KAOLIN OR CHINA CLAY.
Kaolin is the name given by the Chinese to the fine white clay used in making porcelain. It is furnished by the decom position of granite and other rock, the constituents of which are quartz, mica, and feldspar, the mouldering or decomposition being caused by the joint action of air and water.
A much similar clay, to which the Chinese name has been given, occurs near St. Anstel, in Cornwall, a county in the southwest of England.
The kaolin of Cornwall, as well as that near Limoges, in France, and which latter clay was discovered in 1768, is pro duced by the decomposition of pegmatite, a granite in which there is very little of mica or quartz.
The clays which are much valued by porcelain-makers may be represented by the formula Al203,3SiO3+2H0 ; all clays being silicates or hydrated silicates of alumina.
The term kaolin, when used by practical potters, usually means the finer and white qualities, or such as will make the grades of ware known as white granite or stone porcelain— called China clays. Such clays are found in many parts of the
United States, notably in Maryland, Pennsylvania, and Dela ware. The actual quantity is great, as there are fifty to sev enty-five potteries in the United States, which use from ioo to 3000 or 400o tons per annum each. The price is governed by color and plasticity. Some clays are highly valued for their color only, others for their plastic property. The demand is large and steady for good China clays. To ascertain the value of such clays send a piece just as it comes from the earth to any practical potter. Potters test these matters by fire and water. Vessels coming from Liverpool are now bringing En glish clays as ballast, and they are sold at about the same price as American kaolin. Flint or ground quartz, or feldspar, or ground feldspathic rock, is largely used by potters, and is in good demand.