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The following has been extracted by very kind permission of Phil Newman from his book

The Dartmoor Tin industry - A Field Guide. Published 1998.

(Text by Phil Newman & Photos by Chris Kelland)

Blowing Mills                    Stamping & Crazing Mills                    Dressing Floors

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Tin Mills

Once removed from the ground, tin ore needs to undergo a complex series of processes, including crushing, refining and smelting, before becoming a usable commodity. In the earliest days of the tin industry on Dartmoor, the rich and easily accessible alluvial gravels provided a very pure form of ore. These gravels would need a minimum of crushing, if any, before being smelted in small, out-door furnaces, probably very near the tinworking site, leaving an imperceivable level of archaeological evidence. By late medieval times, the economic benefit of high tin production, brought with it the need to exploit less rich deposits, intensifying the effort involved in tin processing, especially crushing. This in turn led to a degree of mechanisation for both crushing and smelting and from about the mid 14th century onwards, most of these processes were carried out in small mills.

Structural remains from over 50 tin mills survive on Dartmoor as well as a further 30 sites where artefacts or other evidence indicate the former location of a mill. This represents a remarkable survival rate for Devon tin mills, especially compared with Cornwall where despite the far higher level of tin production and known documentation for a great many mills, there is surprisingly only a small handful of sites where field evidence has been recorded. ¹

Tin mills are roughly rectangular buildings, within which the tin processing machinery was housed, powered by a waterwheel. The walls are always of a dry-stone construction, using unhewn or roughly dressed moorstone, built randomly or sometimes in approximate courses. Their condition today varies greatly between the well-preserved, extant walls at Week Ford (Photo No.1), and the mostly destroyed building at Gobbett, now recognisable mainly by some artefacts visible there (Photo No.2). Other mills, such as one example in the Plym Valley (Photo No.3), are completely covered by turf and survive as little more than an earthwork. The interior dimensions of the mills varies between 9.8m by 4.7m at a large mill, such as Middle Merrivale, and the small Plym mill already mentioned, which is only 4.5m by 2.5m.

No.1          Week Ford - Tinners Mill	No.2          Gobbett Mill - Mould Stone	No.3          River Plym Mill (Lower Hartor)

The mills are most commonly sited close to a stream and built into the base of the natural, or sometimes artificial, slope which forms the edge of a river valley. This would provide a sufficient drop in height for water, diverted from the stream via a leat, to be delivered onto the top of the waterwheel, using either an overshot or pitchback arrangement. These were probably the only types used in the Dartmoor tin mill where water supplies, though reliable, could not supply the volume necessary for an overshot wheel. Waterwheels were usually housed in narrow stone wheelpits attached to the exterior of the main mill structure or, more rarely, could be contained within the mill building. Examples of the former are Upper and Middle Merrivale and Colesmill on the River Plym, while the latter may be seen may be seen at the Taw River mill. R.H. Worth who compiled statistics based on his research, was able to suggest that the average waterwheel was of 9ft 3" (2.8m) diameter, with the largest being 10ft (3m) and the smallest 8ft (2.5m), with a breast of between 1ft 6" (46cm) and 2ft (60cm). ²

Water was conducted from the leat onto the wheel via a wooden channel or launder. In a well-positioned mill, at the foot of a steep slope, the launder would be fairly short, as for example at the Nosworthy left bank mill, where there is a drop of about 3m from the leat to the wheelpit. When at the foot of a more gentle gradient, a wedge shaped embankment was built above the mill, coaxial with the wheelpit, over which the final section of leat was diverted. A very fine leat embankment of this type survives at Colesmill.

Power from the waterwheel was transmitted to the machinery inside the building by means of a drive shaft which passed through an aperture in the wall adjacent to the wheelpit. At Broad Falls and at Week Ford lower mill (Photo No.4) these roughly square apertures, capped by lintels have survived.

The waterwheels, drive shafts and any other rotating components were supported using iron axles, bedded into primitive stone bearings. These are semi-circular polished slots usually about 20-30mm diameter by approximately 50-90mm long on the face of a granite stone. The polishing was caused by wear resulting from an iron axle rotating in the slot. The adjacent sides of the stone will often have concentric, arc-shaped wear marks, suggesting that a larger rotating component, such as the waterwheel itself perhaps, was rubbing the stone. At the Nosworthy left bank mill, several examples have come to light as a result of the archaeological excavations. At Colesmill (Photo No.5), the bearing stone of the waterwheel is still in-situ on the edge of the wheelpit, utilising previously discarded mortarstones (Photo No.6). Not all waterwheels used these primitive bearings, for there is evidence that 'plumber block' bearings were used as well. In this case a small rectangle was cut into the supporting stones into which an iron bearing could be set. One such stone may be seen in situ in one side of the Nosworthy left bank wheelpit and another was recovered from the wheelpit at Upper Merrivale. It is likely that these bearings are a later innovation.

No.4        Week Ford - Tinners Mill (Lintel)	No.5        Colesmill - Bearing Stone (Left)	No.6     Colesmill - Abandoned Mortarstones

No windows have survived in tin mills but several mills have well preserved entrances. At Black Tor Falls for example (Photo No.7), the granite lintel above the door is still in place and the uprights of the door lining have vertical grooves cut into them to accommodate the woodwork of the door jambs, as well as slots and holes for iron fittings and hinges.

Tin mills fall broadly into two categories - blowing and stamping. Blowing mills or blowing houses contained small blast furnaces for which a forced draught was supplied by waterwheel-powered bellows, and where prepared cassiterite was smelted and cast into ingots. The cassiterite was prepared for smelting in stamping mills or 'knacking/knocking' mills. The stamping mill contained mechanical crushing equipment, activated by a waterwheel. Some mills have evidence of both blowing and stamping though we can never be certain that both were occurring simultaneously. It is often more likely that two phases of separate activity at the same site are represented by the different evidence.