Chemical Characterization of Roman Glass Vessels, Enamels and Tesserae
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CHEMICAL CHARACTERIZATION OF ROMAN GLASS VESSELS, TESSERAE
ENAMELS AND
JULIAN HENDERSON Research Laboratory for Archaeology and the History of Art, Oxford University, 6 Keble Road, Oxford, OXI 3QJ, U.K. ABSTRACT Chemical characterization of Roman glass vessels, enamels and tesserae has revealed compositional relationships between enamels and tesserae including the use of high magnesia red and orange glass, normally considered to have gone out of use hundreds of years earlier. The conservative use of glass recipes over a 300 year period is reflected in the compositions; the results of mass-production versus more restricted production are discussed. 1. INTRODUCTION When glass was used for the manufacture of Roman vessels it was almost invariably translucent, often produced in large volumes and commonly recycled. The working properties of the glass were closely linked to its chemical compositions, so that once established, a glass with a composition that could be blown was recycled. Chemical analysis has revealed that for the glass that has survived, the variations in composition from the normal soda-lime-silica glass with a low magnesia content (or LMG) tend to be related to producing visual effects. This is also true of the use of glass for enamel and tesserae where the use of contrasting colours is essential to function. Since the glass used for tesserae and enamels was manufactured in smaller volumes than for glass vessels and once used as enamel decoration or for the creation of mosaics, it is less likely to have been recycled we might expect to be able to characterize products from specific enamel and tesserae factories, possibly on a regional basis. This is making the the assumption that the artisans involved in the manufacture of tesserae and enamels somehow had access to a wider and/or different range of raw materials from vessel producers and that the products would in some instances be aimed at a slightly different market. These technical considerations were behind the aims of this project which were: (1) to identify compositional relationships between the glass used in vessels, enamels and tesserae; (2) to monitor changes in composition over time and (3) to investigate possible relationships between chemical characteristics and production zones. This paper will consider material of Ist-4th centuries A.D. in date, taken from a range of archaeological contexts within Europe. The champlevde enamels were used in a range of brooch types, metal vessels and seal boxes. Previous studies of Roman enamels [1,2] have concentrated on the colorants used; in addition the present study will consider the full range of oxides. Since very few compositional data for Roman tesserae have been published, these will be amongst the first to be considered. Discussion of Roman vessel glass compositions will mainly draw on data produced by the author for well-provenanced glass. Mat. Res. Soc. Symp. Proc. Vol. 185. ,1991 Materials Research Society
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2.
TECHNIQUES
OF ANALYSIS
Micro-samples of enamel, tessera and glass vessel
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