Blowing house

A blowing house or blowing mill was a building used for smelting tin in Cornwall and on Dartmoor in Devon, in South West England. Blowing houses contained a furnace and a pair of bellows that were powered by an adjacent water wheel, and they were in use from the early 14th century until they were gradually replaced by reverberatory furnaces in the 18th century. The remains of over 40 blowing houses have been identified on Dartmoor.

The blowing house method of smelting tin was probably introduced early in the 14th century to replace the earliest method of smelting which had to be done in two stages – a first smelting probably took place near to the tinworks and the roughly smelted metal was taken to a stannary town to be smelted again to produce the final refined product. Each of these smeltings was taxed separately until 1303 when they were replaced by a single tax on the finished product. It is likely that this tax change was due to the improved smelting process provided by the blowing houses.

Documentation confirms the existence of blowing houses in Cornwall as early as 1402, but the earliest reference for Dartmoor is not found until the early 16th century, though it is likely that they were in use on the moor earlier. In Devon there are many references to blowing mills throughout the 16th and 17th centuries, reflecting the boom time in tin-mining on Dartmoor. However, by 1730 there were only two blowing mills working in the whole of the county: at Sheepstor and Plympton.

From the beginning of the 18th century, this method was gradually superseded by reverberatory furnace smelting, which used higher temperatures and powdered anthracite as fuel and had the advantage of not requiring a forced draught of air. The smelting house at Eylesbarrow tin mine which was in operation during the first half of the 19th century had two furnaces, one of each type.

On Dartmoor, blowing houses were rectangular buildings between around 16 to 32 feet (4.9 to 9.8 m) in length and around half this in width. They were made of unmortared granite blocks with walls often 2 feet 6 inches (0.76 m) or more thick and probably had turf or thatch roofs which were periodically burnt to retrieve the particles of tin that would have been driven into the roof through the blast from the bellows. Blowing houses are typically located on or near the bank of a stream where the fall of water was enough for a leat to be built to work a small (9 ft or 2.7 m) diameter overshot water wheel which developed around 0.66 hp (0.5 kW) to operate the bellows.

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