Pigments, Paint and Painting: A practical book for practical men

by George Terry

Charlton White.—One of the names applied to a white pigment, containing zinc oxide and sulphide, and described under zinc whites, p. 254.

China Clay.—This substance is also known as kaolin, porcelain clay, and Cornish clay. It arises from the natural decomposition of felspar in soft disintegrating granite, gneiss, and porphyry, the rocks which are rich in soda-felspar yielding it most abundantly. The main supplies of this country are derived from Cornwall and Devon; in continental Europe, beds of good quality exist in France, Bavaria, Saxony, Prussia, Bohemia, Bornholm island, and Hungary; in China, it is very plentiful; and in the United States, it occurs in many localities.

The approximate composition of china clay may be stated as silica, 47·2; alumina, 39·1; water, 13·7 per cent. Often a little iron, lime, and potash or soda are left in the prepared article by the imperfection of the cleansing process. The most important characters are colour, plasticity, and a capacity for hardening under the influence of heat.

The china-clay industry of Cornwall and Devon has been admirably described by J. H. Collins, F.G.S., in a paper recently read before the Society of Arts.

Occurrence.—The natural clay rock is almost always covered with a thick layer of stones, sand, or impure and discoloured clay, known as “overburden.” This capping often much resembles glacial drift; but it never contains any scratched or glaciated stones, or travelled blocks. It varies in thickness from 3 feet to 40 feet, and must, of course, be removed before the clay can be wrought. The clay rock, being a decomposed granite, consists of china-clay, irregular crystals of quartz, and flakes of mica, with sometimes a little schorl and undecomposed felspar.

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 case 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-hole” 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 placed in the shaft, with an engine to work them, unless water-power is obtainable.

For water, 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 one 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-whim, 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 drags, 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 inches wide, ten or a dozen in number, and 100 feet 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 feet diameter and 7 to 10 feet deep. These pits are built of rough 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 each outlet, however, is fixed an upright launder some 4 inches 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.