Different furnaces have different structures.
- In blast furnace, both fire box and hearth are combined in big chamber, which accommodate both ore and fuel. It has a cup and cone arrangement at its top to send the ore into the furnace.
- Reverberatory furnace has both fire box and hearth separated, but the flames obtained due to the burning of the fuel touch the ore in the hearth and heat it.
- In furnaces like retort furnaces, there is no direct contact between the hearth and fire box and even the flames do not touch the ore.