Multi-Analytical Study of Copper-Based Historic Pigments and their Alteration Products

Abstract

Copper-containing materials such as verdigris are commonly found in historic and artistic works of art, often at advanced states of decay. Applied on paper as inks and watercolors, many of which needed a binder such as gum arabic, the intrinsic instability of this pigment was known since the medieval period. The decay of verdigris (a mixture of copper acetates) as a pigment, as watercolor, and as a dye, was studied using a combination of vibrational (Fourier transform infrared) and X-ray photoelectron spectroscopy (XPS) and X-ray diffraction (XRD) instrumental techniques. Changes in the copper oxidative states and the formation of copper oxide were monitored during accelerated degradation as powdered solids and applied on mockup samples (with and without binder). Accelerated aging of both commercially available and synthesized verdigris pigments showed the presence of an intermediate species, Cu(CH3COO)2•3Cu(OH)2•2H2O, which points to the beginning of the decay processes, that culminates in the formation of Cu(II) oxide. However, the presence of gum arabic seems to delay deterioration, by temporarily reducing Cu(II) to Cu(I), even when the final product includes Cu(II). This novel application of XPS and supporting techniques has significant implications in art conservation, as the identified behavior helps explain the better preservation state of some works of art.

Recent advances and applications to cultural heritage using ATR-FTIR spectroscopy and ATR-FTIR spectroscopic imaging

“…The initial (original) DS of CN and CA obtained with ATR-FTIR spectroscopy used in this research were 2.26 and 2.3-2.4, respectivley. 155,157 Artificial and accelerated ageing is a treatment for investigating degradation of research samples 158 in a shorter time period compared to natural degradation. The objective of artificial accelerated aging is to speed up the changes in the properties of samples because natural ageing is often a slow process that would take several or even a hundred years.…”

Scientific study of the pigments and binders used in mural painting from the 16th‐centuryceSt Mary's Church of Cheriapally, Kottyam, Kerala, and its relevance to conservation

“…Corrosion processes of malachite, sulfates, chlorides, and other copper salts were primarily investigated, although examination of more works of art revealed that a large range of copper salts was also used as a pigment. Several environmental and intrinsic factors are correlated with the formation of chlorides, sulfates, phosphates, and mixed salts, such that the presence of particular secondary minerals is associated with local conditions, leading to degradation and stabilization of the compounds (Kanth & Singh, 2019a, 2019b; Švarcová et al, 2009; Wiggins et al, 2019).…”

Architectural Polychromy on the Athenian Acropolis: An In Situ Non-Invasive Analytical Investigation of the Colour Remains

“…Atacamite is a copper oxychloride, Cu 2 Cl(OH) 3 , and is known to have been used as a green pigment of natural provenance [65] or as a synthetic product [66] in other parts of the world and in later historical periods. This is not surprising because atacamite occurs as a natural mineral in the oxidation zones of copper deposits [67][68][69]. It is also known that atacamite occurs as a chloride corrosion product on copper and bronze objects [70,71].…”