Synchrotron-based x-ray spectromicroscopy used for the study of an atypical micrometric pigment in 16th century paintings

Grunewald is a famous German painter of the 16th century, whose celebrity is associated with his unique skill in handling colors. This article presents the analysis of materials used to render a metallic aspect in the Isenhein Altarpiece and the Basel's Crucifixion. Such samples are challenging objects for microanalysis due to both chemical and physical complexity. Their study by synchrotron-based X-ray microscopy techniques was made possible thanks to recent developments carried out at the ID21 beam line (European Synchrotron Radiation Facility, ESRF). A submicron X-ray fluorescence probe revealed the main presence of lead, sulfur, antimony, and calcium. The fluorescence-line interferences (in particular K-lines of sulfur with M-lines of lead, and K-lines of calcium with L-lines of antimony) were resolved with the fitting program, PyMCA. 2D-mapping highlighted the presence of micrometer grains of sulfur and antimony into a lead matrix. XANES measurements were performed at both the sulfur K-edge and the antimony ledge to refine information from an atomic to a molecular level. Beam stability was a key point in this study to selectively probe micrometer pigment grains, dispersed in the lead matrix. They confirm that the grains are made of stibnite (antimony sulfide), a very atypical pigment. Chemical mapping of sulfides is perfectly correlated with antimony mapping and provides a clear visualization of the stibnite pigments, in addition to their identification. Besides its artistic relevancy, this work aims at illustrating developments of synchrotron X-ray microprobe methods for the chemical characterization and observation of complex and micrometer-scale materials.