Photo-oxidative kinetics of solvent and oil-based terpenoid varnishes

Here, the photo-oxidative degradation of several terpenoid varnishes usually encountered in conservation of cultural heritage has been investigated. Samples were prepared by dissolving dammar, mastic, colophony, sandarac and bleached shellac in suitable solvents or by heating mastic, colophony, sandarac and Manila copal with linseed oil. The alteration effects of this broad set of samples induced by light exposure in an ageing chamber, were thoroughly characterized using transmission FTIR spectroscopy, colorimetry, and gravimetric measurements. The various varnishes exhibited a similar saturating photo-oxidative kinetics upon the exposure time but significant quantitative differences were pointed out. A decreasing of the methyl and methylene stretching bands was observed for both solvent and oil based varnishes, which suggests the alteration effects begin with hydrogen atom abstractions. The consequent reaction with oxygen, catalyzed by irradiation and temperature rise lead to the increase of carbonyl and hydroxyl functionalities, thus suggesting the formation of new compounds containing carboxylic, ketone and ester groups and lactonised structures. At the same time, cross-linking and extensive polymerization were clearly observed for sandarac and shellac solvent varnishes. Measurement and comparison of the photo-oxidative and discoloration effects of the present set of samples, provide a general picture of the ageing phenomenology of the natural varnishes, which can be usefully exploited in characterization and conservation of easel paintings and other artworks.