Fracture saturation in paintings makes them less vulnerable to environmental variations in museums

Abstract

Understanding paintings as physical systems is fundamental for advancing environmental specifications that would allow for effective management of museum environments in terms of reducing energy use and carbon emissions while maintaining high standards of collection care. The current environmental specifications were derived using the criterion of the crack initiation in undamaged, usually new, material. In reality, historical paintings exhibit complex crack patterns called craquelures. The present paper analyses painted wood which is among the category of cultural objects most vulnerable to relative humidity and temperature fluctuations and frequently found in museum collections of various kinds. Fracture toughness determined experimentally for the most brittle component of pictorial layer—the ground layer (gesso) is used as a ‘failure criterion’. Comparison of energy release rate—calculated for the model of the gesso laid on a wooden substrate using finite element analysis—with the structure toughness, allowed the fracture saturation expressed as the ratio of spacing between cracks S to gesso layer thickness t to be determined for various combinations of the gesso stiffness and geometries of structural flaws at which cracks initiate. For flat geometry of a panel painting and panel thickness of 40 mm, representing the worst-case, largest stresses in the gesso layer, the fracture saturation occurs when S/t is larger than 5, even if flaws in the gesso layer are present. The paper shows that the fracture saturation significantly changes vulnerability of paintings to climate variations—a panel painting with developed craquelure network is significantly less vulnerable to climate variations than an undamaged one.