Background: The terms “parchment” and “vellum” are often used interchangeably. This investigation uses the "vellum" to remain consistent with the enduring use of the term for the binding structures under study, specifically, limp-vellum and stiff-board vellum bindings.
Parchment/vellum is a material found in many special collections in libraries, archives, historical societies, and museums. Western maps, deeds and other important documents, medieval manuscripts, and bindings are often made of vellum. True vellum is animal skin that has been altered (but not tanned) through chemical and physical means to resist putrefaction. It can be made from different animals (but is usually goat, cattle, or sheep) and traditionally has been produced in different ways in various regions and times. These differences may result in disparate aging and reaction characteristics.
Most important to this investigation is the extremely hygroscopic nature of vellum, which can cause vellum to deform rapidly, radically, and irreversibly in response to changes in relative humidity. For example, a vellum map that is secured to its mat or a vellum covering on a book may shrink and subsequently break or warp if the environment becomes too dry. Storage of books with vellum bindings pose particular challenges since optimal environmental conditions for the long-term preservation of paper is considered too dry for vellum. In addition, significant dimensional changes may occur in vellum bindings exposed to wide fluctuations in relative humidity, such as 25% and 60% RH.
Hansen, E. F., and H. Sobel. “Factors to be Considered in Determining the Optimum Relative Humidity for the Display and Storage of Parchment.” In Postprints of the Book and Paper Group Specialty Session, 29th Annual Meeting of the AIC, Albuquerque, 1991. Washington, D.C.: American Institute for Conservation, 1991.
Project Description:The overarching goal of the Library’s current project is to determine optimal storage conditions for vellum materials in order to keep distortion to a minimum, in particular without placing additional stress on auxiliary supports, especially binding structures. The Preservation Research and Testing Division has been creating a series of naturally and accelerated aged parchment samples from a range of animal vellums, in a matrix of different environmental conditions. The environments include temperature, relative humidity, visible and ultra-violet light, and various treatments. This will enable a better understanding of the degradation mechanisms that occur under different environmental conditions. A further component will include a series of controlled tests exposing various types of selected vellum substrate samples to changing RH at constant temperatures, noting any occurrence of planar distortion. Subsequent experiments will determine the effects of variables, such as boxing to protect samples from distortion in fluctuating conditions, and the effects of RH on different binding structures.