468

DYEING AND CALICO PRINTING .

being greater than that of the cast-iron cylinders, the result, on motion beingimparted to the hollow cylinder, is that the inner cast-iron cylinders turn in adirection opposite to that of the large hollow cylinder, and the consequence isan enormous fridtion, which reduces the softened indigo to a pulp. Thisapparatus works very well if indigo of superior—or at least good ordinary—quality is to be operated upon, since these are readily and uniformly penetratedby water; but low qualities of indigo are often with difficulty, or even not atall, penetrated by water, and if brought into this apparatus the result is toconvert the ware into a plastic, tough, semi-dry, clay-like mass, which sooncauses the stoppage of the proper working of the apparatus. In order toguard against this contingency the previously softened ware is submitted to a ,preliminary grinding process, by means of a mechanical contrivance consistingof two circular cast-iron discs, placed horizontally and near to each other, butwith a perceptible open space left between : by means of suitable machinery arapid movement is imparted, one of the discs moving from right to left, theother from left to right; on the internal surfaces of the discs grooves are cut,somewhat like those on the stones used for grinding corn. The indigo is ad-mitted to the spaces between the two discs, and is squeezed out at thecircumference as a kind of thickish paste. This machinery may be appliedfor the pulverisation of other solid substances, and ads very satisfadorily.

Indigo can only be used for colouring textile fibres after having been pre-viously rendered soluble, and this can, as yet, be effected only by the twomethods already described,—the redudion process, and the adion of concen-trated sulphuric acid. In the first method the process of dyeing is based uponthe regeneration of the indigotin, which then adheres to, and probably uniteswith, the fibre: by the second method the colouring matter has itself enteredinto a combination from which it cannot be again separated, so that the dyeingprocess is really carried on with a substance altogether distind from theoriginal indigo, and possessing other properties.

Fixing Indigotin by the Reduction Method.

The hydrogenisation of indigotin and the combination of white indigo withan alkali, or an alkaline earth, render it soluble, so that it can be made topenetrate textile fibres, and when the white indigo is, by suitable oxidisingagents, re-converted into blue indigotin, that substance, becoming insoluble,is fixed in the pores of the textile fibres in such a manner as to adhere firmlyand be capable of withstanding washing and soap. This readion is employedfor the purposes of dyeing in the following manner :—

1. The indigotin is reduced and dissolved by an alkali, and the fabric im-

mersed in the solution ; ordinary indigo-vat.

2. The solution is printed or painted on some portions only of the fabric,

by the aid of a pencil—so-called pencil-blue. Pencil-blue is supposedto be so called because originally used for the “enlummage” of madderchintz work, being pencilled into white spaces left for that purpose,such as leaves, &c. The pencil was simply a small, flat, blunt-pointedpiece of wood.

3. The white indigo is precipitated in the form of a paste, in combination or

mixture with a very strongly reducing metallic oxide (commonly hy-drated protoxide of tin), whereby the too rapid oxidation of the white