Fig. 4

Optical micrograph of scanned area (200 × 110 μm²) of cross sectioned paint sample R36-3 (top). In-depth distribution of primary compounds obtained by μ-XRPD scanning (see Fig. 1 for sampling location): RGB composite maps of some primary compounds (middle panel) and secondary compounds (lower panel). As a result of the degradation processes that oxidize the original orpiment present in layer ③ and hydrolyze the lead white in layer ②, followed by migration of the resulting arsenate, sulfate, calcium and lead ions, at the interface between layers ② and ③ (indicated as ❷′ and ❸′) but also at the surface of the paint (indicated by ④) secondary compounds such as palmierite, syngenite, schultenite and mimetite precipitate