EXTRACTION OF INDIGO FROM WOAD. 309 



animal portions of the plant, so that the indigo disengaged 

 from its several combinations can be oxidated and precipi- 

 tated more easily, and in a state of greater purity. 



The first result of this process appears to be a much 

 smaller quantity of indigo than is obtained by employing a 

 volume of lime-water equal to that of the liquor. But the 

 indigo obtained is purer, being equal in quality to the kind 

 which bears the highest price in commerce. This process 

 may be employed in all cases ; even when the infusion of 

 leaves is at 122° Fahr. The length of time during which 

 beating must be continued in those cases in which it can 

 alone be employed, is much diminished ; and yet the indigo 

 obtained is equally as pure. 



When all the indigo has been precipitated, the water is 

 drawn off. The precipitated fecula requires some further 

 operations to bring it to the requisite degree of perfection. 



The precipitated indigo still contains a greater or less 

 portion of particles which are not sufficiently oxidated, and 

 consequently it has neither the color nor properties which 

 characterize gpod indigo. Prolonged beating would, it is 

 true, bring these portions to the desired state ; but it would 

 likewise cause those particles which had been first oxida- 

 ted to imbibe an additional quantity of oxygen, by which 

 their color would be too much deepened, and indigo of 

 this quality would be rejected in commerce as burnt ; it 

 is therefore better to give to the imperfectly oxidated 

 particles the degree of oxidation required, in the following 

 manner. 



Stir the liquid fecula strongly, and throw over the whole 

 mass a volume of warm water, double that of the fecula ; by 

 this means the perfect indigo will be precipitated, and the 

 other will be held in suspension by the water. This water 

 is to be drawn off and treated with lime-water, by which 

 the green color becomes of a yellow brown, and the indigo 

 being rendered insoluble is precipitated. 



It sometimes happens, that the liquor which has been 

 treated with lime-water, and beaten, if the operations have 

 not been well conducted, still retains a portion of indigo 

 in solution ; this can be ascertained by adding lime-water 

 to a small portion of it, to see if it will become brown. 



That indigo may have the purity and brilliancy belonging 

 to it, it must be twice washed, once in cold, and once in 

 hot water. 



To perform the first washing, collect all the fecula in an 



