Page:The New International Encyclopædia 1st ed. v. 05.djvu/19

* CLAY. CLAY. woiknien handle each spadeful of clay ; the first merely loosens it up, while the second cuts out any nodules of pyrite or other foreign matter, and then throws the clay onto the platform, where it is sorted for ware. l>rick, or whatever product it is best suited. When clay is very hard, it is first loosened with a pick. Dynamite is sometimes employed to break up a bank into loose pieces. Occasionally, mining undergrouud is necessary to reach a desired quality of clay; and this method will he more and more conunon as the best grades of superficial clays are ex- liausted. The clay having been mined and sorted, it is transported to the factory for further manipulation. Clay-irorking or tempering of some sort is generally required before clay can be used, and particularly Iwfore it can be molded into brick, pottery, sewer-pipe, or tiles. Reduction to a plastic state may be efTeeted by wet or dry grinding, sci-eening, pugging, washing, or by the more natural process of weathering; or, a com- bination of two or more of these methods may be employed. The screening and washing may be so arranged as to remove foreign material. It must be understood that the term 'clay-work- ing,' as here used, is limited to the preparation of the raw material for molding or forming, the other processes being treated separately, under the various clay products, as will be the matter of drying; while burning, for the most part, will be discussed imder Kilns. Weafhcriuff is a self-explanatory term. The time involved may range from months to years, but is more often the shorter period. ^'ith im- provements in machinery and methods, less de- pendence is i)laced on this process than formerly. Soaking, like weathering, is a natural process, but it is now used only under primitive condi- tions, and ^here the clay is molded in a soft form, without other working. The clay is simply shoveled into pits, say 4X6 feet in extent, and soaked in water over night. Ring-pits are 25 to .30 feet in diameter, 3 feet deep, lined with brick or boards. An iron wheel is passed over or through the clay, back and forth, mixing in the sand, in case any is used. Clay for some 30.000 bricks may be tem- pered in six hours. DEV-P.S CLAY-ORINDINH MACEnNE. Grinding is accomplished by passing the clay between rolls, or in dry pans, the former process being particularly applicable to shales. The pans are 7 to 9 feet in diameter, with either I)erfi)rated floors or sides, through which the material falls as soon as it has reacheil the de- sired fineness. The pan revolves horizontally and by means of friction motion is im])arled to two iron wheels, niountcil in the pan, << to 14 inches wide, weighing 2000 to CiOO pounds each. A pan with onc-cighthiiuh holes has an average capacity of 100 tuns of clay per day of ten hours. Screening is sometimes employed for clay which has passed the dry pan. Screens may ho inclined sieves, cither fixed or shaking, and ro- tary cylindrical or octagonal in form. They de- mand much attention to prevent clogging, and re- quire heavy repairs, but nevertheless arc cheap and simple in operation. Wet pans are nuich like dry pans, only their bottoms are not i)erforated, and scrapers are placed in front of the wheels, to throw u]) the clay. They may be discharged through a trap- door or by means of an automatic shovel. One of their chief advantages is rapidity, only two or three minutes being required to temper a charge for brick and foiu" or five minutes for sewer-pipe. I'ng-miils appear to be used more than other classes of tempering machines. They are either vertical or horizontal, but in either case they have a central revolving .shaft, fitted with radial knives extending nearly to the surrounding cvl- COMBINED MlXIXe AND pnGGIKS MACHINE. inder in which the clay is placed, or else fitted with a worm screw. Both knives and screw force the clay forward, as well as work it thor- oughly, and by changing the angle of the adjusta- ble knives the speed of the passing clay may be regulated. Water is admitted as desired and needed. When the clay is deficient in redness after burning, hematite may l>e added to the jnig with clay. Pug-mills are compact, and re- quire less power than ring-pits. Washing is effected by a variety of processes, ranging from simple to complex. All of them involve a reduction of the clay in water to a semi-fluid state, or even to a state of suspen- sion, which may be brought about by revolving paddles or blades, mounted on a. shaft in a cylinder or trough, or by any thorough stirring. The heavier, coarser impurities may be deposited in a vat by sedimentation. The water, with the remaining clay, may be removed by siphoning or carefully poured off, and the clay allowed to settle. This process is sometimes called ebulli- tion, particularly where it is repeated many times to recover fine-grained material. The pro- cess of sedimentation may be altogether too slow, especially where large quantities of clay are to be treated, and in such a case the clay and water, after being reduced, as described, to what is known as slip, may he passed through shak- ing-screens of wirecloth, having forty meshes to