ARTIFICIAL CHINA-CLAY.—The principal supplies of china-clay are obtained, as has been described, through the agency of natural decomposing influences in granitic 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.
fire-clays are 60 named from their great fire-resisting properties. Unlike most of the other clays, which occur•obiefly in surface beds, fire-clay is derived exclusively frorn the coal-measures, where it commonly forms the floor or "under-clay" of coal seams, varying in thick ness from 1 to 4 ft. Fire-clays are abundant in Great Britain, the principal works being situate in the counties of Stafford, Worcester, Northumberland, Durham, Lanark, and Mid-Lothian. The essential qualities of a fire-clay are a uniform texture, a somewhat greasy feel, and the presence of only very small proportions of lime, iron, alkaline earths, or other impurities that would cause it to yield to intense heat. Nevertheless, considerable variety of composition is manifested by fire-clays, taken even from different parts of the same seam. Samples taken from Newcastle, Glasgow, Dowlais, Cool Island, Stanuington, and HOwth, allowed the following maximum and minimum percentages of the principal ingredients :—Silica, 67.96 to 43.00 ; alumina, 40.9 to 21.18 ; oxide of iron, 8.4 to 1.19 ; water, 15.1 to 3.14. The celebrated Stourbridge clay contains:—Siliea, 60 to 66 ; alumina, 25 to 31 ; oxide of iron, 2 to 6 per cent. Most fire-clays exhibit impressions and carbonized remains of plants.
Extraction and Preparation.—The beds are worked by the same shafts and levels as the coal seams, except where their outcropping admits of open quarrying, which is rarely the case. After extraction, the clay is first exposed in spoil heaps, over the greatest available area, for from three to eighteen months, according to the season. The action of frost is very serviceable in disintegrating the tough lumps. In dry weather, frequent watering is necessary ; in wet weather, three months' exposure may suffice to render it " mellow," or "ripe," as it is called when it falls to powder. It is next ground in " edge-runner " mills, and carried on endless bands to the " riddles," whose mesh varies according to the destined application of' the clay-4 to 6 for fire-bricks, 6 to 10 for fine cement clay, and 12 to 14 for glass-house pot-elay. The pieces that will not paas through the riddle
are carried back on an endlesa band, to be reground. After being riddled, the clay is " tempered," or brought to the proper degree of plasticity, by the addition of water, and is then thoroughly stirred and kneaded in a " pug-mill," to fit it for moulding.
Uses.—Fire-elay is applied principally to the manufacture of a variety of articles which are required to withstand great awl long continued heat, such as bricks and " lumps," for fw-naces, fire-grates, ovens, retorts, flues, and the linings of fireproof safes, also smelting pots, crucibles, Sze. For sewage pipes, it is quite equal to any other material ernployed, and, perhaps, superior on the score of durability.
Statisties.—According to Hunt's Miaeral Statistiea for 1878, the total production of fire-clay in the United Kingdom exceeded two and a quarter million tons; of this quantity, Durham and North umberland yielded over 350,000 tons, and Scotland nearly as much ; Staffordshire, 180,000; York shire, 100,000 ; Lancashire, 60,000; Denbighshire, 23,000; Comberland, 23,000 ; Shropshire, 17,000 ; Leicestershire, 15,000; Glamorganshirc, 15,000.
Meerschaum.—Thia mineral is a hydrous silicate of magnesia, occurring in veina, or in uniform nodules, whose size varies from that of a nut to a cubic foot or more, in the serpentinous rocks of many countries, but chiefly at Kilellik, Eski-Shehr, and Boman, in Asia Minor, in the island of Negropont, and at Baldissero, in Piedmont. In 1872, the dise,overy of a bank of excellent quality waa announced from Southern California. Its approximate composition is—Silica, 60; magnesia, 28 ; water, 12 per cent. ; it seems to arise from the dec,omposition and waste of the carbonate in older rocks, Extraction and Preparation.—The deposits at Eaki-Shehr are worked by pits and galleries at a depth of 4 to 5 fathoms. When freshly exhumed, the mineral is eovered with red, oily earth, and is so soft as to be easily eut by a knife. Aft,er being scraped, to remove this coating, it is dried in the sun for five or six daya, or in a hot chamber for eight or ten daya, again scraped, and then polished with wax. It is then sorted iuto ten different grades, and is carefully packed with wool in boxea for export.