INDIGO-VATS.

475

washed in warm water, wherein chalk is kept suspended, thereby to saturatethe excess of acid.

Instead of employing as discharges the salts of chromic acid just named,chromate of lead is sometimes applied, being printed on with a suitablethickener, and, after having become dry, the goods are passed through arather strong solution of moderately warm hydrochloric acid. In this case itis not so much chromic acid as chlorine set free, which adts as a dischargeand decolourises the indigo. Instead of chromate of lead hydrated peroxideof manganese is sometimes applied.

M. Mercer discovered that a mixture of red prussiate of potassa (ferricyanideof potassium) and caustic potassa or soda would discharge indigo-blue in avery perfedt manner; but on account of difficulties in the application and of thecostliness of this process it has not been much employed. If the materials bemixed and printed on a blue the colour is at once discharged, and only requireswashing off; but the mixture has a very energetic adlion upon all thickeningmatters, and is very irregular in its results; on the other hand, it does notimpair the strength of the fibre. The discharges upon indigo-blue can bemade to adt as mordants, or colours to produce a coloured design instead of awhite one. For example, if acetate of alumina be mixed with the discharge,and the alumina fixed in the washing off, it may be dyed up in madder orgarancin to produce a coloured discharge. Sometimes the discharge andresist are combined together; for instance, on light blue ground are printedsimultaneously in the first place an ordinary resist, and, secondly, the sameresist, to which has been added bichromate of potassa and hydrochloric acid;when the fabric so prepared is again vatted the result will be a deep blueground, whereon is a pattern of light blue and white.

In order to give an idea of the composition of the materials used for dis-charging indigo-blue we quote the following:—Chrome liquor consists of—Water, 2 litres, wherein are dissolved 500 grms. of yellow chromate. The acidcomposition contains—Tartaric acid, 3 kilos.; oxalic acid, 250 grms.; calcinedstarch, 4 kilos.; nitric acid, 500 grms.; water, 4 litres. This last composition isprinted on by rollers ; the wet pieces, rolled up between a loosely-woven cloth,are immersed momentarily in water, and as soon as white becomes apparentare placed at once in tepid water containing chalk.

Before we proceed to consider the combinations of other colours which maybe formed by the aid of the vat-dyed materials, we give here a short outlineof what has been said on dyeing with indigo :—The usual method of dyeingcotton self blue is to fill with water large stone or wooden tanks, and dissolvingtherein 2 parts of sulphate of protoxide of iron, adding 1 part of finely-groundindigo, and 3 parts of hydrate of lime. After having stirred the whole wellfor several hours, pieces of calico which have been hooked on a frame anddipped in lime-water,* in order to moisten them equally all over, are plungedfor some fifteen minutes into the vat, when the blue indigo which has beenconverted into white indigo by the protoxide of iron, and rendered soluble bythe excess of lime, fixes itself on the fibre, and on the latter being afterwardsexposed to the atmosphere re-absorbs oxygen, and becomes blue. Whenwhite patterns are required the pieces are printed before dyeing with what is

* This is often used in preference to ordinary water, in order to make sure that there beexcess of lime.