ALUM 



100 



poso a boiling hot solution of alum with a solution of potash, till the mixture appears 

 nearly neutral by litmus-paper. This insoluble or basic alum exists native in the 

 alum-stone of Tolfa, near Civita Vecchis. (See below.) 



Ores or Saw Material, The chief difficulty in manufacturing alum has been the 

 solution of the alumina. This substance is generally combined with silica in such a 

 strong combination, that even powerful acids cannot remove it without assistance. 

 The older methods, however, took no notice of these difficulties, and obtained the 

 alum more or less directly from nature. The method now practised at the Solfatara 

 di Pozzuoli and in the island Vulcano is simply to take the efflorescence and the earth 

 containing it, wash it with water, and concentrate. But it very seldom contains a 

 sufficient amount of potash to form alum. A salt of potash is then added, chiefly 

 a carbonate. To transform this into a sulphate, a portion of the sulphate of alumina 

 is decomposed. The use of a carbonate is a wasteful method of modern times ; the 

 ancients would have felt no difficulty, but boiled all down, and so obtained the whole 

 alumina there. Their product, therefore, would have been basic sulphate of alumina, 

 which it evidently was when this practice was resorted to. "When they merely con- 

 centrated and then crystallised, they got pure alum ; but they lost a great deal of 

 their alumina. 



Alum occurs ready formed in nature in the alum-stones of Italy, &c., as an efflo- 

 rescence on stones, and in certain mineral waters in the East Indies. (See ALUM, 

 NATIVE.) The alum of European commerce is manufactured artificially, either from 

 the alum-schists or stones, or from clay. The mode of manufacture differs according 

 to the nature of these earthy compounds. Some of them, such as the alum-stone, 

 contain all the elements of the salt, but mixed with other matters, from which it 

 must be freed. The schists contain only the elements of two of the constituents, 

 namely, clay and sulphur, which are convertible into sulphate of alumina, and this 

 may be then made into alum by adding the alkaline ingredient. To this class belong 

 the alum-slates, and other analogous schists, containing brown coal. Alum has of 

 late years been very extensively prepared by Spenco's process, in which the raw 

 material is a carbonaceous shale from the coal-measures. Quito recently a new 

 method of alum manufacture has been introduced, in which the raw material is a 

 siliceous phosphate of alumina and iron from Ecdonda in the West Indies. Each of 

 these methods of manufacturing alum will now be separately described. 



I. Manufacture of Alum from the Alum-Stone. The alum-stone or alunite is a 

 mineral of limited occurrence, being found in moderate quantity at Tolfa (near Civita 

 Vecchia, in the Koman States), and in larger quantity in Hungary, at Beregszaz and 

 Muszay, where it forms entire beds in a hard substance, partly characterized by 

 numerous cavities, containing drusy crystallisations of pure alum-stone or basic alum. 

 It is also found in the Isle of Milo and elsewhere in the Grecian Archipelago. 

 The alum-stone appears to be confined to volcanic districts, where it is formed by 

 the action of sulphurous acid gas and steam on trachytic and other felspathic rocks. 

 The ordinary alum-stone is a massive rock, often cellular in texture, and sometimes 

 sufficiently hard to be employed as a millstone. 



The composition of ordinary alum-stone is fairly represented by the following 

 selection of analyses : 



The older analysts examined the rock as a whole, including all impurities, and 

 hence the proportion of silica in their determinations appears much higher than in 

 the more recent analyses, which relate to the alunito alone, separated as far as 

 possible from mechanically-associated quartzose matter. 



The purest specimens of alunite, which exhibit the mineral in rhombohodral 

 crystals, consist of a basic sulphate of alumina with sulphate of potash, referable to 

 the formula : KO. S0 + 3 (Al O 1 . S0') + 6 HO (KA1 S 8* O" + 3H 7 O). 



