INDIGO-VATS.

479

is obtained from the pastel or woad, which is sometimes added in large quantityto the liquid in the vats ; this substance then also yields the indigotin itcontains.

In those vats wherein pastel is not employed the nitrogenous matter isderived from the bran : sometimes putrefying human urine is employed, for thesake partly of the alkali it contains (viz., carbonate of ammonia), and also forthe organic nitrogenous matter, which ads as a ferment, while the formerplays the part of a solvent for the white indigo.

Since the phenomena attending fermentation are of a complex nature, evenin the most simple instance, it cannot be wondered at that, in such aninstance as is here given, there is not—as regards the success of the processon an industrial scale—that certainty which attends the working of the limecopperas-vats, and only a long pradice and a lucky empiricism make sure ofsuccess in a method which, scientifically considered, is not by any means fullyelucidated. It is on this account that the details which follow on this subjedare less extensive as to theory.

Pastel-vats, rarely employed in this country, vary in size from 2 to 2‘5 metresin diameter by 3 metres depth. The ingredients employed are 100 kilos, ofpastel, in the shape of balls ; the vat or tank is next filled with tepid water;after which there are added 10 kilos, of madder,* 3 to 4 kilos, of bran, and4 kilos, of slaked lime, well mixed with water to a thick creamy liquid; some-times also weld is added: this mixture is left standing for eight hours, afterwhich time the contents are thoroughly raked up, an operation which isrepeated every three hours : gradually a peculiar ammoniacal smell is givenoff, and the surface of the liquid in the vat becomes covered with a bluishscum, under which deeper blue-coloured veins are seen. Since these symptomsindicate that the indigotin of the pastel has become dissolved, there is addedto the vat a quantity of 10 kilos, of finely-ground indigo, previously mixed toa paste with water. If the fermentation becomes too adtive, as evidenced bythe size and quantity of the gas given off, it is moderated by the addition of asmall quantity of lime, while, if it is too feeble, it is quickened by the additionof bran. A pleasant odour, neither too strong nor too weak, and abundanceof blue scum, are the signs of a proper condition of the contents of the vats.The fabrics which are first immersed in a freshly-made up vat are less beauti-fully dyed than those done afterwards, because the wool withdraws from thebath some drab, brown, or yellow matters, which are the result of the rea&ionof the various substances contained in the vat on each other. The woollengoods, either pieces or yarns, are kept immersed in the vat for two hours, afterwhich time they are aired, and next washed : 100 kilos, of wool consume from8 to 12 kilos, of indigo ; it is therefore evident that this loss or consumptionof indigo has to be restored after each day’s work, and this is done by Addingevery night a sufficient quantity of indigo along with some lime. After havingbeen thus kept up for some months, the vat, becoming gradually foul, isexhausted,—that is to say, the dyeing process is continued until no morecolour is imparted to the wool; after which the contents of the vat are run off,the tank cleaned, and a fresh vat made up.

Another kind of vat is made up in the following manner:—8000 litres of

* Madder in this case is only employed as a substance rich in sugar, and also because it containssome nitrogenous matter.