Risk of climate-induced damage in historic parchment

Abstract

AbstractMoisture adsorption and related dimensional change were examined in several samples of historic and contemporary parchment. The tensile behaviour was determined for contemporary parchment and two selected historic materials. The moisture-related data for most parchments are close to the contemporary material while aging and past treatments may lower adsorption of moisture and, in consequence, the dimensional change induced by changes in the moisture content. Contemporary parchment exhibited larger water vapour adsorption and moisture-related response compared to most historic materials and, therefore, can be regarded as the worst-case material in terms of the climate-induced risks to parchment. Tensile parameters of parchment varied significantly with increasing relative humidity (RH). Elasticity modulus declined from on average 1200 to 400 MPa and strain at failure doubled when RH increased from 30 to 85%. Parchment’s critical strain at which permanent deformation occurred decreased dramatically with increasing RH reaching zero at 80%. Irreversible curling produced by variations in RH to which flat parchment specimens were subjected were measured by scanning the specimen surface with the use of a laser triangulation sensor. The degree of curling was expressed quantitatively as standard deviation of local curvatures in the parchment sheet. The study opens a perspective of using the relationship between degree of curling and magnitude of RH variations to derive categories of risk to parchment from indoor climate variations, under the condition that quantitative loss of aesthetical/display value of parchment objects resulting from increased curling is agreed. Historical parchment documents generally demonstrating considerable curling engendered by uncontrolled storage conditions in the past are not vulnerable to further distortion when subjected to variations in RH even of considerable magnitude.