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BOOK XI.
543
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placed apart by themselves, of which one basketful is mixed with the precious
thorns to be re-melted. The exhausted liquation cakes are " dried " at the same time as other good exhausted liquation cakes. The thorns which are drawn off from the lead, when it is separated from
silver in the cupellation furnace34, and the hearth-lead which remains in the crucible in the middle part of the furnaces, together with the hearth material which has become defective and has absorbed silver-lead, are all melted together with a little slag in the blast furnaces. The lead, or rather the silver-lead, which flows from the furnace into the fore-hearth, is poured out into copper moulds such as are used by the refiners ; a centumpondium of such lead contains four unciae of silver, or, if the hearth was defective, it contains more. A small portion of this material is added to the copper and lead when liquation cakes are made from them, if more were to be added the alloy would be much richer than it should be, for which reason the wise 34The skimmings from the molten lead in the early stages of cupellation have been
discussed in Note 28, p. 539. They are probably called thorns here because of the large amount of copper in them. The lead from liquation would contain 2% to 3% of copper, and this would be largely recovered in these skimmings, although there would be some copper in the furnace bottoms—hearth-lead—and the litharge. These " thorns " are apparently fairly rich, four unciae to the centumpondium being equivalent to about 97 ozs. per ton, and they are only added to low-grade liquation material. |
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