Final answer:
The film emulsion is hardened during the fixing stage, which makes the image on the film permanent by removing unexposed silver halide crystals.
Step-by-step explanation:
The film emulsion is hardened during the fixing stage of the photographic development process. This step comes after development and rinsing and involves submerging the film in a fixer solution, which removes the unexposed silver halide crystals from the emulsion, making the image permanent and insensitive to light. The fixation process thus ensures that the film will no longer react to light and can be viewed under normal lighting conditions after drying. Photographers have historically faced challenges with various development processes, such as the collodion process, which required the film to be exposed and developed while the chemical coating was still wet. The images on film are created by silver reduced from radiation-sensitive silver compounds in the film emulsion, a property utilized in both historical and modern photographic techniques.