The glassy slag is formed by the combination of the alumina, silica, and lime contained in the ore and flux. Being lighter than the iron, the slag floats on the molten metal and is run off near the bottom of the furnace. The chemical action which takes place in the furnace is as follows: Part of the carbon of the fuel burns to carbon monoxide and combines with the iron oxide, setting free the iron; the remainder combines with the oxygen in the air and with the ore, thus forming carbon monoxide (CO) and carbon dioxide (CO2) which pass out through a pipe at the top. The burned and unburned gases are allowed to enter other furnaces where they give off great heat on burning. This heat is used to raise the temperature of the air that enters the tuyeres.