Dyeing with Indigo

In a specially designed process, warp yam dyeings with indigo can be imitated [79],... 

Dyeings with indigo sulfonic acid (C.I. Natural Blue 2), which were produced from indigo and sulfuric acid since 1740, also bleed... 

In the air, indigo white solution oxidises immediately, insoluble indigo blue separating. This property is extensively employed in dyeing with indigo, and also serves for the isolation of pure indigo from the commercial product. 

USEu/As reducing agent, particularly in dyeing with indigo andrvat dyes bleaching soaps, straw removing dyes from dyed, fabrics. 

This method of dyeing with indigo is very ancient, having been practiced by the Egyptians. 

Textile dyes were, until the nineteenth century invention of aniline dyes, derived from biological sources plants or animals, eg, insects or, as in the case of the highly prized classical dyestuff Tyrian purple, a shellfish. Some of these natural dyes are so-caUed vat dyes, eg, indigo and Tyrian purple, in which a chemical modification after binding to the fiber results in the intended color. Some others are direct dyes, eg, walnut sheU and safflower, that can be apphed directly to the fiber. The majority, however, are mordant dyes a metal salt precipitated onto the fiber facUitates the binding of the dyestuff Aluminum, iron, and tin salts ate the most common historical mordants. The color of the dyed textile depends on the mordant used for example, cochineal is crimson when mordanted with aluminum, purple with iron, and scarlet with tin (see Dyes AND DYE INTERMEDIATES). 

Vat Dyes. Vat dyes are insoluble in water. Indigo, for example, an ancient blue dye, is probably the best-known example of an ancient vat dye others include woad and Tyrian purple. Since the process of dyeing requires that the dye be in solution, dyeing with a vat dye (or vat dyeing, as the process is known) is possible only after the vat dye has been made soluble by a relatively long and somewhat complicated chemical procedure. The terms vat dye and vat dyeing are probably derived from the large tanks or "vats", in which the process was carried out in ancient times. 

Table 12.35 Continuous dyeing of cotton yarn with indigo using sodium dithionite or hydroxyacetone [212]...

Alizarin or l 2-dihydroxyanthraquinone is one of the most important dyes. Like indigo, the dye occurs in the plant (the madder root) as the glucoside of the leuco-compound. The cultivation of the madder plant, which, chiefly in southern France, extended over large areas, was brought to an end by the synthesis of the dye from the anthracene of coal-tar (Graebe and Liebennann, 1869). By distillation with zinc dust according to the method of Baeyer, these two chemists had previously obtained anthracene from alizarin. 

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