The detection of copper resinate pigment in works of art: contribution from Raman spectroscopy † Claudia Conti, a * Jana Striova, b Irene Aliatis, c Elena Possenti, a Geneviève Massonnet, c,d Cyril Muehlethaler, c,d Tommaso Poli e and Matteo Positano f Copper resinate is a green pigment widely used by the 16th century painters, as many surveys on Italian and European paintings proved. The pigment is a transparent green glaze, and its color is due to copper salts of resin acids. The oldest recipes suggest the preparation of copper resinate by mixing verdigris with terpenic resins as Venice turpentine (conifer resins) on hot ashes. The detection of copper resinate in paintings is up to this time an analytical challenge. We examined the Raman features of copper resinate (powder and in a mock paint film with linseed oil) and compared them with verdigris. Six laser sources (488, 514, 532, 633, 785, and 830 nm) were used in different laboratories to highlight the drawbacks and advantages of a specific excitation source. The obtained results were applied in the analyses of a famous Caravaggio’s painting. For a detailed interpretation and full exploitation of the Raman spectra, Fourier transform infrared spectroscopy measurements were carried out as well. Copyright © 2014 John Wiley & Sons, Ltd. Keywords: copper resinate; verdigris; Raman spectroscopy; green painted layers; Caravaggio Introduction The term ‘copper resinate’ (CR) commonly refers to a transparent green glaze made up of copper salts of resin acids. The pigment was introduced in Dutch painting in the 15th century and in Italian easel painting in the 16th century, [1] both as a glazing lacquer and as a green pigment mixed with pure oil or oil and egg binders. [2] Laurie [3] reports the occurrence of a transparent copper-containing pigment in the green paints in illuminated manuscripts dated from the 8th to 14th century, but no analytical data proved the use of CR. [1] The glassy brilliant optical effects of CR were particularly appreciated by painters such as Caravaggio, [4–7] Jan Vermeer, and Arnold Böcklin. [1] The pigment fell gradually into disuse starting from the 18th century because of its tendency to discolor. [1,2] The earliest known recipe of CR dates back to the 17th century. De Mayerne [1] suggested the use of verdigris, a green pigment composed of copper salts of acetic acid (Cu(CH 3 COO) 2 ·H 2 O or (xCu(CH 3 COO) 2 · yCu(OH) 2 zH 2 O)) [1,8] widely used by the artists since antiquity. [9] By dissolving a verdigris pigment in Venice turpentine with spirit of turpentine on hot ashes, a very viscous and green mass was formed, which dried in a few months. Nowa- days, CR is commonly obtained in two ways: by reaction of a copper salt solution with an aqueous solution of sodium resinate or by melting natural resins, such as Venice turpentine (mainly abietic acid), with reactive copper compounds (copper acetate, copper carbonate or copper oxide, and hydroxide). [1,2] The resulting prod- uct is a mixture of copper carboxylate complexes of resin acids. It can be used as a liquid or as a dried powder, commonly bound with a drying oil. Depending on the synthesis conditions, the pigment color varies from greenish to green-blue or bluish; by admixing the pigment with organic yellow ones, such as yellow lake, saffron, or gamboges, its hue becomes warmer. [3,10] Different degradation processes of CR were investigated: [1,2,11–14] high temperatures decompose the pigment to black copper oxide and carbon; expo- sure to daylight causes the color surface to fade; ultraviolet irradiation and thermal treatments cause optical alteration due to photochemical phenomena [2] and/or local modification of the copper coordination structure. [13] Moreover, green glazes containing copper complexes can form spontaneously by the reaction of copper pigments with the media (lipids, proteins and waxes). [1,3,15] * Correspondence to: Claudia Conti, Istituto per la Conservazione e la Valorizzazione dei Beni Culturali (ICVBC), Consiglio Nazionale delle Ricerche (CNR), Via Cozzi 53, 20125, Milano, Italy. E-mail: c.conti@icvbc.cnr.it † This article is part of the special issue of the Journal of Raman Spectroscopy entitled “Raman in Art and Archaeology 2013” edited by Polonca Ropret and Juan Manuel Madariaga. a Istituto per la Conservazione e la Valorizzazione dei Beni Culturali (ICVBC), Consiglio Nazionale delle Ricerche (CNR), Via Cozzi 53, 20125, Milano, Italy b Consiglio Nazionale delle Ricerche (CNR), Istituto Nazionale di Ottica and LENS, Università di Firenze, Via Nello Carrara 1, 50019, Sesto Fiorentino, Italy c Dip. DIFEST, Università di Parma, Parco Area delle Scienze 7/A, 43124, Parma, Italy d School of Forensic Science (ESC), Institut de Police Scientifique, University of Lausanne, Batochime, UNIL-Sorge, CH-1015 Lausanne-Dorigny, Switzerland e Università degli Studi di Torino, Dip. Chimica I.F.M., Via Giuria 7, 10125, Torino, Italy f Emmebi Diagnostica Artistica s.r.l., Via S.Francesco di Sales 80, 00165, Roma, Italy J. Raman Spectrosc. (2014) Copyright © 2014 John Wiley & Sons, Ltd. Research article Received: 28 November 2013 Revised: 23 December 2013 Accepted: 30 January 2014 Published online in Wiley Online Library (wileyonlinelibrary.com) DOI 10.1002/jrs.4455