Low relative humidity causes the binder layer to shrink and crack and the photograph to curl. High temperatures speed the rate of deterioration of photographs, especially at high levels of relative humidity. Fluctuations in temperature and humidity are especially damaging, causing the movement of moisture in and out of a photograph. This, in turn, speeds the rate of chemical deterioration of the support layer and promotes the breakdown of the binder layer. The combination of high temperature and humidity may contribute to mold growth.
Mold digests the paper support and gelatin emulsion and can cause stains and image loss.
By measuring the relative humidity and temperature in various parts of your home, you can determine the best places to store photographs. In general, it is best to avoid damp basements and garages, hot attics, or locations near air vents, radiators or other heat sources. The paper is impregnated with cobalt chloride, a cobalt salt, and other inorganic salts that change color with variations in moisture content. Paper humidity indicators are reasonably accurate and fairly long lasting, although their accuracy can be affected by dust and dirt and will decrease over time. Light Excessive exposure to light can cause chemical changes in all of the layers of a photograph, especially the image layer.