208 OX THE CHINA-STONE AND CHINA-CLAYS OF CORNWALL. 



clay streamers are supplied ; tlie sand is thus separated from the clay 

 and mica, which are carried on by the water, and the sand is then carried 

 by rail or carted to the top of the work, whence it is taken to be spread 

 over the drying grounds, or is thrown into the pits and pans. 



The water to be supplied to the clay stope should consist of two- 

 thirds of spring to one-third of rain-water, this mixture causing a deposit 

 of the suspended clay much more readily than any other. Great atten- 

 tion is often necessary in this part of the process, as from an excess of 

 rain-water it is often requisite that it should be saturated with some 

 earthy base ; common alum is at present used for this purpose, though 

 any other cheaper salt would answer the purpose, as it is only necessary 

 to saturate the water fully with earthy bases, when the clay speedily 

 becomes thrown down : a law not generally known. 



As a substitute for this, I have at times had recourse to finely-ground 

 peat, or wood charcoal, which, thrown over the surface of a pit, on which 

 it floats, by a process of angular attraction or repulsion, causes the clay 

 to be deposited, even from distilled water, far more readily than by the 

 addition of any soluble earths, as may be demonstrated, with ease, by 

 experiment in two or three tumblers. But as I am rather in advance of 

 the water in which I left the clay and mica suspended at the bottom 

 level of the clay work, I must return thither, till, by the aid of wooden 

 or iron pumps, from 40 to 80 feet deep, worked by a powerful water- 

 wheel, this milky looking fluid is elevated to the level of the large mica 

 launders, where the clay, being lighter than it, leaves it deposited in 

 these inclined pits, which are generally three or four in number, placed 

 in tiers, with a slight elevation at the upper end of each ; they vary in 

 length from 10 to 20 feet, are generally 3 feet in breadth, and 6 or 9 

 inches deep, though both the number, size, and degree of inclination 

 vary with the size and rapidity of flow of the shear of water, though no 

 less than with the amount of mica contained in the stope. In some 

 clay works the shear is so large that most of the mica is carried on with 

 the clay, so that it possesses, when fused, a greater degree of tenacity, 

 though of an inferior quality as to whiteness, plasticity, &c. In the 

 separation of the best clays, these pits require that the motion of the 

 shear through them should be slow and equable ; the shear of small size, 

 and the launders should be tapped or cleared out once in every six or 

 seven hours ; a careful attention to which well repays any amount of 

 labour in the production of a good article. That portion of the mica 

 collected in the first of these launders often being mixed with its scales 

 and crystals of hornblende, or diallage, is thrown aside as useless, while 

 that collected in the others is generally sold as a second quality clay. 



The clay water, having left the mica, now flows on to a large circular 

 or oval collecting pit, 30 or 40 feet in circumference, and from 6 to 10 

 feet deep, where the clay subsides, forming an under stratum of the con- 

 sistence of cream, the supernatant water flowing off from the top of 

 the pit, until it is filled. As soon as this happens the clay is allowed to 



