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 4000 tons of clay yearly will cost from 2500l. to 5000l. 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, 1s. in removing overburden, 1s. in removing sand, and 1s. for management and office expenses, leaving 3s. 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, 4s. for transit and placing on board ship, and 1s. for agencies, commission, bad debts, and sundries, making the average actual cost amount to 17s. Some favourably situated works can no doubt save 2s. or even 3s. on this 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 rate are unremunerative.
Nature and Utilisation of Waste Products.—Besides the clay proper, there are certain waste or pseudo-waste substances 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 case in many works. Sometimes, however, it is collected, dried in the manner of clay 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, there being 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 washed 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 run 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 tin, and formerly the proportion was much greater. The proportions of these waste materials, as compared with the fine clay procured, are thus stated:—
For every 1 ton of fine clay there is removed—from 3 to 7 tons of sand, average about 3½ tons; from 2 to 5 cwt. of coarse mica, average 3 cwt.; from 1 to 3 cwt. of fine mica, average 2 cwt.; from 0 to 1 cwt. of stones, average ¼ cwt.
A cubic fathom of clay rock, of average quality, will yield about 2½ tons of fine clay; and about half a fathom of overburden must be removed to get it.