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

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.

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