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, h ardhead and slag which can be used to determine whether an ore was sui table 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 furna ce metal. Recycling the slag to increase recovery would appear not to have been practicable because of its high iron content. One slag was c ompletely misidentified as the furnace metal associated with it contai ned only 1% tin, but two other slags were associated with furnace meta l that was more than 99% tin. There are still major questions regardin g the design and operation of the iron age furnaces and the fate of th e tin slag. Possibly this was charged to iron furnaces resulting in so me iron artefacts from this period being contaminated with tin.