Characterisation of rouaite, an unusual copper-containing pigment in early modern English wall paintings, by synchrotron micro X-Ray diffraction and micro X-Ray absorption spectroscopy

Although the use of verditer, a synthetically produced copper carbonate pigment chemically analogous to azurite (Cu₃(CO₃)₂(OH)₂), in early modern wall paintings is regularly reported in grey literature, its detection in wall paintings is typically unavailable in a public forum. This article focusses on two particular samples (M9 and T1) initially suspected to contain copper blue pigments from decorative wall painting schemes in domestic houses across a range of status groups, in eastern and south-eastern England, dated to 1550-1650. Interest in verditer synthesis procedures is found in textual sources including both historic manuals and current scientific literature. The unreliability of the synthesis, particularly for blue verditer, however, is widely documented. Here, we report a multimodal study employing SEM-EDX, SR-\(\mu\)PXRD mapping, and SR-\(\mu\)XANES spectroscopy to characterise the phases present in two wall painting samples, one c. 1600/early 1600s (M9) and the other c. 1600 (T1). In addition to assessment of the presence of verditer, this study presents the first identification of rouaite, a synthetic copper hydroxynitrate mineral (Cu₂(NO₃)(OH)₃), in wall painting samples. Rouaite, shown in a previous publication to be a common byproduct of replications of the historical verditer synthetic recipes, appears in combination with verditer and alone. Its infrequent identification in paintings and murals is considered in light of analytical challenges. The presence of copper hydroxychlorides is also discussed in the context of rouaite stability and degradation. These results represent a significant development in our knowledge of the early modern palette in England and historical verditer synthesis and advances our understanding of how these painting schemes may be best preserved in the future.