Atacamite discolouration under the influence of arsenates in wall paintings in the Kizil Grottoes, Xinjiang, China

This study was focused on a specific blue‒green pigment applied on the edges of the green blocks of wall paintings in the Kizil Grottoes, the earliest Buddhist cave complex in China containing many wall paintings. Based on in situ portable XRF analyses, μ-Raman spectroscope, elemental analysis (SEM–EDS and EPMA-WDS) and μ-XRPD analyses, the pigment was proven to be lavendulan [NaCaCu₅(AsO₄)₄Cl·5H₂O], which has not been reported in ancient wall painting studies before. Lavendulan was found to coexist with atacamite [Cu₂(OH)₃Cl] in some of the samples in the study and showed a nanoneedle-like morphology. Further investigations of lavendulan revealed that it was probably not originally used as a pigment but was generated by the transformation of atacamite under the influence of arsenates [As(V)] and Na⁺. This phenomenon and the related mechanism were discovered in this study and are discussed in this paper. As an important cultural heritage site in the Silk Road, the Route Network of the Chang’an-Tianshan Corridor and the Kizil Grottoes are listed as UNESCO World and Natural Heritage sites. The pigment types and preservation conditions of wall paintings are similar to those of many wall paintings found along the Silk Road, and further attention should be given to this discolouration mechanism for heritage site conservation.