The Changing Mechanical Properties of Aging Oil Paints

Abstract

The stiffening and embrittlement of oil paints over time has been a real concern for those responsible for the long term care and preservation of paintings. This paper examines the effects of time, pigments, relative humidity (RH), temperature and solvents on the mechanical properties of traditional oil paints. In this way it is possible to determine the role of each factor in causing the paints to become brittle. Even after 14 years the oil paints show little evidence that the long term "maturing" processes have begun to slow down. It is shown that there seems to be little correlation between the time paint requires to "dry-to-the-touch" and the longer term mechanical properties. Both low and high temperature levels can increase the stiffness of the paints though the mechanisms are quite different. Considerable hydrolysis of the paints occurs early in their history and the ones that hydrolyze most quickly are the ones that remain the most flexible.

Nanoindentation and the micromechanics of Van Gogh oil paints

“…Zinc white paint is usually described as a very brittle paint compared to paints based on other pigments. Indeed, previously reported that zinc white oil paint is a stiff and brittle paint, with a very high Young modulus (1.7 GPa), significantly higher than lead white paint (Young modulus of 0.3 GPa) [5]. Our nanoindentation results are therefore fully consistent with the tensile test results.…”

“…Our nanoindentation results are therefore fully consistent with the tensile test results. Fuesers and Zumbühl (2008) studied the impact of up to 14 year aging on the mechanical properties of free paint films [5,11]. No saturation of the mechanical response is observed over this long aging time so that extrapolation to centuries is difficult.…”

“…From sample VG1, it appears that the layers which exhibit a higher reduced modulus (between 11 and 14 GPa) are characterized by a high relative content of lead white (around 55 and 90 weight percent of lead (sample VG1, respectively layers 1 and 2)). indicate that the presence of zinc white, even as secondary pigment in lead white paint or in light color paint, often induces brittleness of the oil paint film within as early as three years [5]. It is therefore especially surprising that layer 3 of VG1, which contains a majority of zinc white and cobalt blue, together with some lead white (Zn 33 % atomic, Al 37% atomic and Pb 13% atomic of the mineral fraction) exhibits a quite low reduced modulus (6.1 GPa) in comparison to the layers from which zinc white is absent.…”

“…Here we focus our study on the paint multilayer itself. In the life of a painting, a primary source of stress is the tensile residual stress resulting from paint cross-linking and drying [1,2] while additional stresses arise from variation of environmental factors such as relative humidity or temperature [3][4][5][6][7]. Stresses also result from the deformation of the support, whether deliberately applied during restoration processes (restretching, lining) or accidentally (handling).…”

“…Centimetric strips of free paint films were then removed from the support and characterized by tensile testing. From such studies, valuable information was gained on the influence of the type of pigments, aging of paint, temperature, relative humidity and solvents on the mechanical properties of paint films [5,6,[8][9][10][11]. Effects of solvents and of relative humidity on paint properties were also investigated by thermomechanical measurements on small samples of reconstructed paint, providing information indirectly related to mechanical properties, such as the compressibility as a function of temperature or viscoelastic behavior [12,13].…”