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The fracture toughness in opening mode GIc for selected oil paints from Mecklenburg's Paint Reference Collection after approximately 30 years of natural ageing was determined using the procedure adopted from the ASTM D5528–13 standard in which double cantilever beam specimens are split in tensile tests. The careful reglueing procedure allowed multiple fracturing tests to be carried out that not only improved statistics of the measurements but also provided insight into the variation of the fracture toughness across the paint film observed for some paints. The latter was due to pigment sedimentation or chemical change of the oil binder as a result of the easier access of oxygen from the side open to the air. For the lead white paint, used over centuries both in paint films and oil grounds, the GIc values more than doubled from 18 to 39 J/m2 for three consecutive cracks formed at an increasing distance from one side of the paint film. The study demonstrated that fracture toughness corresponding to the first crack formation in aged oil paints ranged between 10 and 40 J/m2, relatively low values compared to the animal glue-based ground in paintings. In consequence, oil paints are more vulnerable to crack initiation and growth induced by tensile stress than the ground layer. The measurements filled the gap in the knowledge required for the analysis of fracturing or delamination processes in paintings. [ABSTRACT FROM AUTHOR]

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]