Stability of silver images

Film images are composed of silver, silver salts, a number of other metallic salts, or organic dyes. Their stability with time is dependant largely on the inherent stability of the image chemical and its ability to withstand extremes of use or storage. This makes the image depend on the history of the film, and in particular:
1 - How the film has been processed in the printing laboratory. The most decisive factor in conservation, particularly of silver images, is how effectively it has been washed. Black and white images of silver are very stable, and the image is probably considerably more stable than the base or the emulsion matrix.
One major problem can cause destruction of this silver image, and lead to an image life less than that of the base.
The most common chemical used for fixing, the removal of the unused silver salts from processed film, is sodium or ammonium thiosulphate in an aqueous solution called "fixer" or "hypo".
Over the short period of the process these salts have no effect on the metallic silver image, but if the salt is not washed out of the gelatine layer in the subsequent water washing stage and even small quantities remain, in time the silver image fades as the silver is converted to a low-density thiosulphate salt.

In some instances the fading caused by hypo may be corrected by bleaching [with a ferricyanide colour bleach back to a white halide image and redevelopment. This is not as successful in every case but does significantly smooth out the blotchiness of some of these faded images

The importance of washing cannot be over emphasised. Provided the fixer salt is reduced to a threshold level, fading is virtually prevented. Some laboratories use a separate process stage called a scavenger or hypo eliminator to ensure the removal of all the fixer salts in black and white processes for archive films. Chemical analysis of film after processing can show how much fixer salt remains, and this is sometimes used to test the effectiveness of the washing process. Some of this problem is dealt with in the Processing section, including mention of chemical and physical techniques to improve the washing process.