archaeological watching brief and sample excavation was undertaken between 3rd July and 1st October 2013, during topsoil and subsoil stripping as part of extension works at Torr Quarry. The total area monitored comprised approximately 6 hectares. These excavations have produced the only archaeological evidence for medieval iron smelting, dated by large quantities of 12th to 13th century pottery, and associated activities on Mendip. The nearest contemporary sites are at Hemyock in Devon and Tisbury in Wiltshire.
At least 7 bloomery furnaces have been recorded as well as a similar number of charcoal-rich pits. There is also evidence for quarrying for limestone and iron ore. Extensive areas of slag waste have been recorded and sampled, working hollows, pits, ditches, stake holes from temporary structures and evidence for a substantial timber-built structure.
The site has the potential to significantly further our understanding of the development of iron bloomery furnaces and their associated industrial processes during the medieval period.
These ore constituted pods of pure botryoidal haematite. This type of haematite takes a globular form, resembling a bunch of grapes. Botryoidal haematite has a high iron content (70%) and would have been highly prized as smelting ore.
The types of furnaces excavated at Leighton are known as Bloomeries. This is because the heat generated within them was not sufficient to melt the iron completely. Small particles of iron formed during the smelt stuck together into a ‘spongy’ mass known as a bloom which needed further heating and forging by hmmering to remove the substantial impurities within it.
There are two common types of medieval furnaces –
tapping furnaces – where the slag is channelled to run out of a hole in the base of the furnace along specially created ‘runs’ or in pits, and
non-tapping furnaces where the slag collected at the base of the furnace, sometimes in numerous small ‘runs’ or in a large ‘lump’ Both types of furnaces were excavated in 2013.
The advantages of a tapping furnace are numerous. Experimental archaeology has shown that tapping free-running slag with a high iron content enabled the addition of further ore throughout the smelting process. This led to a great improvement in the quality and forgeability of the iron produced.
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