Indigo

Indigo is the name given to plant species from the genus Indigofera, which is part of the legume family. Some species contain the indigo pigment, which has already been used to produce a blue dye for thousands of years. Indigo is also the name of the dye itself, which produces various blue shades, depending on intensity and dyeing time. Both the plant species and the dye, which is very important for the dyeing of textiles, are intertwined in complex and often asymmetrical dependency relationships. They extend through history for thousands of years and around the globe several times.

Indigo as a plant

The genus Indigofera includes more than 700 species, which thrive in tropical and subtropical regions. They owe their name to the pigment contained in some species (e.g. Indigofera tinctoria, Indigofera suffruticosa, Indigofera articulata, Indigofera arreacta). Indigofera tinctoria comes from South Asia and was used in India for thousands of years to dye textiles (figs. 1-3). From there, the dye traveled via trade contacts to Europe, where it was highly sought-after and was given the name “indigo,” referring to its origins in India.

In the modern era, two species were very important for the production of the blue textile dye: Indigofera tinctoria and the species Indigofera suffruticosa from Central America/Guatamala.

Through trade contacts and the European colonization of Africa, Asia, and the Americas, they spread around the world and were cultivated wherever there were suitable climatic conditions. Indigo cultivation was closely linked to asymmetrical dependency relationships in many places. In the Caribbean and North America, indigo was cultivated by enslaved people on plantations. These plantations generally belonged to Europeans or their descendants, while the workers either had African roots or belonged to the indigenous peoples of the Americas. In South Asia/Bengal, the poor rural population was forced to cultivate indigo by the British colonial power and its henchmen in the 19th century. The small farmers had to use their best land for this, which led to food shortages, severe famine, and ultimately to resistance against indigo cultivation.

Fig. 1: Herbarium page of woad (Isatis tinctoria), Herbarium Haussknecht (JE) (photo: B. Ihde, 2025).

Indigo as a dye

The blue dye was obtained from the indigo plant in a multi-stage chemical process, which took place in what were known as “indigoteries.” They consisted of a series of interconnected vats and a plant to dry the dye (fig. 4).

Fig. 4: Indigo production: “Depiction of a simple indigo mil, the digestion vat of which is filled and covered with wood, but the beating vat with its assembled parts is prepared for beating with the beating bar.”. Beauvais Raseau (1772), The Art of the Indigo Maker. IV. Copper, p. 397.

The leaves, or even the whole indigo plant, were first soaked in a container filled with water and left to ferment. The fermented solution was then filled into the next container. Oxygen was added by hitting the surface of the water with sticks. This triggered oxidation, which caused the color pigments contained in the solution to precipitate out. The color pigments settled on the bottom of the container as silt. This silt was then filled into molds for drying. This resulted in what were known as cakes, which were easy to package for transport (fig. 5).

Fig. 5: Indigo cakes: dried indigo pressed into cubes (photo: Shutterstock-ID 1980643148, license 2024).

In the Caribbean and North America, the indigoteries were generally located directly on the indigo plantations. This meant that the indigo was processed by the same people who had previously cultivated it. The situation was different in India, where indigo farmers rarely had anything to do with obtaining the dye. This took place in central indigoteries or factories to which the farmers delivered their indigo harvest (fig. 6).

Fig. 6: Indigo factory in India (photo: Oscar Mallitte 1877).

Synthetic indigo

Since the end of the 19th century, it has also been possible to produce indigo dye synthetically (fig. 7). From 1865, Adolf von Baeyer began to develop processes to synthesize indigo, achieving his breakthrough in 1878. This brought the production of natural indigo to a rapid end at the end of the 19th century. It led to the end of a long chapter of asymmetrical dependency in the context of indigo production from plant-based raw materials.

Fig. 7: Structural formula of indigo (Yikrazuul 2008, Public domain, via Wikimedia Commons).

Indigo dye in other plants

The indigo dye is also contained in other plants. These include, for example, woad (Isatis tinctoria), Japanese indigo (Persicaria tinctoria), also known as dyer’s knotweed, and wild indigo (Baptisia tinctoria) (figs. 8-10).Through globalized trade contacts, the Indigofera dye replaced dyes obtained from other plants in the course of the modern age. Their color intensity was either much weaker or they were unable to compete with the Indigofera dye from an economic viewpoint.

Fig. 8: Blooming woad (Isatis tinctoria) (photo: aga7ta / Shutterstock.com).

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Further Reading

Neumüller, Kerstin und Douglas Luhanko, 2020. Indigo. Anbau, Färbetechniken, Projekte. Bern: Hauptverlag.

Splitstoser, Jeffrey C., et al., 2016. Early pre-Hispanic use of indigo blue in Peru. In: Science Advances, Bd. 2.  doi:10.1126/sciadv.1501623.

Steingruber, Elmar, 2004: Indigo and indigo colorants. In: Ullmann’s Encyclopedia of Industrial Chemistry. Wiley-VCH, Weinheim.

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[i] https://commons.wikimedia.org/wiki/File:Indigofera_tinctoria_jd_plt_Paris.jpg