update notifier cnet download lang ru

RAW MATERIALS. SILICA is introduced generally as flint or powdered quartz or pure river sand. It occurs in three conditions : hexagonal in quartz, and its minor varieties, rock crystal, quartzite, Lydian stone, amethyst, cairngorm, smoke topaz, morion, cat's-eye, avanturine, hornstone, jasper, and bloodstone ; rhombic in tridymite and asmanite ; amorphous in opal, hyalite, hydrophane, sinter, and gey- serite. Then mixtures of quartz and amorphous silica form flint, chalcedony, carnelian, sard, plasma, chrysoprase, agate, and onyx. Sand usually consists of quartz grains. When indurated it becomes sandstone, being cemented by lime or iron compounds with fragments of other minerals. Silica may now be obtained on the market as river sand containing 99.5 per cent, silica. BITSTONE is a coarse sharp-grained quartz which is used in saggers so that the point fused into the glaze on the bottom of a piece of ware may be as inconspicuous as possible. It should be of uniform grain to carry the ware level. CHERT is a silicious rock occurring chiefly in limestone. It is less splintery in fracture than flint, and is used not so much as an ingredient in pottery as for runners of wet grinding mills. H. L. Terry (1906) gives the following analyses of chert sold to the Potteries for this purpose : Derbyshire. Swaledale.