Iron ore, coke, and limestone are added at the top of the furnace, and air is blown in at the bottom. The oxygen of the air reacts with the hot coke to form carbon monoxide. This gas passes up the furnace, changing the iron oxide to iron as it moves down. Liquid iron collects at the bottom of the furnace, covered by a layer of molten slag. The iron is tapped off at the bottom of the furnace and allowed to solidify, forming pig iron. Pig iron is hard but very brittle. The molten slag is run off separately.