Indigo is one of the oldest dyes known, and it's the reason jeans look, age and fade the way they do.
A very old blue
Indigo was extracted from plants, most famously from the Indigofera family, for thousands of years across Asia, Africa and the Americas. In Europe, a related dye came from the woad plant. Indigo was so valuable that it was traded across continents for centuries.
The switch to synthetic
In the late 19th century, the German chemist Adolf von Baeyer worked out how to synthesise indigo, and commercial production of synthetic indigo began in the 1890s. Today almost all denim uses synthetic indigo, though some specialist mills, particularly in Japan, still use natural indigo.
Why indigo behaves differently
Indigo doesn't dissolve in water. To dye with it, it's chemically converted into a soluble, yellow-green form. The yarn is dipped, then exposed to air, where oxygen turns it back to insoluble blue, locked onto the fibre. Repeat the dips and the blue deepens.
Why jeans fade
Indigo sits mostly on the surface of each yarn rather than soaking right through, so the white core stays undyed ("ring dyeing"). As the surface wears away through friction and washing, the white core shows through. That's why fades appear exactly where you move: whiskers at the hips, honeycombs behind the knees.
Why indigo rubs off
Because it sits on the surface, new dark denim can transfer blue onto light sofas, bags and white trainers. A cold soak or wash before you first wear dark jeans reduces the transfer.
Keep the colour longer
Wash less, wash cold and inside out, and dry out of direct sunlight. See how to wash jeans.