Redox conditions in early glass making
Most early glass is not fully colourless and transparent. Most glass of the Roman period is transparent but tinted green or blue, depending on the oxidation state of the iron which is a contaminant form the raw materials, while there is a range of deliberately-produced strongly coloured glasses, many of which depend on achieving the appropriate oxidation conditions of iron, copper or manganese.
Pliny the Elder in his Natural History (c. 70 AD), as well as the Price Edict of Diocletian (301 AD), provide strong indications that the value of glass depended on its colour. Furthermore, it would be very useful archaeologically if it were possible to use the colour of glass to relate it to a specific production centre or workshop.
The present project aims to improve our understanding of the generation of glass colours, focussing on the first instance on glass of the first millennium AD. We have sampled a range of glasses from near eastern production centres and determined the Fe3+/Fe2+ ratios using Mössbauer spectrometry and X-ray Absorption Spectroscopy. While there is agreement between these two techniques over a range of oxidising conditions, there are discrepancies in glasses with lower Fe3+/Fe2+ ratios.
Even so it is clear that in colourless glasses the iron is more-or-less fully oxidised by the addition of antimony or manganese decolourants, that conditions in primary tank furnaces are relatively reducing and that remelting of primary glass for the production of vessels typically does not result in significant oxidation.
Related to this work is the investigation of more strongly coloured glasses, in particular red glasses which are reducing to produce cuprous oxide particles or metallic copper nanoparticles which colour the glass red. Strategies to achieve this condition show significant variation, which may allow us to trace technological traditions and the trajectory of glass making skills.
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