Aizome (Japan Blue)

Japan Blue — awa-ai, natural fermentation indigo from Tokushima. Green from the vat, blue in the air; true fermentation indigo is now about 1% of what is sold.

Green when it leaves the vat, blue when it meets the air. Japanese natural indigo — "Japan Blue" — is not a dye you simmer. The leaves of tade-ai hold a colourless compound; only after the leaves are dried and fermented for roughly 100 days into sukumo, then "built" in an alkaline vat by living microbes, does the colour become usable. Cloth is dipped, lifted, and held to the air: the indigo oxidises from green to deep blue in seconds, and depth is built by dipping again and again.

The river made the colour
Tokushima, on the flood-fed Yoshino River, became Japan's great indigo country — awa-ai — under Edo-period patronage. The river was too wild for rice, but the silt it left was perfect for indigo. Cheap imported and synthetic indigo nearly killed the craft; true fermentation indigo is now said to be about 1% of what is sold. One valley still keeps the vats alive.

See it
At Waza-no-Yakata in Kamiita, Tokushima, you can see the indigo vats and dye a cloth by hand — and watch white turn green, then blue, in the air.