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Understanding how the evolving molecular composition of an oil paint layer on its transition to an aged solid film affects its dimensional change and mechanical properties is fundamental to the assessment of material durability and more broadly risk of degradation of oil paintings. Tensile properties—modulus of elasticity and strain at break—as well as cumulative shrinkage were determined for a selection of oil paints from Mecklenburg's Paint Reference Collection now after approximately 30 years of drying. The oil paints were found to get stiffer and more brittle with diminishing plastic deformation and increasingly elastic behaviour. For some paints, the increases in stiffness and decreases in the strain at break were dramatic during the late stage of drying. The observations modify the current physical model of paintings in which the mismatch in the response of glue-based ground layer and unrestrained wood or canvas support to variations in relative humidity (RH) has been identified as the worst-case condition for the fracturing of the entire pictorial layer. This study demonstrated that some paints were more brittle than the glue-based ground layer and as a consequence more vulnerable to cracking. The shrinkage of paints due to molecular relocation and/or evaporation of organic medium as they dry and age was measured. This shrinkage can exceed their strain at break and lead to fracturing of the oil paint layer if it is restrained by a dimensionally stable substrate. Consequently, after long-term drying, the cumulative shrinkage can cause oil paints to crack even in absence of fluctuations in RH or temperature. An example of cracking developed in an oil paint layer on the top of an undamaged ground layer in a historic panel painting was made evident by the X-ray microtomography. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]