Bloomery

A bloomery is a type of furnace that was once widely used for smelting oxides. Bloomeries were the first smelters able to smelt iron. The result of using a bloomery is a mass of iron and slag called a bloom. This is called sponge iron, because it is porous like a sponge. Sponge iron is usually refined into wrought iron. Bloomeries are no longer used in smelting, because blast furnaces are better.
Process.
A bloomery consists of a chimney with heat-resistant walls made of earth, clay, or stone. Near the bottom, pipes (made of clay or metal) enter through the side walls. The pipes allow air to enter.
To use a bloomery, charcoal is inserted into the chimney, then heated. Then roughly the same volume of iron is inserted. A common product of this process is fayalite, which occurs when iron mixes with slag.
History.
Iron began to be used around 1200 BC.
China appeared to not use bloomeries, seemingly skipping to blast furnaces. However, bloomeries have been found in China, though these were likely only brought from other locations, and not built in China.
Bloomeries had been used in Sub-Saharan Africa before 500 BC.
European Bloomeries started very small, smelting about 1kg of iron with each use, but got larger over time, smelting about 14kg on average in the fourteenth century.