3.1.5 Film and related materials
All photographic materials have in common a substrate of gelatin which carries the emulsion of silver halide particles that produce the image. The film base may be nitrate, acetate, polyester, glass or paper and the format may be a negative, a photograph, or a reel of microfilm, but all have a gelatin layer. As with the gelatin sizes used in paper, photographic gelatin provides a nutrient for mold growth, which can penetrate the emulsion layer, damaging the image. The polymers that provide the base for contemporary film stock are generally very resistant to fungal attack,6 however paper and glass supports are both vulnerable. Glass plate negatives can actually be etched by fungi, and combined with the damage to the silver halide layer, can render the negative completely useless.
Gelatin is relatively stable as long as it is kept dry. In high humidities gelatin begins to swell and if exposure is prolonged, becomes sticky. This can occur at relative humidities as low as 60%.7