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.
Wealden iron industry
The Lamberhurst Foundry is believed to have been the maker in 1710–14 of some of the earliest cast-iron railings produced in England, which they made for St Paul's Cathedral, despite the objections of Christopher Wren, who did not want a fence around the Cathedral at all, and said that if there had to be one it should be of wrought rather than cast iron. The railings surrounded the cathedral, including seven gates. It weighed two hundred tons and cost six pence a pound. The total cost was £11,202. No further railings are known to have been cast in the Weald. Other early uses of cast iron railings were at Cambridge Senate House and at St Martin-in-the-Fields, London.