182 On the Porcelain Earth of Cornwall. [March, 



beneath the vegetable soil there is commonly a bed of lighter 

 coloured matter : and under this, bounded at top by an irregular 

 waving outline, the china clay occurs. 



The depth to which the clay itself extends is also various. In 

 one of the works that I examined, the bottom of the pit in which 

 it was dug was eight or nine feet from the surface ; and the clay 

 " held down," as the workmen expressed it, for about ten feet more. 

 In another work, situated rather lower, in the course of a stream, 

 the thickness of the " overburden," or incumbent soil and earth, 

 was about nine feet. At Trethmsa, one of the principal works, the 

 depth of the " clay" varied from one to eighteen feet. 



The clay as it is found has somewhat the appearance and colour 

 of mortar newly made : there are dispersed through it many angular 

 fragments of quartz ; a proof, I suppose, that it has not been con- 

 veyed from any considerable distance ; and throughout the mass 

 there appear large irregular patches and stripes of a reddish brown 

 colour, of which also some small veins occur. This coloured part, 

 called by the workmen " weed," is, in digging, carefully separated 

 from the rest and thrown aside. 



The process by which the clay is prepared is the elutriation of 

 chemists, performed on a very large scale j the detail of the various 

 steps as follows : — 



The " overburden " being removed to a considerable extent, the 

 clay itself is dug progressively in steps, each four or five feet deep, 

 the vertical faces of which are cut down with pickaxes and shovels, 

 and the whiter parts conveyed in wheelbarrows to be " washed." 

 At some of the works the clay is carefully mixed, in one large 

 heap, before the washing ; but in others this mixture is dispensed 

 with, and it is removed directly from the pit to smaller heaps, on 

 •which a stream of vratcr is allowed to pour, while the mass is fre- 

 quently turned and supplied by a man or boy. The water in passing 

 through the heap becomes charged with particles of clay, and is 

 conveyed by wooden spouts to what are called the "pits" and 

 ''ponds" leaving the coarser parts behind. 



These pits and ponds are merely rectangular excavations dug 

 from the surface, and rendered water-tight by a floor and walls of 

 cut granite, bedded in mortar made with lime from Aberthaw,* 

 ^t'hich has the property of forming a strong cement under water. 

 The pits are in general about five or six feet by tour, and about four 

 feet in depth ; the povds, about twenty feet long by twelve in width, 

 and four or five feet deep. At the middle of one side of each pond 

 there is let into the wall a vertical board, pierced with two rows of 

 holes placed alternately, and furnished with plugs, for the purpose 

 of letting oft' the water gradually : and on the outside of the pond 

 there is a small excavation lined with stone, with steps to enable a 

 j^prkman to descend and adjust the plugs, and an opening at the 



• A village on the coast of GlaraorgaBeliire. 



