Solvent Leaching Effects on Aged Oil Paints

This paper presents a survey of significant developments in research on the leaching of oil paint films by organic solvents, from the first systematic investigations carried out in the 1950s to more recent “clinical” studies involving the sampling and analysis of paintings during treatment. Key studies are discussed in the context of an improved knowledge of oil paint chemistry that has developed in recent decades and advances in analytical techniques that have allowed a more informed and precise analysis of the solventextractable components of aged oil paints. Current research related to solvent cleaning effects, including studies of the formation of metal soaps and their migration in oil paints and the leaching effects of aqueous cleaning systems on acrylic paint media, is considered. The challenges of interpreting experimental data in relation to cleaning practice are outlined and illustrated with reference to discussions of research on solvent effects in the conservation literature. There must have been many . . . who wishfully hoped the phenomenon [leaching] could be dismissed as an insignificant curiosity only of interest to the scientist, perhaps conveniently to be soon forgotten. —Rees Jones (1973:43) Organic solvents have long been the most widely used materials for the removal of discolored or deteriorated varnish and retouchings from paintings, a complex operation prosaically known as “cleaning.” The leaching of oil paint films by solvents, that is, the extraction of soluble, noncrosslinked components of the binding medium, was first described in detail by Nathan Stolow in a series of influential publications in the 1950s– 1970s (Stolow, 1957a, 1957b, 1963, 1971). The findings of Stolow and other researchers were widely discussed by conservators and museum scientists and became a controversial component of the more general debate on cleaning (for a detailed review of solvent cleaning studies, see Phenix and Sutherland, 2001). The above remarks by Stephen Rees Jones were made in a review of the second edition of the book On Picture Varnishes and Their Solvents, in which Stolow summarized his research, and they convey the uncertainty with which the findings on leaching were initially received. Unlike the related phenomenon of swelling, which carries the acute risk of softening and disruption of paint, the risks associated with leaching are less immediately tangible to the conservator and have proved to be more contentious in discussions of cleaning. In the early experiments by Stolow and others, carried out on laboratoryprepared oil paint films, leaching was found to result in embrittlement of the films and optical changes (blanching or desaturation) of paint surfaces. Although highly significant, such risks are difficult to assess in practice. Changes Solvent Leaching Effects on Aged Oil Paints