Note 1 — BOOK XI.

[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.

The discussion of detailed features of the process has been reserved to notes attached to the actual text, to which the reader is referred. As to the general result of liquation, Karsten (see below) estimates the losses in the liquation of the equivalent of 100 lbs. of argentiferous copper to amount to 32-35 lbs. of lead and 5 to 6 lbs. of copper. Percy (see below) quotes results at Lautenthal in the Upper Harz for the years 1857-60, showing losses of 25% of the silver, 9.1% of the copper, and 36.37 lbs. of lead to the 100 lbs. of copper, or say, 16% of the lead; and a cost of £8 6s. per ton of copper. The theoretical considerations involved in liquation have not been satisfactorily determined. Those who may wish to pursue the subject will find repeated descriptions and much discussion in the following works, which have been freely consulted in the notes which follow upon particular features of the process. It may be mentioned that Agricola's treatment of the subject is more able than any down to the 18th century. Ercker (Beschreibung Allerfürnemsten Mineralischen, etc., Prague, 1574). Lohneys (Bericht vom Bergwercken, etc., Zellerfeldt, 1617). Schlüter (Gründlicher Unterricht [Pg 492]von Hütte-Werken, Braunschweig, 1738). Karsten (System der Metallurgie V. and Archiv für Bergbau und Hüttenwesen, 1st series, 1825). Berthier (Annales des Mines, 1825, II.). Percy (Metallurgy of Silver and Gold, London, 1880).

Nomenclature.—This process held a very prominent position in German metallurgy for over four centuries, and came to have a well-defined nomenclature of its own, which has never found complete equivalents in English, our metallurgical writers to the present day adopting more or less of the German terms. Agricola apparently found no little difficulty in adapting Latin words to his purpose, but stubbornly adhered to his practice of using no German at the expense of long explanatory clauses. The following table, prepared for convenience in translation, is reproduced. The German terms are spelled after the manner used in most English metallurgies, some of them appear in Agricola's Glossary to De Re Metallica.

English.Latin.German.

Blast furnacePrima fornaxSchmeltzofen

Liquation furnaceFornax in qua argentum et plumbum ab aere secernunturSaigernofen

Drying furnaceFornax in qua aerei panes fathiscentes torrenturDarrofen

Refining hearthFornax in qua panes aerei torrefacti coquunturGaarherd

Cupellation furnaceSecunda fornax, or fornax in qua plumbum ab argento separaturTreibherd

LeadingMisturaFrischen

LiquatingStillare, or distillareSaigern

"Drying"TorrereDarren

RefiningAes ex panibus torrefactis conficereGaarmachen

Liquation cakesPanes ex aere ac plumbo mistiSaigerstock

Exhausted liquation cakesPanes fathiscentesKiehnstock, or Kinstocke

"Dried" cakesPanes torrefactiDarrlinge

Slags:

from leadingRecrementa (with explanatory phrases)Frischschlacke

" drying " " "Darrost

" refining " " "Gaarschlacke

Liquation thornsSpinae (with explanatory phrases)Saigerdörner, or Röstdörner

Thorns from "drying" " " "Darrsöhle

" " cupellation " " "Abstrich

Silver-lead or liquated silver-leadStannumSaigerwerk or saigerblei

Ash-coloured copperAes cinereumPickschiefer or schifer

Furnace accretions or "accretions"CadmiaeOffenbrüche

[Pg 494] Historical Note.—So far as we are aware, this is the first complete discussion of this process, although it is briefly mentioned by one writer before Agricola—that is, by Biringuccio (III, 5, 8), who wrote ten years before this work was sent to the printer. His account is very incomplete, for he describes only the bare liquation, and states that the copper is re-melted with lead and re-liquated until the silver is sufficiently abstracted. He neither mentions "drying" nor any of the bye-products. In his directions the silver-lead alloy was cupelled and the copper ultimately refined, obviously by oxidation and poling, although he omits the pole. In A.D. 1150 Theophilus (p. 305, Hendrie's Trans.) describes melting lead out of copper ore, which would be a form of liquation so far as separation of these two metals is concerned, but obviously not a process for separating silver from copper. This passage is quoted in the note on copper smelting (Note on p. 405). A process of such well-developed and complicated a character must have come from a period long before Agricola; but further than such a surmise, there appears little to be recorded. Liquation has been during the last fifty years displaced by other methods, because it was not only tedious and expensive, but the losses of metal were considerable.