Ore Processing and Smelting - Crushing Mills

Two types of mechanical crushing mill were known in the Tamar Valley, both dating from the 19th century. The first was the edge-roller method, where a pair of circular granite millstones on edge were rolled around a circular flat base-stone. Edge roller mills were commonly used in agriculture, for rolling grain and crushing apples to make cider. These were large and cumbersome, with a single roller, pulled round in a circle in a deep channel. Powered by horse or ox, they were common in south-west England, the Channel Islands and Brittany from the 17th to 19th centuries. This working example is at the Hamptonne Rural Life Museum in Jersey, Channel Islands.

Industrial edge roller mills were first used in Britain in the 18th century for crushing and amalgamating sulphur, saltpetre and charcoal to make gunpowder. Examples of gunpowder mills which used this method survive at Faversham, Kent and near Postbridge on Dartmoor, but none were located in the Tamar Valley. Instead, three mills are known which ground manganese ore, at Morwellham, Shillamill near Tavistock and Slimeford near Calstock. The ore was carted from mines at Quither, Chillaton and Hogstor, Brentor in Lamerton, Lifton, Milton Abbot and Mary Tavy parishes. Manganese oxide was used as a purple colourant in dye and paint making; it was not until the West Devon sources of it had run out by the 1880s that its properties as a hardening alloy in steel were discovered.

This cross-section of the Morwellham manganese mill was found in IK Brunel's papers in the 1970s; it is assumed that he sketched it in the 1840s or 1850s while involved in railway projects in the Plymouth area. A plan of the Shillamill manganese mill from an 1841 lease is also shown.

The second type of crushing mill originated in West Devon and became the standard method of crushing ore which had previously been broken and sorted on the dressing floor. This was the roller crusher, often referred to as a 'grinder'. Despite being commonly referred to by mining historians as 'Cornish Rolls', the first ever roller crusher was created in 1808 in Devon at Wheal Crowndale copper mine near Tavistock, then managed by John Taylor. His experimental machine was water powered and apparently used two pieces of cast iron rising main (pumping pipes), but chilled cast iron became the standard material, due to the considerable force needed to crush the mineral.

Ore was fed from a hopper between the two rolls, which were geared to turn inwards against each other, crushing the ore. This method was used from the 18th century with granite rolls for crushing apples to make cider, and it is possible that this is where Taylor got the idea. All ores were capable of being put through a roller crusher, but the most common ores treated in this way were copper and lead.

The example seen here is an engineering drawing from the Tavistock Foundries Collection, owned jointly by the Tamar Mining Group and Tavistock Local History Society. It relates to Nicholls & Williams' Foundry in Bannawell Street between 1841 and 1867. The long arm had a weight hung on it, which controlled how firmly the rollers were held together on the slide bars seen in the drawing. If very hard material was being crushed, lighter weights would be used to avoid damaging the rollers.

Many mines in the Tamar Valley are known to have used roller crushers, but few water powered crusher houses have survived regionally, in contrast with steam powered crushers, of which several are known.

This well-preserved roller crusher house at South Bedford Mine, lies on the Devon bank of the Tamar opposite Gunnislake. It was driven by a 24ft diameter by 5ft breast water wheel and was in operation between c.1844 and the late 1860s. It is typical of such crusher houses in being very tall and narrow, as it contained three levels; ore being wheeled into the top floor and dropped into a hopper, feeding the rolls on the middle floor; the crushed material dropping into a 'trommel' or perforated rotating cylinder on the lower floor. The holes in the trommel allowed fully crushed material to pass through to a collecting bin below, while a raff wheel redirected larger material back up to the crusher above. This process is examined in more detail in Rick Stewart's 'Ore Dressing at Devon Great Consols' (see Further Reading).

Steam crusher houses survive well at Gawton Mine, where a pair dating from the 1880s can be found, driven by a rotative engine in a house as seen in this 1960s photograph. A similar arrangement can be found at the dressing floors adjoining Skinners Shaft at Gunnislake Clitters Mine, although only one crusher house was used here.