Powdered indigo pigment extracted from Indigofera tinctoria or Isatis tinctoria (woad) leaves through fermentation and oxidation. The blue colour comes from indigotin (C₁₆H₁₀N₂O₂). Must be chemically reduced (with an alkaline vat) to become soluble for dyeing — the reduced form is yellow-green, turning blue on oxidation in air.