CHINA (ware) : see POTTERY AND PORCELAIN.
so called because it is an essential ingredient in the manufacture of china or porcelain. It is also known as kaolin, a corruption of the Chinese Kau-ling, meaning "High Ridge," the name of a hill east of King-te-chen, whence the earli est samples of the clay sent to Europe were obtained by the Pere d'Entrecolles, a French Jesuit missionary in China in the early part of the 18th century. His specimens, examined in Paris by R. A. Reaumur, showed that true porcelain was manufactured from a paste containing two essential ingredients—china-clay, a pure white clay, and china-stone, a variety of granite consisting mainly of orthoclase felspar. The china-clay confers plasticity on the paste and secures retention of form for the ware when exposed to the heat of the kiln; while the china-stone gives the translu cency so characteristic of porcelain. Some of the earliest discov eries of china-clay in Europe were at Aue, near Schneeberg in Saxony, and at St. Yrieux, near Limoges in France. In England it was discovered in Cornwall about the year 175o by William Cookworthy, of Plymouth ; and in '768 he took out his patent for making porcelain from china-clay and china-stone. These raw materials were found first at Tregonning hill, near Breage, and afterwards at St. Stephen's in Brannel, near St. Austell; and their discovery led to the manufacture of hard paste, or true porcelain, at Plymouth and subsequently at Bristol.
The china-clay rock of Cornwall and Devon is a granite with its orthoclase felspar decomposed to a soft white powdery sub stance which, when wet, acquires the characteristic plasticity of clay. The other components of the granite, the quartz and the mica, are unaffected and, although they are subordinate to the de composed felspar, or kaolin, their presence retains for the rock the appearance of a weathered granite. Although kaolinization can undoubtedly result from weathering, the evidence in this case indicates that the alteration has been effected at a high tem perature by vapours carrying fluorine and boron; and minerals containing these elements (e.g., tourmaline and fluorspar) often occur in association with the china-clay. The alteration consists in the removal of the alkalies and the formation of a hydrated silicate of aluminium having the formula This substance, known as kaolinite (q.v.), constitutes the bulk of re fined china-clay, the impurities of the commercial article being finely divided quartz and mica (muscovite). It is not possible to remove the whole of these impurities by washing and this accounts for the difference in composition between pure kaolinite and a refined Cornish china-clay as shown by the analyses in the next column.
(Note : 1% of alkalies in china-clay is equivalent to 8.5% of muscovite.) The qualities for which china-clay is esteemed in the arts are whiteness and fineness of grain when it is used as a filler for paper or for dressing fabrics ; and plasticity and infusibility when it is used as an ingredient of the paste of porcelain manufacturers. The absence of iron (which would be introduced by dark mica) is particularly important from the colour point of view ; while the presence of light mica (muscovite) is undesirable since the alkalies thus introduced tend to lower the fusing point. Since in china-stone the felspar is only partially decomposed it contains a much higher percentage of alkalies than china-clay. This makes it more fusible and one of its uses is to impart a glaze to porce lain during the final firing. In order to prepare china-clay for the market the china-clay rock is subjected to the action of a stream of water. The bulk of the quartz and undecomposed felspar is arrested in suitably situated "sand pits," while the clay is carried away in suspension to the "button hole launder," whence it is pumped to the surface and conveyed to the "drags," which are a series of channels so arranged as to catch the coarser impurities. From the drags the clay-water passes to the "micas," a series of pits where the finer particles of mica and quartz settle down. Thus purified the liquid is led into "settling pits," or tanks, in which the finely-divided clay is slowly deposited. When it has acquired sufficient consistency it is dug out in square blocks and transferred to the "drys," i.e., sheds heated by flues, where the moisture is expelled and the china-clay obtained as a soft white earthy substance. The material obtained in the micas is useful for some purposes (e.g., as an absorbent and detergent in oil refining) and is sold as an inferior china-clay. China-stone is quarried and ground without levigation. The china-clay industry of Cornwall and Devon has attained to large proportions and is very profitable. The output amounted in 1926 to 825,954 tons, three-quarters of which was exported. The number of persons employed was 4,682. The United States takes one half of the total exports. Other considerable importers are Belgium, France, the Netherlands, Germany, Italy and India. (F. H. HA.) United States.—In the United States kaolins of the type of china-clay are found in North Carolina, Maryland, Delaware, Pennsylvania, Connecticut, Colorado and other States. The prin cipal source of primary kaolin is North Carolina. The clay from this State differs from the true china-clay in having a coarser grain structure and so is more refractory and porous at the same degree of firing. The chemical composition approaches very closely that of typical kaolin. When properly washed, the North Carolina kaolin develops a very good white colour in firing. It is commonly used in conjunction with English china-clay in the manufacture of white earthenware pottery, wall and floor tiles, since it tends to hold the articles straight during the firing process. In addition, there are in the United States large deposits of secondary kaolin, derived from the primary kaolin of the Appalachian region. These are very fine grained and more plastic than the latter but, as a rule, fire to a faint cream colour though there are exceptions which pro duce a good white colour. Good examples are the deposits at Okahumpka, Fla., and other places in that State, and those of the Dry Branch and McIntyre districts of Georgia. A somewhat different secondary kaolin occurs in South Carolina. There is an increased tendency to replace the imported china-clays by these native plastic kaolins. Further west, in the States of Kentucky, Tennessee, Mississippi and Arkansas the secondary kaolins are changed to still more fine-grained, very plastic and strong clays, the so-called ball clays. A number of these fire to a light colour and for this reason are introduced in large quantities in earthen ware and porcelain bodies. The increased use of these clays has done much to reduce the consumption of the kaolinitic type of materials.
In Bollinger county, Mo., white, kaolin-like clays are f ound which are the residue of decomposed limestone and carry a large amount of fine silica. A peculiar type of an indurated, hard clay, having very nearly the composition of kaolin, is found in Edwards county, Texas, and at King mountain, Nev. These clays require to be reduced to powder before they can be made to develop plasticity. When ground in water they show colloidal properties and dry to a horn-like strong mass. They fire to a pure white colour. The Texas "kaolin" has the peculiarity of producing trans lucency in bodies in which it is used even though the firing is not carried close to the point of vitrification. (A. V. B.)