INDIGO is a well known and exceedingly valuable blue dyeing material. The substance has been known among Western communities from an early period, being mentioned by Dioscorides as ________, and by Pliny as Indicum; when it made its appearance in England it was called indico. As all these names show, the material in its origin and production is closely related to India, among the commer-cial commodities of which it has always occupied a distinc-tive and important place. It was not, however, till after the establishment of the Cape route to India that indigo came to be largely used in the dyeing establishments of Western Europe, woad having in earlier times been utilized for purposes to which indigo was subsequently applied.
As a commercial substance indigo is entirely obtained from the vegetable kingdom, although it may be produced, in minute quantity, from a principle contained in urine, and its synthetical formation has also been accomplished. The number of plants from which indigo may be procured is known to be large, but only from a very few is it pre-pared in practice. These are various species of the leguminous genus Indigo/era, especially the four species /. tinctoria, I. Anil, I. dispcrma, and /. argentea; and it is said that in China Polygonum tinctorium and other non-leguminous plants are used as the source of lan or Chinese indigo. The woad plant, Isatis tinctoria, owes its value as a dye to the presence of indigo matter, although indigo is not actually prepared from it. The most important source of indigo, and that most generally cultivated, is /. tinctoria, which is an herbaceous plant growing 3 to 5 feet high, and having bipinnate leaves. It is in the leaves that the indigo-yielding principle chiefly resides, and these are most gorged with it at the period when the flower-buds are about to open. It is then that the plant is cut down ; and in some regions the same stock yields in one year a second and even a third crop of stems.
Two principal methods of preparing indigo are pursued, dried leaves being operated on in the one, while the fresh green stalks and leaves are used in the other. It will be sufficient to describe the latter, the more important process, as it is conducted in Bengal, where the most highly esteemed varieties of indigo are made. The cut leaves and stems are tied up into small bundles and conveyed at once to the factory, in which there are two ranges of large tanks or vats, one series being at a lower level than the other. In the upper or fermenting vats the bundles are sub-merged ; and cross-bars are fixed over the vats. A fermen-tation more or less rapid ensues, its completion occupying, according to the temperature, from nine to fourteen hours. The progress of the operation is judged by the tint assumed by the water, which under favourable conditions should at the end of this stage be of a fine clear yellow colour. In this condition the liquid is run off into the lower vats, in which it is necessary to maintain it in a state of violent agitation. For this purpose a number of men, armed with long bamboos, enter the vats and lash the water incessantly for two or three hours, thus constantly exposing new surfaces to the air, Gradually the liquid assumes a green colour, and indigo appears in broadish flakes, which as it forms begin to sink. After this transformation is complete, the liquid is allowed to settle, and as the indigo sinks the clear liquid is drawn off in a series of discharges by pipes at different heights in the vat. The deposit of indigo is then placed in a boiler, and, to prevent any further fermentation, is raised to the boiling point. After resting for about a day, it is boiled for three or four hours, and then filtered over a thick filtering cloth, and the paste is dried by pressure. The cakes formed during the pressing are then put away to dry gently in the shade, and in a few days are ready for packing.
Bengal indigo of good quality forms a porous earthy mass, light and easily pulverized; and when newly fractured it has that brilliant purple-blue colour distinctively known as indigo, with a kind of coppery lustre. Experts distin-guish upwards of forty qualities of Bengal indigo, princi-pally characterized by varying shades of colour,the in-ferior qualities being dull in hue, with greenish or greyish tones, hard, dense, and not readily broken. The varieties of indigo which come into the European markets are classi-fied according to their sources : the classes most frequently met with are Bengal, Oude, Madras, Manila, Java, Egypt, Guatemala, Caraccas, Mexico, and Brazil. The best qualities are the Bengal, Java, and Guatemala.
The condition in which the indigo-yielding principle exists in the fresh plants has been a subject of some speculation and controversy. Dr Schunck has investi-gated the leaves of the woad (Isatis tinctoria), the Chinese indigo-plant (Polygonum tinctorium), and others, and from all these has isolated a glucoside body indican, which, under the influence of dilute mineral acids, is decomposed, forming indigotin or indigo-blue and a variety of glucose which he calls indiglucin. It has been assumed that the same principle resides in Indigofera as in these other plants, and is the efficient source of the dye-stuff from that genus. In the decomposition of indican there are formedin addition to indigo-blueindigo-red (indigo-rubin or indigo-purpurin) isomeric with the blue, indigo-brown, and indigo-gluten, all of which, forming part of the precipitate, modify the colour of the product, and render commercial indigo a compound body. Indigo also contains a certain amount of inorganic matter and other non-tinctorial constituents, so that the proportion of indigo-blue may vary from about 72 down to 12 or 14 per cent, of the mass. Pure indigo-blue or indigotin, Ci6H10N2O2, is a neutral body of a deep blue colour, destitute of taste and odour, and insoluble in water, dilute acids, and alkalies, and in cold alcohol and ether. Boiling alcohol, ether, and aniline dissolve it, as do also petroleum, benzene, chloroform, and phenol, melted sperma-ceti and stearic acid, and several oils. It sublimes at 290°-300° C, giving off violet vapours which, condense into right rhomboidal prisms possessing a brilliant coppery-lustre. By destructive distillation, indigotin yields, among other products, aniline,-a circumstance to which that now well-known body owes its name (from the Sanskrit nili through the Portuguese anil, indigo). Treated with oxidizing agents, indigotin takes up oxygen, and is con-verted into isatin, thus :
C16H10Ni!O2 + 2O = C16H10N2O4 ;
and by further oxidation nitro-salicylic acid and picric acid are evolved. The most valuable character, however, of indigotin is found in its behaviour under the influence of hydrogenizing or reducing agents. In the presence of nascent hydrogen indigotin absorbs that element and is converted into white indigo, a colourless body which is readily soluble in alkaline or earthy alkaline solutions, and by simple exposure to the air re-oxidizes and reverts to its original blue condition indigotin. The reduction to white indigo is thus formulated :
Advantage is taken of these properties in dyeing with indigo as detailed under DYEING, vol vii. pp. 576-7. See also CALICO-PRINTING, vol. iv. pp. 689-90.
Indigo when dissolved in strong sulphuric acid, forms with it two acid compounds, both of which have limited industrial applications. These are (1) sulphindigotic acid, C16H8N204(S03H)2, known also as sulphate of indigo or soluble blue indigo, and (2) sulphophcenicic acid, sul-phopurpuric acid, or indigo purple, C16HoN202(S03H). These bodies are formed together in the sulphuric acid solution of indigo ; but, as the second is insoluble in weak acids, it precipitates when the solution in which it is formed is largely diluted with water. Both these acids are soluble in water. The first was formerly used in dyeing Saxon-blue on wool and silk, a style now little known; and the sodium salt of the second is known as red indigo carmine.
The synthetical preparation of indigo is a subject which has long occupied the attention of chemists, as obviously any means by which the substance might be artificially obtained on a commercial scale could not fail to be of great industrial value. The numerous efforts made in this direction appear at last (1881) to be crowned with success ; and there is now little doubt that artificial indigo will soon become a commercial product. It is to Professor Adolf Baeyer of Munich that the measure of success already attained in manufacturing indigo is due. For many years he has patiently investigated the molecular constitution of indigotin and its derivatives. From isatin, prepared by the oxidation of indigotin, Baeyer and Knob produced successively di-oxindol, C16H14N204, oxindol, C16H14N202, and indol, C16H14N2. Baeyer at a later period, with the assistance of Emmerling, succeeded in producing indol from cinnamic acid, and as that body can be prepared from coal-tar a new connecting chain was established between indigo at one extreme and coal-tar at the other, meeting in indol just as at a much earlier date they had similarly met in aniline. The task remained of reconverting these deriva-tives of indigotin into that body, and towards that, in 1870, Baeyer and Emmerling, by heating isatin with phosphorus trichloride, acetyl chloride, and phosphorus, succeeded in obtaining a mixture of indigotin and indigo-rubin. In 1878 the further steps necessary to complete the cycle were accomplished by Baeyer, when from phenyl-acetic acid he prepared oxindol. Acting on oxindol by nitrous acid he produced nitrosoxindol, which in its turn, by treatment with nascent hydrogen, was transformed into amidoxindol, a body which on oxidation yielded isatin. Thus the series of transformations was complete; but they were effected by a process so roundabout and elaborate as to preclude all hope of any commercial issue from the method. Quite recently Baeyer, coming back to the use of cinnamic acid, has devised the much simpler and more direct process which now promises to become, and indeed already is in operation as, a method for the com-mercial preparation of indigo. By treating cinnamic acid with nitric acid, ortho-nitro-cinnamic acid is prepared, which on exposure to bromine vapour readily combines with that body, forming ortho-nitro-dibrom-hydro-cinnamic acid. This substance when treated with caustic alkali is converted by the loss of the bromine into ortho-nitro-phenyl-propiolic acid, which, lastly, when heated in an alkaline solution of grape sugar develops into indigotin. The steps in the process are therefore represented thus :
(1) C6H5C2H2COOH + N02HO - C6H4(N02)C2H2COOH + H20 .
Cinnamic acid. Nitric acid. Nitro-cinnamic acid. Water.
(2) C6H4(NOa)CsHaCOOH + 2Br = CfiH4(N02)C2H2Br2COOH. Nitro-cinnamic acid. Bromine. Nitro-dibrom-einnamic acid.
(4) 2C9H5N04 + 2H2 = CleH10NaOa + 2COa + 2H20 .
The nitro-propiolic acid is now being manufactured by the Badische Anilinfabrik as a material for indigo printing. The acid has simply to be printed on the cloth with a thickening containing grape sugar and alkali, and, by steaming, indigo is developed in the fibre. This reaction is in itself a matter of no small importance, seeing that the printing of indigo direct is an extremely troublesome operation. Hitherto indigo in mass has not been produced, but there can be little doubt that the remaining difficulties, among which is the present expensiveness of cinnamic acid, will soon be overcome, and that artificial indigo will take its place among ordinary chemical manufactures, (J. PA.)