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 the early post-medieval period, most
substantial deposits susceptible to streamworking had long 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 the lode exposures at the bedrock surface,
often by pits called lode-back pits and sometimes enlarged to form longer
openworks along the lodes. 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 the 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 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, adding a further range of distinctive processing and refining
components to some mines. 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 early 20th century arsenic works at the Devon Great Consols mine
survives very well. Despite some relatively limited areas of collapse and
demolition, most individual structures at the works retain a good
proportion of their original features of which some, including the bottle
furnace, the internal structure of the arsenic mill, and the condenser,
are remarkable survivals. Collectively, however, they have even greater
significance. With such an unusually broad range of processing structures
remaining on a compact site, this remains one of the most complete
examples to survive nationally of an arsenic production works employing
later 19th-early 20th century technology: the most productive period for
arsenic manufacture in England. The survival of the works' physical
remains is considerably enhanced by a wealth of documentary detail,
especially in the foremen's workbooks, bearing on their construction and
the context of their operation.
Details
The monument includes a 1920s arsenic works at Wheal Anna Maria in the
Devon Great Consols mine near Tavistock in west Devon. The works occupies
part of the site of a larger later 19th century arsenic works, mostly
demolished in 1903 but with limited survivals reused in the 1920s works.
The arsenic works in this scheduling is a Listed Building Grade II.
The main processing area of the arsenic works occupies two terraces on a
south easterly slope: the crushing, calcining and refining structures are
arranged along the lower terrace, from which flues rise to a condenser on
the upper terrace. From the condenser a main flue extends over 250m to a
chimney. North west of the condenser, a reservoir, reused from the 19th
century works, fed water to dressing-floor equipment and a water wheel
south east of the works' terraces. The physical remains are complemented
by the mine foremen's workbooks documenting the works' construction,
mostly from late in 1921 to 1923, and the activity around the site until
its closure in 1925.
On the lower terrace, the north eastern structure is a largely intact
bottle furnace, mostly rubble-built with brick-lined openings for its two
shaft furnaces. Flues link the furnace directly with the main flue to the
west. To the south west, two timber beams span the lower terrace with a
third beam supported on a wall to their south west. The beams supported
an ore-crusher, no longer present, powered by a steam engine whose
granite-capped rubble base survives to the south west, with a similar,
more slender base for its boiler. Beyond the boiler base is the arsenic
mill, where arsenic crystals were ground to a fine powder ready to pack in
barrels. The mill reuses an earlier two-floor rubble building, extensively
refurbished in brick to house the mill but now partly collapsed. Its pair
of millstones remain in place on the first floor, with a metal band
binding the upper half-stones and a collecting channel around the lower
stone. To support the millstones, the west of the first floor was
reinforced on a substantial timber frame which survives intact, along with
most floor joists and some floorboarding above it.
South west of the mill, the superstructure of the arsenic refinery has
largely collapsed to give a dome of reddened brick and firebrick rubble
around whose edges are exposures of the intact circular furnace wall with
its double entrance on the south east side. Remains of a slate-built wall
enclosing the refinery may derive from an earlier building: other walls
rebuilt in brick accommodate the refinery flues in the terrace retaining
wall.
Beyond the refinery are two Brunton calciners, in which crushed ore was
roasted on a rotating hearth in a brick-lined chamber on the first floor.
The hearth was heated by fire-boxes and turned by a vertical iron shaft
housed in a power vault beneath. Both calciners are rubble-built, near
square in plan and survive extensively to full height, with brick quoins
and brick-arched openings to the power vaults and stoke-holes. Their power
vaults retain their drive shafts, pivoted on low brick plinths. Their
structural relationships suggest the north eastern calciner was the
earlier built of the two. Beyond the calciners, at the south west of the
lower terrace, is a flat-bed reverberatory furnace, now partly collapsed
with rubble masking most features, but part of its firebrick floor is
still visible with its original flue rising to the condensor, later
blocked and replaced by a secondary flue. The upper terrace is dominated
by the works' original condenser, 28m long, NNE-SSW, by 6m wide overall.
Of slate-rubble with brick quoins, brick-arched doorways and brick
internal walling, the condenser is divided into two rows of vaulted
chambers by a lengthwise internal wall. From that wall, baffle walls
extend to each side, alternating with baffles built from the outer walls.
Arched doorways between each pair of outer-wall baffles, eight along each
side of the condenser, gave access to clean out the deposited arsenic.
Flues from the Brunton calciners and flat-bed furnace rise to two arched
flue entries with iron shutter-frames at the SSW of the condenser; further
entries were later added for the re-aligned flue of the flat-bed furnace
and for the refinery flue. Late in 1922 a second condenser serving the
refinery was built on the upper terrace immediately north of original one.
This condenser was later demolished though traces of its floor do survive.
The main flue runs 250m upslope from the original condenser. Rubble-walled
with a brick vault, the flue interior was 1m wide by about 1.5m high
though much of its vault has collapsed apart from some lengths on the
north. At intervals along the flue's southern and northern thirds, where
it runs partly or wholly above ground, its wall is pierced by doorways for
inspection and cleaning; its central third runs underground allowing no
such access. The flue generally survives well but a portion near the
centre has been destroyed by a modern track. On leaving the condenser the
flue extends 45m to the NNE, then realigns to head almost due north to the
chimney stack. At a point 95m south of the stack the flue passes through a
rectangular chamber, a legally required washing-tower where residual
arsenic was removed before the fumes passed to the chimney. The chimney
survives to almost full height, about 35m, and is rubble-built with a
brick-arched flue entry, an iron shutter-guide and a second opening called
a fingle hole which regulated the draught.
North west of the condenser is a sub-rectangular reservoir in an earth
bank, overall about 60m long, NNE-SSW, by 30m wide. The reservoir fed
water to a large water wheel, no longer present, which powered the works'
arsenic mill and was sited south east of it. The water also served
ore-separating equipment on a dressing floor north of the wheel. Although
this area is now masked by later deposits, the infilled wheel pit will
survive beneath.
This 1920s arsenic works was a late revival of the most productive copper
and arsenic mine in south west England. Discovery of the copper lode in
1844 and its confirmation over 2.5km led to the establishment of five
mines along the lode by 1848, together forming the Devon Great Consols
mine and achieving record outputs of copper ore in the 1850s. Returns from
copper fell rapidly in the 1860s; to offset this and meet growing demand
for arsenic, the company built a large arsenic works south of the central
mine, Wheal Anna Maria, which, by 1869, introduced half the world's
arsenic. However,arsenic prices declined in the 1890s and slumped in 1901:
in 1902 the mine closed and most of its buildings were levelled in 1903.
In 1915, arsenic mining recommenced to meet shortages in World War I but
dressed ore was taken to Cornwall for calcining and refining. After a
slump in 1921, work began on the arsenic works in this scheduling. The
works took ore mined in the west of Devon Great Consols, supplemented by
ore reworked from 19th century dumps. After further price slumps, arsenic
mining had ceased by 1925 but the works continued using dump ore until
final closure in about May 1925.
All modern signs and signposts are excluded from the scheduling, although
the ground beneath them is included.
MAP EXTRACT
The site of the monument is shown on the attached map extract.
It includes a 2 metre boundary around the archaeological features,
considered to be essential for the monument's support and preservation.
