REDUCTION OF INDIGO.

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indigo is prevented. This paste, properly thickened, is printed on thefabric, which is next passed through an alkali-bath, lime water or sodasolution, which has the effedt of replacing the oxide of tin and forminga soluble combination of white indigo, which latter penetrates the fibreand becomes fixed by a subsequent oxidation process ; so-called preci-pitate or fast blue.

4. Indigo, previously pulverised, is printed on the tissue, and this latter isnext placed in such condition that the indigo may become dissolvedand reduced locally, after which it is fixed by a subsequent oxidation ;so-called China blue.

It is quite clear that for each of these methods any of the known processesof reduction may be used, care being, of course, taken to apply that which ispradtically the most advantageous.

The greatest consumption of indigo is for forming the blue vats in whichwoollen or cotton goods are dyed, by simply immersing them in the solutionof white indigo. The same vat is not equally adapted for wool and calico,and, as will be seen in the following details, there is a wide difference in theircomposition.

Reduction of Indigo; Theory of the Indigo-Vats.

The most convenient method of reducing indigo for dyeing calico and cottonyarn is based upon the adtion of hydrated protoxide of iron in the presence oflime, since it possesses, when recently precipitated, the property of reducingindigo, provided lime be simultaneously present. Any other caustic alkaliwould answer the purpose, but lime is preferable on account of its cheapness,and because it gives to the vats greater stability, the oxidation of the indigobeing greatly lessened, owing to the formation of a thin film of carbonate oflime on the surface of the liquid, and, secondly, because with lime thechemical activity is less intense. The very rapid alteration of vats made upwith caustic soda prohibits its use in dyeing yam and tissues in the indigo-vat(vat-blue), and also prohibits the use of reducing agents, which are moreadvantageous than hydrated protoxide of iron, but do not adt in the presenceof lime. Lastly, but not least, we ought to observe that experience hastaught that the lime compound of white indigo yields its colouring mattermore readily and completely to the fibre than the corresponding soda com-pound, and, as a consequence, the vat is more completely exhausted, evenwhen quite cold. The hydrated protoxide of iron is formed in the vatitself, by the adtion of the slaked lime upon the green copperas employed.This latter salt should not contain any sulphate of copper, because the oxideof that metal adts as a most energetic oxidising agent on the reduced indigo,which, therefore, would be precipitated in the vat itself. Basic sulphate ofperoxide of iron and sulphate of alumina also ought not to be present in thecopperas, because these substances would seize upon a portion of the limeand adt as inert matter.

Copperas in solution is readily purified by boiling it with dean metallic iron,which precipitates any copper present in the state of metal, while any basicsalt of peroxide of iron is reduced to a protoxide salt.

The ingredients of the copperas-vat are—Water; pure or previously purifiedgreen vitriol; indigo reduced to a homogeneous and impalpable creamy paste ;