Water Diffusion and Transport in Oil Paints as Studied by Unilateral NMR and ¹H High-Resolution MAS-NMR Spectroscopy

Heavy metal carboxylate degradation severely affects thousands of oil paintings. Relative humidity has been reported to accelerate the rate of the reactions. To evaluate its role further, water diffusion and molecular mobility of protons in linseed oil-based lead white paints were studied by unilateral NMR and ¹H HRMAS spectroscopy. The results indicate that exposure to high %RH for relatively long times affects the dynamics of the oil paint's mobile fraction and that the effect is more pronounced as the thickness of the film increases. It was found that the paint can absorb appreciable amounts of water and has a porosity of approximately 6 % available for the diffusion of water, for which a regime of restricted diffusion was observed. Furthermore, the presence of bound and free-moving water, due to the possible formation of hydrated ionic-group clusters, supports the hypothesis of a polymeric/ionomeric network, as well as regions of essentially water free to move as in the bulk. The findings allow a better understanding of the role of water as a factor activating the degradation process in linseed oil-based lead white paints.