Page:The New International Encyclopædia 1st ed. v. 10.djvu/672

* INDIGO. 586 INDIGO. altaining a height of about six feet, have pinuately compound lenves. and usually pink or purple Mowers. The pods of Indigofcnt anil are sickle-shaped, short and compressed, while those of Indiijofera tinctoria are straight, cylindrical, and many-seeded. When cultivated the indigo plant requires a rich, friable soil, well watered, but not too wet. The s<'eds are sown in drills about a foot apart, and the plants are cut when beginning to llower. In llie tropics this can be done at frequent intervals, and four or five crops a year obtained. After cutting, the crop in handled in various ways, fermented in tanks, as described below_. and the in<ligo e.tracted. Although the plants are perennial, the greatest yields are obtained from annual plantings. In addition to the species mentioned alxive. at least half a dozen other species of Indigofera are known to produce indigo of good character. It is also produced by species of Sophora, I3aptisia, Amorpha, Tephrosia. and Galega. all belonging to the order Leguminos.-p. Baptisia tinctoria and Amorpha fruticosa are known as 'false in- digo* in the I'nited States, where Ihey are widely distributed. Plants of other families produce indigo, as Isatis tinctoria. a cruciferous plant which was cultivated in Europe during the Mid- dle Ages and is still planted in .*<outhern France (see VoAD) ; Wriglitia, and Neriiim. which be- long to the family A]iocynaceip; Marsdenia tinc- toria and Oynmema tingens. of the natural order Aselepiadaeea': Polygonum tinctoritiin. of the order Polygonaeea? : Strobilanthes flaceidifolius, of the order Acanthaceie; Spilanthes tinctoria (Composita;), and Scabiosa succisa (Dipsa- caceici, as well as many others, representing widely separated orders of plants. Neither the indigo plant nnr Isatis tinctoria contains the dyestufT (indigotin) ready formed, but rather a colorless glucoside. indicanc. which breaks up. by fermentative processes, into indi- gotin, and a glucose sugar called . indiglucin. It is a curious fact that indicane is a normal constituent of human urine, and becomes very plentiful in certain diseases. Natural indigo is prepared for the market in the following fashion: The plants are cut down ju^t liefore reaching the flowering stage, and are thrown into vats, where they are steeped in water and allowed to ferment for 12 to l.'i hours, practically out of contact with air. This produces the soluble indirio vhilr, which is taken tip by the water; the liquid is then drawn off into "ben ting- vats,' where it is violently agitated by machinery, in order to promote contact with the air. so that the indigo white is o.xidized to the insoluble iii- rlifio liliir, which forms a thick scum on the surface and then sinks to the bottom as a bluish mud. After settling, the clear liquid is drawn ofT. and the indigo blue is collected, squeezed between cloths, and drieil in the air. It is .sold in irregular lumps, which differ in tint, size, and textuKe according to the various localities in which they are produced. The amount of available dyestuff ranges from 20 to 90 per cent, of the commercial product, some of the impurities boing accidental, although intentional adulteration is common enough. Tliis irregu- larity of composition in the natural product is one of the causes that have advanced the manu- facture of artificial indigo, which is always chem- ically pure. There are a large number of patented processes for the preparation of synthetic indi- gotin, chiefly held by the German color-factories, and largely based upon the researches of Adolf IJaeyer, who showed, about 1880, what the exact chemical constitutiim of this Huhstance is, and indicated the general principles for its prepara- tion from aniline derivatives. At present, indigo is manufactured on the largest scale by the lleuinaiin i>ri)ccss. Plithalic acid, from naphtha- liiic, is converted into anthranilic (ortho-amido- benzoic) acid by way of the phthalimide, and ' the anthranilic acid is transformed, by treatment with chloracetic acid and fusion with sodium hydroxide, into indoxyl, which readily oxidizes to indigo. According to the United States t'ensus for 1!K)0, three million pounils, or one quarter of the world's supply of indigo, are annually pro- duced by synthesis. Pure indigotin, whether ex- tracted from natural indigo or prepared artili- cially. is crystalline, with a coppery lustre and a very characteristic odor: when crushed, it forms a blue powder, and when fixed upon the fibre it constitutes a remarkably fast and bril- liant 'navy-blue' dye. A few chemical compounds, analogous to indigo, and. of slightly different shades, have been produced; but they are as yet , of very slight im|)ortance. Mention may also be made of the so-called iiuliruhin, a red coloring matter of insignificant tinctorial value, which is found in natural indigo. DvKi.Mi WITH I.NDIGO. As indigo blue is insolu- ble in water, it must first be converted into indigo white, which is soluble in alkaline liquids. This change is produced by the chemical addition of hydrogen, .so that the blue, CnHioNjO,, goes over into the white, CnHi-N.O,. "This is effected in cold vats, which are employed for the dyeing of cotton and linen, by metallic reducing agents, such as sulphate of iron, zinc dust, and alkaline sulphites; while wool and silk are dyed in irnrm vats, where the reduction is caused by a fer- mentation of vegetable substances, preferably woad. in a manner analogous to that by which indicane is converted into indigo white. These vats are always alkaline. When the reduction of the indigo is completed, the liquid is colorless, with a light bluish scum on top. The scoured materials are then drawn through the vats and exposed to the air while drying. The atmospheric oxygen immediately reconverts the white into the blue, by removing the extr.a hyilrogen atoms. Indigo olue is also soluble in fuming sulphuric acid, forming 'indigo-sulphonic acid' or Saxon blue, which was formerly known as a dyestuff, but has l)ecn replaced in modern practice by its sodium salt, imlifjo carmine. The solution of this carmine produces a blue precipitate upon the fibre with an alum mordant, but the color is neither as deep nor as fast as that of the un- altered indigotin from the reducing-vats. With artificial indigo, the final synthesis can be pro- duced on the fibre itself, as when ortho-nitro- phenylpropiolic acid and potassium xanthoKcnate are applied separately and the doubly impreg- nated cloth is then steamed, whereupon indigo blue is formed within the fibre, and the by- products can be washed out. Printino whth iNDifio. Indigotin is not suited to printing in pigment form, and figured goods are produced by what may be considered as dyeing processes. To procure a bliie-and-white pattern, for instance, the whole fabric may first 1* dyed blue, and the white produced by printing on bleaching agents which will remove the color.