162 . Dr. Ure on Indigo, 



dry by three o'clock p.m., to permit the leaves to be separated 

 from the stem by threshing. The leaves are now thoroughly 

 jdried by exposure to the sun, then coarsely bruised, or rather 

 ground to powder in a mill, and packed up for the use of the 

 manufacturer of indigo. 



From these powdered leaves, the dye stuff is extracted 

 either by simply digesting them in water, heated to 150° or 

 180° F., in as small a proportion as may be practicable, and 

 subsequently beating the infusion with paddles till the blue 

 indigo granulates, as Roxburgh recommended ; or by mashing 

 'the ground leaves with twice their bulk of water, at the atmos- 

 pheric temperature, drawing off the liquor into a vat, where it 

 speedily undergoes fermentation, and is beat as above with 

 paddles or oars, till the blue indigo forms. Some persons 

 prescribe the addition of lime-water at this stage of the pro* 

 cess ; others reject its use. 



In operating on the recent plant, it is laid in bundles in the 

 steeping trough {trempoir)^ which contains sufficient water to 

 stand about two inches above plants slightly pressed down by 

 crossing bars of wood. A brisk fermentation soon begins, with 

 copious extrication of air-bubbles. This process is suffered to 

 proceed till the liquor has become green, and casts up a pellicle 

 of a copper-red hue. A sour smell is now perceived, and the 

 blue colouring particles seem ready to separate. This happens 

 commonly at the end of from ten to twenty hours, according 

 to the temperature of the weather. The hquor is then run off 

 into the beating vat, and lime-water is added, or not, according 

 to the fancy of the operator. In all cases of fermentation, 

 whether the dried leaves or the recent plant be employed, it is 

 proper to watch the progress of that change with solicitude ; 

 because, when too violent, it not only decomposes entirely 

 some of the indigo blue, but introduces much foreign vegetable 

 matter into the precipitate ; when too feeble, it is said to leave 

 45ome indigo unextracted. 



From the differences which exist in the nature and culture 

 of the indigoferaf and of its treatment by the manufacturer, the 

 product, indigo, as found in commerce, differs remarkably in 

 quality and chemical composition. In this respect, it forms a 

 complete contrast to the simple crystaUine product sugar. 



