Copper Complexes in Verdigris Painting Mixtures: An Electron Paramagnetic Resonance Characterization

Copper complexes, also called "copper soaps" when referred to carboxylate compounds, have become a concern in the conservation of historical artefacts. These complexes are suspected to play a significant role in degradation processes such as darkening and cracking of painting layers. Here we propose the Continuous Wave Electron Paramagnetic Resonance spectroscopy (CW-EPR) as a tool for the identification of these species. EPR spectroscopy is a micro-invasive and non-destructive technique that is highly sensitive to paramagnetic species, including radicals, crystal defects, and transition metal ions, e.g., Cu(II) and Fe(III). We have selected the verdigris (copper acetate) pigment combined with many organic binders, e.g., egg tempera and gum Arabic, as mock-up systems because of the known degradation tendency of the copper-based pigment. We would like to demonstrate that a precise analysis of the experimental spectra assisted with the calculus of the main magnetic quantities could lead to a good characterization of the copper complexes (or "soaps").