Indigo dye

Natural indigo is produced by woad (Isatis tinctoria) and dyer's knotweed[?] (Polygonum tinctorum) in temperate climates and Indigofera species in the tropics. The indigofera species yield more dye and this led to Europeans establishing indigo plantations in warmer climates. The two principal commercial indigo species were indigofera tinctoria (native to India and Asia) and indigofera suffructiosa (native to South and Central America). Indigo was a major crop in Jamaica and South Carolina although most indigo came from India and the East Indies. France and Germany outlawed imported indigo in the 1500's to protect the local woad dye industry.

Indigo dye was traditionally set with stale urine. This reduces the water-insoluble indigo to indigo white and produces a yellow-green solution. Fabric dyed in the solution turns blue after exposure to oxygen in the air converts the soluble indigo white back to indigo. Synthetic urea to replace urine became available in the 1800's.

Indigo does not strongly bond to the fiber and wear and repeated washing slowly removes the dye.

In 1897 BASF produced a synthetic form of indigo and they remain a major producer. In 2002, 17,000 tons of synthetic indigo was produced worldwide.

Indigo treated with sulfuric acid produces a blue-green color and became available in the mid-1700's. Sulphonated indigo is also referred to as Saxon Blue or Indigo Carmine.

See also indigo.