FURNACE 843 eluded in the same category. These are almost invariably air furnaces, though sometimes air under pressure is used ; as for example in the combustion of small anthracitic coal, where a current of air from a fan-blower is sometimes blown under the grate to promote combustion. Crucible Melting Furnaces. Figs. 7 and 8 represent a series of one of the simplest furnaces of this class, the ordi nary crucible air furnace, or pot meltint/ hole, according to the Sheffield term used in cast-steel works. It is a chamber of brick work a, either straight in the sides or more generally some what barrel-shaped, with a grate at the bottom, and of sufficient capacity to hold one or two crucibles b, containing from 50 to 70 K) weight of steel each, enough room being left around them for coke to bring the con- , tents up to the melting point in || five or six hours. The crucibles are supported upon disks, or "cheeses" of fire-brick, placed upon the grate; the draught is i_l.;_ -^ ^ maintained by a chimney c about 30 feet high, communicating Fin. 7. Crucible Melting Fur- with the furnace by a short flue liace (longitudinal section), near the top of the latter. The furnace is placed nearly or quite level_with the floor of the casting shop /, and covered with a square fire tile or quarry g, set in an iron frame with a projecting handle, the fire grate being accessible from the cellar below. Assayers Muffle Furnace. The construction of a muffle furnace as used by assayers has already been described in the article ASSAYIXG, vol. ii. p. 726. It is simply a small square air furnace with a D-shaped chamber of fire clay fixed in the middle, so as to be surrounded with incandescent fuel, a current of air being drawn through it by a series of draught holes or slits in the roof or sides. Larger-sized muffles are used by enamellers and painters, and in the production of enamelled iron goods, as Fio. 8. -Crucible Melting Furnace well as for calcining minerals (transverse section), containing arsenic where it is to be collected for sale, and in the production of metallic colours where the material has to be kept free from the influence of flame and smoke. Furnace Materials. The materials used in the construc tion of furnaces are divisible into two classes, namely, ordinary, and refractory or fire-resisting. The former are used principally as casing, walls, pillars, or other supporting parts of the structure, and includes ordinary red or yellow bricks, clay-slate, granite, and most building stones ; while the latter are reserved for the parts immediately in contact with the fuel and flame, such as the lining of the fire-place, the arches, roof, and flues, the lower part if not the whole of the chimney lining in reverberatory furnaces, and the whole of the internal walls of blast furnaces. 1 Among such substances are fire-clay and fire-bricks, certain sand stones, silica in the form of ganister, and Dinas stone and bricks, ferric oxide and alumina, carbon (as coke and 1 In the figures fire-brick work is represented by closer, and casing walls in ordinary bricks by more open shading. graphite), magnesia, lime and oxide of chromium, their relative importance being indicated by their order, the last two or three indeed being only of limited use. The most essential point in good fire-clays, or in tho bricks or other objects made from them, is the power of re sisting fusion at the highest heat to which they may be exposed. This supposes them to be free from metallic ox ides, forming easily fusible compounds with silica, such as lime or iron, the presence of the former even in compara tively small proportion being very detrimental. As clays they must be sufficiently plastic to be readily moulded, but at the same time possess sufficient stiffness not to contract too strongly in drying, whereby the objects produced would be liable to be warped or cracked before firing. In most cases, however, the latter tendency is guarded against, in making up the paste for moulding, by adding to the fresh clay a certain proportion of burnt material of the same kind, such as old bricks or potsherds, ground to a coarse powder. Coke dust or graphite is used for the same pur pose in crucible making. The most highly valued fire-clays are derived from the Coal Measures. Among the chief localities are the neighbour hood of Stourbridge in Worcestershire and Stannington near Sheffield, which supply most of the materials for crucibles used in steel and brass melting, and the pots for glass houses ; Newcastle-on-Tyne, and Glenboig near Glasgow, where heavy blast furnace and other fire-bricks, gas retorts, &c., are made in large quantities. Coarse-grained but very strong fire-bricks are also made of the waste of the china clay works at Lee Moor, Devon. See FIRE-CLAY. In Belgium, the clay raised at Andenne is very largely used for making retorts for zinc furnaces, not for local use alone, as it is exported for the same purpose to England and other countries. The principal French fire-clays are derived from the Tertiary strata in the south, and more nearly resemble porcelain clays than those of the Coal Measures. They give wares of remarkably fine texture and surface, combined with high refractory character. The principal centre of manufacture is in Paris, where small crucibles, tubes, furnaces, and other articles for the use of assaying and chemical laboratories, as well as for gold and silver refining, are produced in large quantities. In Germany, Ips and Passau on the Danube, and Gross Almerode in Hesse, are the best known localities producing fire-clay goods, the crucibles from the last-mentioned place, known as Hessian crucibles, going all over the world. These, though not showing a great resistance to extreme heat, are very slightly affected by sudden alternations in heating, as they may be plunged cold into a strongly heated furnace without cracking, a treatment to which French and Stourbridge pots cannot be subjected with safety. The Cornish crucibles used in copper assaying, made at Eedruth, are generally similar in quality and behaviour to the Hessian, but are not quite so rapidly perforated by corrosive fluxes. Plumbago or graphite is largely used in the production of crucibles, not in the pure state but in admixture with fire-clay ; the proportion of the former varies with the quality from 25 to nearly 50 per cent. These are the most enduring of all crucibles, the best lasting out 70 or 80 meltings in brass foundries, about 50 with bronze, and 8 to 10 in steel melting. The most important manufactory is that of the Patent Plumbago Crucible Company, Batter- sea, on the Thames, where the best Ceylon graphite is the basis of the composition employed. They are also made in all the principal crucible works of the continent of Europe and in the graphite-producing localities of Canada and the United States. Silica is used in furnace-building in the forms of sand, ganister, a finely-ground sandstone from the Coal Measures
