New highlights on degradation process of verdigris from easel paintings |
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Authors: | Carlotta Santoro Karim Zarkout Anne-Solenn Le Hô François Mirambet Didier Gourier Laurent Binet Sandrine Pagès-Camagna Solenn Reguer Sigrid Mirabaud Yann Le Du Pascal Griesmar Nadège Lubin-Germain Michel Menu |
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Institution: | 1. C2RMF, Centre de Recherche et Restauration des Musées de France, Palais du Louvre, Paris, France 2. SOSCO, Laboratoire de Synthèse Organique Sélective et de Chimie bioOrganique, EA 4505 CNRS-Université de Cergy-Pontoise, Cergy-Pontoise Cedex, France 3. Chimie-ParisTech, Institut de Recherche de Chimie-Paris-IRCP, UMR8247 CNRS, Paris Cedex 05, France 4. Ligne DiffAbs, Synchrotron SOLEIL, Saint-Aubin, France 5. INP, Institut national du patrimoine, Saint-Denis, France 6. SATIE, laboratoire Système de l’Application des Technologies de l’Information et de l’Energie UMR 8029 CNRS, ENS, Université de Cergy-Pontoise, Cergy-Pontoise Cedex, France
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Abstract: | Verdigris is a green copper organometallic pigment, widely used in paintings during the fifteenth and sixteenth centuries. With ageing, chromatic modifications like browning or darkening can be observed on those green painted layers. An original but crucial approach has been developed based on the characterization of a reference neutral verdigris pigment—anhydrous copper acetate—and model samples, made of verdigris and linseed oil. Samples have undergone artificial ageing (temperature, light) to reproduce the color change effect. They were analysed before and after accelerated ageing tests by a complementary set of classical techniques: colorimetry, electron paramagnetic resonance, X-ray absorption spectroscopy, and UV–visible absorption. Our experiments revealed that the incorporation of the verdigris pigment in linseed oil induces a transformation of the copper acetate bimetallic structure, with the formation of monomeric species. These monomers, however, are not directly responsible for the darkening. The chromatic alteration seems instead linked to the transient formation of Cu(I) in the copper complexes of the pigment/oil system. This formation could be initiated by ambient light absorption through ligand-to-metal charge transfer, which favors the decarboxylation of the copper complexes leading to the reduction of Cu(II) into Cu(I). Moreover, dioxygen can react with partially decarboxylated dimers to form peroxy-Cu dimer complexes that can be responsible for the darkening. |
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