Conservation Issues of Modern Oil Paintings: A Molecular Model on Paint Curing

20th- and 21st-century oil paintings are presenting a range of challenging conservation problems that can be distinctly different from those noted in paintings from previous centuries. These include the formation of vulnerable surface ‘skins’ of medium and exudates on paint surfaces, efflorescence, unpredictable water and solvent sensitivity and incidence of paint dripping which can occur within a few years after the paintings were completed. Physicochemical studies of modern oil paints and paintings in recent years have identified a range of possible causal factors for the noted sensitivity of painting surfaces to water and protic solvents, including the formation of water-soluble inorganic salts and/or the accumulation of diacids at the paint surface, which are oxidation products of the oil binder. Other studies have investigated the relationship between water sensitivity and the degree of hydrolysis of the binder, the proportions of free fatty and dicarboxylic acids formed, as well as the relative content of free metal soaps. Thus far, data indicate that the qualitative and quantitative composition of the non-polymerised fractions of the oil binder cannot be solely or directly related to the solvent sensitivity of the paint film. Conclusions therefore indicate that the polymeric network, formed upon the curing of the oil plays a fundamental role; suggesting that water sensitivity – at least in some cases - may be related to the poor development, and/or polar nature of the formed polymeric network rather than the composition of the non-polymerised fractions. Poorly developed polymeric networks, in combination with the migration of polar fractions i.e. dicarboxylic and hydroxylated fatty acids towards the paint surface, can be related to other degradation phenomena, including the separation and migration of the paint binder which can lead to the presence of observable skins of medium, as well as the more alarming phenomenon of liquefying or dripping oil paints. It is thus crucial to understand the molecular composition of these paints and their physicochemical behaviour to aid the further development of appropriate conservation and preservation strategies, as the risks currently associated with surface cleaning treatments and other conservation procedures can be unacceptably high. This account reviews the relationships between the degradation phenomena associated with modern oil paintings and the chemical composition of the oil binder; and proposes a molecular model for the development of water sensitivity and other noted degradation phenomena. It is suggested that water sensitivity (and possibly other degradation phenomena) are consequences of processes that take place upon curing, and in particular to the rate of formation and decomposition of alkoxyl and peroxyl radicals. These reactions are strongly dependent on the type of oil present, ambient environmental conditions, and the chemical and physical nature of the pigments and additives present in the paint formulation. When the curing environment is oxidising, the chemistry of peroxyl radicals dominates the reaction pathways, and oxidative decomposition of the paint film overwhelms cross-linking reactions.