Transparent high lead and tin-opacijied lead-alkali glazes have been extensively used throughout Europe and the Near East from theirjrst appearance in the Roman era and the tenth-to eleventh-century Islamic world, respectively, up until the present day. Using, to a large extent, information which is widely scattered through a diverse range of literature, the methods employed in the production of these two glaze types arejrst outlined and their merits are then compared with those of alkali glazes in terms of ease of preparation of the glaze mixture, ease of application of the glaze, ease ofjring, cost of production, glaze-body j t and visual appearance. The principal advantages of transparent high lead glazes as compared to alkali glazes are shown to be ease ofpreparation and application of the glaze suspension, low susceptibility to glaze 'crazing' and 'crawling' and high optical brilliance. Factors that influence the choice of tin-opacijied lead-alkali glazes include ease of production of tin oxide by melting tin and lead metals together; a reduced risk of reduction of lead oxide to lead metal and consequent blackening of the glaze; and, again, low susceptibility to 'crazing' and 'crawling'. Limits of current knowledge regarding these two glaze types and requirements for future research are outlined.

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Tin-based opacifiers (lead stannate yellow and tin oxide white) were first used in glass production for a short period in Europe from the second to the first centuries BC , and then again throughout the Roman and Byzantine Empires from the fourth century AD onwards. Tin oxide was also used in the production of Islamic opaque glazes from the ninth century AD , and subsequently in enamels applied to Islamic and Venetian glasses from the 12th century AD onwards. A selection of published analytical data for the tin-opacified glasses, enamels and glazes is summarized, and the methods used in their production are reassessed. The phase transformations occurring when mixtures of lead oxide, tin oxide and silica are fired are investigated with high temperature X-ray diffraction (XRD) using a synchrotron radiation source, and these results are used to explain the observed differences in the glass, enamel and glaze compositions. Possible reasons for the use of tin-based opacifiers in the second to first centuries BC , and for the switch from antimony-to tin-based opacifiers in the fourth century AD are suggested, and the possible contexts in which tin-based opacifiers might have been discovered are considered. The introduction of tin-opacified glazes by Islamic potters in the ninth century AD is discussed in terms of technological transfer or independent invention.

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