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Tin-based opacification by tin oxide and lead-tin-oxide particles was used in glass production since the first millennium BC and in ceramic glazes since the eighth century AD. Opacification process is often characterised by significant amounts of tin oxide and lead oxide dispersed into glassy matrices or by identification of the opacifying particles by means of microstructural or (micro-)XRD analyses. The processes of opacification and manufacture are usually more difficult to establish from compositional and microstructural analyses because they leave little diagnostic traces. This review aims to integrate compositional data on archaeological glass and glazes and in particular the Pb/Sn values, with descriptions of the opacification processes in historical treatises, observations at traditional workshops, and the results of previous replication experiments to shed further light on technological issues underlying these methods of opacification and highlight new research perspectives.

a b s t r a c tThe generally accepted theory is that the demand for Islamic glazed pottery started in Abbasid Iraq in the 9th century AD with the production of a range of glazed wares in response to the import of Chinese stonewares and porcelains. However, Oliver Watson has recently proposed that the demand for Islamic glazed pottery first occurred in Egypt and Syria in the 8th century AD resulting in the production of opaque yellow decorated wares. Using a combination of SEM analysis of polished cross-sections, and surface analysis using hand-held XRF or PIXE, Coptic Glazed Ware from Egypt, Yellow Glazed Ware from Syria, and comparable wares from Samarra, Kish and Susa have been analysed. The analyses show that the opaque yellow decoration was the result of lead stannate particles in a high lead glaze, which it is suggested was produced using a lead-silica-tin mixture. The use of lead stannate in the production of yellow opaque glazes is explained in terms of technological transfer from contemporary Islamic glassmakers who continued the Byzantine tradition of glassmaking. It is further argued that the introduction of opaque yellow glazed pottery into Mesopotamia could have provided the social context for the sudden emergence of tin-opacified white glazed pottery in Abbasid Iraq in the 9th century AD. However, in view of the very different glaze compositions employed for the yellow and white opaque glazes, it seems probable that the white tin-opacified glazes used for Abbasid cobalt blue and lustre decorated wares represent a separate but parallel technological tradition with its origins in the production of Islamic opaque white glass.

Ore samples collected from the cobalt mine of Qamsar, Kāshān, were studied using optical microscopy (OM), qualitative X‐ray diffraction analysis (XRD), semi‐quantitative X‐ray fluorescence spectrometry (XRF) and quantitative electron micro‐probe with wavelength‐dispersive spectrometry (EMP–WDS). These analytical data provide the characteristics of the Kāshān cobalt ore, which can be compared with the composition of the blue pigments and colourants applied to glass and glazes, not least those of Islamic and Chinese blue‐and‐white wares. Preliminary replication studies of the pigment were also conducted according to the descriptions given in the Persian and European historical texts reviewed in a previous paper. The resulting phases were examined under an optical microscope and a semi‐quantitative energy‐dispersive spectrometer (EDS) attached to a scanning electron microscope (SEM). The ore was shown to contain high Fe and As concentrations and negligible Zn and Ni concentrations. Cobaltite (CoAsS) and erythrite (Co3(AsO4)2.8H2O) made up the two main cobalt‐bearing minerals in the ore. The washing processes that were conducted based on a historical text rendered the ore more enriched with erythrite. In light of the new data on the Kāshān cobalt ore, the origins of the blue pigments used in some of the Islamic and Chinese ceramics are reconsidered.