It will be easier to understand why photos deteriorate if we understand what makes up the photograph itself. Support - This is the backing of the photograph.
Old photographs can have a support of glass, tin, and other solid materials.
Current photos use a paper based backing and though much more convenient, is one of the main culprits in a photograph deteriorating. Binder - This is a gelatin like material that holds the final image to the support. Final Image - Made of silver, color dyes, or other materials and is held in the binder layer. Please keep in mind that the three components we just discussed and the photographic process that was used will determine how long the photograph prints will last and what you can do to give them some longevity. Like say an attic or a cellar, if so, this can be a very bad place to store them.