INDIGQ, a well known and much esteemed blue dye. Under the article DYEING (where see Indigo in the In dex) will be found an account of the history of its introduc tion into Europe, and of the processes by which it is ap plied in practice. Under CHEMISTRY, (see Index,) will be found a minute account of its chemical properties, with brief notices of its natural history. But as the importance of indigo renders the history of it, and the processes which it undergoes preparatory to its exportation from the coun tries which produce it, interesting to those who cultivate or respect the useful arts, we shall, under the present ar ticle, give an ampler account of these subjects than ap peared conformable to the rank which it held under those more comprehensive heads. At the same time, as even in this instance we avoid unnecessary repetitions, we ad vise those readers who are deeply interested in the sub ject, to peruse also the accounts there given.
This substance, or its basis, is contained in the iaatia tinctoria, winch grows in temperate climates, and has been used for imparting a blue colour to the animal and vege table fibre. It is also contained in some leguminous plants of warmer latitudes, particularly various species of the in digofera. When it exists in small quantity along with the other constituent parts or the plant, it may be extracted by water, and detained in that menstruum a sufficient length of time to admit of being used as a dye. These plants, therefore, were, in former times, merely dried and com minuted fur the market, and the processes of fermentation and exposure to an absorption of oxygen were left to the care of the dyer. The isatis tinctoria and lusitanica were, indeed, often subjected to a partial fermentation previously to drying- In Hindostan, for thousands of years back, the colouring matter of the indigo plant has been extracted, oxygenated, and precipitated in a comparativ,ly pure state, for sale. In the plant, the basis of indigo is I; colour less, destitute of the requisite proportion of oxygen, and is then extremely liable to decomposition and destruction ; hut, after it has received that addition, its qualities are re markably permanent. This property is mentioned by Dr Bancroft, as manifested by the simple experiment of squeezing the juice of the fresh leaves on calico. The co lour is at first green, but, by the influence ofraxygen from the atmosphere, it is converted into a permanent blue.
This, however, only affords an iniperfeLL, though a sim ple, exemplification of the principles on which the prepa ration of indigo depends. We shall give an account of the process by which this is effected on the large scale.
The plants from which it is produced are, 1. The Indi
gofera and, a large American plant, which affords indigo of good quality. 2. Indigofera tinctoria, a Chinese and In dian plant, which has been carried to America. This is less hardy, but more productive than the preceding. 3. Indigofera disperma. This affords the Guatimala Its produce is uncommonly fine, but less abundant : and, 4. Indigofera argentea, which affords the indigo batard of the French. All these plants are small shrubs, which emit a smell offensive to cattle. The stems with the leaves are cut, and, either immediately, or after more or less drying, are pressed down into large vats or sleepers, then covered with water, which is generally cold. A slight motion, in crease of bulk, and evolution of heat, are in a few hours perceived : air-bubbles rise, and patches of froth are form ed on the surface, together with a violet or copper-coloured cream, which soon after acquires a fine blue ; the body of the liquor becomes green, but in no degree turbid, till fully exposed to the action of the air. Instead of the peculiar smell of the recent plant, a putrescent and extremely of fensive odour is generated. In the first part of this pro cess, it has been ascertained that there is an absorption of oxygen ; the gas which is separated in bubbles is carbonic acid. While the liquor first becomes green, it deposits a fine green precipitate when treated with alkalies : at a later period, it deposits, when thus treated, a substance of a light sky-blue. When the fermentation is completed, the green liquor is drawn off into the beating vat, in which it is agi tated or churned by machinery. It now absorbs oxygen, and the colouring matter is formed into a floculent sub stance, which is merely suspended in the fluid, while the other principles are still in a state of solution. Lime-wa ter, or caustic alkali, is added, to promote the precipita tion of the indigo, as soon as it is observed to be distinctly granulated. At this stage, the froth, from being blue, is rendered colourless by the subsiding of the blue matter. When this addition is made, a further absorption of oxygen takes place ; and, after it has settled a little, the evolution of carbonic acid wholly ceases. When the precipitation is complete, the supernatant liquor is of a clear brandy or Madeira wine colour. A greenish or olive tint is an evi dence of imperfect precipitation, and arises either from its being united to a portion of carbonic acid, or its not having absorbed a sufficiency of oxygen, most probably the latter, as a further exposure to the atmosphere by churning causes it to precipitate an additional portion of indigo when treated with alkalies.