Note note 10, p. 564 — BOOK XII.

[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.

The third process is essentially the same as the first, except that the decomposition of the pyrites was hastened by roasting. The following obscure statement of some interest occurs in Agricola's De Natura Fossilium, p. 209:—"... alum is made from vitriol, for when oil is made from the latter, alum is distilled out (expirat). This absorbs the clay which is used in cementing glass, and when the operation is complete the clay is macerated with pure water, and the alum is soon afterward deposited in the shape of small cubes." Assuming the oil of vitriol to be sulphuric acid and the clay "used in cementing glass" to be kaolin, we have here the first suggestion of a method for producing alum which came into use long after.

"Burnt alum" (alumen coctum).—Agricola frequently uses this expression, and on p. 568, describes the operation, and the substance is apparently the same as modern dehydrated alum, often referred to as "burnt alum."

Historical Notes.—Whether the Ancients knew of alum in the modern sense is a most vexed question. The Greeks refer to a certain substance as stypteria, and the Romans refer to this same substance as alumen. There can be no question as to their knowledge and common use of vitriol, nor that substances which they believed were entirely different from vitriol were comprised under the above names. Beckmann (Hist. of Inventions, Vol. I., p. 181) seems to have been the founder of the doctrine that the ancient alumen was vitriol, and scores of authorities seem to have adopted his arguments without inquiry, until that belief [Pg 566]is now general. One of the strongest reasons put forward was that alum does not occur native in appreciable quantities. Apart from the fact that the weight of this argument has been lost by the discovery that alum does occur in nature to some extent as an aftermath of volcanic action, and as an efflorescence from argillaceous rocks, we see no reason why the Ancients may not have prepared it artificially. One of the earliest mentions of such a substance is by Herodotus (II., 180) of a thousand talents of stypteria, sent by Amasis from Egypt as a contribution to the rebuilding of the temple of Delphi. Diodorus (V., 1) mentions the abundance which was secured from the Lipari Islands (Stromboli, etc.), and a small quantity from the Isle of Melos. Dioscorides (V., 82) mentions Egypt, Lipari Islands, Melos, Sardinia, Armenia, etc., "and generally in any other places where one finds red ochre (rubrica)." Pliny (XXXV., 52) gives these same localities, and is more explicit as to how it originates—"from an earthy water which exudes from the earth." Of these localities, the Lipari Islands (Stromboli, etc.), and Melos are volcanic enough, and both Lipari and Melos are now known to produce natural alum (Dana. Syst. Min., p. 95; and Tournefort, "Relation d'un voyage du Levant." London, 1717, Lettre IV., Vol. 1.). Further, the hair-like alum of Dioscorides, repeated by Pliny below, was quite conceivably fibrous kalinite, native potash alum, which occurs commonly as an efflorescence. Be the question of native alum as it may—and vitriol is not much more common—our own view that the ancient alumen was alum, is equally based upon the artificial product. Before entering upon the subject, we consider it desirable to set out the properties of the ancient substance, a complete review of which is given by Pliny (XXXV., 52), he obviously quoting also from Dioscorides, which, therefore, we do not need to reproduce. Pliny says:—

"Not less important, or indeed dissimilar, are the uses made of alumen; by which name is understood a sort of salty earth. Of this, there are several kinds. In Cyprus there is a white alumen, and a darker kind. There is not a great difference in their colour, though the uses made of them are very dissimilar,—the white alumen being employed in a liquid state for dyeing wool bright colours, and the dark-coloured alumen, on the other hand, for giving wool a sombre tint. Gold is purified with black alumen. Every kind of alumen is from a limus water which exudes from the earth. The collection of it commences in winter, and it is dried by the summer sun. That portion of it which first matures is the whitest. It is obtained in Spain, Egypt, Armenia, Macedonia, Pontus, Africa, and the islands of Sardinia, Melos, Lipari, and Strongyle; the most esteemed, however, is that of Egypt, the next best from Melos. Of this last there are two kinds, the liquid alumen, and the solid. Liquid alumen, to be good, should be of a limpid and milky appearance; when [Pg 568]rubbed, it should be without roughness, and should give a little heat. This is called phorimon. The mode of detecting whether it has been adulterated is by pomegranate juice, for, if genuine, the mixture turns black. The other, or solid, is pale and rough and turns dark with nut-galls; for which reason it is called paraphoron. Liquid alumen is naturally astringent, indurative, and corrosive; used in combination with honey, it heals ulcerations.... There is one kind of solid alumen, called by the Greeks schistos, which splits into filaments of a whitish colour; for which reason some prefer calling it trichitis (hair like). Alumen is produced from the stone chalcitis, from which copper is also made, being a sort of coagulated scum from that stone. This kind of alumen is less astringent than the others, and is less useful as a check upon bad humours of the body.... The mode of preparing it is to cook it in a pan until it has ceased being a liquid. There is another variety of alumen also, of a less active nature, called strongyle. It is of two kinds. The fungous, which easily dissolves, is utterly condemned. The better kind is the pumice-like kind, full of small holes like a sponge, and is in round pieces, more nearly white in colour, somewhat greasy, free from grit, friable, and does not stain black. This last kind is cooked by itself upon charcoal until it is reduced to pure ashes. The best kind of all is that called melinum, from the Isle of Melos, as I have said, none being more effectual as an astringent, for staining black, and for indurating, and none becomes more dry.... Above all other properties of alumen is its remarkable astringency, whence its Greek name.... It is injected for dysentry and employed as a gargle." The lines omitted refer entirely to medical matters which have no bearing here. The following paragraph (often overlooked) from Pliny (XXXV., 42) also has an important bearing upon the subject:—"In Egypt they employ a wonderful method of dyeing. The white cloth, after it is pressed, is stained in various places, not with dye stuffs, but with substances which absorb colours. These applications are not apparent on the cloth, but when it is immersed in a caldron of hot dye it is removed the next moment brightly coloured. The remarkable circumstance is that although there be only one dye in the caldron yet different colours appear in the cloth."

It is obvious from Pliny's description above, and also from the making of vitriol (see Note 11, p. 572), that this substance was obtained from liquor resulting from natural or artificial lixiviation of rocks—in the case of vitriols undoubtedly the result of decomposition of pyritiferous rocks (such as chalcitis). Such liquors are bound to contain aluminum sulphate if there is any earth or clay about, and whether they contained alum would be a question of an alkali being present. If no alkali were present in this liquor, vitriol would [Pg 569]crystallize out first, and subsequent condensation would yield aluminum sulphate. If alkali were present, the alum would crystallize out either before or with the vitriol. Pliny's remark, "that portion of it which first matures is whitest", agrees well enough with this hypothesis. No one will doubt that some of the properties mentioned above belong peculiarly to vitriol, but equally convincing are properties and uses that belong to alum alone. The strongly astringent taste, white colour, and injection for dysentry, are more peculiar to alum than to vitriol. But above all other properties is that displayed in dyeing, for certainly if we read this last quotation from Pliny in conjunction with the statement that white alumen produces bright colours and the dark kind, sombre colours, we have the exact reactions of alum and vitriol when used as mordants. Therefore, our view is that the ancient salt of this character was a more or less impure mixture ranging from alum to vitriol—"the whiter the better." Further, considering the ancient knowledge of soda (nitrum), and the habit of mixing it into almost everything, it does not require much flight of imagination to conceive its admixture to the "water," and the absolute production of alum.

Whatever may have been the confusion between alum and vitriol among the Ancients, it appears that by the time of the works attributed to Geber (12th or 13th Century), the difference was well known. His work (Investigationes perfectiones, IV.) refers to alumen glaciale and alumen jameni as distinguished from vitriol, and gives characteristic reactions which can leave no doubt as to the distinction. We may remark here that the repeated statement apparently arising from Meyer (History of Chemistry, p. 51) that Geber used the term alum de rocca is untrue, this term not appearing in the early Latin translations. During the 15th Century alum did come to be known in Europe as alum de rocca. Various attempts have been made to explain the origin of this term, ranging from the Italian root, a "rock," to the town of Rocca in Syria, where alum was supposed to have been produced. In any event, the supply for a long period prior to the middle of the 15th Century came from Turkey, and the origin of the methods of manufacture described by Agricola, and used down to the present day, must have come from the Orient.

In the early part of the 15th Century, a large trade in alum was done between Italy and Asia Minor, and eventually various Italians established themselves near Constantinople and Smyrna for its manufacture (Dudae, Historia Byzantina Venetia, 1729, p. 71). The alum was secured by burning the rock, and lixiviation. With the capture of Constantinople by the Turks (1453), great feeling grew up in Italy over the necessity of buying this requisite for their dyeing establishments from the infidel, and considerable exertion was made to find other sources of supply. Some minor works were attempted, but nothing much [Pg 570]eventuated until the appearance of one John de Castro. From the Commentaries of Pope Pius II. (1614, p. 185), it appears that this Italian had been engaged in dyeing cloth in Constantinople, and thus became aware of the methods of making alum. Driven out of that city through its capture by the Turks, he returned to Italy and obtained an office under the Apostolic Chamber. While in this occupation he discovered a rock at Tolfa which appeared to him identical with that used at Constantinople in alum manufacture. After experimental work, he sought the aid of the Pope, which he obtained after much vicissitude. Experts were sent, who after examination "shed tears of joy, they kneeling down three times, worshipped God and praised His kindness in conferring such a gift on their age." Castro was rewarded, and the great papal monopoly was gradually built upon this discovery. The industry firmly established at Tolfa exists to the present day, and is the source of the Roman alum of commerce. The Pope maintained this monopoly strenuously, by fair means and by excommunication, gradually advancing the price until the consumers had greater complaint than against the Turks. The history of the disputes arising over the papal alum monopoly would alone fill a volume.

By the middle of the 15th Century alum was being made in Spain, Holland, and Germany, and later in England. In her efforts to encourage home industries and escape the tribute to the Pope, Elizabeth (see Note on p. 283) invited over "certain foreign chymistes and mineral masters" and gave them special grants to induce them to "settle in these realmes." Among them was Cornelius Devoz, to whom was granted the privilege of "mining and digging in our Realm of England for allom and copperas." What Devoz accomplished is not recorded, but the first alum manufacture on a considerable scale seems to have been in Yorkshire, by one Thomas Chaloner (about 1608), who was supposed to have seduced workmen from the Pope's alum works at Tolfa, for which he was duly cursed with all the weight of the Pope and Church. (Pennant, Tour of Scotland, 1786).