How to fire up a bloomery furnace
manufacture and metallurgy. At the Chiltern Open Air Museum (COAM, we have reconstructed an Iron Age bloomery furnace based on research from experimental archaeology, excavations and the help of Jack Greene, historical educator.
Iron smelting is a complex process requiring huge amounts of heat. We extract iron from ores which are minerals containing iron. The bloomery method of smelting does not melt the iron, but instead, the tiny particles of iron stick together, separating from the mineral and forming a ‘bloom’. The waste material (the slag) falls to the bottom of the furnace.
The furnace is a cone shape with a circular shaft running down the middle and is made of a material called daub. Daub is a mixture consisting of a clay and water slurry, straw, soil and horse manure (fresh from our shire horses). The Daub is stuck onto a circular wattled frame, with a hole left for the ‘tuyere’ to allow bellows to be attached.
Once the structure is dry (which usually takes a day) the leather and bone bellows, can be attached.
A fire is started in the bottom of the structure and when well alight, the forge is filled half way with charcoal topped by crushed iron ore. The rest of the furnace is filled with charcoal. Bellows achieve the high temperatures needed to smelt iron. The furnace is then broken open to extract the iron.