MISTRY.
The salt, which now receives the name of Alum, was not known to the ancients ; the ?/iimen of the Romans, from the description that has been given of it, appear ing to have been rather a sulphate of iron, or at least to have contained a considerable portion of that metal. The art of extracting and preparing alum, was brought into Europe from the Levant. The most ancient of the alum-works known, is that of Rocca in Syria, now called Edessa ; whence the name of Rock alum, applied to a particular variety of this salt, has been derived. From this place, the alum used in Europe continued to be imported, until in the fifteenth century, some Genoese, who had learned in the Levant the mode of fabricating it, were fortunate enough to discover alum ores in Italy, and to extract it from them. The alum works at Tolfa, near Civita Vecchia, attained celebrity and still afford a large quantity of this salt. In the six teenth century, the art of fabricating alum was known in other parts of Europe: The manufacture appears to have been established in England during the reign of Elizabeth, at Whitby, in Yorkshire, where it is still carried on.
The properties of the peculiar earth which forms the basis of alum, were first pointed out by Geoffroy and Hellot, and it was shown by these chemists to be the same with that which is the basis of pure clay ; which, from modern chemists, has received the name of argil, or alumine. This earth, Pott and Margraaf proved to be contained in alum. The acid combined with it, was known to be the sulphuric ; and this acid being present in excess, or the salt being always sensi bly acidulous, alum was regarded as a supersulphate of argil.
More light, however, was thrown on the composition of this salt a few years ago by the researches of Vauque lin, and of Chaptal. It had been known, that, in some of the processes for manufactut ing alum, it was neces sary to add to the saline Icy, to cause it to crystallize, a quantity of potash, or of ammonia, or of some substance capable of affording either of these alkalis. This was supposed to operate by correcting the too great excess of acidity, which it was imagined prevented the crystalli zation. Bergman, however, observed, that soda, or
lime, which would equally correct any excess of acid, cannot be employed with advantage ; and Vauquelin and Chaptal, by an analysis of the different varieties of alum of commerce, proved, that potash is always an ingredient in them, and is essential to the composi tion of the salt. From the experiments of these che mists it appears, that the varieties of alum which are met with in commerce, are not altogether uniform in composition. Some of them consist of sulphuric acid, argil, and potash ; the composition of this variety, as stated by Vauquelin, being 49 of sulphate of argil, 7 of sulphate of potash, and 44 of water. In others, am monia is present, as is evident from the ammoniacal smell which they exhale on the addition of lime to the solution. Frequently, too, they contain a portion of iron.
The knowledge of the composition of this salt, enables us better to understand the nature of the processes by which it is prepared. These vary considerably accord ing to the nature of the substance from which it is pro cured. This substance is usually a kind of slate, which, from affording this product, has received the name of alum ore, or alum slate, and which contains the elements necessary to the formation of alum in a more or less perfect state.
The following is the process followed at the celebra ted alum works of Tolfa :—The ore is blown up with gunpowder: it is separated from the pieces of the rock that adhere to it : it is calcined in furnaces, nearly in the same manner as lime is burned : in six or seven hours, being sufficiently calcined and friable, it is taken out, and laid on pavements of a long shape, surrounded with walled trenches: on these it is laid in heaps of a moderate height, which are watered for forty clays with water from the trenches. The ore being thus de composed, it is boiled in large cauldrons ; and when the water is saturated to a certain point, it is poured into the crystallizing pans ; where, after it is cold, it deposits the alum in large crystalline masses. Journal des Mmes, No. 117. p. 179.