Heritage Science

Featured article - Shrinkage and mechanical properties of drying oil paints

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.

Image: X-ray microtomographs of pictorial layer in a historic panel painting; cross-sections parallel to the surface: a paint layer, b ground. For presentation, the X-ray absorbing material is represented on a grey scale in which white and black colours correspond to highly and least absorbing materials, respectively. The black spots in the X-ray microtomograph of the paint layer correspond to grains of the blue pigment, most probably natural ultramarine

Featured collection - Preventive Conservation, Predictive Analysis and Environmental Monitoring

Heritage Science is pleased to present this article collection, supported by the Universitat Politecnica de Valencia in Spain. This set of articles aims to show the ongoing research and latest technological advances in the study of the behavior and aging of cultural heritage materials, environmental monitoring, and their contribution in the design of preventive conservation strategies developed in the framework of CollectionCare (www.collectioncare.eu), an EU-funded project involving 18 partners from 9 countries between 2019 and 2022. Selected contributions from the conference “CollectionCare: New Challenges in Preventive Conservation, Predictive Analysis and Environmental Monitoring” are presented, alongside contributions from research groups working in relevant areas.