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Castell Mine was originally developed around 1785 by Thomas Bonsall for mining blende or zinc ore. The surface workings occupied two sites, the main one immediately south west of the junction between the A44 and the minor road to Devil's Bridge, and a subsidiary shaft, known as East Castell mine, sunk in 1874, some 0.6km (0.4 mile) to the east. The complex has a chequered history of closure and reopening, its longest operating period being from 1898 to 1908. The mine was only 49m (27 fathoms) deep and exploited a lode or vein extending to the Rheidol valley west of Aberffrwd; the principal product was blende of which over 5000 tonnes were mined with about 140 tonnes of galena or lead ore and 25t of copper.

The most prominent structure still visible at the main site is the shell of the crusher house, dating from 1898, with an adjacent waterwheel pit 13m x 1.5m (43ft x 5ft). To the south east are open-cut workings and nearby is the Engine Shaft, now covered with a steel grill. Remains at the east site consist of a capped shaft, a waterwheel pit 10.6m x 1.5m (35ft x 5ft), and a building now adapted for agricultural use.

Waterwheels were employed at the mine for driving crushing machinery, for pumping excess water from the underground workings, and possibly for winding in the shafts. The practice was widespread in the Ceredigion metal mines at the time and the techniques well-established as water was cheap and plentiful whereas coal for steam power was not. Water was also needed for washing the fine stone waste from the metal, and was brought to the site by means of leats. These are open ditches carrying water from a weir across a stream or river and are most carefully designed and constructed on a very slight gradient to maintain a constant flow. Some leats in the area are of a considerable length, up to 30km (19 miles) long, and represent an enormous engineering achievement, particularly in such mountainous terrain. There are four leats at Castell of a more typical length, one nearly 0.75km (0.5 mile) long and running from a purpose-built reservoir to the south and, unusually, three closely-spaced and parallel channels 2.5km (1.5 miles) long from weirs on the Afon Castell to the northeast, each representing different periods in the mine's development.

As the crusher house is such a conspicuous landmark, it is perhaps worth explaining its function in more detail. Once common at metal mines, no complete examples are now left in situ in Britain. The building contained machinery for crushing the mined ore so enabling the metal and waste stone to be separated, a technique is known as dressing which was carried out manually with hammers until the end of the nineteenth century. In Wales water-powered stamps performing the same task were developed by the mid-seventeenth century and by the 1830s crushing rolls were introduced. The machinery comprised two cast-iron rollers, side by side, one driven directly by a waterwheel (or steam engine in some areas) and the other mounted on sliding bearings to allow sideways movement. The rolls were kept pressed together by a weight suspended from a lever attached to the sliding roller, and as they revolved, lumps of ore up to 40mm (1.5") in size were fed between the rollers from above and the crushed material collected underneath. Design of the machines was improved continually, for example, rubber then metal springs were used to regulate more closely the separation of the rolls and geared rolls allowing higher speeds were developed. It is most likely that machinery of this sort was installed at Castell, as Green's foundry of Aberystwyth were supplying 'off-the-shelf' crushing-roll sets by the 1880s. A set of Cornish crushing rolls are displayed at the nearby Llywernog Silver-lead Mining Museum, which is well worth a visit to see the kind of equipment that would have been in use at Castell.

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