It might come as a great surprise that ‘blue jeans’ are dyed by first submerging denim in a pale yellow solution – but that is the mysterious nature of indigo.
Dyeing with indigo is different from any other process used to color cloth because the darkly colored blue dye is actually insoluble in water. To make the dye soluble, the colorant is first reduced to leucoindigo, “white indigo,” which is a pale, yellow color. It must take an act of faith to submerge cloth or a pair of jeans into a vat containing a nearly colorless dye, and expect the result to come out blue. In truth, however, the process is even more complicated. When the dyed fabric first emerges, it is a bright green. The wet fabric must then be exposed to air and the indigo reoxidized before the characteristic deep blue color appears.
The same colorant that characterizes indigo is also contained in the plant, Isatis tinctoria, from which the dye woad is made. Woad was famously used by Celtic warriors who painted their faces blue to appear more terrifying in battle. Woad and indigo are, however, treated as separate substances since they came from different plants on different continents. Still they hold an important commonality: both colorants are isolated from plants through fermentation. In the modern day world, using natural dyes generally carries a positive environmental association. But woad processing from Isatis tinctoria not only released noxious fumes, the plants themselves rapidly depleted the soil of nutrients, which necessitated the woad cultivators to relocate frequently, leaving infertile fields behind.
Indigo as a colorant was first introduced in Europe in the late 15th century following the voyages of Vasco de Gama, who established trade routes to India, where indigo, was the preferred blue dye. In order to protect the woad industry, French and German governments declared dyeing with indigo prohibited by law – even punishable by death in some locales; in 1654 in Germany, an imperial edict declared indigo the “devil’s dye.” It wasn’t until 1737 that dyeing with the more strongly colored indigo, which is derived from the plant Indigofera tinctoria, was legalized.
Centuries later, a German chemist, Adolf von Baeyer, began exploring alternate paths away from natural indigo and pioneered a synthetic dye. The German company BASF eventually found an economically viable route starting with aniline, such that by the early 20th century, synthetic indigo was dominating the market.
More information about indigo may be found at http://en.wikipedia.org/wiki/Indigo_dye