Generally speaking, higher temperatures accelerate the chemical reactions that cause deterioration in paper-based collections. Thus, cooler temperatures are preferred for collections with long-term value.
Temperature plays a particularly crucial role in the fading of chromogenic color photographs. The higher the temperature, the more movement there is on the molecular level, and the more likely it is that the molecules will be rearranged, resulting in dye fading. For this reason, cold storage is recommended for long-term preservation of color photographs.
The rate of most chemical reactions doubles with an increase of 18 degrees F.
Humidity (moisture in the air) provides water to fuel the chemical reactions that cause deterioration. The more moisture there is in the air, the more quickly chemical deterioration proceeds.
High humidity is a primary factor in hydrolysis, which accelerates the deterioration of paper (causing ferrotyping, shiny blotches on the emulsion that result from pressure on the softened binder material; and accelerating), black and white photographs (softening gelatin binders, causing ferrotyping and accelerating the yellowing of albumen binders), and color photographs (accelerating dye fading). Parchment will ripple in overly humid conditions. Mold and many insects also flourish under hot and humid conditions. Mold causes staining and, in extreme cases, disintegration of paper, while insects can eat holes in paper-based collections.
If humidity is too low, materials such as adhesives and book leathers may become brittle and more susceptible to cracking, particularly during handling. Extended storage at low humidity can cause photographic emulsions to shrink and crack, supports to curl, and mounts to warp. Parchment will split and warp in overly dry conditions.
Fluctuating humidity levels also cause damage. Most materials expand and contract with small changes in relative humidity (RH). These changes weaken physical bonds and set up stresses that can shorten the life of most materials.
Always keep in mind that temperature and relative humidity are interrelated. Given a specific amount of moisture in the air, relative humidity will decrease if the temperature increases. If the temperature falls, the relative humidity will rise.