Cast iron
Cast iron is made from pig iron, which is the product of smelting iron ore in a blast furnace. Cast iron can be made directly from the molten pig iron or by re-melting pig iron, often along with substantial quantities of iron, steel, limestone, carbon (coke) and taking various steps to remove undesirable contaminants. Phosphorus and sulfur may be burnt out of the molten iron, but this also burns out the carbon, which must be replaced. Depending on the application, carbon and silicon content are adjusted to the desired levels, which may be anywhere from 2–3.5% and 1–3%, respectively. If desired, other elements are then added to the melt before the final form is produced by casting.
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An American architectural innovation, cast iron was cheaper to use for facades than materials such as stone or brick. Molds of ornamentation, prefabricated in foundries, were used interchangeably for many buildings, and a broken piece could be easily recast. The buildings could be erected quickly; some were built in four months. Despite the brief construction period, the quality of the cast-iron designs was not sacrificed. Bronze had previously been the metal most frequently used for architectural detail. Architects found that the relatively inexpensive cast iron could provide intricately designed patterns. Classical French and Italian architectural designs were often used as models for these facades. Because stone was the material associated with architectural masterpieces, cast iron, painted in neutral tints such as beige, was used to simulate stone.