A multimodal analysis of early synthetic Cu-based pigments in early modern wall paintings by Cu and Cl XANES, µ-XRF, and µ-XRD

A study of historical wall painting cross sections suspected to contain early examples of synthetic Cu-based blue pigments is presented, employing Cu and Cl K-edge XANES alongside µ-XRF and µ-XRD. Blue verditer, a synthetic basic copper carbonate, has been identified in paintings from the 16th-18th centuries, but has not been adequately studied scientifically. These samples are highly heterogeneous, necessitating spatially resolved µ-XRF and µ-XANES to clarify the identity of pigments and understand the presence of chlorine, which may be due to intentional use of copper chloride pigments or degradation/alteration processes. Mock paintouts aged under high Cl conditions are studied to contextualise the presence of chlorine. This work improves our understanding of early modern pigment use, production, and degradation. Basic copper carbonates and chlorides are also relevant to catalyst production and geochemistry, so this research has utility beyond cultural heritage as well.