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 circumstances. At the present time, about one ton of water has to be driven off from each ton of clay 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 in fuel has been effected, by lengthening the kiln; but in none has it been brought so low as 1½ cwt. to the ton of clay.
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 impermeability 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 2s. a ton for fuel, would have to be placed the wear and tear of the calico.
In washing the clay from the stope, some 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 stopes 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 cost of pumping, say 250 gals. a minute to a height of 150 feet in a standpipe, would be very slight, as the extra power required is little more than that of one 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 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 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 utilisation 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.
Artificial China Clay.—The principal supplies of china clay are obtained, as has been described, through the agency of natural decomposing influences in granite rocks. In one instance, however, at Betleek, County Fermanagh, it is procured by calcining the red orthoclase granite of the district. The felspar is whitened by the process, and the iron becomes separated in a metallic state, and is removed by magnets.
Characters.—Being virtually a hydrated silicate of alumina, china clay is a remarkably stable pigment. Not only is it unaffected by prolonged exposure to strong light and impure air, but is insoluble in water, weak acids, and alkalies. It is moreover very much lighter in weight than any other white pigment, an advantage on the score of cost when buying by weight. Its covering power in distemper work and as a water colour is good; but the addition of oil reduces its capacity. The best qualities are exceedingly fine in grain and pure in tint, but inferior samples sometimes have their yellowness “corrected” by the addition of a little ultramarine.
Enamelled White.—Another name for the finest kinds of baryta white, [see p. 170.]
English White.—A synonym for whiting, see [p. 246].
Gypsum.—This very common and abundant mineral is a hydrated sulphate of lime, occurring in several forms, of which only the opaque white variety is useful as a pigment.
The native mineral is quarried, dressed, ground, and levigated, in all which operations there is nothing special to be noted.