What is residual thiosulfate and why is it important?
Thiosulfate, also called fixer or hypo, is a chemical used in processing photographic film. It fixes, or makes permanent, the image, hence the name fixer. However, if this chemical is not washed completely from the film the residual thiosulfate can cause problems. Stains can occur on inadequately washed film images, the image contrast is reduced significantly, and the images will fade in a relatively short period of time. Consequently, it is essential to wash the film according to the film manufacturer’s directions in fresh water. The water should be changed daily, or more often depending on the volume of films processed, in tanks with no water flow.
