Clay

Soil particles of less than 0.005 millimeters in diameter or rock composed mainly of clay particles. There are ceramic clays, clay shales, mudstones, glacial clays, deep-sea clays, and soils - which are plastic when wet and hard when dry. No other natural material has so wide an importance or such extended uses as does clay. The use of clay in potterymaking antedates recorded human history, and pottery remains provide a record of past civilizations. As building materials, bricks (baked and as adobe) have been used in construction since earliest time.

Strictly, the word clay describes the size of the particles which go to make up a sediment, soil or similar material. Clay particles are those that are less than 0.002 mm (BS 1377) in diameter (see particle size). Thus a clay is a sediment consisting largely of claysized particles; strictly used, the term has no implications about colour, organic content or any property other than particle size or texture. However, there may be other connotations. The minerals most frequently found as clay-sized particles are the ‘clay minerals’ such as kaolinite, illite or montmorillonite. Clay as a material frequently consists largely of one or more of these clay minerals. In general usage, ‘clay’ may imply the material used for making pottery, bricks or tiles. These, however, frequently contain large quantities of other particle sizes, such as silt or sand. ‘Clay’ in this context merely implies that the material is more or less mouldable when wet, but on drying out becomes hard. This property is shared by most materials that contain any significant quantity of claysized particles (see pottery).