The result is extremely hard, white and translucent. For centuries, potters tried to imitate porcelain using other clays. 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. Alternatively, decoration may be added to the surface of the pot. 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.