Ceramics are made by baking various types of clay in a kiln. The type of pottery produced depends on the clay and the temperature of firing in the kiln.
There are three basic types of pottery: earthenware, stoneware and porcelain. (Pottery is any ceramic shaped as a vessel, such as a pot or vase.)
Earthenware is fired at the lowest temperature – between 700 and 1200 degrees C. At this temperature, the particles of clay do not fuse together completely. As a result earthenware is porous and can be scratched with a knife. Terracotta is a common type of earthenware.
To make earthenware hold water, it must be glazed. A glaze is simply a coating of glass. The most common glazes, called “lead glazes” are essentially the same glass as is used in lead-light windows. The glaze is applied to the pot either as a powder or as a suspension in water. This can be done either before the pot is fired or after an initial “biscuit” firing. The pot is then fired to melt the glaze onto its surface.
A less common method of glazing is to throw common salt over the pots in the kiln when it is at its highest temperature. The salt reacts chemically with the silicates in the surface clay and produces a very thin layer of glaze.
Stoneware is fired between 1200 and 1350 degrees C. This fuses the clay particles together so the stoneware only needs to be glazed for aesthetic reasons.
Porcelain is made from china clay (called “kaolin” in China) and china stone (felspar) fired at between 1250 and 1400 degrees C. The result is extremely hard, white and translucent. Porcelain made in this way is called “hard- paste” porcelain.
For centuries, potters tried to imitate porcelain using other clays. These products are called “soft-paste” porcelain.
One development from porcelain is bone china, which has up to 50% powdered bone added to the china clay and china stone.
After the basic pot has been shaped by moulding or throwing on a potter’s wheel, it can be decorated by cutting or by pressing patterns into its surface. This is usually done at the “leather-hard” stage, when the clay has dried but not been fired.
Alternatively, decoration may be added to the surface of the pot. Most often the decoration is moulded from clay and stuck on with “slip”, a mixture of clay and water before the pot is fired.
Painting pottery presents a difficulty because paint applied after glazing easily comes off, whereas colours added before glazing, or coloured glazes, have to withstand the high temperature of the kiln. Pigments are usually metal compounds. In most kilns, the metals in the pigments, and in any impurities in the clay or glaze, are converted to the metal oxide – which is likely to be an entirely different colour. It is also possible to design kilns which have the opposite effect, reducing any metal oxides to the base metal. Sometimes pots are fired in both oxidising and reducing kilns to produce particular decorative effects, such as the metallic sheen of lustre ware.
Because of its low firing temperature, it is relatively easy to produce brightly coloured earthenware. But finding the pigments and processes necessary to decorate stoneware and porcelain is much more difficult.