Coke is primarily used to smelt iron ore and other bearing materials in blast furnaces, acting both as a source of heat and as a chemical reducing agent to produce pig iron or hot metal. Coke, iron-ore and limestone are fed into blast furnace, which runs continuously. Hot air blown into the furnace burns the coke, which serves as source of heat and as an oxygen reducing agent to produce metallic iron. Limestone acts as a flux and also combines with impurities to form slag. Coke also serves as a structural material to support the deep bed of coke/iron oxide/limestone that makes up much of the furnace volume. It is in this last role that its properties are crucial. It is important that it does not degrade (eg break up into small particles ) during its descent through the oxidizing hot gases passing through the stock region of the furnace.