[Pg 511][16] The latter part of this paragraph presents great difficulties. The term "refining furnace" is given in the Latin as the "second furnace," an expression usually applied to the cupellation furnace. The whole question of refining is exhaustively discussed on pages [530] to [539]. Exactly what material is meant by the term red (rubrum), yellow (fulvum) and caldarium copper is somewhat uncertain. They are given in the German text simply as rot, geel, and lebeter kupfer, and apparently all were "coarse" copper of different characters destined for the refinery. The author states in De Natura Fossilium (p. 334): "Copper has a red colour peculiar to itself; this colour in smelted copper is considered the most excellent. It, however, varies. In some it is red, as in the copper smelted at Neusohl.... Other copper is prepared in the smelters where silver is separated from copper, which is called yellow copper (luteum), and is regulare. In the same place a dark yellow copper is made which is called caldarium, taking its name among the Germans from a caldron.... Regulare differs from caldarium in that the former is not only fusible, but also malleable; while the latter is, indeed, fusible, but is not ductile, for it breaks when struck with the hammer." Later on in De Re Metallica (p. [542]) he describes yellow copper as made from "baser" liquation thorns and from exhausted liquation cakes made from thorns. These products were necessarily impure, as they contained, among other things, the concentrates from furnace accretions. Therefore, there was ample source for zinc, arsenic or other metallics which would lighten the colour. Caldarium copper is described by Pliny (see note, p. [404]), and was, no doubt, "coarse" copper, and apparently Agricola adopted this term from that source, as we have found it used nowhere else. On page [542] the author describes making caldarium copper from a mixture of yellow copper and a peculiar cadmia, which he describes as the "slags" from refining copper. These "slags," which are the result of oxidation and poling, would contain almost any of the metallic impurities of the original ore, antimony, lead, arsenic, zinc, cobalt, etc. Coming from these two sources the caldarium must have been, indeed, impure.
[Pg 512][17] The liquation of these low-grade copper "bottoms" required that the liquated lead should be re-used again to make up fresh liquation cakes, in order that it might eventually become rich enough to warrant cupellation. In the following table the "poor" silver-lead is designated (A) the "medium" (B) and the "rich" (C). The three charges here given are designated sixth, seventh, and eighth for purposes of reference. It will be seen that the data is insufficient to complete the ninth and tenth. Moreover, while the author gives directions for making four cakes, he says the charge consists of five, and it has, therefore, been necessary to reduce the volume of products given to this basis.
| 6th Charge. | 7th Charge. | 8th Charge. | |
| Amount of copper bottoms | 176.5 lbs. | 176.5 lbs. | 176.5 lbs. |
| Amount of lead | 282.4 lbs. (slags) | 564.8 lbs. of (A) | 635.4 lbs. of (B) |
| Amount of de-silverized lead | 494.2 lbs. | 211.8 lbs. | 141.2 lbs. (A) |
| Weight of each cake | 238.3 lbs. | 238.3 lbs. | 238.3 lbs. |
| Average value of charge per ton | 22 ozs. 5 dwts. | 35 ozs. 15 dwts. | 50 ozs. 5 dwts. |
| Per cent. of copper | 18.5% | 18.5% | 18.5% |
| Average value per ton original copper | 97 ozs. 4 dwts. | 97 ozs. 4 dwts. | 97 ozs. 4 dwts. |
| Average value per ton of | 90 ozs. 2 dwts. (slags) | 28 ozs. 5 dwts. (A) | 28 ozs. 5 dwts. (A) |
| Average value per ton of | 3 ozs. 1 dwt. (lead) | 3 ozs. 1 dwt. (lead) | 42 ozs. 10 dwts. (B) |
| Weight of liquated lead | 550.6 lbs. | ||
| Average value of the liquated lead per ton | 28 ozs. 5 dwts. (A) | 42 ozs. 10 dwts. (B) | 63 ozs. 16 dwts. (C) |
| Weight of exhausted liquation cakes | 225.9 lbs. | ||
| Average value of the exhausted liquation cakes per ton | 12 ozs. 3 dwts. | ||
| Weight of liquation thorns | 169.4 lbs. | ||
| Average value of the liquation thorns per ton | 18 ozs. 4 dwts. | ||
| Extraction of silver into the liquated lead | 71% |
[Pg 520][18] For the liquation it was necessary to maintain a reducing atmosphere, otherwise the lead would oxidize; this was secured by keeping the cakes well covered with charcoal and by preventing the entrance of air as much as possible. Moreover, it was necessary to preserve a fairly even temperature. The proportions of copper and lead in the three liquation products vary considerably, depending upon the method of conducting the process and the original proportions. From the authors consulted (see note p. [492]) an average would be about as follows:—The residual copper—exhausted liquation cakes—ran from 25 to 33% lead; the liquated lead from 2 to 3% copper; and the liquation thorns, which were largely oxidized, contained about 15% copper oxides, 80% lead oxides, together with impurities, such as antimony, arsenic, etc. The proportions of the various products would obviously depend upon the care in conducting the operation; too high temperature and the admission of air would increase the copper melted and oxidize more lead, and thus increase the liquation thorns. There are insufficient data in Agricola to adduce conclusions as to the actual ratios produced. The results given for the 6th charge ([note 17, p. 512]) would indicate about 30% lead in the residual copper, and would indicate that the original charge was divided into about 24% of residual copper, 18% of liquation thorns, and 57% of liquated lead. This, however, was an unusually large proportion of liquation thorns, some of the authors giving instances of as low as 5%.
[Pg 522][19] The first instance given, of 44 centumpondia (3,109 lbs.) lead and one centumpondium (70.6 lbs.) copper, would indicate that the liquated lead contained 2.2% copper. The second, of 46 centumpondia (3,250 lbs.) lead and 11/2 centumpondia copper (106 lbs.), would indicate 3% copper; and in the third, 120 centumpondia (8,478 lbs.) lead and six copper (424 lbs.) would show 4.76% copper. This charge of 120 centumpondia in the cupellation furnace would normally make more than 110 centumpondia of litharge and 30 of hearth-lead, i.e., saturated furnace bottoms. The copper would be largely found in the silver-lead "which does not melt," at the margin of the crucible. These skimmings are afterward referred to as "thorns." It is difficult to understand what is meant by the expression that the silver which is in the copper is mixed with the remaining (reliquo) silver. The coppery skimmings from the cupellation furnace are referred to again in [Note 28, p. 539].
[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%.
[21] If Roman weights, this would equal from 6,360 lbs. to 7,066 lbs.
[Pg 529][22] One half uncia, or three drachmae of silver would equal either 12 ozs. or 9 ozs. per ton. If we assume the values given for residual copper in the first four charges (note p. [506]) of 34 ozs., this would mean an extraction of, roughly, 65% of the silver from the exhausted liquation cakes.