Raw silk, which contains the proteins sericin and fibroin, absorbs atmospheric water to a greater extent than degummed silk in which most of the sericin has been removed. Changes in relative humidity affect the mechanical properties of raw silk to a greater degree. Both raw and degummed silk exhibit increased loss of tensile strength and elongation when exposed to relative humidity (RH) above 40 to 50% RH. This has been demonstrated in raw silk by long-term storage at ambient conditions, and in degummed silk through accelerated aging. The deterioration rate of raw silk is expected to be higher than degummed silk at these elevated relative humidities because of the greater hygroscopicity of raw silk. Studies of the condition of Edo Period (1596-1644) Japanese silk seem to demonstrate this effect historically. These studies indicate the need to store silk at relative humidities below 50%. However, if we are to fully understand and implement preventive conservation, we need to analyze the chemical reactivity and structural changes in silk caused by exposure to water, heat, and radiant energy so as to understand better the modifications occurring on the molecular level. Historic silks should reveal a wide variety of deterioration rates and mechanisms because different methods for degumming, reeling and weaving, dyeing, weighting, finishing, and past environmental exposure can produce changes in chemical composition and structural integrity. -- AATA