The term ceramic is used to describe a variety of fired clay objects. The transformation of an earthy substance by fire produces a durable material, examples of which have survived from ancient times until the present.
The basic criteria for categorising ceramics are clay composition, firing process and glazing. Clays are formed from the weathering of metamorphic and igneous rocks in the earth’s crust, by the action of hot gases, chemical and physical erosion. Primary clays are found at their site of formation and are quite pure. China clay for example, is 98 % kaolinite (hydrated aluminium silicate - Al2O3.2SiO2.2H2O). Secondary (or sedimentary) clays are deposited far from their site of origin. They have smaller particles and more impurities which result in higher plasticity and a range of colours. Examples of secondary clays include ball clays, red clays, earthenware clays, marls and fire clays.
Clay minerals have a relatively small particle size and are able to adsorb water chemically. The water acts as a lubricant between clay particles and as the water evaporates, the particles are drawn closer together until a rigid form is created. Fillers, such as sand, ground shells or grog, are added to clay bodies to control shrinkage and aid in shape retention. Fluxes, such as alkaline metal earth oxides, are added to control the fusion point of clay during firing.
The firing cycle includes an initial or biscuit firing of the clay vessel followed by glaze firing. Biscuit firings are usually within the range 900 to 1100 °C. In biscuit firing, chemically bound water is driven off at 400 to 600 °C and the clay particles are joined by sintering. At 800 °C vitrification or the fusion of fluxes and free silica begins and the ceramic attains maximum porosity. At higher firing temperatures, 1100 to 1300 °C, the ceramic undergoes shrinkage and fusion of the clay body continues until all pores are closed.
Glazes, essentially a thin layer of glass fired onto the ceramic ware, are composed of silica, metal oxide fluxes (such as sodium, potassium, calcium, magnesium and lead) and stabilisers (commonly aluminium oxide). The glaze is usually applied to the biscuit ware as a slip (ground ingredients suspended in water) and then fired when dry. Fusion of the glaze begins at 600 °C and the body and glaze of a ceramic start to integrate at 1100 °C. Glaze colourants are produced from metallic compounds and may be applied under the glaze, incorporated into the glaze slip or applied as enamels on glazed ware. Under glaze colours are more durable, whereas the lower firing temperatures of enamels (750 °C) render them more vulnerable to damage.
The major ceramic categories are earthenware, stoneware and porcelain. Earthenware bodies are derived from naturally occurring secondary clays. Low firing temperatures (600 – 1100 °C) produce a porous ware (> 15 % porosity) and a range of colours (yellow, buff, grey, red and brown). Earthenware includes terracotta (usually unglazed red ceramic), ancient and medieval wares, tin glazed earthenware (Majolica, Faience and Delft ware) and transfer printed Creamware.
Stoneware bodies are composed of modified secondary clays, fired to high temperatures of 1200 to 1300 °C to produce a vitrified, hard and durable ceramic. The porosity of stoneware is < 3 % and colours include white, buff, grey and black.
A hard-paste porcelain body is made from 50 % kaolinite, 25 % feldspar and 25 % quartz or flint. It is fired to a high temperature of 1400 °C to produce a vitreous, white, non-porous ceramic with a glassy fracture surface. Porcelain was first made in China and then developed in Europe in 1709. Soft-paste porcelain is fired to a lower temperature (1150 °C) than hard-paste porcelain and is slightly more brittle, more porous and fractures with a grainy texture. Bone china, as developed in the late 18th century in England, contains 50 % bone ash, 25 % kaolinite and 25 % Cornish stone. It is fired to 1250 °C, producing a non-porous, pure white translucent body.
Many further distinctions may be made within these groups according to the type of body, decoration, style or function of the object. It is important to assess ceramic objects before treatment, as the type of ceramic obviously affects the treatment options.