Note 20 — BOOK XI.

[Pg 523][20] A further amount of lead could be obtained in the first liquation, but a higher temperature is necessary, which was more economical to secure in the "drying" furnace. Therefore, the "drying" was really an extension of liquation; but as air was admitted the lead and copper melted out were oxidized. The products were the final residual copper, called by Agricola the "dried" copper, together with lead and copper oxides, called by him the "slags," and the scale of copper and lead oxides termed by him the "ash-coloured copper." The German metallurgists distinguished two kinds of slag: the first and principal one, the darrost, and the second the darrsöhle, this latter differing only in that it contained more impurities from the floor of the furnace, and remained behind until the furnace cooled. Agricola possibly refers to these as "more liquation thorns," because in describing the treatment of the bye-products he refers to thorns from the process, whereas in the description of "drying" he usually refers to "slags." A number of analyses of these products, given by Karsten, show the "dried" copper to contain from 82.7 to 90.6% copper, and from 9.4 to 17.3% lead; the "slag" to contain 76.5 to 85.1% lead oxide, and from 4.1 to 7.8% cuprous oxide, with 9 to 13% silica from the furnace bottoms, together with some other [Pg 524]impurities; the "ash-coloured copper" to contain about 60% cuprous oxide and 30% lead oxide, with some metallic copper and minor impurities. An average of proportions given by various authors shows, roughly, that out of 100 centners of "exhausted" liquation cakes, containing about 70% copper and 30% lead, there were about 63 centners of "dried" copper, 38 centners of "slag," and 61/2 centners of "ash-coloured copper." According to Karsten, the process fell into stages; first, at low temperature some metallic lead appeared; second, during an increasing temperature for over 14 to 15 hours the slags ran out; third, there was a period of four hours of lower temperature to allow time for the lead to diffuse from the interior of the cakes; and fourth, during a period of eight hours the temperature was again increased. In fact, the latter portion of the process ended with the economic limit between leaving some lead in the copper and driving too much copper into the "slags." Agricola gives the silver contents of the "dried" copper as 3 drachmae to 1 centumpondium, or equal to about 9 ozs. per ton; and assuming that the copper finally recovered from the bye-products ran no higher, then the first four charges (see note on p. 506) would show a reduction in the silver values of from 95 to 97%; the 7th and 8th charges (note on p. 512) of about 90%.