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The light-induced aging of natural triterpenes used in varnishes was studied using graphite-assisted laser desorption/ionization mass spectrometry. This method was found to be superior to conventional matrix-assisted laser desorption/ionization for these materials because of higher signal levels and less chemical noise from the matrix. The dammar and mastic raw materials were found to be partially oxidized compared to their nominal composition, with up to six oxygens incorporated. The light-induced aging process leads initially to polymerization, up to trimers. Further aging leads to a decrease in average molecular weight. These observations are supplemented by FTIR and 13C NMR spectra which show complete reaction of keto groups to acids and peroxy species. A general mechanism for the process is proposed, and the consequences for properties of varnish films are investigated.

Photochemical and thermal aging of triterpenoid dammar and mastic resins used as varnishes on paintings were studied using graphite-assisted laser desorption/ionization mass spectrometry. This extends an earlier study on similar materials (Zumbuhl et al., Anal. Chem. 1998, 70, 707-715) that focused on photoaging. Progressive aging results in development of groups of signals spaced by 14 and 16 Da, indicating incorporation of oxygen as well as simultaneous loss of hydrogen. Oligomers up to tetramers are formed, while cleavage reactions lead to increased signal intensities in the mass ranges between the oligomers and below the monomers. No major differences were found between the mass spectra of samples aged in light or darkness, except that deterioration was faster in light. Electron paramagnetic resonance spectroscopy revealed similar and significant amounts of radicals in films of dammar stored either in light or in darkness. It is concluded that oxidative radical reactions also take place in darkness and that differences in light and dark aging pathways are minor, although rates may differ. These findings lead to a unified explanation for yellowing of natural resin varnishes, one of the major degenerative changes in the appearance of paintings. It is also shown that the commercially available, nominally fresh resins are already in an advanced stage of oxidation and degradation. Energy-rich substances are formed upon irradiation with sunlight and are believed to restart the autoxidative chain reactions, regardless of storage conditions. As a result, varnishes are oxidized quite quickly (months) even when kept in darkness.