Note 21 — BOOK X.

[Pg 458][21] Historical Note on Parting Gold and Silver. Although the earlier Classics contain innumerable references to refining gold and silver, there is little that is tangible in them, upon which to hinge the metallurgy of parting the precious metals. It appears to us, however, that some ability to part the metals is implied in the use of the touchstone, for we fail to see what use a knowledge of the ratio of gold and silver in bullion could have been without the power to separate them. The touchstone was known to the Greeks at least as early as the 5th Century B.C. (see note 37, p. 252), and a part of Theophrastus' statement (LXXVIII.) on this subject bears repetition in this connection: "The nature of the stone which tries gold is also very wonderful, as it seems to have the same power as fire; which is also a test of that metal.... The trial by fire is by the colour and the quantity lost by it, but that of the stone is made only by rubbing," etc. This trial by fire certainly implies a parting of the metals. It has been argued from the common use of electrum—a gold-silver alloy—by the Ancients, that they did not know how to part the two metals or they would not have wasted gold in such a manner, but it seems to us that the very fact that electrum was a positive alloy (20% gold, 80% silver), and that it was deliberately made (Pliny XXXIII, 23) and held of value for its supposed superior brilliancy to silver and the belief that goblets made of it detected poison, is sufficient answer to this.

To arrive by a process of elimination, we may say that in the Middle Ages, between 1100 and 1500 A.D., there were known four methods of parting these metals: a, parting by solution in nitric acid; b, sulphurization of the silver in finely-divided bullion by heating it with sulphur, and the subsequent removal of the silver sulphide in a regulus by melting with copper, iron, or lead; c, melting with an excess of antimony sulphide, and the direct conversion of the silver to sulphide and its removal in a regulus; d, cementation of the finely-divided bullion with salt, and certain necessary collateral re-agents, and the separation of the silver by absorption into the cement as silver chloride. Inasmuch as it can be clearly established that mineral acids were unknown to the Ancients, we can eliminate that method. Further, we may say at once that there is not, so far as has yet been found, even a remote statement that could be applied to the sulphide processes. As to cementation with salt, however, we have some data at about the beginning of the Christian Era.

Before entering into a more detailed discussion of the history of various processes, it may be useful, in a word, to fix in the mind of the reader our view of the first authority on various processes, and his period.

(1) Separation by cementation with salt, Strabo (?) 63 B.C.-24 A.D.; Pliny 23-79 A.D.

(2) Separation by sulphur, Theophilus, 1150-1200 A.D.

(3) Separation by nitric acid, Geber, prior to 14th Century.

(4) Separation by antimony sulphide, Basil Valentine, end 14th Century, or Probierbüchlein, beginning 15th Century.

(5) Separation by antimony sulphide and copper, or sulphur and copper, Probierbüchlein, beginning 15th Century.

(6) Separation by cementation with saltpetre, Agricola, 1556.

(7) Separation by sulphur and iron, Schlüter, 1738.

(8) Separation by sulphuric acid, D'Arcet, 1802.

(9) Separation by chloride gas, Thompson, 1833.

(10) Separation electrolytically, latter part 19th Century.

Parting by Cementation. The following passage from Strabo is of prime interest as the first definite statement on parting of any kind (III, 2, 8): "That when they have melted the gold and purified it by means of a kind of aluminous earth, the residue left is electrum. This, which contains a mixture of silver and gold, being again subjected to the fire, the silver is separated and the gold left (pure); for this metal is easily dissipated and fat, and on this account gold is most easily molten by straw, the flame of which is soft, and bearing a similarity (to the gold) causes it easily to dissolve, whereas coal, besides wasting a great deal, melts it too much, by reason of its vehemence, and carries it off (in vapour)." This statement has provoked the liveliest discussion, not only on account of the metallurgical [Pg 459]interest and obscurity, but also because of differences of view as to its translation; we have given that of Mr. H. C. Hamilton (London, 1903). A review of this discussion will be found in Percy's Metallurgy of Gold and Silver, p. 399. That it refers to cementation at all hangs by a slender thread, but it seems more nearly this than anything else.

Pliny (XXXIII, 25) is a little more ample: "(The gold) is heated with double its weight of salt and thrice its weight of misy, and again with two portions of salt and one of a stone which they call schistos. The virus is drawn out when these things are burnt together in an earthen crucible, itself remaining pure and incorrupt, the remaining ash being preserved in an earthen pot and mixed with water as a lotion for lichen (ring-worm) on the face." Percy (Metallurgy Silver and Gold, p. 398) rightly considers that this undoubtedly refers to the parting of silver and gold by cementation with common salt. Especially as Pliny further on states that with regard to misy, "In purifying gold they mix it with this substance." There can be no doubt from the explanations of Pliny and Dioscorides that misy was an oxidized pyrite, mostly iron sulphate. Assuming the latter case, then all of the necessary elements of cementation, i.e., vitriol, salt, and an aluminous or silicious element, are present.

The first entirely satisfactory evidence on parting is to be found in Theophilus (12th Century), and we quote the following from Hendrie's translation (p. 245): "Of Heating the Gold. Take gold, of whatsoever sort it may be, and beat it until thin leaves are made in breadth three fingers, and as long as you can. Then cut out pieces that are equally long and wide and join them together equally, and perforate through all with a fine cutting iron. Afterwards take two earthen pots proved in the fire, of such size that the gold can lie flat in them, and break a tile very small, or clay of the furnace burned and red, weigh it, powdered, into two equal parts, and add to it a third part salt for the same weight; which things being slightly sprinkled with urine, are mixed together so that they may not adhere together, but are scarcely wetted, and put a little of it upon a pot about the breadth of the gold, then a piece of the gold itself, and again the composition, and again the gold, which in the digestion is thus always covered, that gold may not be in contact with gold; and thus fill the pot to the top and cover it above with another pot, which you carefully lute round with clay, mixed and beaten, and you place it over the fire, that it may be dried. In the meantime compose a furnace from stones and clay, two feet in height, and a foot and a half in breadth, wide at the bottom, but narrow at the top, where there is an opening in the middle, in which project three long and hard stones, which may be able to sustain the flame for a long time, upon which you place the pots with the gold, and cover them with other tiles in abundance. Then supply fire and wood, and take care that a copious fire is not wanting for the space of a day and night. In the morning taking out the gold, again melt, beat and place it in the furnace as before. Again also, after a day and night, take it away and mixing a little copper with it, melt it as before, and replace it upon the furnace. And when you have taken it away a third time, wash and dry it carefully, and so weighing it, see how much is wanting, then fold it up and keep it."

The next mention is by Geber, of whose date and authenticity there is great doubt, but, in any event, the work bearing his name is generally considered to be prior to the 14th, although he has been placed as early as the 8th Century. We quote from Russell's translation, pp. 17 and 224, which we have checked with the Latin edition of 1542: "Sol, or gold, is beaten into thin plates and with them and common salt very well prepared lay upon lay in a vessel of calcination which set into the furnace and calcine well for three days until the whole is subtily calcined. Then take it out, grind well and wash it with vinegar, and dry it in the sun. Afterwards grind it well with half its weight of cleansed sal-armoniac; then set it to be dissolved until the whole be dissolved into most clear water." Further on: "Now we will declare the way of cementing. Seeing it is known to us that cement is very necessary in the examen of perfection, we say it is compounded of inflammable things. Of this [Pg 460]kind are, all blackening, flying, penetrating, and burned things; as is vitriol, sal-armoniac, flos aeris (copper oxide scales) and the ancient fictile stone (earthen pots), and a very small quantity, or nothing, of sulphur, and urine with like acute and penetrating things. All these are impasted with urine and spread upon thin plates of that body which you intend shall be examined by this way of probation. Then the said plates must be laid upon a grate of iron included in an earthen vessel, yet so as one touch not the other that the virtue of the fire may have free and equal access to them. Thus the whole must be kept in fire in a strong earthen vessel for the space of three days. But here great caution is required that the plates may be kept but not melt."

Albertus Magnus (1205-1280) De Mineralibus et Rebus Metallicis, Lib. IV, describes the process as follows:—"But when gold is to be purified an earthen vessel is made like a cucurbit or dish, and upon it is placed a similar vessel; and they are luted together with the tenacious lute called by alchemists the lute of wisdom. In the upper vessel there are numerous holes by which vapour and smoke may escape; afterwards the gold in the form of short thin leaves is arranged in the vessel, the leaves being covered consecutively with a mixture obtained by mixing together soot, salt, and brick dust; and the whole is strongly heated until the gold becomes perfectly pure and the base substances with which it was mixed are consumed." It will be noted that salt is the basis of all these cement compounds. We may also add that those of Biringuccio and all other writers prior to Agricola were of the same kind, our author being the first to mention those with nitre.

Parting with Nitric Acid. The first mention of nitric acid is in connection with this purpose, and, therefore, the early history of this reagent becomes the history of the process. Mineral acids of any kind were unknown to the Greeks or Romans. The works of the Alchemists and others from the 12th to the 15th Centuries, have been well searched by chemical historians for indications of knowledge of the mineral acids, and many of such suspected indications are of very doubtful order. In any event, study of the Alchemists for the roots of chemistry is fraught with the greatest difficulty, for not only is there the large ratio of fraud which characterised their operations, but there is even the much larger field of fraud which characterised the authorship and dates of writing attributed to various members of the cult. The mention of saltpetre by Roger Bacon (1214-94), and Albertus Magnus (1205-80), have caused some strain to read a knowledge of mineral acids into their works, but with doubtful result. Further, the Monk Theophilus (1150-1200) is supposed to have mentioned products which would be mineral acids, but by the most careful scrutiny of that work we have found nothing to justify such an assertion, and it is of importance to note that as Theophilus was a most accomplished gold and silver worker, his failure to mention it is at least evidence that the process was not generally known. The transcribed manuscripts and later editions of such authors are often altered to bring them "up-to-date." The first mention is in the work attributed to Geber, as stated above, of date prior to the 14th Century. The following passage from his De Inventione Veritatis (Nuremberg edition, 1545, p. 182) is of interest:—"First take one libra of vitriol of Cyprus and one-half libra of saltpetre and one-quarter of alum of Jameni, extract the aqua with the redness of the alembic—for it is very solvative—and use as in the foregoing chapters. This can be made acute if in it you dissolve a quarter of sal-ammoniac, which dissolves gold, sulphur, and silver." Distilling vitriol, saltpetre and alum would produce nitric acid. The addition of sal-ammoniac would make aqua regia; Geber used this solvent water—probably without being made "more acute"—to dissolve silver, and he crystallized out silver nitrate. It [Pg 461]would not be surprising to find all the Alchemists subsequent to Geber mentioning acids. It will thus be seen that even the approximate time at which the mineral-acids were first made cannot be determined, but it was sometime previous to the 15th Century, probably not earlier than the 12th Century. Beckmann (Hist. of Inventions II, p. 508) states that it appears to have been an old tradition that acid for separating the precious metals was first used at Venice by some Germans; that they chiefly separated the gold from Spanish silver and by this means acquired great riches. Beckmann considers that the first specific description of the process seems to be in the work of William Budaeus (De Asse, 1516, III, p. 101), who speaks of it as new at this time. He describes the operation of one, Le Conte, at Paris, who also acquired a fortune through the method. Beckmann and others have, however, entirely overlooked the early Probierbüchlein. If our conclusions are correct that the first of these began to appear at about 1510, then they give the first description of inquartation. This book (see appendix) is made up of recipes, like a cook-book, and four or five different recipes are given for this purpose; of these we give one, which sufficiently indicates a knowledge of the art (p. 39): "If you would part them do it this way: Beat the silver which you suppose to contain gold, as thin as possible; cut it in small pieces and place it in 'strong' water (starkwasser). Put it on a mild fire till it becomes warm and throws up blisters or bubbles. Then take it and pour off the water into a copper-bowl; let it stand and cool. Then the silver settles itself round the copper bowl; let the silver dry in the copper bowl, then pour the water off and melt the silver in a crucible. Then take the gold also out of the glass kolken and melt it together." Biringuccio (1540, Book VI.) describes the method, but with much less detail than Agricola. He made his acid from alum and saltpetre and calls it lacque forti.

Parting with Sulphur. This process first appears in Theophilus (1150-1200), and in form is somewhat different from that mentioned by Agricola. We quote from Hendrie's Translation, p. 317, "How gold is separated from silver. When you have scraped the gold from silver, place this scraping in a small cup in which gold or silver is accustomed to be melted, and press a small linen cloth upon it, that nothing may by chance be abstracted from it by the wind of the bellows, and placing it before the furnace, melt it; and directly lay fragments of sulphur in it, according to the quantity of the scraping, and carefully stir it with a thin piece of charcoal until its fumes cease; and immediately pour it into an iron mould. Then gently beat it upon the anvil lest by chance some of that black may fly from it which the sulphur has burnt, because it is itself silver. For the sulphur consumes nothing of the gold, but the silver only, which it thus separates from the gold, and which you will carefully keep. Again melt this gold in the same small cup as before, and add sulphur. This being stirred and poured out, break what has become black and keep it, and do thus until the gold appear pure. Then gather together all that black, which you have carefully kept, upon the cup made from the bone and ash, and add lead, and so burn it that you may recover the silver. But if you wish to keep it for the service of niello, before you burn it add to it copper and lead, according to the measure mentioned above, and mix with sulphur." This process appears in the Probierbüchlein in many forms, different recipes containing other ingredients besides sulphur, such as salt, saltpetre, sal-ammoniac, and other things more or less effective. In fact, a series of hybrid methods between absolute melting with sulphur and cementation with salt, were in use, much like those mentioned by Agricola on p. 458.

Parting with Antimony Sulphide. The first mention of this process lies either in Basil Valentine's "Triumphant Chariot of Antimony" or in the first Probierbüchlein. The date to be assigned to the former is a matter of great doubt. It was probably written about the end of the 15th Century, but apparently published considerably later. The date of the Probierbüchlein we have referred to above. The statement in the "Triumphal Chariot" is as follows (Waite's Translation, p. 117-118): "The elixir prepared in this way has the same power of penetrating and pervading the body with its purifying properties that antimony has of penetrating and purifying gold.... This much, however, I have proved beyond a possibility of doubt, that antimony not only purifies gold and frees it [Pg 462]from foreign matter, but it also ameliorates all other metals, but it does the same for animal bodies." There are most specific descriptions of this process in the other works attributed to Valentine, but their authenticity is so very doubtful that we do not quote. The Probierbüchlein gives several recipes for this process, all to the same metallurgical effect, of which we quote two: "How to separate silver from gold. Take 1 part of golden silver, 1 part of spiesglass, 1 part copper, 1 part lead; melt them together in a crucible. When melted pour into the crucible pounded sulphur and directly you have poured it in cover it up with soft lime so that the fumes cannot escape, and let it get cold and you will find your gold in a button. Put that same in a pot and blow on it." "How to part gold and silver by melting or fire. Take as much gold-silver as you please and granulate it; take 1 mark of these grains, 1 mark of powder; put them together in a crucible. Cover it with a small cover, put it in the fire, and let it slowly heat; blow on it gently until it melts; stir it all well together with a stick, pour it out into a mould, strike the mould gently with a knife so that the button may settle better, let it cool, then turn the mould over, strike off the button and twice as much spiesglas as the button weighs, put them in a crucible, blow on it till it melts, then pour it again into a mould and break away the button as at first. If you want the gold to be good always add to the button twice as much spiesglass. It is usually good gold in three meltings. Afterward take the button, place it on a cupel, blow on it till it melts. And if it should happen that the gold is covered with a membrane, then add a very little lead, then it shines (plickt) and becomes clearer." Biringuccio (1540) also gives a fairly clear exposition of this method. All the old refiners varied the process by using mixtures of salt, antimony sulphide, and sulphur, in different proportions, with and without lead or copper; the net effect was the same. Later than Agricola these methods of parting bullion by converting the silver into a sulphide and carrying it off in a regulus took other forms. For instance, Schlüter (Hütte-Werken, Braunschweig, 1738) describes a method by which, after the granulated bullion had been sulphurized by cementation with sulphur in pots, it was melted with metallic iron. Lampadius (Grundriss Einer Allgemeinen Hüttenkunde, Göttingen, 1827) describes a treatment of the bullion, sulphurized as above, with litharge, thus creating a lead-silver regulus and a lead-silver-gold bullion which had to be repeatedly put through the same cycle. The principal object of these processes was to reduce silver bullion running low in gold to a ratio acceptable for nitric acid treatment.

Before closing the note on the separation of gold and silver, we may add that with regard to the three processes largely used to-day, the separation by solution of the silver from the bullion by concentrated sulphuric acid where silver sulphate is formed, was first described by D'Arcet, Paris, in 1802; the separation by introducing chlorine gas into the molten bullion and thus forming silver chlorides was first described by Lewis Thompson in a communication to the Society of Arts, 1833, and was first applied on a large scale by F. B. Miller at the Sydney Mint in 1867-70; we do not propose to enter into the discussion as to who is the inventor of electrolytic separation.