IRON IN ANCIENT TIN FROM ROOIBERG, SOUTH-AFRICA

Tin and iron oxides when smelted together form an undesirable tin/iron alloy called hardhead. An equilibrium exists between furnace metal, hardhead and slag which can be used to determine whether an ore was suitable for tin smelting and if a slag was produced by a tin smelt. Two ores, found near iron age furnaces at the Rooiberg mining and smelting complex in South Africa, could have been smelted to recover 68-78% of the tin if the smelters were willing to tolerate 1% iron in the furnace metal. Recycling the slag to increase recovery would appear not to have been practicable because of its high iron content. One slag was completely misidentified as the furnace metal associated with it contained only 1% tin, but two other slags were associated with furnace metal that was more than 99% tin. There are still major questions regarding the design and operation of the iron age furnaces and the fate of the tin slag. Possibly this was charged to iron furnaces resulting in some iron artefacts from this period being contaminated with tin.