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A cupola furnace is a cylindrical device that is used to heat and melt metals such as iron and bronze. These blast furnaces are typically between 1.5 and 13 feet (about 0.5 to four meters) in diameter, and have the outward appearance of a smokestack. In order to operate a cupola furnace, the cylinder is usually filled with alternating layers metal and coke, and flux materials, such as lime or carbon adjuncts, can also be added. When the metal melts, it flows down and can be released from a tap on the front of the furnace. After the run is complete, these furnaces can be emptied and prepared for future use by opening a dump gate on the bottom to remove remaining coke and other materials.
Archaeological evidence suggests that cupola designs may have been used in China during the third century BCE. Early examples of cupola furnaces were used in China during the Chunqiu period to cast bronze. Cupola furnaces were also used for cast iron in Europe from the 17th century on. Until the middle of the 20th century, the cupola furnace remained the primary method of casting iron. Most foundries eventually switched over to induction furnaces, though cupolas are still used in some instances.
When a cupola furnace is in use, it is typically referred to as a campaign. At the start of a campaign, coke is placed into the furnace and then ignited. Ports called tuyeres are used to introduce air to the burning coke, which creates a type of blast furnace. The introduction of air to the coke results in it becoming very hot, at which point metal can be set into the furnace. Additional layers of coke are added so that the heating process will continue, and lime or other fluxing agents can be added to reduce oxidation.
Heat from the lower layers of burning coke typically rises through the cylinder, heating the upper layers of metal and melting them. Carbon from the coke can bond with the liquid metal as it runs through the lower layers and pools at the bottom. In some cases, adjuncts such as silicon carbide may be added to increase carbon content. After enough molten metal has pooled in the bottom part of a cupola furnace, the operator can open a spigot to drain it into a collection vessel. There is typically another spigot at a higher point on the back of the device that can drain slag materials.