Nanoindentation and the micromechanics of Van Gogh oil paints

Understanding the mechanical properties of ancient paintings is a major issue for conservation and restoration. One strategy is to measure the mechanical properties of reconstructed paints: however, the aging process is poorly known, so it is also desirable to measure mechanical properties directly on ancient paint samples. Using nanoindentation, we have characterized submillimetric samples recovered from restoration of two Van Gogh paintings and compared the results with reconstructed paint samples. We demonstrate that the reduced modulus and hardness of historical paints can be measured at a very local scale, even differentiating between each paint layer. Our reconstructed paint samples exhibit elastic moduli comparable to values of the literature, but the values measured on the two 19th century paint samples are found to be significantly larger. Similarly, the compositional dependence of the elastic modulus is consistent with literature results for our reconstructed samples while our preliminary results for ancient samples do not readily fall into the same pattern. These results all point out to a significant impact of long term aging, in a manner which is difficult to predict in our present state of understanding. They demonstrate that nanoindentation is a very adequate tool to improve our knowledge of art paint mechanics and aging.