A prehardener is a chemical solution applied to photographic film or paper to strengthen the gelatin in the emulsion before development. By hardening the emulsion, it becomes more resistant to physical damage, swelling, and uneven chemical action during processing.
Prehardening is especially important in traditional black-and-white and colour photographic techniques, where precise control over emulsion integrity ensures consistent image quality. Using a prehardener helps prevent scratches, streaks, and other processing defects, contributing to sharper, more durable photographic prints.