphate and ferrous EDTA, a powerful chelating agent.
After fixation, washing is important to remove the exhausted chemicals from the emulsion, which cause image deterioration if left in place.
- Sowerby, A.L.M. (ed.) (1961). Dictionary of Photography: A Reference Book for Amateur and Professional Photographers. London: Illife Books Ltd. pp. 324–326.
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