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Pottery is usually made of clay, which can be moulded into a variety of shapes while wet. Once the form has been established, a piece of pottery is placed in a hot kiln, where the high temperature hardens the clay and gives it permanence. In order to give pottery and ceramics a smooth, shiny finish, one of several glazes must be applied. It can also be used to make some kinds of pottery less porous.An underglaze is a type of glaze used to describe the decoration applied to pottery, while an overglaze is used to create a shiny surface over the decoration. Not only does the overglaze provide a protective layer, it also leaves colours vibrant. When using this type of glaze, the piece of pottery is first fired in a kiln, the overglaze is applied, and the item is then re-fired. The heat helps to blend the colour pigments with the glaze and creates a glass-like, shiny surface.Ceramic glazes usually consist of silica, a component of quartz. Normally, a very high heat is required to melt quartz, so a flux is added to the glaze. Flux is a substance that lowers the melting temperature of a particular mineral or compound so it can be fired at a lower heat, the most common of which are lead, potassium, sodium, and lime and calcium oxide. Pieces of pottery can be dipped into the glaze, sprayed or applied with a brush. Glaze can also be poured directly over pottery.Lead glazes have existed for centuries, although some controversy exists over its use. Many potters maintain that they cannot achieve the proper colour shade or brightness without the use of a lead glaze, while the opposition argues that lead is not an appropriate alternative for household use. It is often strongly suggested that, as a safety precaution, consumers refrain from purchasing cookware or dishes that contain a lead coating. Even edible plants are susceptible to acquiring lead residue if grown in a container glazed with lead. It also helps to know the origin of a piece of pottery, as underdeveloped countries do not yet understand the implications of using lead and sometimes fire their pieces at low enough temperatures that would allow toxic release into food or drink.Pottery containing alkali glazes, using salt and/or potassium, have also been found throughout history; it is not unusual to find a piece that features a glazed mix of lead and alkali. Regardless of the type of glaze chosen, pottery is normally fired at temperatures between 950�C and 1430�C. Glazes are found on a variety of household items, including toilets and sinks, tableware, figurines and giftware, and tiles.
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