How it began
The furnaces were found at Samanalawewa in southern Sri Lanka and reported by the archaeologist Gill Juleff in Nature in 1996 as a previously unknown furnace type. All stand on the western margins of hills and ridges, where they face the strong monsoon winds. Survey recorded more than eighty such sites in about 70 square kilometers. Radiocarbon dates place a first phase in the seventh to ninth centuries and a second in the ninth to eleventh centuries CE. Older furnaces of other types in the same area go back to between the fourth century BCE and the first century CE.
Before it existed
Before the blast furnace, iron was smelted by reducing ore below the melting point of the metal, which left a bloom of low-carbon iron and slag. The bellows-operated shaft furnace was usually seen as the peak of this early technology. Natural-draught furnaces, where a tall shaft pulls air through, were also known but were regarded as inefficient by comparison.
How it works
Each furnace had a permanent curved back wall of clay and a straight front wall, rebuilt for every smelt, set between two upright stones. Clay pipes called tuyeres passed through the front wall. Wind blowing up and over the front wall created a pressure difference between the top of the wall and the mouths of the tuyeres, which drew air through the charge. Juleff described this as a wind-based air supply distinct from both forced and natural draught. Trials with replica furnaces gave about 17 kilograms of metal from about 120 kilograms of ore, including slag-free high-carbon steel.
Where it stands today
The technology sustained a major iron and steel industry in the Samanalawewa area during the first millennium CE. Juleff suggested it may have contributed to South Asia's early pre-eminence in steel production. The industry collapsed in the eleventh century, when Cholan invasions from South India ended political stability and the royal court of Ruhuna and its control of trade fell. The furnaces are known today through excavation and field experiments, which Juleff's project presented under the name Monsoon Steel.
