Casting Iron in a Cupola Furnace — Re-Melting Pig Iron into Finished Castings

The blast furnace makes pig iron by the tonne, but pig iron in bars is not yet a useful object. To turn it into pots, pipes, stoves, and machine frames, a foundry re-melts it and pours it into moulds — and the workhorse that does the re-melting is the cupola furnace.

A cupola is a tall iron chimney lined with firebrick, charged from the top with alternating layers of coke, pig iron, and limestone, and blown with air near its base. Unlike the blast furnace it does not smelt ore; it simply melts iron that is already made, quickly and cheaply, keeping a steady stream of molten metal ready to tap. The melting iron picks up a little carbon from the coke and runs out of the tap hole as grey cast iron.

Poured into sand moulds shaped from wooden patterns, that iron sets into castings of almost any form. Cast iron is brittle but immensely strong in compression and cheap to pour into complex shapes, which made the cupola the beating heart of Victorian industry — casting everything from cooking pots and drainpipes to the columns and machines of the factories themselves.