Ore Processing and Smelting - Arsenic Refineries

By far the largest and most prolific types of ore processing works found in the Tamar Valley were those which produced arsenic oxide.

These crushed and burnt arsenical pyrites and collected the 'soot' resulting from channelling the fumes up a complex series of flues. This 'soot' consisted of arsenic oxide and could be further refined by re-roasting in a bottle furnace. The resulting white powder was 99.5% pure arsenic, which was used as a fixer in dyes and paints, as an alloying agent in bronze, and later as a pesticide. In the latter half of the 19th century and the first quarter of the 20th, Devon and Cornwall produced most of the world's supply of arsenic, which had to be regulated to avoid flooding the market.

Several refineries existed in the Tamar Valley, but by far the largest and most complex were those at Devon Great Consols Mine, operating between c.1865 and 1930, and at Greenhill near Chilsworthy, from 1879 to 1930. Others were located at Rumleigh (1885-c.1890) and Gawton (1887-1903) near Bere Alston, New Consols at Luckett (1860s-1877), Okel Tor near Calstock (1883-87) and Wheal Friendship near Mary Tavy (1880s-1925).

The arsenic pyrites was crushed, usually with a roller-crusher to a fine sand. This was trammed or barrowed to a Calciner, where it was roasted. Two different types were used in the Tamar Valley: the Oxland Cylinder, and the Brunton Calciner.

The Oxland Cylinder was, as its name suggests, a long iron tube, set at about 10 degrees from horizontal, and made to revolve slowly in a brick-lined chamber. A fire in a grate at one end produced hot fumes which were channeled into the chamber, passing along the tube. The arsenic pyrites vapourised and was carried away up flues, while the waste rock and other minerals fell out of the lower end onto the floor below.

No complete Oxland Cylinders survive and their houses seem to have been rather lightweight structures, but Brunton calciners are more common, as are their square, solidly built houses, which often contain partially complete mechanisms, as here at Devon Great Consols, dating from the 1920s. (below)

This better-preserved example (above) is in Cornwall, in Tucking Mill Valley near Camborne, and is included to show what a complete one looks like. The ore was dropped onto a revolving metal plate in a brick-lined chamber, while fumes from a coal fire in an adjoining grate were directed around it. The slowly revolving plate could be driven by a water wheel or a steam engine - early 20th century examples were sometimes driven by oil engines or even electricity.

This photograph (left) shows the vertical driveshaft of the eastern calciner of the 1920s group at Devon Great Consols. It passes into the combustion chamber via a hole in the brick arched roof of the drive chamber beneath. A more complete version can be seen (right) at the Tucking Mill Valley calciner.

The arsenic ore went directly to gaseous form in the calciner, not assuming a metal form on the way. It was directed into a set of flues, which led to a labyrinth: a flue which ran in a zig-zag manner back and forth in a long low building.

This example at Devon Great Consols dates from the 1920s, but earlier examples are of the same basic design, such as these at Wheal Friendship, Mary Tavy, of the 1880s. In the labyrinth the fumes were cooled and the arsenic oxidised onto the walls in the form of a 'soot'. This was scraped off at the completion of a burn, by heavily muffled and suited men with brushes and shovels.

It was then re-roasted in a bottle-shaped kiln or rectangular reverberatory furnace, where the fumes from the fuel did not come directly into contact with (and therefore contaminate) the arsenic oxide. The oxide vapourised again and was led into a second labyrinth, where it cooled again and crystallised out on the walls as virtually pure white arsenic oxide. This was again scraped out by men, who accessed the flues via doors in their sides.

The still-poisonous fumes were led away uphill via long vaulted flues, with occasional smaller labyrinths or expansion chambers. If a water supply was available, a waterfall chamber might be placed across the flue. Typically, this would form an 8-10ft step in the flue; the gases entering at the bottom and leaving at the top, having to pass through tightly-packed bundles of brushwood, through which a constant stream of water was spraying.

This 'scrubber' would wash out much of the poisonous element of the smoke. The final part of the system consisted of a tall chimney stack, designed to discharge the smoke into the atmosphere away from people who might inhale it. Even this did not remove acid vapours and many trees died; the valley's vegetation being stunted well into the 20th century. A postcard of Gawton Mine in about 1900 shows many dead trees around the terminal chimney stack.

The arsenic soot was ground between millstones, identical to those used in flour mills. These photographs (right) are of the 1920s grinding mill at Devon Great Consols mine, while (below) at New Consols, near Luckett in East Cornwall, this still roofed example survives, the small vertical boiler which supplied a steam engine for grinding the arsenic still standing in one corner. Astonishingly, this building erected in the 1870s has no statutory protection, despite being the best-preserved arsenic grinding house in Britain.

The layout of arsenic processing works varied. The 1860s-1900s Devon Great Consols works was essentially a rectangle with a triangular projection off its uphill side; a line of calciners feeding into a long parallel block of labyrinths of similar length. The flues converging on the stack about 250m to the north-west formed the triangle. This site is best understood from 19th century maps and plans, as it was comprehensively destroyed in 1903, only slight traces surviving today.

A similar layout seems to have been developed at New Consols between 1870 and 1877, where eight well-preserved Brunton calciners, with occasional iron fittings such as furnace doors and a draught control lever, survive in scrub woodland. The associated labyrinths have vanished in dense gorse thickets, from which the final flues emerge, converging on one of two sequential stacks, both of which survive well. The grinding house described above lies near the lower stack.

This plan of the small Rumleigh Arsenic Works, operated by Thomas Westlake & Co, who ran the adjoining brickworks, was made in 1885 to record the company's illegal encroachments onto adjoining land. It is interesting as it shows how an arsenic works could be made to fit a small site - in this case by running the sequence of flues round four sides of a square. The circular feature is the Hoffman Kiln belonging to the adjoining brickworks - (see Brick and Tile Making page)

For further details of the history and operation of these refineries, I would recommend reading Rick Stewart's 'Ore Dressing at Devon Great Consols' (see Further Reading) and Richard de Nul's Greenhill Arsenic Works which is available on CD

As noted above, the Devon Great Consols arsenic works was the most impressive, and its vast extent is shown well in this extract from the 1867 Bedford Estate Map.

A smaller works at Rumleigh, adjoining Gawton Quay, built in 1885, is shown on the above plan in the author's possession, made to record its construction without permission by Thomas Westlake on land leased primarily as a brickworks!
The works was comprehensively destroyed on the orders of the Bedford Estate after the mine closed in 1903: only traces of the flue systems (with bricks by Calstock and Bealswood Brickworks, and Martin of Lee Moor on Dartmoor), some brick yard-surfacings (by Westlake of Greenhill Brickworks) and occasional fragments of walling, surviving.
This seems rather short-sighted, as within 20 years, the Estate had itself recommenced arsenic processing on the site, and had to build a completely new set of calciners and flues! It is this 1920s works which now survives on the site, and has recently been conserved.