Just What is Indigo?

Just what is indigo? Most of us start out thinking we know, but do we?

The term indigo is, of course, used as the name of a color. To me it is
a deep navy blue with an ever-so-slightly greenish cast. But this is the description of the color called ai (藍) in Japanese. When indigo appears as one of the seven colors of the rainbow in the West it exhibits a slightly reddish cast.

Indigofera tinctoria – “true” indigo from India

Next, most of us would list it as a plant. But, which plant? Perhaps Indigofera tinctoria, commonly called true indigo? This is the one associated with the blues of India. It grows as a shrub and is in the legume family of plants along with beans and wisteria. It likes a tropical climate. The basic material we extract from the leaves to create blue is called indican.

Indigofera suffruticosa from the Americas

But indican is found in many other plants and in many other parts of the world, such as Indigofera suffruticosa. Even though it is commonly called Guatemalan indigo, this plant is native to most of the tropical and subtropical areas of the Americas including the Southern United States and all the way down to the northern reaches of Argentina.

Indigofera bracteolata – West Africa

Indigofera bracteolata grows well in the moist savannah lands of West Africa and thrives in sandy soil.

Isatis tinctoria – woad, covering Europe and the Middle East

Woad, Isatis tinctoria, is another common source of indican. It is native to the steppe and desert areas of the Middle East and Central Asia, and is related to mustard and cabbage. It is the plant associated with the blues of Britain, going all the way back back to the Celts.

Marsdenia tinctoria – covering Southeast Asia

Marsdenia tinctoria is commonly found in subtropical regions including Nepal, Thailand, Burma, and Indonesia. It is a winding shrub or vine that can grow up to fifteen feet long. It is related to a plant called dogbane.

Persicaria tinctoria – tadeai, or Japanese (buckwheat) indigo

And in Japan there is tadeai (蓼藍), (Persicaria tinctoria), also known as Polygonum tinctorium, or Japanese indigo. It is an annual that grows up to about three feet in height and grows well in temperate climates. It is classed with buckwheat.

Mercurialis leiocarpa – Japanese yama-ai

Strobilanthes cusia – Ryuukyuu-ai, Okinawan indigo

But even in Japan there are more plants than just tadeai that contain indican. There is yama-ai (山藍), Mercurialis leiocarpa, a slender perennial that hugs the ground.  Ryuukyuu-ai (琉球藍), Strobilanthes cusia, has leaves as large as a person’s hand. It is in the acanthus family and has been a long-time favorite source of blue for collectors of Okinawan textiles.

And of course, there is synthetic indigo. It made its first real commercial appearance at the tail end of the 19th century (1897) and quickly replaced the more laboriously produced natural sources. The actual core colorant of all of these sources, including the synthetic, are chemically identical. Slight visual variations can be appreciated among the numerous natural sources brought about by differing impurities in the plants or unique extraction techniques.