AN INTRO TO CLAY: Clay is a naturally occurring substance consisting mainly of alumina and silica with varying elemental and/or organic impurities. These variables account for the existence of many dramatically different kinds of clay (kaolin, ball clay, fire clay, etc.). CERAMIC is a general expression that may be used to describe anything made of clay. A CLAY BODY is a combination of clays, often with the addition of colouring oxides and occasionally synthetic materials. These materials are combined to produce a workable substance with particular qualities such as low shrinkage, good plasticity, high firing temperature, specific colour or texture, etc. Some examples of commonly used clay bodies are: EARTHENWARE: Earthenware is porous (it can be penetrated by water). It matures at a low temperature and is available in a variety of colours, including red-brown (or terra cotta) and white. Temperature range: 800°C - 1150°C. STONEWARE: Stoneware is virtually non-porous. When fired, it is stronger than earthenware bodies. Stoneware is available is a wide range of colours including buff, grey, and brown. Temperature range: 1200°C - 1350°C. PORCELAIN: Porcelain is very strong. It is usually smooth, white and non-porous when fired. Temperature range: 1250°C - 1360°C. FORMING METHODS: Clay can be formed in many ways. The first three methods described here are referred to as HAND BUILDING and require very few tools, if any. PINCH: This is the oldest method of forming in which a ball of soft clay is literally pinched into shapes such as small bowls. COIL: Coiling is the most commonly used method of forming vessels on any scale. This method is used all over the world. Soft clay is rolled into long rope-like pieces. These "coils" are then wound around and around on a clay base, building up the walls of the vessel. They are usually "welded" together by smearing clay from one coiled to the next. SLAB: Using this method, slabs of clay are used either soft or damp to form organic-looking shapes, or firm (leather-hard) and less damp to construct crisp, flat-sided shapes. SOLID BUILDING: The form is modelled from a solid lump of clay. It is then cut in half, hollowed our and put back together, all while it is still wet. In some cases the form is not hollowed but is fired solid. The most common tool is the POTTER'S WHEEL. Clay is form by the pinch method on a spinning wheel using water as a lubricant. Pots made on the wheel are said to be "THROWN" Throwing allows for the quick production of rounded, symmetrical forms. Another common tool is the EXTRUDER. Soft clay is extruded through dies producing long, hollow or solid tubes, even shapes. Extruded pieces are often using in combination with other methods of formation. Plaster and wood MOULDS are also commonly used in the production of ceramics. An original object is crafted using any of the methods described above and a mould is made around the object. Then liquid clay, called SLIP, can be poured into the mould and allowed to set on its interior walls. Excess slip is poured out, and the hollow shell of clay is unmoulded when set. SURFACE TREATMENTS: Clay can be textured easily before firing. Once fired, the clay is irreversibly hardened. It is then ready to receive various other surface treatments. GLAZES are coatings usually applied as a liquid. The object can be brushed or dipped into the glaze or glaze may be poured or painted onto the piece. Glazes are composed of powdered components such as clay and silica and often contain metallic oxides such as iron or cobalt for colour. These ingredients are mixed with water and, once applied dry to chalky finish and are then fired. Glazes are designed to melt and bond with the clay when fired, forming a vitreous surface for impermeability and decoration. Glazed surfaces can be heavily textured or smooth, dry, satiny or glossy. Glazes can be clear, translucent or opaque. Clay forms of all types are FIRED or baked in kilns. There are numerous types of kilns and the resulting effects vary greatly. The kiln can be as basic as a pit dug into the earth, filled with combustible materials and set ablaze. More commonly the kiln is a box-like or cylindrical brick structure fired by electric coils or a fuel such as gas, wood or propane. More important that the type of kiln used is the gaseous atmosphere in which the pots are fired. Firing can be done either in OXIDATION, where oxygen is present throughout the firing; or in REDUCTION where, for a portion of the process, the pots are deprived of oxygen. Colour changes in this reducing environment can be dramatic. For example, when a copper glaze is fired in oxidation the result will be greenish, like an oxidised penny or a weathered copper roof. The same glaze in reduction can yield a range of copper reds and pinks. RAKU is an ancient but still popular process. Red hot pots are pulled from the kiln using tongs and immediately placed in a covered contained filled with combustibles such as leaves, sawdust, paper etc. This creates a reducing atmosphere and the smoke produced can be absorbed into any exposed clay, turning it black. Ceramic objects are occasionally treated with non-ceramic embellishments such as paints, and can be combined with other materials (glass, enamel, wood, fibres, plastic, etc.) for mixed media works. Suggested further reading: Preaud, Tamara & Serge Gauthier, Ceramics of the 20th Century, New York: Rizzoli, c1982 Cameron, Elizabeth, Encyclopaedia of Pottery and Porcelain 1800 - 1960*, New York: Facts of File, c1986 Ayers, John, David Boston et al., World Ceramics*, Robert Charleston, ed., New York: Crescent Books, 1981 *These books are available in the Canadian Clay & Glass Gallery Library and Resource Centre. The Library and Resource Centre is available by appointment only. To arrange an appointment, please contact the Administrative Assistant at the Canadian Clay & Glass Gallery.