Copper blue in an ancient glass bead: a XANES study
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The blue colour in ancient soda-lime glasses has been attributed to the presence of copper and/or cobalt but the origin of different shades is not yet fully interpreted. As a contribution to this question, a non-destructive X-ray absorption study at [ Cu]K-edge was undertaken on the blue (turquoise) layer from a “Nueva Cadiz” type tubular glass bead dated pre-XVII century where copper is the unique colouring agent. Minerals configuring two distinct blue tonalities due to Cu (2+) in similar square coordination were selected as basic model compounds: azurite, which is a classical navy-blue pigment used in ancient wall paintings over plaster, and chalcanthite, displaying exactly the same turquoise-blue tonality of tubular glass beads manufactured since the Egyptian Antiquity. Theoretical modelling of the XAFS spectra was undertaken using the FEFF code. The IFEFFIT software package was used for fitting the calculated spectra to experimental data. EXAFS results are discussed in view of the crystal structures of copper minerals chosen to model the speciation state and structural situation of that element prevailing in the turquoise-blue archaeological glass. Special attention is focused on the difficulties in theoretical modelling [ Cu]K-XANES spectra of ancient glasses with different colourings.
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