_Fermentation Method._—Until recently the methods used for reducing the indigo—i.e., changing the solid blue into the soluble white—were just about the same as those used by the ancients, and were based upon some kind of fermentation, usually alcoholic. It was found out at a very early date that if indigo, ground up with water to a paste and rendered alkaline by the addition of wood ashes, lime, or other simple alkali, were mixed with grape juice or any other sugary liquid, and then kept warm and allowed to ferment, the resulting fluid would contain the dyestuff dissolved in a form suitable for dyeing. The vessel in which this process was conducted was known as a vat, and the process of indigo dyeing is still called “Vat Dyeing.” _Disadvantages._—At the very best this method is slow, uncertain, and difficult to manage, especially on a small scale. In wool dyeing, to this day, a few vats are still to be found where syrup, ground madder root or, in some instances, woad, wheat bran, and other materials which ferment readily in the presence of alkali, are stirred up with warm water and soda, and then allowed to stand. In two or three days they are in active fermentation, and the indigo, in the form of paste, is added and well stirred in. After much further delay, if all goes well, the indigo is finally “reduced,” and, if the amount of alkali, the temperature, the concentration of the vat, and various other factors are carefully attended to, the bath can be used for several days, or even weeks, without being made over again; fresh indigo and other ingredients being added, from time to time, as needed. Cotton, linen, wool, and even silk can be dipped in this bath, which should be light greenish yellow in color, with a blue or bluish-green scum or coating, where the indigo is oxidized on the surface. Goods immersed in this bath turn yellow, and then, when taken out, wrung free from loose liquor, and exposed to the air, the yellow color quickly changes to a permanent blue. A serious drawback to all these various fermentation vats is that a good deal of the dyestuff is always spoilt—i.e., decomposed into colorless compounds which can never be regenerated or made useful. Indeed, the loss from this cause frequently amounts to 20% or 25% of all the dye used, and occasionally, especially in hot weather, and on a small scale, to far more. But, apart from the actual loss in valuable dyestuff, there is a much more serious drawback to this method of indigo dyeing, namely, the waste of time and energy involved. There is always a considerable delay in getting a fermentation vat fairly started, even where all the conditions are favorable; and when it is running smoothly, the reducing process is a very slow one. Furthermore, the indigo, not being dissolved in the liquid but only suspended in it, has a constant tendency to sink to the bottom in the form of a blue mud, and thus escape the chemical action of the fermentation gases entirely.