The detrimental effects of incorrect temperature (either too high or too low) are often observed after considerable time has passed and so the slow deterioration that results is often underestimated. Temperature is a measure of heat energy. High temperatures promote faster chemical reactions and so the overall degradation of organic materials occurs more quickly. Temperature is also important in that it directly affects relative humidity, given the same moisture in the air. If the temperature goes up, the relative humidity goes down and vice versa. A fluctuating temperature leads to a fluctuation in relative humidity.
- Enough heat should be provided to keep the relative humidity below 60% to prevent mould and discourage infestations and corrosion.
- A stable temperature helps prevent relative humidity fluctuations and the damage they cause. The optimum temperature range for museum objects is often given as 68-72 degrees F.
- High temperature can cause softening of meltable materials like waxes and resins, and general aging of organic materials, which leads to embrittlement and weakening.
- High temperatures can also promote faster desiccation of organics leading to loss of flexibility and cracking. Composite objects (those composed of two or more types of materials) have different rates of thermal expansion and contraction and this can cause physical damage resulting in cracks and splits along join lines.
- Generally, cooler temperatures are beneficial to most artifacts. Any temperature above that required for human comfort can be viewed as too high. However, while cold storage is a good preservation technique for some types of unstable collections, and most collections would benefit from cooler temperatures than normally used in museums, temperatures that are too cold can cause embrittlement, hazing, cracks in other types of artifacts, in particular those with resins and varnishes.
- Cultural institutions often have to compromise between the temperatures that are best for the preservation of the collection, what is economical in terms of equipment and energy policy, and what is best for the comfort of staff, and visitors.