De Re Metallica-Recovery of Precious Metals Page 495



	460	BOOK X.

	pound and is stirred; afterward it is poured into another crucible, first warmed and lined with tallow, and then violently shaken. The rest is performed according to the process I have already explained. Gold may be parted without injury from silver goblets and from other gilt vessels and articles²², by means of a powder, which consists of one part of sal-ammoniac and half a part of sulphur. The gilt goblet or other article is smeared with oil, and the powder is dusted on ; the article is seized in the hand, or with tongs, and is carried to the fire and sharply tapped, and by this means the gold falls into water in vessels placed underneath, while the goblet remains uninjured. " 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 J ameni, 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 ²²There were three methods of gilding practised in the Middle Ages—the first by hammering on gold leaf ; the second by laying a thin plate of gold on a thicker plate of silver, expanding both together, and fabricating the articles out of the sheets thus prepared ; and the third by coating over the article with gold amalgam, and subsequently driving off the mercury by heat. Copper and iron objects were silver-plated by immersing them in molten silver after coating with sal-ammoniac or borax. Tinning was done in the same way.