116 ALUM 



forred as a precipitant, -when it can be procured at a cheap rato. It has, also, the 

 advantage of decomposing the sulphate of iron present into a chloride, a salt very 

 difficult of crystallisation, and, therefore, lees apt to contaminate the crystals of alum. 

 Of late years chloride of potassium has been largely obtained from carnallite, and 

 sulphate of potash has been procured from kainite, two minerals found in great 

 abundance in the salt-mines of Stassfurt, in Prussia. The quantity of alkaline salts 

 requisite to precipitate the alum, in a granular powder, from the lixivium, depends 

 on their richness in potash or ammonia, on the one hand, and on the richness of the 

 liquors in sulphate of alumina on the other ; and this must be ascertained, for each 

 large quantity of product, by a preliminary experiment in a precipitation glass. Here, 

 an aliquot measure of the aluminous liquor being taken, the liquid precipitant must 

 be added in successive portions, as long as it causes any cloud, when the quantity 

 added will be indicated by the graduation of the vessel. A very exact approximation 

 is not practicable upon the great scale ; but, as the mother-waters are afterwards 

 mixed together in one cistern, any excess of the precipitant at one time is corrected 

 by excess of aluminous sulphate at another, and the resulting alum-meal is collected 

 at the bottom. When the precipitated saline powder is thoroughly settled and 

 cooled, the supernatant mother-water must be drawn off by a pump, or rather a 

 syphon or stopcock, into a lower cistern. The more completely this drainage is 

 effected, the more easily and completely will the alum be purified. 

 100 parts of alum are formed from the sulphate of alumina liquor, 



by 18-32 of sulphate of potash, 

 13-86 of sulphate of ammonia, 

 or 15'69 of chloride of potassium. 



Sulphate of ammonia is soluble in 1 of hot and 2 of cold water ; sulphate of potash in 

 nearly 10, and chloride of potassium in 3, of water of ordinary temperature ; alum, 

 in 13 parts of water. A portion of the alum formed will remain in solution ; this 

 will depend on the quantity of liquid ; the rest falls as a powder. 



This mother-liquor has generally a specific gravity of 1*4 at a medium tempera- 

 ture of the atmosphere, and consists of a saturated solution of sulphate or muriate of 

 black and red oxide of iron with sulphate of magnesia, in certain localities, and 

 chloride of sodium, when kelp salts have been used as a precipitant, as also a saturated 

 solution of sulphate of alumina. By adding some of it, from time to time, to the 

 fresh lixivia, a portion of that sulphate is converted into alum ; but, eventually, the 

 mother-water must be evaporated, so as to obtain from it a crop of ferruginous 

 crystals ; after which it becomes capable, once more, of giving up its alum to the 

 alkaline precipitants. 



When the aluminous lixivia contain a great deal of sulphate of iron, it may be good 

 policy to withdraw a portion of it by crystallisation before precipitating the alum. 

 With this view the liquors must be evaporated to the density of 1 !, and then run 

 off into crystallising stone cisterns. After the green vitriol has crystallised, the liquor 

 should be pumped back into the evaporating pan, and again brought to the density of 

 1-4. On adding to it, now, the alkalino-salino precipitants, the alum will fall down 

 from this concentrated solution, in a very minute crystalline powder, easy to wash 

 and purify. But this method requires more vessels and manipulation than- the 

 preceding, and should only be had recourse to from necessity ; since it compels us to 

 carry on the manufacture of both the valuable alum and the lower-priced salts at the 

 same time ; moreover, the copperas extracted at first from the schist liquors carries 

 with it, as we have said, a portion of the sulphate of alumina, and acquires thereby a 

 dull aspect ; whereas the copperas obtained after the separation of the alum is of a 

 brilliant appearance. 



5. The Washing, or Edidcoration, of the Alum Powder. This crystalline pulveru- 

 lent matter has a brownish colour, from the admixture of the ferruginous liquors ; but 

 it may be freed from it by washing with very cold water, which dissolves not more 

 than one-eighteenth of its weight of alum. After stirring the powder and the 

 water well together, the former must be allowed to settle, and then the washing 

 must be drawn off. A second washing will render the alum nearly pure. The 

 less water is employed and the more effectually it is drained off, the more complete 

 is the process. The second water may be used in the first washing of another 

 portion of alum powder, in the place of pure water. These washings may be 

 added to the schist lixivia. This powder is now extensively Bold without further 

 manipulation. 



6. The Crystallisation. The washed alum is put into a lead pan, with just enough 

 water to dissolve it at a boiling heat ; fire is applied, and the solution is promoted by 

 stirring. Whenever it is dissolved in a saturated state, it is run off into the 

 crystallising vessels, which are called roching casks, These casks are about five 



