DEEPLY COLOURED AND BLACK GLASS IN THE NORTHERN PROVINCES OF THE ROMAN EMPIRE: DIFFERENCES AND SIMILARITIES IN CHEMICAL COMPOSITION BEFORE AND AFTER ad 150*

In this work we attempt to elucidate the chronological and geographical origin of deeply coloured and black glass dating between 100 bc and ad 300 on the basis of their major and trace element compositions. Samples from the western and eastern parts of the Roman Empire were analysed. Analytical data were obtained by means of a scanning electron microscope - energy-dispersive system (SEM-EDS, 63 samples analysed) and laser ablation - inductively coupled plasma - mass spectrometry (LA-ICP-MS, 41 samples analysed). Among the glass fragments analysed, dark brown, dark purple and dark green hues could be distinguished. Only among the dark green fragments could a clear compositional distinction be observed between fragments dated to the periods before and after ad 150. In the early samples (first century bc to first century ad), iron, responsible for the green hue, was introduced by using impure sand containing relatively high amounts of Ti. In contrast, a Ti-poor source of iron was employed, containing Sb, Co and Pb in trace quantities, in order to obtain the dark green colour in the later glass samples. The analytical results obtained by combining SEM-EDS and LA-ICP-MS are therefore consistent with a differentiation of glassmaking recipes, detectable in glass composition, occurring in the period around ad 150.