A large holding furnace for molten pig iron. The capacity of these furnaces, which are of the tilting type, is up to 1,400 st (1,270 t). Hot metal mixers may be active (that is, the pig iron is partially refined while in the furnace) or inactive (that is, the pig iron is merely kept molten until it is required for transfer to a steelmaking furnace). In either case, the bottom and walls of the furnace are made of magnesite refractories and the roof of silica refractories.