Indigo, Sul′phate of. Syn. Sulphindylic acid, Sulphindigotic a., Saxony blue, Soluble indigo.
Prep. By gradually adding indigo (in fine powder), 1 part, to fuming sulphuric acid (Nordhausen sulphuric acid), 5 parts, or oil of vitriol, 8 parts contained in a stone-ware vessel placed in a tub of very cold water, to prevent the mixture heating; the ingredients are stirred together with a glass rod at short intervals until the solution is complete, after which the whole is allowed to repose for about 48 hours, by which time it becomes a homogeneous pasty mass of an intense blue colour, which in a dull light appears nearly black.
Obs. In this state it forms ‘Barth’s blue,’ or the ‘CHEMIC BLUE’ or ‘INDIGO COMPOSITION’ of the dyer. Diluted with about twice its weight of soft water, it is converted into the ‘Saxony blue’ or ‘LIQUID BLUE’ of the shops, also used for dyeing. When commercial sulphate of indigo is diffused through a large quantity of water, nearly boiling, and wool (old white flannel rags, &c.) is macerated in it for some time, the latter absorbs the whole of the sulphate and is dyed blue, whilst the liquor assumes a greenish-blue colour. Wool, so prepared, when well rinsed in cold water, and boiled for some minutes in a large quantity of that liquid containing 1% or 2% of carbonate of potassa, or a quantity equal to about 1-3rd that of the indigo originally employed, gives up its blue colour, and becomes of a dull brown. The liquid is now a rich blue-coloured solution of sulphindylate of potassa, from which the salt may be obtained by cautious evaporation. This compound is prepared on the large scale, by diluting sulphate of indigo with about 12 times its weight of soft water, and imperfectly saturating the solution with carbonate of potassa; the sulphindylate falls down as a dark-blue
coppery-looking powder, soluble in 140 parts of cold water and in about 90 parts of boiling water. This substance is kept both in the moist and dry state, and is known in commerce under the respective names of ‘DISTILLED INDIGO,’ ‘PRECIPITATED INDIGO,’ ‘SOLUBLE INDIGO,’ ‘INDIGO PASTE,’ ‘BLUE CARMINE,’ ‘DISTILLED BLUE,’ ‘SOLUBLE BLUE,’ &c. It is extensively used in dyeing; and when mixed with starch, whilst in the moist state, and made into cakes or knobs, it constitutes the finest variety of the ‘BLUE’ used by laundresses for tinging linen. The ammonia and soda salts may be prepared in the same way as the potassa salt, by substituting the carbonates of those bases for carbonate of potassa. The ammonia salt is very soluble.
INDIGO BLUE. See Indigotin.
INDIGO DYE. There are two methods of preparing solutions of indigo for dyeing.—1. By deoxidising it, and then dissolving it in alkaline menstrua.—2. By dissolving it in sulphuric acid. The former method is used in preparing the ordinary INDIGO VAT of the dyers.
1. a. (Cold vat.) Take of indigo, in fine powder, 1 lb.; green copperas (clean cryst.), 21⁄2 to 3 lbs.; newly slaked lime, 31⁄2 to 4 lbs.; triturate the powdered indigo with a little water or an alkaline lye, then mix it with some hot water, add the lime, and again well mix, after which stir in the solution of copperas, and agitate the whole thoroughly at intervals for 24 hours. A little caustic potassa or soda is frequently added, and a corresponding portion of lime omitted. For use, a portion of this ‘preparation vat’ is ladled into the ‘dyeing vat,’ as wanted. After being employed for some time, the vat must be refreshed with a little more copperas and fresh-slaked lime, when the sediment must be well stirred up, and the whole thoroughly mixed together. This is the common vat for cotton.
b. (Potash vat.) Take indigo, in fine powder, 12 lbs.; madder, 8 lbs.; bran, 9 lbs.; ‘potash,’ 24 lbs.; water at 125° Fahr., 120 cubic feet; mix well; at the end of about 36 hours add 14 lbs. more potash, and after 10 or 12 hours longer further add 10 lbs. of potash, and rouse the whole up well; as soon as the fermentation and reduction of the indigo are well developed, which generally takes place in about 72 hours, add a little fresh-slaked lime. This vat dyes very quickly, and the goods lose less of their colour in alkaline and soapy solutions than when dyed in the common vat. It is well adapted for woollen goods.
c. (Wood vat.) As the last, but employing wood instead of madder; the vat is ‘set’ at 160° Fahr., and kept at that temperature until the deoxidation and solution of the indigo has commenced. The last two are also called the ‘warm vat.’
d. (Pastel vat.) This is ‘set’ with a variety of wood which grows in France, and which is richer in colouring matter than the plant commonly known as ‘wood.’