Cornish stamps

A stamp mill (or stamp battery or stamping mill) is a type of mill machine that crushes material by pounding rather than grinding, either for further processing or for extraction of metallic ores. Breaking material down is a type of unit operation.

A stamp mill consists of a set of heavy steel (iron-shod wood in some cases) stamps, loosely held vertically in a frame, in which the stamps can slide up and down. They are lifted by cams on a horizontal rotating shaft. As the cam moves from under the stamp, the stamp falls onto the ore below, crushing the rock, and the lifting process is repeated at the next pass of the cam.

Each one frame and stamp set is sometimes called a "battery" or, confusingly, a "stamp" and mills are sometimes categorised by how many stamps they have, i.e. a "10 stamp mill" has 10 sets. They usually are arranged linearly, but when a mill is enlarged, a new line of them may be constructed rather than extending the line. Abandoned mill sites (as documented by industrial archaeologists) will usually have linear rows of foundation sets as their most prominent visible feature as the overall apparatus can exceed 20 feet in height, requiring large foundations. Stamps are usually arranged in sets of five.

Some ore processing applications used large quantities of water so some stamp mills are located near natural or artificial bodies of water. For example, the Redridge Steel Dam was built to supply stamp mills with process water.

The main components for water-powered stamp mills – water wheels, cams, and hammers – were already known by the Greeks in Hellenistic times.Ancient cams are in evidence in early water-powered automata from the third century BC. A passage in the Natural History of the Roman scholar Pliny (NH 18.23) indicates that water-driven pestles had become fairly widespread in Italy by the first century AD: "The greater part of Italy uses an unshod pestle and also wheels which water turns as it flows past, and a trip-hammer [mola]". These trip-hammers were used for the pounding and hulling of grain. Grain-pounders with pestles, as well as ordinary watermills, are also attested as late as the middle of the fifth century in a monastery founded by Romanus of Condat in the remote Jura region, indicating that the knowledge of trip hammers continued into the early Middle Ages. Apart from agricultural processing, archaeological evidence also strongly suggests the existence of trip hammers in Roman metal working. In Ickham in Kent, a large metal hammer-head with mechanical deformations was excavated in an area where several Roman water-mills and metal waste dumps have also been traced.

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