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The Lead Mines at Charterhouse in the Mendip Hills

Last Updated: 22nd Aug 2016

By Chris Popham

Mining probably began here during the iron age but became an industry within a few years of the arrival of the Romans with archaeological evidence dating their occupation at the site from 49 CE. A number of pigs of lead weighing up to 200lbs each have been recovered from Charterhouse clearly stamped to show their origin and with the names of emperors dating from 49 CE to 200CE. Others similarly stamped to indicate they originated at Charterhouse have been found as far afield as Bath, Bristol and even The Somme in France. Pottery and coins abound and from the latter it is apparent that from around 200CE Roman influence at the site declined but there are still a few coins right up to around 400CE when the Romans finally quit Britain.

The Romans were not only interested in the lead but also the silver that may account for up to 0.4% of some lead ores so they took firm control of the site and built a small fort intending to use the silver to pay soldiers' wages. Britain was one of the prime sources of lead for the Roman Empire, Mendip was one of the 3 main sites in Britain and of all the historical lead mines on Mendip, Charterhouse it seems was the pre-eminent Roman site. They clearly planned to stay and built a small amphitheatre or gymnasium, the only one known to be associated with a mine in the south-west of England. However, the silver yield proved to be low and within a short time mine management had been transferred from the army to independent agents.

The Mendip Hills form a prominent northwest-southeast ridge through the middle of Somerset stretching from beyond Shepton Mallet almost to the fringe of Sailsbury Plain in the east to Weston-Super-Mare on the Bristol Channel coast though in the west the line of hills becomes very broken. The structure is a substantial anticline capped with Carboniferous Limestone, punctured in places by Old Red Sandstones and on the flanks are deep gorges choked with dolomitic limestone conglomerate eroded from above the present day surface.

The mineralization occurs as veins of loose rock, clay and calcite 'spar', with stones, granules and intergrowths of lead ore in fissures that follow the structures in the resistant Carboniferous limestones. This neat packaging makes it possible to excavate the galena, the sulphide ore of lead with great efficiency leaving little in the excavation. The gulleys thus created are locally termed 'rakes' and at Charterhouse these are the Ubley's and Blackmoor Rakes. In addition to the rakes the ground around is also disturbed with exploratory 'costean' pits dug to find the strike of the lodes and numerous rubble tips. Collectively the local term 'gruffy ground' is used to describe the resulting landscape.

To take a tour of the site; from the parking place along the top of the road embankment (NGR ST503554) walk up through the gruffy ground to a prominent rake on the crest of the hill in the south-east corner of the 'liberty' as the mining areas were known. The site is otherwise laid out along a shallow south-west to north-east valley stretching nearly 2km. From here a substantial part of the site can be seen including a row of 3 buddles close under the road embankment. This was where crushed ore was separated from powdered rock using running water. The overall layout of the site is shown in the drawing below.

The rake is several 100m in length, up to 20m in width and comprised a number of intersecting channels of varying widths and up to 5m deep. Notably however the southernmost wall of the rake is sharp and near vertical suggesting a fault plane with the beds in the rake dipping at about 25° south-south-east towards the fault. Some individual blocks of stone show random strike and dip which may be a consequence of natural forces or settling after being undermined, as the blocks must weigh many 10s of tons in some cases. The pale grey 'Black Rock Limestone' contains many crinoid fossils along with solitary Caninia and branching Syringopora corals but no trace of any mineralisation.

Cross over the stile in the wall and walk down to one of the dressing floors and the site of the works where silver was refined from molten lead by the Pattinson process, a form of cupellation. The lead was melted in very large, up to 1m diameter, iron bowls and then allowed to cool very slowly with constant, gentle stirring. In this way the lead solidifies in large crystals that can be scooped from the mass. The mixture is melted again and crystallisation repeated over a number of cycles until the silver content reaches about 2.5%. Zinc is then melted into the mix and the silver dissolves into the zinc phase which floats on the lead and is scooped off. The zinc is then distilled from the silver. The advantage of the Pattinson process over older methods was that in those the lead was exposed to a stream of air causing it to oxidise and form a worthless scum (litharge) that was raked off and discarded. In the Pattinson process oxidation was controlled and restricted to only a small fraction of that which would otherwise occur. Purified lead recovered from the Pattinson process is also considerably more malleable than it would have otherwise been thus making it suitable for sheet work.

From this dressing floor follow the path to the small car park on the edge of the 'watercourse' and walk north-east along the track beside Blackmoor Rake crossing over onto the Avon Group Lower Limestone Shales in the process. This transition is apparent in the water retentiveness of the ground; the areas of the watercourse underlain by Black Rock Limestone are very well drained and do not hold running water in normal conditions, whereas numerous ponds have been created on the poorly draining Avon Group Shales. However, in 1968 there was an extreme rainfall event which produced a torrent of water in the valley sufficient to sweep away the road embankment. More importantly the water also found a way into a shallow depression on the eastern side of the valley bottom just onto the Black Rock Limestone and opened a sink hole. This was excavated in 1981 and found to be Grebe Swallet Mine, extending underground towards Ubley's Rake as far as the Pattinson plant to a depth of 55m. In 2008 a blockage was cleared revealing a further 3.5km of cave passages to a depth of 220m, making it the 4th deepest cave in the England.

Records from the 1700s in particular refer to and in some cases describe a number of huge caverns that were broken in to during mining. But even by the early 1800s when interest was renewed most of these could not be found. It is possibly the case, miners being miners, that these caverns were back filled with waste rock as it is much simpler to dump it underground than haul it to the surface.

The Roman fort and workings are set in the hillside below St Hugh's Church on the north-western side of the site, Ubley's Rake is on the opposing south-east hill and the account of the 1968 flash flood together with the layout of the site perhaps made it an ideal location for the Roman practice of hushing. Water would be held behind a dam until a sufficient head had been accumulated whereupon it was let go as a flood, scouring away topsoil and small rocks and stones revealing lodes in situ and also leaving behind the dense galena stones. Tin miners on Dartmoor were still using this technique in their open cast 'beams' until the 1800s when underground mining via shafts put an end to the practice.

The miners during the Roman occupation and those that followed in the middle ages had worked only close to the surface finding that the lodes pinched out with depth and not having the technology for deep mining. This is somewhat typical of ore deposits produced by metasomatic enrichment in soluble rock such as the carboniferous limestones. As the groundwater rises it dissolves the rock and fans out. In doing so it also alters from acidic to alkaline which precipitates the metal minerals. In contrast veins formed in the granite, typically along faults, tend to remain reasonably uniform and the minerals are deposited along a falling temperature gradient. Pinching out is also seen in mines surrounding the granite masses: Wheal Exmouth in the Teign Valley being a typical example.

In the mid 1600s lead output from the mines was at its peak. Every man was able to stake a claim where he found a new outcrop of ore and despite laws to the contrary even highways were dug away. In many cases the mines followed the ore veins frequently taking advantage of the naturally occurring swallets and pot holes, which made access difficult and dangerous. In places where the ground was harder the miners resorted to fire setting to break up the boulders but this left the mine unworkable for days due to the heat and poor ventilation. Ventilation was as much of a problem as drainage and was achieved by putting up cowls on the surface to direct any breeze into wooden ducts leading it down into the mine.

Water, despite being scarce on the surface where it was in great demand for ore processing, was a considerable problem beneath the surface. but the technology of the 1600s was not up to the task of draining the mines. Unlike many of the mines in Devon and Cornwall which lie along river valleys where drainage via adits can be undertaken, the Mendip Mines lie in the middle of a plateau. Some drainage could be achieved by taking water through adits into the natural swallets but as most of the mines were concerns run by a few freemen working together and because of a peculiarity of the Mendip Mines customs of working which set very clear limits on the size of mines underground, this was often quite impossible.

Unable to reach the ore deeper in the mines the miners stripped away every last vestige of the ore they could reach and by 1700 lead mining had died out on the Mendips.

In the late 1700s plans were drawn up to reopen the mines. Drainage technology had improved with the coming of the age of steam and the Cornish Miners had experience of finding deeper ores where lodes passed from country rock into the granite and this is no doubt why they came to the area in the mid-1700s reviving the mines with deep shafts. However, even though the deepest passed 55 fathoms (roughly equivalent to 100m but only shallow compared to some Devon and Cornwall mines which often passed 600 fathoms or sometimes even 800 fathoms) mineralisation was minimal below 50m and work was abandoned.

Despite the failure underground the spoil heaps from earlier times were found to contain large quantities of galena, up to 55% in places. It seems the early workers had hand sorted out the galena stones taking only the easiest to find ore, leaving the loose grit and any disseminations in harder rocks. Setting up a very large reservoir at Blackmoor (on the site of a reservoir used by the Romans for hushing perhaps?), a network of leats, raised launders, at least one waterwheel, numerous buddles and a tramroad, the miners were able to extract considerable amounts of lead. In addition the previously smelted slag waste was found in some cases to contain up to 20% by weight of lead and a smelter was set up at Blackmoor in the upper end of the valley. It was during this reprocessing stage that much of the Roman archaeological evidence was found and simultaneously lost or destroyed. Many of the Roman lead artefacts including whole pigs of lead were simply melted down.

The first indication of the scale of the operation are the vast dumps of broken, glassy, black smelter slag, not at all unlike obsidian and still in some instances viciously sharp. These dumps, made up not only of many golf ball size pieces but also the same small pieces welded into cubes as much as 1m square, extend for several 100m forming the tiered banks of the reservoir. The blocks must have been formed as the result of the slag pieces settling into the base of the furnace in such a way that they could be drawn out once cooled. Before reprocessing began at Priddy in the 1800s one estimate put the quantity of workable slag there at 400 000 cubic yards which would yield 500 000 tons of lead at £1 per ton. By all accounts a similar amount of material was available at Charterhouse.

A lead smelter was in operation for many years at Weir Quay on the Bere Alston Peninsula in Devon but there are no similar dumps at that site though rarely pieces of brown fused slag are found on the shoreline. This may reflect the fact that at Charterhouse there was a ready supply of the Black Rock Limestone which could be used as a good flux whereas around Bere Alston the country rock is all shales and mudstones and so the 2 sites produced different wastes. The erection of such a large smelter at Charterhouse and others nearby such as St Cuthbert's at Priddy, was probably also only possible because of the local source of coal in Somerset. In Devon and Cornwall it was necessary to import coal by barge around Land's End from South Wales and so it was more economically viable to send the ore to smelters in South Wales. For the last 10 years of the Charterhouse mines that was also how this operated, exporting ore out through a waterway port at Cheddar to a smelter in Bristol. The availability of coal may also explain why there are no large slag heaps at Bere Alston; the smelter there may have been in use for a long time but the throughput may have been limited only to ore from which it was viable to recover the silver (thereby keeping the silver in the hands of the mine owners and not trusting it to the honesty of agents).

Galena is smelted in a 2 step process. Initially it is roasted to produce lead oxides and sulphur dioxide gas. The solid oxides are then roasted with charcoal, coal or coke producing a reducing atmosphere giving lead metal and carbon dioxide. Smelting can be achieved with only low efficiency as a one step process perhaps explaining why the wastes from early smelting operations had a high lead content (as oxide?). These wastes were later re-smelted (for some wastes this was the third time around) in the more efficient 19th century reverberatory furnaces.

In addition to the slag heaps there are the remains of a condensing labyrinth. This is not at all like the arsenic labyrinths seen in Devon and Cornwall where the fumes are made to zig-zag along their course, depositing metal oxides on the brickwork. These are instead linear passages approximately 0.5m wide, just tall enough for an adult to walk in without bending, 100m in length and open at both ends. Lead oxides from the fumes condensed on the walls and were collected for reprocessing to recover the lead metal. This also reduced pollution and contamination of surrounding farmland but still a number of successful compensation cases were brought against lead smelters including at Charterhouse where the mine company was forced to buy up nearby farmland.

Arsenic minerals are not found in the Mendip mines as arsenic mineralisation in the UK is only associated with the granite masses of Devon and Cornwall, in Cumbria and in Derbyshire. Consequently, there is no hint of the hexagonal, platy crystallisations of arsenic oxides on the stonework of the flues, so much a feature of these structures in Devon and Cornwall. There are however white and bright orange 'botryoidal' encrustations on the north facing walls outside the arched over tunnel sections. Modern analysis of some of the Roman lead pigs has shown traces of arsenic, gold, copper and antimony but only traces.
The oxides of lead are often collectively termed litharge after the oxide scum formed during cuppellation. The white oxide (actually very pale yellow when prepared in the laboratory) is the lead II monoxide PbO, an orthorhombic dimorph sometimes called 'massicot. The orange form is either the lead II monoxide tetragonal dimorph for which the term litharge is properly reserved though sometimes also called 'red lead'. Or it could be the tetroxide Pb3O4 (or 2PbO.PbO2) sometimes called 'minium', which also takes a tetragonal form.

Neither oxide is commonly occurring in nature. However, lead II oxide is the intermediary in the production of lead from sulphide bearing ores and lead tetroxide is formed by the calcination of lead II oxide at a temperature around 450°C. Consequently it is not hard to see how both oxides could be present in the labyrinth and how, given that lead oxides are poorly soluble in water, there could be a slow accumulation on the walls by leeching.

Lead oxides are however soluble in stomach hydrochloric acid whence they are highly toxic. The white lead oxide soot from the smelters poisoned a great deal of farmland, many livestock and even people when it was carried away in watercourses so do take appropriate precautions when handling.

Though stable lead compounds typically have poor solubility in water it seems hard to believe that plants could survive at all in this toxic environment. But it is as a direct consequence of the contamination that smelter sites have been colonised by extremely rare, heavy metal tolerant plants, which are additionally protected from grazing, as until recently there was an embargo on producing food for human consumption in areas around the mines and smelters as lead can bioaccumulate in plants.
Finally at Blackmoor there is a ruin of an ore washing building complete with a fragment of brick flooring containing a half round drainage channel.

By the 1870s falling metal values and low returns were taking their toll on mine profits and the venture could only be kept in operation with bail outs from share holders. This was not helped by the fact that Mendip lead is not very malleable and is often termed 'hard lead'. This made it a poor cousin to imported lead though it was popular for bullets and shot. After the revival in the mid 1800s when 300 were employed the mine finally closed in 1885.

The site is more extensive than described here as it stretches about a further kilometre below the road embankment through an area known as Velvet Bottom where another reprocessing smelter was built. Viewed on Google Earth not only can the full extent of the site be taken in but also the Roman settlement can be seen along with the run of 3 buddles by the road embankment and 2 runs of 5 in Velvet Bottom.

Refs.: The Habitat and Origin of Lead Ore in Grebe Swallet Mine, W.I. Stanton, Proc. Univ. Bristol Spelaeol. Soc (1991) Vol.19.1, 43-65
Industrial Monuments in the Mendip etc., Field Guide No4, Neil Cossons, (1967) Bristol Archaeological Research Group.
The Geology of Somerset, P. Hardy, (1999) Ex Libris Press.
The Mines of Mendip, J.W. Gough (1967), David & Charles, Newton Abbot, Devon.
BGS various on-line publications.

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