Shrinkage and mechanical properties of drying oil paints

Abstract

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.

Hierarchical craquelure patterns in paintings on panels

“…In contrast to gesso, drying oils used as binders in oil paints solidify and harden through chemical reactions with oxygen. As indicated earlier, the molecular system of oil paints is continually evolving even after 30 years of drying under room conditions, and oil paints not only get stiffer and more brittle with the drying time but also experience increased cumulative shrinkage [4]. The hierarchical patterns visible in Fig.…”

“…The assumed magnitudes of drying shrinkage were indicated in Fig. 2 varying between 0.5% and 9% as determined for oil paint with lead white in [4]. The assumed ranges of evolving modulus of elasticity and strain at break were 1 -10 GPa and 1 -0.1 %, respectively.…”

“…For the sake of clarity, the drying times in years are indicated just for the paint with lead white. Data from [4].…”

Drying Drops of Paint Suspension: From “Fried Eggs” to Quasi-Homogeneous Patterns

“…Better control and understanding of the mechanisms governing the drying process of paint suspensions and their rheological properties present an undeniable interest for fundamental investigations. Recent studies deal with shrinkage, and the mechanical properties of drying oil paint or mechanical properties of artists paint-dried illustrate well the potential interest of the proposed investigations to practical applications.…”

A Numerical Model for Predicting the Time for Crack Initiation in Wood Panel Paintings Under Low-Cycle Environmentally Induced Fatigue

“…Barbera et al [7] performed mechanical characterisation of 30-year naturally aged linseed-oil-based paint showing that the linear increase in stiffness for the first ten years is followed by an exponential trend in stiffness thereafter. In addition, Janas et al [33] recently performed a mechanical characterisation study on 29-year aged linseed-oil-based paints. The results of these studies showed that even after decades of ageing, the mechanical properties of these paints were still changing (becoming stiffer and more brittle).…”