Cupola
The Encyclopedia of Founding (1892)
A cupola is simply a foundry melting-furnace. Those now in common use are usually composed of an outer wrought-iron shell, inside of which is built a lining of fire-bricks carefully set in their fire-clay, as close together as possible, and in close proximity to the shell. Cupolas may be from two to eight feet in diameter up to the charging-door, according to requirements, beyond which point they can assume a conical shape for the chimney. Formerly all cupolas rested on a solid foundation of brick or stone-work; but they are now, as a rule, supported on four iron columns, by which means the residue may be dropped clean out of the inside when through melting, instead of raking it out, as must be the case when a solid bottom is used. This is accomplished by providing hinged iron doors under the bottom, which, when the heat is over, may be dropped and thus allow the slag and cinders to fall through into the pit below. For the former, a large breast-hole must be provided at the bottom, either front or back of the cupola, through which to rake out the cinder, etc.; but for the improved ones a hole about seven inches square is all that is required, to which is attached the spout for leading the molten iron into the ladles. The spout and tap-hole are invariably made good with sand or fire-clay mixture for each heat. In some instances, for very small cupolas, the blast enters therein through one or two pipes; but for larger ones a continuous belt or wind-box encircles the cupola, out of which as many tuyeres as may be considered necessary are served, the wind-box being simply a continuation of the main blast-pipe leading from the blower or fan, which, if up to the required capacity and skilfully managed, will be regulated to supply a blast of such volume and pressure as will ensure perfect combustion of the fuel with sufficient rapidity to produce the intensity of heat required for melting the metal which has been charged. See, under the following list, for details of cupola construction and management: Bed-fuel. Blast. Blast-gate. Blast-gauge. Blast-pipe. Blast-pressure. Blowers. Bott-clay. Bott-stick. Breast. Bugs. Burnt-iron. Carbon. Carbonic Acid. Carbonic Oxide. Carrying-bar. Cast Iron. Charge. Charging-door. Charging-hole. Charging the common cupola. Coal. Coke. Coke-fork. Combustion. Crusted-ladle. Cupola. Daubing. Drop-bottom. Eye-piece. Fan. Fire-brick. Fire-clay. Flame. Flux. Fuel. Grades of Pig-iron. Greiners cupola. Grouting. Hard Iron. Height of cupola. Hood. Hot-blast. Ladles. Lighting the cupola. Lining ladles. Lining the cupola. Oxygen. Pig-iron. Piston-blower. Pressure-blower. Pressure-gauge. Ratio of fuel to iron. Repairing the cupola. Sand-bed. Scaffolding. Scrap. Shank. Silicon. Slag. Softeners. Soft-iron. Spout. Tapping-bar. Tuyeres. Wind-box.
What Has Changed in the Last Century
The cupola described here in 1892 — wrought-iron shell, firebrick lining, wind belt feeding a ring of tuyeres, drop-bottom doors on four columns — is still a recognisable description of a cupola, and the type is still built and run, mainly in large iron foundries producing grey and ductile iron. What has changed is how rarely it is the melting unit of choice: since the middle of the twentieth century most foundries, especially small and medium ones, have shifted to electric melting, above all the coreless induction furnace, which needs no coke, starts and stops at will, and gives much closer control of composition. Surviving cupolas have also been much modified: water-cooled shells (sometimes running with little or no refractory), preheated hot blast, oxygen enrichment and long continuous campaigns rather than a single day's heat. The other great difference is the law — the open charging stack and free-venting hood of 1892 have given way, under modern air-pollution rules, to closed tops with afterburners, scrubbers or baghouses to capture dust and carbon monoxide.
Written for this edition, 2026.