6 CONCLUSIONS

IT HAS been shown previously on the basis of accelerated aging studies that certain methacrylate polymers will tend to cross-link and lose their ability to be removed efficiently. In order to monitor this effect, a program of varnish removability testing was initiated. A combination of infrared spectroscopy, visual examination, and fluorescence in ultraviolet light was used to monitor removability of poly(isoamyl methacrylate) and poly(n-butyl methacrylate) varnish coatings. The results of these tests show that some cross-linking of these varnishes has occurred under natural aging conditions with ultraviolet-free light, but that although the varnishes were applied between 10 and 40 years ago, they are still removable. Usually the application of solvents such as cyclohexane-toluene mixtures causes the varnish to gel but does not remove the varnish completely. Solvents of higher polarity (toluene or toluene-acetone mixtures) were necessary for complete, effective varnish removal. The word effective needs to be stressed here. Although a varnish may be eventually removed in a milder solvent, a slightly stronger solvent may be preferred to avoid the need for excessively vigorous rubbing with the swab. It must also be noted that this qualitative removability test does not indicate the amount of cross-linked material or the expected lifetime of the coating.

The insolubility of the varnish on the painting exposed to unfiltered sunlight above the laylights demonstrates the effect of ultraviolet filters and reduced light levels. The National Gallery of Art has ultraviolet filters over the skylights. The fact that these varnishes could be removed after 30 to 40 years does not imply that varnishes on paintings in other galleries that do not have these filters would be as easily removed after this exposure time. Care should be taken, therefore, in extrapolating this observation to paintings in other collections. Nor should the observation be interpreted as proof that varnishes remain removable indefinitely. Based on the published results for accelerated aging tests (Feller et al. 1985), it must be assumed that varnishes will eventually become insoluble. Continued monitoring of selected paintings at regular intervals will therefore be undertaken at the National Gallery of Art.

ACKNOWLEDGEMENTS

JIA-SUN TSANG AND JOHANNE PERRON were members of the initial study group formed to investigate the synthetic surface coatings. Although they have both since left the National Gallery of Art, their hard work, enthusiasm, and dedication are gratefully acknowledged. The substance of this paper was presented to the Paintings Group at the 1988 meeting of the American Institute for Conservation.