[15] As 144 siliquae are 1 uncia, then 1/4 siliqua in 8 unciae would equal one part silver in 4,608 parts gold, or about 999.8 fine.
[Pg 448][16] The object of this treatment with sulphur and copper is to separate a considerable portion of silver from low-grade bullion (i.e., silver containing some gold), in preparation for final treatment of the richer gold-silver alloy with nitric acid. Silver sulphide is created by adding sulphur, and is drawn off in a silver-copper regulus. After the first sentence, the author uses silver alone where he obviously means silver "containing some gold," and further he speaks of the "gold lump" (massula) where he likewise means a button containing a great deal of silver. For clarity we introduced the term "regulus" for the Latin mistura. The operation falls into six stages: a, granulation; b, sulphurization of the granulated bullion; c, melting to form a combination of the silver sulphide with copper into a regulus, an alloy of gold and silver settling out; d, repetition of the treatment to abstract further silver from the "lump;" e, refining the "lump" with nitric acid; f, recovery of the silver from the regulus by addition of lead, liquation and cupellation.
The use of a "circle of fire" secures a low temperature that would neither volatilize the sulphur nor melt the bullion. The amount of sulphur given is equal to a ratio of 48 parts bullion and 9 parts sulphur. We are not certain about the translation of the paragraph in relation to the proportion of copper added to the granulated bullion; because in giving definite quantities of copper to be added in the contingencies of various original copper contents in the bullion, it would be expected that they were intended to produce some positive ratio of copper and silver. However, the ratio as we understand the text in various cases works out to irregular amounts, i.e., 48 parts of silver to 16, 12.6, 24, 20.5, 20.8, 17.8, or 18 parts of copper. In order to obtain complete separation there should be sufficient sulphur to have formed a sulphide of the copper as well as of the silver, or else some of the copper and silver would come down metallic with the "lump". The above ratio of copper added to the sulphurized silver, in the first instance would give about 18 parts of copper and 9 parts of sulphur to 48 parts of silver. The copper would require 4.5 parts of sulphur to convert it into sulphide, and the silver about 7 parts, or a total of 11.5 parts required against 9 parts furnished. It is plain, therefore, that insufficient sulphur is given. Further, the litharge would probably take up some sulphur and throw down metallic lead into the "lump". However, it is necessary that there should be some free metallics to collect the gold, and, therefore, the separation could not be complete in one operation. In any event, on the above ratios the "gold lump" from the first operation was pretty coppery, and contained some lead and probably a good deal of silver, because the copper would tend to desulphurize the latter. The "powder" of glass-galls, salt, and litharge would render the mass more liquid and assist the "gold lump" to separate out.
The Roman silver sesterce, worth about 21/8 pence or 4.2 American cents, was no doubt used by Agricola merely to indicate an infinitesimal quantity. The test to be applied to the regulus by way of cupellation and parting of a sample with nitric acid, requires no explanation. The truth of the description as to determining whether the gold had settled out, by using a chalked iron rod, can only be tested by actual experiment. It is probable, however, that the sulphur in the regulus would attack the iron and make it black. The re-melting of the regulus, if some gold remains in it, with copper and "powder" without more sulphur, would provide again free metallics to gather the remaining gold, and by desulphurizing some silver this button would probably not be very pure.
[Pg 449] From the necessity for some free metallics besides the gold in the first treatment, it will be seen that a repetition of the sulphur addition and re-melting is essential gradually to enrich the "lump". Why more copper is added is not clear. In the second melting, the ratio is 48 parts of the "gold lump", 12 parts of sulphur and 12 parts copper. In this case the added copper would require about 3 parts sulphur, and if we consider the deficiency of sulphur in the first operations pertained entirely to the copper, then about 2.5 parts would be required to make good the shortage, or in other words the second addition of sulphur is sufficient. In the final parting of the "lump" it will be noticed that the author states that the silver ratio must be arranged as three of silver to one of gold. As to the recovery of the silver from the regulus, he states that 66 librae of silver give 132 librae of regulus. To this, 500 librae of lead are added, and it is melted in the "second" furnace, and the litharge and hearth-lead made are re-melted in the "first" furnace, the cakes made being again treated in the "third" furnace to separate the copper and lead. The "first" is usually the blast furnace, the "second" furnace is the cupellation furnace, and the "third" the liquation furnace. It is difficult to understand this procedure. The charge sent to the cupellation furnace would contain between 3% and 5% copper, and between 3% and 5% sulphur. However, possibly the sulphur and copper could be largely abstracted in the skimmings from the cupellation furnace, these being subsequently liquated in the "third" furnace. It may be noted that two whole lines from this paragraph are omitted in the editions of De Re Metallica after 1600. For historical note on sulphur separation see page [461].
[Pg 451][17] There can be no doubt that in most instances Agricola's stibium is antimony sulphide, but it does not follow that it was the mineral stibnite, nor have we considered it desirable to introduce the precision of either of these modern terms, and have therefore retained the Latin term where the sulphide is apparently intended. The use of antimony sulphide to part silver from gold is based upon the greater affinity of silver than antimony for sulphur. Thus the silver, as in the last process, is converted into a sulphide, and is absorbed in the regulus, while the metallic antimony alloys with the gold and settles to the bottom of the pot. This process has several advantages over the sulphurization with crude sulphur; antimony is a more convenient vehicle of sulphur, for it saves the preliminary sulphurization with its attendant difficulties of volatilization of the sulphur; it also saves the granulation necessary in the former method; and the treatment of the subsequent products is simpler. However, it is possible that the sulphur-copper process was better adapted to bullion where the proportion of gold was low, because the fineness of the bullion mentioned in connection with the antimonial process was apparently much higher than the previous process. For instance, a bes of gold, containing 5, 6, or 7 double sextulae of silver would be .792, .750 or .708 fine. The antimonial method would have an advantage over nitric acid separation, in that high-grade bullion could be treated direct without artificial decrease of fineness required by inquartation to about .250 fine, with the consequent incidental losses of silver involved.
The process in this description falls into six operations: a, sulphurization of the silver by melting with antimony sulphide; b, separation of the gold "lump" (massula) by jogging; c, re-melting the regulus (mistura) three or four times for recovery of further "lumps"; d, re-melting of the "lump" four times, with further additions of antimony sulphide; e, cupellation of the regulus to recover the silver; f, cupellation of the antimony from the "lump" to recover the gold. Percy seems to think it difficult to understand the insistence upon the addition of copper. Biringuccio (IV, 6) states, among other things, that copper makes the ingredients more liquid. The later metallurgists, however, such as Ercker, Lohneys, and Schlüter, do not mention this addition; they do mention the "swelling and [Pg 452]frothing," and recommend that the crucible should be only partly filled. As to the copper, we suggest that it would desulphurize part of the antimony and thus free some of that metal to collect the gold. If we assume bullion of the medium fineness mentioned and containing no copper, then the proportions in the first charge would be about 36 parts gold, 12 parts silver, 41 parts sulphur, 103 parts antimony, and 9 parts copper. The silver and copper would take up 4.25 parts of sulphur, and thus free about 10.6 parts of antimony as metallics. It would thus appear that the amount of metallics provided to assist the collection of the gold was little enough, and that the copper in freeing 5.6 parts of the antimony was useful. It appears to have been necessary to have a large excess of antimony sulphide; for even with the great surplus in the first charge, the reaction was only partial, as is indicated by the necessity for repeated melting with further antimony.
The later metallurgists all describe the separation of the metallic antimony from the gold as being carried out by oxidation of the antimony, induced by a jet of air into the crucible, this being continued until the mass appears limpid and no cloud forms in the surface in cooling. Agricola describes the separation of the silver from the regulus by preliminary melting with argols, glass-gall, and some lead, and subsequent cupellation of the lead-silver alloy. The statement that unless this preliminary melting is done, the cupel will absorb silver, might be consonant with an attempt at cupellation of sulphides, and it is difficult to see that much desulphurizing could take place with the above fluxes. In fact, in the later descriptions of the process, iron is used in this melting, and we are under the impression that Agricola had omitted this item for a desulphurizing reagent. At the Dresden Mint, in the methods described by Percy (Metallurgy Silver and Gold, p. 373) the gold lumps were tested for fineness, and from this the amount of gold retained in the regulus was computed. It is not clear from Agricola's account whether the test with nitric acid was applied to the regulus or to the "lumps". For historical notes see p. [461].
[Pg 453][18] As will be shown in the historical note, this process of separating gold and silver is of great antiquity—in all probability the only process known prior to the Middle Ages, and in any event, the first one used. In general the process was performed by "cementing" the disintegrated bullion with a paste and subjecting the mass to long-continued heat at a temperature under the melting point of the bullion. The cement (compositio) is of two different species; in the first species saltpetre and vitriol and some aluminous or silicious medium are the essential ingredients, and through them the silver is converted into nitrate and absorbed by the mass; in the second species, common salt and the same sort of medium are the essentials, and in this case the silver is converted into a chloride. Agricola does not distinguish between these two species, for, as shown by the text, his ingredients are badly mixed.
[Pg 454] The process as here described falls into five operations: a, granulation of the bullion or preparation of leaves; b, heating alternate layers of cement and bullion in pots; c, washing the gold to free it of cement; d, melting the gold with borax or soda; e, treatment of the cement by way of melting with lead and cupellation to recover the silver. Investigation by Boussingault (Ann. De Chimie, 1833, p. 253-6), D'Elhuyar (Bergbaukunde, Leipzig, 1790, Vol. II, p. 200), and Percy (Metallurgy of Silver and Gold, p. 395), of the action of common salt upon silver under cementation conditions, fairly well demonstrated the reactions involved in the use of this species of cement. Certain factors are essential besides salt: a, the admission of air, which is possible through the porous pots used; b, the presence of some moisture to furnish hydrogen; c, the addition of alumina or silica. The first would be provided by Agricola in the use of new pots, the second possibly by use of wood fuel in a closed furnace, the third by the inclusion of brickdust. The alumina or silica at high temperatures decomposes the salt, setting free hydrochloric acid and probably also free chlorine. The result of the addition of vitriol in Agricola's ingredients is not discussed by those investigators, but inasmuch as vitriol decomposes into sulphuric acid under high temperatures, this acid would react upon the salt to free hydrochloric acid, and thus assist to overcome deficiencies in the other factors. It is possible also that sulphuric acid under such conditions would react directly upon the silver to form silver sulphates, which would be absorbed into the cement. As nitric acid is formed by vitriol and saltpetre at high temperatures, the use of these two substances as a cementing compound would produce nitric acid, which would at once attack the silver to form silver nitrate, which would be absorbed into the melted cement. In this case the brickdust probably acted merely as a vehicle for the absorption, and to lower the melting point of the mass and prevent fusion of the metal. While nitric acid will only part gold and silver when the latter is in great excess, yet when applied as fumes under cementation conditions it appears to react upon a minor ratio of silver. While the reactions of the two above species of compounds can be accounted for in a general way, the problem furnished by Agricola's statements is by no means simple, for only two of his compounds are simply salt cements, the others being salt and nitre mixtures. An inspection of these compounds produces at once a sense of confusion. Salt is present in every compound, saltpetre in all but two, vitriol in all but three. Lewis (Traité Singulier de Métallique, Paris, 1743, II, pp. 48-60), in discussing these processes, states that salt and saltpetre must never be used together, as he asserts that in this case aqua regia would be formed and the gold dissolved. Agricola, however, apparently found no such difficulty. As to the other ingredients, apart from nitre, salt, vitriol, and brickdust, they can have been of no use. Agricola himself points out that ingredients of "metallic origin" corrupt the gold and that brickdust and common salt are sufficient. In a description of this process in the Probierbüchlein (p. 58), no nitre is mentioned. This booklet does mention the recovery of the silver from the cement by amalgamation with mercury—the earliest mention of silver amalgamation.