Furnace (Lat. fornax), a structure containing a fireplace, intended for maintaining intense heat. In many of the useful arts the first requisite is the means of obtaining a very high temperature. In all metallurgic operations, the object of which is the reduction of the ores and treatment of the metals, and in almost every art involving the use of fire, a furnace of some kind for producing this heat is in demand. The ancient Greeks employed furnaces for casting statues of bronze; Homer makes mention of a blast furnace with 20 crucibles (II. xviii. 470). The Egyptians are known to have made use of melting pots, but we have no knowledge of their furnaces. An ancient smelting furnace was discovered near Aries, in southern France, which was shaped like an inverted bell, having under the surface of the ground a channel for the discharge of the melted metal. Strabo speaks of furnaces built in Spain, which were raised to a great height for conveying off the noxious fumes; they were also furnished with long flues and chambers for collecting the oxides and other sublimed matters. The forms and dimensions of modern furnaces vary greatly according to the different purposes they are designed to serve.

The iron manufacturer, smelting the ores upon a vast scale, builds an immense structure with a capacity of hundreds of tons, and furnishes it with heavy machinery for supplying the great volume of air blown in almost without ceasing, as the operation is continued during a single blast of two years or more.-All furnaces employed in melting refractory materials-Mhose for assaying, as well as those operating upon a large scale-require a free supply of air, proportional in quantity to the amount of fuel they consume. The generation of heat depends upon the rapid chemical combination of carbon with oxygen, and a sufficient supply of the latter element is as essential as is that of the former. Every pound of good bituminous coal, according to Dr. Thomson, requires 150 cubic feet of air, or allowing one third more for waste, there should be supplied at least 200 cubic feet. So immense is the quantity of this invisible element consumed and wasted in the large furnaces for smelting iron ores, that its weight even is greater than that of all the other materials, ores, coal, and flux, introduced; and the power required to force this volume of air through the dense column of heated matters far exceeds that expended in charging the furnace with its solid contents, even adding to this the power involved in the removal of the products of the operation.

To provide for this large supply is then a matter of the first consequence to furnaces; and according to the mode in which this is effected they are separated into two classes. The kind called air or wind or reverberatory furnaces receive their supply by means of the current produced by a tall chimney, the heated column rushing upward through the flue. To fill the space in the lower part of the flue, air presses in from without through every aperture; and none being allowed except those leading through the receptacle for the fuel, the supply of air is thus secured, heat is generated for the purposes required, and a portion is expended in furnishing the mechanical power involved in the movement of the current of air. Fireplaces, stoves, and grates are examples of air furnaces; and by means of the blower, which causes the air admitted into the chimney to pass first through the fire, the flue is prevented from becoming chilled by the entrance of cold air, the column ascends more rapidly, an increased supply of air is furnished to every portion of the body of fuel, and the chemical process goes on with augmented intensity and generation of heat.

The other classes of furnaces are supplied with air through bellows or other blowing apparatus. (See Blowing Machines.) They are called for this reason blast furnaces, and are used when the resistance opposed to the passage of the current of air by the density of the contents of the furnace is so great, that sufficient quantity cannot penetrate to keep up thorough combustion throughout the mass; or when the operations do not admit of the large openings beneath the fire, which the free admission of such bodies of air would require; or again, when the nature of the operation demands an intensity of heat concentrated in one spot. The blast in this case acts like the jet of the blowpipe, and its effect is in many cases greatly increased by its being conveyed through iron pipes which are highly heated by exposure in suitable ovens to the waste heat of the smoke and gases which escape from the chimney. It thus restores to the interior of the furnace, in the form of highly heated air, a portion of the caloric that would otherwise be lost. Furnaces of both classes are often used in the chemical laboratory; but the blast furnace is rather preferable because it can always be more perfectly controlled.

The one commonly employed for general purposes is a wind furnace, built of fire brick, and strongly secured with iron rods and straps. It has a flat top, with two or more openings, and on these are placed pans of cast iron for holding sand in which vessels are placed for exposure to moderate heat. The furnace has under the flue that leads into the chimney an oven for drying. With a good draught this furnace produces sufficient heat for many crucible operations. These are, however, better conducted in smaller furnaces, either wind or blast, constructed specially for this use.-The construction and manner of using the various kinds of reverbe-ratory, blast, and assay furnaces will be found described under the heads Assaying, Bloom-ary, Casting, Copper Smelting, Iron Manufacture, and others which treat of processes involving the use of these furnaces.-Gas furnaces employ gas instead of solid fuel, and are constructed in a variety of forms, but always upon the principle of the Bunsen's burner. (See Flame.) Griffin's blast gas furnace, for metallurgic operations requiring high heat, is shown in section in fig. 1. Two fine clay cylinders, a, a, form the body of the furnace. They rest upon a perforated fire-clay plate, b, into which the gas burner, c, is introduced.