Transition metal chromophores in glass beads of the classical and Hellenistic period: Bonding environment and colouring role

Abstract

The delicate blue and green hue of ancient coloured beads from the Classical and Hellenistic period originate from the different transition metal (TM) ions added in the batch materials during glass production. The chemical composition in conjunction with the TM redox are fingerprints of the colour and vitrification process. We report on the structural and colouring/decolouring role of TM in blue and green-blue soda-lime beads from Thebes, central Greece dated back to the Classical and Hellenistic period using non-destructive X-Ray Absorption Fine Structure (XAFS) spectroscopies. Concerning the TMs redox and their consequent influence on colour, it is established that the major blue colourants are Co2+ and Cu2+, colourless Cu+ dominates in the blue beads, whereas chromophores Fe3+ and Cu2+ give rise to a green-blue hue. The Mn2+ content in the blue beads, whose occurrence is attributed to unintentional addition via the Co rich mineral colourant, counteracts the effect of the moderately higher content of Fe2+ in the raw materials. Based on the XAFS analysis results, we propose that the fractions of all TM under question (Mn, Fe, Co and Cu) were chosen and adjusted properly not only to control the desired colour, but also to ensure the stability of the glass matrix. Indeed, the integrity of the silica matrix should be further enhanced due to the presence of intermediate Fe oxides linking to Cu ions in either planar or linear geometry, while Mn and Co act as typical network formers.