Speciation and weathering of copper in “copper red ruby” medieval flashed glasses from the Tours cathedral (XIII century)

Abstract Three “copper ruby red” (or “flashed”) glasses from the St Gatien cathedral in Tours (windows from the XIII century) were investigated at the Cu K-edge by synchrotron X-ray fluorescence microscopy and μ-XANES/EXAFS spectroscopies. The spectra are compared to XANES/EXAFS spectra collected for modern glasses synthesized at various O 2 fugacities. Two main types of red glasses are present in Tours, which show a distinct Cu speciation. In the first type (“plaques”), Cu is present as sub-micron metallic nucleates, which coexist with monovalent Cu linear moieties (30:70 at.%). In contrast, the glasses of the other type (“feuilletes”) show mostly monovalent Cu (together with some evidence for metallic Cu in the edge region of the XANES). The plaques glasses appear to have been synthesized at lower O 2 fugacity and higher temperatures to promote homogeneous nucleation of metallic Cu. A relative enrichment in Al (provided by the addition of orthoclase in the melt) greatly enhances the glass durability by decreasing the melt peralkalinity by a factor of ∼4. In contrast, the feuilletes were equilibrated at much higher O 2 fugacities. Hence, metallic Cu, despite being scarce in that glass, is not at equilibrium and could have well been added on purpose during the melt cooling. It is proposed that a technique called ‘tourage’ could have been used to help create the micron scale greenish and red layers. The feuillete glasses are also weathered at their surface, promoting the formation of amorphous Cu(II) species related to a Cu sulphate.