810 CLAY 



there is not the slightest attempt. Bat we do not know by what process the decom- 

 position of the solid granite could hare been effected to a depth from the surface 

 of upwards of one hundred feet, and then, as it often does, suddenly to cease. This, 

 however, is a question into which wo cannot at present enter. The largest quantity 

 of porcelain or China-clay is manufactured in Cornwall, especially about St. Austell 

 and St. Stephen's. 



A spot being discovered where this substance abounds, the operation is commenced 

 by removing the vegetable soil and substratum, called by the workmen the overburden, 

 which varies in depth from about three to ten feet. The lowest part of the ground is 

 thus selected, in order to secure an outlet for the water used in washing the clay. 

 The overburden being removed, the clay is dug up in stopes : that is, in successive 

 layers or courses, and each one being excavated to a greater extent than the one 

 immediately below it, the stopet resemble a flight of irregular stairs. The depth of 

 the China-clay pits is various, extending from twenty feet to fifty feet. 



The clay when first raised has the appearance and consistence of mortar ; it contains 

 numerous grains of quartz, which are disseminated throughout in the same manner 

 as in granite. In some parts the clay is stained of a rusty colour, from the presence 

 of veins and imbedded portions of shorl and quartz ; these are called by the workmen 

 weed, caple, and shell, which are carefully separated. The clay is next conveyed to 

 the floor of the washing place, and is then ready for the first operation of the process. 



A heap of the clay being placed on an inclined platform, on which a little stream of 

 water falls from the height of about six feet, the workman constantly moves it and 

 turns it over with a jriggle and shovel, by which means the whole is gradually 

 carried down into an oblong trench beneath, which is also inclined, and which ends 

 in a covered channel that leads to the catch-pits about to be described. In the trench 

 the grains of quartz are deposited, but the other parts of the clay, in consequence of 

 their greater levity, are carried away in a state of suspension. 



This water is conducted into a series of pits, each of which is about eight feet long, 

 four in breadth and in depth, and is lined on the sides and bottom with cut moorstone, 

 laid in a waterproof cement. In these pits the porcelain earth is gradually deposited. 

 In the first pit the grosser particles collect ; and being of a mixed nature, are always 

 rejected at the end of each day's work by an opening provided for that purpose at the 

 bottom of the pit. When the .water has filled the first pit, it overflows into the 

 second, and in like manner into the third ; and in these pits, particularly in the second, 

 a deposit also takes place, which is often preserved, and is called by the workmen 

 mica. The water, still holding in suspension the finer and purer particles of porcelain 

 clay, next overflows into Larger pits, called ponds, which are of the same depth as the 

 first pits, but about three times as long and wide. Here the clay is gradually 

 deposited, and the clear supernatant water is from time to time discharged by plug- 

 holes on one side of the pond. This process is continued until, by successive accumu- 

 lations, the ponds are filled. At this stage the clay is in the state of a thick paste, 

 and to complete the process, it remains to be consolidated by drying. Formerly, the 

 clay was dried by exposure to atmospheric influences only ; but the demand for clay 

 has been so largo, that artificial heat is now applied to the brick tanks, so as to 

 quicken the evaporation of the water. When sufficiently dry, the clay is cut into 

 oblong cakes, and is ready for the market. 



The following remarks on the clays and plastic strata of Great Britain are from 

 the pen of Mr. George Maw, who has brought to bear upon the subject much geological 

 and chemical knowledge, coupled with groat experience as a practical potter. 1 



' Plastic strata may be defined as beds of mechanical origin, containing alumina as 

 an essential constituent, which have undergone little or no consolidation, or been 

 subject to metamorphic action. 



' Although common to various geological formations from the palaeozoic to the most 

 recent deposits, a very large proportion of plastic strata applicable to ceramic manu- 

 factures occurs in the recent and tertiary beds. 



' Plastic strata diminish in frequency as the older deposits are approached : in the 

 earlier palaeozoic formations, the beds which were at the time of decomposition soft 

 clays and marls, occur for the most part as shales and slates, or have undergone 

 further metamorphism into hard porcelainites and other altered rocks unavailable for 

 the potters' use. Indeed, the very changes which the potter effects by artificial heat 

 have, as regards the earlier rocks, been anticipated in the laboratory of nature, 

 pressure in combination with heat having altered their original soft and plastic condi- 

 tion, changing them into the hardest rocks. 



' It must not, however, be supposed that all clays of economic applicability occur in 

 a soft and plastic state, as every gradation exists between hard metamorphic rocks 



1 These remarks are taken, with slight alterations, from Mr. Maw's Appendix to he ' Catalogue of 

 Britibh Pottery in the Museum of Practical Geology.' 2nd edition, 1871. 



