Clay is useful because it changes. It is plastic and malleable when wet and a potter can shape it to his or her need. After one or two firings, the clay is hard, waterproof, and inflexible.
Major types of clay include porcelain, earthenware and stoneware. These clays differ in their hardness, texture, and color. They are worked using slightly different techniques, and fired at different temperatures.
The chemical composition of clay varies widely, with additives included for strength, color, or the ability to stand high firing temperatures or vitrify at low temperatures. Clay is manipulated mechanically before it is shaped, to improve its physical characteristics. In the kiln, raw clay becomes useful and beautiful, and hardens into the shape its creator intended for it.
Chemical composition of clays
Most clay is aluminum oxide, silicon dioxide, and water. These are the primary elements which fuse together to form a ceramic object. However, many additives can go into clay, affecting its sturdiness, color, texture, or firing range.
Sand may be added to some clay, for toughness, viscosity, and texture. Sand is also rich in vitrifying silica. Vitrification is hardening, the process that makes a mass of clay a durable (though brittle) object. Such additions as iron can color clays as well as lowering their firing temperature.
Many of the additions to clay can be described as flux. Fluxes lower the firing temperatures needed to harden the clay. They include various feldspars, nephelene syenite, barium carbonate, iron, and rarely, lead compounds. Talc is a flux, and is commonly used, in spite of possible dangers to health.
Grog is added to clay for tensile strength, and to add a gritty texture sometimes called toothiness. Its grit opens up the clay to allow gases to escape.
Smooth porcelain clay is mostly kaolin. It makes a smooth piece, but it is not very plastic, so it is hard to work. After firing, it becomes glass and mullite, and is tough, impervious, and often translucent.
Kaolin is a clay that is mined around the world, but is most common in tropical regions. When heated it passes through several chemical stages before becoming the mullite that forms needles in the clay body and contributes to its strength and durability. Bentonite may be added to kaolin for plasticity. It is weathered volcanic ash. Many porcelains use bone ash, which may contribute to translucency. Various forms of feldspar or quartz provide silica, which becomes glassy in firing.
Manipulation of clay
Potters often wedge their clay, though it can be purchased pre-wedged. Wedging removes air bubbles, which can destroy a piece in the kiln. It also homogenizes the clay, so that the desired consistency is uniform throughout. Some potters believe it aligns the clay particles better for throwing on the wheel. A vacuum pug is a machine for removing air from clay.
Various wedging techniques exist, some of which resemble kneading bread. Potters who do not personally wedge their clay often pound it, theoretically to make it denser and likelier to hold its form.
After preparation, the potter shapes the clay. Hand building is shaping clay freeform. A potter's wheel spins the clay, while the potter shapes it into radially symmetrical forms. Slabs may be formed into tiles, boxes or similar forms, and joined with slip, which is very wet clay. Slip is also used in slip casting, in which liquid clay is poured into plaster molds to shape it.
After forming, clay must dry before firing. Water in clay that is fired too wet can explode. Clay that is still damp feels cool, so a potter may wait until his piece warms to fire it. Clay may be decorated at this stage, by incising patterns into the surface, staining, or glazing. Often a piece is fired once before glazing.
Clay in the kiln
Clay that has not been fired is greenware. It is flexible and fragile. The kiln drives residual water from greenware, and either vitrifies it or prepares it for further processing. Pieces that have been fired once are called bisque, if they will be glazed and fired again. The chemical composition of clay is permanently altered by firing.
Firing may take place in an oxygen atmosphere, or in reduction. This will affect the color of the glazes or the clay by affecting the chemical compounds at the surface. Iron, for example, tints ceramics the familiar terra cotta color of roof tiles if fired at a low temperature in oxygen, khaki at a higher temperature, brown at a still higher temperature, but green in a reduction atmosphere.
Iron oxide, formed with oxygen, is the familiar color of rust, but without abundant oxygen, true rust does not form. Intakes on the kiln are closed to exclude oxygen, and the firing strips oxygen molecules from the surface of the pottery, producing reduction.
Earthenware is fired to 1200 Celsius or less. Stoneware perhaps to 1300, and porcelain perhaps to 1400. However, the chemical changes that produce finished ceramics are not solely due to the highest temperatures reached, but also to the time spent at each temperature. Leaving pieces at a given temperature for prolonged periods is soaking. A potter may soak pieces at one temperature for hours before raising them higher or lowering the temperature. In addition, in some kilns, temperatures are not uniform, and may also vary day-to-day.
Since the maturity of ceramics is not merely a matter of temperature, but of length of time at different temperatures, pyrometric cones are used in firing. These cones are meant to be a visual indication of how the clay in the kiln is doing. Cones are clay spikes manufactured to bend with a standardized amount of change to the conditions in the kiln.
In low-fired pottery, the clay particles are fused together, sintered. In high-fired stoneware and porcelain, the chemical constituents of the ceramic piece are greatly altered. The soft malleable clay becomes in effect, a new stone, which is hard, impervious, and sometimes quite beautiful