Environment - Heat
Heat makes materials age faster. In fact, there is a rough rule, the Arrhenius equation, which states that for every increase of 18 degrees °F (10 degrees °C) the activity rate of most chemical and physical processes doubles. For paper the situation is even worse: those activities which lead to the breakdown of paper double with every increase of about 10 degrees °F. Oxidation and hydrolysis reactions speed up, as do the rates of physical processes, such as evaporation of water, which leads to embrittlement. Mold growth and other biological activities also accelerate.
These are aging processes. In conjunction with the other four factors (light, high RH, pollution and biological agents) high temperature increases the degradation rate of organic materials.
Heat is also an indirect damaging agent in that changes in temperature affect changes in relative humidity. A spot light trained on a work during the day, for example, will heat up the interior of the frame, and lower the RH inside the frame. In tests we performed at Bark Frameworks we found the differences to be significant. At night when the light is switched off, the RH will quickly rise, causing hygroscopic materials to swell. The next morning they will contract again. The dangers of humidity cycling have already been described.