Lead Glass - Lead Glazes

Lead Glazes

The fluxing and refractive properties valued for lead glass also make it attractive as a pottery or ceramic glaze. Lead glazes first appear in first century BC to first century AD Roman wares, and occur nearly simultaneously in China. They were very high in lead, 45–60% PbO, with a very low alkali content, less than 2%. From the Roman period, they remained popular through the Byzantine and Islamic periods in the Near East, on pottery vessels and tiles throughout medieval Europe, and up to the present day. In China, similar glazes were used from the twelfth century for colored enamels on stoneware, and on porcelain from the fourteenth century. These could be applied in three different ways. Lead could be added directly to a ceramic body in the form of a lead compound in suspension, either from galena (PbS), red lead (Pb₃O₄), white lead (2PbCO₃·Pb(OH)₂), or lead oxide (PbO). The second method involves mixing the lead compound with silica, which is then placed in suspension and applied directly. The third method involves fritting the lead compound with silica, powdering the mixture, and suspending and applying it. The method used on a particular vessel may be deduced by analysing the interaction layer between the glaze and the ceramic body microscopically.

Tin-opacified glazes appear in Iraq in the eighth century AD. Originally containing 1–2% PbO, by the eleventh century high-lead glaze had developed, typically containing 20–40% PbO and 5–12% alkali. These were used throughout Europe and the Near East, especially in Iznik ware, and continue to be used today. Glazes with even-higher lead content occur in Spanish and Italian maiolica, with up to 55% PbO and as low as 3% alkali. Adding lead to the melt allows the formation of tin oxide more readily than in an alkali glaze, which precipitates into crystals in the glaze as it cools, creating its opacity.

The use of lead glaze has several advantages over alkali glazes in addition to their greater optical refractivity. Lead compounds in suspension may be added directly to the ceramic body. Alkali glazes must first be mixed with silica and fritted prior to use, since they are soluble in water, requiring additional work input. A successful glaze must not crawl, or peel away from the pottery surface upon cooling, leaving areas of unglazed ceramic. Lead reduces this risk by reducing the surface tension of the glaze. It must not craze, forming a network of cracks, nor peel. This is caused when the thermal contraction of the glaze and the ceramic body do not match properly. Ideally, the glaze contraction should be 5–15% less than the body contraction, as glazes are stronger under compression than under tension. A high-lead glaze has a linear expansion coefficient of between 5 and 7×10−6/°C, compared to 9 to 10×10−6/°C for alkali glazes. Those of earthenware ceramics vary between 3 and 5×10−6/°C for non-calcareous bodies and 5 to 7×10−6/°C for calcareous clays, or those containing 15–25% CaO. Therefore the thermal contraction of lead glaze matches that of the ceramic more closely than an alkali glaze, rendering it less prone to crazing. A glaze should also have a low enough viscosity to prevent the formation of pinholes as trapped gasses escape during firing, typically between 900–1100 °C, but not so low as to run off. The relatively low viscosity of lead glaze mitigates this issue. It may also have been cheaper to produce than alkali glazes. Lead glass and glazes have a long and complex history, and continue to play new roles in industry and technology today.