Darkroom Tips: What Fixer Does.
After the developer has done its work the black and white image on the negative is visible but very perishable. If exposed to light it vanishes into total blackness. To prevent this, the fixer sets the black and white image permanently on the black and white film and prepares it for black and white printing - if it is used properly.
The fixing agent is needed because the negative black and white image is surrounded by the leftover crystals of silver bromide; since they represent shadows in the scene, they were not exposed and they should not be converted to dark metallic silver. But, if allowed to remain, these leftover crystals will darken in the light and black out almost the entire black and white image. The fixer prevents this by getting rid of the unneeded crystals. It dissolves them out of the emulsion.
Finding the right solvent to accomplish this took a third of a century and was a major stumbling block in the early evolution of black and white photography. The man who solved the problem was Sir John normal negative Frederick Herschel, the famous English astronomer and an early black and white photographic chemist. In 1839 he suggested using sodium thiosulfate, and no better fixer for general use has yet been found. For a long time black and white photographers thought they were using sodium hyposulfate, and hypo is still a common name for fixer.
One other important substance, although not technically part of the fixing operation, is added to the solution. This is the hardener compound, usually potassium alum, which prevents the black and white film emulsion from becoming so soft or swollen that it is damaged during the washing that follows fixing. Under normal conditions the fixer dissolves only the unexposed crystals of silver bromide and does not affect the metallic silver making up the black and white image itself. But if the black and white film is left too long in the fixer a side reaction converts the silver into a compound that will, in time, cause the black and white image to dissolve. This upper limit to the fixing period is quite long - about three times the recommended maximum.
Ordinarily the danger is not too much fixing but too little. If the fixer is not given sufficient time to complete its work, silver bromide crystals are left undissolved and they later darken. Often, the fault is simply repeated use of the same solution: a given amount of fixer can dissolve only so much silver bromide. Then it stops working.