Exploring the early stages of verdigris-linseed oil mixture curing by EPR spectroscopy

• Spin trapping method provides hints on amount and chemistry of radical formed in verdigris-linseed oil pictorial mixtures • Copper carboxylates are formed in the early staged of pictorial layer curing • The Cu(II) coordination changes over curing time • The copper acetate dissolves as a bimetallic species in the oil matrix, and then evolves to monometallic complexes • The light plays a critical role on Cu(II) monometallic complexes reactivity Copper acetate, commonly known as verdigris, is a greenish bimetallic Cu(II) pigment that has been used since ancient times as a drier in oil-based pictorial layers. Recent studies have explored the interaction between verdigris and linseed oil, revealing the formation of monometallic Cu(II) complexes with free fatty acids. However, the detailed mechanism of the drying process and the early formation of these copper complexes during the initial stages of curing remains largely unexplored. In this study, we investigate the evolution of radical species in verdigris-linseed oil systems during the early stages of the curing process. We employ the spin-trapping technique combined with electron paramagnetic resonance (EPR) spectroscopy to monitor radical formation alongside the kinetic transformation of Cu(II) bi- and monometallic complexes. Based on experimental evidence, we propose the following mechanism: copper acetate initially dissolves as a bimetallic complex within the oil matrix, gradually transforming into monometallic Cu(II) complexes. Upon light exposure, these monometallic species promote radical formation, acting as catalysts for the polymerization reactions that lead to network formation in the curing paint layer.