62 



METALLURGY. 



Metal!ur- also necessary, that the stamping be not clone too rapid- 

 ly, otherwise the stampers are apt to throw up pieces 

 ~^ T ~~ of the ore, not sufficiently pulverised, which flow out 

 with the fine powder, and prove detrimental in the sub- 

 sequent part of the process. The stamping should 

 therefore be performed slowly, and the fresh ore as it 

 is put in, must be placed below the middle stamper, 

 where it is subjected to the greatest force, and from 

 which it is thrown under the other stampers. 



The vessels into which the water flows from the cis- 

 tern, are arranged in a particular way, and a great deal 

 depends on the performance of this part of the process ; 

 for by altering the velocity of the current of water in 

 them, the rapidity with which the powder is deposited 

 also alters. The vessels are of different dimensions, 

 that nearest the cistern being about 12 feet long, and 

 9 inches broad, and as many deep. The others gra- 

 dually enlarge as they retire from the cistern. They 

 also vary in their inclination, the first having a slope of 

 about 3 inches, the second about 1 inch, the third and 

 fourth about ^ an inch, and the fifth and sixth being 

 level. Each vessel is furnished with a groove at the 

 extremity farthest from the cistern, into which pieces 

 of wood are placed, varying in height, according to the 

 quantity of ore that is collected in the vessel, and each 

 vessel communicates with that beyond it. 



When the water flows from the cistern into the first 

 vessel, the heavy part of the ore is deposited and col- 

 lected where the wood is put into the groove ; as the 

 powder reaches the top of this, another piece is put 

 over the former, and so on till the vessel is nearly filled. 

 When this is the case, the water is allowed to flow into 

 the second vessel, and from this, when full, into the 

 third, by which means the different parts of the ore are 

 collected, according to their specific gravity in the dif- 

 ferent vessels. The parcels of ore thus procured, are 

 still farther freed from impurities by washing. For 

 this purpose, each parcel is put into a wicker basket, 

 into which a stream of water slowly flows, and which, 

 as it issues, is received on inclined tables, grooved in 

 various directions. By this means, the ore is still farther 

 separated, according to the specific gravity of the par- 

 ticles, the heaviest being deposited in the upper groove. 

 Some of the ores of gold are subjected to another wash- 

 ing, in a vessel similar in shape to a fire-shovel, called 

 a huddle. This is immersed in water, and a particular 

 motion is given to it by the workman, by which the 

 lighter particles are thrown out, and the heavy metallic 

 matter is left in the buddle. If the gold procured by 

 this process, be mixed only with earthy matter, it is 

 subjected to amalgamation, as already described ; or it 

 is fused in crucibles with nitre. If, however, it be 

 mixed with inflammable matter, as sulphur, it is roast- 

 ed, by which the sulphur is expelled. It is then mixed 

 with lime and galena, in quantity proportionate to the 

 gold contained in it, which is ascertained by assaying 

 it, and kept at a red heat, in a reverberatory furnace, 

 till part of the sulphur is expelled. The heat is then 

 increased, till the whole becomes fluid, after which it 

 is poured into moulds of sand. The product of this 

 operation consists of scoria above, and beneath of a 

 metallic matter, composed of gold, silver, copper, lead, 

 iron, and a little sulphur. By repeated fusions the 

 gold is obtained, alloyed only with silver, copper, and 

 lead, from which it is freed by the process of refining. 



The refining of gold is performed in different ways, 

 according to the metals with which it is mixed. Of 

 these the most common is cupellation, or testing, the 

 former term being employed when the operation is 



performed on a small, the latter, when it is practised Metallur. 

 on a large scale. gy. 



Gold is one of the metals which is not oxidated by v *"y ' 

 heat and air, while the other metals, with which it is 

 usually alloyed, except silver, pass into the state of 

 oxide, at a high temperature. This constitutes the pro- 

 cess of cupellation, which is merely the separation of 

 the precious from the baser metals, by means of oxida- 

 tion. When, however, the base metals are in small 

 quantity, compared with that of the gold, the oxidation is 

 not complete ; but if a metal, which is easily oxidated, 

 and the oxide of which is very fusible, be added to the 

 alloy, the oxidation of the base metals is promoted, 

 and they are completely separated. The metal always 

 employed is lead, and the quantity added depends on 

 the proportion of gold which the alloy contains. ( For 

 a particular account of the process of refining, see 

 Silver, in this article, p. 63.) 



If lead be the only metal with which the gold is al- 

 loyed, the process of cupellation is sufficient for its 

 purification ; if however, which is usually the case, sil- 

 ver and copper be also present, other methods are ne- 

 cessary to free it from these metals. When copper 

 only exists, the gold is mixed with silver and lead, 

 and then subjected to cupellation, by which the base 

 metals are removed, but the gold still retains the silver ; 

 it is necessary, therefore, to have recourse to means for 

 separating these two metals. When the gold amounts 

 to about one-fourth of the alloy, the process is car- 

 ried on by means of nitric acid. Silver is easily 

 acted on by nitric acid, while gold is not; if, however, 

 the gold exceed what is stated above, the whole of the 

 silver is not dissolved ; it is necessary, therefore, for 

 this process, that the alloy employed do not contain 

 more than one-fourth of gold ; if it do, it must be 

 fused with poorer alloy, i.e. alloy which contains a 

 great deal of silver. When in this state, it is poured 

 into cold water, by which it is reduced to powder, or 

 granulated as it is called. The granulated alloy is 

 then put into parting-glasses, which are vessels of a pear- 

 shape, about 12 inches long, and 7 wide at the bot- 

 tom. Into each of these about 40 ounces of the alloy 

 are put along with nitric acid, so that the acid cover 

 the alloy to about the depth of two or three inches. These 

 are gently heated on a sand bath, and when the action 

 has ceased, the solution is poured off, and another quan- 

 tity of acid is added, and the process is repeated a third 

 time, which is, in general, sufficient to dissolve the 

 whole of the silver. The last portion of acid, as con. 

 taining little metal, is used as the first quantity, in the 

 next purification ; the two first are decomposed by the 

 immersion of copper plates, by which the silver is de- 

 posited in the metallic state. The gold left after the 

 action of the acid, is washed with warm water, till the 

 fluid which comes off does not alter the colour of a 

 piece of copper, and is then fused in a crucible with 

 nitre and borax, by which all impurities are removed. 



When the proportion of gold in an alloy is much 

 smaller than that mentioned, a different process is some- 

 times followed for separating it from the silver. For 

 this purpose, the alloy is melted and granulated. About 

 seven-eighths of it are then mixed with about one- 

 eighth of the flowers of sulphur, and the mixture heated 

 in a covered crucible, first slowly, and afterwards till 

 the whole is brought into fusion, in which state it is 

 kept on the fire for about an hour ; one-third of the 

 alloy kept out, is then added and thoroughly stirred 

 with it by means of a wooden rod, and the whole is 

 again heated for about an hour. Another third of the 



