THE SWELLING OF ARTISTS' PAINTS IN ORGANIC SOLVENTS. PART 2, COMPARATIVE SWELLING POWERS OF SELECTED ORGANIC SOLVENTS AND SOLVENT MIXTURES
1 INTRODUCTION: BACKGROUND AND CONTEXT
At the first IIC congress in Rome in 1961, Nathan Stolow presented a distillation of his doctoral research work, completed a few years earlier at the Courtauld Institute, on the effects of organic solvents on pigmented and unpigmented films of linseed oil, in the context of the cleaning of paintings (Stolow 1955, 1963). Stolow's findings, ultimately compiled in his chapter of On Picture Varnishes and Their Solvents, remain possibly the most important source that describes the action of solvents on oil paints (Feller et al. 1985). Stolow investigated two principal effects that were understood to contribute to risk in cleaning with solvents: the physical swelling of the organic binder phase caused by sorption of solvent, and leaching, i.e., the extraction of soluble, low molecular weight components of the organic binder phase.
Perhaps partly because the problem is amenable to investigation by conventional techniques of analytical organic chemistry (gas chromatography, gas chromatography—mass spectroscopy, liquid chromatography—gas chromatography, etc.), the issue of leaching of paint films by organic solvents has received a fair amount of research attention since Stolow's pioneering gas chromatographic investigations of the 1960s (Stolow 1965). This aspect of risk in cleaning is rightly continuing as an important theme in the theoretical investigation of the cleaning process (Erhardt and Tsang 1990; White and Roy 1998; Sutherland and Shibayama 1999; Tumosa et al. 1999). However, in the practical situation of actually removing varnish from a picture with organic solvents, it is arguable whether leaching can ever be anything more than a notional concern, for it is unlikely that a conservator can actually sense if organic molecules are being extracted during the cleaning process. At the present time there is no simple test the practical conservator may perform to detect or quantify extracted compounds at the point of cleaning. In the practice of cleaning, then, the magnitude of risk associated with leaching is extremely difficult, if not impossible, for the conservator to gauge. The constitution of the paint and its environmental and treatment histories will all have a marked influence on the presence, or not, of an abundant solvent-extractable organic phase. Paint films of different types have been shown to vary appreciably in their degrees of extractability.
From the point of view of the practical conservator, it could be argued that the most directly useful parts of Stolow's research are his investigations of the swelling power of various organic solvents on reference paint films. Swelling is a real, tangible concern in the practice of cleaning paintings. If swollen to a significant degree due to sorption of solvent, the paint medium has a reduced binding power and the pigment is vulnerable to removal by the mechanical action of the cleaning swab applied to the surface. The preliminary cleaning tests routinely conducted by conservators, such as Ruhemann's Safety Margin Test, are essentially directed toward assessing and controlling the risk presented by swelling of the paint set against the solubility of the varnish (Ruhemann 1968). The degree of equilibrium swelling was shown by Stolow to be an indicator of the magnitude of solvation interaction between a liquid and the organic phase of the paint. A key finding was the variation in the degree of swelling of reference paint films by organic solvents in relation to their Hilde-brand solubility parameter ∂ (Stolow 1963, 1976). Solvents and solvent mixtures of intermediate polarity, i.e., those having, for example, ∂ values in the range ca. 8.8–11 (cal/cm3)1/2 [18–22.5 MPa1/2], produced the greatest degrees of swelling. This observation is comparable with the results of the very few other paint swelling studies that exist in the paint technology literature, notably Browne (1956), Brunt (1964), and Eissler and Princen (1968, 1970).
The application of oil paint swelling data to actual cleaning practice owes much to later interpretations using more sophisticated treatments of the solubility parameter element. Of particular importance is the work of Hedley (1980), who took some of Stolow's published swelling data and combined it with the empirical approach of Ruhemann (1968) to develop a broad framework for solvent selection in varnish removal. Using the Teas fractional solubility parameter chart, Hedley identified for oil paint a peak swelling region, which represented a zone of solvent power associated with increased risk in varnish removal. This approach was developed further by Michalski (1990). Despite shortcomings in the Teas fractional solubility parameter approach, which are now well established (Blank and Stavroudis 1989; Michalski 1990; Phenix 1998b), there is no doubt that these interpretations have been valuable, influential contributions to the technical and theoretical foundations of solvent cleaning (Banik and Krist 1984; Stoner 1994; Cremonesi 2000). It must be emphasized, however, that they are constrained by the limitations of the swelling data on which they are based. These limitations are significant, and it is perhaps now appropriate to reconsider the validity of the Hedley-Stolow peak swelling region.