clay

Water runs off a clay surface. Clay is an impermeable rock. This means that water flows over it. (A little water can enter the clay, but this only causes the clay particles to expand and become even more impermeable.) The more water that falls, the greater the flow over the surface; this causes small gullies to form on the clay. In addition to the amount of rain, a steeper gradient will also increase the quantity of overland runoff.

Very fine-grained sedimentary deposit that has undergone a greater or lesser degree of consolidation. When moistened it is plastic, and it hardens on heating, which renders it impermeable. It may be white, grey, red, yellow, blue, or black, depending on its composition. Clay minerals consist largely of hydrous silicates of aluminium and magnesium together with iron, potassium, sodium, and organic substances. The crystals of clay minerals have a layered structure, capable of holding water, and are responsible for its plastic properties. According to international classification, in mechanical analysis of soil, clay has a grain size of less than 0.002 mm/0.00008 in.

Types of clay include adobe, alluvial clay, building clay, brick, cement, kaolinite, ferruginous clay, fireclay, fusible clay, puddle clay, refractory clay, and vitrifiable clay. Clays have a variety of uses, some of which, such as pottery and bricks, date back to prehistoric times.

clay

Fine-grained sediment (mud) used as the basic material in ceramics. The term can refer to a variety of mixtures and textures. When moistened and wet, clay is plastic (flexible) and easily moulded. When dry, it becomes brittle and can crumble; if heated at high temperatures in a kiln, clay becomes very hard or ceramic (cannot be recycled with water). Clay may be white, grey, red, yellow, blue, or black, depending on its mineral composition. The most common types of clay are earthenware and terracotta; stoneware, with feldspar and a high silica content; porcelain clay, also known as kaolin or china clay; and bone china, a mixture of bone ash and kaolin. Other types of clay include raku, black clay, and Egyptian clay. In ceramics, clay type will often dictate final form.

Earthenware Earthenware is fired at a low temperature (1,200°C/2,200°F), and includes terracotta, white, and buff clays. The name terracotta comes from the Italian, meaning ‘baked earth’, and is usually associated with a red clay. Terracotta is the cheapest clay, and is found in large deposits in many parts of the world. It is used for tiles, bricks, and domestic ware. White and buff earthenware, while not as common as the terracotta earthenware, is also used for both decorative and functional ware. Different from terracotta, white and buff earthenware is prepared from a basis of ball clay (a fine-textured clay).

Stoneware Stoneware is fired at a higher temperature ((1,200–1,280°C/2,190–2,350°F) than earthenware. It is so-called because its composition is similar to stone, the only difference being that stones are formed naturally, whereas stoneware clay is made with ceramic materials and plastic (recyclable with water) clay. Once fired it produces a non-porous, glass-like ceramic, which is used in both the arts and industry. Domestic stoneware pots tend to be heavier and more durable than either porcelain or earthenware.

Porcelain and bone china Porcelain, also known as china clay or kaolin, is fired at high temperatures (1,400°C/2,550°F), and can be opaque or translucent, depending on the make-up of the clay body. Porcelain is very difficult to handle and is usually only used by expert potters. Once fired porcelain is very tough and has a smooth, shiny surface. Bone china, a mixture of 95% kaolin and 5% bone ash (calcined bone), is a high-fire clay (over 1,240°C/2,260°F), known for its extreme whiteness and strength. These qualities come from the bone ash, which acts as a binding agent helping the ingredients fuse together like glass when fired. The material is only used by expert ceramicists.

Raku Raku is a form of soft earthenware, originally developed in Japan and now used all over the world. The clay is fired at low temperatures (starting at 750°C/1,380°F), and is finished with lead glaze. Ceramicists can achieve a number of unusual finishes and effects using the raku process, making it a popular art form.

Black clay Black clays can be made by adding a black stain or other colourant to any clay body, ranging from earthenware to porcelain. The material is usually used in the same way as terracotta, the ceramicist using the natural unglazed quality and colour of the clay as part of the work.

Egyptian paste Egyptian paste is a specially-prepared clay body in which the colour and glaze are drawn out as the clay dries. The glaze is supplied by soluble salts of soda, which rise to the surface in the drying process; the soda's reaction with the specific oxide in the clay gives the work its final colour. Egyptian paste is usually only used in the creation of decorative ware.