96 



DRESSING OF ORES 



STAMPS. 



Tin and somo other of the more valuable ores are usually associated with and 

 minutely disseminated in a hard crystalline gangue, requiring to bo reduced to a fine 

 powder before the valuable portions can bo extracted. 



Various contrivances have been employed for this purpose, but none of them seem to 

 have entered into successful competition with the stamping mill. This apparatus essen- 

 tially consists of a number of cast-iron pestles, each measuring about 20 inches high, 

 and 6 by 10 inches in cross section. These are secured either to a wrought-iron or 

 wooden lifter; a projecting arm is placed towards the top on each lifter, which may be 

 slidden up and down so as to meet the wear of tho pestle or any other irregularity. 

 These lifters are retained in their vertical position by suitable metal or wooden supports. 

 Motion is communicated by a revolving shaft in front, fitted with four or five projecting 

 cams, each of which takes the arm, and lifting tho pestle from 8 to 10 inches, lets 

 it suddenly fall on the substances which may be underneath. The bottom on which 

 the heads fall is formed by introducing hard stones or other suitable material, 

 and pounding it until it becomes sufficiently solid. In most parts of tho Continent of 

 Europe, on the contrary, stamping mills are provided with solid cast-iron bottoms ; 

 these, however, require frequent renewal. 



Around the pestles a wooden box or coffer is constructed, and covered in at the top ; 

 the back is partly open at the bottom in order to admit the vein-stuff. On each side 

 one, and in front two openings are made, 7 or 8 inches square, which are fitted with 

 wrought-iron frames, for the reception of perforated iron, copper, or brass plates, the 

 bur of the punch or drill being towards the inside. As a precaution against the speedy 

 destruction of the coffer from the constant scattering of fragments of stone, the inside 

 is partially lined with sheet iron. The stuff to be stamped is supplied on an inclined 

 plane, connected with a hopper at the back, in the front of which is a launder for 

 affording a stream of water to the coffer. The stamped stuff passes through tho grates 

 into launders, and is thus directed to the floors. When water is the motive power, the 

 number of heads is limited by the volume and fall of water available ; three heads 

 are the least number used, but a larger number is generally preferred. When steam- 

 power is employed, a battery of heads sometimes includes 100 or more pestles. When 

 in action these are elevated from 40 to 80 times per minute, according to the cha- 

 racter of the stuff to be reduced. The pulverisation is said to be greatly facilitated 

 by having four heads in the same chest or coffer, about 2 inches apart. Each head 

 is lifted separately, and the cams by which this is done are so disposed on the axle as 

 to make the blows in regular succession. Great care is also taken whether it bo in 

 a largo or small battery, to prevent any two pestles falling at the same instant ; 

 the object being to secure an equal load against the power. Practical dressers 

 are not well decided as to the order in which the lifting of four heads in one coffer 

 should take place, whether one of the inner pestles should precede the other, or 



whether a side pestlo should be first lifted. A preference, however, seems to be 

 given to tho following method : supposing a spectator to stand in front of a 4-hoad 

 stamps, left side pestle first, right side second, right middle third, left middle last. 



