Indigo is a so-called vat dye, which means that it needs to be reduced to its water soluble leuco-form before dyeing. The reduced form is absorbed into the fibres, and when oxidized back to its blue form it stays within the fibre. Earlier the reduction and dyeing was done with fermentation. Nowadays, the most of the reduction has been done chemically by sodium dithionite. It is considered environmentally unfavourable since it produces sulphite, sulphate, thiosulphate and toxic sulphides as degradation products, which then contaminate the waste waters from the dyeing plants. Therefore there has been interest to find new possibilities to reduce indigo.Indigo has been used in dyeing textile materials for thousands of years. Because indigo pigment is insoluble in water, it must be de-oxidized, or “reduced,” to a water-soluble white form known as “leuco indigo” before it can be used in dyeing. Leuco indigo is unstable; it oxidizes and returns to its blue pigment form when exposed to oxygen. Thus, leuco indigo solution needs to be kept in an oxygen-free environment, or otherwise stabilized, if it is not being used immediately for dyeing.
For many years, dyehouses commonly reduced indigo in-house through a process known as hydrosulfite reduction. Dyers created a “stock vat,” in which indigo is reduced in water with sodium hydrosulfite and solubilized with an alkali, e.g., sodium hydroxide. The resulting leuco indigo solution is then transferred into a feed tank and fed into the dyebath. After the dyebath is prepared, the textile material is dyed through a process known as “dipping” and “skying.” In “dipping,” the textile material is contacted with leuco indigo in the dyebath; in “skying,” the dyed textile material is introduced to the air, causing the indigo to convert back to its blue pigment form.
A second common method of indigo reduction, catalytic hydrogenation, was patented by Andre Brochet in 1917. See U.S. Patent No. 1,247,927 (“Brochet”). The superficial difference between hydrosulfite reduction and catalytic hydrogenation is that the latter uses gaseous hydrogen, rather than sodium hydrosulfite, as a reducing agent. Catalytic hydrogenation allowed “economical production of concentrated solutions of leuco derivatives free from impurities and mineral salts”; when left to settle, the solution naturally separates from nickel or another catalytic metal and can be “drawn off and is ready for use”.