Effect of photodegradation on the identification of natural varnishes by FT-IR spectroscopy

In this study, natural varnishes used in ancient painting techniques were recreated following ancient recipes. They were irradiated by artificial sunlight and analysed by FT-IR all along the accelerated ageing. The main natural resins used were: sandarac, copal, colophony, mastic and dammar in alcohol, spirit turpentine and oil media. This study aims to understand and follow the evolution of characteristic absorption bands for the differentiation of varnish types by FT-IR analyses. Kinetics and evolutions of the compositions of the varnishes during photo-ageing were also studied. Photodegradation reactions lead to the loss of many characteristic absorption bands and the apparition of new ones. To assess the specific fingerprints of different resins which do not depend on the degradation state, results were analysed by statistical treatment (Principal Component Analyses). This multi-disciplinary approach leads to the identification of new criterions including the decrease of specificity caused by the photochemical reactions.