How indigo dye is made from plants

The blue in a living indigo vat is born yellow and turns blue only when it meets air.

stations: 3
things downstream: 2

The short version

True indigo is a shrub whose leaves contain a colorless compound called indican. Soaking cut leaves in a vat for 12 to 15 hours releases it, and it turns into a yellow soluble form. Cloth dipped in that vat comes out yellow-green and turns blue as the air hits it.

Cut the leafy shrubs

Indigofera tinctoria is a shrub one to two meters high with light green leaves. Leaves hold between 0.2 and 0.8 percent of the colorless precursor, so a lot of foliage is needed.

0.2 to 0.8% indican in leaves
Soak the leaves in a vat

Pressed into a vat and soaked, the leaves release the precursor. After 12 to 15 hours of fermentation the liquid holds the yellow, water-soluble leucoindigo.

12 to 15 hours
Air it blue

Exposure to oxygen turns the yellow liquid into insoluble blue indigo. The pigment is collected as a paste, which can be dried into cakes.

End of the lineIndigo Next it goes into denim.

Worth repeating

Most indigo today is synthetic, about 80,000 tonnes each year as of 2023.
A pair of blue jeans uses only 3 to 12 grams of indigo.
Blue colorants are relatively rare in nature, which is part of why indigo was so prized.