Characterization of copper-based pigment preparation and alteration products

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.

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

“…This is consistent with previous results in Wiggins et al. 38 Although this result could be interpreted as a mixed-valence compound being present, it is possible the ultrahigh vacuum required to run XPS dehydrated the samples, resulting in a partial reduction.…”

“…3–5,8 However, similar studies to those made on verdigris with oil media remain scant. Herein, we report on the results that build onto preliminary work 38 and discuss detailed accelerated degradation studies of verdigris using gum arabic on cellulose, a common binding agent and media in writing and illumination. The present article reports on the application of a multi-analytical approach, including X-ray photoelectron spectroscopy (XPS), for tracking oxidation state changes that lead to the decay of verdigris in cultural heritage applications; ongoing investigations focus on substrates other than cellulose.…”

Polymorph identification in green Chinese architectural paints using Raman imaging and multivariate curve resolution

“…The preparation method reported in Europe involves covering a copper plate in honey and salt (NaCl) and then suspending it over vinegar to produce the pigment. Studies that have replicated this process have found only atacamite present 3,4,31 . These varying pigment products warrant additional studies into “copper green,” as it is not clearly understood and it possibly plays a larger role in Chinese architecture than “mineral green.” Evidence, while limited, from this study and others 4–6,16,17 show a transition from the mined “mineral green” to synthesized “copper green” pigments around the 16th century.…”

Multidisciplinary Learning: Redox Chemistry and Pigment History

“…Namely, X-ray diffraction (XRD), Raman and Fourier transform infrared (FTIR) spectroscopies, and X-ray photoelectron spectroscopy (XPS) were used to to track chemical changes. 25,29,32,33 ■ EXPERIMENTAL SUMMARY See Supporting Information for full lab experiments. Both handouts describe the synthesis and can be used for both upper-undergraduate and graduate classes on chemistry and heritage conservation science.…”

“…For an upper-level undergraduate chemistry or a graduate art conservation course, characterization techniques can be easily added to the experimental design. Namely, X-ray diffraction (XRD), Raman and Fourier transform infrared (FTIR) spectroscopies, and X-ray photoelectron spectroscopy (XPS) were used to to track chemical changes. ,,, …”