There is a remarkable survival of the field evidence for mining technology from that period. Providing a unique physical link with advances in drainage techniques in the mid 15th century.
Silver bearing ores have been worked on the Bere Ferrers or Birland peninsula, between the Tamar and Tavy rivers south of Tavistock, for nearly six centuries. By the middle of the 19th century there were at least a dozen mines, some consolidated into larger setts. The majority of which worked silver-lead deposits found in north-south crosscourses.1 Two of those crosscourses, at South Hooe (Tamar Silver-Lead) and Cleave (South Tamar Consols), proved particularly rich whilst another to the east, north of Lopwell, was of only marginal economic importance. However, only the deposits on the crosscourse north from Cleave to Goldstreet were known to the medieval miner and worked extensively from 1292 to the end of the 15th century. By the early part of the 14th century there were four mines at work, South, Middle, Fershull and the Old Mine.
Working of silver-bearing ores in the medieval period was in the hands of the Crown which held the prerogative on all precious metals plus copper and tin. Initially the mines at Bere Ferrers were worked directly by the Crown but after 1350 they were granted on lease to outside interests, with the Crown retaining a royalty and right of pre-emption on the produce.
The miners themselves held none of the customary rights enjoyed by the tinners of Devon and Cornwall or lead miners on Mendip and elsewhere. If necessary they were pressed into service and worked under the direct control of Crown officers or the lessee. An initial lack of adequate local expertise in mining and smelting meant a workforce from areas as far as North Wales, the Derbyshire Peak, Mendip, the Forest of Dean, and Cornwall was assembled in south Devon, bringing with them techniques suited to less complex ore deposits. It was only with experimentation, particularly in smelting, that processes adequate to the silver bearing ores were developed.
Known workable deposits of silver-bearing ores were a limited resource and the Devon mines, at Bere Ferrers and Combe Martin, dominated 'English' production from the late 13th century until new mines were opened up in mid Wales in the late 16th century.2 Output peaked in the early years, 1297 and 1306, but even then only at a little over 23,000 ozs per annum. Average production was around a tenth of that figure. Nevertheless, the demand for silver was such that the mines remained at the forefront of technological advances throughout the medieval period.
Within a few years of being opened the Bere Ferrers mines were beset with drainage problems which restricted ore extraction in the winter months. Galleries (adits) were soon introduced to allow the free drainage of water to surface. Despite neglect during the tenure of the Friscobaldi at the turn of the 13th century, subsequent capital work programmes ensured that free drainage was available as the mines worked deeper. This gradually reduced the reliance on manual water haulage using ropes and leather buckets.
By the mid 15th century the workings were so deep that drainage was once again a problem. In an area where few deep valleys cut the productive deposits, deeper free drainage could only be achieved by the use of long crosscutting adits. A problem aggravated by the increased cost and scarcity of labour brought about by a prolonged period of population decline. In the 1470's, with interest apparently centred on a possible source of rich ore north of Lockridge Hill, mechanisation using water powered suction lift pumps was introduced in an attempt to overcome the problems of drainage. That is only 50 years after suction lift pumps were first attested to in Italian documents and only two decades after their first use in the mines of central Europe.3
With no system of power transmission available it was necessary to bring the power source, water, into the mine at the shaft head. That entailed a leat, or watercourse, 16 kilometres long tapping tributaries of the River Lumburn at Millhill west of Tavistock and careful surveying was required to bring the water over a shallow saddle in the ridge, between the Tavy and Tamar rivers, near Higher Gawton. The line of that leat can be traced over much of its route particularly on the steep west bank of the Tavy where it has been cut through solid rock. A unique physical link with the introduction of innovative technology in the late medieval period.
The preparation, smelting and refining required to process the silver-bearing ores were an integrated part of Crown operations at Bere Ferrers. During the first few years of operations in the late 13th century production was based on the wood fired, wind blown 'bole' smelting technique brought from the lead mining areas of northern England. However the bole was not capable of efficiently smelting all the ore mined and was soon augmented by charcoal fired, bellows blown furnaces developed through a period of experimentation. Transport by horse and by river played an important part in the processing operations. Ore was moved from the mines to washing sites where waste was removed by simple gravitational separation. From there it was taken to one of a number of smelting sites. Bole smelting sites were located as far afield as Milton to the east of the Tavy. Residues from smelting were removed for crushing and washing to separate the waste. Those parts still rich in lead and silver were then re-smelted in the furnace.
Until 1301 the lead produced was refined at a mill near Martinstowe (Maristow), on the Tavy estuary.4 Refining, furnace smelting and most of the bole smelting operations were then moved to Calstock, on the Cornish bank of the Tamar. Remaining there until circa 1318 when they were returned to Martinstowe. The smelting / refining operations consumed large amounts of fuel drawn from woods at Warleigh, Bickham, Halsere (in Bere Ferrers) and Morwellham. When the mines were granted wood from the manor of Calstock it was easier to move the ore to the fuel and centre activity around the church there, although some smelting was carried out in outlying parts of the manor as far north as Greenscombe.
Working during the early modern period appears to have been concentrated at Buttspill, on the crosscourse north of the medieval workings, where a 'silver mine' was active in the 1690s. However, much of the ground worked during the medieval period was re-examined during the late 18th and 19th centuries when, with the advent of powerful steam pumping engines, workers were able to get to new ground below the medieval workings. A testament to the drainage skills of the earlier miners. As a consequence later activity was concentrated on certain deep shafts leaving much of the old surface workings untouched.
Walking north from Weirquay along the public footpath, which follows the line of the crosscourse, there is ample evidence of shaft mounds marking the early workings. Adits which once drained the medieval workings are now largely covered over but in the woodland to the north of Whitsam Down is the lobby of a shallow adit probably dating from that period. The best surviving evidence of medieval activity is to be found well away from the mines themselves in Shillamill and Blackmoorham woods, where cuttings and tunnels mark the course of the mid 15th century leat.5