An Iron Age furnace discovered in 1963 at heritage site and nature reserve Melville Koppies, in Johannesburg, required a significant amount of air and temperature control, which is an art that is difficult to be imitated by modern technology, but can be done with practice.

This is according to former head of the archaeological research unit at the University of the Witwatersrand Revil Mason and nonprofit organisation Friends of Melville Koppies chairperson Wendy Carstens, who say African settlers had no measuring instru-ments and relied only on experience, making the use of this furnace an art.

The furnace, which was made using clay and has a diameter of about 1 m, was dis-covered by Mason and is between 200 and 300 years old.

According to Carstens, it was used by early African settlers to smelt iron-ore to extract the iron to forge hunting tools, such as spearheads, and agricultural tools, such as hoes.

The discovery of the furnace led to the nature reserve, established in 1959, being declared a Johannesburg heritage site. The furnace is protected by shatterproof glass and a steel cage of about 4 m in size.

Carstens explains that settlers in the Iron Age migrated from the north to the Koppies, owing to population expansion that forced settlers south of the Vaal river into the Free State and south-east into KwaZulu-Natal and the Transkei in the Eastern Cape.

Her husband, retired engineer David Carstens says the furnace was found with bits of iron slag, broken tuyeres – a pipe that is blown into to keep the flames burning in the furnace – and charcoal, which was made using trees in the area.

He says tree branches were burnt in a hole in the ground and covered with sand, while still burning to exclude oxygen. This turned the wood into charcoal, which burns at a higher temperature than wood. The charcoal was layered with iron-ore in the furnace, after which the furnace was fired.

The tuyeres were used to blow oxygen into the furnace for at least 16 hours to keep it burning at about 1 000 ºC.

The produced iron was then taken to a forge, reheated and beaten into required shapes. After forging, and while still hot, the implements were probably tempered by immersing the iron in goat fat, blood or urine when needed.

“The use of such a furnace slowly became a lost art, as a result of the cheap iron goods that were imported from Europe. There-fore, the use of old technologies, such as the furnace at Melville Koppies, fell away. To produce iron in this way today is expen-sive and time consuming, especially as beneficiated iron can be bought at a cheap price,” Wendy Carstens states.