A molecular perspective on the cleaning of oil paintings

Scientific investigations have shown that the exposure of oil paintings to organic solvents for the removal of unwanted varnish layers, can affect the chemical and physical properties of oil paintings in undesired ways. To assess the effects of solvents on the molecular mechanisms of paint alteration, a thorough understanding of the chemistry of oil paint is needed.
This thesis commences with IR spectroscopic studies of solvent diffusion and swelling of oil paint models in water and in a range of organic solvents. The reaction between saturated fatty acids and metal ions in paint films to form crystalline metal soaps is studied. Subsequently, the mechanism by which zinc white pigment dissolves into the oil paint binding medium, by associating to carboxylic acid groups and forming zinc carboxylates, is described. Next, an in-depth characterisation of zinc carboxylates in oil paint, which unravels the coordination environment around zinc ions, is presented. The transport of saturated fatty acids between paint layers is then investigated and found to be accelerated by solvent exposure, stimulating metal soap formation after long solvent exposure times. Fourier transform laser speckle imaging is used to obtain spatial information on solvent transport within the paint layers by measuring the motion of scattering pigment particles. The relation between solvent transport and paint ageing is studied and a quantitative comparison of solvents and solvents application methods is made. Finally, a physicochemical model describing the processes involved in solvent action on oil paint is presented.