Page:Encyclopædia Britannica, Ninth Edition, v. 14.djvu/11



AOLIN, a name applied to the pure white clay which forms an important ingredient in the manufacture of porcelain, and which is, therefore, known also as china clay. Large quantities are raised in Cornwall, whence it is frequently termed Cornish clay. The name kaolin is said to be derived from a hill near King-tih-chin, in China, named Kao-ling or "lofty ridge." The clay from this locality was first sent to Europe, under the name of kaolin, by the Père d'Entrecolles, a Jesuit missionary who resided at King-tih-chin in the early part of the last century. A similar white clay was soon afterwards found at Aue, near Schneeberg in Saxony, and was used by Böttcher in the manufacture of porcelain, thus laying the foundation of the factory at Meissen for producing the famous Dresden china. In England kaolin was first detected in Cornwall by William Cookworthy of Plymouth about 1755, a discovery which resulted in the manufacture of hard paste china at Plymouth and Bristol. In Cookworthy's writings the clay is called "caulin." Kaolin is found in Nebraska, and in several of the eastern States of the American Union.

Certain clays, when examined under the microscope, are seen to contain crystalline pearly scales of a mineral which Messrs Johnson and Blake have described as kaolinite (American Journal of Science, ser. ii. vol. xliii., 1867, p. 351). They regard this crystalline substance as a distinct mineral species, a hydrated silicate of aluminum, forming the basis of pure kaolin. Its composition appears to agree with Forchhammer's formula for true kaolin, viz.,. Mr Collins regards the crystalline scales which are associated with the Cornish kaolin, not as kaolinite, but as a white lithia-mica or lepidolite.

Kaolin is almost invariably a product of the alteration of felspar, and is therefore always found in association with felspathic rocks, usually granite. The china-clay rocks of Cornwall and Devon are simply granites in which the orthoclase-felspar has become decomposed or kaolinized. Such rocks are termed by Mr Collins carclazite, after the Carclaze mine, near St Austell, where typical varieties occur. The production of kaolin from felspar is rather difficult to explain, inasmuch as the alteration is sometimes observed under conditions which appear to preclude the operation of atmospheric agencies. It is not simply the effect of water charged with carbonic acid, whereby the felspar might be decomposed and its alkaline silicate removed as a soluble carbonate, while the silicate of aluminium remained behind, in a hydrated condition, as kaolin or china clay. Many chemists have been inclined to attribute the decomposition to the effect of water or watery vapour at a high temperature, charged with hydrofluoric and boric acids. It is certain that minerals containing fluorine and boron—such as fluor spar, lepidolite, and schorl—are common associates of kaolin.

The localities from which kaolin is obtained in Britain are all situated in Cornwall and Devon; in the former county the workings are principally in the neighbourhood of St Austell, St Stephen's, and Breague, while in Devon they are situated at Lee Moor and Meavy, on the south of Dartmoor. In working the clay the "overburden" or superficial deposit is first removed, in order to reach the clay-bearing rock. The rock is broken up by the pick, and water is introduced to wash out the clay. A quantity of sand is left behind, and requires to be constantly removed. The water containing the clay in suspension is either pumped to the surface up a shaft, or, if the working be upon a hill-side, is run out at an adit level. This clay-water is led into channels called "drags," where the sand and coarser flakes of mica are deposited. From the drags the liquid passes into another set of channels called "micas," in which further deposition of suspended matter occurs. Thus purified, the clay-water is conducted into a series of pits and tanks, where the finely-divided particles of clay slowly subside. In the tanks it is allowed to settle until it acquires a thick creamy consistency, when it is transferred to the drying house or "dry." Formerly the clay was dried naturally by exposure to sun and air, but it is now always artificially dried by means of heated flues, and the preparation of the clay is thus greatly facilitated.

China clay is not only used in the manufacture of pottery, but is also extensively employed by the paper-maker and by the calico-bleacher. It is likewise used to a small extent in the manufacture of alum, artificial ultramarine, and some other chemical products. In 1880 the quantity of china clay raised in Cornwall amounted to 278,572 tons, and in Devon to 25,370 tons.

See Collins, in Journal of the Society of Arts, May 5, 1876; and Treatise on China Clay, by D. Cock, 1880.

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