A Study of the Factors Affecting the Deteioration of Old Paint Media

Abstract

This thesis is concerned with the factors which cause old paint media to deteriorate over a long period of time. The first part of study is concerned with the development of techniques for evaluating the changes occurring during photo-oxidation and two have been found to be particularly useful. The first involves a measurement of the decrease in IR_absorbance of the -CH2- stretching vibration at 2930 cm-1 which correlates with weightloss. The second monitors the formation of scission product (dicarboxylic acids). The second part of the investigation is concerned with the mechanism of action of some of the common pigments used in old paint-media. Verdigris (basic copper acetate) was found to be an effective photo-stabiliser due to the ability of copper acetate to behave as a regenerative antioxidant in the paint film. This leads to the formation of conjugated unsaturation during ageing which is the reason for the discolouration associated with this pigment. Vermilion on the other hand is an effective sensitiser for photo-oxidation due to the formation of hydroperoxides. The probable intervention of singlet oxygen and hydrogen peroxide via radical intermediates in this process is indicated. Vermilion pigmented paint film is considered to be a useful monitor of photo-oxidation. It is highly active in the wavelength range 300-365 nm and it is considered unlikely that physical screening of incident light over that already employed in art galleries will achieve significant additional protection. The study suggests that chemical protectants may provide a long-term solution to the problem of paint deterioration.