Influence of Sunlight during Harvest on the Oxidation and Yellowing of Natural Mastic Resins Used as Varnishes on Artwork

Abstract

The natural resin mastic, composed largely of triterpenes, is used as a varnish on artwork. This study investigates the influence of light on the autoxidation and yellowing of mastic, both during harvest and after application as a film. The nature of photoinitiation reactions is considered, as is the propagation of oxidative processes in both light and darkness. Oxidation, radical content and yellowing were studied by graphite-assisted laser desorption mass spectrometry, EPR and UV/VIS spectrometry, respectively. Exposure to sunlight during harvesting is found to strongly affect the resin. The radical content increases dramatically, and oxidation is accelerated. These differences are also observed during artificial aging under a range of conditions. Mastic that is harvested without exposure to sunlight deteriorates less quickly in all respects. This is attributed to lack of sunlight-generated radicals and/or labile radical precursors, which are very long-lived in the viscous resin or solid film. Remarkably, radicals are found to be nearly as prevalent in dark-aged films as those aged in light. Oxidation in the dark is also nearly as fast as with continuous light exposure. These results suggest that dark and light aging are not fundamentally different, in contrast to the conventional model.