DRESSING.] MINING 463 to be rendered fit for sale, at other times the washing is necessary as a preliminary process previous to sorting by hand. The operation is performed either by raking the ore backwards and forwards upon a grating under a stream of water, or in a box containing water, or, thirdly, by means of an inclined revolving iron drum worked by hand or any other motive power. The machines used for this purpose, known as washing trommels, are revolving cylinders or truncated cones of sheet-iron provided with teeth inside. The ore is fed in at one end, is subjected to the action of a stream of water, and is discharged at the other end. The stuff, i.e., the mixed ore, veinstone, and country rock, having been cleansed, it is now possible to make a separation by hand. Women and children are generally employed for this work, as their labour is cheaper and their sight sharper than that of men. The stuff is spread out on a table, and various classes are picked out according to the nature of the products furnished by the mine. Thus in a lead mine we may have (a) clean galena, (6) mixed ore, i.e., pieces consisting partly of galena and partly of barren veinstone, (c) barren veinstone and country rock. This is a most simple case ; very frequently we have to deal with a vein producing ores of two metals, especially in the case of lead and zinc, and then the classification into various qualities becomes more com plicated. (2) Reduction in size is necessary for two reasons. Even when an ore is sufficiently clean for the smelter, the large lumps are often crushed by the miner for the sake of obtaining a fair sample of the whole, or supplying a product which is at once fit for the furnace. The chief reason, however, for disintegration lies in the fact that the particles of ore are generally found enclosed in or adhering to particles of barren veinstone. The disintegration is effected by hand or by machinery. Large blocks of ore and veinstone are broken by men with large sledge hammers, and the reduction in size is continued very often by women with smaller hammers. Sometimes the blow of the hammer is directed so as to separate the good from the poorer parts, and hand-picking accompanies this process, called cobbing. Ore may be crushed fine by a flat-headed hammer (bucking iron) on an iron plate. The machines used for reducing ores to smaller sizes are very numerous ; here it is impossible to do more than briefly call attention to those most commonly used. These are stone-breakers, stamps, rolls, mills, and centrifugal pulverizers. 3 oue- The stone-breaker, or rock-breaker, is a machine with
- >reak?r. two jaws, one of which is made to approach the other, and
FIG. 98. Blake s Stonebreaker, improved by Marsden. so crack any stone which lies between them. The best- known stone-breaker is the machine invented by Blake, which has rendered inestimable services to the miner for the last twenty years, and the introduction of which con stituted a most important step in advance in the art of ore-dressing. Its mode of action is very simple. When the shaft A (fig. 98) revolves, an excentric raises the " pitman " B, and this, by means of the toggle-plates C, C, causes the movable jaw D to approach the fixed jaw E by about inch at the bottom. When the pitman descends the jaw is drawn back by an india-rubber spring. The jaws are usually fluted, the ridges of one jaw being opposite the grooves of the other, and they are so con structed that the wearing parts are quickly and easily replaced. Mr Marsden of Leeds has lately introduced a pulverizer, constructed on the principle of the stone-breaker, which will reduce large stones to the finest powder in one opera tion. The moving jaw has an up-and-down as well as the old backwards-and-forwards motion, and the stones are first cracked and then ground by the double action. Stamps are pestles and mortars worked by machinery, stamps The construction of the modern California stamp mill with revolving heads is explained in GOLD, vol. x. p. 747, and the description need not be repeated. In Cornwall the older form with rectangular heads still prevails. It is impossible to give any correct average figures representing .the work done by a stamping mill, because this varies with the hardness of the stuff treated and the fineness to which it must be reduced. However, it is usual in Cornwall to reckon 1 ton of tinstuff and in California 1 to 1 ton of gold quartz stamped per horse-power in twenty-four hours. Stamps are principally used in dressing the ores of gold, silver, and tin, but are occasionally employed for those of copper and lead. The stamps described at vol. x. p. 747 act simply by gravity. Another form, which has met with favour in the Lake Superior district, is the direct- acting or Ball stamp, which works like a steam hammer, the blow of the head being assisted by the pressure of steam. At the Calumet and Hecla Mill, Lake Superior, each Ball stamp is capable of crushing 130 tons in twenty- four hours. In a third kind of stamps, the heads are lifted by a crank and the power of the up-stroke compresses a cushion of air (pneumatic stamps) or a spring, storing up power which makes the down-stroke strike a heavier blow. Revolving rolls were introduced in the west of England Rolls. in the early part of the present century to replace bucking by hand. The machine, now often known as the Cornish crusher, consists of two cast-iron or steel cylinders which revolve towards each other, whilst at the same time they are kept pressed together by levers or springs. The cylinders or rolls are generally from 18 inches to 2 feet or 2 feet 8 inches in diameter and 12 to 22 inches wide. Stone mills constructed like flour mills are employed in Mills. some countries for reducing ores to powder; and the arrastra, which consists of heavy stones dragged round upon a stone bed, has rendered good service in grinding and amalgamating gold and silver ores, in spite of its being slow and cumbersome. Edge-runners (Chilian mills) also deserve mention. Iron mills, known as pans, with grinding surfaces made of chilled cast-iron and arranged so that they can be quickly and easily replaced when worn out, are greatly in vogue in the United States for the treatment of ores of gold and silver ; the ore delivered to them is already finely divided, and they are intended, not only still further to reduce the size of the particles, but also and more especially to effect the amalgamation of the precious metals with quicksilver. The pulverizers used in Cornwall for grinding grains of tin ore with a little waste still adhering to them are also iron mills.
The centrifugal pulverizers are machines by which the