Page:The New International Encyclopædia 1st ed. v. 05.djvu/18

* CLAY. c-lay is said to be residual in its nature. The material, however, is often washed down into the lakes or ocean by the surface-waters, and there spread out over the bottom as an alumi- nous sediment: such a clay deposit being known as a sedimentary one, which is not onlj' strati- tied, but may also l>e more extensive than a residual. Scilimcntary clay sometimes becomes consolidated by the pressure of other sediments which have been deposited on top of it, arid it is then termed shale. These shales, on grinding and nii.xing with water, develop the same plas- ticit}' as does soft clay. The chief chemical constituents of clay are silica and alumina; but, in addition to these, variable ijuanlities of iron oxide, lime, mag- nesia, alkalies, water, and even rarer substances are often present. These ingredients affect the physical properties of the clay; such as its color when bui-ned, air and fire shrinkage, refractori- ness, plasticity, and thus indirectly its uses. The lime, magnesia, iron oxide, and alkalies exert a fluxing action in burning, and the great- er their quantity the lower the fusing-point of the clay. Silica decreases the air and fire shrinkage of a clay, while alumina and water have the re- verse effect. The property of plasticity, to- gether with that of hardening under fire, makes clay an article of great value in the plastic arts. Those clays which are low in plasticity are said to be 'lean.' while the highly plastic varieties are 'fat.' Clay does not fuse suddenly, but soft- ens gradually under the influence of heat. In ery fusible clays this sintering may begin at 1500° or 1700° F., while in very refractory kinds it does not take place imtil a temperature of 3000° F. or more is reached. The red color of a burned clay is due to considerable iron oxide, while buff is produced by a small quantity of iron, or by an excess of lime. The following table gives the composition of several grades of clay : SiOs AUO3 Ti,03 CaO MgO Alkalies... H.O Moisture.. % 62.40 26.51 1.14 .86 .01 .10 a3.35 I % I % 4.5.78 54.23 36.46 I 32.80 1.08 .50 .04 13.40 2.06 4 5 % % 68.54 63.31 18.49 16.57 3.38 4.06 1.03 1.11 .88 1.10 2,37 3.16 4.62 6.89 1.52 3.76 % 60.59 12.46 5.79 6.84 3.28 4.39 4.36 1.46 (1) Crude kaolin, Webster, N. C; (3) Washed kaolin. Webster, N. C; (3) Fire-clay, Wymp's Bap, Pa.: (4) Paving-, brick shale, Kansas City, Mo.: (5) Brick-clay, Indianola, Iowa: (6) Calcareous slip clay, -Albany, N. T. Clay is used in the manufacture of common, pressed, and paving brick; terracotta, fireproof- ing, terra-cotta lumber; roofing, floor, and glazed tile; firebrick, retorts, crucibles, muffles, and other refractory goods; all grades of pottery, stoves, sewer-pipe, door-knobs, electrical insu- lators, turbine-wheels, closets, and bathtubs and washtubs: filters, mineral paint, food-adulter- ants, Portland cement, paper - fillers, emery- wheels (as a cement therein), ultramarine, mod- eling, soap. etc. Clay is widely distributed geographically, and also geologically — i.e. in the rock-formations of different ages. In the I'nited States deposits are found at a great number of localities. The Cre- taceous clays of New .lersey are much used in the manufacture of refractory goods and white- 6 CLAY. ware, while the Carboniferous clays of Pennsyl- vania and Ohio are also extensively employed lor making firebrick. Kaolin is (juarried at several points in North Carolina, Georgia, Jliiry- land, and Wisconsin : much white ball-clay is louiul in Florida, and stoncwarc-clay in Illinois and Missouri. Clays suitable for brick, terra- cotta, and sewer-pipe occur at many points all over the United States. In the Central States, clays suitable for the manufacture of vitrified ]iaving-brick are actively worked. With all this supply, however, much ball-clay and kaolin is imported, the material coming chiefly from Eng- land. About $70,000,000 worth of clay products are produced annually in the United States alone. The different varieties of clay are as follows: K.LIN (q.v. ) or CniXA-Ci.AY. A veiy white burning clay, used in the manufacture of porce- lain and white earthenware. It is of residual origin, and often occurs in the form of veins. Fire-Cl.ys (q.v.). Clays containing a low percentage of fluxes, and hence capable of re- sisting high temperatures. They are used in the manufacture of all classes of refractory goods, and sometimes also for making pressed brick and terra-cotta. Flikt-Clay. a dense, hard, non-plastic fire- clay, often found in the Carboniferous forma- tions. Pipe-Clat. a term applied to many smooth, highly plastic clays. Brick-Clay. A term including almost any iminire clay which can be molded into bricks. Terra-Cotta Clay. A grade of clay used for making terra-cotta (q.v.). It includes many varieties. Slip-Clay. An easily fusible clay, which melts to a translucent glass, and is used for glazing the cheaper grades of pottery. Pottery^-Clay. a term applied to any clay used in the manufacture of pottery. Saggar - Clay. A grade of fire - clay used for making saggars, or vessels in which fine pottery is placed during baking in the kiln. Alum-Clay'. a clay containing a large amount of alum. ]SI.RL-s Clay. A clay containing from 20 to 30 per cent, of lime carbonate. Gumbo Cl.^.y, A very plastic, sticky clay, found in many localities in the Central States, and often used in the manufacture of railroad ballast, Ball-Clay. a plastic, white burning clay, used as a bonding ingredient in white-ware and porcelain bodies. Clay Mining and Working. The prepara- tion of clay for use in the plastic arts is a simple process, owing to the accessibilitj' ot clay-banks and strata, and the ease with which the material can be separated from the other substances with which it is commonly found. After preliminary exp-loration and testing, the first thing to be done is to remove in ears, wagons, or carts the top layer of dirt. The dig- ging out of the clay itself is usually done in successive pits, the dirt from the pit under ex- cavation being thrown into the pit that has just been dug. In digging clay, a gouge-spade is used, which differs from the ordinary spade in having the blade cylindrical and the upper edge broader. A platform of boards is placed beside the pit, on which the clay is thrown and sorted. Two