Alum is a common chemical used in photographic processing, particularly in acid hardening fixing baths for film and paper. It functions as a hardening agent, strengthening the gelatin emulsion and helping it resist damage during subsequent handling, washing, or drying. Alum is essential for maintaining the durability, longevity, and archival quality of negatives and prints, while also contributing to even development and consistent image quality. Proper use of alum ensures that photographic materials remain stable, minimizing the risk of emulsion softening or scratching in both black-and-white and color processing workflows.