IRON is usually melted in a blast furnace, or, as it is more commonly called, a cupola; although the cupola or dome leading
First; a rough model of the figure is made in clay, but somewhat smaller than its intended size, it is covered over with wax which is modelled to the required form, or the wax is worked up in separate pieces and afterwards attached: various rods or cylinders of wax to make the apertures for the runners and air holes, are fixed about the figure and led upwards. The whole is now surrounded with a coating of loam and similar materials, the inner portion of which is ground very fine and laid on with a brush like paint; and the outer part is secured with iron bands. When all has been partially dried a fire is lighted beneath the grating on which the figure is built, to cause the wax to run out through one or more apertures at the base, which are afterwards stopped, and all is thoroughly dried and secured in the pit, after which the charge of the furnace is let into the cavity left by the wax.
Secondly; the finished figure is modelled in clay, and stuck full of brass pins just flush with its surface, which surface is now scraped away as much as the thickness required in the metal; the reduced figure is now covered with wax mixed with pitch or rosin, which is worked to the original size with all the exactness possible. The other stages are the same as in the foregoing; the metal studs or pins prevent the mould and core from falling together, and they afterwards melt, becoming a part of the metal constituting the figure.
Thirdly; the finished figure is modelled in plaster, and a piece-mould is made around it, the blocks of which consist internally of a layer of sand and loam 1 1/2inch thick, and externally of plaster one foot thick. The mould when completed is taken to pieces, dried, and rebuilt in the casting pit; it is now poured full of a composition suitable for the coro, the mould is again taken to pieces, the core is dried and scraped to leave room for the metal, and all is then put together for the last time, secured in the pit. and the statue is cast
The first plan is the most wasteful of metal, the third the least so, although it is the most costly when the time occupied is also taken into account; but it has the advantage of saving the original work of the artist to the chimney, from which it would appear to have derived its name, is frequently omitted, the two or three furnaces being often built side by side in the open foundry.
At the basement there is a pedestal of brickwork about 20 to 30 inches high, upon which stands a cast-iron cylinder from 30 to 40 inches diameter, and 5 to 8 feet high; this is lined with road-drift, which contracts its internal diameter to 18 or 24 inches. The furnace is open at the top for the escape of the flame and gases, and for the admission of the charge, consisting of pig-iron, waste or old metal, coke and lime, in due proportion. The lime acts as a flux, and much assists the fusion; chalk is considered to answer the best, but oyster-shells are very commonly used where they are abundant.
At the back of the furnace there are three or four holes one above the other for the blast, which is urged by bellows or by a revolving fan. No crucible is used, and as the fluid metal collects at the bottom of the furnace, the blast pipe is successively removed to a higher hole, and the lower blast-hole is stopped with sand, which partly fuses and secures the blast-hole very effectually.
The front aperture of the furnace through which the metal is allowed to flow into the ladles or trough, is usually made sufficiently large for the purpose of clearing or raking out rapidly the fuel and slag, as the process is most laborious owing to the excessive heat. This aperture is closed by a guard-plate, fixed on by staples attached to the iron-case of the furnace, in the center of which plate the tapping hole is made: during the time the metal is fusing the tap hole is closed by sand well rammed in, and this, if well done, is never found to fail.
Many iron furnaces are made octangular, and in separate parts bound together by hoops, so that in the event of the charge becoming accidentally solidified in the cupola, the latter may be taken to pieces for its removal, and thus avoid the necessity of destroying the furnace. There is frequently a light framing or grating above the furnace, upon which the small cores are placed that require to be dried.
In some foundries the cupolas are built just outside the moulding-shop, beneath one or more chimneys or shafts, which carry off the fumes; in such cases the fronts of the furnaces arc accessible through an aperture in the foundry wall, with which they are nearly flush; when the furnaces are lofty there is a felling stage at the buck, from which the charge is thrown in. For heavy iron castings, which sometimes amount to thirty tons and upwards in one piece, reverberatory or air furnaces are also commonly used; the ordinary charge for these is four to six tons of iron, and five or six furnaces arc commonly built close together, so that they may be simultaneously tapped in the production of such enormous works.*
For melting iron in the small way, good air furnaces may be used, and also some of the black-lead furnaces, which arc blown with bellows, but this is one of the processes that is not successful upon a limited scale.
Considerable judgment is required in proportioning the charge for the iron furnace, which always consists of at least two, and often of half-a-dozen kinds of new pig-iron mixed together, (as adverted to in the foot note, page 185,) and to which new iron, a small proportion of old cast-iron is usually added. The kinds and quantities used are greatly influenced by local and other circumstances, so that nothing can be said beyond a few general remarks.