Photoreactivity of Oil and Alcoholic Diterpenic Varnishes: Key Role of the Polymerization Process

The conservation of artworks is a key issue in cultural heritage. The alterations of the varnish could be linked mainly to the photopolymerization of the natural resin. The photodegradation of sandarac and rosin under simulated solar light was studied using a multianalytical approach using a second derivative FT‐IR spectroscopy and gas chromatography coupled to mass spectroscopy. The photodegradation of sandarac and rosin under simulated solar light undergo a two steps reaction. In a first step, main terpene molecules underwent esterification and perestererifications via the formation of hydroperoxides preferentially formed on tertiary carbons. In a second step, these esters and perester bonds underwent photoscissions by Norrish I and II reactions, leading to the production of free terpenes in the varnishes.