Photographic fixer
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 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 severe 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.[1]
Fixer is used for processing all commonly used films, including black and white films, Kodachrome and chromogenic films - colour and black and white C41 negative films and colour reversal films E6. In chromogenic films, the remaining metallic silver must be removed by solution in a chemical called a bleach fix or blix, which contains a mixture of ammonium thiosulfate and ferrous EDTA, a powerful chelating agent.
Washing
A 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"[2]:
- 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 non-hardening fixers are used.
Over-washing 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.[3]
References
- ^ Sowerby (Ed.), A.L.M. (1961). Dictonary of Photography: A Reference Book for Amateur and Professional Photographers. London: Illife Books Ltd.. pp. 324–326.
- ^ Ilford Rapid Fixer Fact Sheet, August 2002
- ^ Washing - Silvergrain Labs
Related
- Film developing