A Study of Metal Ion Migration in Oil Paint Films with Iron Oxide, Cobalt, and Organic Pigments

Abstract

Metal soap formation and metal ion migration are two drying phenomena observed in historical and modern oil paintings. Conservation professionals have observed that the formation of metal soaps in oil paintings can be problematic for their care, and these metalorganic compounds have been linked to cracking and delamination on the surfaces of paintings. Metal ion migration can facilitate this process by the transfer of metal ions into adjacent paint layers which may induce metal soap formation in paints that would otherwise not undergo this process. The focus of this thesis was to determine whether metal ion migration and metal soap formation can be observed during the intial stages of the curing process of oil paint films. Laboratory samples consisting of paint films of a selection of colours and pigment content were studied using analytical methods including attenuated total reflectance-Fourier-transform infrared spectroscopy (FTIR-ATR), portable X-ray fluorescence spectroscopy, and environmental scanning electron microscopy coupled with energy dispersive spectroscopy (ESEM-EDS), along with two other methods, the scribe test and the percent weight change tests. In particular, the results from the scribe test strongly suggests that interactions between adjacent paint films occur and can affect the drying times of individual paint types. Analyses conducted on the laboratory samples served to study the drying behaviour of paint films early in the paint-curing process in the immediate period following their initial casting. While metal ion migration and metal soap formation were not confirmed to have taken place through instrumental methods, interactions between different paint types were found to have occurred through the scribe and weight tests. A case study was also undertaken as part of this study to understand the long-term drying behaviour of oil paints better. Two paintings by the French-Canadian artist Jean-Paul Riopelle from the Agnes Etherington Art Centre were analyzed using FTIR-ATR, scanning XRF, and ESEM-EDS. Two crack patterns observed on the surfaces of the paintings could be attributed to the presence of cobalt and calcium in the paint layers.