Photographic fixer

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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.

Most fixers are based on the thiosulfate ion, especially ammonium thiosulfate. Up until the 1970s, sodium thiosulfate or 'hypo' was the commonly used fixer. Both fixers work best in acid conditions and this is usually created using small quantities of acetic acid.

In the case of chromogenic (color) films, the elemental silver left behind after development must be subsequently removed by solution in a chemical cocktail called a bleach fix or blix. This contains a mixture of ammonium thiosulfate and ferrous EDTA, a powerful chelating agent.

[edit] Washing

One disadvantage of the use of thiosulfate as a fixer is its ability to dissolve elemental silver at a very slow rate. If films or papers are inadequately washed after fixing, any residual fixer can slowly bleach or stain the photographic image. For prints on high grade fibre papers, a period of continuous washing in clean, cold water for up to 40 minutes may be required. For modern plastic (resin) coated papers, washing for as little as 2 minutes in warm water can be sufficient to eliminate residual fixer. Washing aids (also called hypo clearing agents) can be used to make the process of removing fixer faster and more thorough.

A quick, water-saving, and archival technique for washing film fixed with nonhardening fixer in a spiral tank is the popular "Ilford method"^[1]:

Fill the developing tank with tap water at the same temperature as the fixer (+/-5 ºC or 9ºF)—maintaining a constant bath temperature during processing is necessary to avoid reticulation of the emulsion;
Invert the tank five times and drain it completely;
Fill the tank again, invert it ten times, and drain it completely;
Fill the tank again, invert it twenty times, and drain it completely.
The film is now washed.

More conventional darkroom practice recommends washing film for 30 minutes or longer, with a flow of water sufficient to change the water in the washing container at least three times. This is not needed when nonhardening fixers are used.

Overwashing can actually reduce the archival properties of film, as thiosulfate in very small concentrations has been shown to have a beneficial effect on film image stability.^[2]