Influence of sunlight during harvest on the oxidation and yellowing of natural mastic resins used as varnishes on artwork

The natural resin mastic, composed largely of triterpenes, is used as a var nish on artwork. This study investigates the influence of light on the auto xidation and yellowing of mastic, both during harvest and after application as a film. The nature of photoinitiation reactions is considered, as is th e propagation of oxidative processes in both light and darkness. Oxidation, radical content and yellowing were studied by graphite-assisted laser deso rption mass spectrometry, EPR and UV/VIS spectrometry, respectively. Exposu re to sunlight during harvesting is found to strongly affect the resin. The radical content increases dramatically, and oxidation is accelerated. Thes e differences are also observed during artificial aging under a range of co nditions. Mastic that is harvested without exposure to sunlight deteriorate s less quickly in all respects. This is attributed to lack of sunlight-gene rated radicals and/or labile radical precursors, which are very long-lived in the viscous resin or solid film. Remarkably, radicals are found to be ne arly as prevalent in dark-aged films as those aged in light. Oxidation in t he dark is also nearly as fast as with continuous light exposure. These res ults suggest that dark and light aging are not fundamentally different, in contrast to the conventional model.