A mass spectrometry and electron paramagnetic resonance study of photochemical and thermal aging of triterpenoid varnishes

Photochemical and thermal aging of triterpenoid dammar and mastic resins us ed as varnishes on paintings were studied using graphite-assisted laser des orption/ionization mass spectrometry, This extends an earlier study on simi lar materials (Zumbuhl et al,, Anal Chem. 1998, 70, 707-715) that focused o n photoaging, Progressive aging results in development of groups of signals spaced by 14 and 16 Da, indicating incorporation of oxygen as wed as simul taneous loss of hydrogen. Oligomers up to tetramers are formed, while cleav age reactions lead to increased signal intensities in the mass ranges betwe en the oligomers and below the monomers, No major differences were found be tween the mass spectra of samples aged Zn light or darkness, except that de terioration was faster in light. Electron paramagnetic resonance spectrosco py revealed similar and significant amounts of radicals in films of dammar stored either in light or in darkness. It is concluded that oxidative radic al reactions also take place in darkness and that differences in light and dark aging pathways are minor, although rates may differ. These findings le ad to a unified explanation for yellowing of natural resin varnishes, one o f 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 a re formed upon irradiation with sunlight and are believed to restart the au toxidative chain reactions, regardless of storage conditions, As a result, varnishes are oxidized quite quickly (months) even when kept in darkness,