A XANES study of chromophores: the case of black glass

Abstract

We studied the Fe K-edge X-ray absorption near edge (XANES) spectra of several Roman black glass fragments in order to determine the Fe³⁺/ΣFe ratio of these materials. The selected archaeological glass samples cover the period 1^st–5^th century AD in nine different sites of the North Western provinces of the Roman Empire. The fragments belong to two different compositional groups demonstrating a diachronic evolution: early Roman HMG (High Magnesia Glass) and Roman Imperial LMG (Low Magnesia Glass). The first group contains natural Fe levels (below 2 wt% as Fe₂O₃), while the LMG has concentrations above 5 wt%. This difference is also reflected by Fe³⁺/ΣFe values. Low iron glass was produced under strongly reducing conditions in order to obtain the black colour, with average Fe³⁺/ΣFe values ≈ 0.17. LMG glass is somewhat more oxidised (Fe³⁺/ΣFe ≈ 0.4–0.5). While HMG glass required active control of the furnace environment, LMG was made under ambient atmosphere and its higher oxidation degree is mainly determined by the chemistry of the raw glass.