Definition: Clay

clay (kl³) n. 1.a. A fine-grained, firm earthy material that is plastic when wet and hardens when heated, consisting primarily of hydrated silicates of aluminum and widely used in making bricks, tiles, and pottery. b. A hardening or nonhardening material having a consistency similar to clay and used for modeling. 2. Geology. A sedimentary material with grains smaller than 0.002 millimeters in diameter. 3. Moist, sticky earth; mud. 4. The human body as opposed to the spirit. --clay“ey (kl³“¶) or clay“ish adj.

Properties of the clays include plasticity, shrinkage under firing and under air drying, fineness of grain, color after firing, hardness, cohesion, and capacity of the surface to take decoration. Clay is one of the three principal types of soil, the other two being sand and loam. A certain amount of clay is a desirable constituent of soil, since it binds other kinds of particles together and makes the whole retentive of water. Excessively clayey soils, however, are exceedingly difficult to cultivate. Clays are divided into two classes: residual clay, found in the place of origin, and transported clay, also known as sedimentary clay, removed from the place of origin by an agent of erosion and deposited in a new and possibly distant position.

Clay consists of soil particles the diameters of which are less than 0.005 millimetre; also a rock that is composed essentially of clay particles. Rock in this sense includes soils, ceramic clays, clay shales, mudstones, glacial clays (including great volumes of detrital and transported clays), and deep-sea clays (red clay, blue clay, and blue mud). These are all characterized by the presence of one or more clay minerals, together with varying amounts of organic and detrital materials, among which quartz is predominant. Clay materials are plastic when wet, and coherent when dry. Most clays are the result of weathering.

No other earth material has so wide an importance or such extended uses as do the clays. They are used in a wide variety of industries. As soils, they provide the environment for almost all plant growth and hence for nearly all life on the Earth's surface. They provide porosity, aeration, and water retention and are a reservoir of potassium oxide, calcium oxide, and even nitrogen.

The use of clay in pottery making 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. Impure clays may be used to make bricks, tile, and the cruder types of pottery, while kaolin, or china clay, is required for the finer grades of ceramic materials. Another major use of kaolin is as paper coating and filler; it gives the paper a gloss and increases the opacity. Refractory materials, including fire brick, chemical ware, and melting pots for glass, also make use of kaolin together with other materials that increase resistance to heat. Certain clays known as fuller's earth have long been used in wool scouring. In rubber compounding, the addition of clay increases resistance to wear and helps eliminate molding troubles.
Clay materials have a wide variety of uses in engineering. Earth dams are made impermeable to water by adding suitable clay materials to porous soil; water loss in canals may be reduced by adding clay. The essential raw materials of portland cement are limestone and clays, commonly impure. After acid treatment, clays have been used as water softeners; the clay removes calcium and magnesium from the solution and substitutes sodium. A major use of clay is as drilling mud--i.e., heavy suspension consisting of chemical additives and weighting materials, along with clays, employed in rotary drilling.

Structural clay products are ceramic products that are intended for use in building construction. Typical structural clay products are building brick, paving brick, terra-cotta facing tile, roofing tile, and drainage pipe. These objects are made from commonly occurring natural materials, which are mixed with water, formed into the desired shape, and fired in a kiln in order to give the clay mixture a permanent bond. Finished structural clay products display such essential properties as load-bearing strength, resistance to wear, resistance to chemical attack, attractive appearance, and an ability to take a decorative finish.

Structural clay products are made from 35 to 55 percent clays or argillaceous (clayey) shales, 25 to 45 percent quartz, and 25 to 55 percent feldspar. As with all traditional ceramic products, the clay portion acts as a former, providing shaping ability; the quartz (silica) serves as a filler, providing strength to the formed object; and the feldspar serves as a fluxing agent, lowering the melting temperatures of the clay and quartz during firing. The proportions cited above are often found directly in shale deposits, so that blending is often not necessary. In addition, little or no beneficiation, or crushing and grinding of the mined material, is employed. Local clays or shales of highly variable composition are used in order to keep transportation costs as low as possible. The colour of the finished product derives from impurities, most notably iron oxides, present in the raw materials. Colours can range from buff and other light shades of brown through red to black, depending upon whether an oxidizing or reducing atmosphere exists in the kiln.

The properties exhibited by structural clay products are determined by particle size, firing temperature, and ultimate microstructure. Compared with finer ceramic products such as whitewares, much coarser filler particles are used, and lower firing temperatures are employed--typically in the range of 1,050 C (approximately 1,925 F). At such low temperatures the filler particles (usually crushed quartz) are normally not affected. Instead, the clay or shale ingredients contain sufficient impurities to melt and form a glass, thus bonding the particles together. As is the case with whitewares, crystalline mullite needles grow into this glassy phase. The resulting microstructure consists of large secondary particles embedded in a matrix of fine-grained mullite and glass--all containing a substantial volume of large pores.

Because of the presence of large and small particles in their microstructures, fired clay products have relatively high compressive strengths. This ability to bear relatively heavy loads without fracture is the prime property qualifying these products for structural applications. The size and number of pores is also important. If underfired, structural clay products have low strength and poor resistance to frost and freezing, owing to the presence of many small pores in the clay regions. Overfired ware, on the other hand, has too much glass. It is strong but brittle and is susceptible to failure under mechanical and thermal stress. Furthermore, it is impossible to obtain a good bond when glassy products are used with mortars. Small pores and high glass content are desirable, however, when chemical resistance and imperviousness to water penetration are required.

By some estimates structural clay products make up as much as 50 percent of the entire ceramics market. The industry is highly conservative, with development aimed primarily at automation and labour minimization rather than the introduction of new products.

There is a wide variety of structural clay products, broadly classified as facing materials, load-bearing materials, paving materials, roofing tile, and chemically resistant materials. Examples of facing materials are face brick, terra-cotta, brick veneer, sculptured brick, glazed brick and tile, and decorative brick. Building brick, hollow brick, and structural tile for floors and walls are examples of load-bearing materials. Paving materials include light traffic pavers, quarry tile, and paving brick--this last product once in more common use than at present. Roofing tiles are quite common in many parts of the world, red and black colours being of particular note. Chemically resistant materials include sewer pipe, industrial floor brick, drain tile, flue liners, chimney brick, and chemical stoneware.