The coloured glass of Iulia Felix

Research on the cargo of glass in the Roman ship Iulia Felix, wrecked off the town of Grado (province of Udine, North Italy) in the first half of the 3rd century AD and composed of recycling cullet carefully selected for colour and type, provided much information on Roman glass production technology. A combined approach, involving analytical, statistical, and archaeological evidence, included chemical analyses (X-ray fluorescence, electron microprobe), which indicated that the coloured samples were all soda-lime-silica in composition, with natron as flux, although cluster analysis identified many compositional groups. Comparisons among the compositional groups of the colourless glass, previously studied, and those of coloured glass showed that the production technologies of the colourless glass vessels constitute two well-defined technological end-members, also related to group types, into which those of the coloured glass samples fall. In particular, coloured glass samples - all bottles, low-status vessels - were produced with mainly beach siliceous-feldspar-calcareous sand. However, unlike the colourless type, strict control of raw materials and decolourising processes was not adopted, and recycling was also practised, as demonstrated by the Sb2O3, Cu and Pb patterns. In this context, a ‘‘recycling index’’ (RI) - [(Sb2O3)X/(Sb2O3)Ref]*100 - proposed here for the first time, in order to quantify the extent of recycling of antimony colourless glass in the batch. RI is valid for glass containing abundant Sb from an end-member of colourless glass. In conclusion, although it cannot be stated unequivocally that the identified compositional trends are related to different production centres or different raw materials, the strong evidence of compositional variability among all the Iulia Felix glass samples, both colourless and coloured, supports the dispersed production model for Roman glassware and the common practice of recycling in Roman imperial times, especially for low-status vessels.