The distinction between pastel and woad is not very clear. Schutzenberger says: “Pastel, woad, and satis tinctoria is a plant of the family of the crucifera. It would seem, however, that the term pastel as used by the old French dyers is applied to the leaves of the woad which have been fermented, formed into paste, and afterwards into balls, and which contain much blue coloring matter. And the term woad as distinguished from pastel is applied to the unfermented plant.”
SICKNESS OF THE WARM VAT FOR DYEING WOOL, AND ITS REMEDIES.
We have to thank that excellent practical magazine, “The American Chemist,” for the following notes on the sicknesses of the warm vat, by F. W. Kugler, translated from Reimann’s Färberzeintung:—
In the wool indigo vat, among the principal “sicknesses” is the blackening of the vat, or “sharpening.” This arises from the presence of too much lime. When “sharpened,” the liquor, instead of having a waxy yellow color with a dense blue film on its surface, has no film; while the liquor is a dark blackish-green, and on being stirred shows a gray or white scum on its surface, while it emits at the same time a pungent odor. If the vat is only slightly affected, it is sufficient to add some bran and madder and to let it stand over night. If it has not quite recovered by morning, it may be necessary to heat it up, agitate it, and let it stand for a couple of hours, after which perhaps the addition of a little lime will be necessary.
If the vat is much sharpened, it is recommended to sink in it a bag of bran, and leave it over night, when the fermentation will have restored the vat in a considerable degree; but it will be necessary to add lime cautiously and by degrees, to bring it to a proper state for working.
The theory of the souring of the vat is given. Butyric fermentation takes place under certain circumstances, butyric acid being formed; and hydrogen is set free, which reduces the indigo. The addition of lime makes the vat too strongly alkaline, and sets ammonia free, which gives the pungent odor of the soured (verschäften) vat. Simultaneously the lime with the white indigo forms a difficultly soluble compound, which settles, and thus interferes with the working of the vat. The excess of lime must be removed, which is accomplished by introducing bran, which causes a lactic fermentation; and the lactic acid neutralizes the excess of lime, and destroys the lime compound with indigo which had been formed. The lime may be neutralized by the use of mineral acids, but there is danger in that case of precipitating the indigo.
A second “sickness” is “becoming too sweet.” The symptoms are,—the blue veins and surface film disappear on stirring, the foam gives a rustling sound, the bath assumes a reddish-yellow color, blue goods placed in the bath lose their color, and the vat has an unpleasant odor.
The vat when “too sweet” needs to be brought to the regular temperature, and lime to be added cautiously until the vat is brought to its normal state. It is safer to add an excess of lime and “sour” the vat, and then bring it back according to the directions under that head, than to add too little, as less indigo is lost. To use up all the dye and to dye a light blue, as little lime should be present as is consistent with the workings of the vat.
The cause of the “falling away” of the vat is a too active fermentation, which produces considerable lactic acid, from which butyric acid forms, setting free hydrogen, thereby making white indigo, which, if the action is allowed to continue, changes to a compound from which the indigo cannot be recovered. If lime is added, the lactic and butyric acids unite with it and precipitate it, while the excess precipitates the white indigo, which is slowly recovered, as fermentation progresses, which forms lactic acid, which, taking the place of the white indigo, sets it free. Besides the sicknesses, there are various results of mismanagement, of which the first is overwarming, which causes the bath to turn brown, which is the beginning of the souring.
When the bath begins to sour from overheating, some logwood should be added and then bran, and the vat left to itself over night. The reason of it is that the temperature is too high for the desired fermentation to operate. The vat sometimes suddenly turns green, and even when indigo and the other necessary ingredients are added it remains of this color. This is called the “breaking up of the vat.” The reason is that the temperature is too low; to remedy it, it is necessary to add logwood and bran, warm it up, and stir, when it should stand for some hours.