CHAMBERS'S INFORMATION FOR THE PEOPLE. 



and nearly pure. It is then melted and cast into 

 ingots. The European amalgamation process 

 differs from the Mexican chiefly in the mixture of 

 silver ore with the sulphide of iron and copper 

 and common salt being roasted previous to the 

 addition of mercury. It is also more carefully 

 conducted. 



Silver is most conveniently extracted from rich 

 ores by fusing them with lead, and then extracting 

 the silver from the alloy s*o obtained by cupella- 

 tion. When lead is heated with free access of air, 

 it rapidly oxidises ; while silver is not oxidised at 

 any temperature short of a very high one, even in 

 presence of melted oxide of lead. But this oxide 

 (litharge) in the molten state dissolves the oxides 

 of several other metals, and consequently these 

 can be separated along with the lead on a cupel. 

 The cupel is a bone-ash tray filled with the melted 

 alloy of lead and silver, and exposed to a current 

 of air, which gradually oxidises the lead, and 

 blows it off in the state of litharge, leaving, when 

 the operation is conducted long enough, the silver 

 as a cake on the cupel. Nearly all lead obtained 

 by the ordinary smelting processes contains silver, 

 of which 8 or 10 oz. to the ton is a very 

 common proportion. Some kinds, however, con- 

 tain only 2 or 3, while others have occasion- 

 ally as much as 300 oz. per ton. By a process 

 patented by the late Mr H. L. Pattinson of 

 Newcastle-on-Tyne, the silver can now be profit- 

 ably extracted from even the poorest qualities we 

 have named. It is known as Pattinson's de- 

 silvering process, and is based on the fact, which 

 he discovered, that when lead containing silver is 

 melted, and allowed to cool slowly, a portion of 

 the lead separates first, in the solid form, as small 

 crystals. These contain less, and therefore the 

 portion which remains liquid contains more silver 

 than the original lead. Suppose, then, we began 

 by melting a ton of lead containing 10 oz. of silver 

 in a pot, and, allowing it to cool slowly, removed 

 two-thirds of it in the solid state as the crystals 

 formed. We should then have a crystallised por- 

 tion containing only 5, and a liquid' portion (one- 

 third) containing 20 oz. of silver per ton. By 

 repeating the operation with both portions, we at 

 last get crystals containing almost no silver, and 

 a liquid portion very rich in that metal. In prac- 

 tice, 10 tons of lead are melted into one pot, of 

 which there are usually from six to nine in a row, 

 and the concentration of the silver is not often 

 carried further than 600 oz. to the ton. The rich 

 lead is then submitted to the cupellation process 

 described above. Nearly all the silver produced 

 in Great Britain, amounting of late years to about 

 800,000 oz. annually, is obtained by the desilverisa- 

 tion of lead. 



Copper. 



The use of copper by ancient nations is well 

 known, through the weapons and other objects of 

 bronze an alloy of copper and tin which have 

 been collected so largely by archaeologists. It was 

 obtained by the ancients from various places. 

 Copper is the only metal with a red colour, 

 and is very malleable and ductile, but in tenacity 

 it is far inferior to iron. Its melting-point is 

 between those Of gold and silver, and when heated 

 closely up to it, it becomes very brittle, a property 

 taken advantage of by founders when they wish to 

 break up ingots of the metal. 



401 



Native copper, although frequently met with 

 wherever copper ore occurs, both as a constituent 

 of the ore itself, and also in separate arborescent 

 pieces, laminae, and irregular lumps or blocks, is 

 yet rarely found in sufficient quantity to be syste- 

 matically worked. The chief locality for it is Lake 

 Superior, where in some years as many as 6000 tons 

 have been obtained. Dr Percy, in his large work 

 on Metallurgy, states, on the authority of a well- 

 known mining engineer, 'that at Minnesota, in 

 1854, not fewer than forty men were engaged 

 during twelve months in cutting up a single mass 

 of native copper, weighing about 500 tons !' Cop- 

 per ores are found in a great many countries, but 

 it is only in some that they are smelted. Large 

 quantities of ore from Australia and South America, 

 for example, are smelted in England. The richest 

 ore of copper is the red oxide, which contains, 

 when pure, nearly 90 per cent, of the metal. Green 

 carbonate of copper is a widely distributed ore, 

 occurring largely in Australia and Russia. In a 

 pure state it contains 57 per cent, of copper, while 

 the blue carbonate frequently associated with it 

 contains only 55 per cent. These carbonates yield 

 a high quality of copper. Some of the sulphides 

 are also valuable copper ores ; two of them, namely, 

 gray sulphide and purple copper, contain respec- 

 tively, when pure, 80 and 55^ per cent, of metallic 

 copper. Copper pyrites a sulphide of iron and 

 copper which is the most abundant ore of the 

 metal, and the one chiefly found in Cornwall, con- 

 tains in the pure state 35 per cent, of copper, 

 although, on account of impurities, the average 

 yield of what is obtained in England is not more 

 than 12 per cent. 



In practice, the process of smelting copper from 

 ores like the Cornish is somewhat complicated, 

 but in theory it is comparatively simple. The 

 main impurities of the ore are quartz, iron, sulphur, 

 and very commonly arsenic. The process is con- 

 ducted with the view of separating the iron and 

 quartz as a fusible slag, and of dissipating the 

 sulphur and arsenic, by converting them into 

 sulphurous and arsenious acids, through oxidation 

 in the furnace. At Swansea, which is the seat of 

 copper-smelting in this country, reverberatory 

 furnaces are used, and these are of two kinds, 

 called respectively calciners and melting fur- 

 naces. A section of a melting furnace is given 

 in fig. i. There are never fewer than six opera- 

 tions in the Welsh process, and when so limited, 



Fig. I. Section of Copper-melting Furnace : 

 A, fireplace ; B, fire-bridge ; C, bed of sand ; D, melted copper. 



a favourable admixture of ores is necessary. In 

 theyfr^/, the ore is calcined in a furnace for at 

 least twelve hours, by which time the greater part 



