DYEING 341 from the acid. It was formerly effected with cotton goods by hanging them up after mordant- ing in a warm room ; but now it is more quickly performed by combining the action of moisture with that of heat. An excessive consumption of coloring matter is avoided by the removal of superfluous mordant; and in the case of printed goods this is necessary to prevent the spreading of the color over the unprinted por- tions of the surface. Wool may be dyed in the fleece, in the yarn, or in the cloth. It is usual, for fast colors, to mordant the goods with a solution of alum and cream of tartar ; or the latter salt may be used with chloride of tin or with sulphate of iron, the selection depending upon the color which may be desired. For dyeing wool blue the best material is indigo, as it yields the finest and fastest color. This substance is not ready formed in the plants from which it is obtained, but is generated by exposing the juice to the action of the air. It is technically called indigotine, and some- times oxidized indigo, and is precisely the same substance which exists in the dyed cloth and gives it its color. That it may be de- posited in the fibre it is, however, necessary to reduce it to a soluble condition. This is effected by what has been termed deoxida- tion, but which is more strictly the absorption of hydrogen, as will presently be explained. Soluble indigo, called indigogene, is white, and soluble in alkaline liquors ; but upon exposure to the air it parts with the hydrogen which had been absorbed, and again becomes indigo- tine or blue indigo. It was thought by Che- vreul that the juice of the living plant con- tained indigo white, but this idea cannot be maintained in the light of the fact that white indigo is only soluble in alkaline liquids, where- as the juices of plants are acid. Moreover, the investigations of Schunck show that the or- ganic substance in the living plant which is converted into indigo by fermentation is differ- ent from indigo white ; he has named it indi- can. (See INDIGO.) The process of dyeing wool with indigo is accomplished in what are called blue vats, the formation of which differs somewhat in different countries. The German vat, which is very manageable, and may be used for two years by merely replenishing, is composed as follows: To 2,000 gallons of water, heated to 130 F., are added 20 Ibs. of crystals of common carbonate of soda, 2 pecks of bran, and 12 Ibs. of ground indigo. The mixture being well stirred, fermentation com- mences in about 12 hours, the liquid' assuming a green color. There are then added 2 Ibs. of slaked lime, and the vat is again heated and covered for 12 hours longer, when about the same quantities of soda, bran, and indigo, with some lime, are again added. The vat is ready to be worked in about 48 hours after the com- mencement. It is usual also to add five or six pints of molasses to promote fermentation. Should this become too active, it is repressed by the addition of more lime ; and if it is not active enough, more bran and molasses are added. The bran and molasses are called re- ducing agents, because the fermentation which they undergo decomposes a portion of the water, abstracting the oxygen, while the hy- drogen is taken up by the insoluble indigotine, or blue indigo, Ci 6 Hi N 2 O 2 , which is thus converted into white indigo, or indigogene, CieHiaNaOs, which is soluble in alkaline solu- tions, but not in pure water. The dyeing is performed by suspending the fabric on frames and moving it about in the vat for about two hours. It is then washed with water and treated with very dilute hydrochloric or sul- phuric acid, to remove the alkali. In England blue vats for wool are made with water 500 gallons, indigo 20 Ibs., carbonate of potash 30 Ibs., bran 9 Ibs., madder 9 Ibs. The water is heated to just below the boiling point, and the potash, bran, and madder are put in first, and then the indigo, which in all cases should be previously very finely ground. Cold water is added to reduce the temperature to 90 F., which is maintained constantly by a steam pipe. The ingredients are well stirred every 12 hours, and the vat is ready to be worked in 48 hours. It cannot, however, be used for more than a month, and is also more expensive than the German vat, on account of the pot- ash. For dyeing cotton goods, especially if they are heavy, the German vat is used with advantage ; but most calicoes are dyed with the cold lime and copperas vat, which is made with 900 gallons of water, 60 Ibs. of green copperas, 36 Ibs. of ground indigo, and 90 Ibs. of slaked lime, stirred every half hour for three or four hours, then left 12 hours to settle and stirred again, when it is ready for dyeing. The reac- tions are the formation of sulphate of lime and protoxide of iron, which latter body, by reason of its affinity for oxygen, removes this from a portion of the water, leaving the hydrogen to unite with the blue and form white indigo, which is soluble in the excess of lime. For dyeing Saxony blue sulphindigotic acid is em- ployed, a substance formed by the action of concentrated sulphuric acid on indigo. It is made by treating one part of indigo with about five parts of fuming sulphuric acid. The solu- tion is then poured into a vessel containing water, and in this the wool is immersed for 24 hours, when there will be formed upon it in- soluble sulphindigotate of alumina. Wool is dyed with Prussian blue by immersing it in a solution of persulphate or pernitrate of iron and then passing it through a solution of ferro- cyanide of potassium acidulated with sulphuric acid, by which brilliant, blue, insoluble ferro- cyanide of iron, or Prussian blue, is formed. Wool is generally dyed yellow with weld, fus- tic, or quercitron bark. The yellow coloring matter of weld, luteoline, is extracted by boil- ing water, to which sometimes caustic lime or alkalies are added to increase the solvent power. Luteoline dissolves with a deep yellow color in caustic alkalies and alkaline carbon-
