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

Abstract

Distinguishing the polymorphs of basic copper chlorides is useful to inform how these materials were used as pigments in Western artworks. Raman imaging spectroscopy and multivariate curve resolution‐alternating least squares (MCR‐ALS) were employed to distinguish the minerals used in green pigments of several Chinese architectural paintings (12th–19th centuries). MCR‐ALS generated component spectra for the Raman datasets, which allowed the resolution of trace materials separated from background features. This method established the mixtures of atacamite and botallackite polymorphs and their relative ratios in five paint samples and identified the mineral azurite, a basic copper carbonate in one sample. These findings identify and suggest a shift to synthetic basic copper chloride pigments in Chinese architectural paints based on the spectroscopically resolved spectra of these mixtures. Thus, this methodology has successfully been applied to paint cross sections to distinguish components and to inform on the materiality and history of objects.

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].…”

Application of chemometrics on Raman spectra from Mars: Recent advances and future perspectives

“…Learning from terrestrial applications, the information extrapolated from Raman spectra can be optimized by the use of chemometric strategies. In recent years, the chemometric analysis of Raman datasets has found a fruitful application in many disciplines, including forensic science, 28,29 environmental pollution, 30,31 conservation science, 32,33 food chemistry, 34,35 pharmaceutic, 36,37 and medical science. 35,38 As a result, chemometric analysis in conjunction with Raman spectroscopy proved to be a reliable approach for spectra calibration, 39,40 identification, 41,42 classification, 43,44 and quantification 45,46 purposes.…”