Improving the characterization of red coloring matter from prehistoric cave art by means of laboratory confocal XRF depth profiling combined with synchrotron XRF imaging

Chemical in situ study of red coloring matter in cave art is challenging because characteristic trace elements can be present both in the matter and in the wall support, and the latter has a quite heterogeneous composition.In this study, a stalactite presenting red iron oxide coloring matter from La Garma cave, Northern Spain, has been analyzed with complementary techniques. The aims are, on the one hand, confirming for the first time the feasibility of confocal XRF (CXRF) depth-resolved scans to successfully separate the complex stratigraphy of red prehistoric coloring matter on a calcitic support. On the other hand, finding differentiation criteria and improving the characterization of this red iron-based coloring matter would help understand and virtually separate it from its support, as well as improve future in situ analyses in cave sites.CXRF depth-resolved scans were performed using the LouX3D device available in our laboratory. With this technique, we can precisely determine the chemical depth composition of an object, layer by layer, with a spatial resolution in the micrometer scale. These measurements were combined with a synchrotron induced micro-X-ray fluorescence (SR-µXRF) chemical mapping performed at the PUMA beamline, SOLEIL synchrotron. These highly sensitive measurements not available with any other non-destructive lab-based technique allow seeing a greater number of trace and minor elements associated with Fe. Complementing them with the direct depth-resolved information given by CXRF, it was possible to find differentiation criteria distinguishing the wall support and the coloring matter, finely characterize this red iron-based matter, and confirm the feasibility as well as the advantages of applying CXRF for the non-invasive technical study of prehistoric cave figures.