New highlights on degradation process of verdigris from easel paintings

On the stability of mediaeval inorganic pigments: a literature review of the effect of climate, material selection, biological activity, analysis and conservation treatments

“…It promotes oil curing and it retards its oxidation [20]. Verdigris (neutral and basic), as well as the copper carbonate pigments azurite and malachite, shows the tendency to react with the oil binding media to form copper soaps [35,67,145], to brown and darken, as Cu + is formed by ambient light absorption (photoreduction), and oxygen promotes the formation of brown peroxide species [146]. Cu-salts of organic acidic compounds are formed in verdigris-containing paint layers (or at the interface with the varnish), as fatty and resin acids can extract copper ions from the verdigris pigment [133,147].…”

“…Copper acetates and resinates catalyse the oxidation process of oil films and influences the curing of the oil [133,134,147,148]. In tempera, verdigris seems less prone to discolouration, but still reactions of Cu-extraction can take place [133,146]. Copper acetates are very soluble in acidic conditions, and copper oxalates are formed in presence of oxalic acid from various sources [32].…”

On verdigris, part II: synthesis of the 2-1-5 phase, Cu3(CH3COO)4(OH)2·5H2O, by long-term crystallisation from aqueous solution at room temperature

“…Historic verdigris samples are even more complex as most of them are multiphase mixtures, as well as they are usually already affected by degradation processes. 18 This emphasis the need for reliable reference data for the pure x − y − z phases which can serve as a basis for phase identification, e.g. by vibrational spectroscopy or quantification by X-ray powder diffraction (XRPD).…”

Tracing the Alteration of Verdigris Pigment through Combined Raman Spectroscopy and X-ray Diffraction, Part I

“…Optical microscopy of the pigment from clear green areas is similar to, but does not match, reference verdigris, and samples from both types of greens exhibit Raman spectra with highly fluorescent backgrounds and only very broad, weak peaks with maxima near 1580, 1440, and 1350 cm −1 (Figure 1b) (Albro et al 2011(Albro et al , 2012Brostoff et al 2011Brostoff et al , 2013. Due to the well-known behavior of copper salts to combine with proteinaceous and resinous media (Scott 2002;Gunn et al 2002;Aceto et al 2010;Santoro et al 2014), such copper-containing green pigments without other identifying characteristics are often described as organo-copper complexes and presumed to be altered verdigris (Kühn 1993;Scott 2002;Mairinger et al 1980;Banik et al 1981;Ponahlo 1982/1983;Chaplin et al 2006Nastova et al 2012Chaplin et al 2005;Groen 1975). However, it is also known that in some cases artists made amorphous copper-based green colorants or glazes intentionally (Conti e.g.…”

“…Previous investigations of verdigris primarily focus on characterization of the unaged reference salts, or on verdigris alteration in oil, resin, or proteinaceous media (e.g. Cartechini et al 2018;Gunn et al 2002;Mendes et al 2008;Richardin et al 2011;Santoro et al 2014;Švarcová et al 2014;Zoleo et al 2014). Studies of verdigris in aqueous media are fewer in number, and often focus on the deleterious effects of copper ions on cellulose deterioration (Ahn et al 2015;Ponahlo 1982/1983;Bhownik 1970;Henniges et al 2006;Kühn 1993;Shahani and Hengemihle 1986) or the evaluation of stabilization treatments (Henniges et al 2006;Maitland 2009;Kolar et al 2008;Ahn et al 2014, Hofmann et al 2015Malešič et al 2015;Hofmann et al 2016).…”