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BOOK XI.
511
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in the latter, and in the front of the furnace, three feet above the floor of
the building, is the mouth out of which the re-melted copper flows into a forehearth and a dipping-pot. On the left side of the mouth is an aperture, through which beech-wood may be put into the furnace to feed the fire. If in a centumpondium of copper there were a sixth of a libra and a semi-uncia of silver, or a quarter of a libra, or a quarter of a libra and a semi-uncia—there is re-melted at the same time thirty-eight centumpondia of it in this furnace, until there remain in each centumpondium of the copper " bottoms " a third of a libra and a semi-uncia of silver. For example, if in each centumpondium of copper not yet re-melted, there is a quarter of a libra and a semi-uncia of silver, then the thirty-eight centumpondia that are smelted together must contain a total of eleven librae and an uncia of silver. Since from fifteen centumpondia of re-melted copper there was a total of four and a third librae and a semi-uncia of silver, there remain only two and a third librae. Thus there is left in the " bottoms," weighing twenty-three centumpondia, a total of eight and threequarter librae of silver. Therefore, each centumpondium of this contains a third of a libra and a semi-uncia, a drachma, and the twenty-third part of a drachma of silver; from such copper it is profitable to separate the silver. In order that the master may be more certain of the number of centumpondia of copper in the " bottoms," he weighs the " tops " that have been drawn off from it; the " tops " were first drawn off into the dipping-pot, and cakes were made from them. Fourteen hours are expended on the work of thus dividing the copper. The " bottoms," when a certain weight of lead has been added to them, of which alloy I shall soon speak, are melted in the blast furnace; liquation cakes are then made, and the silver is afterward separated from the copper. The " tops" are subsequently melted in the blast furnace, and re-melted in the refining furnace, in order that red copper shall be made16; and the " tops " from this are again smelted in the blast furnace, and then again in the refining furnace, that therefrom "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 kwpfer, 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. |
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