De Re Metallica-Recovery of Precious Metals Page 489



	454	BOOK X.

	consist of half a libra of brick dust, a quarter of a libra of salt, an uncia of saltpetre, half an uncia of sal-ammoniac, and half an uncia of rock salt. The bricks or tiles from which the dust is made must be composed of fatty clays, free from sand, grit, and small stones, and must be moderately burnt and very old. Another cement is made of a bes of brick dust, a third of rock salt, an uncia of saltpetre, and half an uncia of refined salt. Another cement is made of a bes of brick dust, a quarter of refined salt, one and a half unciae of saltpetre, an uncia of sal-ammoniac, and half an uncia of rock salt. Another has one libra of brick dust, and half a libra of rock salt, to which some add a sixth of a libra and a sicilicus of vitriol. Another is made of half a libra of brick dust, a third of a libra of rock salt, an uncia and a half of vitriol, and one uncia of saltpetre. Another consists of a bes of brick dust, a third of refined salt, a sixth of white vitriol¹⁹, half an uncia of verdigris, and likewise half an uncia of saltpetre. Another is made of one and a third librae of brick dust, a bes of rock salt, a sixth of a libra and half an uncia of sal-ammoniac, a sixth and half an uncia of vitriol, and a sixth of saltpetre. Another contains a libra of brick dust, a third of refined salt, and one and a half unciae of vitriol. 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 Boussingalt (Ann. De Chimie, 1833, p. 253-6), D'Elhuyar (Bergbaukunde, Leipzig, 1790, Vol. 11, 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, 1743,11, 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. ¹⁹While a substance which we now know to be natural zinc sulphate was known to Agricola (see note 11, p. 572), it is hardly possible that it is referred to here If green vitriol be dehydrated and powdered, it is white.