Okeltor 19th century arsenic, copper and tin mine


Heritage Category: Scheduled Monument

List Entry Number: 1019440

Date first listed: 31-May-2000


Ordnance survey map of Okeltor 19th century arsenic, copper and tin mine
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The building or site itself may lie within the boundary of more than one authority.

District: Cornwall (Unitary Authority)

Parish: Calstock

National Grid Reference: SX 44543 68927


Legacy Record - This information may be included in the List Entry Details.

Reasons for Designation

For several millennia the south west peninsula has been one of the major areas of non-ferrous metal mining in England, its more important and prolific products including copper and tin along with a range of minor metals and other materials, notably arsenic, which occur in the same ore bodies. Before the 16th century, exploitation of this region's non-ferrous metal resources almost exclusively involved tin. Extraction focussed along valley floors and hillslopes on and around the granite uplands of the south west where tin ore had accumulated after natural erosion from the parent lodes. These accumulations were exploited by streamworks, using carefully controlled flows of water to expose and then concentrate the ore, leaving behind distinctively deepened valley floors with various patterns of spoil heaps. By this method Dartmoor became Europe's chief source of tin in the 12th century, with a further peak of production between about 1450 and 1550. By the early post-medieval period, most substantial deposits susceptible to streamworking had been exhausted and exploitation transferred to the mineralised lodes themselves, a change which marks the appearance of copper as an important product of the south western mining industry. The early post-medieval exploitation of the lodes was restricted by the ability to drain the cut, resulting in relatively shallow workings directly into lode exposures at the bedrock surface, often by pits called lode-back pits and sometimes enlarged to form longer openworks along the lodes. Tin ore from later medieval and early post-medieval extraction sites was processed and smelted using water powered mills sited nearby. By the 18th century, ore extraction and processing rapidly expanded to meet growing demands, aided and promoted by technological development. Surface workings became larger and more extensive, and deeper extraction was achieved from shafts, the water pumped from larger mines by early steam engines or drained through near-horizontal tunnels called adits which also served to access the lodes. Horse-powered winding engines lifted ore from the shafts while larger and more efficient water-wheels served ore-processing areas. By such means, west Cornwall became England's main producer of copper ore in the 18th-early 19th century. Intensification accelerated in the late 18th-early 19th century with more efficient steam-powered pumping engines allowing deeper shafts from which extensive underground workings spread out. By the mid-19th century, steam also powered winding and ore-processing operations, the engines, boilers and ancillary machinery housed in distinctive masonry buildings grouped around the main shafts and dressing areas. Later in the century, compressed air was used for underground extraction equipment, fed from steam-powered compressors on the surface. Ore-processing became increasingly mechanised, along with the development of more effective methods of separating and retaining different ores, particularly in the production of arsenic which became a major saleable product in the 19th century, the south west mines forming the world's main producer. Arsenic production added a further range of distinctive ore-processing and refining components, notably calciners of various types and flue systems, to the suite of built structures at some mines. Transport systems serving mines were also revolutionised in the 19th century, with tramways replacing packhorse and cart tracks; later, railways were built specifically to provide fuel and materials to some mining districts and to export their ore. With these advances, east Cornwall and west Devon became one of the world's main sources of copper and arsenic ore until the later 19th century, while in west Cornwall, copper ores became exhausted and replaced as that area's main product by the tin ores present at deeper levels. From the 1860s, the south western mining industries began to decline in competition with cheaper sources of copper and tin ore overseas, especially from South America, leading to a major economic collapse and widespread mine closures in the 1880s, though limited ore extraction and spoil reprocessing continued into the 20th century.

The 19th century arsenic, copper and tin mine at Okeltor survives well with an extensive proportion of its original structures, many retaining an unusually good range of detailed features. The diversity of structures at this mine, pertaining to arsenic, copper and tin ore processing, shows well the integration of varied production methods dictated by the exploitation of such complex ore bodies. Of particular merit, the surviving arsenic processing and refining works at this mine are among the best-preserved of this date nationally, the shaft kiln being an especially rare survival. The structural remains are enhanced by good survival of the mine's overall layout, little modified by later activity. This shows clearly the chronological development across the valley side, the modifications required to the underlying slope, and the very extensive nature of the flue systems which served arsenic refineries at this date. The surviving key elements of the elaborate drainage system for the stamps-complex tailings provide a good example of early measures to control pollution into the river system. The physical remains at this mine are complemented by a wealth of historical documentation articulating the stages and causes of this mine's development and confirming the critical role of the wider economic context in determining the mine's eventual decline and closure.


Legacy Record - This information may be included in the List Entry Details.


The monument includes a 19th century mine producing arsenic and copper with significant quantities of tin and some lead, located across the north bank of a loop in the River Tamar east of Calstock in south east Cornwall. The earliest recorded working at the mine involved an adit dug from near the riverbank in about 1845, producing arsenic and copper ore; probable blocking of this adit occurs by a small quay below the mine's later western area. Renewed activity in 1850 extended the adit and a vertical engine shaft was begun on a midslope terrace about 40m north west of the adit's present blocking. This shaft, the main pumping, winding and access route for most of the mine's productive life, eventually reached a depth of at least 166m. The structures on the shaft's terrace, in the west of the mine's later overall extent, formed the mine's chief focus of activity prior to 1870. In 1853, a pumping engine house was built by the north of the shaft to house a 50ft steam engine; it survives fairly intact except for extensive collapse of its west wall. Its pump rod balance-bob mounting slot survives on the west of the shaft. The boiler house, north of the engine house, and an adjoining coal store east of the engine house, occupy an area deeply levelled into the slope, partly into bedrock, their surviving walling revetting the cut on the north and east sides. The boiler house's west gable is intact, from which its chimney rises to full height. It is constructed of rubble with a brick-built upper quarter. A reservoir to the east of this western building complex provided water for the boilers and ore-dressing equipment around the shaft. The pumping engine house and its boiler house chimney are Listed Buildings Grade II. For much of the 1850s the chief products were arsenic ore with some lead, however cross-cuts from the shaft reached lodes giving large and increasing quantities of copper ore from 1859, stimulating further investment in machinery and buildings which more than doubled the mine's copper production between 1864 and 1870. By about 1864, a cluster of buildings operating east of the engine shaft included a winding engine house, an ore crusher house and a boiler house or roofed dressing floor: partly collapsed rubble walls survive from each of these structures. Between this cluster and the reservoir is the partly walled levelled stance of a jigger house, with a choked wheel pit to its south, where ore from the crusher house was concentrated; the jigger house was rebuilt at least once in the mine's later life. By the mid-1860s the small quay had been quarried out by the original riverside adit to serve some of the mine's supply and export needs. Further structures of this phase appear on contemporary mine drawings. South west from the pumping engine boiler house chimney are the miner's dry and smithy, both surviving in modified forms and both situated immediately beyond this scheduling. South west of the smithy is the count house, a slate-roofed building little altered from its 1860s depiction, which is excluded from the scheduling, although the ground beneath is included. The smithy and count house, which are Listed Buildings Grade II, lie beside the main track to the mine's early structural complex. Between the count house and the track's entrance to the Calstock-Harewood lane, a rectangular wheel pit or saw pit occupies the east of a carpenter's shop built between 1867 and 1875 but which leaves no evident walling: the west of its site is now occupied by a horticultural packing shed which is excluded from the scheduling. The early 1870s saw major growth in the mine's productive capacity, notably in arsenic refining and tin processing. Already by about 1870 the structural complex had expanded eastward, terraced onto lower levels, with the construction of a Brunton calciner in which arsenic ore was roasted on a rotating hearth, the arsenic-laden fumes carried off by exhaust flues for condensing. A second such calciner was built close by in about 1872. Both calciners present extensive remains, including their brick-arched power vaults. East of the calciners and on their alignment, two adjoining two- storied buildings with door and window openings were ancillary to the arsenic refinery, possibly a cooperage and barrel store. South east of these at a lower level are walls of an arsenic grinding mill; drive from that mill's engine may have been transferred west to power an ore-dressing house whose walling is located below the eastern calciner. East of the grinding mill is a shaft kiln where coarse arsenic ore and fuel were burnt in cylindrical brick faced and lined shafts, 1m across internally. Remains of six shafts survive, three to each side of a partly collapsed central brick-arched vault which may have served for ore-drying; exhaust flues converge north from the shaft kiln. Terraced on the slope east of the shaft kiln are walls of three further buildings, the western two are of uncertain function while the eastern shows evidence for housing a reverberatory furnace for refining arsenic soot. Across the south of these buildings a levelled track leads east to a tin stamps complex. By 1872, the ore-heating sites were served by about 450m of arsenic flues which survive mostly intact, but largely roofless, with doorways giving access for removing arsenic soot. Flues linked the ore-heating sites with rectangular labyrinths and condensing chambers upslope of the mine's central building complex; from there, a single flue extends west, passing beneath the Calstock-Harewood lane close to an earlier well, and gradually ascends the slope to end at a chimney, now lacking its upper section, at the west of the scheduling. To its south, the chimney overlooks a small quarry: a possible source for the mine's building stone, the quarry also provided a safe site for the mine's powder magazine whose walls define a small squarish building against the quarry's north face. Contemporary with the expanded arsenic refining facilities, the discovery and extraction of tin ore at the mine required the provision of a stamps and dressing floor complex in 1872-3 at the east of the mine's overall spread of buildings. Central to this complex is the platform, originally smaller than that which survives, on which the stamps were installed. To its west, the stamps engine house has walls surviving almost complete to roof height and retains much fine detail including timber lintels, window frames and glazing bars. Its flywheel loadings and crank pit, and condensor housing also survive almost intact. East of the stamps platform, a large building, 19m by 10m internally, accommodated the engine house boilers with sufficient space also for an ore-dressing plant; its walls and gables survive extensively, with limited collapse over a blocked entrance in its south east corner. Steam reached the engine house via a channel north of the stamps platform; a flue also led north from the boiler house to its surviving rubble-built chimney with a brick upper quarter. Beyond the chimney, a large reservoir, now dry, provided water for the boilers and an ore-dressing in the stamps complex. Across the south of the stamps complex, the surviving tin dressing floor includes brick settling channels and round buddles. Falling ore prices coupled with the cost of installing the stamps complex led to the mine's closure and sale of its machinery in September 1873. Work soon restarted but records indicate the only significant production was of arsenic and its ore between 1876 and 1881. Formation of a new company gave renewed investment from 1881, focussing on two newly discovered lodes: an arsenic/copper lode 43m north of the engine shaft and a tin lode 365m east of the engine shaft. To avoid the ore transport problems that had beset the 1872-3 production, two new shafts were sunk in 1882. The western shaft, directed at the arsenic/copper lode, was sited by the south of the western jigger house, which was replaced by a timber building leaving no known trace. The eastern shaft, easing exploitation of the tin lode, was sunk from low- lying mudflats in the south east of the mine's sett. Within a raised platform about 25m across, the shaft's flooded mouth still contains remnants of timberwork; it eventually reached over 224m deep by 1887, probably using a portable steam engine for pumping and winding this shaft. Ore from the shaft was taken to the stamps complex by tram, crossing the mudflats on a causeway built of mine waste which survives with only minor breaks. The 1882 refurbishment of the stamps complex led to enlargement of the stamps platform to its present visible extent, occupying most of the area between the engine and boiler houses, revetted and retained against the slope by rubble walls. The dressing floor south of the stamps was extended by further buddles and settling channels built on the earlier spoil heaps. To minimise pollution from the dressing floor waste, its tailings were diverted through channels and ponds created in the eastern corner of the site, from which a channel led downslope almost to the river before being directed west behind a levee to allow further settling before final discharge into the river. This increased activity from 1882 saw a peak in arsenic production, enhanced by its processing of arsenical pyrites brought up river by barge from the Danescoombe Valley Mine, west of Calstock. The increased production was facilitated by altering the earlier flue system. The condensing chambers were removed and a new flue was cut roughly parallel with, and north of, the earlier single flue, passing beneath the Calstock-Harewood lane and uniting with the earlier flue about 110m before reaching the western chimney. A new arsenic labyrinth was built between the flues west of the lane. This pattern allowed continuous ore-refining, with one flue route always available while another was closed to remove condensed arsenic from its walls. Although the post-1882 activity brought successful returns in arsenic production, tin and copper production remained poor. Against uncertain market prices for those metals, the mine was wound up in 1885 but work restarted under a new company in 1886, producing 40 tons of tin in 1886-7. However tin prices were falling and the mine's arsenic output was well below that of previous years and also falling. With prospects in decline the mine's final recorded production is in 1887, though its company remained in operation until 1889. Excluded from the scheduling are the count house, the smithy verandah and log store, the modern garage/store, the septic tank and its foul-sewer pipework, inspection chambers and trenches, all existing utilities fittings, cables, pipes and their trenches in current use, the modern shoring and shaft-mouth safety installations, all modern sheds and garden furniture, all modern fences, gates and their fittings, the modern surfaces and metalling of all tracks and footpaths, all modern steps, seats, waymarkers, signposts and notices, the ground beneath all these features is, however, included.

MAP EXTRACT The site of the monument is shown on the attached map extract.


The contents of this record have been generated from a legacy data system.

Legacy System number: 15549

Legacy System: RSM


Arsenic Co25 A-B, Brown, A, MPP Step 4 Report, Non Ferrous Metals Industries, SW peninsula, (1998)
CAU Report to Cornwall County Council, Buck, C, Okel Tor Works Archaeological Assessment, (1999)
Cornwall Arsenic 25 A-B, Cranstone, D, MPP Arsenic Industry Step 3 site assessment for Okel Tor Mine, (1994)
In conversation with MPPA on 9/3/2000, Lake, J, Clarification of List Entry interpretation for Okel Tor Mine, (2000)
List Entry: Okeltor Mine SX 46 NW 4/137 Calstock parish,
Title: 1:10000 Ordnance Survey Map SX 46 NW Source Date: 1982 Author: Publisher: Surveyor:

End of official listing