Archaeometry 47, 4 (2005) 763– 780. Printed in Singapore *Received 18 August 2004; accepted 14 December 2004. © University of Oxford, 2005 Blackwell Publishing, Ltd. Oxford, UK ARCH Archaeometry 0003-813X © University of Oxford, 2005 November 2005 47 4 ORIGINAL ARTICLE Making colourless glass in the Roman period C. M. Jackson MAKING COLOURLESS GLASS IN THE ROMAN PERIOD * C. M. JACKSON Department of Archaeology, University of Sheffield, Northgate House, West Street, Sheffield S1 4ET, UK This paper discusses the compositional analysis of Roman colourless glass from three sites in Britain. The findings suggest that two broad compositional groups can be identified on the basis of the choice of the initial raw materials selected for glass production, in particular the sand. The largest of these groups is inherently different from the naturally coloured, blue–green glasses of the same period, while the other group is compositionally similar. Further subgroups are apparent on the basis of the decolorizers used. These glass groups are explored in the light of the current theories concerning the organization of glass production in the Roman world. KEYWORDS: BRITAIN, ROMAN, COLOURLESS GLASS, ANTIMONY, MANGANESE, SAND *Received 18 August 2004; accepted 14 December 2004. INTRODUCTION Roman glass is recognized as a masterful feat of technology in terms of both manufacture and design. The most striking visible sign is the control over colour, where colorizers and decolor- izers are used skilfully, in conjunction with the control of furnace parameters, to provide a wide variety of hues. This study argues that colourless glasses are a good medium through which to show the skill of the Roman glassmaker and illustrate the influence of the choice of raw materials upon the final glass composition. These choices are explored in the light of current theories relating to diachronic changes in the nature of colourless glass manufacture and the organization of glass production in the Roman world. Colourless glass is known from the early periods in glass-making history (Bimson and Freestone 1988), but its popularity increased during the Roman period. From the late first century ad and into the second century it was produced in volume for high-quality tablewares, its popularity only declining in the late third century (Price and Cottam 1998, 16). In fact, Pliny specifically mentions it, suggesting that ‘the most highly valued glass is colourless and transparent, as closely as possible resembling rock-crystal . . .’ (NH XXXVI, 200). Such glass forms, which are clear and ‘sparkle’, can be found in high-quality vessels such as facet-cut beakers and dietrata. However, the statement also implies that colourless glasses could be found in a number of different qualities, from the truly colourless to those that have a slight blue or green tint. This range of hues within colourless glass has been noted by other authors (e.g., Sayre 1963) and is explored in this paper. The greatest proportion of Roman glass is blue–green (Price and Cottam 1998, 15) owing to iron impurities introduced to the glass from the raw materials (Bamford 1977, 79). A