Most earthenwares have traditionally been made of secondary clays, receiving relatively little purification after being mined. Common clays tend to contain iron oxide and other impurities, which reduce firing temperatures to between 800–1100°C. Iron, a natural flux, facilitates the melting together of the other ingredients. Earthenware clay can be highly plastic or less plastic because of sand or other rocky fragments. After firing the clay remains unfused and is therefore porous, opaque and less sturdy than higher-fired wares. Earthenware clays rich in iron oxide fire from buff to tan, red, brown or black, depending on the clay and firing conditions. They take low-temperature glazes, such as certain alkaline and lead-oxide types, to render them non-porous. In the simplest method the body and glaze are hardened during a single firing. Finer, more pure earthenware bodies, some with higher firing temperatures, require an initial (biscuit) firing to harden the unglazed body. A lower temperature glaze or ‘glost’ firing leaves the wares impervious to liquid. This two-firing system facilitates the execution of crisp underglaze decoration, which is fixed in the body-firing and is thus less likely to run when the glaze is fired. Such overglaze decoration as enamelling or gilding is hardened in subsequent lower temperature firings.
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