Cost of Production.—Where the conditions of production vary so greatly, there must necessarily be great differences of cost ; but, after having been at some pains to determine the cost under average conditions, Collins thinks the following figures and statements may be relied upon. A work capable of producing say 1000 tons of clay yearly will cost from 2500/. to 5000/. To get the clay in the linhay ready for the market, will cost about 9s. a ton, of which about 2s. 6d. must be expended in fuel for pumping and drying, ls. in removing overburden, ls. in removing sand, and ls. for management and office expenses, leaving as. 6d. as the net labour cost of washing and drying a ton of clay. To the 9s. net cost of clay, must be added an average of 3s. for royalties, Is. for transit and placing on board ship, and Is. for agencies, commission, bad debts, and sundries, making the average actual cost amount to 17s. Some favourably situated works can no doubt saie two or even three shillings on tbis account ; in others, the cost may amount to 20s. or even 22s. As to the selling price, this varies much more widely than the cost of production, ranging from 14s. to 35s. f.o.b. Clays sold at the lower rates are unremunerative.
Nature and Utilization of Waste Products.—Besidee the clay proper, there are certain waste or pseudo-waste products, produced in very large quantities. These are as follows :— Fine mica.—This is deposited in the " micas "; a few years since, it was thrown away or rather washed away, as is still the cEtse in many works. Sometimes, however, it is collected, dried in the manner of clEty proper, and sold to the makers of soft paper, paste-board, inferior pottery, &c., at a low price.
Coarse mica.—This is invariably washed away or thrown away, tberebeing at present no demand for it. It, however, contains a very beautiful material, which might be applied to many ornamental purposes.
Sand.—This consists of broken quartz crystals, mostly white or pale brownish ; when wsshed clean, it is the finest building sand known, as the angles are all sharp. Mixed with one-eighth of Portland cement, it forms a concrete as hard as stone.
Discoloured clay.—This has to be dug out from among the good white clay in many places. It has been successfully used in the manufacture of white bricks, for building purposes. In some instances, a quantity of the sand already mentioned is mixed with the refuse clay, and produces an excellent fire-brick. The same material is used in the manufacture of the tiles used as a floor for drying the clay. The manufacture of bricks and tiles from this debris is a growth, it is believed, of the last twelve years.
Overburden.—The upper part of this consists of soil, or " meat earth " ; this is usually removed and carefully preserved. Underneath is a hard, often stony or sandy layer, which, in districts where tin is worked, often contains enough tin to pay for washing. With this stony or sandy layer, is usually a considerable thickness of discoloured clay suitable for brick-making.
Branches.—These are stony veins which ruu through the clay stopes in various directions. Sometimes they are quite worthless ; but, in a few instances, they are veritable tin lodes, and contain enough tin to pay for stamping and dressing. Thus at Carclaze, near St. Austell, each 1000 tons of clay yields something like one ton of oxide of tiu, and formerly the proportion was much greater. The propoitions of these waste materials as compared with the fine clay prooured arc thus stated :— For every one ton of fine clay there is removed—from 3 to 7 tons of sand, average about 31 tons ; from 2 to 5 owt. of coarse mica, average 3 cwt.; from 1 to 3 cwt. of fine mica, average 2 cwt.; from
0 to 1 owt. of stones, average I cwt.
A oubio fathom of clay rock, of average quality, will yield about 2i tons of fine clay ; and about half a fathom of overburden must be removed to get it.
Suggested Improvements in Preparing. — Collins thinks that there is still much room for improvement in the preparation of china-clay, but that such must be a growth of time and oircumstances. At the present time, about one ton of water has to be driven off from each ton of olay in the dry, and this uses at least 2 cwt. of coals on an average, and costs from 8d. to 10d. in labour. In a few modern drys, a small economy iu fuel has been effected by lengthening the kiln ; but in none has it been brought so low as lf cwt. to the ton of clay. Mr. Stocker, in 1862, suggested the use of filter beds, and also devised a centrifugal dryer ; but neither of these contrivances has come into use, and the first would seem quite inapplicable, on account of the extreme fineness of the particles of clay, and the imperrneability of even a thin layer of that substance. Some economy might perhaps result from the use of hydraulic filters of calico, such as are used in the potteries for drying the slip ; but it is very doubtful if any saving would be effected, as the labour would be about the same, and, against the 28. a ton for fuel, would have to be plated the wear and tear of the calico. In washing the clay from the stope, sorne benefit might accrue from the use of a jet of water under a pressure of from 50 to 100 lb. per square inch, as in the so-called hydraulic mining. This could only be applied to dopes of even quality, where very little picking out of inferior portions was required ; but it would supersede the services of the " breakers" on the stope, and. greatly lessen the labour of the washers. It is but rarely that a natural head of water is obtainable equal to the required pressure ; but where machinery is used for pumping, the additional coat of pumping, say 250 gallons a minute to a height of 150 ft. in a standpipe, would be very slight, as the extra power required is little more than that of ono horse.
Statistics.—From statistics obtained from many sources, it is evident that the production has very largely increased from 1809 to 1874-2919 tons against 226,309. In 1810, Trethosa (one of the largest works) produced 300 tons per annum, and employed thirteen persons, viz. eight in removing burden and raising (breaking) clay (at per fathom), three washing, two attending ponds and packing. In 1874, one of the largest works near St. Austell produced 9000 tons, employing about thirty men. Many works produced 6000 tons, employing twenty men. The quantity sent annually from Cornwall must average at least 150,000 tons. It goes not only to Staffordshire, but also largely to France, Belgium, and other foreign countries. The extensive clay works recently opened in several departments of northern France have done much to curtail the export of Cornish clay to that country ; and the large deposits of the island of Bornholm have lately been worked upon to supply the needs of Denmark, Sweden, and Germany ; while similar utilization of native clays has been carried out in America. Nevertheless, the growth of home industries which depend in a measure upon this article will, doubtless, counteract the influence of decreasing exports.