Clay, an earthy material composed of very small particles of weathered rock. The particles are less than 0.00015 inch (0.004 mm) in diameter, much smaller than sand particles. Dry clay is usually powdery and feels oily when rubbed between the fingers. When mixed with water, clay becomes plastic—that is, it can be readily molded.
Clay consists of such minerals as kaolinite, montmorillonite, attapulgite, and illite. These minerals contain various proportions of alumina and silica, usually combined with water to form hydrous aluminum silicates. Some of the minerals also contain calcium, sodium, magnesium, iron, or other elements.
Clay in small amounts is necessary for good soil. Clay serves to hold water and other substances that are necessary for plant growth. However, too much clay makes soil stiff and hard to cultivate. A soil with too much clay is rocklike when dry and sticky when wet.
According to the way it is formed, clay is classified into two types: residual and sedimentary. Residual clay comes directly from the weathering of rock into very fine particles. The particles become mixed with water and materials from the surrounding soil. Sedimentary clay is formed when particles of weathered rocks are carried from the place in which they were formed, usually by streams of water, and deposited in another place. It occurs in layers.
Clays are also commonly classified according to their general composition and properties into six types: kaolin, ball clay, fire clay, common clay, bentonite, and fuller's earth. Kaolin is a fine white clay consisting chiefly of the mineral kaolinite. Ball clay consists of kaolinite and certain micas, and has strong bonding properties. Fire clay is basically kaolinite with some iron oxides, magnesia, and alkalies. It can resist high temperatures. Common clay contains more impurities than fire clay, and does not have as great a resistance to heat. Bentonite consists largely of montmorillonite. Some types that contain sodium swell when mixed with water. Fuller's earth is composed of montmorillonite and attapulgite and is high in magnesia.
The main use of clay is for making pottery and ceramics, including such products as dinnerware, bricks, sewage pipes, and tiles.
Kaolin is used for fine pottery and porcelain. Its chief use, however, is as a filler and coating material for paper. Ball clay is widely used in making pottery; it is sometimes called potter's clay. Fire clay is used primarily for making firebrick, which is used to line furnaces, kilns, and crucibles. Common clay is used for brick, clay pipe, clay floor and wall tiles, and other building materials. Bentonite is used in molds for casting metal and in materials called drilling muds that are used in drilling for petroleum. Bentonite that expands when mixed with water is used in engineering work to seal leaks. Fuller's earth is used to filter impurities from oil and grease. Some is also used in pet litter, pesticides, and other products.