10 Dr. Thomas Thomson on [Jan. 



aluminous and iron mordants, and then dyeing with 

 madder. 



Indigo, oxide of manganese, catechu, &c. are colours per 

 se, and therefore, require no mordant. 



II. DISCHARGERS OF COLOURS. 



Most colours are fixed to the cloth by mordants ; or if 

 they be metallic oxides, they retain their affinity only at a 

 particular state of oxidizement. # Thus madder is fixed 

 by alumina, and cochineal by means of oxide of tin. Man- 

 ganese adheres to the cloth only when in the state of 

 sesquioxide, and is washed away by water the moment it is 

 converted into protoxide. Hence, when the printers wish 

 to discharge a colour from cloth, they employ something 

 that will dissolve the mordant, or which will deoxidize the 

 oxide, or colouring matter, if no mordant be present. The 

 dischargers are either acids, or substances having a strong 

 affinity for oxygen ; the former being employed to dissolve 

 the mordants, and the latter to deoxidize the oxides. The 

 chief of these are the following : — 



1 . Citric acid is much used to dissolve alumina, and per- 

 oxide of iron, and thus to prevent the formation of colour 

 on particular parts of the cloth, by removal of the mordant, 

 which would otherwise produce them. It is obtained by 

 evaporating lemon juice, and thickening it with gum-sene- 

 gal for the cylinder, or with gum and pipe-clay for the 

 block. Its action is occasionally assisted by bisulphate of 

 potash, or sulphuric acid. 



• Sometimes the citric acid is first printed on white cloth, 

 and afterwards the aluminous or iron mordant is applied 

 slightly thickened. It is dried immediately to prevent the 

 swelling of the acid figures. At other times, the mordants 

 are first applied, and the acids printed over them. 



In both cases, the goods are afterwards passed through 

 hot water, containing cow dung, and well washed before 



* Almost every thing which can he applied to cloth, in a state of solution, and 

 which becomes afterwards insoluble in water, either by precipitation, or spon- 

 taneous decomposition, sticks to the cloth when it is washed. Water, therefore, 

 does not remove protoxide of Manganese, and the protochloride of tin alluded to 

 at the conclusion of this section, as a means of removing the sesquioxide or 

 peroxide of Manganese, not only takes away their oxygen, but converts them 

 into a soluble chloride. 



