The tin mining industry on Dartmoor, Devon, England, is thought to have originated in pre-Roman times,Newman 1998, p.4. and continued right through to the 20th century, when the last commercially worked mine (Golden Dagger Mine) closed in November 1930 (though it saw work during the Second World War), and "composite mines" such as those producing tin as a by- product, such as Hemerdon Mine in Plympton continue to operate today. From the 12th century onwards tin mining was regulated by a stannary parliament which had its own laws.
Tin is smelted from cassiterite, a mineral found in hydrothermal veins in granite, and the uplands of Dartmoor were a particularly productive area. The techniques used for the extraction of tin from Dartmoor followed a progression from streaming through open cast mining to underground mining. Today, there are extensive archaeological remains of these three phases of the industry, as well as of the several stages of processing that were necessary to convert the ore to tin metal.
Stannary law
Mining became such an important part of life in the region that as early as the 12th century, tin miners developed their own set of laws (stannary law) and, ultimately, their own parliaments (stannary parliaments). These laws applied to anyone involved in the industry.
Stannaries were established in Tavistock, Ashburton and Chagford by King Edward I in 1305. Plympton followed soon after. The Devon stannary parliament met in an open-air forum at Crockern Tor from 1494.Finberg 1950, p.296.
Anyone who broke a stannary law could find himself imprisoned in the gaol at Lydford. The stannary courts were abolished in 1836.
Geology The majority of the tin mines on Dartmoor are located in granite country rock and most of the lodes trend ENE–WSW and are of limited length, though there are exceptions.Dines 1956, p.719 In the underground workings, the tin ore, cassiterite, was usually found in association with large amounts of tourmaline, and in central Dartmoor with much specular haematite.Dines 1956, p.720 In the southern part of the moor, the cassiterite was usually found in relatively large grains, but the lodes were of very variable quality.Dines 1956, p.728 These factors, combined with the fact that none of the underground workings was found to be profitable at depth, are typical of the deepest zone of tin mineralisation. The once very extensive alluvial deposits of tin ore, that were the first deposits to be mined, also point to the vast quantity of ore that once existed in lodes that have been eroded from above the granite since it was emplaced in the Carboniferous period.
Mining methods
Streaming
The earliest means of recovery, known as streaming or streamworking, involved the collection of alluvial deposits from river and stream beds where they had accumulated after being eroded from the ore-bearing lodes. The geological processes that resulted in the deposition of the cassiterite in the stream beds often resulted in very pure tin gravel which was mixed with gravels of other, unwanted, minerals such as quartz, mica and feldspar, collectively known as gangue.Newman 1998, p.11. It was relatively easy to separate these minerals on the basis of their very different specific gravities – cassiterite about 7 and gangue 3 or less. The separation was performed by passing a stream of water over the gravels: the gangue would be washed away faster than the wanted tin gravel.Newman 1998, p.12.
Once a tin-bearing valley had been identified, the stream-workers would arrange a stream of water, probably carried by a leat from higher up the valley, and starting at the lowest end of the deposit they would dig a trench (known as the "tye") as deep as possible to allow the finer gangue to be washed away.Newman 1998, p.13. (Quoting Hitchens' & Drew's History of Cornwall 1824.) From there they would start working up the valley, using the stream of water to wash over the debris they had loosened from the bed with picks. This method of working leaves characteristic evidence in the valleys - a series of ridges of the larger gangue material, sometimes roughly perpendicular to, sometimes coaxial with the line of the valley, sometimes apparently haphazard, all bounded by a scarp which marked the edge of the worked ground and whose height relates to the depth of the deposits.Newman 1998, p.13-16.
Eluvial deposits—those that had weathered from the lode in the usual way, but had not then been transported by flowing water—were also worked; these tended to be poorer deposits due to the lack of sorting that a stream provides, and they usually did not have such a ready supply of water available to work them. They are found on gently sloping hillsides. Where possible the water needed to work these deposits was carried by a leat from the nearest available river, or if the site was above such supplies, reservoirs were constructed to collect rainwater and runoff from hillsides.Newman 1998, p.20.
A study has shown that a phase of aggradation occurred in the Erme valley between the 4th century and 7th century, providing evidence of late Roman or early post-Roman tin mining activity in this valley.Geoarchaeology, 2004, Vol 19, Issue 3, pp 219-236, Wiley InterScience. The same study confirmed what documentary evidence also relates: that there was a boom in tin production in the 13th century. The earliest written record of tin streaming comes from the 12th century.
Open cast
By the 15th century, the sources of stream tin were becoming scarce and since the demand for tin was as strong as ever, the direct working of lodes was considered viable despite the greater effort needed in both extraction and refining.Newman 1998, p.20-21. "Beamworks" was the name given by the tinners to workings where the lode was followed by digging down from the surface, and the massive gullies that resulted remain prominent features of the moor today. Several of these gullies retain names that include the word beam: Gibby Beam, Willabeam and Scudley Beam, for example. They are also known as "openworks" or "gerts".Newman 1998, p.22.
Another method of surface mining involved digging pits at intervals along the lode – this was known as "lodeback-work". At times these pits were dug in pairs along the back of the lode and the lode followed underground between them.Newman 1998, p.26. This may have been the precursor to beamworks in a number of locations, but enough lodeback pits survive to indicate that it was a separate technique. The issue that mine managers had to decide was whether it was worth removing the extra overburden to form a gulley in order to be able to mine deeper into the lode.
In a similar fashion to streamworking, much use was made of water for the removal of the overburden and gangue. When digging large gullies the overburden, which consisted at least partly of decomposed granite ("growan"), was loosened with picks, and then water was used to wash away the unwanted material instead of manhandling it. Means of collecting, diverting and storing water were always associated with openworks. In some cases when only poor sources of water were available complex systems of reservoirs were built.
Underground
The Dartmoor tin industry declined in importance during the early 18th century and had fallen to nothing by 1730.Worth 1967, p. 287 This was due to a number of factors, but the most important was likely to be the exhaustion of the easily accessible deposits.Newman 1998, p.55. It was not until the Industrial Revolution fuelled demand for all metals and also provided the technology to mine them that mining resumed on the moor on any scale.
Although underground mining technology had been available for many centuries, it is likely that the joint problems of the hardness of the granite rock and the preponderance of underground water together with the relatively easy pickings from near the surface made deep mining unviable until the late 18th century.Newman 1998, p.56.
Many small mining enterprises started up on Dartmoor in the late 18th and 19th centuries – 48 mines are known to have produced some tin during this period.Harris 1972, p. 45. Many of these ventures were unsuccessful, despite being given optimistic names like Wheal Fortune, Wheal Lucky and Wheal Prosper (the common prefix Wheal is a Celtic word meaning "mine" or "works"). Some larger mines, however, such as Eylesbarrow and the Vitifer – Birch Tor complex were productive for many years.
Almost all of the underground mines re-worked lodes that had already been mined from the surface. Because of the great quantity of underground water, it was necessary to dig horizontal adits into the hillsides to de-water the mines. Dartmoor's topography, cut with deep valleys, helped there and in many cases it was possible to dig to a reasonable depth without the need to pump out water.Harris 1972, p.38. These adits connected with shafts that were either sunk vertically downwards or followed the line of the steeply dipping lode. Working the lode then took place by stoping from the horizontal levels in the usual manner.Harris 1972, p. 40–41.
When it became necessary to go deeper than the lowest adit, it was essential to pump out the water. Large waterwheels were used for this and where the shaft was higher than a good water supply, the waterwheel was located lower down the hillside and the power transferred up to the shaft by a "flatrod" system. Evidence for these systems survives as double rows of stones with grooves on their tops – these held the pulleys that guided the metal rods. Such stone rows are still visible at Eylesbarrow and at Hexworthy.Newman 1998, p. 59.
The discovery of extensive tin deposits in Malaya in the later 19th century had a major impact on the Dartmoor industry, and many miners emigrated. The last tin mine on Dartmoor eventually closed just before the Second World War, although Hemerdon Mine, or Hemerdon Ball, was mined for tungsten and tin on a trial basis in the 1980s, leading to the re-opening this mine with first production in 2015.
Processing After collection the tin ore had to be crushed, concentrated and then smelted. Over time a series of ever more sophisticated processes were used for these operations.
Crushing
Early streamworkers operating on a small scale used a block of hard stone as a mortar and perhaps a metalbound piece of wood or a ball of stone as a pestle to break up the ore when necessary,Harris 1972, p.28. but the rich gravels would have required little or no crushing before concentration.Newman 1998, p.40. A later technique called "crazing" employed a pair of circular stones used like millstones, the top one rotating on the fixed lower stone. The coarse gravel or crushed ore was introduced into a hole in the centre of the top stone and was rendered to a fine sand. Only three examples of such crazing mills have been found; at Gobbet mine both stones are still visible.Newman 1998, p.44-45.
As it became necessary to regularly process pieces of ore-bearing rock that were too large to be directly ground in a crazing mill, stamping was introduced. This involved vertical hammers powered by a waterwheel in a Stamp mill, of which at least 60 are known to have existed on Dartmoor. The first documentary evidence for a stamping mill on Dartmoor is dated 1504, though they would almost certainly have been in use earlier than this (the first reference for Cornwall is from 1400, for instance).Newman 1998, p.42. Stamping mills were also known as "knacking" or "knocking" mills; Knacking Mill Gulf, a shallow side valley in the upper reaches of the River Erme, attests to the existence of such a mill there at one time.Worth 1967, p.321.