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Herb to Know: Indigo

| February/March 1998

  • Photograph by Rita Buchanan

• Indigofera tinctoria, I. suffruticosa
• (In-dih-GOFF-er-uh tink-TOR-ee-uh/ suh-froo-tih-KO-suh)
• Family Leguminosae
• Tender subshrubs

Indigo, best known to Americans as the traditional colorant of blue jeans, has been esteemed and used for centuries throughout the world. Long before synthetic dyes became available, this blue pigment was being extracted from a variety of species belonging to ­several families of plants.

The Japanese preferred dyer’s knotweed (Polygonum tinctorium), a member of the buckwheat fam­ily; Europeans favored woad (Isatis tinctoria), a mustard native to the Middle East; while West Africans obtained their indigo dye from Lonchocarpus cyanescens, a vine of the pea family.

However, the most commercially significant indigo sources throughout history have been Indi­gofera tinctoria, pictured above, which is native to Southeast Asia, and I. ­suffruticosa, from the New World Tropics. Both are known by the common name indigo.

The genus Indigofera (the name is Latin for “indigo-bearing”) comprises about 700 species of trees, shrubs, and annual or perennial herbs of the pea family native to tropical and subtrop­ical regions. The dye plants I. tinc­toria (tinctoria is Latin for “dyer’s”) and I. suffruticosa (suffruticosa refers to ­having a woody base and herbaceous top) are both perennial deciduous ­subshrubs that are grown as annuals, at least in cooler climates. Both grow 3 to 6 feet tall, have compound leaves 2 to 3 inches long with many pairs of rounded leaflets. If you tear a leaflet, the cut edges will turn blue. Clusters of tiny coppery pink pealike flowers are produced in late summer, followed by small seedpods. Those of I. tinctoria are straight, those of I. suffruticosa, curved.

Indigo Dye

The chemistry of indigo is complex. In living plants of the genus Indigofera, indigo occurs as a colorless, water- soluble compound known as indican. When the leaves are placed in water, the indican leaches out and is transformed into an insoluble compound called indoxyl. With the addition of oxygen (as by stirring up the water), the blue dye indigo is formed, but it, too, is insoluble. To dye yarn or fabric with it, you have to add a reducing agent (which removes oxygen) to convert it to a yellowish water-soluble form called indigo white. In olden times, stale urine—vats of it—was used as the reducing agent, but today, Spectralite and Rit Color Remover serve as convenient substitutes. You soak your yarn or fabric in a warm bath of indigo white for about twenty minutes. As you remove it from the dyebath, the dye will react with the oxygen in the air and turn blue—at last. Repeated soakings and airings deepen the color. Because indigo coats the fibers rather than chemically bonding with them as other dyes do, no mordant is needed to ensure colorfastness.

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