280 The Art of Dijeing. 



If, on the other hand, blue spots appear, the alum contains 

 iron. This test is quite certain when the alum is introduced 

 in pieces into the solution. Powdered alum and alum solu- 

 tion cannot be tested in this way with certainty. 



It is often difficult to obtain alum free from iron. The 

 dyer may employ the following method advantageously. 

 Let 100 lbs. of alum and 150 lbs. water be boiled in a tinned 

 vessel, and be poured off into a wooden vessel which con- 

 tains 150 lbs. of cold water, and be agitated until the whole 

 is cooled. Alum free from iron precipitates like flour, 

 while a portion remains in solution with the iron. If the 

 alum-flour is now dissolved in water and the solution allowed 

 to cool, pure alum is obtained in crystals. This will not be 

 rendered blue by the addition of Prussiate of potash. 



The mother liquors which remain in both cases are em- 

 ployed for the exhibition of mixed mordants to dye brown, 

 olive or gray. 



The alum dissolves copper, and gives dull colours. 

 Copper vessels must not, therefore, be employed to dissolve 

 it. Alum also takes up tin. This is, however, rather use- 

 ful than hurtful to the dye. 



If we boil copper in a solution of alum, wherein tin is 

 soldered, no copper is dissolved ; but tin is taken up. We 

 can therefore protect colours containing alum when boiled 

 in copper, thoroughly, from the injurious action of the cop- 

 per, by soldering a piece of tin in the inside of the vessel. 



The solubility of alum in water is not great. It requires 

 13 lbs. of cold water, at 54°J, to dissolve 1 lb. of alum. 

 This is the case especially in winter when the water is 

 colder, and dissolves still less. There is, however, a method 

 of remedying this disadvantage to the dyer. This is done 

 by taking away from the alum a portion of its acid. Dis- 

 solve 32 lbs. alum in 80 lbs. boiling water, and add gradually 

 4 lbs. chalk-powder suspended in 80 lbs. of hot water, and 

 heat them until effervescence ceases. A small quantity of 

 alum only separates on cooling, while, without the addition 

 of chalk, 16 or 20 lbs. of alum are separated on cooling 

 from the hot solution. If 11 lbs. of crystallized carbonate 

 of soda are employed instead of the chalk, scarcely any alum 

 separates during the first 24 hours. The acid which the 

 alum contains is the sulphuric. Hence, the alum may 



