This section is from the book "Spons' Mechanics' Own Book: A Manual For Handicraftsmen And Amateurs", by Edward Spon. Also available from Amazon: Spons' Mechanics' Own Book.
Furnaces for melting brass or bronze may be built of common brick and lined with fire-brick; but the best are made with a boiler-plate caisson, 20-30 in. diam. and 30-10 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 admits 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 1 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; 3 or 4 such furnaces are commonly arranged in sets giving a graduated scale of sizes, to suit the needs of large or smaller castings. When the quantity of metal used is large, a blast is generally employed.
The common brass furnace usually depends on a natural draught and connects by a flue with a chimney stack at the back; 3 or 4 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 hens' eggs; coke is preferable as heating more rapidly, and thus lessening the oxidation of metal, but gas-coke from cannel coal 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 cast-iron having one large hole d in the centre (for the withdrawal of ashes and clinkers), and 12 smaller ones e arranged symmetrically around it. Below the bed-plate is the ash-pit f leading to an arched air passage g, which supplies air to the ash-pit. Tapering cast-iron nozzles, 6 in. high, 3 in. diameter at the bottom, 1 1/4 in. at the top, and about 3/4 in. thick, are placed over the 12 small holes c. The space between the top of the bed-plate and the top of the nozzles is built up with fire-brick and fire-clay until it forms a surface perfectly level with the top of the small nozzles, leaving the central 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 h is built in over the air-way in front of the bed-plate, and resting on the walls forming the sides of the air-way, to give support.
The dimensions of the furnace shown are 3 ft. 6 in. diameter, and 3 ft. 6 in. height from furnace bed to crown of arch.
The ordinary molting furnace is shown in Fig. 2. The fire-place a is lined throughout with fire-brick, as well as the opening d into the flue and a portion of the flue e itself; b is the ash-pit; c, register-door of ash-pit, by which the draught is partially regulated; f, fire-brick cover for the furnace; g, fire-bars. It is built all round with common brick; and as many as 6 may use the same stack.
Fig. 3 illustrates the circular melting furnace, consisting of an iron plate a pierced, in the centre by a circular hole of the size of the interior of the furnace, and crossed by, the fire-bars; b is a fheet-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, fire-place; d, flue leading to stack; e, iron grating for admitting air beneath the furnace; f, ash-pit; g, 4 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 open or closed by a lever.
A reverberatory furnace is illustrated in Fig. 4: a, fire-place; b, ash-pit; c, bridge; d, melting furnace; e, fire-door; f, flue leading to stack; g, door for feeding in and ladling out metal. The draught is regulated by the fire-door and the damper on the top of the stack.