Dartmoor tin mining
Tin is smelted from cassiterite, a mineral found in hydrothermal veins in granite, and the uplands of Dartmoor were a particularly productive area.
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.
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.
[3] 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.
[4] In the southern part of the moor, the cassiterite was usually found in relatively large grains, but the lodes were of very variable quality.
[5] These factors, combined with the fact that none of the underground workings was found to be profitable at depth,[notes 1] are typical of the deepest zone of tin mineralisation.
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.
[14] "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.
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.
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.
Evidence for these systems survives as double rows of stones with grooves on their tops – these held the pulleys that guided the metal rods.
[23] 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.
[30] Improvements to the technology in the 18th and 19th centuries involved increasing the number of stamping heads and replacing the granite mortarstone with a thick bed of crushed quartz contained in a masonry or iron box.
[32] Although individual grains or pebbles of alluvial tin collected by streaming were often of high purity it was usually still necessary to remove the unwanted "gangue" material before the ore could be smelted.
The principle of concentration was a refined version of that used by the early tin-streamers: it depended on the large difference in specific gravity between the wanted tin ore and the gangue.
An early separation method was the rectangular buddle which was simply a slightly inclined plane over which the crushed material was washed by a gentle stream of water.
As there was a gradation in quality from the top to the bottom, the mineworker's skill lay in deciding where to make the dividing line between the wanted material and the waste.
[33] The crushed ore, suspended in water was introduced onto a central cone and spread outwards over a slightly inclined conical surface.
[20] The effects of the large scale of early tin streaming were felt on the coast, as several harbours silted up due to the amount of fine material that was washed down the rivers.
Walkers on the moor will often stumble upon ruined blowing houses and mortar stones, featuring rows of semi-circular depressions, in which the ore was crushed.
The three hares is a circular motif which appears in sacred sites from the Middle and Far East to the churches of south west England (where it is often referred to as the "Tinners’ Rabbits").
In South Western England there are nearly thirty recorded examples of the Three Hares appearing on 'roof bosses' (carved wooden knobs) on the ceilings in medieval churches in Devon, (particularly Dartmoor).
