Characterization of copper-based pigment preparation and alteration products
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MRS Advances © 2018 Materials Research Society DOI: 10.1557/adv.2018.53
Characterization of copper-based pigment preparation and alteration products Marcie B. Wiggins,1 Jocelyn Alcántara-García,2 Karl S. Booksh1 1
Department of Chemistry & Biochemistry, University of Delaware, Newark, DE 19716
2
Department of Art Conservation, University of Delaware, Newark, DE 19716
ABSTRACT
Copper-based pigments are common in works of art that show signs of decay on green and blue areas and are frequently associated with the degradation of organic substrates and/or media (drying oils, cellulose, etc.). The exact causes of degradation remain unknown. This prompted us to study possible starting and degradation products of one especially reactive copper pigment, verdigris (copper acetate), as well as pigments of the same family (salt and soap greens). Preparation of pigments using historical methods was followed by spectroscopic and crystallographic characterization using Fourier-transform infrared (FTIR) spectroscopy, X-ray diffraction (XRD), and X-ray photoelectron spectroscopy (XPS). Synthesis of verdigris and verdigris-like pigments resulted in a mixture of starting polymorphs of verdigris, including neutral and basic verdigris. With accelerated aging, pigments degraded to a polymorph of basic verdigris when not affected by organic media, whereas pigments on cellulosic substrates showed oxidized copper species. With this study, we are beginning to understand verdigris starting materials and highlight the complex interactions between pigments and substrates that influence pigment degradation pathways.
INTRODUCTION: Blue and green copper-containing pigments have been used in art since antiquity. While there are many copper-based pigments, the most common are copper carbonates (azurite, malachite), and copper acetates (verdigris). Of these pigments, verdigris is unique, as it cannot be found in nature as a mineral, but must be synthesized. As a result, verdigris can be found as several variations of copper acetates, all used in artworks [1-3]. Objects containing verdigris include, but are not limited to, manuscripts, oil paintings, and mural paintings; all of which the degradation and instability of verdigris are well-documented [4-8]. There are reports of verdigris and its preparation dating back to antiquity, for instance, ancient Greeks and Romans, Theophrastus, Vitruvius, and Pliny. Historically, reports of these pigments are not limited to art technology texts, but include those found in medicinal and natural history references [2, 9-11]. Copper ions embedded in organic binders and/or on organic substrates are very reactive. In fact, there is evidence suggesting basic verdigris species are more reactive than neutral verdigris [7, 12]. Herein, we report on the degradation studies related to the interactions between this complex family of pigments (including verdigris-like pigments, such as salt and soap greens) and an organic substrate. Historic recipes of verdigris and verdigris-like pigments were replicated and charact
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