The colour blue is rare in nature in a form that can easily be transferred to cloth. Woad (Isatis tinctoria), a plant native to Europe, provided a pale, somewhat grey-blue for centuries. True indigo — the deep, saturated blue extracted from plants of the Indigofera genus, particularly Indigofera tinctoria — was a different proposition entirely. Traded from India and later cultivated in the Americas, it produced a colour of an intensity and richness that European dyers had never been able to achieve locally. When Portuguese traders began importing Indian indigo directly in the 16th century, the European woad industry lobbied hard to have it banned. In parts of Germany and France, the ban succeeded for a time; indigo was falsely labelled ‘devil’s dye’ and declared corrosive to cloth. The charge was entirely unfounded, but commercial interest is a powerful driver of regulation.
The Vat and the Air
What makes indigo chemically unusual is that it is insoluble in water. You cannot simply dissolve it and dip cloth in the resulting liquid, as you can with many dyes. To use indigo, you must first reduce it — remove oxygen from the molecule — to produce a pale greenish-yellow water-soluble form called leucoindigo. Cloth dipped in this solution emerges a yellowish-green. Then, as it is lifted from the vat and exposed to air, oxidation converts the leucoindigo back to indigo, and the cloth turns blue before your eyes. The transformation is rapid — seconds, not minutes — and was, for centuries before anyone understood the chemistry, both practically useful and slightly magical-seeming.
Traditional indigo vats used fermentation to achieve the reduction: a mixture of indigo, water, alkaline material (wood ash, urine, limestone), and a sugar source (bran, dates, figs) was left to ferment for days or weeks, with microbial activity gradually deoxygenating the liquid. Maintaining a vat in good condition was skilled work, and the vat was sometimes treated with an almost superstitious care — the bacteria inside were doing the chemistry, but the dyer couldn’t see them doing it. A healthy vat had a characteristic coppery iridescent sheen on its surface; an experienced dyer could read the state of the vat by its smell, colour, and feel.
Why It Fades
Indigo does not bond to most fibres chemically in the way that mordant dyes do. Instead, the insoluble indigo molecules are mechanically trapped within the fibres when the leucoindigo oxidises back to its insoluble form. This means the dye sits on and within the fibre rather than being permanently bonded to it, and it gradually wears away from the surface with use and washing. This is why old denim fades: the indigo is wearing off the outer cotton fibres, revealing undyed white fibre beneath.
For much of history, this fading was considered a defect — a limitation of the dye. The development of synthetic indigo in 1897 by Adolf von Baeyer (who won the Nobel Prize partly for this work) produced a chemically identical molecule made from coal tar derivatives rather than plants, and eventually made indigo cheap enough to dye the enormous quantities of cotton used in workwear. The American denim industry was built on synthetic indigo.
The fading that was once a problem has since become a feature. The way a pair of jeans fades over months and years of wear — darkest at the seams, lighter at the knees and thighs, fading in patterns that record the specific way a specific body has moved in them — is now considered part of the appeal. The denim industry has developed elaborate mechanical and chemical processes to simulate artificial ageing, selling pre-worn fades as a premium product. The chemistry that old dyers struggled against has become, in the 21st century, a selling point.