CLAY MINING AND WORKING. The prepara tion of clay for use in the plastic arts is a simple process, owing to the accessibility of 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 exploration 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 workmen handle each spadeful of clay; the first merely loosens it up, A‘Ilile 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. brick, 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 underground is necessary to reach a desired quality of clay; and this method will be more and more common as the best grades of superficial clays are ex hausted. The clay having been mined and sorted, it is transported to the factory for further manipulation.
Clay-n=orking or tempering of sonic sort is generally required before clay can be used, and particularly before it can be molded into brick, pottery, sewer-pipe, or tiles. Reduction to a plastic state may be effected by wet or dry grinding, screening. 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 a.t 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 under X1LNS.
Weathering is a self-explanatory term. The time involved may range from months to years, but is more often the shorter period. With im protemonts in machinery and methods, less de pendence is placed on this process than formerly.
Soaking. like weathering, is a natural process, hut it is now used only under primitive condi tions, and where the clay is molded in a soft form. without other working. The clay is simply shoveled into pits, say 4 X 6 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 he tem pered in six hours.
Grinding is accomplished by passing the clay between roils, or in dry pans. the former process being particularly applicable to shales. The pair, are 7 to 9 feet in diameter, with either perforated floors or sides. through Which the material falls as soon as it has reached the de sired fineness. The pan revolves horizontally and by means of friction motion is imparted to two iron wheels, mounted in the pan, Ii to 14 inches wide, weighing 2000 to 6500 pounds each. A pan with one-eighth-inch holes has all average capacity of 100 tons of (day per day of ten hours.
Screening is sometimes employed for clay which has passed the dry pan. Screens may be inclined sieges, either fixed or shaking, and ro• tart' cylindrical or octagonal in form. They de. wand much attention to prevent (dogging, and re quire heavy repairs, but neverthelevs are cheap and simple in operation.
Wet puns are much like dry pans, only their bottoms are not perforated, and scrapers are placed in front of the wheels, to throw up 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 four or five minutes for sewer-pipe.