Furnaces for melting alloys may be built of common brick and lined with fire-brick; but the best are made with a boiler-plate caisson, 20 to 30 in. diam. and 30 to 40 in. high, usually set down in a pit, with the top only 10 or 12 in. above the floor of the foundry. The ash-pit, or opening around the furnace, is covered by a loose wooden grating, that aim its of the ashes being removed. The iron caisson is lined with fire-brick, the same as a cupola, the lining being usually 6 in. or more thick. The inside diameter of the furnace should not exceed the outside diameter of the crucible by more than 4 or 5 in., as greater space will require greater expenditure of fuel. These furnaces are liable to burn hollow around where the crucible rests; to avoid waste of fuel, they should be kept straightened up with fire-clay and sand. Sometimes these furnaces are built square inside, but they are inferior to the circular form and consume more fuel. Three or four such furnaces are commonly arranged in sets giving a graduated scale of sizes, to suit the needs of larger or smaller castings.
The common brass furnace usually depends on a natural draught, and connects by a flue with a chimney stack at the back. Three or four commonly share a single stack, each having a separate flue and damper. When the chimney does not give sufficient draught, the ash-pit may be tightly closed and a mild blast turned into the pit, to find its way up through the grates. The fuel may be hard coal or coke, broken into lumps about the size of hen's eggs; rapidly, and thus lessening the oridacoal is not admissible.
The ordinary cupola furnace is shown in Fig. 1. It consists of a circular chamber a built of fire-brick, rising in the form of a dome, in the top of which is a circular opening, carrying a cast-iron ring b, through which the pots and fuel are introduced. At the bottom is a bed-plate c, which is a circular plate of cut-iron having one targe hole d in a arranged symmetrically around it. Below the bed-plate is the ash-pit /, lending to an arched air-passage g, which supplies air to the ash-pit. Tu-pering cast-iron nozzles, 6 in. high, the tap, and about 3/4 in. thick, ate placed over the twelve email holes e. The space between the top of the bedplate and the top of the nozzlei is built of with fire-brick and fire-clay until it forms a surface perfectly level with the top of the small nozzles, leaving the centra] hole free. These nozzles do the duty of a fire-grate, by admitting the air that supports combustion. The whole construction is enclosed in a solid mass of brickwork, and an iron bar A is built in over the air-way in front of the bed-plate, and resting on the walls forming the sides of the air-war, to give support.
The dimensions of the furnace shown are 3 ft. 6 in. diameter, and 3 ft. 6 in. height from funwee-bed to crown of arch.
The ordinary melting furnace is shown in Fig 2, The fire-place a is lined throughout with the well as the opening of into the due and a portion of the fine Itself; is the ashpit; c, register-door of ash-pit, by which the drought is partially regulated; f, fire-brick cover for the furnace; g, fire-barv It is built ail round with common brick; and as many as six may use the same stack.
Fig, 3 illustrates the circular melting furnace, consisting of an iron piste a pierce 1 in the centre by a circular hole of the size of the interior of the furnace, and crossed by the fire-burs; b is a sheet-iron drum riveted together, forming the shell of the furnace, and resting on the bed-plate; it is first lined on the inside with 4 1/2 in. of ordinary brick, and next with 9 in. of fire-brick; c, fireplace; d, flue lending to stack; e, iron grating for admitting air beneath the furnace; ash-pit; g, four small brickwork pillars, about 18 in. high, supporting the bed-plate; h, fire-brick cover to furnace. The draught is regulated by a damper in the flue or on the stack. The latter is an iron plate large enough to entirely cover the top of the stack, hinged at one edge, and opened or closed by a lever.
A reverberatory furnace is illustrated in Fig. 4;-a, tire-place; b, ash-pit; c, bridge; d, melting furnace; e, fire-door; /, flue leading to stack; g, door for feeding in and ladling out metal. The drought is regulated by the fire-door and the damper on the top of the stack.