Evaluation of the hygroexpansion of Japanese papers by digital image correlation (3D-DIC) and application to the study of the impact of inlay mounting on the strain of an albumen photographic print

Abstract

• Inlay-mounted albumen photographic prints become cockled during exhibition. • The mass and the in-plane and out-of-plane displacement of different papers were evaluated by DVS and 3D-DIC as the humidity fluctuated. • The Japanese papers tested have lower strain than the Western paper and, for some samples, very similar hygroexpansion in the machine and cross-machine direction. • The differences in hygroexpansion of the materials may explain the formation of cockles. • DIC can assist in the decision-making process when selecting materials and techniques for conservation treatment. During an exhibition of photographs, a temporary excess of humidity caused waviness to form on some inlay-mounted albumen prints, which persisted and required conservation treatment. In order to understand the origin of this phenomenon, dynamic water sorption (DVS) measurement and digital image correlation (DIC) have been applied to assess the weight and the mechanical behavior of the materials when the humidity varies. The 3D-DIC has been performed using common digital cameras and commercial software. The in-plane hygroexpansion and out-of-plane displacements of various paper samples and of a 19 th -century albumen photographic print, subjected to humidity fluctuation, made it possible to explain the origin and the persistence of waviness of the photograph in an inlay mount. This non-contact and multi-scale technique is therefore particularly useful for assessing the mechanical behavior of cultural heritage materials. It permitted the evaluation of the compatibility or incompatibility between materials depending on the climatic conditions, whether from the point of view of in-plane or out-of-plane displacements, and enabled potential risks linked to thermo-hygrometric variations to be foreseen.