Washing and stamping of tin ores at Polgooth, near St. Austle

—The stamps or pestles are of wood, 6 inches by 5 1 ⁄ 2 in the square: they carry lifting bars b , secured with a wooden wedge and a bolt of iron, and they terminate below in a lump of cast iron A , called the head, which is fastened to them by a tail, and weighs about 2 1 ⁄ 2 cwts. The shank of the pestle is strengthened with iron hoops. A turning-shaft communicates motion to the stamps by cams stuck round its circumference, so arranged that the second falls while the first and third of each set are uplifted. There are 4 cams on one periphery, and the shaft makes 7 turns in the minute. Each stamp, therefore, gives 28 strokes per minute, and falls through a space of 7 1 ⁄ 2 inches. The stamp chest is open behind, so that the ore slips away under the pestles, by its weight, along the inclined plane with the stream of water. The bottom of the troughs consists of stamped ores. With 6 batteries of 6 pestles each, at Poldice, near Redruth, 120 bags of ore are stamped in 12 hours; each bag containing 18 gallons of 282 cubic inches; measuring altogether 352 cubic feet, and 864 cubic inches. IMG:4147767755307473660_illo1245.png:Mining apparatus The openings in the front sides of the troughs are nearly 8 inches by 7 1 ⁄ 2 : they are fitted with an iron frame, which is closed with sheet iron, pierced with about 160 holes in the square inch, bored conically, being narrower within. The ore, on i uing, deposits its rough in the first basin, and its slimes in the following basins. The rough is washed in buddles (see Lead , page 751 ), and in to ing tubs ; the slimes in trunks , and upon a kind of twin tables, called racks . Into the to ing-tub , or dolly , fig. 1143. , the stamped ore is thrown, along with a certain quantity of water, and a workman stirs it about with an iron shovel for three or four minutes. He then removes a little of the water with a handled pitcher, and strikes the sides of the tub for 8 or 10 minutes with a hammer, which hastens the subsidence of the denser parts. The water is next poured off by inclining the tub to one side. In one operation of this kind, four distinct strata of the ores may be procured, as indicated by the lines a b , c d , e f g , h i k , in the figure. The portion A is to be washed again in the trunking-box , figs. 1144 , 1145. ; B is to be washed upon the German chests or racks, fig. 1146. ; C , the most considerable, is put aside, as schlich fit for the market; D , forming a nucleus the centre of the tub, is to be pa ed through sieves of copper wire, having 18 meshes in the square inch. This product thus affords a portion D ′, which pa es through the sieve, and D ′′ which remains upon it; the latter is sometimes thrown away, and at others is subjected to the operation called the tie , viz., a washing upon the sloping bottom of a long trough. The slimes are freed from the lighter mud in the trunking-box, figs. 1144 , 1145. ; which is from 7 to 8 feet long. Being accumulated at M , the workman pushes them back with a shovel from a towards b . The metallic portion is carried off, and deposited by the stream of water upon the table; but the earthy matters are floated along into a basin beyond it. The product collected in the chest is divided into two portions; the one of which is washed once, and the other twice, upon the rack , fig. 1146. This is composed of a frame C , which carries a sloping board or table, susceptible of turning round to the right or left upon two pivots, K , K . The head of the table is the inclined plane T . A small board P , which is attached by a band of leather L , forms the communication with the lower table C , whose slope is generally 5 inches in its whole length of 9 feet; but this may vary with the nature of the ore, being somewhat le when it is finely pulverized. The ore is thrown upon T , in small portions of 20 or 25 lbs. A woman spreads it with a rake, while a stream of water sweeps a part of it upon the table, where it gets washed. The fine mud falls through a cro slit near the lower end, into a basin B . After working for a few minutes, should the schlich seem tolerably rich, the operative turns the table round its axis K , K , so as to tumble it into the boxes below. The mud is in B ; an impure schlich in B ′, which must be washed again upon the rack ; and a schlich fit for roasting in B ′′. The slope of the rack-table for washing the roasted tin ore, is 7 3 ⁄ 4 inches in the 9 feet. IMG:4147767755307473660_illo1246a.png:Crushers