Fixation is a critical chemical stage in the traditional photographic processing sequence, in which a photographic film or print is immersed in a fixing solution - commonly known as fixer or hypo - that renders the developed image permanently stable and resistant to further change when exposed to light. Without fixation, the unexposed and undeveloped silver halide crystals remaining in the emulsion after development would continue to react to light, causing the image to darken and eventually become completely obscured.
During fixation, the fixing agent - most commonly sodium thiosulphate or ammonium thiosulphate - reacts chemically with the residual unexposed silver halides in the emulsion, converting them into soluble silver complexes that can be dissolved and washed away from the film or paper base in the subsequent washing stage. The developed metallic silver that forms the visible image is not affected by the fixer and remains securely embedded in the emulsion, leaving a clean, stable image against a clear or white background.
The duration of fixation is an important consideration in processing. Insufficient fixing time leaves residual silver halides in the emulsion, which will cause the image to discolour and deteriorate over time when exposed to light. Over-fixation, on the other hand, can begin to attack and bleach the developed silver image itself, particularly in prints, leading to a loss of density and shadow detail. Most fixers have a recommended working life measured by the number of films or prints processed, after which their effectiveness diminishes and fresh fixer should be prepared.
Following fixation, thorough washing in clean running water is essential to remove all traces of the soluble silver complexes and residual fixer chemicals from the emulsion. Incomplete washing can lead to long term staining and deterioration of the image, while proper washing ensures the stability and archival permanence of the finished negative or print. Hypo clearing agents are sometimes used between fixation and washing to accelerate and improve the efficiency of the washing process, particularly important for fibre based photographic papers which absorb fixer more deeply than resin coated materials.