How are Temperature and Relative Humidity related?

The greater the temperature, the more moisture the air can hold. There is no single range of relative humidity that is ideal for all artifacts. Different materials have different ideal relative humidities and temperatures. It has been shown that lowering the relative humidity and temperature will increase the life of plastics and other organic materials. Relative humidity should never fluctuate rapidly. Many museums have their relative humidity at 45%. Lowering temperatures greatly increase the longevity of collections, but are hard on museum visitors.

Changing relative humidities cause stress on materials and damages a larger variety of materials than does temperature, especially to hygroscopic materials. Seasonal drifts in relative humidity are less harmful than abrupt changes. High relative humidity can cause mold and metal corrosion and low relative humidity can cause materials to become brittle. It is important to maintain a relative humidity between 25% and 65% as humidity above 65% will cause mold to grow, and lower than 25% will cause artifacts to lose structurally important water.

Temperature is a major factor in the speed of natural aging. Materials last longer when kept as low temperatures. High temperatures increase deterioration reaction rates and melt heat-susceptible materials. The rate of decay is unacceptably fast at temperatures that humans find comfortable.

To predict how an item may deteriorate, we must know the composition, the type of collection, how the artifact decays, and how it is stored or exhibited. Every artifact requires an ideal environment. Temperature and relative humidity affect artifacts in three decay processes: chemical, biological, and mechanical. Chemical deterioration is also called natural aging, and is when a chemical reaction occurs and damages the artifact. Plastic and organic materials chemically deteriorate spontaneously, whose rates are determined by temperature and relative humidity. Chemical reaction rates increase with higher temperature, increased concentration of reactants and increased pressure. Temperature affects chemical degradation the most. Biological deterioration is damage caused by living organisms. Mechanical deterioration is related to either the amount of water absorbed by an organic artifact or thermal expansion in inorganic materials. These things cause the object to change in size and shape rapidly, warping, cracking and splitting the artifact. These consequences are amplified with artifacts that are made out of mixed materials. Different materials react differently with increased temperature or humidity, which would cause more warping and the materials to split apart.