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Early streamworkers operating on a small scale used a block of hard stone as a mortar and perhaps a metalbound piece of wood or a ball of stone as a pestle to break up the ore when necessary,but the rich gravels would have required little or no crushing before concentration. A later technique called "crazing" employed a pair of circular stones used like millstones, the top one rotating on the fixed lower stone. The coarse gravel or crushed ore was introduced into a hole in the centre of the top stone and was rendered to a fine sand. Only three examples of such crazing mills have been found; at Gobbet mine both stones are still visible.
As it became necessary to regularly process pieces of ore-bearing rock that were too large to be directly ground in a crazing mill, stamping was introduced. This involved vertical hammers powered by a waterwheel in a stamping mill, of which at least 60 are known to have existed on Dartmoor. The first documentary evidence for a stamping mill on Dartmoor is dated 1504, though they would almost certainly have been in use earlier than this (the first reference for Cornwall is from 1400, for instance). Stamping mills were also known as "knacking" or "knocking" mills; Knacking Mill Gulf, a shallow side valley in the upper reaches of the River Erme, attests to the existence of such a mill there at one time.
The hammers or "stamps" in a stamping mill consisted of vertical balks of timber, iron-shod at the bottom, which were lifted by cams attached to the waterwheel drive shaft, and repeatedly dropped onto the ore which lay on a block of granite known as a mortarstone. There were usually two or three of these stamps in a row, powered by the same drive and operating in sequence. At first this process operated on the dry ore, which was shovelled in and removed by hand. However in a 16th century innovation from Europe, the stamp heads were surrounded by a wooden box with a finely perforated grill at one end and the ore was washed into the box by a stream of water which also washed out the crushed ore once it was just fine enough to pass through the grill. This was a vast improvement over dry stamping because it was a continuous process that also stopped the production of unwanted very fine dust.
The characteristic indicator of a former stamping mill is the mortarstone. These are blocks of granite up to a metre long with flat faces bearing two, three or (rarely) four circular or elliptical hollows usually around 17cm in diameter and up to about 10cm deep. Many of these mortarstones have hollows on more than one face, showing that they were turned and reused once the hollows became too deep for effective stamping. Improvements to the technology in the 18th and 19th centuries involved increasing the number of stamping heads and replacing the granite mortarstone with a thick bed of crushed quartz contained in a masonry or iron box.
Although individual grains or pebbles of alluvial tin collected by streaming were often of high purity it was usually still necessary to remove the unwanted "gangue" material before the ore could be smelted. The need for this process, which was known as dressing the ore, increased as the poorer sources of lode tin were exploited. The principle of concentration was a refined version of that used by the early tin-streamers: it depended on the large difference in specific gravity between the wanted tin ore and the gangue. Many different mechanical methods were used, including rectangular and circular buddles, Wilfley tables and revolving slime tables, kieves, trommels, and even magnetic separation.
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