Sixth operation. Melting of the calcined fine metal

—This fusion is conducted like that of the first matt. The black copper, or coarse copper, which it produces, contains from 70 to 80 per cent. of pure metal; it is run into ingots, in order to undergo the operation of roasting. The scoriæ are rich in copper; they are added to the fusion of the calcined coarse metal of the fourth operation. In the smelting houses of Me rs. Vivian, at Hafod, near Swansea, the fifth and sixth operations have been omitted of late years. The second matt is run into pigs, under the name of blue metal , to be immediately exposed to the roasting. The disposition of the canal a a′ , fig. 302. , which introduces a continuous current of air to the hearth of the furnace, accelerates and facilitates the calcination of the matt; an advantage which has simplified the treatment, by diminishing the number of calculations. Seventh operation. Roasting of the coarse copper, the product of the sixth operation. The chief object of this operation is oxidize ment; it is performed either in an ordinary roasting furnace, or in the one belonging to fig. 302. , which admits a constant current of air. The pigs of metal derived from the preceding melting are exposed, on the hearth of the furnace, to the action of the air, which oxidizes the iron and other foreign metals with which the copper is still contaminated. The duration of the roasting varies from 12 to 24 hours, according to the degree of purity of the crude copper. The temperature should be graduated, in order that the oxidize ment may have time to complete, and that the volatile substances which the copper still retains may escape in the gaseous form. The fusion must take place only towards the end of the operation. The charge varies from a ton and a quarter to a ton and a half. The metal obtained is run out into moulds of sand. It is covered with black blisters, like steel of cementation; whence it has got the name of blistered copper. In the interior of these pigs, the copper gases during the moulding. The copper being now almost entirely purged from the sulphur, iron, and the other substances with which it was combined, is in a fit state to be refined. This operation affords some scoriæ; they are very heavy, and contain a great deal of oxide of copper, sometimes even metallic copper. These scoriæ, as well as those of the third melting and of the refining, are added to the second fusion, as we have already stated, in describing the fourth operation. In some works, the roasting is repeated several times upon the blue metal , in order to bring it to a state fit for refining. We shall afterwards notice this modification of the treatment.