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.
Cast-iron architecture
It was in 18th-century Britain that new production methods first allowed cast iron to be produced cheaply enough and in large enough quantities to regularly be used in large building projects. New production methods included using steam engine powered blast air, which allowed higher blast furnace temperatures, which in turn allowed the use of more limestone to be added with the iron ore charge. The higher furnace temperatures made the slag produced with the additional lime to flow more freely. The calcium and magnesium in the lime helped tie up sulfur, which allowed the use of coke for fuel. The higher furnace temperatures also increased the furnace capacity.