THE EFFECTS OF RELATIVE HUMIDITY ON SOME PHYSICAL PROPERTIES OF MODERN VELLUM:
ERIC F. HANSEN, STEVE N. LEE, & HARRY SOBEL
This article is a discussion of the factors to be considered in determining an optimum level of relative humidity for the display and storage of parchment and other skin artifacts, based upon both a consideration of the physical chemistry and chemical reactivity of collagen and upon physical tests of modern vellum over a range of relative humidities. The Charters of Freedom of the United States (the Constitution and Declaration of Independence) and the Dead Sea Scrolls will be used as examples for this discussion. Indeed, the arguments presented for the optimum level of relative humidity for display and storage, 30% RH, do not differ significantly from those proposed by the National Bureau of Standards for the Charters of Freedom in 1951 (NBS 1951). The effects of light and concentrations of air pollutants on the deterioration of collagen, although important factors that are in all probability also dependent upon the concentration of oxygen and atmospheric moisture, are not included in the scope of this discussion.
In a review of storage conditions for archives and libraries, Wilson (1986) noted that conditions for parchment and vellum could not be discussed in as concise a way as for leather and paper because systematic data on the physical properties of parchment and the degradation rates of parchment over a wide range of relative humidities were not available. In an effort to alleviate this situation, data were acquired and are reported here for the following physical properties of three types of modern vellum over a range of relative humidities from 11% to 60%:
- tensile properties of equilibrated samples fractured at a constant rate of elongation
- a measure of the force developing in a restrained sample that is exposed to successively lower relative humidities.