DYEING. 181 



When alum is used as a mordant, it is dissolved 

 in water, and sometimes a quantity of tartar is 

 added. The cloth is put into this solution, and 

 kept till it has absorbed as much alumine as is ne- 

 cessary. It is then taken out, and is washed and 

 dried. A quantity of alumine has by this process 

 combined with the fibres of the cloth, which is 

 perceived by the latter weighing more than before. 

 The addition of the tartar, or tartrate of potash, is 

 made on two accounts ; the potash which it contains 

 combines with the sulphuric acid of the alum, and 

 thus prevents that very corrosive substance from 

 injuring the texture of the cloth : the tartareous 

 acid, on the other hand, combines with part of the 

 alumine, and forms a tartrate of alumine, which is 

 more easily decomposed by the cloth than alum. 



Acetate of alumine is used as a mordant for 

 cotton and linen, which have a much less affinity 

 than wool for alumine. The alumine is retained less 

 powerfully in a state" of combination by the acetic 

 than by the sulphuric acid ; and, therefore, cotton 

 and linen are better able to separate it and attach 

 it : also the acetic acid being volatile, gradually 

 leaves the earthy basis, and allows the alumine to 

 unite to the stuff. 



This mordant is now prepared by pouring acetate 

 of lead into a solution of alum ; on which a double 

 decomposition takes place ; the sulphuric acid com- 

 bines with the lead, and the sulphate of lead pre- 

 cipitates in the form of an insoluble powder, while 

 the alumine combines with acetous acid, and 

 remains in the liquor. 



This mordant gives a richer colour than alum. 



Lime is also sometimes employed as a mordant : 

 but it does not answer so well in genera^ not giving 

 so good a colour. It is used either in the state 



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