Photochemical Origin of the Darkening of Copper Acetate and Resinate Pigments in Historical Paintings Marion Alter, ‡,† Laurent Binet, †,‡ Nadia Touati, †,‡ Nade ̀ ge Lubin-Germain, § Anne-Solenn Le Hô , ‡,† Franç ois Mirambet, ‡,† and Didier Gourier* ,†,‡ † CNRS, Institut de Recherche de Chimie de Paris (IRCP), Chimie-ParisTech, PSL University, 75005 Paris, France ‡ Centre de Recherche et de Restauration des Musé es de France (C2RMF), Palais du Louvre, 75001 Paris, France § Laboratoire de Chimie Biologique (LCB), EA4505, Université de Cergy-Pontoise, 95510 Cergy-Pontoise, France * S Supporting Information ABSTRACT: Copper acetate and copper resinate pigments are bimetallic Cu II complexes in which metal atoms are bridged by four carboxylate ligands (acetate or abietate). Prepared with lindseed oil as binder, these green pigments were particularly used in easel paintings between the 15th and 17th centuries. Unfortunately, they had the tendency to darken in an irreversible way, explaining why they fell into disuse. The darkening mechanism of films of copper pigments in linseed oil is studied by electron paramagnetic resonance (EPR) and by optical absorption spectroscopy (OAS). EPR and OAS reveal different chemical and photochemical behaviors depending on the type of copper complex and on the binding oil. The effect of light is investigated by illuminating the films at ∼410 nm in the bridging ligand-to-metal charge transfer (LMCT) transition. The photodarkening manifests itself as the appearance of an optical absorption band around 22 000 cm −1 and a decrease of the EPR intensity of bimetallic copper complexes. These effects are explained by the photoinduced substitution of acetate (or abietate) bridging ligands by dioxygen molecules from ambient atmosphere. The resulting peroxo-Cu II dimer is characterized by a red shift of the LMCT and an increase of the exchange interaction in the ground state, which is responsible for the decrease of the EPR intensity due to the depletion of the paramagnetic S = 1 state. This mechanism explains the differences in darkening intensity observed with different pigment compositions (resinate versus acetate, raw linseed oil versus boiled linseed oil). 1. INTRODUCTION Copper acetate (also known as verdigris) and copper resinate are bimetallic green copper pigments used in easel paintings between the 15th and 17th centuries in Europe. 1−8 Appreciated for its brilliance and pure green tone, copper acetate (Cu 2 (CH 3 COO) 4 ·2H 2 O) was prepared by corroding copper plates with acetic acid (vinegar). 1−4 Copper resinate (Cu 2 (C 19 H 29 COO) 4 ·2H 2 O) was synthesized by mixing copper acetate with abietic acid coming from terpenic resins, as colophony or Venice turpentine. 1,8 Despite its bright glassy appearance, it was used by painters for a shorter period than copper acetate, ceasing to be used around the end of the 16th century. At the molecular scale, both copper acetate and copper resinate have the same structure, based on bimetallic carboxylate complexes with a bidentate bridging coordination, as determined by X-ray and neutron diffraction for solid copper acetate 9,10 and by UV−visible, Fourier transform infrared (FTIR), and X-ray absorption spectroscopy (XAS) analyses for viscous copper resinate. 3 Their molecular structures are presented in Figure 1. The binder used most frequently by painters during the Renaissance and Early Modern periods was linseed oil, extracted from linen seeds. It is composed of a mixture of fatty acids and triesters of fatty acids and glycerol. 11 This siccative oil was appreciated for its ability to improve the drying of pictorial films, which involves the formation of several degradation products such as alcohols, aldehydes, ketones, and esters. 12,13 Sometimes painters also mixed their pigments with boiled linseed oil (BLO), which was heat- treated in a vessel to dry faster than raw linseed oil (LO). As BLO is prepolymerized, it is more stable than LO due to a molecular network resulting from the reactive functions and to the advanced oxidation state of the matter. 11 In spite of the very much appreciated shades of green obtained with copper acetate and resinate, they were quickly given upcopper resinate earlier than copper acetate because of their tendency to progressively darken with time. 1,4 The coexistence of brown and green areas on the same painting can also be encountered. However, even though this non-systematic degradation has been known for long times, it is still not well understood. Various hypotheses have Received: July 6, 2019 Article pubs.acs.org/IC Cite This: Inorg. Chem. XXXX, XXX, XXX-XXX © XXXX American Chemical Society A DOI: 10.1021/acs.inorgchem.9b02007 Inorg. Chem. XXXX, XXX, XXX−XXX Downloaded via GARDNER WEBB UNIV on September 17, 2019 at 13:42:08 (UTC). See https://pubs.acs.org/sharingguidelines for options on how to legitimately share published articles.