Colour and Chemistry of the Glass Finds in the Roman Villa of Treignes, Belgium

Optical spectroscopy allows the identification of ionic species and, under certain conditions, the quantification of Fe + and Fe3+. The ratio of the oxidation states of iron gives an insight into the technological aspects of production. Moreover from the transmission spectra it is possible to calculate the CIE Lab colour coordinates and the UV absorption edge. The latter parameter is strongly related to the presence of heavy elements because they disrupt the silica polymer network. The optical parameter highlights differences in the sample population allowing the definition of subgroups. A comparison between colour coordinates, iron redox ratios, UV absorption edge and the chemical composition is presented. The results provide important information about the proportion between different compositional groups available from the archaeological site and underline the potentiality of UV-Vis-NIR absorption spectroscopy as a first-step screening method for large sets of archaeological or historical glass fragments. The present case-study demonstrates the results of optical spectroscopy on a selection of 16 late Roman "naturally" coloured glass fragments from the Roman villa complex 'les Bruyeres' in Treignes (Belgium).