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 17 cm in diameter and up to about 10 cm 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. By the 1900s there were several different types of stamps in use: Cornish, Californian, Holman's Pneumatic, etc. Concentration 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. An early separation method was the rectangular buddle which was simply a slightly inclined plane over which the crushed material was washed by a gentle stream of water. The heavier ore would settle near the top of the incline while the lighter material would settle lower down or be washed off the buddle altogether. As there was a gradation in quality from the top to the bottom, the mineworker's skill lay in deciding where to make the dividing line between the wanted material and the waste. The division was often made in three parts: the "heads", which represented the best quality ore; the "middles", which was often reprocessed; and the "tailings", which was dumped. From about 1848 a circular form of the buddle was introduced. The crushed ore, suspended in water was introduced onto a central cone and spread outwards over a slightly inclined conical surface. The "heads" would settle near the centre