THE EFFECTS OF RELATIVE HUMIDITY ON SOME PHYSICAL PROPERTIES OF MODERN VELLUM:
ERIC F. HANSEN, STEVE N. LEE, & HARRY SOBEL
This study has sought to determine the optimum relative humidity for objects containing collagen as a principal component. Decreasing the water content reduces both the possibility of biodeterioration and the probability of future environmentally induced chemical modifications of collagen. A lower limit has been discussed for collagen; storage or display below around 25% RH is not indicated. Because of possible cycling effects of ± 5%, a slightly higher value, 30% RH, is suggested as the optimum condition for an object for which the long-term preservation of the intact collagen is of greatest concern.
However, there are other considerations, particularly in regard to the degree to which a skin object must be manipulated or in determining the possibility of the loss of inks or colorants. In this regard, determining the optimum relative humidity for remains derived from skin has some parallel in the history of conservation to determining safe levels of illumination. When determining the optimum light levels for the display of light-sensitive objects, the illumination recommended is the lowest that will allow the required visibility. In the case of remains derived from skin, the optimum relative humidity is the lowest amount of atmospheric moisture (above 25% RH) that will allow for mechanical requirements, the consideration of other composite elements, and the aesthetic requirements.
Previous discussions of optimum relative humidity levels for parchment that have recommended 50% RH or higher have focused primarily upon the pliability and other physical properties of parchment. The work just reported has shown that for organic materials, such as proteins, a knowledge of the chemical changes that occur over time may be as important as a knowledge of the physical properties in determining the most reasonable display and storage conditions.
It can be stated that the conditions for maintenance of collagen containing objects should be geared either for the intent of its exhibition or use (long-term display or short-term handling), keeping in mind that relative humidities below 25% are incompatible with long-term maintenance, as are relative humidities above 40%, which increase hydrogen bond breakage, gelatinization, and biological growth.
The contents of this paper have also been published in the AIC Book and Paper Group Annual (1992). We would like to thank Neville Agnew, Mary Chase, A. David-Baynes Cope, Michele Derrick, Robert Feller, Mary-Lou E. Florian, William Ginell, Betty Haines, and David Scott for their comments and suggestions in preparing this manuscript; Andrew Kim and Raphael Garcia for assistance in the laboratory; Jack Gromek for software support in data collection and analysis; and Jim Davies for assistance in manufacturing the testing chamber.