Different furnaces have different structures.
1) 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.
2) 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.
3) 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.