3 DESCRIPTION OF TESTING PROCEDURE

PAINTINGS WERE selected on the basis of the simplicity of varnish stratification and the age of the coating, as indicated by the National Gallery of Art's conservation files. A representative sampling of at least 25 paintings believed to be coated with each varnish was selected. The exhibition history of each painting was examined to compare the time exposed to light under exhibition conditions to the time in storage, since cross-linking is a light-induced phenomenon. The paintings were examined under ultraviolet light, where areas of retouch were noted (de la Rie 1982), as was the color of the fluorescence. Most natural resins fluoresce green under long-wavelength ultraviolet light (a notable exception is shellac, which fluoresces orange), while synthetic resins often have a milky fluorescence. The paintings' appearance under visible light was also carefully noted in an effort to keep a record, albeit subjective, of the aesthetic appearance of these 30-year-old synthetics.

At least two test spots were selected on each painting, usually one in a light area and one in a dark area. Solvents of increasing polarity were applied to the areas with swabs, while the areas were viewed under a stereo microscope. In order of increasing polarity, the solvents used were: cyclohexane, cyclohexane-toluene mixtures, pure toluene, toluene-acetone mixtures, pure acetone (Feller 1978, 1976; Feller et al. 1985), and a mixture of isopropanol and methyl ethyl ketone (MEK) (table 2). The mildest solvent was applied first and the solvent progression was continued until the varnishes were completely removed, as monitored by infrared spectroscopy.

TABLE 2 Solvent mixtures for removability studies, with dispersion factors

Following the application of solvents, changes in the varnish were noted, including blanching, matteness, and gelling. The observations were recorded in a standard format, and reports were compiled on each painting. The removal of all of the varnish layers was determined by visual examination and by comparison of the fluorescence of the spot with the surrounding area. The swabs were saved and independently analyzed by FTIR.