The concentrates are of two kinds—precious and base.[33] The first are obtained from the accretions of the blast furnace, when liquation cakes are made from copper and lead, or from precious liquation thorns, or from the better quality "slags," or from the best grade of concentrates, or from the sweepings and bricks of the furnaces in which exhausted liquation cakes are "dried"; all of these things are crushed and washed, as I explained in [Book VIII]. The base concentrates are made from accretions formed when cakes are cast from base thorns or from the worst quality of slags. The smelter who makes liquation cakes from the precious concentrates, adds to them three wheelbarrowsful of litharge and four barrowsful of hearth-lead and one of ash-coloured copper, from all of which nine or ten liquation cakes are melted out, of which five at a time are placed in the furnace in which silver-lead is liquated from copper; a centumpondium of the lead which drips from these cakes contains one uncia of silver. The liquation thorns are placed apart by themselves, of which one basketful is mixed with the precious thorns to be re-melted. The exhausted liquation cakes are "dried" at the same time as other good exhausted liquation cakes.
The thorns which are drawn off from the lead, when it is separated from silver in the cupellation furnace[34], and the hearth-lead which remains in the crucible in the middle part of the furnaces, together with the hearth material which has become defective and has absorbed silver-lead, are all melted together with a little slag in the blast furnaces. The lead, or rather the silver-lead, which flows from the furnace into the forehearth, is poured out into copper moulds such as are used by the refiners; a centumpondium of such lead contains four unciae of silver, or, if the hearth was defective, it contains more. A small portion of this material is added to the copper and lead when liquation cakes are made from them, if more were to be added the alloy would be much richer than it should be, for which reason the wise foreman of the works mixes these thorns with other precious thorns. The hearth-lead which remains in the middle of the crucible, and the hearth material which absorbs silver-lead, is mixed with other hearth-lead which remains in the cupellation furnace crucible; and yet some cakes, made rich in this manner, may be placed again in the cupellation furnaces, together with the rest of the silver-lead cakes which the refiner has made.
The inhabitants of the Carpathian Mountains, if they have an abundance of finely crushed copper[35] or lead either made from "slags," or collected from the furnace in which the exhausted liquation cakes are dried, or litharge, alloy them in various ways. The "first" alloy consists of two centumpondia of lead melted out of thorns, litharge, and thorns made from hearth-lead, and of half a centumpondium each of lead collected in the furnace in which exhausted liquation cakes are "dried," and of copper minutum, and from these are made liquation cakes; the task of the smelter is finished when he has made forty liquation cakes of this kind. The "second" alloy consists of two centumpondia of litharge, of one and a quarter centumpondia of de-silverized lead or lead from "slags," and of half a centumpondium of lead made from thorns, and of as much copper minutum. The "third" alloy consists of three centumpondia of litharge and of half a centumpondium each of de-silverized lead, of lead made from thorns, and of copper minutum contusum. Liquation cakes are made from all these alloys; the task of the smelters is finished when they have made thirty cakes.
The process by which cakes are made among the Tyrolese, from which they separate the silver-lead, I have explained in [Book IX].
Silver is separated from iron in the following manner. Equal portions of iron scales and filings and of stibium are thrown into an earthenware crucible which, when covered with a lid and sealed, is placed in a furnace, into which air is blown. When this has melted and again cooled, the crucible is broken; the button that settles in the bottom of it, when taken out, is pounded to powder, and the same weight of lead being added, is mixed and melted in a second crucible; at last this button is placed in a cupel and the lead is separated from the silver.[36]
There are a great variety of methods by which one metal is separated from other metals, and the manner in which the same are alloyed I have explained partly in the eighth book of De Natura Fossilium, and partly I will explain elsewhere. Now I will proceed to the remainder of my subject.
END OF BOOK XI.
FOOTNOTES:
[Pg 491][1] The whole of this Book is devoted to the subject of the separation of silver from copper by liquation, except pages [530]-[9] on copper refining, and page [544] on the separation of silver from iron. We believe a brief outline of the liquation process here will refresh the mind of the reader, and enable him to peruse the Book with more satisfaction. The fundamental principle of the process is that if a copper-lead alloy, containing a large excess of lead, be heated in a reducing atmosphere, above the melting point of lead but below that of copper, the lead will liquate out and carry with it a large proportion of the silver. As the results are imperfect, the process cannot be carried through in one operation, and a large amount of bye-products is created which must be worked up subsequently. The process, as here described, falls into six stages. 1st, Melting the copper and lead in a blast furnace to form "liquation cakes"—that is, the "leading." If the copper contain too little silver to warrant liquation directly, then the copper is previously enriched by melting and drawing off from a settling pot the less argentiferous "tops" from the metal, liquation cakes being made from the enriched "bottoms." 2nd, Liquation of the argentiferous lead from the copper. This work was carried out in a special furnace, to which the admission of air was prevented as much as possible in order to prevent oxidation. 3rd, "Drying" the residual copper, which retained some lead, in a furnace with a free admission of air. The temperature was raised to a higher degree than in the liquation furnace, and the expelled lead was oxidized. 4th, Cupellation of the argentiferous lead. 5th, Refining of the residual copper from the "drying" furnace by oxidation of impurities and poling in a "refining furnace." 6th, Re-alloy and re-liquation of the bye-products. These consist of: a, "slags" from "leading"; b, "slags" from "drying"; c, "slags" from refining of the copper. All of these "slags" were mainly lead oxides, containing some cuprous oxides and silica from the furnace linings; d, "thorns" from liquation; e, "thorns" from "drying"; f, "thorns" from skimmings during cupellation; these were again largely lead oxides, but contained rather more copper and less silica than the "slags"; g, "ash-coloured copper," being scales from the "dried" copper, were cuprous oxides, containing considerable lead oxides; h, concentrates from furnace accretions, crushed bricks, &c.