A blue dye made from plants.

A well-known blue dye made from plants is indigo. Indigo dye is extracted from the leaves of certain plant species, most notably those from the genus Indigofera, which are native to tropical climates. The process of making indigo dye traditionally involves several steps:

1. Harvesting: The plants are harvested at the peak time when the leaves contain the highest level of the precursor to indigo, called indican.

2. Wilt and ferment: The leaves are soaked and allowed to ferment in water; through fermentation, indican undergoes hydrolysis to release indoxyl.

3. Oxidation: The fermented leaf solution is then exposed to air, during which the indoxyl is oxidized to form indigo particles, which are insoluble in water and appear as a blue sediment.

4. Filtering and processing: This sediment is filtered out and pressed into cakes, dried, and ground into a powder that is the indigo dye.

Indigo dye has a rich history and has been highly valued for its ability to produce a wide range of blues on textiles. Historically, it was a significant commodity in the trade between European countries and their colonies.