Roman glasses coloured by dissolved transition metal ions: redox-reactions, optical spectroscopy and ligand field theory

Abstract

A collection of Roman glass samples from Ancient Messene, Greece, was analysed non-destructively for colouring elements and non-colouring additives such as fining agents, opacifiers and decolourizers, by comparing the chemical composition with the observed optical spectra. The resulting information on ion speciation is important for the discussion of technological parameters such as the applied melting temperatures or the prevalent redox conditions. Speciation also helps to distinguish intentionally added dopants from impurities. This knowledge might be used in provenancing the raw materials. The Roman assemblage under study included purple coloured Mn3+-containing glasses, but also samples with high levels of colourless Mn2+. Manganese ions can either be considered a residue of recycling, or are indicative of an intentional addition, either as a fining or as a decolouring agent. Antimony oxide was either added as a fining agent, resulting in good quality transparent glasses, or as crystalline antimonates which act in glass mosaic tesserae as opacifiers. The much weaker molar extinction coefficient and the partial reduction of blue Cu2+ to colourless Cu+ explain why CuO is a weaker colouring agent than CoO. The colours of iron-bearing glasses range from light blue to green and from yellow to dark brown, reflecting a complex interplay between redox conditions and the presence of sulphur in the glass melt.Quantitative speciation of dopants will be reviewed on the basis of the ligand field theory. The different transition probabilities will be demonstrated on differently doped soda lime silicate glasses which were prepared for this purpose in the laboratory.