One of a series of seven articles forming a practical guide to HVAC for museums and historical renovation. Discusses the principal effects of moisture and temperature on mechanical and chemical stability of collections. Considers the effects of relative humidity on mechanical properties of cultural materials and explains with examples how their mechanical properties can be measured by conducting tests at extremely slow strain rates. Notes the effects of the rate of change of relative humidity on materials and the effects of temperature on the mechanical properties. Based on chemical considerations, illustrates in a diagram the safe allowable environmental ranges for photographic materials. Gives an equation for the rates of chemical reaction of museum objects. Notes deterioration by hydrolysis and cross-linking reactions. Considers permissible ranges of relative humidity and temperature, based on a multi-year effort to study the effects of the environment on materials and objects in collections.

Article appeared in the Practical Guide to HVAC for Museums and Historic Renovation, which was included with the vol. 41, no. 4, April 1999 issue.