[Pg 503][9] By this expression is apparently meant the "bottoms" produced in enriching copper, as described on p. [510].
[Pg 504][10] The details of the preparation of liquation cakes—"leading"—were matters of great concern to the old metallurgists. The size of the cakes, the proportion of silver in the original copper and in the liquated lead, the proportion of lead and silver left in the residual cakes, all had to be reached by a series of compromises among militant forces. The cakes were generally two and one-half to three and one-half inches thick and about two feet in diameter, and [Pg 505]weighed 225 to 375 lbs. This size was wonderfully persistent from Agricola down to modern times; and was, no doubt, based on sound experience. If the cakes were too small, they required proportionately more fuel and labour; whilst if too large, the copper began to melt before the maximum lead was liquated. The ratio of the copper and lead was regulated by the necessity of enough copper to leave a substantial sponge mass the shape of the original cake, and not so large a proportion as to imprison the lead. That is, if the copper be in too small proportion the cakes break down; and if in too large, then insufficient lead liquates out, and the extraction of silver decreases. Ercker (p. 106-9) insists on the equivalent of about 3 copper to 9.5 lead; Lohneys (p. 99), 3 copper to 9 or 10 lead. Schlüter (p. 479, etc.) insists on a ration of 3 copper to about 11 lead. Kerl (Handbuch Der Metallurgischen Hüttenkunde, 1855; Vol. III., p. 116) gives 3 copper to 6 to 7 parts lead. Agricola gives variable amounts of 3 parts copper to from 8 to 12 parts lead. As to the ratio of silver in the copper, or to the cakes, there does not, except the limit of payability, seem to have been any difficulty on the minimum side. On the other hand, Ercker, Lohneys, Schlüter, and Karsten all contend that if the silver ran above a certain proportion, the copper would retain considerable silver. These authors give the outside ratio of silver permissible for good results in one liquation at what would be equivalent to 45 to 65 ozs. per ton of cakes, or about 190 to 250 ozs. per ton on the original copper. It will be seen, however, that Agricola's cakes greatly exceed these values. A difficulty did arise when the copper ran low in silver, in that the liquated lead was too poor to cupel, and in such case the lead was used over again, until it became rich enough for this purpose. According to Karsten, copper containing less than an equivalent of 80 to 90 ozs. per ton could not be liquated profitably, although the Upper Harz copper, according to Kerl, containing the equivalent of about 50 ozs. per ton, was liquated at a profit. In such a case the cakes would run only 12 to 14 ozs. per ton. It will be noticed that in the eight cases given by Agricola the copper ran from 97 to over 580 ozs. per ton, and in the description of enrichment of copper "bottoms" the original copper runs 85 ozs., and "it cannot be separated easily"; as a result, it is raised to 110 ozs. per ton before treatment. In addition to the following tabulation of the proportions here given by Agricola, the reader should refer to footnotes [15] and [17], where four more combinations are tabulated. It will be observed from [Pg 506]this table that with the increasing richness of copper an increased proportion of lead was added, so that the products were of similar value. It has been assumed (see [footnote 13 p. 509]), that Roman weights are intended. It is not to be expected that metallurgical results of this period will "tie up" with the exactness of the modern operator's, and it has not been considered necessary to calculate beyond the nearest pennyweight. Where two or more values are given by the author the average has been taken.
| 1st Charge. | 2nd Charge. | 3rd Charge. | 4th Charge. | |
| Amount of argentiferous copper | 211.8 lbs. | 211.8 lbs. | 211.8 lbs. | 211.8 lbs. |
| Amount of lead | 564.8 " | 635.4 " | 776.6 " | 847.2 " |
| Weight of each cake | 193.5 " | 211.5 " | 247.1 " | 264.75 " |
| Average value of charge | 56 ozs. 3 dwts. | 62 ozs. 4 dwts. | 64 ozs. 4 dwts. | 66 ozs. 7 dwts. |
| Per cent. of copper | 27.2% | 25% | 21.4% | 20% |
| Average value of original copper per ton | 207 ozs. 4 dwts. | 251 ozs. 3 dwts. | 299 ozs. 15 dwts. | 332 ozs. 3 dwts. |
| Weight of argentiferous lead liquated out | 423.6 lbs. | 494.2 lbs. | 635.4 lbs. | 706 lbs. |
| Average value of liquated lead per ton | 79 ozs. | 79 ozs. | 79 ozs. | 85 ozs. |
| Weight of residues (residual copper and thorns) | 353 lbs. | 353 lbs. | 353 lbs. | 353 lbs. |
| Average value of residues per ton | 34 ozs. | 34 ozs. | 34 ozs. | 34 ozs. to 38 ozs. |
| Extraction of silver into the argentiferous lead | 76.5% | 73.4% | 79% | 85.3% |
[Pg 509][12] An analysis of this "slag" by Karsten (Archiv. 1st Series IX, p. 24) showed 63.2% lead oxide, 5.1% cuprous oxide, 20.1% silica (from the fuel and furnace linings), together with some iron alumina, etc. The pompholyx and spodos were largely zinc oxide (see note, p. [394]).
[13] This description of a centumpondium which weighed either 1331/3 librae, or 1463/4 librae, adds confusion to an already much mixed subject (see [Appendix C].). Assuming the German pfundt to weigh 7,219 troy grains, and the Roman libra 4,946 grains, then a centner would weigh 145.95 librae, which checks up fairly well with the second case; but under what circumstances a centner can weigh 1331/3 librae we are unable to record. At first sight it might appear from this statement that where Agricola uses the word centumpondium he means the German centner. On the other hand, in the previous five or six pages the expressions one-third, five-sixths, ten-twelfths of a libra are used, which are even divisions of the Roman 12 unciae to one libra, and are used where they manifestly mean divisions of 12 units. If Agricola had in mind the German scale, and were using the libra for a pfundt of 16 untzen, these divisions would amount to fractions, and would not total the sicilicus and drachma quantities given, nor would they total any of the possibly synonymous divisions of the German untzen (see also page [254]).
[14] If we assume Roman weights, the charge in the first case can be tabulated as follows, and for convenience will be called the fifth charge:—
| 5th Charge (3 cakes). | |
| Amount of copper | 211.8 lbs. |
| Amount of lead | 635.4 lbs. |
| Weight of each cake | 282.4 lbs. |
| Average value of charge | 218 ozs. 18 dwts. |
| Per cent. of copper | 25% |
| Average value of original copper per ton | 583 ozs. 6 dwts. 16 grs. |
| Weight of argentiferous lead liquated out | 494.2 lbs. |
| Average value of liquated lead per ton | 352 ozs. 8 dwts. |
| Weight of residues | 353 lbs. |
| Average value of residues per ton | 20 ozs. (about). |
| Extraction of silver into the argentiferous lead | 94% |
The results given in the second case where the copper contains 2 librae and a bes per centumpondium do not tie together at all, for each liquation cake should contain 3 librae 91/2 unciae, instead of 11/2 librae and 1/2 uncia of silver.
[Pg 510][15] In this enrichment of copper by the "settling" of the silver in the molten mass the original copper ran, in the two cases given, 60 ozs. 15 dwts. and 85 ozs. 1 dwt. per ton. The whole charge weighed 2,685 lbs., and contained in the second case 114 ozs. Troy, omitting fractions. On melting, 1,060 lbs. were drawn off as "tops," containing 24 ozs. of silver, or running 45 ozs. per ton, and there remained 1,625 lbs. of "bottoms," containing 90 ozs. of silver, or averaging 110 ozs. per ton. It will be noticed later on in the description of making liquation cakes from these copper bottoms, that the author alters the value from one-third librae, a semi-uncia and a drachma per centumpondium to one-third of a libra, i.e., from 110 ozs. to 97 ozs. 4 dwts. per ton. In the Glossary this furnace is described as a spleisofen, i.e., a refining hearth.