A BRITISH TIN SLAG MYSTERY: BLAST OR REVERBERATORY FURNACE?

The southwest region of Great Britain was a prominent source of tin for centuries. Cassiterite, extracted from stream deposits or from local granite in the counties of Devon and Cornwall, was smelted in either a blast furnace or a reverberatory furnace to create pure metallic tin. Tin smelting sites are scattered throughout Southwest Great Britain, and these sites are marked by remains of the smelter buildings or by slag finds. In rare cases, furnaces are found intact. However, in situations where no part of the furnace exists, one must look to old smelting documents or to other remains at the site to shed light on the smelting processes. Unfortunately, documentation of the tin smelters in Cornwall and Devon is poor, and it is necessary to look for other indicators for the processing, namely, slag. Slag is the waste product formed by smelting and in many instances, the sole indicator for metallurgical activity at a site. Determination of the smelting processes used at a tin smelting site is invaluable for clarifying the metallurgical past of Devon and Cornwall-a history that is often poorly documented and contested among archaeologists. Such information is also important in delineating the technological advances in the field of tin metallurgy in Britain.

Tin slags from 9 smelting sites in Great Britain were studied with specific attention to their morphology, mineralogy, and chemistry. Compilations of observations helped differentiate between slags formed by blast furnaces and reverberatory processes. In general, blast furnace slags are smaller in size and tend to exhibit ropy macroscopic features due to formation from highly viscous melts. Reverberatory slags are more massive and flow features are rare. Conventional light microscopy, XRD, and electron microprobe analyses indicate that blast furnace tin slag is more vitreous and often contains zircon. Reverberatory slags, in contrast, rarely contain zircon, but often feature small prills of metallic iron or pyrite. Finally, blast furnace slags contain higher amounts of SiO₂ and lower concentrations of FeO compared to reverberatory tin slags. A combination of morphology, mineralogy, and chemistry clearly distinguishes slags formed by the two furnace types.