"Salt made from the ashes of musk ivy" (sal ex anthyllidis cinere factus,—Glossary, salalkali). This would be largely potash.
[7] This wondrous illustration of soda-making from Nile water is no doubt founded upon Pliny (XXXI., 46). "It is made in almost the same manner as salt, except that sea-water is put into salt pans, whereas in the nitrous pans it is water of the Nile; these, with the subsidence of the Nile during the forty days, are impregnated with nitrum."
[8] This paragraph displays hopeless ignorance. Borax was known to Agricola and greatly used in his time; it certainly was not made from these compounds, but was imported from Central Asia. Sal-ammoniac was also known in his time, and was used like borax as a soldering agent. The reaction given by Agricola would yield free ammonia. The following historical notes on borax and sal-ammoniac may be of service.
Borax.—The uncertainties of the ancient distinctions in salts involve borax deeply. The word Baurach occurs in Geber and the other early Alchemistic writings, but there is nothing to prove that it was modern borax. There cannot be the slightest doubt, however, that the material referred to by Agricola as borax was our borax, because of the characteristic qualities incidentally mentioned in [Book VII]. That he believed it was an artificial product from nitrum is evident enough from his usual expression "chrysocolla made from nitrum, which the Moors call borax." Agricola, in De Natura Fossilium (p. 206-7), makes the following statements, which could leave no doubt on the subject:—"Native nitrum is found in the earth or on the surface.... It is from this variety that the Venetians make chrysocolla, which I call borax.... The second variety of artificial nitrum is made at the present day from the native nitrum, called by the Arabs tincar, but I call it usually by the Greek name chrysocolla; it is really the Arabic borax.... This nitrum does not decrepitate nor fly out of the fire; however, the native variety swells up from within." The application of the word chrysocolla (chrysos, gold; colla, solder) to soldering materials, and at the same time to the copper mineral, is of Greek origin. If any further proof were needed as to the substance meant by Agricola, it lies in the word tincar. For a long time the borax of Europe was imported from Central Asia, through Constantinople and Venice, under the name of tincal or tincar. When this trade began, we do not know; evidently before Agricola's time. The statement here of making borax from alum and sal-ammoniac is identical with the assertion of Biringuccio (II., 9).
Sal-ammoniac.—The early history of this—ammonium chloride—is also under a cloud. Pliny (XXXI., 39) speaks of a sal-hammoniacum, and Dioscorides (V., 85) uses much the same word. Pliny describes it as from near the temple of Ammon in Egypt. None of the distinctive characteristics of sal-ammoniac are mentioned, and there is every reason to believe it was either common salt or soda. Herodotus, Strabo, and others mention common salt sent from about the same locality. The first authentic mention is in Geber, who calls it sal-ammoniacum, and describes a method of making, and several characteristic reactions. It was known in the Middle Ages under various names, among them sal-aremonicum. Agricola (De Nat. Fos., III., p. 206) notes its characteristic quality of volatilization. "Sal-ammoniac ... in the fire neither crackles nor flies out, but is totally consumed." He also says (p. 208): "Borax is used by goldsmiths to solder gold, likewise silver. The artificers who make iron needles (tacks?) similarly use sal-ammoniac when they cover the heads with tin." The statement from Pliny mentioned in this paragraph is from XXXIII., 29, where he describes the chrysocolla used as gold solder as made from verdigris, nitrum, and urine in the way quoted. It is quite possible that this solder was sal-ammoniac, though not made in quite this manner. Pliny refers in several places (XXXIII., 26, 27, 28, and 29, XXXV., 28, etc.) to chrysocolla, about which he is greatly confused as between gold-solder, the copper mineral, and a green pigment, the latter being of either mineral origin.
[Pg 561][9] Saltpetre was secured in the Middle Ages in two ways, but mostly from the treatment of calcium nitrate efflorescence on cellar and similar walls, and from so-called saltpetre plantations. In this description of the latter, one of the most essential factors is omitted until the last sentence, i.e., that the nitrous earth was the result of the decay of organic or animal matter over a long period. Such decomposition, in the presence of potassium and calcium carbonates—the lye and lime—form potassium and calcium nitrates, together with some magnesium and sodium nitrates. After lixiviation, the addition of lye converts the calcium and magnesium nitrates into saltpetre, i.e., Ca(NO3)2 + K2CO3 = CaCO3 + 2KNO3. The carbonates precipitate out, leaving the saltpetre in solution, from which it was evaporated and crystallized out. The addition of alum as mentioned would scarcely improve the situation.
The purification by repeated re-solution and addition of lye, and filtration, would eliminate the remaining other salts. The purification with sulphur, however, is more difficult [Pg 562]to understand. In this case the saltpetre is melted and the sulphur added and set alight. Such an addition to saltpetre would no doubt burn brilliantly. The potassium sulphate formed would possibly settle to the bottom, and if the "greasy matter" were simply organic impurities, they might be burned off. This method of refining appears to have been copied from Biringuccio (X., 1), who states it in almost identical terms.
Historical Note.—As mentioned in [Note 6 above], it is quite possible that the Ancients did include efflorescence of walls under nitrum; but, so far as we are aware, no specific mention of such an occurrence of nitrum is given, and, as stated before, there is every reason to believe that all the substances under that term were soda and potash. Especially the frequent mention of the preparation of nitrum by way of burning, argues strongly against saltpetre being included, as they would hardly have failed to notice the decrepitation. Argument has been put forward that Greek fire contained saltpetre, but it amounts to nothing more than argument, for in those receipts preserved, no salt of any kind is mentioned. It is most likely that the leprosy of house-walls of the Mosaic code (Leviticus XIV., 34 to 53) was saltpetre efflorescence. The drastic treatment by way of destruction of such "unclean" walls and houses, however, is sufficient evidence that this salt was not used. The first certain mention of saltpetre (sal petrae) is in Geber. As stated before, the date of this work is uncertain; in any event it was probably as early as the 13th Century. He describes the making of "solvative water" with alum and saltpetre, so there can be no doubt as to the substance (see Note on p. [460], on nitric acid). There is also a work by a nebulous Marcus Graecus, where the word sal petrosum is used. And it appears that Roger Bacon (died 1294) and Albertus Magnus (died 1280) both had access to that work. Bacon uses the term sal petrae frequently enough, and was the first to describe gunpowder (De Mirabili Potestate Artis et Naturae 1242). He gives no mention of the method of making his sal petrae. Agricola uses throughout the Latin text the term halinitrum, a word he appears to have coined himself. However, he gives its German equivalent in the Interpretatio as salpeter. The only previous description of the method of making saltpetre, of which we are aware, is that of Biringuccio (1540), who mentions the boiling of the excrescences from walls, and also says a good deal about boiling solutions from "nitrous" earth, which may or may not be of "plantation" origin. He also gives this same method of refining with sulphur. In any event, this statement by Agricola is the first clear and complete description of the saltpetre "plantations." Saltpetre was in great demand in the Middle Ages for the manufacture of gunpowder, and the first record of that substance and of explosive weapons necessarily involves the knowledge of saltpetre. However, authentic mention of such weapons only begins early in the 14th Century. Among the earliest is an authority to the Council of Twelve at Florence to appoint persons to make cannon, etc., (1326), references to cannon in the stores of the Tower of London, 1388, &c.
[Pg 564][10] There are three methods of manufacturing alum described by Agricola, the first and third apparently from shales, and the second from alum rock or "alunite." The reasons for assuming that the first process was from shales, are the reference to the "aluminous earth" as ore (venae) coming from "veins," and also the mixture of vitriol. In this process the free sulphuric acid formed by the oxidation of pyrites reacts upon the argillaceous material to form aluminium sulphate. The decomposed ore is then placed in tanks and lixiviated. The solution would contain aluminium sulphate, vitriol, and other impurities. By the addition of urine, the aluminium sulphate would be converted into ammonia alum. Agricola is, of course, mistaken as to the effect of the addition, being under the belief that it separated the vitriol from the alum; in fact, this belief was general until the latter part of the 18th Century, when Lavoisier determined that alum must have an alkali base. Nor is it clear [Pg 565]from this description exactly how they were separated. In a condensed solution allowed to cool, the alum would precipitate out as "alum meal," and the vitriol would "float on top"—in solution. The reference to "meal" may represent this phenomenon, and the re-boiling referred to would be the normal method of purification by crystallization. The "asbestos" and gypsum deposited in the caldrons were no doubt feathery and mealy calcium sulphate. The alum produced would, in any event, be mostly ammonia alum.
The second process is certainly the manufacture from "alum rock" or "alunite" (the hydrous sulphate of aluminium and potassium), such as that mined at La Tolfa in the Papal States, where the process has been for centuries identical with that here described. The alum there produced is the double basic potassium alum, and crystallizes into cubes instead of octahedra, i.e., the Roman alum of commerce. The presence of much ferric oxide gives the rose colour referred to by Agricola. This account is almost identical with that of Biringuccio (II., 4), and it appears from similarity of details that Agricola, as stated in his [preface], must have "refreshed his mind" from this description; it would also appear from the [preface] that he had himself visited the locality.