the all-seeingeyeThreats to Photographs: Light and Heat

Light and Fading

The ultraviolet component of ordinary daylight accelerates any number of reactions-most notably the one which breaks down cellulose molecules in the presence of water and mildly acid compounds. UV light also affects dyes and many pigments. It is the UV light which is largely responsible for fading of pictures hanging on the wall. The color shift that occurs at the same time is due to different dyes being more or less susceptible to UV light. Long-term storage of photographs should be in a dark place.

Heat

I've spoken of the damage which can occur when paper is exposed to certain chemicals and moisture. This damage gets faster the higher the temperature. A good general rule is that the speed of a reaction doubles for every ten degrees centigrade temperature increase.

A photograph damaged by high heat.

Heat is also a problem in itself. Photographs have two (or more) layers. There is a backing-usually paper, and an image layer. These are made of significantly different materials and expand and contract at different rates as the temperature changes. This is what makes photographs curl. In extreme cases-especially if very dry as well, heat can causes cracking and peeling of the image layer.

Disclaimer: I'm neither a chemist nor physicist, I only play one on the web.