|
452
|
BOOK X.
|
||
|
|
|||
|
a siciliens of copper is added ; and if it contains no copper, then half an
uncia, because copper must be added to stibium in order to part gold from silver. The gold is first placed in a red hot earthen crucible, and when melted it swells, and a little stibium is added to it lest it run over ; in a short space of time, when this has melted, it likewise again swells, and when this occurs it is advisable to put in all the remainder of the stibium, and to cover the crucible with a lid, and then to heat the mixture for the time required to walk thirty-five paces. Then it is at once poured out into an iron pot, wide at the top and narrow at the bottom, which was first heated and smeared over with tallow or wax, and set on an iron or wooden block. It is shaken violently, and by this agitation the gold lump settles to the bottom, and when the pot has cooled it is tapped loose, and is again melted four times in the same way. But each time a less weight of stibium is added to the gold, until finally only twice as much stibium is added as there is gold, or a little more ; then the gold lump is melted in a cupel. The stibium is melted again three or four times in an earthen crucible, and each time a gold lump settles, so that there are three or four gold lumps, and these are all melted together in a cupel. To two librae and a half of such stibium are added two librae of argol
and one libra of glass-galls, and they are melted in an earthen crucible, where a lump likewise settles at the bottom ; this lump is melted in the cupel. Finally, the stibium with a little lead added, is melted in the cupel, in which, after all the rest has been consumed by the fire, the silver alone remains. If the stibium is not first melted in an earthen crucible with argol and glass-galls, before it is melted in the cupel, part of the silver is consumed, and is absorbed by the ash and powder of which the cupel is made. The crucible in which the gold and silver alloy are melted with stibium,
and also the cupel, are placed in a furnace, which is usually of the kind frothing," and recommend that the crucible should be only partly filled. As to the copper,
we suggest that it would desulphurize part of the antimony and thus free some of that metal to collect the gold. If we assume bullion of the medium fineness mentioned and containing no copper, then the proportions in the first charge would be about 36 parts gold, 12 parts silver, 41 parts sulphur, 103 parts antimony, and 9 parts copper. The silver and copper would take up 4.25 parts of sulphur, and thus free about 10.6 parts of antimony as metallics. It would thus appear that the amount of metallics provided to assist the collection of the gold was little enough, and that the copper in freeing 5.6 parts of the antimony was useful. It appears to have been necessary to have a large excess of antimony sulphide ; for even with the great surplus in the first charge, the reaction was only partial, as is indicated by the necessity for repeated melting with further antimony. The later metallurgists all describe the separation of the metallic antimony from
the gold as being carried out by oxidation of the antimony, induced by a jet of air into the crucible, this being continued until the mass appears limpid and no cloud forms in the surface in cooling. Agricola describes the separation of the silver from the regulus by preliminary melting with argols, glass-gall, and some lead, and subsequent cupellation of the lead-silver alloy. The statement that unless this preliminary melting is done, the cupel will absorb silver, might be consonant with an attempt at cupellation of sulphides, and it is difficult to see that much desulphurizing could take place with the above fluxes. In fact, in the later descriptions of the process, iron is used in this melting, and we are under the impression that Agricola had omitted this item for a desulphurizing reagent. At the Dresden Mint, in the methods described by Percy (Metallurgy Silver and Gold, p. 373) the gold lumps were tested for fineness, and from this the amount of gold retained in the regulus was computed. It is not clear from Agricola's account whether the test with nitric acid was applied to the regulus or to the " lumps". For historical notes see p. 461. |
|||
|
|
|||