If the copper is not good, the master draws off the "slags" twice, or three times if necessary—the first time when some of the cakes have been melted, the second when all have melted, the third time when the copper has been heated for some time. If the copper was of good quality, the "slags" are not drawn off before the operation is finished, but at the time they are to be drawn off, he depresses the bar over both bellows, and places over both a stick, a cubit long and a palm wide, half cut away at the upper part, so that it may pass under the iron pin fixed at the back in the perforated wood. This he does likewise when the copper has been completely melted. Then the assistant removes the iron plate with the tongs; these tongs are four feet three palms long, their jaws are about a foot in length, and their straight part measures two palms and three digits, and the curved a palm and a digit. The same assistant, with the iron shovel, throws and heaps up the larger pieces of charcoal into that part of the hearth which is against the little wall which protects the other wall from injury by fire, and partly extinguishes them by pouring water over them. The master, with a hazel stick insertedinto the crucible, stirs it twice. Afterward he draws off the slags with a rabble, which consists of an iron blade, wide and sharp, and of alder-wood; the blade is a digit and a half in width and three feet long; the wooden handle inserted in its hollow part is the same number of feet long, and the alder-wood in which the blade is fixed must have the figure of a rhombus; it must be three palms and a digit long, a palm and two digits wide, and a palm thick. Subsequently he takes a broom and sweeps the charcoal dust and small coal over the whole of the crucible, lest the copper should cool before it flows together; then, with a third rabble, he cuts off the slags which may adhere to the edge of the crucible. The blade of this rabble is two palms long and a palm and one digit wide, the iron part of the handle is a foot and three palms long, the wooden part six feet. Afterward he again draws off the slags from the crucible, which the assistant does not quench by pouring water upon them, as the other slags are usually quenched, but he sprinkles over them a little water and allows them to cool. If the copper should bubble, he presses down the bubbles with the rabble. Then he pours water on the wall and the pipes, that it may flow down warm into the crucible, for, the copper, if cold water were to be poured over it while still hot, would spatter about. If a stone, or a piece of lute or wood, or a damp coal should then fall into it, the crucible would vomit out all the copper with a loud noise like thunder, and whatever it touches it injures and sets on fire.Copper RefiningA—Crucible. B—Board. C—Wedge-shaped bar. D—Cakes of copper made by separating them with the wedge-shaped bar. E—Tongs. F—Tub.[Pg 537]Subsequently he lays a curved board with a notch in it over the front part of the crucible; it is two feet long, a palm and two digits wide, and a digit thick. Then the copper in the crucible should be divided into cakes with an iron wedge-shaped bar; this is three feet long, two digits wide, and steeled on the end for the distance of two digits, and its wooden handle is three feet long. He places this bar on the notched board, and, driving it into the copper, movesit forward and back, and by this means the water flows into the vacant space in the copper, and he separates the cake from the rest of the mass. If the copper is not perfectly smelted the cakes will be too thick, and cannot be taken out of the crucible easily. Each cake is afterward seized by the assistant with the tongs and plunged into the water in the tub; the first one is placed aside so that the master may re-melt it again immediately, for, since some "slags" adhere to it, it is not as perfect as the subsequent ones; indeed, if the copper is not of good quality, he places the first two cakes aside. Then, again pouring water over the wall and the pipes, he separates out the second cake, which the assistant likewise immerses in water and places on the ground together with the others separated out in the same way, which he piles upon them. These, if the copper was of good quality, should be thirteen or more in number; if it was not of good quality, then fewer. If the copper was of good quality, this part of the operation, which indeed is distributed into four parts, is accomplished by the master in two hours; if of mediocre quality, in two and a half hours; if of bad quality, in three. The "dried" cakes are re-melted, first in the first crucible and then in the second. The assistant must, as quickly as possible, quench all the cakes with water, after they have been cut out of the second crucible. Afterward with the tongs he replaces in its proper place the iron plate which was in front of the furnace, and throws the charcoal back into the crucible with a shovel. Meanwhile the master, continuing his work, removes the wooden stick from the bars of the bellows, so that in re-melting the other cakes he may accomplish the third part of his process; this must be carefully done, for if a particle from any iron implement should by chance fall into the crucible, or should be thrown in by any malevolent person, the copper could not be made until the iron had been consumed, and therefore double labour would have to be expended upon it. Finally, the assistant extinguishes all the glowing coals, and chips off the dry lute from the mouth of the copper pipe with a hammer; one end of this hammer is pointed, the other round, and it has a wooden handle five feet long. Because there is danger that the copper would be scattered if thepompholyxandspodos, which adhere to the walls and the hood erected upon them, should fall into the crucible, he cleans them off in the meantime. Every week he takes the copper flowers out of the tub, after having poured off the water, for these fall into it from the cakes when they are quenched.[26]
The bellows which this master uses differ in size from the others, for the boards are seven and a half feet long; the back part is three feet wide; the front, where the head is joined on is a foot, two palms and as many digits. The head is a cubit and a digit long; the back part of it is a cubit and a palm wide, and then becomes gradually narrower. The nozzles of the bellows are bound together by means of an iron chain, controlled by a thick bar, one end of which penetrates into the ground against the back of the long wall, and the other end passes under the beam which is laid upon the foremost perforated beams. These nozzles are so placed in a copper pipe that they are at a distance of a palm from the mouth; the mouth should be made three digits in diameter, that the air may be violently expelled through this narrow aperture.
There now remain the liquation thorns, the ash-coloured copper, the "slags," and thecadmia.[27]Liquation cakes are made from thorns in the following manner.[28]There are taken three-quarters of acentumpondiumof thorns, which have their origin from the cakes of copper-lead alloy when lead-silver is liquated, and as many parts of acentumpondiumof the thorns derived from cakes made from once re-melted thorns by the same method, and to them are added acentumpondiumof de-silverized lead and half acentumpondiumof hearth-lead. If there is in the works plenty of litharge, it is substituted for the de-silverized lead. One and a halfcentumpondiaof litharge and hearth-lead is added to the same weight of primary thorns, and half acentumpondiumof thorns which have their origin from liquation cakes composed of thorns twice re-melted by the same method (tertiary thorns), and a fourth part of acentumpondiumof thorns which are producedwhen the exhausted liquation cakes are "dried." By both methods one single liquation cake is made from threecentumpondia. In this manner the smelter makes every day fifteen liquation cakes, more or less; he takes great care that the metallic substances, from which the first liquation cake is made, flow down properly and in due order into the forehearth, before the material of which the subsequent cake is to be made. Five of these liquation cakes are put simultaneously into the furnace in which silver-lead is liquated from copper, they weigh almost fourteencentumpondia, and the "slags" made therefrom usually weigh quite acentumpondium. In all the liquation cakes together there is usually onelibraand nearly twounciaeof silver, and in the silver-lead which drips from those cakes, and weighs seven and a halfcentumpondia, there is in each anunciaand a half of silver. In each of the threecentumpondiaof liquation thorns there is almost anunciaof silver, and in the twocentumpondiaand a quarter of exhausted liquation cakes there is altogether one and a halfunciae; yet this varies greatly for each variety of thorns, for in the thorns produced from primary liquation cakes made of copper and lead when silver-lead is liquated from the copper, and those produced in "drying" the exhausted liquation cakes, there are almost twounciaeof silver; in the others not quite anuncia. There are other thorns besides, of which I will speak a little further on.
Those in the Carpathian Mountains who make liquation cakes from the copper "bottoms" which remain after the upper part of the copper is divided from the lower, in the furnace similar to an oven, produce thorns when the poor or mediocre silver-lead is liquated from the copper. These, together with those made of cakes of re-melted thorns, or made with re-melted litharge, are placed in a heap by themselves; but those that are made from cakes melted from hearth-lead are placed in a heap separate from the first, and likewise those produced from "drying" the exhausted liquation cakes are placed separately; from these thorns liquation cakes are made. From the first heap they take the fourth part of acentumpondium, from the second the same amount, from the third acentumpondium,—to which thorns are added one and a halfcentumpondiaof litharge and half acentumpondiumof hearth-lead, and from these, melted in the blast furnace, a liquation cake is made; each workman makes twenty such cakes every day. But of theirs enough has been said for the present; I will return to ours.
The ash-coloured copper[29]which is chipped off, as I have stated, from the "dried" cakes, used some years ago to be mixed with the thorns produced from liquation of the copper-lead alloy, and contained in themselves, equally with the first, twounciaeof silver; but now it is mixed with the concentrates washed from the accretions and the other material. The inhabitants of the Carpathian Mountains melt this kind of copper in furnaces in which are re-melted the "slags" which flow out when the copper is refined; but as this soon melts and flows down out of the furnace, two workmen are required forthe work of smelting, one of whom smelts, while the other takes out the thick cakes from the forehearth. These cakes are only "dried," and from the "dried" cakes copper is again made.
The "slags"[30]are melted continually day and night, whether they have been drawn off from the alloyed metals with a rabble, or whether they adhered to the forehearth to the thickness of a digit and made it smaller and were taken off with spatulas. In this manner two or three liquation cakes are made, and afterward much or little of the "slag," skimmed from the molten alloy of copper and lead, is re-melted. Such liquation cakes should weigh up to threecentumpondia, in each of which there is half anunciaof silver. Five cakes are placed at the same time in the furnace in which argentiferous lead is liquated from copper, and from these are made lead which contains half anunciaof silver to thecentumpondium. The exhausted liquation cakes are laid upon the other baser exhausted liquation cakes, from both of which yellow copper is made. The base thorns thus obtained are re-melted with a few baser "slags," after having been sprinkled with concentrates from furnace accretions and other material, and in this manner six or seven liquation cakes are made, each of which weighs some twocentumpondia. Five of these are placed at the same time in the furnace in which silver-lead is liquated from copper; these drip threecentumpondiaof lead, each of which contains half anunciaof silver. The basest thorns thus produced should be re-melted with only a little "slag." The copper alloyed with lead, which flows down from the furnace into the forehearth, is poured out with a ladle into oblong copper moulds; these cakes are "dried" with base exhausted liquation cakes. The thorns they produce are added to the base thorns, and they are made into cakes according to the method I have described. From the "dried" cakes they make copper, of which some add a small portion to the best "dried" cakes when copper is made from them, in order that by mixing the base copper with the good it may be sold without loss. The "slags," if they are utilisable, are re-melted a second and a third time, the cakes made from them are "dried," and from the "dried" cakes is made copper, which is mixed with the good copper. The "slags," drawn off by the master who makes copper out of "dried" cakes, are sifted, and those which fall through the sieve into a vessel placed underneath are washed; those which remain in it are emptied into a wheelbarrow and wheeled away to the blast furnaces, and they are re-melted together with other "slags," over which are sprinkled the concentrates from washing the slags or furnace accretions made at this time. The copper which flows outof the furnace into the forehearth, is likewise dipped out with a ladle into oblong copper moulds; in this way nine or ten cakes are made, which are "dried," together with bad exhausted liquation cakes, and from these "dried" cakes yellow[31]copper is made.
Copper RefiningA—Furnace. B—Forehearth. C—Oblong moulds.[Pg 543]Thecadmia,[32]as it is called by us, is made from the "slags" which the master, who makes copper from "dried" cakes, draws off together with other re-melted base "slags"; for, indeed, if the copper cakes made from such "slags" are broken, the fragments are calledcadmia; from this and yellow copper is madecaldariumcopper in two ways. For either two parts ofcadmiaare mixed with one of yellow copper in the blast furnaces, and melted; or, on the contrary, two parts of yellow copper with one ofcadmia, so that thecadmiaand yellow copper may be well mixed; and the copper which flows down from the furnace into the forehearth is poured out with a ladle into oblong copper moulds heated beforehand. These moulds are sprinkled over with charcoal dust before thecaldariumcopper is to be poured into them, and the same dust is sprinkled over the copper when it is poured in, lest thecadmiaand yellow copper should freeze before they have become well mixed. With a piece of wood the assistant cleanses each cake from the dust, when it is turned out of the mould. Then he throws it into the tub containing hot water, for thecaldariumcopper is finer if quenched in hot water. But as I have so often made mention of the oblong copper moulds, I must now speak of them a little; they are a foot and a palm long, the inside is three palms and a digit wide at the top, and they are rounded at the bottom.
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 inBook 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; acentumpondiumof the lead which drips from these cakes contains oneunciaof silver. The liquation thorns areplaced 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; acentumpondiumof such lead contains fourunciaeof 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 wiseforeman 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 twocentumpondiaof lead melted out of thorns, litharge, and thorns made from hearth-lead, and of half acentumpondiumeach of lead collected in the furnace in which exhausted liquation cakes are "dried," and of copperminutum, 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 twocentumpondiaof litharge, of one and a quartercentumpondiaof de-silverized lead or lead from "slags," and of half acentumpondiumof lead made from thorns, and of as much copperminutum. The "third" alloy consists of threecentumpondiaof litharge and of half acentumpondiumeach of de-silverized lead, of lead made from thorns, and of copperminutum 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 inBook IX.
Silver is separated from iron in the following manner. Equal portions of iron scales and filings and ofstibiumare 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 ofDe 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 pages530-9on copper refining, and page544on 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 (seebelow) 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 (seebelow) 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.andArchiv 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 toDe Re Metallica.English.Latin.German.Blast furnacePrima fornaxSchmeltzofenLiquation furnaceFornax in qua argentum et plumbum ab aere secernunturSaigernofenDrying furnaceFornax in qua aerei panes fathiscentes torrenturDarrofenRefining hearthFornax in qua panes aerei torrefacti coquunturGaarherdCupellation furnaceSecunda fornax, orfornax in qua plumbum ab argento separaturTreibherdLeadingMisturaFrischenLiquatingStillare, ordistillareSaigern"Drying"TorrereDarrenRefiningAes ex panibus torrefactis conficereGaarmachenLiquation cakesPanes ex aere ac plumbo mistiSaigerstockExhausted liquation cakesPanes fathiscentesKiehnstock, orKinstocke"Dried" cakesPanes torrefactiDarrlingeSlags:from leadingRecrementa(with explanatory phrases)Frischschlacke" drying" " "Darrost" refining" " "GaarschlackeLiquation thornsSpinae(with explanatory phrases)Saigerdörner, orRöstdörnerThorns from "drying"" " "Darrsöhle" " cupellation" " "AbstrichSilver-lead or liquated silver-leadStannumSaigerwerkorsaigerbleiAsh-coloured copperAes cinereumPickschieferorschiferFurnace 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. InA.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.[2]Paries,—"Partition" or "wall." The author uses this term throughout in distinction tomurus, usually applying the latter to the walls of the building and the former to furnace walls, chimney walls, etc. In order to gain clarity, we have introduced the term "hood" in distinction to "chimney," and so far as possible refer to thepariesof these constructions and furnaces as "side of the furnace," "side of the hood," etc.[Pg 495][4]From this point on, the construction of the roofs, in the absence of illustration, is hopeless of intelligent translation. The constant repetition of "tignum," "tigillum," "trabs," for at least fifteen different construction members becomes most hopelessly involved, especially as the author attempts to distinguish between them in a sort of "House-that-Jack-built" arrangement of explanatory clauses.[Pg 496][5]In the original text this is given as the "fifth," a manifest impossibility.[Pg 500][6]Chelae,—"claws."[7]If Roman weights, this would be 5.6 short tons, and 7.5 tons if Germancentneris meant.[Pg 501][8]This is, no doubt, a reference to Pliny's statement (XXXIII, 35) regarding litharge at Puteoli. This passage from Pliny is given in the footnote on p.466. Puteoli was situated on the Bay of Naples.[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 footnotes15and17, 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 (seefootnote 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 copper211.8 lbs.211.8 lbs.211.8 lbs.211.8 lbs.Amount of lead564.8 "635.4 "776.6 "847.2 "Weight of each cake193.5 "211.5 "247.1 "264.75 "Average value of charge56 ozs. 3 dwts.62 ozs. 4 dwts.64 ozs. 4 dwts.66 ozs. 7 dwts.Per cent. of copper27.2%25%21.4%20%Average value of original copper per ton207 ozs. 4 dwts.251 ozs. 3 dwts.299 ozs. 15 dwts.332 ozs. 3 dwts.Weight of argentiferous lead liquated out423.6 lbs.494.2 lbs.635.4 lbs.706 lbs.Average value of liquated lead per ton79 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 ton34 ozs.34 ozs.34 ozs.34 ozs. to 38 ozs.Extraction of silver into the argentiferous lead76.5%73.4%79%85.3%[Pg 507][11]See p.356.[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. Thepompholyxandspodoswere largely zinc oxide (see note, p.394).[13]This description of acentumpondiumwhich weighed either 1331/3librae, or 1463/4librae, adds confusion to an already much mixed subject (seeAppendix C.). Assuming the Germanpfundtto weigh 7,219 troy grains, and the Romanlibra4,946 grains, then acentnerwould weigh 145.95librae, which checks up fairly well with the second case; but under what circumstances acentnercan weigh 1331/3libraewe are unable to record. At first sight it might appear from this statement that where Agricola uses the wordcentumpondiumhe means the Germancentner. On the other hand, in the previous five or six pages the expressions one-third, five-sixths, ten-twelfths of alibraare used, which are even divisions of the Roman 12unciaeto onelibra, and are used where they manifestly mean divisions of 12 units. If Agricola had in mind the German scale, and were using thelibrafor apfundtof 16untzen, these divisions would amount to fractions, and would not total thesicilicusanddrachmaquantities given, nor would they total any of the possibly synonymous divisions of the Germanuntzen(see also page254).[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 copper211.8 lbs.Amount of lead635.4 lbs.Weight of each cake282.4 lbs.Average value of charge218 ozs. 18 dwts.Per cent. of copper25%Average value of original copper per ton583 ozs. 6 dwts. 16 grs.Weight of argentiferous lead liquated out494.2 lbs.Average value of liquated lead per ton352 ozs. 8 dwts.Weight of residues353 lbs.Average value of residues per ton20 ozs. (about).Extraction of silver into the argentiferous lead94%The results given in the second case where the copper contains 2libraeand abespercentumpondiumdo not tie together at all, for each liquation cake should contain 3librae91/2unciae, instead of 11/2libraeand1/2unciaof 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-thirdlibrae, asemi-unciaand adrachmapercentumpondiumto one-third of alibra,i.e., from 110 ozs. to 97 ozs. 4 dwts. per ton. In the Glossary this furnace is described as aspleisofen,i.e., a refining hearth.[Pg 511][16]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 pages530to539. Exactly what material is meant by the term red (rubrum), yellow (fulvum) andcaldariumcopper is somewhat uncertain. They are given in the German text simply asrot,geel, andlebeter kupfer, and apparently all were "coarse" copper of different characters destined for the refinery. The author states inDe 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 isregulare. In the same place a dark yellow copper is made which is calledcaldarium, taking its name among the Germans from a caldron....Regularediffers fromcaldariumin 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 inDe 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.Caldariumcopper 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 page542the author describes makingcaldariumcopper from a mixture of yellow copper and a peculiarcadmia, 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 thecaldariummust have been, indeed, impure.[Pg 512][17]The liquation of these low-grade copper "bottoms" required that the liquated lead should be re-used again to make up fresh liquation cakes, in order that it might eventually become rich enough to warrant cupellation. In the following table the "poor" silver-lead is designated (A) the "medium" (B) and the "rich" (C). The three charges here given are designated sixth, seventh, and eighth for purposes of reference. It will be seen that the data is insufficient to complete the ninth and tenth. Moreover, while the author gives directions for making four cakes, he says the charge consists of five, and it has, therefore, been necessary to reduce the volume of products given to this basis.6th Charge.7th Charge.8th Charge.Amount of copper bottoms176.5 lbs.176.5 lbs.176.5 lbs.Amount of lead282.4 lbs. (slags)564.8 lbs. of (A)635.4 lbs. of (B)Amount of de-silverized lead494.2 lbs.211.8 lbs.141.2 lbs. (A)Weight of each cake238.3 lbs.238.3 lbs.238.3 lbs.Average value of charge per ton22 ozs. 5 dwts.35 ozs. 15 dwts.50 ozs. 5 dwts.Per cent. of copper18.5%18.5%18.5%Average value per ton original copper97 ozs. 4 dwts.97 ozs. 4 dwts.97 ozs. 4 dwts.Average value per ton of90 ozs. 2 dwts. (slags)28 ozs. 5 dwts. (A)28 ozs. 5 dwts. (A)Average value per ton of3 ozs. 1 dwt. (lead)3 ozs. 1 dwt. (lead)42 ozs. 10 dwts. (B)Weight of liquated lead550.6 lbs.Average value of the liquated lead per ton28 ozs. 5 dwts. (A)42 ozs. 10 dwts. (B)63 ozs. 16 dwts. (C)Weight of exhausted liquation cakes225.9 lbs.Average value of the exhausted liquation cakes per ton12 ozs. 3 dwts.Weight of liquation thorns169.4 lbs.Average value of the liquation thorns per ton18 ozs. 4 dwts.Extraction of silver into the liquated lead71%[Pg 520][18]For the liquation it was necessary to maintain a reducing atmosphere, otherwise the lead would oxidize; this was secured by keeping the cakes well covered with charcoal and by preventing the entrance of air as much as possible. Moreover, it was necessary to preserve a fairly even temperature. The proportions of copper and lead in the three liquation products vary considerably, depending upon the method of conducting the process and the original proportions. From the authors consulted (see note p.492) an average would be about as follows:—The residual copper—exhausted liquation cakes—ran from 25 to 33% lead; the liquated lead from 2 to 3% copper; and the liquation thorns, which were largely oxidized, contained about 15% copper oxides, 80% lead oxides, together with impurities, such as antimony, arsenic, etc. The proportions of the various products would obviously depend upon the care in conducting the operation; too high temperature and the admission of air would increase the copper melted and oxidize more lead, and thus increase the liquation thorns. There are insufficient data in Agricola to adduce conclusions as to the actual ratios produced. The results given for the 6th charge (note 17, p. 512) would indicate about 30% lead in the residual copper, and would indicate that the original charge was divided into about 24% of residual copper, 18% of liquation thorns, and 57% of liquated lead. This, however, was an unusually large proportion of liquation thorns, some of the authors giving instances of as low as 5%.[Pg 522][19]The first instance given, of 44centumpondia(3,109 lbs.) lead and onecentumpondium(70.6 lbs.) copper, would indicate that the liquated lead contained 2.2% copper. The second, of 46centumpondia(3,250 lbs.) lead and 11/2centumpondiacopper (106 lbs.), would indicate 3% copper; and in the third, 120centumpondia(8,478 lbs.) lead and six copper (424 lbs.) would show 4.76% copper. This charge of 120centumpondiain the cupellation furnace would normally make more than 110centumpondiaof litharge and 30 of hearth-lead,i.e., saturated furnace bottoms. The copper would be largely found in the silver-lead "which does not melt," at the margin of the crucible. These skimmings are afterward referred to as "thorns." It is difficult to understand what is meant by the expression that the silver which is in the copper is mixed with the remaining (reliquo) silver. The coppery skimmings from the cupellation furnace are referred to again inNote 28, p. 539.[Pg 523][20]A further amount of lead could be obtained in the first liquation, but a higher temperature is necessary, which was more economical to secure in the "drying" furnace. Therefore, the "drying" was really an extension of liquation; but as air was admitted the lead and copper melted out were oxidized. The products were the final residual copper, called by Agricola the "dried" copper, together with lead and copper oxides, called by him the "slags," and the scale of copper and lead oxides termed by him the "ash-coloured copper." The German metallurgists distinguished two kinds of slag: the first and principal one, thedarrost, and the second thedarrsöhle, this latter differing only in that it contained more impurities from the floor of the furnace, and remained behind until the furnace cooled. Agricola possibly refers to these as "more liquation thorns," because in describing the treatment of the bye-products he refers to thorns from the process, whereas in the description of "drying" he usually refers to "slags." A number of analyses of these products, given by Karsten, show the "dried" copper to contain from 82.7 to 90.6% copper, and from 9.4 to 17.3% lead; the "slag" to contain 76.5 to 85.1% lead oxide, and from 4.1 to 7.8% cuprous oxide, with 9 to 13% silica from the furnace bottoms, together with some other[Pg 524]impurities; the "ash-coloured copper" to contain about 60% cuprous oxide and 30% lead oxide, with some metallic copper and minor impurities. An average of proportions given by various authors shows, roughly, that out of 100centnersof "exhausted" liquation cakes, containing about 70% copper and 30% lead, there were about 63centnersof "dried" copper, 38centnersof "slag," and 61/2centnersof "ash-coloured copper." According to Karsten, the process fell into stages; first, at low temperature some metallic lead appeared; second, during an increasing temperature for over 14 to 15 hours the slags ran out; third, there was a period of four hours of lower temperature to allow time for the lead to diffuse from the interior of the cakes; and fourth, during a period of eight hours the temperature was again increased. In fact, the latter portion of the process ended with the economic limit between leaving some lead in the copper and driving too much copper into the "slags." Agricola gives the silver contents of the "dried" copper as 3drachmaeto 1centumpondium, or equal to about 9 ozs. per ton; and assuming that the copper finally recovered from the bye-products ran no higher, then the first four charges (see note on p.506) would show a reduction in the silver values of from 95 to 97%; the 7th and 8th charges (note on p.512) of about 90%.[21]If Roman weights, this would equal from 6,360 lbs. to 7,066 lbs.[Pg 529][22]One halfuncia, or threedrachmaeof silver would equal either 12 ozs. or 9 ozs. per ton. If we assume the values given for residual copper in the first four charges (note p.506) of 34 ozs., this would mean an extraction of, roughly, 65% of the silver from the exhausted liquation cakes.[Pg 530][23]Seenote 29, p. 540.[Pg 533][24]Assuming Roman weights:2centumpondia=141.3lbs.21/2"=176.6"3"=211.9"31/2"=248.2"6"=423.9"[Pg 535][25]This description of refining copper in an open hearth by oxidation with a blast and "poling"—thegaarmachenof the Germans—is so accurate, and the process is so little changed in some parts of Saxony, that it might have been written in the 20th century instead of the 16th. The best account of the old practice in Saxony after Agricola is to be found in Schlüter'sHütte Werken(Braunschweig, 1738, Chap.CXVIII.). The process has largely been displaced by electrolytic methods, but is still in use in most refineries as a step in electrolytic work. It may be unnecessary to repeat that the process is one of subjecting the molten mass of impure metal to a strong and continuous blast, and as a result, not only are the impurities to a considerable extent directly oxidized and taken off as a slag, but also a considerable amount of copper is turned into cuprous oxide. This cuprous oxide mostly melts and diffuses through the metallic copper, and readily parting with its oxygen to the impurities further facilitates their complete oxidation. The blast is continued until the impurities are practically eliminated, and at this stage the molten metal contains a great deal of dissolved cuprous oxide, which must be reduced. This is done by introducing a billet of green wood ("poling"), the dry distillation of which generates large quantities of gases, which reduce the oxide. The state of the metal is even to-day in some localities tested by dipping into it the point of an iron rod; if it be at the proper state the adhering copper has a net-like appearance, should be easily loosened from the rod by dipping in water, is of a reddish-copper colour and should be quite pliable; if the metal is not yet refined, the sample is thick, smooth, and detachable with difficulty; if over-refined, it is thick and brittle. By allowing water to run on to the surface of the molten metal, thin cakes are successively formed and taken off. These cakes were the article known to commerce over several centuries as "rosetta copper." The first few cakes are discarded as containing impurities or slag, and if the metal be of good quality the cakes are thin and of a red colour. Their colour and thinness, therefore, become a criterion of purity. The cover of charcoal or charcoal dust maintained upon the surface of the metal tended to retard oxidation, but prevented volatilization and helped to secure the impurities as a slag instead. Karsten (Archiv., 1st series, p. 46) gives several analyses of the[Pg 536]slag from refining "dried" copper, showing it to contain from 51.7 to 67.4% lead oxide, 6.2 to 19.2% cuprous oxide, and 21.4 to 23.9 silica (from the furnace bottoms), with minor quantities of iron, antimony, etc. The "bubbles" referred to by Agricola were apparently the shower of copper globules which takes place upon the evolution of sulphur dioxide, due to the reaction of the cuprous oxide upon any remaining sulphide of copper when the mass begins to cool.Historical Note.—It is impossible to say how the Ancients refined copper, beyond the fact that they often re-smelted it. Such notes as we can find are set out in the note on copper smelting (note 42, p. 402). The first authentic reference to poling is in Theophilus (1150 to 1200A.D., Hendrie's translation, p. 313), which shows a very good understanding of this method of refining copper:—"Of the Purification of Copper. Take an iron dish of the size you wish, and line it inside and out with clay strongly beaten and mixed, and it is carefully dried. Then place it before a forge upon the coals, so that when the bellows act upon it the wind may issue partly within and partly above it, and not below it. And very small coals being placed round it, place the copper in it equally, and add over it a heap of coals. When by blowing a long time this has become melted, uncover it and cast immediately fine ashes of coals over it, and stir it with a thin and dry piece of wood as if mixing it, and you will directly see the burnt lead adhere to these ashes like a glue, which being cast out again superpose coals, and blowing for a long time, as at first, again uncover it, and then do as you did before. You do this until at length by cooking it you can withdraw the lead entirely. Then pour it over the mould which you have prepared for this, and you will thus prove if it be pure. Hold it with the pincers, glowing as it is, before it has become cold, and strike it with a large hammer strongly over the anvil, and if it be broken or split you must liquefy it anew as before. If, however, it should remain sound, you will cool it in water, and you cook other (copper) in the same manner." Biringuccio (III, 8) in 1540 describes the process briefly, but omits the poling, an essential in the production of malleable copper.[Pg 538][26]Pompholyxandspodoswere impure zinc oxides (seenote 26, p. 394).The copper flowers were no doubt cupric oxide. They were used by the Ancients for medicinal purposes. Dioscorides (V, 48) says: "Of flowers of copper, which some call the scrapings of old nails, the best is friable; it is gold-coloured when rubbed, is like millet in shape and size, is moderately bright, and somewhat astringent. It should not be mixed with copper filings, with which it is often adulterated. But this deception is easily detected, for when bitten in the teeth the filings are malleable. It (the flowers) is made when the copper fused in a furnace has run into the receptacle through the spout pertaining to it, for then the workmen engaged in this trade cleanse it from dirt and pour clear water over it in order to cool it; from this sudden condensation the copper spits and throws out the aforesaid flowers." Pliny (XXXIV, 24) says: "The flower, too, of copper (æris flos) is used in medicine. This is made by fusing copper, and then removing it to another furnace, where the repeated blast makes the metal separate into small scales like millet, known as flowers. These scales also fall off when the cakes of metal are cooled in water; they become red, too, like the scales of copper known as 'lepis,' by use of which the flowers of copper are adulterated, it being also sold for it. These are made when hammering the nails that are[Pg 539]made from the cakes of copper. All these methods are carried on in the works of Cyprus; the difference between these substances is that thesquamae(copper scales) are detached from hammering the cakes, while the flower falls off spontaneously." Agricola (De Nat. Fos., p. 352) notes that "flowers of copper (flos æris) have the same properties as 'roasted copper.'"[27]It seems scarcely necessary to discuss in detail the complicated "flow scheme" of the various minor bye-products. They are all re-introduced into the liquation circuit, and thereby are created other bye-products of the same kindad infinitum. Further notes are given on:—Liquation thornsNote28.Slags"30.Ash-coloured copper"29.Concentrates"33.Cadmia"32.There are no data given, either by Agricola or the later authors, which allow satisfactory calculation of the relative quantities of these products. A rough estimate from the data given in previous notes would indicate that in one liquation only about 70% of the original copper came out as refined copper, and that about 70% of the original lead would go to the cupellation furnace,i.e., about 30% of the original metal sent to the blast furnace would go into the "thorns," "slags," and "ash-coloured copper." The ultimate losses were very great, as given before (p.491), they probably amounted to 25% of the silver, 9% copper, and 16% of the lead.[28]There were the following classes of thorns:—1st.From liquation.2nd.From drying.3rd.From cupellation.In a general way, according to the later authors, they were largely lead oxide, and contained from 5% to 20% cuprous oxide. If a calculation be made backward from the products given as the result of the charge described, it would appear that in this case they must have contained at least one-fifth copper. The silver in these liquation cakes would run about 24 ozs. per ton, in the liquated lead about 36 ozs. per ton, and in the liquation thorns 24 ozs. per ton. The extraction into the liquated lead would be about 80% of the silver.[Pg 540][29]The "ash-coloured copper" is a cuprous oxide, containing some 3% lead oxide; and if Agricola means they contained twounciaeof silver to thecentumpondium, then they ran about 48 ozs. per ton, and would contain much more silver than the mass.[Pg 541][30]There are three principal "slags" mentioned—1st.Slag from "leading."2nd.Slag from "drying."3rd.Slag from refining the copper.From the analyses quoted by various authors these ran from 52% to 85% lead oxide, 5% to 30% cuprous oxide, and considerable silica from the furnace bottoms. They were reduced in the main into liquation cakes, although Agricola mentions instances of the metal reduced from "slags" being taken directly to the "drying" furnace. Such liquation cakes would run very low in silver, and at the values given only averaged 12 ozs. per ton; therefore the liquated lead running the same value as the cakes, or less than half that of the "poor" lead mentioned inNote 17, p. 512, could not have been cupelled directly.[Pg 542][31]SeeNote 16, p. 511, for discussion of yellow andcaldariumcopper.[32]Thiscadmiais given in the Glossary and the German translation askobelt. A discussion of this substance is given in the note on p.112; and it is sufficient to state here that in Agricola's time the metal cobalt was unknown, and the substances designatedcadmiaandcobaltumwere arsenical-cobalt-zinc minerals. A metal made from "slag" from refining, together with "base" thorns, would be very impure; for the latter, according to the paragraph on concentrates a little later on, would contain the furnace accretions, and would thus be undoubtedly zincky. It is just possible that the termkobeltwas used by the German smelters at this time in the sense of an epithet—"black devil" (seeNote 21, p. 214).[33]It is somewhat difficult to see exactly the meaning of base (vile) and precious (preciosum) in this connection. While "base" could mean impure, "precious" could hardly mean pure, and while "precious" could mean high value in silver, the reverse does not seem entirelyapropos. It is possible that "bad" and "good" would be more appropriate terms.[Pg 543][34]The skimmings from the molten lead in the early stages of cupellation have been discussed inNote 28, p. 539. They are probably called thorns here because of the large amount of copper in them. The lead from liquation would contain 2% to 3% of copper, and this would be largely recovered in these skimmings, although there would be some copper in the furnace bottoms—hearth-lead—and the litharge. These "thorns" are apparently fairly rich, fourunciaeto thecentumpondiumbeing equivalent to about 97 ozs. per ton, and they are only added to low-grade liquation material.[Pg 544][35]Particulis aeris tusi. Unless this be the fine concentrates from crushing the material mentioned, we are unable to explain the expression.[36]This operation would bring down a button of antimony under an iron matte, by de-sulphurizing the antimony. It would seem scarcely necessary to add lead before cupellation. This process is given in an assay method, in theProbierbüchlein(folio 31) 50 years beforeDe Re Metallica: "How to separate silver from iron: Take that silver which is in ironplechen(plachmal), pulverize it finely, take the same iron orplecone part,spiesglasz(antimony sulphide) one part, leave them to melt in a crucible placed in a closedwindtofen. When it is melted, let it cool, break the crucible, chip off the button that is in the bottom, and melt it in a crucible with as much lead. Then break the crucible, and seek from the button in the cupel, and you will find what silver it contains."
[Pg 491][1]The whole of this Book is devoted to the subject of the separation of silver from copper by liquation, except pages530-9on copper refining, and page544on 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 (seebelow) 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 (seebelow) 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.andArchiv 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 toDe Re Metallica.English.Latin.German.Blast furnacePrima fornaxSchmeltzofenLiquation furnaceFornax in qua argentum et plumbum ab aere secernunturSaigernofenDrying furnaceFornax in qua aerei panes fathiscentes torrenturDarrofenRefining hearthFornax in qua panes aerei torrefacti coquunturGaarherdCupellation furnaceSecunda fornax, orfornax in qua plumbum ab argento separaturTreibherdLeadingMisturaFrischenLiquatingStillare, ordistillareSaigern"Drying"TorrereDarrenRefiningAes ex panibus torrefactis conficereGaarmachenLiquation cakesPanes ex aere ac plumbo mistiSaigerstockExhausted liquation cakesPanes fathiscentesKiehnstock, orKinstocke"Dried" cakesPanes torrefactiDarrlingeSlags:from leadingRecrementa(with explanatory phrases)Frischschlacke" drying" " "Darrost" refining" " "GaarschlackeLiquation thornsSpinae(with explanatory phrases)Saigerdörner, orRöstdörnerThorns from "drying"" " "Darrsöhle" " cupellation" " "AbstrichSilver-lead or liquated silver-leadStannumSaigerwerkorsaigerbleiAsh-coloured copperAes cinereumPickschieferorschiferFurnace 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. InA.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.
[Pg 491][1]The whole of this Book is devoted to the subject of the separation of silver from copper by liquation, except pages530-9on copper refining, and page544on 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 (seebelow) 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 (seebelow) 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.andArchiv 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 toDe Re Metallica.
English.Latin.German.Blast furnacePrima fornaxSchmeltzofenLiquation furnaceFornax in qua argentum et plumbum ab aere secernunturSaigernofenDrying furnaceFornax in qua aerei panes fathiscentes torrenturDarrofenRefining hearthFornax in qua panes aerei torrefacti coquunturGaarherdCupellation furnaceSecunda fornax, orfornax in qua plumbum ab argento separaturTreibherdLeadingMisturaFrischenLiquatingStillare, ordistillareSaigern"Drying"TorrereDarrenRefiningAes ex panibus torrefactis conficereGaarmachenLiquation cakesPanes ex aere ac plumbo mistiSaigerstockExhausted liquation cakesPanes fathiscentesKiehnstock, orKinstocke"Dried" cakesPanes torrefactiDarrlingeSlags:from leadingRecrementa(with explanatory phrases)Frischschlacke" drying" " "Darrost" refining" " "GaarschlackeLiquation thornsSpinae(with explanatory phrases)Saigerdörner, orRöstdörnerThorns from "drying"" " "Darrsöhle" " cupellation" " "AbstrichSilver-lead or liquated silver-leadStannumSaigerwerkorsaigerbleiAsh-coloured copperAes cinereumPickschieferorschiferFurnace 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. InA.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.
[2]Paries,—"Partition" or "wall." The author uses this term throughout in distinction tomurus, usually applying the latter to the walls of the building and the former to furnace walls, chimney walls, etc. In order to gain clarity, we have introduced the term "hood" in distinction to "chimney," and so far as possible refer to thepariesof these constructions and furnaces as "side of the furnace," "side of the hood," etc.
[2]Paries,—"Partition" or "wall." The author uses this term throughout in distinction tomurus, usually applying the latter to the walls of the building and the former to furnace walls, chimney walls, etc. In order to gain clarity, we have introduced the term "hood" in distinction to "chimney," and so far as possible refer to thepariesof these constructions and furnaces as "side of the furnace," "side of the hood," etc.
[Pg 495][4]From this point on, the construction of the roofs, in the absence of illustration, is hopeless of intelligent translation. The constant repetition of "tignum," "tigillum," "trabs," for at least fifteen different construction members becomes most hopelessly involved, especially as the author attempts to distinguish between them in a sort of "House-that-Jack-built" arrangement of explanatory clauses.
[Pg 495][4]From this point on, the construction of the roofs, in the absence of illustration, is hopeless of intelligent translation. The constant repetition of "tignum," "tigillum," "trabs," for at least fifteen different construction members becomes most hopelessly involved, especially as the author attempts to distinguish between them in a sort of "House-that-Jack-built" arrangement of explanatory clauses.
[Pg 496][5]In the original text this is given as the "fifth," a manifest impossibility.
[Pg 496][5]In the original text this is given as the "fifth," a manifest impossibility.
[Pg 500][6]Chelae,—"claws."
[Pg 500][6]Chelae,—"claws."
[7]If Roman weights, this would be 5.6 short tons, and 7.5 tons if Germancentneris meant.
[7]If Roman weights, this would be 5.6 short tons, and 7.5 tons if Germancentneris meant.
[Pg 501][8]This is, no doubt, a reference to Pliny's statement (XXXIII, 35) regarding litharge at Puteoli. This passage from Pliny is given in the footnote on p.466. Puteoli was situated on the Bay of Naples.
[Pg 501][8]This is, no doubt, a reference to Pliny's statement (XXXIII, 35) regarding litharge at Puteoli. This passage from Pliny is given in the footnote on p.466. Puteoli was situated on the Bay of Naples.
[Pg 503][9]By this expression is apparently meant the "bottoms" produced in enriching copper, as described on p.510.
[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 footnotes15and17, 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 (seefootnote 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 copper211.8 lbs.211.8 lbs.211.8 lbs.211.8 lbs.Amount of lead564.8 "635.4 "776.6 "847.2 "Weight of each cake193.5 "211.5 "247.1 "264.75 "Average value of charge56 ozs. 3 dwts.62 ozs. 4 dwts.64 ozs. 4 dwts.66 ozs. 7 dwts.Per cent. of copper27.2%25%21.4%20%Average value of original copper per ton207 ozs. 4 dwts.251 ozs. 3 dwts.299 ozs. 15 dwts.332 ozs. 3 dwts.Weight of argentiferous lead liquated out423.6 lbs.494.2 lbs.635.4 lbs.706 lbs.Average value of liquated lead per ton79 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 ton34 ozs.34 ozs.34 ozs.34 ozs. to 38 ozs.Extraction of silver into the argentiferous lead76.5%73.4%79%85.3%
[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 footnotes15and17, 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 (seefootnote 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 copper211.8 lbs.211.8 lbs.211.8 lbs.211.8 lbs.Amount of lead564.8 "635.4 "776.6 "847.2 "Weight of each cake193.5 "211.5 "247.1 "264.75 "Average value of charge56 ozs. 3 dwts.62 ozs. 4 dwts.64 ozs. 4 dwts.66 ozs. 7 dwts.Per cent. of copper27.2%25%21.4%20%Average value of original copper per ton207 ozs. 4 dwts.251 ozs. 3 dwts.299 ozs. 15 dwts.332 ozs. 3 dwts.Weight of argentiferous lead liquated out423.6 lbs.494.2 lbs.635.4 lbs.706 lbs.Average value of liquated lead per ton79 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 ton34 ozs.34 ozs.34 ozs.34 ozs. to 38 ozs.Extraction of silver into the argentiferous lead76.5%73.4%79%85.3%
[Pg 507][11]See p.356.
[Pg 507][11]See p.356.
[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. Thepompholyxandspodoswere largely zinc oxide (see note, p.394).
[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. Thepompholyxandspodoswere largely zinc oxide (see note, p.394).
[13]This description of acentumpondiumwhich weighed either 1331/3librae, or 1463/4librae, adds confusion to an already much mixed subject (seeAppendix C.). Assuming the Germanpfundtto weigh 7,219 troy grains, and the Romanlibra4,946 grains, then acentnerwould weigh 145.95librae, which checks up fairly well with the second case; but under what circumstances acentnercan weigh 1331/3libraewe are unable to record. At first sight it might appear from this statement that where Agricola uses the wordcentumpondiumhe means the Germancentner. On the other hand, in the previous five or six pages the expressions one-third, five-sixths, ten-twelfths of alibraare used, which are even divisions of the Roman 12unciaeto onelibra, and are used where they manifestly mean divisions of 12 units. If Agricola had in mind the German scale, and were using thelibrafor apfundtof 16untzen, these divisions would amount to fractions, and would not total thesicilicusanddrachmaquantities given, nor would they total any of the possibly synonymous divisions of the Germanuntzen(see also page254).
[13]This description of acentumpondiumwhich weighed either 1331/3librae, or 1463/4librae, adds confusion to an already much mixed subject (seeAppendix C.). Assuming the Germanpfundtto weigh 7,219 troy grains, and the Romanlibra4,946 grains, then acentnerwould weigh 145.95librae, which checks up fairly well with the second case; but under what circumstances acentnercan weigh 1331/3libraewe are unable to record. At first sight it might appear from this statement that where Agricola uses the wordcentumpondiumhe means the Germancentner. On the other hand, in the previous five or six pages the expressions one-third, five-sixths, ten-twelfths of alibraare used, which are even divisions of the Roman 12unciaeto onelibra, and are used where they manifestly mean divisions of 12 units. If Agricola had in mind the German scale, and were using thelibrafor apfundtof 16untzen, these divisions would amount to fractions, and would not total thesicilicusanddrachmaquantities given, nor would they total any of the possibly synonymous divisions of the Germanuntzen(see also page254).
[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 copper211.8 lbs.Amount of lead635.4 lbs.Weight of each cake282.4 lbs.Average value of charge218 ozs. 18 dwts.Per cent. of copper25%Average value of original copper per ton583 ozs. 6 dwts. 16 grs.Weight of argentiferous lead liquated out494.2 lbs.Average value of liquated lead per ton352 ozs. 8 dwts.Weight of residues353 lbs.Average value of residues per ton20 ozs. (about).Extraction of silver into the argentiferous lead94%The results given in the second case where the copper contains 2libraeand abespercentumpondiumdo not tie together at all, for each liquation cake should contain 3librae91/2unciae, instead of 11/2libraeand1/2unciaof silver.
[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 copper211.8 lbs.Amount of lead635.4 lbs.Weight of each cake282.4 lbs.Average value of charge218 ozs. 18 dwts.Per cent. of copper25%Average value of original copper per ton583 ozs. 6 dwts. 16 grs.Weight of argentiferous lead liquated out494.2 lbs.Average value of liquated lead per ton352 ozs. 8 dwts.Weight of residues353 lbs.Average value of residues per ton20 ozs. (about).Extraction of silver into the argentiferous lead94%
The results given in the second case where the copper contains 2libraeand abespercentumpondiumdo not tie together at all, for each liquation cake should contain 3librae91/2unciae, instead of 11/2libraeand1/2unciaof 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-thirdlibrae, asemi-unciaand adrachmapercentumpondiumto one-third of alibra,i.e., from 110 ozs. to 97 ozs. 4 dwts. per ton. In the Glossary this furnace is described as aspleisofen,i.e., a refining hearth.
[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-thirdlibrae, asemi-unciaand adrachmapercentumpondiumto one-third of alibra,i.e., from 110 ozs. to 97 ozs. 4 dwts. per ton. In the Glossary this furnace is described as aspleisofen,i.e., a refining hearth.
[Pg 511][16]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 pages530to539. Exactly what material is meant by the term red (rubrum), yellow (fulvum) andcaldariumcopper is somewhat uncertain. They are given in the German text simply asrot,geel, andlebeter kupfer, and apparently all were "coarse" copper of different characters destined for the refinery. The author states inDe 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 isregulare. In the same place a dark yellow copper is made which is calledcaldarium, taking its name among the Germans from a caldron....Regularediffers fromcaldariumin 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 inDe 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.Caldariumcopper 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 page542the author describes makingcaldariumcopper from a mixture of yellow copper and a peculiarcadmia, 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 thecaldariummust have been, indeed, impure.
[Pg 511][16]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 pages530to539. Exactly what material is meant by the term red (rubrum), yellow (fulvum) andcaldariumcopper is somewhat uncertain. They are given in the German text simply asrot,geel, andlebeter kupfer, and apparently all were "coarse" copper of different characters destined for the refinery. The author states inDe 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 isregulare. In the same place a dark yellow copper is made which is calledcaldarium, taking its name among the Germans from a caldron....Regularediffers fromcaldariumin 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 inDe 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.Caldariumcopper 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 page542the author describes makingcaldariumcopper from a mixture of yellow copper and a peculiarcadmia, 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 thecaldariummust have been, indeed, impure.
[Pg 512][17]The liquation of these low-grade copper "bottoms" required that the liquated lead should be re-used again to make up fresh liquation cakes, in order that it might eventually become rich enough to warrant cupellation. In the following table the "poor" silver-lead is designated (A) the "medium" (B) and the "rich" (C). The three charges here given are designated sixth, seventh, and eighth for purposes of reference. It will be seen that the data is insufficient to complete the ninth and tenth. Moreover, while the author gives directions for making four cakes, he says the charge consists of five, and it has, therefore, been necessary to reduce the volume of products given to this basis.6th Charge.7th Charge.8th Charge.Amount of copper bottoms176.5 lbs.176.5 lbs.176.5 lbs.Amount of lead282.4 lbs. (slags)564.8 lbs. of (A)635.4 lbs. of (B)Amount of de-silverized lead494.2 lbs.211.8 lbs.141.2 lbs. (A)Weight of each cake238.3 lbs.238.3 lbs.238.3 lbs.Average value of charge per ton22 ozs. 5 dwts.35 ozs. 15 dwts.50 ozs. 5 dwts.Per cent. of copper18.5%18.5%18.5%Average value per ton original copper97 ozs. 4 dwts.97 ozs. 4 dwts.97 ozs. 4 dwts.Average value per ton of90 ozs. 2 dwts. (slags)28 ozs. 5 dwts. (A)28 ozs. 5 dwts. (A)Average value per ton of3 ozs. 1 dwt. (lead)3 ozs. 1 dwt. (lead)42 ozs. 10 dwts. (B)Weight of liquated lead550.6 lbs.Average value of the liquated lead per ton28 ozs. 5 dwts. (A)42 ozs. 10 dwts. (B)63 ozs. 16 dwts. (C)Weight of exhausted liquation cakes225.9 lbs.Average value of the exhausted liquation cakes per ton12 ozs. 3 dwts.Weight of liquation thorns169.4 lbs.Average value of the liquation thorns per ton18 ozs. 4 dwts.Extraction of silver into the liquated lead71%
[Pg 512][17]The liquation of these low-grade copper "bottoms" required that the liquated lead should be re-used again to make up fresh liquation cakes, in order that it might eventually become rich enough to warrant cupellation. In the following table the "poor" silver-lead is designated (A) the "medium" (B) and the "rich" (C). The three charges here given are designated sixth, seventh, and eighth for purposes of reference. It will be seen that the data is insufficient to complete the ninth and tenth. Moreover, while the author gives directions for making four cakes, he says the charge consists of five, and it has, therefore, been necessary to reduce the volume of products given to this basis.
6th Charge.7th Charge.8th Charge.Amount of copper bottoms176.5 lbs.176.5 lbs.176.5 lbs.Amount of lead282.4 lbs. (slags)564.8 lbs. of (A)635.4 lbs. of (B)Amount of de-silverized lead494.2 lbs.211.8 lbs.141.2 lbs. (A)Weight of each cake238.3 lbs.238.3 lbs.238.3 lbs.Average value of charge per ton22 ozs. 5 dwts.35 ozs. 15 dwts.50 ozs. 5 dwts.Per cent. of copper18.5%18.5%18.5%Average value per ton original copper97 ozs. 4 dwts.97 ozs. 4 dwts.97 ozs. 4 dwts.Average value per ton of90 ozs. 2 dwts. (slags)28 ozs. 5 dwts. (A)28 ozs. 5 dwts. (A)Average value per ton of3 ozs. 1 dwt. (lead)3 ozs. 1 dwt. (lead)42 ozs. 10 dwts. (B)Weight of liquated lead550.6 lbs.Average value of the liquated lead per ton28 ozs. 5 dwts. (A)42 ozs. 10 dwts. (B)63 ozs. 16 dwts. (C)Weight of exhausted liquation cakes225.9 lbs.Average value of the exhausted liquation cakes per ton12 ozs. 3 dwts.Weight of liquation thorns169.4 lbs.Average value of the liquation thorns per ton18 ozs. 4 dwts.Extraction of silver into the liquated lead71%
[Pg 520][18]For the liquation it was necessary to maintain a reducing atmosphere, otherwise the lead would oxidize; this was secured by keeping the cakes well covered with charcoal and by preventing the entrance of air as much as possible. Moreover, it was necessary to preserve a fairly even temperature. The proportions of copper and lead in the three liquation products vary considerably, depending upon the method of conducting the process and the original proportions. From the authors consulted (see note p.492) an average would be about as follows:—The residual copper—exhausted liquation cakes—ran from 25 to 33% lead; the liquated lead from 2 to 3% copper; and the liquation thorns, which were largely oxidized, contained about 15% copper oxides, 80% lead oxides, together with impurities, such as antimony, arsenic, etc. The proportions of the various products would obviously depend upon the care in conducting the operation; too high temperature and the admission of air would increase the copper melted and oxidize more lead, and thus increase the liquation thorns. There are insufficient data in Agricola to adduce conclusions as to the actual ratios produced. The results given for the 6th charge (note 17, p. 512) would indicate about 30% lead in the residual copper, and would indicate that the original charge was divided into about 24% of residual copper, 18% of liquation thorns, and 57% of liquated lead. This, however, was an unusually large proportion of liquation thorns, some of the authors giving instances of as low as 5%.
[Pg 520][18]For the liquation it was necessary to maintain a reducing atmosphere, otherwise the lead would oxidize; this was secured by keeping the cakes well covered with charcoal and by preventing the entrance of air as much as possible. Moreover, it was necessary to preserve a fairly even temperature. The proportions of copper and lead in the three liquation products vary considerably, depending upon the method of conducting the process and the original proportions. From the authors consulted (see note p.492) an average would be about as follows:—The residual copper—exhausted liquation cakes—ran from 25 to 33% lead; the liquated lead from 2 to 3% copper; and the liquation thorns, which were largely oxidized, contained about 15% copper oxides, 80% lead oxides, together with impurities, such as antimony, arsenic, etc. The proportions of the various products would obviously depend upon the care in conducting the operation; too high temperature and the admission of air would increase the copper melted and oxidize more lead, and thus increase the liquation thorns. There are insufficient data in Agricola to adduce conclusions as to the actual ratios produced. The results given for the 6th charge (note 17, p. 512) would indicate about 30% lead in the residual copper, and would indicate that the original charge was divided into about 24% of residual copper, 18% of liquation thorns, and 57% of liquated lead. This, however, was an unusually large proportion of liquation thorns, some of the authors giving instances of as low as 5%.
[Pg 522][19]The first instance given, of 44centumpondia(3,109 lbs.) lead and onecentumpondium(70.6 lbs.) copper, would indicate that the liquated lead contained 2.2% copper. The second, of 46centumpondia(3,250 lbs.) lead and 11/2centumpondiacopper (106 lbs.), would indicate 3% copper; and in the third, 120centumpondia(8,478 lbs.) lead and six copper (424 lbs.) would show 4.76% copper. This charge of 120centumpondiain the cupellation furnace would normally make more than 110centumpondiaof litharge and 30 of hearth-lead,i.e., saturated furnace bottoms. The copper would be largely found in the silver-lead "which does not melt," at the margin of the crucible. These skimmings are afterward referred to as "thorns." It is difficult to understand what is meant by the expression that the silver which is in the copper is mixed with the remaining (reliquo) silver. The coppery skimmings from the cupellation furnace are referred to again inNote 28, p. 539.
[Pg 522][19]The first instance given, of 44centumpondia(3,109 lbs.) lead and onecentumpondium(70.6 lbs.) copper, would indicate that the liquated lead contained 2.2% copper. The second, of 46centumpondia(3,250 lbs.) lead and 11/2centumpondiacopper (106 lbs.), would indicate 3% copper; and in the third, 120centumpondia(8,478 lbs.) lead and six copper (424 lbs.) would show 4.76% copper. This charge of 120centumpondiain the cupellation furnace would normally make more than 110centumpondiaof litharge and 30 of hearth-lead,i.e., saturated furnace bottoms. The copper would be largely found in the silver-lead "which does not melt," at the margin of the crucible. These skimmings are afterward referred to as "thorns." It is difficult to understand what is meant by the expression that the silver which is in the copper is mixed with the remaining (reliquo) silver. The coppery skimmings from the cupellation furnace are referred to again inNote 28, p. 539.
[Pg 523][20]A further amount of lead could be obtained in the first liquation, but a higher temperature is necessary, which was more economical to secure in the "drying" furnace. Therefore, the "drying" was really an extension of liquation; but as air was admitted the lead and copper melted out were oxidized. The products were the final residual copper, called by Agricola the "dried" copper, together with lead and copper oxides, called by him the "slags," and the scale of copper and lead oxides termed by him the "ash-coloured copper." The German metallurgists distinguished two kinds of slag: the first and principal one, thedarrost, and the second thedarrsöhle, this latter differing only in that it contained more impurities from the floor of the furnace, and remained behind until the furnace cooled. Agricola possibly refers to these as "more liquation thorns," because in describing the treatment of the bye-products he refers to thorns from the process, whereas in the description of "drying" he usually refers to "slags." A number of analyses of these products, given by Karsten, show the "dried" copper to contain from 82.7 to 90.6% copper, and from 9.4 to 17.3% lead; the "slag" to contain 76.5 to 85.1% lead oxide, and from 4.1 to 7.8% cuprous oxide, with 9 to 13% silica from the furnace bottoms, together with some other[Pg 524]impurities; the "ash-coloured copper" to contain about 60% cuprous oxide and 30% lead oxide, with some metallic copper and minor impurities. An average of proportions given by various authors shows, roughly, that out of 100centnersof "exhausted" liquation cakes, containing about 70% copper and 30% lead, there were about 63centnersof "dried" copper, 38centnersof "slag," and 61/2centnersof "ash-coloured copper." According to Karsten, the process fell into stages; first, at low temperature some metallic lead appeared; second, during an increasing temperature for over 14 to 15 hours the slags ran out; third, there was a period of four hours of lower temperature to allow time for the lead to diffuse from the interior of the cakes; and fourth, during a period of eight hours the temperature was again increased. In fact, the latter portion of the process ended with the economic limit between leaving some lead in the copper and driving too much copper into the "slags." Agricola gives the silver contents of the "dried" copper as 3drachmaeto 1centumpondium, or equal to about 9 ozs. per ton; and assuming that the copper finally recovered from the bye-products ran no higher, then the first four charges (see note on p.506) would show a reduction in the silver values of from 95 to 97%; the 7th and 8th charges (note on p.512) of about 90%.
[Pg 523][20]A further amount of lead could be obtained in the first liquation, but a higher temperature is necessary, which was more economical to secure in the "drying" furnace. Therefore, the "drying" was really an extension of liquation; but as air was admitted the lead and copper melted out were oxidized. The products were the final residual copper, called by Agricola the "dried" copper, together with lead and copper oxides, called by him the "slags," and the scale of copper and lead oxides termed by him the "ash-coloured copper." The German metallurgists distinguished two kinds of slag: the first and principal one, thedarrost, and the second thedarrsöhle, this latter differing only in that it contained more impurities from the floor of the furnace, and remained behind until the furnace cooled. Agricola possibly refers to these as "more liquation thorns," because in describing the treatment of the bye-products he refers to thorns from the process, whereas in the description of "drying" he usually refers to "slags." A number of analyses of these products, given by Karsten, show the "dried" copper to contain from 82.7 to 90.6% copper, and from 9.4 to 17.3% lead; the "slag" to contain 76.5 to 85.1% lead oxide, and from 4.1 to 7.8% cuprous oxide, with 9 to 13% silica from the furnace bottoms, together with some other[Pg 524]impurities; the "ash-coloured copper" to contain about 60% cuprous oxide and 30% lead oxide, with some metallic copper and minor impurities. An average of proportions given by various authors shows, roughly, that out of 100centnersof "exhausted" liquation cakes, containing about 70% copper and 30% lead, there were about 63centnersof "dried" copper, 38centnersof "slag," and 61/2centnersof "ash-coloured copper." According to Karsten, the process fell into stages; first, at low temperature some metallic lead appeared; second, during an increasing temperature for over 14 to 15 hours the slags ran out; third, there was a period of four hours of lower temperature to allow time for the lead to diffuse from the interior of the cakes; and fourth, during a period of eight hours the temperature was again increased. In fact, the latter portion of the process ended with the economic limit between leaving some lead in the copper and driving too much copper into the "slags." Agricola gives the silver contents of the "dried" copper as 3drachmaeto 1centumpondium, or equal to about 9 ozs. per ton; and assuming that the copper finally recovered from the bye-products ran no higher, then the first four charges (see note on p.506) would show a reduction in the silver values of from 95 to 97%; the 7th and 8th charges (note on p.512) of about 90%.
[21]If Roman weights, this would equal from 6,360 lbs. to 7,066 lbs.
[21]If Roman weights, this would equal from 6,360 lbs. to 7,066 lbs.
[Pg 529][22]One halfuncia, or threedrachmaeof silver would equal either 12 ozs. or 9 ozs. per ton. If we assume the values given for residual copper in the first four charges (note p.506) of 34 ozs., this would mean an extraction of, roughly, 65% of the silver from the exhausted liquation cakes.
[Pg 529][22]One halfuncia, or threedrachmaeof silver would equal either 12 ozs. or 9 ozs. per ton. If we assume the values given for residual copper in the first four charges (note p.506) of 34 ozs., this would mean an extraction of, roughly, 65% of the silver from the exhausted liquation cakes.
[Pg 530][23]Seenote 29, p. 540.
[Pg 530][23]Seenote 29, p. 540.
[Pg 533][24]Assuming Roman weights:2centumpondia=141.3lbs.21/2"=176.6"3"=211.9"31/2"=248.2"6"=423.9"
[Pg 533][24]Assuming Roman weights:
2centumpondia=141.3lbs.21/2"=176.6"3"=211.9"31/2"=248.2"6"=423.9"
[Pg 535][25]This description of refining copper in an open hearth by oxidation with a blast and "poling"—thegaarmachenof the Germans—is so accurate, and the process is so little changed in some parts of Saxony, that it might have been written in the 20th century instead of the 16th. The best account of the old practice in Saxony after Agricola is to be found in Schlüter'sHütte Werken(Braunschweig, 1738, Chap.CXVIII.). The process has largely been displaced by electrolytic methods, but is still in use in most refineries as a step in electrolytic work. It may be unnecessary to repeat that the process is one of subjecting the molten mass of impure metal to a strong and continuous blast, and as a result, not only are the impurities to a considerable extent directly oxidized and taken off as a slag, but also a considerable amount of copper is turned into cuprous oxide. This cuprous oxide mostly melts and diffuses through the metallic copper, and readily parting with its oxygen to the impurities further facilitates their complete oxidation. The blast is continued until the impurities are practically eliminated, and at this stage the molten metal contains a great deal of dissolved cuprous oxide, which must be reduced. This is done by introducing a billet of green wood ("poling"), the dry distillation of which generates large quantities of gases, which reduce the oxide. The state of the metal is even to-day in some localities tested by dipping into it the point of an iron rod; if it be at the proper state the adhering copper has a net-like appearance, should be easily loosened from the rod by dipping in water, is of a reddish-copper colour and should be quite pliable; if the metal is not yet refined, the sample is thick, smooth, and detachable with difficulty; if over-refined, it is thick and brittle. By allowing water to run on to the surface of the molten metal, thin cakes are successively formed and taken off. These cakes were the article known to commerce over several centuries as "rosetta copper." The first few cakes are discarded as containing impurities or slag, and if the metal be of good quality the cakes are thin and of a red colour. Their colour and thinness, therefore, become a criterion of purity. The cover of charcoal or charcoal dust maintained upon the surface of the metal tended to retard oxidation, but prevented volatilization and helped to secure the impurities as a slag instead. Karsten (Archiv., 1st series, p. 46) gives several analyses of the[Pg 536]slag from refining "dried" copper, showing it to contain from 51.7 to 67.4% lead oxide, 6.2 to 19.2% cuprous oxide, and 21.4 to 23.9 silica (from the furnace bottoms), with minor quantities of iron, antimony, etc. The "bubbles" referred to by Agricola were apparently the shower of copper globules which takes place upon the evolution of sulphur dioxide, due to the reaction of the cuprous oxide upon any remaining sulphide of copper when the mass begins to cool.Historical Note.—It is impossible to say how the Ancients refined copper, beyond the fact that they often re-smelted it. Such notes as we can find are set out in the note on copper smelting (note 42, p. 402). The first authentic reference to poling is in Theophilus (1150 to 1200A.D., Hendrie's translation, p. 313), which shows a very good understanding of this method of refining copper:—"Of the Purification of Copper. Take an iron dish of the size you wish, and line it inside and out with clay strongly beaten and mixed, and it is carefully dried. Then place it before a forge upon the coals, so that when the bellows act upon it the wind may issue partly within and partly above it, and not below it. And very small coals being placed round it, place the copper in it equally, and add over it a heap of coals. When by blowing a long time this has become melted, uncover it and cast immediately fine ashes of coals over it, and stir it with a thin and dry piece of wood as if mixing it, and you will directly see the burnt lead adhere to these ashes like a glue, which being cast out again superpose coals, and blowing for a long time, as at first, again uncover it, and then do as you did before. You do this until at length by cooking it you can withdraw the lead entirely. Then pour it over the mould which you have prepared for this, and you will thus prove if it be pure. Hold it with the pincers, glowing as it is, before it has become cold, and strike it with a large hammer strongly over the anvil, and if it be broken or split you must liquefy it anew as before. If, however, it should remain sound, you will cool it in water, and you cook other (copper) in the same manner." Biringuccio (III, 8) in 1540 describes the process briefly, but omits the poling, an essential in the production of malleable copper.
[Pg 535][25]This description of refining copper in an open hearth by oxidation with a blast and "poling"—thegaarmachenof the Germans—is so accurate, and the process is so little changed in some parts of Saxony, that it might have been written in the 20th century instead of the 16th. The best account of the old practice in Saxony after Agricola is to be found in Schlüter'sHütte Werken(Braunschweig, 1738, Chap.CXVIII.). The process has largely been displaced by electrolytic methods, but is still in use in most refineries as a step in electrolytic work. It may be unnecessary to repeat that the process is one of subjecting the molten mass of impure metal to a strong and continuous blast, and as a result, not only are the impurities to a considerable extent directly oxidized and taken off as a slag, but also a considerable amount of copper is turned into cuprous oxide. This cuprous oxide mostly melts and diffuses through the metallic copper, and readily parting with its oxygen to the impurities further facilitates their complete oxidation. The blast is continued until the impurities are practically eliminated, and at this stage the molten metal contains a great deal of dissolved cuprous oxide, which must be reduced. This is done by introducing a billet of green wood ("poling"), the dry distillation of which generates large quantities of gases, which reduce the oxide. The state of the metal is even to-day in some localities tested by dipping into it the point of an iron rod; if it be at the proper state the adhering copper has a net-like appearance, should be easily loosened from the rod by dipping in water, is of a reddish-copper colour and should be quite pliable; if the metal is not yet refined, the sample is thick, smooth, and detachable with difficulty; if over-refined, it is thick and brittle. By allowing water to run on to the surface of the molten metal, thin cakes are successively formed and taken off. These cakes were the article known to commerce over several centuries as "rosetta copper." The first few cakes are discarded as containing impurities or slag, and if the metal be of good quality the cakes are thin and of a red colour. Their colour and thinness, therefore, become a criterion of purity. The cover of charcoal or charcoal dust maintained upon the surface of the metal tended to retard oxidation, but prevented volatilization and helped to secure the impurities as a slag instead. Karsten (Archiv., 1st series, p. 46) gives several analyses of the[Pg 536]slag from refining "dried" copper, showing it to contain from 51.7 to 67.4% lead oxide, 6.2 to 19.2% cuprous oxide, and 21.4 to 23.9 silica (from the furnace bottoms), with minor quantities of iron, antimony, etc. The "bubbles" referred to by Agricola were apparently the shower of copper globules which takes place upon the evolution of sulphur dioxide, due to the reaction of the cuprous oxide upon any remaining sulphide of copper when the mass begins to cool.
Historical Note.—It is impossible to say how the Ancients refined copper, beyond the fact that they often re-smelted it. Such notes as we can find are set out in the note on copper smelting (note 42, p. 402). The first authentic reference to poling is in Theophilus (1150 to 1200A.D., Hendrie's translation, p. 313), which shows a very good understanding of this method of refining copper:—"Of the Purification of Copper. Take an iron dish of the size you wish, and line it inside and out with clay strongly beaten and mixed, and it is carefully dried. Then place it before a forge upon the coals, so that when the bellows act upon it the wind may issue partly within and partly above it, and not below it. And very small coals being placed round it, place the copper in it equally, and add over it a heap of coals. When by blowing a long time this has become melted, uncover it and cast immediately fine ashes of coals over it, and stir it with a thin and dry piece of wood as if mixing it, and you will directly see the burnt lead adhere to these ashes like a glue, which being cast out again superpose coals, and blowing for a long time, as at first, again uncover it, and then do as you did before. You do this until at length by cooking it you can withdraw the lead entirely. Then pour it over the mould which you have prepared for this, and you will thus prove if it be pure. Hold it with the pincers, glowing as it is, before it has become cold, and strike it with a large hammer strongly over the anvil, and if it be broken or split you must liquefy it anew as before. If, however, it should remain sound, you will cool it in water, and you cook other (copper) in the same manner." Biringuccio (III, 8) in 1540 describes the process briefly, but omits the poling, an essential in the production of malleable copper.
[Pg 538][26]Pompholyxandspodoswere impure zinc oxides (seenote 26, p. 394).The copper flowers were no doubt cupric oxide. They were used by the Ancients for medicinal purposes. Dioscorides (V, 48) says: "Of flowers of copper, which some call the scrapings of old nails, the best is friable; it is gold-coloured when rubbed, is like millet in shape and size, is moderately bright, and somewhat astringent. It should not be mixed with copper filings, with which it is often adulterated. But this deception is easily detected, for when bitten in the teeth the filings are malleable. It (the flowers) is made when the copper fused in a furnace has run into the receptacle through the spout pertaining to it, for then the workmen engaged in this trade cleanse it from dirt and pour clear water over it in order to cool it; from this sudden condensation the copper spits and throws out the aforesaid flowers." Pliny (XXXIV, 24) says: "The flower, too, of copper (æris flos) is used in medicine. This is made by fusing copper, and then removing it to another furnace, where the repeated blast makes the metal separate into small scales like millet, known as flowers. These scales also fall off when the cakes of metal are cooled in water; they become red, too, like the scales of copper known as 'lepis,' by use of which the flowers of copper are adulterated, it being also sold for it. These are made when hammering the nails that are[Pg 539]made from the cakes of copper. All these methods are carried on in the works of Cyprus; the difference between these substances is that thesquamae(copper scales) are detached from hammering the cakes, while the flower falls off spontaneously." Agricola (De Nat. Fos., p. 352) notes that "flowers of copper (flos æris) have the same properties as 'roasted copper.'"
[Pg 538][26]Pompholyxandspodoswere impure zinc oxides (seenote 26, p. 394).
The copper flowers were no doubt cupric oxide. They were used by the Ancients for medicinal purposes. Dioscorides (V, 48) says: "Of flowers of copper, which some call the scrapings of old nails, the best is friable; it is gold-coloured when rubbed, is like millet in shape and size, is moderately bright, and somewhat astringent. It should not be mixed with copper filings, with which it is often adulterated. But this deception is easily detected, for when bitten in the teeth the filings are malleable. It (the flowers) is made when the copper fused in a furnace has run into the receptacle through the spout pertaining to it, for then the workmen engaged in this trade cleanse it from dirt and pour clear water over it in order to cool it; from this sudden condensation the copper spits and throws out the aforesaid flowers." Pliny (XXXIV, 24) says: "The flower, too, of copper (æris flos) is used in medicine. This is made by fusing copper, and then removing it to another furnace, where the repeated blast makes the metal separate into small scales like millet, known as flowers. These scales also fall off when the cakes of metal are cooled in water; they become red, too, like the scales of copper known as 'lepis,' by use of which the flowers of copper are adulterated, it being also sold for it. These are made when hammering the nails that are[Pg 539]made from the cakes of copper. All these methods are carried on in the works of Cyprus; the difference between these substances is that thesquamae(copper scales) are detached from hammering the cakes, while the flower falls off spontaneously." Agricola (De Nat. Fos., p. 352) notes that "flowers of copper (flos æris) have the same properties as 'roasted copper.'"
[27]It seems scarcely necessary to discuss in detail the complicated "flow scheme" of the various minor bye-products. They are all re-introduced into the liquation circuit, and thereby are created other bye-products of the same kindad infinitum. Further notes are given on:—Liquation thornsNote28.Slags"30.Ash-coloured copper"29.Concentrates"33.Cadmia"32.There are no data given, either by Agricola or the later authors, which allow satisfactory calculation of the relative quantities of these products. A rough estimate from the data given in previous notes would indicate that in one liquation only about 70% of the original copper came out as refined copper, and that about 70% of the original lead would go to the cupellation furnace,i.e., about 30% of the original metal sent to the blast furnace would go into the "thorns," "slags," and "ash-coloured copper." The ultimate losses were very great, as given before (p.491), they probably amounted to 25% of the silver, 9% copper, and 16% of the lead.
[27]It seems scarcely necessary to discuss in detail the complicated "flow scheme" of the various minor bye-products. They are all re-introduced into the liquation circuit, and thereby are created other bye-products of the same kindad infinitum. Further notes are given on:—
Liquation thornsNote28.Slags"30.Ash-coloured copper"29.Concentrates"33.Cadmia"32.
There are no data given, either by Agricola or the later authors, which allow satisfactory calculation of the relative quantities of these products. A rough estimate from the data given in previous notes would indicate that in one liquation only about 70% of the original copper came out as refined copper, and that about 70% of the original lead would go to the cupellation furnace,i.e., about 30% of the original metal sent to the blast furnace would go into the "thorns," "slags," and "ash-coloured copper." The ultimate losses were very great, as given before (p.491), they probably amounted to 25% of the silver, 9% copper, and 16% of the lead.
[28]There were the following classes of thorns:—1st.From liquation.2nd.From drying.3rd.From cupellation.In a general way, according to the later authors, they were largely lead oxide, and contained from 5% to 20% cuprous oxide. If a calculation be made backward from the products given as the result of the charge described, it would appear that in this case they must have contained at least one-fifth copper. The silver in these liquation cakes would run about 24 ozs. per ton, in the liquated lead about 36 ozs. per ton, and in the liquation thorns 24 ozs. per ton. The extraction into the liquated lead would be about 80% of the silver.
[28]There were the following classes of thorns:—
1st.From liquation.2nd.From drying.3rd.From cupellation.
In a general way, according to the later authors, they were largely lead oxide, and contained from 5% to 20% cuprous oxide. If a calculation be made backward from the products given as the result of the charge described, it would appear that in this case they must have contained at least one-fifth copper. The silver in these liquation cakes would run about 24 ozs. per ton, in the liquated lead about 36 ozs. per ton, and in the liquation thorns 24 ozs. per ton. The extraction into the liquated lead would be about 80% of the silver.
[Pg 540][29]The "ash-coloured copper" is a cuprous oxide, containing some 3% lead oxide; and if Agricola means they contained twounciaeof silver to thecentumpondium, then they ran about 48 ozs. per ton, and would contain much more silver than the mass.
[Pg 540][29]The "ash-coloured copper" is a cuprous oxide, containing some 3% lead oxide; and if Agricola means they contained twounciaeof silver to thecentumpondium, then they ran about 48 ozs. per ton, and would contain much more silver than the mass.
[Pg 541][30]There are three principal "slags" mentioned—1st.Slag from "leading."2nd.Slag from "drying."3rd.Slag from refining the copper.From the analyses quoted by various authors these ran from 52% to 85% lead oxide, 5% to 30% cuprous oxide, and considerable silica from the furnace bottoms. They were reduced in the main into liquation cakes, although Agricola mentions instances of the metal reduced from "slags" being taken directly to the "drying" furnace. Such liquation cakes would run very low in silver, and at the values given only averaged 12 ozs. per ton; therefore the liquated lead running the same value as the cakes, or less than half that of the "poor" lead mentioned inNote 17, p. 512, could not have been cupelled directly.
[Pg 541][30]There are three principal "slags" mentioned—
1st.Slag from "leading."2nd.Slag from "drying."3rd.Slag from refining the copper.
From the analyses quoted by various authors these ran from 52% to 85% lead oxide, 5% to 30% cuprous oxide, and considerable silica from the furnace bottoms. They were reduced in the main into liquation cakes, although Agricola mentions instances of the metal reduced from "slags" being taken directly to the "drying" furnace. Such liquation cakes would run very low in silver, and at the values given only averaged 12 ozs. per ton; therefore the liquated lead running the same value as the cakes, or less than half that of the "poor" lead mentioned inNote 17, p. 512, could not have been cupelled directly.
[Pg 542][31]SeeNote 16, p. 511, for discussion of yellow andcaldariumcopper.
[Pg 542][31]SeeNote 16, p. 511, for discussion of yellow andcaldariumcopper.
[32]Thiscadmiais given in the Glossary and the German translation askobelt. A discussion of this substance is given in the note on p.112; and it is sufficient to state here that in Agricola's time the metal cobalt was unknown, and the substances designatedcadmiaandcobaltumwere arsenical-cobalt-zinc minerals. A metal made from "slag" from refining, together with "base" thorns, would be very impure; for the latter, according to the paragraph on concentrates a little later on, would contain the furnace accretions, and would thus be undoubtedly zincky. It is just possible that the termkobeltwas used by the German smelters at this time in the sense of an epithet—"black devil" (seeNote 21, p. 214).
[32]Thiscadmiais given in the Glossary and the German translation askobelt. A discussion of this substance is given in the note on p.112; and it is sufficient to state here that in Agricola's time the metal cobalt was unknown, and the substances designatedcadmiaandcobaltumwere arsenical-cobalt-zinc minerals. A metal made from "slag" from refining, together with "base" thorns, would be very impure; for the latter, according to the paragraph on concentrates a little later on, would contain the furnace accretions, and would thus be undoubtedly zincky. It is just possible that the termkobeltwas used by the German smelters at this time in the sense of an epithet—"black devil" (seeNote 21, p. 214).
[33]It is somewhat difficult to see exactly the meaning of base (vile) and precious (preciosum) in this connection. While "base" could mean impure, "precious" could hardly mean pure, and while "precious" could mean high value in silver, the reverse does not seem entirelyapropos. It is possible that "bad" and "good" would be more appropriate terms.
[33]It is somewhat difficult to see exactly the meaning of base (vile) and precious (preciosum) in this connection. While "base" could mean impure, "precious" could hardly mean pure, and while "precious" could mean high value in silver, the reverse does not seem entirelyapropos. It is possible that "bad" and "good" would be more appropriate terms.
[Pg 543][34]The skimmings from the molten lead in the early stages of cupellation have been discussed inNote 28, p. 539. They are probably called thorns here because of the large amount of copper in them. The lead from liquation would contain 2% to 3% of copper, and this would be largely recovered in these skimmings, although there would be some copper in the furnace bottoms—hearth-lead—and the litharge. These "thorns" are apparently fairly rich, fourunciaeto thecentumpondiumbeing equivalent to about 97 ozs. per ton, and they are only added to low-grade liquation material.
[Pg 543][34]The skimmings from the molten lead in the early stages of cupellation have been discussed inNote 28, p. 539. They are probably called thorns here because of the large amount of copper in them. The lead from liquation would contain 2% to 3% of copper, and this would be largely recovered in these skimmings, although there would be some copper in the furnace bottoms—hearth-lead—and the litharge. These "thorns" are apparently fairly rich, fourunciaeto thecentumpondiumbeing equivalent to about 97 ozs. per ton, and they are only added to low-grade liquation material.
[Pg 544][35]Particulis aeris tusi. Unless this be the fine concentrates from crushing the material mentioned, we are unable to explain the expression.
[Pg 544][35]Particulis aeris tusi. Unless this be the fine concentrates from crushing the material mentioned, we are unable to explain the expression.
[36]This operation would bring down a button of antimony under an iron matte, by de-sulphurizing the antimony. It would seem scarcely necessary to add lead before cupellation. This process is given in an assay method, in theProbierbüchlein(folio 31) 50 years beforeDe Re Metallica: "How to separate silver from iron: Take that silver which is in ironplechen(plachmal), pulverize it finely, take the same iron orplecone part,spiesglasz(antimony sulphide) one part, leave them to melt in a crucible placed in a closedwindtofen. When it is melted, let it cool, break the crucible, chip off the button that is in the bottom, and melt it in a crucible with as much lead. Then break the crucible, and seek from the button in the cupel, and you will find what silver it contains."
[36]This operation would bring down a button of antimony under an iron matte, by de-sulphurizing the antimony. It would seem scarcely necessary to add lead before cupellation. This process is given in an assay method, in theProbierbüchlein(folio 31) 50 years beforeDe Re Metallica: "How to separate silver from iron: Take that silver which is in ironplechen(plachmal), pulverize it finely, take the same iron orplecone part,spiesglasz(antimony sulphide) one part, leave them to melt in a crucible placed in a closedwindtofen. When it is melted, let it cool, break the crucible, chip off the button that is in the bottom, and melt it in a crucible with as much lead. Then break the crucible, and seek from the button in the cupel, and you will find what silver it contains."