While the glass is fragile, the emulsion is usually very stable, rendering beautifully detailed prints. Most 19th-century prints were made on very thin paper but were then mounted on cards or mounts in a variety of sizes. While the mounting board was not acid-free, the paper used then was less acidic than what was later produced. One of today's most challenging preservation questions is how to save the multitudes of negatives produced during the 1920s through the 1950s on nitrate and acetate film stock. Color photographs are even less stable than black and white prints, as the layers of dye tend to fade unless they are kept at near-freezing temperatures.
Photographs are organic documents that interact with their environment.
A binder is used to adhere the emulsion layer to the base. Each of these layers can interact with moisture, heat, light, and chemicals to degrade or destroy the photograph. In addition, some photographic prints, including modern RC prints and negatives such as nitrate film and acetate film, contain agents of self-destruction that cause them to emit gases that accelerate deterioration in photographs stored nearby. Physical damage or destruction from fire or flood are obvious dangers, but the effects of improper handling also can have devastating consequences. Restoration is a highly specialized treatment performed only by a professional conservator who attempts to repair a damaged item or return it to its original condition.