Theory of the Indigo vat

—The large quantity of extractive matter in woad and madder; as also the sugar, starch and gluten in the bran and woad, when di olved in warm water, soon occasion a fermentation, with an absorption of oxygen, from the air, but especially from the indigo of the woad, and from that introduced in a finely ground state. When thus dis oxygenated, it becomes soluble in alkaline menstrua; the red-brown of the indigo being di olved at the same time. When lime is added, the indigo-blue di olves, and still more readily if a little potash is conjoined with it; but whatever indigo-brown may have been di olved by the potash, is thrown down by the lime. Lime in too large a quantity, however, forms an insoluble combination with the reduced indigo, and thus makes a portion of the dye ineffective; at the same time it combines with the extractive. In consequence of the fermentative action, carbonic acid, acetic acid, and ammonia are disengaged; the first two of which neutralize a portion of the lime, and require small quantities of this earth to be added in succe ion; hence also a considerable quantity of the carbonate of lime is found as a deposit on the sides and bottom of the vat. In the sound condition of the indigo vat, no free lime should be perceived, but on the contrary a free acid. Yet when the disengaged carbonic and acetic acids saturate the lime completely, no indigo can remain at solution; therefore a sufficient supply of lime must always be left to di olve the dye, otherwise the indigo would fall down and mix with the extractive matter at the bottom. Goods dyed in the blue vat are occasionally brightened by a boil in a logwood bath, with a mordant of sulpho-muriate of tin, or in a bath of cudbear. Another mode of mounting the indigo vat without woad and lime, is by means of madder, bran, and potash. The water of the vat is to be heated to the temperature of 122° F.; and for 120 cubic feet of it, 12 pounds of indigo, 8 pounds of madder, and as much bran are to be added, with 24 pounds of good potashes; at the end of 36 hours, 12 pounds more of potash are introduced, and a third 12 pounds in other 12 hours. In the course of 72 hours, all the characters of the reduction and solution of the indigo become apparent; at which time the fermentation must be checked by the addition of quick-lime. The liquor has a bright full colour, with a beautiful rich froth. In feeding the vat with indigo, an equal weight of madder, and a double weight of potash should be added. The odour of this vat in its mild but active state is nece arily different from that of the woad vat, as no ammonia is exhaled in the present case, and the sediment is much smaller. The reduced indigo is held in solution by the carbonated potash, while the small addition of quicklime merely serves to precipitate the indigo-brown. A potash vat dyes in about half the time of the ordinary warm vat, and penetrates fine cloth much better; while the goods thus dyed lose le colour in alkaline and soap solutions. This vat may moreover be kept with ease in good condition for several months; is more readily mounted; and from the minute proportion of lime present, it cannot impair the softne of the woollen fibres. It is merely a little more expensive. It is said that cloth dyed in the potash indigo vat, requires one third le soap in the washing at the fulling mill, and does not soil the hands after being dre ed. At Elbœuf and Louviers in France, such vats are much employed. Wool, silk, cotton, and linen may all be dyed in them.