Extraction and Preparation.—The following descriptions apply, with more or less accuracy, to a majority of the larger works of the present day, turning out from 2500 to 8000 tons of clay each yearly. Two somewhat different methods are employed, according to the situation of the " bed " of clay, in relation to the surface contour of the immediate neighbourhood. The most general ease is that in which the clay has to be raised from a veritable pit, the bottom of which is lower than the ground on all sides. The exact situation of the clay is first determined by systematic " pitting," to a depth of several fathoms, or occasionally by boring. A shaft is then sunk either in the clay itself, or, preferably in the granite close to the clay. From the bottom of this shaft, a level is driven out under that part of the clay which it is intended to work first, and a " rise " is put up to the surface, which should, by this time, be partially cleared of its overburden.. A COMMon depth for such a shaft will be from ten to twelve fathoms. As. soon as the rise is completed to surface, a " button-bole" launder is placed in it, and the remainder of the rise is again filled up with clay. In the meantime, a column of pumps has been pla,ced in the shaft, with an engine to work them, unle,ss water-power is obtainable.
For wat,er, many works are almost entirely dependent upon that met with in sinking the shaft and in driving levels ; but, of course, this may be, and is, eked out by catching the rain-water in reservoirs, and by making use of such small streams as may happen to be available. A small constant supply is sufficient even for a large work, as it is used over and over again. The operation is begun by digging a small pit in the clay, around the upper end of the button-hole launder, and running a stream of water over the exposed clay, or "stope," which is broken up with picks. A very large quantity of sand is constantly disturbed, and as constantly shovelled out of the way, while the water, holding the clay and finer impurities in suspension, runs down the launder, along the level, and into the bottom of the shaft, from whence it is pumped up by the engine or water wheel.
As the excavation becomes larger and deeper, more overburden is removed, and the upper portions of the launder are taken away, until at last the stopes reach the level, when the launder is, of course, no longer required. At first, the sand is thrown out by oue or two " throws," but very soon it becomes necessary to put in an inclined road, for pulling up the sand in waggons ; these are worked by a horse-whiu, or by winding gear attached to the engine or water-wheel. As there are from three to eight tons of sand to each ton of clay, its removal, in the cheapest possible manner, is a matter of great importance. Any veins or lodes of stone, or discoloured portions of clay, are raised from the " bottoms " in the same way as the sand. The stream of water, holding in suspension clay, fine sand, and mica, is, in well-arranged works, lifted at once high enough to allow of all subsequent operations being carried out by the aid of gravity. The stream is first led into one or two long channels, the sides of which are built of rough stone. In these channels, called " drags," the current suffers a partial check, and the fine sand and rougher particles of mica are deposited. From these dregs, the stream passes on into other channels much resembling them, but of greater number, so as to divide the stream still further. This second series of channels, known as " micas," are often built of wood, but sometimes of stone. They differ in no essential respect from the drags, but are more carefully constructed and better looked after, and, as the stream is greatly divided and very gentle, the fine mica is deposited in them. The micas are often about 11 in. wide, ten or
a dozen in number, and 100 ft. Or more long. Provision is made, by underground channels and plug holes, for the periodical cleansing of the drags and micas. This may have to be done twice a day, but generally only once. The deposit in the drags is worthless at present, and is always thrown away ; but that from the micas is often saved, and sold as inferior or " mica" clay. The refined stream of clay then passes on to the " pits," which are circular, 30 to 40 ft. diameter and 7 to 10 ft. deep. These pits 8,re built of rougla masonry, and have an outlet at the bottom, opposite the point at which the stream of clay-water is admitted. This outlet is stopped by a gate or " hatch," or by a plug, and is kept closed until the pit is full of clay. In eaoh outlet, however, is fixed an upright launder some 4 in. square, provided with " pin-holes " and wooden pins set close together. As the stream of clay enters on one side, it is constantly depositing its burden, and the water is as constantly drawn off nearly Or quite clear from the pin-holes, the pins being put higher and higher as the clay rises in the pit. The effluent water is conducted directly to small storage reservoirs, and thence over the clay stopes, whence it does its work over again. When the stream of clay-water enters the pits, it contains from 11 to 3 per cent. of clay ; and what is called a good washing stream will carry about one ton of clay an hour. When the pit is full, the " hatch" is drawn, and the clay is " landed " into the tank. The upper portion is sufficiently fluid to run in of itself ; but that near the bottom has to be helped out by men using " shivers " of wood or iron, which resemble large hoes ; they are assisted by a small stream of-water. The tanks are commonly, but not always, rectangular, built of stone, and paved with stone at bottom, often 60 ft. by 30 by 6, or larger. Once in the tank, the clay is left to settle, until it has the consistency of crea.m eheese, the water being drawn off from time to time ; it is than ready to bo trammed into the " dry." The dry is a large building erected in immediate proximity to the tanks. It is always composed of two parts, the dry proper and the " linhay." The floor or " pan" of the dry is composed of fire clay tiles 18 in. square, 5 or 6 in. thick at the fire end, and gradually thinning off to 2 or 2i in. at the stack end. The flues are built of fire-brick about 14 in. wide, 2 ft. deep at the fire end, and 9 in. deep at the stack end. Each flue should be supplied with a damper. The furnaces Etre built in and arched over with best fire-brick ; the fire-bars run longitudinally, end ere about 6 ft. long. The grate surface is abuut 2 ft. 6 in. wide in front, and 4 ft. 6 in. to 6 ft. Ett back, ac,cording as each furnace supplies three or four flues. The clay, brought in from the tanks in trEtm-waggons holding about half a ton, is tipped on to the tiles, and spread in Et layer from 9 in. thick at the fire end to 6 in. thick at the stack end. The fire end is loaded and cleared every day; the other end perhaps twiee or thrice a week, according to the length of the dry, thiclmess of tiles, perfection of draught, Ste. An average size for a first-class dry is perhaps 15 ft. wide and 120 ft. long ; but some have been constructed considerably larger than this. The pan of the dry should be 6 or 8 ft. above the linhay whenever possible, so as to afford storage space for the dry clay, without expending labour in piling. The tiles should be as porous as possible, for very much more water passes through the tiles and into the flues than is driven upwarda in the state of steam. The temperature should never be allowed to rise so high that the workmen cannot walk on the tiles, otherwise the clay may become baked and damEtged.