João Pedro Veiga

Provenance and production of tin in the Ancient World has since long been a major topic of discussion among archaeologists. In Western Europe, where significant tin ore (cassiterite) deposits are known, only a few remains of ancient tin production, such as tin slags, have been detected. In the present work, elemental and microstructural analyses by WDXRF, SEM-EDS and XRD were performed on recently recognised tin slags from the Iron Age Carvalhelhos hillfort located in NW Iberia, a territory that represents the largest extension with tin mineralisation in Western Europe. Elemental and microstructural characterisation of cassiterite collected in a pilot field survey in the region of the hillfort are presented and discussed, as well as two ceramic fragments that could be part of a smelting structure and an iron slag from the settlement. Results show that the tin slags have variable but high contents in Sn, similarly to Pre-Medieval tin slags found in other Western European areas, but also high contents of Ta and Nb, which specifically distinguish them from other tin slags, such as those found in SW Britain. Tin ores from the hillfort region frequently have Ta and Nb in cassiterite solid solution or as inclusions of columbite group minerals, relating well with the Carvalhelhos tin slags. Up to present, the Carvalhelhos slags are amongst the very few ancient tin slags known in Western Europe, and their study can contribute to a better knowledge on ancient tin sources and trade routes.

In the present work microstructural and chemical analysis of tin and slag that resulted from three smelting experiments reconstructing prehistoric conditions are presented. Cassiterite ores were collected from NW Iberian deposits and were smelted in a small open pit. Results show that the loss in tin can be very high, up to 80%, mainly as a result of volatilization of Sn species. The experiments show that using handmade leather bag bellows and clay tuyeres, temperatures can easily reach >1200°C. The tin produced was a very pure (Sn) solid solution, with the presence of dispersed inclusions or phases of various composition. These included FeSn and FeSn2 intermetallics and small metallic W inclusions. Slags could be grouped into three types based on their physical characteristics, and bulk chemical analysis could relate each type as being a product from the reaction of ore material or a product from the reaction with the crucible material. Generally, it was found that in spite of very low recovery rates, the tin produced by this simple technique would have been adequate for a domestic small-scale production.