192 DYEING. 



hour in a solution of five parts of alum, and three 

 of tartar, for every thirty-two parts of cloth. The 

 cloth is then to be put into a bath of sulphate of 

 indigo, diluted according to the strength of shade 

 required, and kept till it has acquired the desired 

 colour. The use of the alum and tartar is not to 

 act as mordants, but to facilitate the decomposition 

 of the indigo. The alcali is added to the sulphate 

 for the same reason. Another use of these* sub- 

 stances is, that they protect the cloth from the action 

 of the sulphuric acid, by neutralizing part of it, 

 otherwise the texture of the cloth might be 

 injured. 



This, however, is not the most common method of 

 dyeing by indigo. The usual method is to deprive 

 the indigo of the oxygen which has been combined 

 with the green fecula, and to which it owes its blue 

 colour, and thus reduce to the green state again. 

 It is then capable of being dissolved in water by 

 means of the alcalies or alcaline earths, which act 

 upon it very readily in that state, 



To dye wool blue, indigo is mixed with wood, 

 bran, and madder, vegetable substances which 

 readily undergo fermentation ; and the whole is 

 boiled together, stirring the mixture frequently. 

 By this a fermentation takes place, and the oxy- 

 gen is separated from the indigo. Quick lime or 

 alcali is then thrown in, which dissolves the green 

 base of the indigo. The Solution of indigo is apt 

 to run into the putrid fermentation, which is known 

 by the putrid vapours which it exhales ; the green 

 colour then disappears, and, indeed, the colouring 

 matter is decomposed. This danger is prevented 

 by adding more lime to correct the putrescent 

 tendency. Sometimes the fermentation does not 

 proceed with sufficient activity, and then more bran 



