Effects of Organic Solvents on Artists’ Oil Paint Films: Swelling

This paper concerns the sensitivity of artists’ oil paints to organic solvents. Specifically, it examines the respective capacities of single organic solvents and mixtures of solvents to cause swelling as a consequence of sorption into the paint and solvency action thereupon. The swelling power of a solvent is essentially an indicator of its relative strength in terms of action on the original (oil) paint material. The paper collates and interprets the large body of experimental data that has been gathered on the solventinduced swelling of two particular reference oil paint samples. These samples comprise uniform films prepared from mixtures of proprietary yellow ochre and flake white tube paints, one portion being artificially aged by exposure to highintensity daylight and the other portion not light aged. The method adopted for measuring swelling employed a combination of lowpower stereomicroscopy and quantitative image analysis. The initial output from the swelling experiments is a series of swelling curves for each solvent that shows the mean percentage change over time in area ∆A % for the group of paint fragments of the particular sample type tested. Various approaches are explored to correlate the respective swelling powers of individual solvents with a range of solvency descriptors and physicochemical constants. INTRODUCTION: BACKGROUND AND CONTEXT Organic solvents rightly remain an important class of chemical tools used by conservators in the cleaning of paintings, particularly for removing aged varnishes and overpaints; and as with all tools, a proficient practitioner must understand how they work in order to be able to use and control them with skill and refinement. The respective effects of organic solvents on varnishes and original paint are key elements in the theoretical and technical frameworks that conservators use to comprehend the cleaning process, to assess the risks and benefits of cleaning, and to diagnose and solve the practical problems presented by specific objects requiring treatment. This study focuses on the sensitivity of artists’ oil paints to organic solvents. It has long been recognized that swelling of the oil binder represents an important aspect of risk in the cleaning of paintings using solvents. If the paint is swollen to a significant degree, it becomes softened and its capacity to bind pigment may be diminished, possibly to the point where the pigment is vulnerable to erosion by the mechanical action of the cleaning swab. Safety in cleaning depends to a large degree on controlling the magnitude and rate of paint swelling. In contrast to the phenomenon and risk factor of leaching (i.e., the extraction of soluble, low molecular weight components of the organic binder phase), which are arguably more notional or abstract concerns for it is improbable that a conservator performing the operation of cleaning could actually sense any organic molecules that are Effects of Organic Solvents on Artists’ Oil Paint Films: Swelling