Clay, a finely levigated silico-aluminous earth, found in its natural state in beds of varying depth, and in lamime of varying thickness. It is formed by the disintegration of feldspathic rocks, and the precipitation in basins from suspension in water of the finely divided, impalpable particles. It may be to a certain extent derived from slate rocks, and in such case is only the original clay from which the slate was formed, reproduced; but the original source was feldspathic or granite rock. The aluminous minerals contained in granite rocks are feldspar, mica, and hornblende. Ordinary granite is composed of quartz, feldspar, and mica. In syenitic granite mica is partly or wholly replaced by hornblende. These minerals contain alumina in varying proportions, in combination with silica, forming silicate of alumina, and they also contain alkaline silicates, either of potassium or sodium. Mica and hornblende generally contain considerable oxide of iron, while feldspar usually yields only traces, or none. Therefore, clays which are derived from feldspar are light-colored or white, while those partially made up of decomposed mica or hornblende are dark, either bluish or red.
As a rule, however, feldspar is the constituent of granite in fine clay, as the particles of the other minerals do not become so finely divided, and therefore are deposited before reaching the basin where the fine clay is formed. Clay may be formed artificially from certain decomposed granites which are found in various parts of the United States, particularly in the vicinity of Washington, D. C. These beds are often found in a sufficiently soft condition to admit of being cut with the spade to the depth of 20 or 30 ft. A mass of the excavated rock is placed in a sluice and drenched with water, which is made to run into a tank. Here the coarser portions subside, while those more finely suspended run out with the water at the top into another tank, in which again there is (mother separation made in a similar manner, the finest particles passing into a third tank, where after a time they are precipitated. The water is then carefully drawn off, and the clay allowed to become sufficiently dry to be cut with a spade and removed. A fine white pipe clay may be procured in this manner. - Clays have varying physical properties, depending on the admixture of other bodies, such as sand, lime, magnesia, alkalies, oxide of iron, and organic matter.
Dry clay is readily separated into lamina?, which, however, especially in the more tenacious varieties, break with a conchoidal fracture. It is non-crystalline, opaque, and of varying degrees of hardness, the harder kinds being nearly of the consistency of chalk. It has a strong affinity for water, which is not destroyed by moderate baking. A new clay pipe taken into the mouth sticks to the tongue and lips by absorbing the moisture from these parts. When moistened, clay emits a peculiar smell, called argillaceous, and also has a peculiar taste, which may also be so called. Mixed with considerable water, the clays are more or less plastic, the degree depending on their purity and peculiarities of composition. When subjected to a white heat they become very hard, so that they will strike fire with steel. Those clays which have a high degree of plasticity are said to be fat or long; and when possessing but little plasticity they are called lean, meagre, or short. They all shrink considerably in drying and burning, the amount depending on the quantity of water which they contain. Pure clay or silicate of alumina is infusible, but when mixed with the alkalies or alkaline earths it becomes fusible in proportion to the admixture.
The principal varieties of clay used for ceramic manufacture may be divided into refractory, to which belong porcelain clays, and fusible, as potter's clay. To these may be added limey and ochrey clays. - Porcelain clay, or kaolin (a word derived from Kao-ling, the name of a hill in China whence the material for making porcelain is obtained), is rather softer and more crumbly than many other kinds of clay, does not form so stiff a paste with water on account of the free silica which it contains, and adheres but slightly to the tongue. It has a specific gravity of about 2'2. When separated from free silica, it has an average composition of 47 per cent, of silica, 40 of alumina, and 13 of water, having the formula Al4O3, 2SiO2+2aq (or A12O3, 3SiO3 + 2 aq). It may be considered as derived from potash feldspar, or orthoclase, which has the formula K2O1AI4O3, 6SiO2, the transformation taking place by the loss of all the potash and two thirds of the silica, and a union with two equivalents of water.
Some varieties, however, have a different composition, as the kaolin of Passau in Bavaria, which contains 43.65 per cent, of silica, 35.93 of alumina, 1 of sesquioxide of iron, 0.88 of carbonate of lime, and 18.5 of water; represented by the formula 4A14O39SiO2 + 12aq. Porcelain clay from Gutenberg, near Halle, contains, according to Bley, 39.02 per cent, of silica, 45 of alumina, 0.07 of carbonate of lime, 3.32 of carbonate of magnesia, 0.19 of sesquioxide of iron, and 10 of water, having approximately the formula 2Al4O3, 3SiO2 + 3aq. The Chinese and Japanese kaolins contain about twice the proportion of silica to alumina that is found in the above European specimens, and are whiter and more unctuous to the touch. The Cornish kaolin of England, however, has an unctuous feel, and is very white and evidently formed by the decomposition of feldspar. Potters' clay, or plastic clay, contains lime and magnesia with more or less oxide of iron, either black or red. Mixed with water, it becomes exceedingly plastic; it is the common modelling material of the sculptor. The finer varieties are used for white earthenware, and the coarser for inferior ware and for drain pipes.
Pipe clay is a pure variety of potters' clay, approaching somewhat in its character to kaolin, although containing less silica, and remaining more porous after baking. Fine pipe clay is found at Poole in Dorsetshire, England, and in the isle of Purbeck. It is semi-fusible, allowing the stems to be bent while hot. - Beds of fine kaolin are found at Brandon, Vt., which is used in the manufacture of paper to give weight, and the colored varieties are used in the adulteration of paint. In New Jersey, near Woodbridge, and also at South Amboy, beds of clay are worked to great extent for the manufacture of stone ware, which also furnish an excellent material for fire brick. For this purpose they must be burned twice; that is, a certain amount of clay is burned, and then ground up with another portion to make it sufficiently plastic for moulding into forms, which are again baked. Similar deposits also compose the banks of the Delaware river between Bor-dentown and Burlington. They all belong to the series of upper secondary rocks, underlying the greensand beds. Common brick clay is usually obtained from alluvial deposits. The color of the brick depends upon the quantity of oxide of iron in the clay.
Clay beds found in Wisconsin near Lake Michigan are so t'vea from this coloring matter that the bricks are of a straw color. - Clay for fire brick is obtained for the most part from the coal formation. Each coal bed, with very rare exceptions, rests upon a stratum of bluish gray clay, which appears of the solidity of structure of stone, though it is found to be easily cut with a knife. It often contains some carbonaceous matter lining its seams and giving them a black color, and it abounds with the fossil stigmaria roots, which are all converted into the same material as the rest of the beds. When ground to powder in a mill, and mixed with quartz sand, obtained from the sandstone beds of the same formation, it makes an excellent quality of fire brick. Old, hard-burned brick, broken up and mixed with the clay, improves its quality, causing it to retain better its shape after moulding. Such a mixture is well adapted for the manufacture of crucibles. The following are analyses of a number of the best foreign fire clays, that of Gross Almerode being the material of which Hessian crucibles are made, with the addition of an equal weight of sand, and the Stourbridge clay that of the fire brick of this name:
Constituents, dried at 100° C.
Brierly Hill, near
Hygromet. ric water.
Oxide of iron
The clay beds of the coal formation are extensively used at Newcastle for the manufacture of large gas retorts as well as of fire brick. A number of different qualities of clay are found, all of which are from beds overlaid by coal and worked with it. Dr. Richardson gives the following analyses of these varieties:
Oxide; of iron..
Water and organic matter.
- A very singular quality of clay was long ago known in Europe, which produced bricks so light and porous that they floated upon water. The clay was found in Spain, and centuries afterward in Tuscany. Its composition was: silica 65 per cent., magnesia 17, alumina 14, and lime 4. The bricks were remarkably porous, light, and such poor conductors of heat that they might be held in the hand by one end while the other was red.hot. Clay of similar properties was afterward met with in central France, which when mixed with .1/20 its weight of ordinary clay produced bricks of this character. When submitted by Ehrenberg to microscopic examination, these clays were found to consist of the shelly coverings of infusoria. Such collections of these microscopic silicious shells forming beds of clay are not uncommon in this country, particularly at the bottom of peat bogs; and it is highly probable they might be in many places turned to useful account. Ehrenberg, knowing that the clay under Berlin was composed of these organic remains, concluded that it was adapted to this use, which proved to be the case.
In the banks of the river Spree it is found sometimes 100 ft. thick, of a light gray color, and almost entirely made up of these little shells, of which it takes no fewer than nine to equal the diameter of a human hair. - Besides its use for brick, tiles, and earthenware, clay is applied to several other purposes, as for the manufacture of mosaic tesserae, buttons, and artificial gems; for the manufacture of alum; for clarifying sugar by the process called claying; for producing a glaze upon paper hangings; for the construction of adobe houses; and for the fulling or scouring of woollen cloths. The clay used for this last purpose is called fullers' earth. It is of an unctuous, soft, and dull quality, and is distinguished from other clays by falling to pieces in water with a crackling noise, instead of forming a paste as other clays do. Applied to cloth, it absorbs the oil and greasy matters. Its use has for some time been gradually giving way to that of soap. Extensive as are the uses of clay in the arts, its importance in agriculture is still greater, forming as it does the most valuable component of fertile soils.
Clay, the name of 16 counties in the United States. I. A central county of West Virginia, intersected by Elk river, and bounded S. E. by Twenty Mile creek; area, 400 sq. m.; pop. in 1870, 2,196, of whom 4 were colored. The surface is diversified. The chief productions in 1870 were 39,093 bushels of Indian corn, 11,497 of oats, and 274 tons of hay. The value of live stock was $63,096. Capital, Clay Court House. II. A S. W. county of North Carolina, bordering on Georgia, watered by several streams; area, about 200 sq. m.; pop. in 1870, 2,461, of whom 142 were colored. The chief productions in 1870 were 6,061 bushels of wheat, 79,985 of Indian corn, 14,408 of oats, 4,618 of Irish and 5,507 of sweet potatoes, 221 tons of hay, and 13,113 lbs. of tobacco. There were 587 horses, 945 milch cows, 2,103 other cattle, 3,154 sheep, and 5,376 swine. Capital, Haysville. III. A S. W. county of Georgia, bounded S. by Colomoke creek, separated from Alabama by the Chattahoochee river, and watered by its tributaries; area, about 200 sq. m.; pop. in 1870, 5,493, of whom 2,849 were colored. The Fort Gaines branch of the Southwestern railroad terminates at the county seat. The surface is level and the soil fertile.
The chief productions in 1870 were 85,014 bushels of Indian corn, 18,124 of sweet potatoes, and 3,220 bales of cotton. There were 375 horses, 845 milch cows, 1,549 other cattle, and 4,281 swine. There were 4 carriage factories and 1 flour mill. Capital, Fort Gaines. IV. A N. E. county of Florida, bounded E. by St. John's river, and watered by its affluents and by numerous lakes; area, 430 sq. m.; pop. in 1870, 2,098, of whom 399 were colored. The Florida railroad passes through the N. W. corner. The surface is level. The chief productions in 1870 were 13,188 bushels of Indian corn, 5,322 of sweet potatoes, 6,690 gallons of molasses, and 65 bales of cotton. There were 130 horses, 1,238 milch cows, 2,620 other cattle, and 1,988 swine. Capital, Green Cove Springs. V. An E. county of Alabama, recently formed from portions of Randolph and Talladega counties, drained by tributaries of the Coosa and Tallapoosa rivers; area, about 700 sq. m.; pop. in 1870, 9,560, of whom 737 were colored. The soil is well adapted to agriculture. The chief productions in 1870 were 38,422 bushels of wheat, 196,886 of Indian corn, 17,005 of oats, 8,209 of sweet potatoes, 1,143 bales of cotton, 123,404 lbs. of butter, and 9,005 of tobacco.
There were 959 horses, 501 mules and asses, 2,580 milch cows, 2,832 other cattle, 3,924 sheep, and 10,271 swine. Capital, Ashland. VI. A.N. W. county of Texas, separated from the Indian territory on the N. by lied river; area, 1,100 sq. m.; pop. in 1860, 109; in 1870 not returned. It is intersected by the Big and Little Wichita rivers, and also watered by tributaries of the W. fork of Trinity river. The surface is broken and hilly; the soil well adapted to wheat and other cereals as well as to stock raising.