When indigo, suspended in water, is brought into contact with certain deox idizing agents, it is deprived of a part of its oxygen, becomes green, and is ren dered soluble in water, and still more so in the alkalies. It recovers its former color, however, on exposure to the air, by again absorbing oxygen of 1-7th or 1-8th of the whole weight of the resulting in digo. Its deoxidizement is effected either by allowing it to ferment along with bran, or other vegetable matter, or by decom posing in contact with it the protosul phate of iron, by the addition of lime. Substances dyed by deoxidized indigo receive a green tint at first, which be comes blue by exposure to the air. This is the usual method of coloring cloths by means of indigo, which, when fully oxi dized, affords a permanent dye, not re movable by soap or by acids.
Indigo, purified by sublimation, is composed of 73.26 carbon, 13.81 nitrogen, 10.43 oxygen, and 2.5 hydrogen.
Employment of indigo in dyeing.—As indigo is insoluble in water, and as it can penetrate the fibres of wool, cotton, silk, and flax, only when in a state of solution, the dyer must study to bring it into this condition in the most complete and eco nomical manner. This is effected either by exposing it to the action of bodies which have an affinity for oxygen superior to its own, such as certain metals and metallic oxydes, or by mixing it with fer menting matters, or, finally, by dissolving it in a strong acid, such as the sulphuric. The second of the above methods is called the warm hue, or pastel vat ; and being the most intricate, we shall begin with it.
Before the substance indigo was known in Europe, woad having been used for dyeing blue, gave the name of woad vats to the apparatus. The vats arc sometimes made of copper, at other times of iron or wood, the last alone being well adapted for the employment of steam. The di mensions are very variable ; but the fol lowing may be considered as the average size: depth, 7# feet ; width below, 4 feet ; above, 5 feet. The vats are built in such a way that the fire does not affect their bottom, but merely their sides half way up ; and they arc sunk much under the floor of the dyehouse, that their upper half only is above it, and is surrounded with a mass of masonry to prevent the dissipation of the heat. About 3 or 31 feet under the top edge an iron ring is fixed, called the champagne by the French, to which a net is attached in order to suspend the stuffs out of contact of the sediment near the bottom.
In mounting the vat the following arti cles are required : 1. woad prepared by fermentation, or woad merely dried, which is better, because it may be made to ferment in the vat, without the risk of becoming putrid, as the former is apt to do ; 2. indigo, previously ground in a proper mill ; 3. madder ; ; 5. slaked quicklime ; 6. bran. In France, weld is commonly used instead of potash.
The vat being filled with clear river water, the fire is to be kindled, the in gredients introduced, and if fermented woad be employed, less lime is needed than with the merely dried plant. Mean while the water is to be heated to the temperature of 160° Fahr., and main tained at this pitch till the deoxidize ment and solution of the indigo begin to show themselves, which, according to the state of the constituents, may happen in 12 hours, or not till after several days. The first characters of incipient solution are blue bubbles, called the flowers, which rise upon the surface, and remain like a head of soap-suds for a consider able time before they fall ; then blue shining veins appear with a like colored froth. The hue of the liquor now passes from blue to green, and an ammoniacal odor begins to be exhaled. Whenever the indigo is completely dis solved, an acetic smelling acid may be recognised in the vat, which neutralizes all the alkali, and may occasion even an acid excess, which should be saturated with quicklime. The time for doing this cannot be in general very exactly defined. When quicklime has been added at the beginning in sufficient quantity, the liquor appears of a pale wine-yellow color, but if not, it acquires this tint on the subsequent introduction of the lime. Experience has not hitherto decided in favor of the one practice or the other.
As soon as this yellow color is formed in the liquor, and its surface becomes blue, the vat is ready for the dyer, and the more lime it takes up without being alkaline, the better is its condition. The dyeing power of the vat may be kept up during six months, or more, according to the fermentable property of the woad. From time to time, madder and bran must be added to it, to revive the fer mentation of the sediment, along with some indigo and potash, to replace what may have been abstracted in the progress of dyeing. The quantity of indigo must be proportional, of course, to the depth or lightness of the tints required.