EFFECT OF “FREEZING” TREATMENTS ON THE HYDROTHERMAL STABILITY OF COLLAGEN
STEPHEN L. WILLIAMS, SARAH R. BEYER, & SAMINA KHAN
Subjecting objects and specimens to temperatures below 0°C has become a common conservation treatment in museum settings for pest control (Florian 1990a; Strang 1992), stabilization of water-saturated materials (Jakes and Mitchell 1992; Koesterer and Geating 1976), temporary preservation of unprepared biological materials (Dickerman and Villa-R. 1964; McConachie 1993), and long-term preservation through freeze-drying procedures (Hower 1970). However, the implications of this process, often referred to as “freezing,” are misleading because water molecules may or may not be in a solid state, depending on the materials and temperatures involved (Florian 1990b). In this paper, “freezing” refers to the process of subjecting materials to temperatures below 0°C, not to the inducing of a phase transition of water molecules.
Although various freezing treatments have been widely and successfully used in museums, it seems that the decision to use this treatment is based on theory or on information extrapolated from a few obscure sources and often does not take its effects into account. Florian (1990b) provides a good review of the current state of knowledge about freezing museum materials. For some materials, however, there is a need for basic, systematic study of material reaction to various freezing treatments.
This need became clear in a recent emerency in which dried skin material had become water-saturated and required treatment. Would freezing treatments for wood, paper, textiles, and even leather be applicable for water-saturated skin material that was normally preserved in a dry condition? The current study analyzes the temperature at which collagen shrinks to gain a better understanding of how fresh and dried skin material responds to freezing treatments. The information gained is relevant to supporting or refuting the wisdom of current freezing practices in museum settings.
Collagen is the most common protein in mammal skin and leather. Thus treatments for its preservation are relevant to collections of history, clothing and textiles, ethnography, and natural sciences as well as libraries. The collagen molecule consists of three left-handed helices twisted together in a right-handed direction. These helices are linked by hydrogen bonding. Under normal conditions high temperatures (65–75°C) are required to break these bonds and cause the collagen to collapse or shrink (Haines 1987). However, the shrinkage temperature (Ts) may be reduced, indicating a loss of collagen stability, by degradation processes and by some preservation treatments. Monitoring Ts change resulting from different treatments provides a mechanism for evaluation (Young and Grimstad 1990; Williams 1991; Larsen et al. 1993).