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Modelling the size of red-colouring copper nanoclusters in archaeological glass beads

The origin of a red colour in ancient soda-lime glasses has been attributed either to the presence of both copper clusters and cuprous oxide or to copper alone. As a contribu- tion to this question, a non-destructive X-ray absorption study at the Cu K -edge was undertaken on the red layer from a singu- lar "rosette"-type archaeological glass bead dated as pre-XVII century. On comparing with data collected from metallic copper and the mineral cuprite, cubic Cu2O, XANES spectra of the red glass are identical to the first. Theoretical modelling of Cu 1s XANES spectra was undertaken using the FEFF code based on a multiple scattering formalism. A hypothetical tetragonal struc- ture was simulated for Cu2O in order to remove the constraints arising from linear O-Cu-O bonds, unstable within the silica glass matrix, and an ideal body-centred array was considered on the basis of real metallic Cu-Cu distances in the metal. Calcu- lations were performed for atom clusters of variable size within real and hypothetical structures. A spherical cluster of about 5 u A radius, capped by 24 copper atoms already provides a calculated Cu 1s XANES spectrum that compares well with data collected from the red glass. Post-edge details are noted in relation to the oxide, considering ionic states and effective valences of copper. The possibility of estimating the size of copper clusters through simulated structures is discussed.