On cooling, we break open the crucible, and, on removing the slag, extract a spherical piece of impure or alloyed copper, according as other metals may chance to occur in the ore. If any intermediate crust should have formed between the button and the slag, the ore was not properly roasted, a part of the copper not reduced, and consequently the assay is worthless.

In a good assay the slag should be black and vitreous in appearance, never of an earthy texture. If striated or speckled with red, we may know that protoxide of copper is dissolved in it, and again that the whole cannot be productive of an accurate result. Much attention is required during this test, and, as already mentioned under the heads of silver and gold, the final result depends entirely upon the care taken by the assayer. If well managed, however, as here directed, he can hardly fail to be successful.

The button, as remarked above, may

vary in purity. It generally contains some iron, and (since these metals frequently exist in copper ores) lead, bismuth, tin, cobalt, nickel, antimony, and arsenic-the latter two in particular, if they were added in the process of reduction. Thus it very rarely, if ever, happens, that a copper ore is sufficiently pure to require no third process. The less admixtures the alloy contains, the less brittle and the more ductile it is. Nickel particularly tends to harden it.

To remove the foreign metals, the button is put in a piece of paper, with sufficient borax to cover it, (one-fourth to one-third part by weight, rather more than less,) and if no lead occur in the ore, with about from five to ten per-cent. of that metal, which amount, however, should increase up to forty, or even more, if there are many impurities in the alloy. Even if this be not the case, it is always safer, and can never produce any bad effect, to add much.

A crucible of the kind illustrated in fig. 3,

Plate II., should be brought to a br.ght white-heat in the muffle. Coals may be placed round to increase the temperature, which should be so great, that the copper, on being introduced, wrapped in paper with borax and lead, may melt in a few minutes.

As long as the tongs held over the button are reflected, or rainbow colours are yet seen to flicker over its surface, lead is still present. As with silver and gold, too great heat ought not to be employed. When the lead has left, we immediately take out the crucible, and immerse it in water, to prevent any copper from oxidating unnecessarily. The button of pure copper is then broken out and weighed. A slight loss cannot be prevented, as it is impossible to prevent some oxide of copper from being formed, and we must therefore grant a larger percentage than the assay would direct.

At least two assays should be made at once, to compare the results, and take the average.

XIV.

Assay of Copper Ores-English Method.

Ar the copper mines in Cornwall, a mode of assaying is employed, which in several of its minutiæ differs considerably from the one just described. It should be remarked that this method is, properly speaking, only applicable to sulphurets, as all copper con: tained in the ore as a pure oxide, or combined with an acid, will pass over into the slag during the reduction process; and that therefore if these latter are present, some means should be employed, as shall be shown hereafter, to extract them from the slags, when the other parts of the assay are concluded.

The ore, being a sulphuret, should be roasted as directed in the foregoing German or Hungarian method, after which it

is mixed with from one to one and a half parts of pounded glass, which should, however, contain no lead or arsenic, from twenty-five to fifty per-cent. of saltpetre, and fifty per-cent. of borax. Together with these, it is exposed to a strong melting heat in a clay crucible. On cooling, the button is removed, as is the case in the other mode of procedure, more common on the continent of Europe.

Having thus reduced the copper, it is necessary to purify it, as was also done in the other assay. I must observe, however, that this part of the Cornwall process, again, can only be applied where very little lead is contained in the copper ore.

For this purpose, some white flux should be prepared, which is done by igniting together equal parts of saltpetre and tartar. Being as susceptible to the effects of atmospheric moisture as black flux, it requires the same precaution as regards its preservation.