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Okeltor 19th century arsenic, copper and tin mine

A Scheduled Monument in Calstock, Cornwall

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Latitude: 50.4993 / 50°29'57"N

Longitude: -4.1928 / 4°11'34"W

OS Eastings: 244579.003446

OS Northings: 68909.157312

OS Grid: SX445689

Mapcode National: GBR NT.KWC2

Mapcode Global: FRA 273Q.VSF

Entry Name: Okeltor 19th century arsenic, copper and tin mine

Scheduled Date: 31 May 2000

Source: Historic England

Source ID: 1019440

English Heritage Legacy ID: 15549

County: Cornwall

Civil Parish: Calstock

Traditional County: Cornwall

Lieutenancy Area (Ceremonial County): Cornwall

Church of England Parish: Calstock

Church of England Diocese: Truro


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
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.

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

Source: Historic England

Reasons for Scheduling

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.

Source: Historic England


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

Source: Historic England

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