Note 34 — BOOK XI. (continued 2)

[Pg 543][34] The 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.