Kaolin

Kaolin, the Chinese name of china-clay, known in Cornwall as growan clay. It is a pure clay (q.v.) or hydrous silicate of alumina, containing, on an average, 46 per cent. of silica, 40 per cent. of alumina, and 14 per cent. of water, and results from the decomposition of white potash-, or soda-, felspar in a granite known in its partly decomposed state as carclayzite, from Carclaze, in Cornwall. It was first found in England in the Hensbarrow district by William Cookworthy about 1750, though the "soft growans" containing it had long before been worked for the veins of tinstone that traverse them. The pure clay is freed from the quartz and mica by streams of water and is then dried. It is packed in tierces of half a ton weight, many thousands of which are now produced annually. Being practically infusible, it forms in porcelain manufacture the "bone" of the ware, the more fusible china-stone or petuntzite (q.v.) forming the "flesh." The clay is also much used for "loading," or adding weight to, paper, calico, etc. Possibly the kaolinisation or decomposition of felspar is partly owing to the presence of fluorine in tourmaline, floor, or some other mineral, in the rock, and not merely to the action of atmospheric carbon dioxide and moisture.