Fixer is a chemical that makes a photographic image permanent. Without it, the image would continue to develop until it turned black. Fixer works by dissolving the unexposed silver halides in the emulsion, leaving only the metallic silver that forms the image. It is the third step in black-and-white processing, after developer and stop bath. The film or paper is immersed in fixer for a set time, then washed to remove residual chemicals.

Fixer has a distinctive smell, often described as vinegary, because it contains acetic acid or ammonium thiosulfate. It exhausts over time as it becomes saturated with silver. Testing with a hypo check solution tells you when it is spent. Fixer can be reused, but not indefinitely. It must be disposed of properly, because it contains silver and other chemicals that are harmful to the environment. Some photographers use rapid fixers, which work faster and are more convenient. Others prefer traditional fixers for archival quality. The choice matters for prints that are meant to last decades.

Fixer facts

• Purpose. Removes unexposed silver halides.
• Position. Third step after developer and stop bath.
• Testing. Hypo check solution reveals exhaustion.
• Disposal. Must be handled as hazardous waste.

Fixer is not glamorous. It is essential. Without it, every photograph would eventually disappear.