C l a y

Plasticity
Impressionability
Absorption
Dissolve [clay can be watered down to a 'slip' to blend and apply as with a brush]
Shrinkage under firing and under air drying
Fineness of grain
Capacity of the surface to take decoration
Color after firing
Hardness [in firing]
Crispness [in firing]
Cohesion
Intensity in tone and hue changes according to the number of firings

What is Clay? [From The New Columbia Encyclopedia] [Clay is a] Common name for a number of fine-grained, earthy materials that become plastic when wet. The individual clay particles are always smaller than 0.004 mm. Clays often form colloidal suspensions when immersed in water, but the clay particles flocculate (clump) and settle quickly in saline water. Clays are easily molded into a form that they retain when when dry, and they become hard and lose their plasticity when subjected to heat.

Chemically, clays are hydrous aluminum silicates, ordinarily containing impurities, e.g., potassium, sodium, calcium, magnesium, or iron, in small amounts. Clay consists of a sheet of interconnected silicates combined with a second sheetlike grouping of metallic atoms, oxygen, and hydroxyl, forming a two-layer mineral such as KAOLINITE. Sometimes the latter sheetlike structure is found sandwiched between two silica sheets, forming a three-layer mineral such as vermiculite.

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. Residual clays are most commonly formed by surface weathering, which gives rise to clay in three ways--by the chemical decomposition of rocks, such as granite, containing silica and alumina; by the solution of rocks, such as limestone, containing clayey impurities, which , being insoluble, are deposited as clay; and by disintegration and solution of shale. One of the commonest processes of clay formation is the chemical decomposition of FELDSPAR. In the lithification process, compacted clay layers can be transformed into shale. Under the intense heat and pressure that may develop in the layers, the shale can be metamorphosed into slate.... From prehistoric times, clay has been indispensable in architecture, in industry, and in agriculture. As a building material, it is used in the form of BRICK, either sun-dried (adobe) or fired. Clays are also of great industrial importance, e.g., in the manufacture of TILE for wall and floor coverings, of porcelain, china, and earthenware, and of pipe for drainage and sewage. Properties of the clays used in such products that must be taken into consideration include plasticity, shrinkage under firing and under air drying, fineness of grain, color after firing, hardness, cohesion, capacity of the surface to take decoration. On the basis of such qualities clays are variously divided into classes or groups; products are generally made from mixtures of clays and other substances. The purest clays are the CHINA CLAYS and kaolins. "Ball clay" is a name for group of plastic, refractory clays used with other clays to improve their plasticity and to increase their strength. Bentonites are clays composed of very fine particles derived usually from volcanic ash. They are composed chiefly of the hydrous magnesium-calcium-aluminum silicate called montmorillonite. Highly absorbent, bentonite is much used in foundry work for facing the molds and preparing the molding sands for casting metals. The less absorbent bentonites are used chiefly in the oil industry, e.g., as filtering and deodorizing agents in the refining of petroleum and, mixed with other materials, as drilling muds to protect the cutting bit while drilling. Other uses are in the making of fillers, sizings, and dressings in construction, in clarifying water and wine, in purifying sewage, and in the paper, ceramics, plastics, and rubber industries.

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. Their stiffness presents resistance to implements, impedes the growth of the plants, and prevents free circulation of air around the roots. They are cold and sticky in wet weather, while in dry weather they bake hard and crack. Clods form very often in clayey soils. Clays can be improved by the addition of lime, chalk, or organic matter; sodium nitrate, however, intensifies the injurious effects. In spite of their disadvantages, the richness of clay soils makes them favorable to the growth of crops that have been started in other soil. R.E. Grim, Clay Mineralogy (2nd ed. 1968); R.W. Grimshaw, The Chemistry and Physics of Clays and Allied Ceramic Materials (4th ed. 1971).

[The New Columbia Encyclopedia. Harris, William H., and Judith S. Levey, eds. New York and London: Columbia University Press, 1975.]

Silicate, chemical compound containing silicon, oxygen, and one or more metals, e.g., aluminum, barium, beryllium, calcium, iron, magnesium, manganese, potassium, sodium, or zirconium. Silicates may be considered chemically as salts of the various silicic acids. For a long time classified as orthometa-, di-, or trisilicates according to the acid from which they are (theoretically) derived, they are now also classified by an X-ray diffraction method according to their crystalline structure. Silicates are widely distributed in nature, making up most of the earth's outer crust. Most of the common rock-forming minerals (e.g., QUARTZ, FELDSPAR, MICA, and PYROXENE) are silicates, as are ASBESTOS, BERYL, AQUAMARINE, EMERALD, SERPENTINE, and TALC. CLAY consists essentially of hydrous aluminum silicates mixed with other substances. GLASS is a mixture of silicates, as is WATER GLASS. See SODIUM SILICATE. [The New Columbia Encyclopedia. Harris, William H., and Judith S. Levey, eds. New York and London: Columbia University Press, 1975.] [The New Columbia Encyclopedia. Harris, William H., and Judith S. Levey, eds. New York and London: Columbia University Press, 1975.]

Silicon, nonmetallic chemical element; symbol Si; at. no. 14; at, wt. 28.086; m.p. 1410 degrees C; b.p. 2355 degrees; sp. gr. 2.33 at 25 degrees C; valence usually + 4. Silicon is the element directly below carbon and above germanium in group IVa of the PERIODIC TABLE. It is more metallic in its properties than carbon; in many ways it resembles germanium. Silicon has two allotropic forms, a brown amorphous form and a dark crystalline form. The crystalline form has a structure like diamond and the physical properties given above. Silicon forms compounds with metals (silicides) and with nonmetals. With carbon it forms SILICON CARBIDE; with oxygen a dioxide, SILICA; with oxygen and metals, SILICATES..... Silicon is the second most abundant element of the earth's crust; it makes up about 28% of the crust by weight. Oxygen, most abundant, makes up about 47%. Aluminum, third in abundance, makes up about 8%. Silicon is widely distributed, occurring in silica and silicates, but never uncombined. Silicon is obtained commercially by heating sand and coke in an electric furnace. It is used in the steel industry in an alloy known as ferrosilicon, and also to form other alloys, such as those with aluminum, copper, and manganese; in these alloys it contributes hardness and corrosion resistance. A purified silicon is used in the preparation of SILICONES. Silicon of very high purity is prepared by thermal decomposition of silanes; it is used in transisters and other SEMICONDUCTOR devices. Silica is widely used in the production of GLASS. Silicates in the form of CLAY are used in pottery, brick, tile, and other ceramics. SIlicon is found in many plants and animals; it is a major component of the test (cell wall) of diatoms*. SILICOSIS is a lung disease caused by inhaling silica dust. Discovery of the element is usually credited to J. J. Berzelius, who in 1824 prepared fairly pure amorphous silicon. [The New Columbia Encyclopedia. Harris, William H., and Judith S. Levey, eds. New York and London: Columbia University Press, 1975.]

Shale, sedimentary ROCK formed by the consolidation of mud or clay, having the property of splitting into thin layers parallel to its bedding planes. Shale tends to be fissile, i.e., it tends to split along planer surfaces between the layers of stratified rock. Shales comprise an estimated 55% of all sedimentary rocks. The composition of shale varies widely. Shales with very high silica content may have been formed when large quantities of DIATOMS and volcanic ash were present in the original sediment. Large numbers of FOSSILS in shales may give them a high calcium content; such shales may grade into LIMESTONES. Shales that contain a large percentage of ALUMINA are used as a source of that mineral in the manufacture of CEMENT. Shales containing abundant carbonaceous matter grade into bituminous COAL. Oil shales are widely distributed in the W United States and my be a future source of petroleum. [The New Columbia Encyclopedia. Harris, William H., and Judith S. Levey, eds. New York and London: Columbia University Press, 1975.]

Slate, fine-grained ROCK formed when sedimentary rocks such as SHALE are metamorphosed by great pressure. Slate splits into perfectly cleaved, broad thin layers; this characteristically regular and planar cleavage is called slaty cleavage. In the formation of slate, pressure causes the flaky minerals within the sedimentary rock, such as mica, clay, and chlorite, to be reoriented; the flat faces of the minerals lie at right angles to the source of the pressure, and the planes of easy cleavage are also at right angle to the source of the pressure. The rock is not necessarily compressed in the same direction as the sedimentary layers were originally laid down, and because the compression crumples and deforms the original sedimentary layers, the planes of slaty cleavage usually cut through the old bedding planes. Slate is intermediate in hardness between mica SCHISTS and shale; the better grades are used for roofing. Its characteristic color is gray-blue. Slate is mined in Maine, Vermont, Pennsylvania, Georgea, Lake Superior, and the Rocky Mts. [The New Columbia Encyclopedia. Harris, William H., and Judith S. Levey, eds. New York and London: Columbia University Press, 1975.]

Diatom, single-celled microscopic form of plant belonging to the group of golden brown ALGAE. Most diatoms exist singly, although some join to form colonies. They are usually yellowish or brownish, and are found in fresh and salt water, in moist soil, and on the moist surface of other plants. They are most abundant in polar and other cold waters. Some 15,000 species are known. The living matter of each diatom is enclosed in a shell of siliceous material that it secretes. Many of the shells (some are round and some elongated and tapering at both ends) show intricate and beautiful sculpturing. Some diatoms can move over moist surfaces by a streaming of the protoplasm, and some aquatic forms can move about in the water. When the aquatic forms die they drop to the bottom, and the shells, not being subject to decay, collect in the ooze and eventually form the material known as diatomaceous earth (sometimes called kieselguhr). When it occurs in a more compact form as a soft, chalky, light-weight rock, it is called diatomite. Deposits of diatomaceous material, formed under water in past geologic time and now exposed above water, are found in all parts of the world. They are scattered over most of the United States, but productive sources exist chiefly in Oregon, California, Washington, Nevada, Florida, New York, and New Jersey.... Most of the earth's limestone has been deposited by diatoms, and much petroleum is of diatom origin. Diatoms, as the principal constituent of plankton (see MARINE BIOLOGY), are an important food source for fish and other aquatic animals, e.g., the baleen whales. The desmids, a much smaller group of plankton sometimes confused with diatoms, are green algae (division Chlorophyta). They are also usually single cells but are not siliceous and never occur in marine habitats. They are noted for their extraordinary symmetry and geometrical beauty. They are found only in fresh (usually still) water. Diatoms are classified in the division CHRYSOPHYTA, call Bacillariophyceae. [The New Columbia Encyclopedia. Harris, William H., and Judith S. Levey, eds. New York and London: Columbia University Press, 1975.]

Feldspar or felspar, mineral of which there are many widely distributed varieties. As constituents of granite, gneiss, basalt, and other crystalline rocks the feldspars form a large part of the earth's crust. Clay is the chief substance formed when weathering decomposes feldspars. Feldspar crystals are either monoclinic or triclinic (see CRYSTAL), and all show clean cleavage planes in two directions. Othoclase feldspars, which have cleavage planes intersecting at right angles, are monoclinic. The triclinic feldspars include the plagioclase feldspars (e.g., albite, anorthite, and labradorite) and microcline, and their cleavage planes form slightly oblique angles. Chemically the feldspars are silicates of aluminum, containing sodium, potassium, calcium, or varium or combinations of these elements. Pure feldspar is colorless and transparent but the mineral is commonly opaque and found in a variety of colors. Orthoclase and microcline are called potassium feldspars or potash feldspars, and usually they range from flesh color to brick red, although white, gray, and other colors are found. They are used in the making of porcelain and as a source of aluminum in making glass. A green variety of microcline known as amazonite, or Amazon stone, is used for ornament when cut and polished. The plagioclase feldspars are most commonly gray and occasionally red. Some labradorite exhibits a play of colors, which makes it useful for decorative purposes. [The New Columbia Encyclopedia. Harris, William H., and Judith S. Levey, eds. New York and London: Columbia University Press, 1975.]