First in order, then, we proceed to the ore-yard, which presents a very motley appearance. Under its capacious roof there were tons upon tons of every variety of ore—native and foreign, blue and red, green and yellow, and all intermediate colours—indiscriminately piled around. There was the beautiful green malachite from Australia, the gray sulphuret from Algiers, the phosphate from Chili, and the hydrous-carbonate from Spain. There was the glistening yellow sulphuret from Cuba, the silicate from Brazil, the bright-blue carbonate from the sunny regions of the south, and the dark-brown oxide from the colder regions of the north. There was regulus from New Zealand, and the good old pyrites from the Cornish mines; some compounds with arsenic, antimony, and numerous other substances; and last, though in one sense not least, there was a solitary specimen of ore from Ireland.
These ores were all in the form of a coarse powder. The regulus we have mentioned is simply the sulphuret deprived, by a preliminary operation, of its extraneous earthy matters; and this is frequently effected in the localities where it is produced, such as New Zealand and Chili, the expense of transport from these places being very considerable.
'And what is this?' we inquired, looking at a black earthy substance the workmen at that moment were discharging from a vessel.
'Ah!' said our friend, 'that is a commodity which, I suspect, you know something about. It is a waste product from certain foundries and chemical works—from Scotland in this case—and it contains a small per cent. of copper. We don't care much about it; we seldom have it; but it is sold at the ticketings regularly. For want of a better name, we term it slag; but it is not slag, properly so called, which you see all around you. A better denomination is that employed in designating it in the Journal—namely, rubbish.'[5]
'You make no kind of distinction in the ore-yard,' we continued. 'Is that unnecessary?'
'Well, practically it is. As these heaps lie, you can perceive that a vertical slice from top to bottom will give us a tolerably even admixture of the different ores. This is always desirable to a certain extent, since the ores being of different constitution, the one materially assists in the reduction of the other. Thus an ore containing a large proportion of fluor-spar may with great advantage be employed to flux another containing felspar or quartz, which substances are almost infusible alone. Indeed, the judicious admixture of ores constitutes the most important vocation of the smelter; and it is to this that the copper-houses of Swansea are indebted for one of their advantages over the proprietors of mines, who, possessing only one kind of ore—rich, probably, but intractable—can never bring it into the state of a metal with any satisfactory profit.'
'What is the value of these ores?'
'That varies much. This gray sulphuret contains about 70 per cent. of copper, and is worth L.35 per ton. This yellow sulphuret, from being mixed with a large quantity of iron and silicious earth, contains only about 12 or 14 per cent. Some malachites contain so much as 50 per cent., and others less pure, 30 to 40 per cent. of copper. But the greater mass of the ores we melt have a far less produce than this. That Cornish ore you see there, for example, contains only 4½ per cent. of metal. The average produce, however, of all the British and foreign ores smelted at Swansea may be given at about 12 per cent. Previous to the great increase of foreign importation, it was much lower.'
We now come to the process of smelting. The theory of reducing metallic ores, of whatever constitution, is to bring them to the state of oxides; and then, by the addition of charcoal, and with the aid of heat, to expel the oxygen in the form of carbonic acid; after which the pure metal is left. In practice, the reduction of copper-ores is slightly different. Here the object is to separate, first, the earthy matters and extraneous metals, by forming them into oxides by calcination: these are subsequently obtained as waste products in the form of slag; while the copper is left in combination with sulphur, which is then dispelled at one operation. According to Mr Vivian, copper undergoes eight, and sometimes nine, distinct operations in its progress from the ore to the ingot; and these consist of alternate calcinations and fusions, extending over a period of from 100 to 120 hours. As, however, some of these are simple repetitions, we may, for convenience' sake, illustrate the process under its three most important steps.
1. Calcination of the Ore.—Having arranged a proper mixture of ores in the yard, it is weighed out in boxes to the calcining-men. This is drawn up an inclined plane over the tops of the furnaces, and from thence emptied through hoppers, 3½ to 4½ tons at a time, into the large calcining furnace. Here it is roasted for a period ranging from twelve to twenty-four hours, after which it is drawn into the ash-pit, where it remains to cool. In this state, the ore is a black, amorphous substance, and is termed calcined ore. The object of this process is to oxidise the extraneous metals, and also to reduce the quantity of sulphur, by driving it off in the form of vapour. It is, therefore, in this and the analogous processes of roasting, that the sulphurous and arsenous vapours are so profusely given off.