70 



METALLURGY. 



MetnUur- long, the tin attracts more oxygen from the air, and 

 K7- does not thus deprive the protoxide of copper of its oxy- 

 SP "V P ' gen. Thus Fourcroy found, that when he exposed an 

 nlloy of 80 of copper and 20 of tin to heat and air till 

 they amounted to 104, 54 parts of copper were obtain- 

 ed by afterwards subjecting the whole in a covered ves- 

 sel to a high temperature. When, however, 100 parts 

 of the same alloy were heated with access of air till they 

 increased to 1 1 7, a very minute quantity of copper was 

 obtained from them by the subsequent heating. Ac- 

 cording to Fourcroy, when copper is to be obtained by 

 the method just mentioned, the alloy, supposing it com- 

 posed of 80 copper and 20 tin, should be heated in the 

 air till it gain about 6 or 7 parts in weight, and then 

 subjected to a high temperature in close vessels. When 

 the alloy has been too much oxidated, it must be mixed 

 with the due proportion of alloy, and then exposed to a 

 high temperature, by which a large quantity of metallic 

 copper will be procured. 



Occasionally a little nitre, or black oxide of manga- 

 nese, is mixed with the alloy, by which the oxidation is 

 more speedily accomplished. Some glass or salt should 

 lso be added, to increase the fusibility of the oxide of 

 tin formed, and thus allow the metallic copper to fall to 

 the bottom. The most accurate experiments on this 

 subject on a large scale, are those of Pelletier and Dar- 

 cet, which were done by exposing the alloy to heat and 

 air, and by the addition of black oxide of manganese. 



In one of these, 400 Ib. of alloy, known to be com- 

 posed of 80 copper and 40 tin, were heated, and con- 

 stantly stirred till they increased to 425 Ib. 2 oz. These 

 were added to 800 Ib. of alloy brought to a state of fu- 

 sion in a reverberatory furnace, and the mixture con- 

 stantly stirred during 20 minutes, and occasionally af- 

 terwards for 9 hours. The fused metal was then drawn 

 off, and amounted to 761 Ib. 12 oz. ; 7 Ib. 4 oz. of me- 

 tal were also obtained during the trials to ascertain its 

 purity, and the scoria yielded 64 Ib. more, making in 

 all 833 Ib. ; that is, very nearly 70 parts of copper from 

 the 100 of alloy, which contained 80 of this metal. 



In another experiment, 800 Ib. of alloy were melted 

 in the furnace, and 25 Ib. of oxide of manganese were 

 added to them. The mixture was then well stirred, 

 and in two hours afterwards 1 5 Ib. more of manganese 

 were thrown in, a similar quantity being added every 

 two hours till the whole amounted to 100 Ib. and the fu- 

 sion and occasional stirring were continued during 10 

 hours. At the expiry of this time, the copper was 

 drawn off, and amounted to 520 Ib. that is, 65 per cent, 

 of the alloy employed. The scoria still retained a good 

 deal of copper, but this was not extracted from it. 



When the alloy contains silver, it may be assayed 

 by dissolving it in nitric acid, precipitating the silver 

 by muriate of soda, and the copper by a plate of iron. 



Tin. 



, jn Different methods are followed in reducing the ores 



of tin. 



The ore, which is procured from the mines of Corn- 

 wall, after being hand dressed, is freed from impurities 

 by stamping, as has been described under gold, after 

 which, it is roasted in a reverberatory, to drive off the 

 sulphur, part of which, however, is acidified, and unites 

 with the copper and iron of the ore. The ore is again 

 washed, by which it is nearly freed from all impu- 

 rities ; it is then mixed with one-fifth of its bulk of 

 culm, and subjected to heat in a reverberatory for 

 about six hours, during which the oxide of tin is re- 

 duced, and the metal collects at the bottom, covered 

 4 



with a black scoria. The tin is then drawn off into a 

 shallow pit, in which it is freed from the scoria which 

 collects on its surface. It is then taken out with 

 ladles, and poured into moulds. 



The metal thus obtained is afterwards exposed to a 

 gentle heat in a small reverberatory furnace, by which 

 the purest part of it melts first, and is drawn off. This 

 forms grain tin ; what is left behind is common tin, 

 which contains a small portion of iron, copper, and ar- 

 senic. 



The water employed in the second washing of the 

 ore contains a considerable quantity of sulphate of 

 copper, on which account it is kept and decomposed 

 by iron. The scoria separated from the tin, when drawn 

 from the reverberatory, retains a good deal of the me- 

 tal it is therefore stamped and melted along with 

 the ore. 



The stream tin stone of Cornwall is melted in a dif- 

 ferent way. As the ore is in a powdery state when 

 procured, it is submitted to a stream of water, by which 

 a great deal of the impurities are removed. It is af- 

 terwards bruised, and passed through wire sieves. 



It is then thrown, with alternate quantities of char- 

 coal, into a blast furnace, in which it is reduced, and es- 

 capes through a channel at the bottom, into pits ; the 

 scoria being removed as it collects, and thrown again 

 into the furnace. 



The metal is then put into a large iron pot, in which 

 it is kept fused. When in this state, pieces of charcoal 

 are plunged into it, which cause a fresh quantity of 

 scoria to be separated. The metal is then tried by re- 

 moving a quantity in a ladle, and pouring it into the 

 pot. If it appear bright like silver, and of uniform 

 consistence, it is pure. After this it is poured into 

 moulds, and forms good grain tin. 



Assay. In assaying an ore of tin, it is first reduced 

 to coarse powder, and then washed, to free it from 

 earthy matter. If it contain arsenic, which is known 

 by its emitting the odour of garlic when fused before 

 the blow-pipe, it must be exposed to heat with char- 

 coal, till the vapours of arsenic cease to be emitted. 

 What remains is then mixed with pitch and saw-dust, 

 subjected to a strong heat in a crucible, lined with 

 charcoal. The metallic button collected at the bottom 



is tin. 



Lead. 



The only ore of lead from which the metal is extract- Lead, 

 ed is galena, the smelting of which is very simple. 



The ore after being brought from the mine is hand- 

 dressed, by which it is freed as much as possible of im- 

 purities. What remains is then washed, to remove 

 still farther any extraneous matter, and is put into a re- 

 verberatory furnace, where it is speedily made red hot. 

 When in this state it is frequently stirred, and when it 

 begins to become soft, the heat is reduced till the whole 

 of the sulphur is expelled. The fire is then made 

 brisk, by which the lead is melted, and collects at the 

 bottom. A little lime is then thrown in to thicken 

 the scoria, and the lead is drawn off into oblong moulds; 

 a sufficient heat is again applied to the scoria, by which 

 another portion of lead is procured. The lead obtain- 

 ed in the first operation is considered the best, as it is 

 more malleable than the other. 



Assay. The assay of galena is very simple. It may Assay. 

 be done either in the dry or the humid way. 



The lead ore is first reduced to powder, and then di- 

 gested in diluted nitric acid. To the solution, after 



