36 Transactions of the 



manufacture of which it is employed prove of an extreme white- 

 ness. Much judgment and experience are necessary in the using 

 of this clay, that its tendency to cracking may be as far as possible 

 obviated by a proportionate admixture. If clay of any description 

 were dried without the addition of any other body after being 

 made sufficiently plastic to be modelled on the potter's wheel, 

 it must inevitably crack, as the evaporation of its water will 

 occasion it to shrink in the proportion of one part in twelve 

 during the process of drying. 



Another desciiption of clay much j^rized for the manufacture 

 of finer kinds of earthenware and porcelain was found in Cornwall 

 by a Mr Cookworthy, and is commonly denominated China Clay. 

 This is very white and unctuous to the touch, and is obviously 

 formed by the gradual disintegration of the felspar of granite. 

 There are large mountains of this mineral, some of which are 

 thus partially decomposed ; this China clay proves on examination 

 to be identical with the kaolin of the Chinese. It is prepared 

 by the clay merchants on the spot where found. The stone is 

 broken up into pieces of a small size and then thrown into a 

 running stream ; there the light argillaceous parts are washed off 

 and held suspended in the water while some ponderous mica and 

 quartz remain at the bottom of the stream. At the end of the 

 rivulet the water is stopped by a dam, and the pure clay gradually 

 subsides. When this deposit is completed, the clear water is 

 drawn off, and the solid matter dug out in square blocks, which 

 are placed on shelves and exposed to a continued current of air 

 until sufficiently dry to be packed in cases for shipment. The 

 clay, which is then in the state of a fine powder, is very smooth 

 and of an extreme whiteness. Mr Wedgewood found by analysis 

 that it contains sixty parts of alumina and twenty parts of silica. 

 In preparing the clay, the first operation of the potter is that of 

 mixing it with water to the consistence of cream. 



It is well known that water collected from springs and from 

 many streams contains various foreign matters, some of which 

 would be injurious to the composition of porcelain. It is there- 

 fore necessary to examine chemically the properties of water, and 

 to counteract any bad qualities that may be found existing, as far 

 as possible. The French manufacturers are accustomed to employ 

 only rain-water, while in Germany, still more precise in their 

 operations, the manufacturers prepare their materials only twice 

 in the year, at the vernal and autumnal equinoxes, acting under 

 the impression that there is some peculiarity in the rain which 

 better qualifies it for the purposes of their manufacture. 



The mixing of the clay, which is called blunging, is effected 

 in a trough five feet long, three feet wide, and two and a half 



