Bloomery

A bloomery was a small furnace of clay, sometimes with stone, with thick walls, in which iron ore was reduced with charcoal to a spongy lump of iron and slag called a bloom. Air was blown in by hand or foot bellows through one or more holes. The bloom was then hammered to drive out slag. In 12th-century England bloomeries were worked by hand; water-powered bellows came in around the 14th century and blast furnaces only from the late 15th.

Sources: historicengland.org.uk