Kaolin or China clay is the basis of porcelain and many pottery clays, and is produced by the decomposition of felspar. Kaolin occurring in the position of the original felspar is called residual kaolin, and frequently it happens that this is carried away by the streams and deposited as sediment in a distant locality, when it is known as transported or sedimentary kaolin. The residual kaolin is likely to contain fragments of crystalline quartz, mica, and un-decomposed spar, with smaller quantities of other minerals; while the transported kaolin is likely to contain iron oxide, lime carbonate, and other impurities intimately diffused 'with it. The residual kaolin furnishes the purer grade, as its impurities may be washed out; whilst the impurities in the sedimentary kaolin are not of such a nature as to be washed out. The common method of mining kaolin in the United States is by means of vertical shafts 25 ft. or 30 ft. in diameter, lined with pieces of wood, each 3 in. by 10 in. or 12 in. by 24 in. The ends are bevelled, so that when the pieces are laid end to end around the sides of the vertical shaft they form a strong wall capable of resisting the great pressure from the clay. As the shaft is sunk, the walls are added to by building from below.

Sometimes the clay is mined from open pits, and in a few instances it has been obtained from underground galleries by using heavy timbers, but in most cases the shafts lined with wood are found to be the safest and most economical method. The different methods of washing the kaolin to remove the coarse impurities are all basedon the same principle, that of notation. The material is thrown into water, and the particles of the clay, being finer and lighter than those of the impurities, remain longer in suspension; hence it is only necessary to increase the length of the troughs through which it is carried or to decrease the rate of flow, or both may be done, to get the required degree of fineness in the kaolin, and remove practically all the foreign ingredients. One method commonly employed is to feed the crude material with a current of water into an ordinary log washer; this consists of a horizontal beam from 10 ft. to 25 ft. or more in length, revolving in a horizontal, rectangular, or semi-cylindrical trough of about twice the diameter of the beam. Mounted on the beam are numerous short arms or knives which cut and stir up the lumps, and at the same time carry it slowly to the other end of the trough.

The current of water carrying the clay passes from the log washer into a trough or a zigzag series of troughs. The length traversed by the current in the washing troughs and the rate of flow may be varied to suit the character of the material used and the grade of kaolin required. The greater portions of the coarse sand and the larger particles are dropped either in a log washer or close to it, and sand wheels are used to remove this and prevent the troughs from being clogged. The finer sand and the mica flakes are deposited in the zigzag troughs, which are usually about 700 ft. long; they are opened and the deposit is scraped out at intervals. The kaolin carried in suspension by the water flowing through this long zigzag channel is run into larger vats or settling tanks. From these, after a time, the clear water is drawn off and the mud is pumped into a filter press and squeezed by hydraulic pressure. The presses consist of a series of flat iron or wood frames, strung on a central iron pipe. Bags of heavy cloth are placed in the spaces between the frames and connected with the central pipe, which is connected with the pump.

The kaolin comes from the filter press in large cakes either round or square, and so that they may dry, these are exposed in racks to the air for several weeks, or put on a floor or in a tunnel and heated by steam or hot air. The cheaper grades of clays are not put through a filter press, being either dried in the settling tanks or transferred, to a drying floor directly from the tanks. Auother method of washing is to put the clay with water into vessels, where it is thoroughly disintegrated by means of plungers. It is stirred up into a slip which is run off through troughs to settling tanks, made preferably of cypress wood. The kaolin slip is carried thence into the other tanks, whence it is pumped into the filter presses. The clay is removed from the press to the drying floor, heated by exhaust steam. To obtain high grade kaolin, such as that used in making paper, it is usually easy to get rid of grit by elutriation and settling in the washing troughs, vats, etc., iron being avoided by the proper selection of material.

The chief trouble is often the presence of almost microscopic plates of mica, which the washing process often fails to eliminate, and which have to be removed by passing the wet material through a very fine silk mesh.