X-ray absorption investigations of copper resinate blackening in a XV century Italian painting

Investigations of the darkening phenomenon of copper resinate observed in a XV century easel painting were carried out by X-ray absorption spectroscopy (XAS) at the GILDA beamline of the European synchrotron radiation facility (ESRF, Grenoble, France). X-ray absorption near edge structure and extended X-ray absorption fine structure (EXAFS) measurements were collected at the Cu K-edge on an original painting sample, as well as on fresh pigment standards and on painting models. The study was aimed at providing structural information of the oxidation states and the local chemical environment (neighbouring atoms and bond distances) of copper in the unaltered and blackened pigments in order to elucidate the discoloration mechanism. Complementary information on optical and molecular properties of copper resinate were obtained by UV-vis and Fourier transform infrared spectroscopies. EXAFS analysis evidenced that the local chemical environment of Cu in copper resinate can be described using neutral copper acetate as a model. It consists, essentially, of bimetal Cu2+ carboxylate complexes with a distorted octahedral coordination. Such a structure is retained in the blackened pigment, although some differences were observed. It has been found that the alteration takes place without change of the valence state of Cu(II) ions, while the formation of the copper oxides CuO and Cu2O responsible for the embrownment is excluded. On the basis of the XAS results we deduced that discoloration of copper resinate may be related to local modification of the copper coordination structure as evidenced by the observation of an increase of the Cu-Cu and Cu-C distances in the EXAFS spectra.