I built a natural draft furnace and smelted iron ore in it, producing a small amount of iron. A natural draft furnace is one where the draft is supplied by convection alone. This is opposed to a forced draft where bellows or fan is used to supply air flow instead. Natural draft furnaces are typically taller than forced draft furnaces to make use of the stack effect. They also often have multiple large air entries or tuyeres to help reduce resistance to air flow. Natural draft furnaces where used to smelt iron in ancient times in Europe and up to more recently in Africa where some are still standing. The lack of the need for bellows reduces labor and complexity.