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The following Articles are copyrighted in the United States of America, and permission to print them in this
volume has been granted exclusively to A. & C. BLACK, Edinburgh :-
KENTUCKY. Copyright, 1882, by John R. Procter.
AOLIN, a name applied to the pure white clay which felspar might be decomposed and its alkaline silicate
forms an important ingredient in the manufacture removed as a soluble carbonate, while the silicate of of porcelain, and which is, therefore, known also as china aluminium remained behind, in a hydrated condition, as day. Large quantities are raised in Cornwall, whence it kaolin or china clay. Many chemists have been inclined to is frequently termed Cornish clay. The name kaolin is attribute the decomposition to the effect of water or watery said to be derived from a hill near King-tih-chin, in China, vapour at a high temperature, charged with hydrofluoric named Kao-ling or "lofty ridge." The clay from this and boric acids. It is certain that minerals containing locality was first sent to Europe, under the name of kaolin, fluorine and boron-such as fluor spar, lepidolite, and schorl 2 King-tin-chin in the early part of the last century. A The localities from which kaolin is obtained in Britain similar white clay was soon afterwards found at Aue, near are all situated in Cornwall and Devon; in the former Schneeberg in Saxony, and was used by Böttcher in the county the workings are principally in the neighbourhood manufacture of porcelain, thus laying the foundation of of St Austell, St Stephen's, and Breague, while in Devon the factory at Meissen for producing the famous Dresden they are situated at Lee Moor and Meavy, on the south china. In England kaolin was first detected in Cornwall of Dartmoor. In working the clay the "overburden" or by William Cookworthy of Plymouth about 1755, a dis- superficial deposit is first removed, in order to reach the covery which resulted in the manufacture of hard paste clay-bearing rock. The rock is broken up by the pick, and china at Plymouth and Bristol. In Cookworthy's writings water is introduced to wash out the clay. A quantity of the clay is called “caulin.” Kaolin is found in Nebraska, sand is left behind, and requires to be constantly removed. and in several of the eastern States of the American Union. The water containing the clay in suspension is either
Certain clays, when examined under the microscope, are pumped to the surface up a shaft, or, if the working be seen to contain crystalline pearly scales of a mineral which upon a hill-side, is run out at au adit level. This clayMessrs Johnson and Blake have described as kaolinite water is led into channels called “drags,” where the sand (American Journal of Science, ser. ii. vol. xliii., 1867, p. and coarser flakes of mica are deposited. From the drags 351). They regard this crystalline substance as a distinct the liquid passes into another set of channels called mineral species, a hydrated silicate of aluminium, forming “micas,” in which further deposition of suspended matter the basis of pure kaolin. Its composition appears to
Thus purified, the clay-water is conducted into a agree with Forchhammer's formula for true kaolin, viz., series of pits and tanks, where the finely-divided particles A1,03.2SiO2 + 2H,0. Mr J. H. Collins regards the crystal- of clay slowly subside. In the tanks it is allowed to settle line scales which are associated with the Cornish kaolin, until it acquires a thick creamy consistency, when it is not as kaolinite, but as a white lithia-mica or lepidolite. transferred to the drying house or “ dry.” Formerly the
Kaolin is almost invariably a product of the alteration clay was dried naturally by exposure to sun and air, but it of felspar, and is therefore always found in association with is now always artificially dried by means of heated flues, felspathic rocks, usually granite. The china-clay rocks of and the preparation of the clay is thus greatly facilitated. Cornwall and Devon are simply granites in which the China clay is not only used in the manufacture of orthoclase-felspar has become decomposed or kaolinized. pottery, but is also extensively employed by the paperSuch rocks are termed by Mr Collins carclazite, after the maker and by the calico-bleacher. It is likewise used to Carclaze mine, near St Austell, where typical varieties a small extent in the manufacture of alum, artificial ultra
The production of kaolin from felspar is rather marine, and some other chemical products. In 1880 the difficult to explain, inasmuch as the alteration is sometimes quantity of china clay raised in Cornwall amounted to observed under conditions which appear to preclude the 278,572 tons, and in Devon to 25,370 tons. operation of atmospheric agencies. It is not simply the See J. H. Collins, in Journal of the Society of Arts, May 5, 1876 ; effect of water charged with carbonic acid, whereby the and Treatise on China Clay, by D. Cock, 1886.