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In traditional painting, the presence of copper based pigments very often combines with the observation of copper oxalates and copper carboxylates as degradation products of the paint layer. Similar alteration phenomena are recurrent also for other pigments containing metal cations mainly in the presence of the oil binder (see [1,2] and reference therein). The reactivity of the system depends mostly on the nature of the pigment due to the different ability of the metal cations to diffuse into the paint organic matrix and to react. This is particularly true for the class of copper based green pigments as they show a great variety of chemical-physical properties related to the very different nature of such pigments ranging from purely inorganic compounds to organo-metallic complexes. In this research work a systematic investigation by infrared spectroscopy of the chemical changes produced into linseed oil model paints by the presence of selected copper green pigments (neutral verdigris, emerald green, chrysocolla and malachite) under different aging conditions is presented. Accelerated aging of the model paints has been performed in conditions of controlled RH% (45% and 90%) and temperature (45°C). Comparative infrared spectroscopic measurements by ATR and reflection acquisition modes have been performed at regular time intervals in order to exploit the different sensitivity to surface properties of the two approaches in monitoring the ongoing alteration process. Spectral data show a remarked reactivity of all the systems in the presence of high humidity leading to the formation of both copper oxalates and carboxylates. Furthermore, a change of the pigment molecular structure was observed both for verdigris and emerald green. Complementary measurements by micro-ATR-FTIR spectroscopy imaging have been performed on transversal cross-sections of the degraded paints to characterize the distribution of the different alteration products in order to help understand the occurring degradation mechanisms. Acknowledgements: This work has been financially supported by the national research projects SICH (Project No. 2010329WPF_001) founded by the Italian PRIN 2010-2011 program. References  L. Monico, F. Rosi, C. Miliani, A. Daveri, B.G. Brunetti, Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 116 (2013) 270–280.  V. Otero, D. Sanches, C. Montagner, M. Vilarigues, L. Carlyle, J.A. Lopes, M. J. Melo, J. Raman Spectrosc. 2014, 45, 1197–1206.
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1 Istituto di Scienze e Tecnologie Molecolari-CNR, Via Elce di Sotto ,8 - 06123 Perugia, Italy, email: firstname.lastname@example.org