Cu and As containing pigments in Zhejiang architecture polychrome paintings: a case study of degradation products of emerald green

Abstract The discoloration of pigments caused by deterioration may significantly alter an artifact's original pattern and design, which could have an impact on its intended meaning. The green pigments containing copper (Cu) and arsenate (As) in paintings are usually recognized as Emerald green or Scheele’s green and are also infrequently reported as cornwallite and lavendulan. Recent studies point out that lavendulan may be the degradation product of Emerald green but not a natural mineral. Overall, there are still relatively few cases of current studies on green pigment with Cu and As. The elemental composition and molecular structure of green minerals containing Cu and As are similar, so it is difficult to identify by Raman spectroscopy. The green pigment samples used in this investigation were taken from architectural paintings in the Wenchang palace and traditional Ziweishan dwellings. The composition of the paint samples was investigated comprehensively by Raman spectroscopy, scanning electron microscope observation, micro-X-ray diffraction (μ-XRD), X-ray photoelectron spectroscopy (XPS) and X-ray absorption near-edge spectroscopy (XANES) analysis. From the results of elemental distribution, the pigments mainly contain Cu, As, and Cl. The morphological results illustrated that the particles in these three pigments are rounded and granular, indicating the synthetic pigment is possibly emerald green. The characteristic vibrational peaks of the As–O, Cu–O and Cu–Cl bond were detected by Raman spectroscopy, and infrared spectroscopy found the presence of oxalate salt. The XPS and XANES analysis results show that As valence is + 5. This study found that the green pigment samples are a degradation product of emerald green. The formation of this product is related to the preservation environment. The results of this work will provide information to understand the degradation processes of emerald green and be a reference for the identification of pigments containing Cu and As elements.