CLAY 809 



pottery kiln, and is infusible at that heat. It causes no effervescence with nitric acid, 

 but falls down quickly in it, and becomes more highly coloured. Its refractoriness 

 allows of a harder glaze being applied to the ware formed from it without risk of the 

 heat requisite for making the glaze flow affecting the biscuit either in shape or colour. 

 ' Most of the plastic clays of France,' says M. Brongniart, ' employed for the same 

 ware, have the disadvantage of reddening a little in a somewhat strong heat ; and 

 hence it becomes necessary to coat them with a soft glaze, fusible by means of excess 

 of lead at a low heat, in order to preserve the white appearance of the biscuit. Such 

 :i glaze has a dull aspect and cracks readily into innumerable fissures by alternations 

 of hot and cold water.' Hence one reason of the vast inferiority of the French 

 stoneware to the English. See DORSETSHIRE CLAY. 



5. PORCELAIN CLAY or KAOLIN EARTH. ( Terre a porcelaine, Fr. ; Porzellanerde, 

 Ger.). Kaolin is the name given by the Chinese to the fine white clay with which they 

 fabricate the biscuit of their porcelains. This is the purest known form of clay, its 

 chemical composition being generally reducible to the formula Al 2 O s .2Si0 2 + 2HO 

 (AT-O 3 .2SiO- + 2K S O), corresponding to silica, 46'3 ; alumina, 39'8; water, 13'9per 

 cent. Ordinary massive kaolin, examined under the microscope, always exhibits certain 

 six-sided scales characteristic of the mineral species called Kaolinite. Kaolin and 

 some allied substances are included by Messrs. Johnson and Blake under this species. 



The term Kaolin appears to have been derived from the name of a mountain 

 Kao-ling (Lofty ridge), from which this argillaceous earth was extracted. The 

 kaolins possess very characteristic properties. They are friable in the hand, meagre 

 to the touch, and difficultly form a paste with water. When freed from the coarse and 

 evidently foreign particles interspersed through them, they are absolutely infusible in 

 the porcelain kiln, and retain their white colour unaltered. They harden with heat 

 like other clays, and perhaps in a greater degree ; but they do not acquire an equal 

 condensation or solidity, at least when they are perfectly pure. Most of the kaolin- 

 clays contain some spangles of mica, which betray their origin from granite. 



This origin may bo regarded as one of their most distinctive features. Almost all 

 the porcelain clays are evidently derived from the felspars contained in granite, 

 principally in those rocks of felspar and quartz called ' graphic granite.' Hence they 

 are to be found only in primitive mountain districts, among banks or blocks of granite, 

 forming seams often of very considerable thickness. In the same bed quartz and 

 mica occur, while some portions of the kaolin retain the external form of felspar. 



The most valuable kaolins have been found 



In China and Japan. The specimens imported from these countries appear pretty 

 white ; but are more unctuous to the touch, and more micaceous than the porcelain 

 clays of France. 



In Saxony. The kaolin employed in the porcelain manufactories of that country 

 has a slight yellow or flesh colour, which disappears in the kiln, proving, as Wallerius 

 observed, that this tint is not owing to any metallic matter. 



In France, at Saint- Yrieix-la-Perche, about 10 leagues from Limoges. The kaolin 

 occurs there in a bed, or perhaps a vein, in beds of granite, or rather of that felspathic 

 rock called Pe-t\in-tse, which exists there abundantly. This kaolin is generally white, 

 but sometimes a little yellowish, with hardly any mica. It is meagre to the touch, 

 and some beds include large grains of quartz, called pebbly by the China manu- 

 facturers. This variety, when ground, affords, without the addition of any fusible 

 ingredient, a very transparent porcelain. 



Near Bayonne. A kaolin possessing the lamellated structure of felspar in many 

 places. The rock containing it is a graphic granite in every stage of decomposition. 



In England, in the counties of Devonshire and Cornwall. The kaolin or China- 

 clay is very white, and more unctuous to the touch than those upon the continent of 

 Europe mentioned above. Like them it is supposed to result from the decomposition of 

 the felspars entering into the composition of granite. 



Nature has, up to a certain point, provided the article which man requires for the 

 elaboration of the most perfect production of the potters' art. The clay China 

 clay, as it is commonly called, or kaolin, as the Chinese have it is quarried from 

 amidst the granitic masses of Dartmoor and of Cornwall. We are not at all satisfied 

 with any of the theories which have been put forward to account for the formation of 

 porcelain clay. It is commonly stated to be a decomposed granite ; this rock, as is 

 well known, consisting of mica, quartz, and felspar, with sometimes schorl and horn- 

 blende. The felspar is supposed to have decomposed ; and, as this forms the largest 

 portion of the mass, the granite is disintegrated by this process. We have, therefore, 

 the mica, quartz, and the clay, forming together a soft mass, lying but a short dis- 

 tance below the surface, but extending to a considerable depth. It is quite evident 

 that this stratum is not deposited ; had it been so, the particles constituting the mass 

 would have arranged themselves in obedience to the law of gravity, towards which 



