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Compositional analyses of ancient and historic glasses have often been interpreted in terms of the use of specific raw materials in glass manufacture. However, the known inhomogeneity of many glass-making raw materials and the insolubilities of some compounds make any explanation of compositional data problematic. This paper looks at three glass-making alkalis with a view to understanding how the compositions of these raw materials are carried through to the final glass. The chemistry and variability of the raw materials are discussed, as is their contribution to the final glass composition. In addition, the choices and decisions made by glassmakers are acknowledged in the final glass compositions. This combination of factors addresses the complexity of predicting the use of specific raw materials from the finished glass composition.

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Ceramic vessels and associated vitreous debris, excavated at Coppergate, York, UK, have been interpreted as the remains of Roman glass-making from the raw materials. This paper reports the results of analysis of this assemblage by XRF, ICPS, XRD, SEM-EDAX and thin-section petrography. These findings suggest that some ceramic vessels, used as crucibles, have been subjected to temperatures up to 1200 ° C, well above the firing temperatures of the local domestic assemblage from which they were selected. Analysis of quartz-rich debris, intimately mixed with glass in some samples and in others interleaved with glassy phases, indicates partially reacted glass-making raw materials, with α -quartz, tridymite and cristoballite phases represented. This could represent evidence either of a failed attempt to frit the raw materials, or a batch that had not fully fused. Variability in the composition of glass at the site is viewed in the light of glass-making technology, and possible interpretations concerning this episode are discussed in the light of the archaeological evidence.

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