Ellen Carrlee
Abstract
Preventive freezing for pest control during the relocation of the ethnographic collections of the National Museum of the American Indian afforded the opportunity to undertake an observational study of the potential damage to vulnerable categories of materials and to investigate the possible causes. The observational study revealed no visible damage to any of the materials frozen, although minor changes on a molecular level are likely. The effects of changing relative humidity and water relationships conventionally thought to be the biggest threat to objects in the freezer seem to be extrapolated from room temperature observations under circumstances distinct from the freezer environment. These moisture issues are less of a threat than are effects related to low temperature alone, such as shrinkage, embrittlement, and molecular alteration. While many of these changes are reversible upon warming, the danger of cumulative effects from repeated preventive freezing of objects is questioned.
The time honored conservation tradition of borrowing information from other fields proves difficult to apply to a low-temperature, low-moisture content closed system. This study contributes to an informed approach for the freezing of composite objects, cracked objects, lamellar objects, and waxy/oily objects. Concepts of condensation, moisture content, freezer burn, concentration effects, glass transition temperature, coefficient of thermal expansion, polymorphism, lipid autoxidation, protein denaturation, ratcheting and shakedown are reviewed.