Relative humidity

The proportion of actual moisture in the air to the maximum amount possible at a specified temperature. It is stated as a percentage, with 100% being air fully saturated with water vapor. Control very important in art conservation because when relative humidity is either too high, too low, or changes dramaticly, relative humidity can damage many types of artworks. Organic materials absorb or give off moisture in a continuous attempt to achieve equilibrium with the atmosphere. These materials tend to be more stable in a moderate relative humidity (45-55%), a condition rarely present in normal exterior or interior environments. Inorganic materials usually are not affected by relative humidity levels unless they contain salts or are otherwise unstable. Metals, however, are best preserved at low relative humidity. Serious damage can occur when materials are subjected to dramatic, sudden changes in relative humidity over short periods. Prolonged exposure of organic materials to relative humidity above 60-65% will encourage the growth of molds and fungi.Also see climate control, condensation, foxing, hygrothermograph, museum, silica gel, and thymol.