De Re Metallica-Recovery of Precious Metals Page 493



	458	BOOK X.

	of prepared sulphur (not exposed to the fire), one siciliens of glass, likewise one sicilicus of saltpetre, and a drachma of sal-ammoniac.²¹ The sulphur is prepared as follows : it is first crushed to powder, then it is heated for six hours in sharp vinegar, and finally poured into a vessel and washed with warm water ; then that which settles at the bottom of the vessel is dried. To refine the salt it is placed in river water and boiled, and again evaporated. The second compound contains one libra of sulphur (not exposed to fire) and two librae of refined salt. The third compound is made from one "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 ^AD· (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 Probierbuchlein, beginning 15th Century. (5) Separation by antimony sulphide and copper, or sulphur and copper, Probierbuchlein, 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 (in, 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