37. _Purification of Madder, by the Separation of its Yellow Colouring Matter_.—The experiments of MM. Kuhlman, Colin, and Robiquet[36], have induced M. G. H. de Kurrer to publish the means which he has resorted to for the purification of madder, by the separation of the yellow colouring matter from it; and thus rendering it more fit to supply the various red, lilac, violet, and brown colours which are required upon wool, silk, cotton and linen. Three tubs or vessels are placed by the side of each other: in summer they may be in the open air under shelter, but in the winter should be placed in an airy cellar where the temperature may be retained at 18° or 20° R. (73° to 77° F.). The first is that in which the soaking and fermentation is to be effected: it should be 2 feet 8 inches deep, and 2 feet 6 inches in diameter, for from fifty to fifty-five pounds of madder. The second, or washing vessel, should be 5-1/2 feet deep, and 3 feet in diameter; it should have three wooden cocks fixed into it, the first 2 feet, the second 3 feet, and the third 4 feet from the bottom. The third tub is for deposition; its height should be 4-1/2 feet, and it should have a cock at 1-1/2 foot from the bottom. [p220]
On commencing the operation, 50 or 55 lb. of pulverised madder are to be put into the first vessel, water is to be added, and stirred into the mass until it stands 1-1/2 inch above the madder. The whole is then to be left until fermentation comes on and has formed a coat of madder at the surface; this usually takes place in 36 hours, and at latest in 48 hours, according to the temperature. The mass should now be transferred into the second vessel, which is then to be filled with water, and being left for two hours, the madder will fall to the bottom. The upper cock is then to be opened, after that the second, and then the third; and the water which runs from the two latter is to be put into the third vessel, that the rest of the madder may separate from it. The madder in the second vessel is then to be washed a second, third, or fourth time until the washing water is colourless. Thus purified, the madder may be used in the processes of dyeing, according to the known methods; but it is important in summer that it should be used immediately, that a new (the vinous) fermentation may be avoided. The madder deposited in the third vessel, when washed and deposited, may be used like the rest. The liquid first separated after the fermentation may be used in the preparation of hot indigo baths, &c. instead of madder.—_Bull. Univ._ P. vii. 352.