Test of Dyeing . 383 



as the calico itself; since it is impossible to purify the 

 pieces on a large scale, in such a complete manner as to 

 prevent any more of the mordant from passing away. 



This is managed during the preparations for dyeing. In 

 madder-dyeing, a cheap or inferior kind of madder is em- 

 ployed, or galls or tan may be used, which renders the 

 mordant more permanent on the calico, by forming gallate 

 and tannate of alumina, which take up the uncombined 

 mordant. 



The operation of passing the cloth through a cow-dung 

 bath is a similar preparation for dyeing. 



In most cases, it is most economical to employ for pre- 

 paration the same colour which is to produce the dye, or a 

 substance which does not dye, as the excrescence on the 

 birch. 



This principle, that the extraction of colouring matter 

 constitutes a discoloration of the fibres of plants which hold 

 their colouring matter in chemical combination, explains 

 the reason why dyeing cannot be well effected with the 

 fresh or green parts of plants. Here the fibres and colour- 

 ing matter of plants, are less intimately combined ; they 

 are still in their natural living state, but the latter will be 

 taken up completely at a boiling temperature. This kills 

 the plants, and puts them in the condition which would be 

 produced in another way by drying. 



In general, the dyeing commences when the solution is 

 lukewarm, and terminates when it is boiling. With colour- 

 ing matters which have a great tendency to unite with the 

 mordanted calico, as logwood, it is proper to place the 

 cloth in the cold solution at 50° or 54°| ; otherwise the 

 cloth will be spotted or unequally dyed. 



In general, slow heating of the solution is an important 

 object for obtaining an equal colour ; therefore, steam- 

 heat is to be preferred, as, by opening or closing the steam- 

 pipe, it is easy to increase or diminish the heat. 



Test of dyeing on a small scale, and mode of estimating the 



necessary quantity of colouring matter to be used in dyeing. 



It is impossible for any one to learn in a dye-work tho- 

 roughly the art of dyeing ; this must be attained by trials 

 upon a small scale, which will give a regular measure for 



