Photographic fixer is a chemical used in the final step in the photographic processing of film or paper. The fixer removes the unexposed silver halide remaining on the negative or photographic paper, leaving behind the reduced metallic silver that forms the image. By removing the unexposed silver halide, the fixer prevents any further reaction of the silver salts and ensures a permanent image. If the film or paper was not fixed, the remaining silver halide would quickly darken and cause severe fogging of the image. The film or paper can be exposed to light after fixing.
Fixer is used for processing all commonly used films, including black and white films, color negative films (C41), color reversal films (E6), and chromogenic films.