478

DYEING AND CALICO PRINTING.

calicos. Since linen and hempen tissues possess, generally speaking, lessaffinity for dyes, some precautions are also necessary when they are to bedyed with indigo by the self blue or vat process. These goods are, therefore, pre-vious to vatting, either first passed through a thin starch pastel, to which somesalt has been added, or through weak lime-water, but rarely or never are thesegoods printed, and the process is, therefore, simply one of uniform blue dyeing.

As regards the application of indigo to woollen fabrics, these are alwaysvatted, and, in fadt, a very large proportion of woollen cloth is grounded bymeans of the indigo vat; more frequently woollen and worsted yarns thanwoollen woven fabrics are vatted, but if the latter are thus treated it is onlya plain blue dye they receive, and no resists are applied.

According to general ideas -prevailing among woollen dyers in this country,the lime and copperas vat is not well adapted for woollen goods, and theusual blue vat is made up in a different manner, viz.—Waterloo gallons;indigo, 20 lbs.; potash or soda, 30 lbs.; bran, g lbs.; madder, 9 lbs. Thewater is heated to near its boiling-point; the potassa, bran, and madder arefirst introduced, and then the indigo previously very finely ground ; coldwater is added to bring the temperature down to 90° F., and the vat is kept atthat heat all through; the ingredients are very well stirred every twelvehours, and the vat should be ready for use in forty-eight hours after settling.This vat does not work longer than a month, and is somewhat expensive onaccount of the loss of potash. Another vat, much more manageable andlasting, since it may be worked for two years without emptying, beingfreshened up as required, and known in France as the German vat, is madeup of 2000 gallons of water, heated to 130° F.; 20 lbs. of crystals of soda,pecks of bran, and 12 lbs. of ground indigo are added, and then the mixturewell raked up ; in twelve hours a fermentation sets in, bubbles of gas arise,the liquid has a sweetish smell, and has become greenish ; 2 lbs, of slakedlime are now added, the liquid well stirred, the vat heated again, commonlyby the aid of steam, and covered up for twelve hours, when a similar quantityof bran, indigo, soda, and again a little lime are added ; in about forty-eighthours after the setting the vat may be worked, but because the reductivepower of bran is rather feeble, an addition of 6 lbs. of molasses is made. Ifthe fermentation becomes too a&ive, it is repressed by the addition of lime;if too sluggish, it is stimulated by the addition of bran and molasses, and,like all other blue vats for wool, is worked hot. In France, however, thecopperas and lime vat is used for wool also, though not exclusively.

Silk is rarely, if ever, dyed blue by the aid of the lime and copperas vat; infa&, generally speaking, the textile fibres of animal origin are dyed by meansof so-called pastel, woad, urine, and Indian vats. The reduction of theindigotin is, in all these instances, the result of a peculiar fermentation calledinto existence in an alkaline liquor by the aid of substances containing muchnitrogen, and also by the aid of suitable carbohydrates (sugar, &c.). We knowthat under these conditions sugar is rapidly converted, provided the tem-perature be sufficiently elevated, into butyric acid, while carbonic acid andhydrogen are simultaneously set free : the latter substance combines in thenascent state with the indigotin, and the white indigo is rendered soluble inthe alkali present in the liquid: the fermentescible carbohydrates arederived from bran, madder, and sometimes molasses ; the nitrogenous matter