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.
Stirling and Dunfermline Railway
Cast iron is rigid and brittle so that deflection of the rails resulted in breakages. In 1785 a wearing surface of Swedish malleable iron, 1¾ inches broad, and ¾ of an inch thick, was tried with great success because of its ability to flex with the timber. The running resistance was also remarkably low.