FURNACE  , a contrivance for the

production and utilization of heat by the combustion of fuel . The word is common to all the Romance tongues, appearing in more or less modified forms of the Latin fornax . But in all those languages the word has a more extended meaning than in English, as it covers every variety of heating apparatus; while here, in addition to furnaces proper, we distinguish other varieties as ovens, stoves and kilns . The first of these, in the form Ofen, is used in German as a general term like the French four; but in English it has been restricted to those apparatus in which only a moderate temperature, usually below a red heat, is produced in a close chamber . Our bakers' ovens, hot-air ovens or stoves, annealing ovens for glass or metal, &c., would all be called fours in French and Ofen in German, in common with furnaces of all kinds . Stove, an equivalent of oven, is from the German Stube, i.e. a heated room, and is commonly so understood; but is also applied to open fire-places, which appears to be somewhat of a departure from the original signification . Furnaces are constructed according to many different patterns with varying degrees of complexity in arrangement; but all may be considered as combining three essential parts, namely,the fire-place in which the fuel is consumed, the heated chamber, laboratory, hearth or working bed, as it is variously called, where the heat is applied to the special work for which the furnace is designed, and the apparatus for producing rapid combustion by the supply of air under pressure to the fire . In the simplest cases the functions of two or more of these parts may be combined into one, as in the smith's forge, where the fire-place and heating chamber are united, the iron being placed among the coals, only the air for burning being supplied under pressure from a blowing engine by a second special contrivance, the tuyere, tuiron, twyer or blast-pipe; but in the more refined modern furnaces, where great economy of fuel is an object, the different functions are distributed over separate and distinct apparatus, the fuel being converted into gas in one, dried in another, and heated in a third, before arriving at the point of combustion in the working chamber of the furnace proper . Furnaces may be classified according as the products of combustion are employed (I) only for heating purposes, or (2) both for heating and bringing about some chemical change . The furnaces employed for steam-raising or for heating buildings are invariably of the first type (see BOILER and HEATING), while those employed in metallurgy are generally of the second . The essential difference in construction is that in the first class the substances heated do not come into contact with either the fuel or the furnace gases, whereas in the second they do . Metallurgical furnaces of the first class are termed crucible, muffle or retort furnaces, and of the second shaft and reverberatory furnaces .

The following is a detailed subdivision: (I) Fuel and substance in contact . (a) Height of furnace greater than

diameter =shaft furnaces . (a) No blast =kilns . (i3) With blast =blast furnaces . (b) Height not much greater than diameter =hearth furnaces . (2) Substance heated by products of combustion=reverberatory furnaces . (a) Charge not melted =roasting or calcining furnaces . (b) Charge melted melting furnaces . (3) Substance is not directly heated by the fuel or by the products of combustion . (a) Heating chamber fixed and forming part of furnace= muffle furnaces . (b) Crucible furnaces . (c) Retort furnaces .

Another

classification may be based upon the nature of the heating agent, according as it is coal (or some similar combustible) oil, gas or electricity . In this article the general principles of metallurgical furnaces will be treated; the subject of gas- and oil-heated furnaces is treated in the article FUEL, and of the electric furnace in the article ELECTROMETALLURGY . For special furnaces reference should he made to the articles on the industry concerned, e.g . GLASS, Gas, § Manufacture, &c . Shaft, Blast and Hearth Furnaces.—The blast furnace in its simplest form is among the oldest, if not the oldest, of metallurgical contrivances . In the old copper-smelting district of Arabia Petraea, clay blast-pipes dating back to the earlier dynasties of ancient Egypt have been found buried in slag heaps; and in India the native smiths and iron-workers continue to use furnaces of similar types . These, when reduced to their most simple expression, are mere basin-shaped hollows in the ground, containing ignited charcoal and the substances to be heated, the fire being urged by a blast of air blown in through one or more nozzles from a bellows at or near the top . They are essentially the same as the smith's forge . This class of furnace is usually known as an open fire or hearth, and is represented in a more advanced stage of development by the Catalan, German and Walloon forges formerly used in the production of malleable iron . Fig . 1 represents a Catalan forge . The cavity in the ground is represented by a pit of square or rectangular section lined with brick or stone of a kind not readily acted on by heat, about 11 or 2 ft. deep, usually somewhat larger above than below, with a tuyere or blast-pipe of copper penetrating one of the walls near the top, with a considerable downward inclination, so that the air meets the fuel some way down .

In iron-smelting the ore is laid in a heap upon the fuel (charcoal) filling up the hearth, and is gradually brought to the metallic

state by the reducing action of the carbon monoxide formed at the tuyere . The metal sinks through the ignited fuel, forming, in the hearth, a spongy mass or ball, which is lifted out by the smelters at the end of each operation, and carried to the forge hammer . The earthy matters form a fusible glass or slag melt, and collect at the lowest point of the nearrn, wnence they are removed ally representing its class . The fire-place A is divided from the by opening a hole pierced through the front wall at the bottom. working bed B by a low wall C known as the fire bridge, and at the The active portion of such a furnace is essentially that above the opposite end there is sometimes, though not invariably, a second blast-pipe, the function of the lower part being merely the collection bridge of less height called the flue bridge D . A short diagonal. flue of the reduced metal; the fire may therefore be regarded as burning in an unconfined space, with the waste of a large amount of its heating power . By continuing the walls of the hearth above the tuyere, into a shaft or stack either of the same or some other section, we obtain a furnace of increased capacity, but with no greater power of consuming fuel, in which the material to be treated can be heated up gradually by loading it into the stack, alternately with layers of fuel, the charge descending regularly to the point of combustion, and absorbing a pro-portion of the heat of the flame that went to waste in the open fire . This principle is capable of very wide extension, the blast furnace being mainly limited in height by the strength the column or up-take E conveys the current of spent flame to the chimney of materials or " burden " has to F, which is of square section, diminishing by steps at two or three resist crushing, under the weight due to the head adopted, and the different heights, and provided at the top with a covering plate or power of the blowing engine to supply blast of sufficient density to overcome the resistance of the closely packed materials to the free passage of the spent gases . The consuming power of the furnace or the rate at which it can burn the fuel supplied is measured by the number of tuyeres and their section . The development of blast furnaces is practically the development of iron-smelting . The profile has been very much varied at different times . The earliest examples were square or rectangular in horizontal section, but the general tendency of modern practice is to substitute round sections, their construction being facilitated by the use of specially moulded bricks which have entirely superseded the sandstone blocks formerly used . The vertical section, on the other hand, is subject to considerable variation according to the work to which the furnace is applied .

Where the operation is simply one of

fusion, as in the iron-founder's cupola, in which there is no very great change in volume in the materials on their descent to the tuyeres, the stack is nearly or quite straight-sided; but when, as is the case with the smelting of iron ores with limestone flux, a large proportion of volatile matter has to be removed in the process, a wall of varying inclination is used, so that the body of the furnace is formed of two dissimilar truncated cones, joined by their bases, the lower one passing downwards into a short, nearly cylindrical, position .