Indigo

by Marnanel Thurman

Rainbow

The end of the rainbow-- it was in Salford all along

I'm pretty sure you were taught the order of the colours of the rainbow-- maybe with "Richard Of York Gave Battle In Vain", or perhaps with someone named "Roy G. Biv". Either way, the standard colour sequence is red, orange, yellow, green, blue, indigo, violet. The obvious question is: what on earth is indigo?

The sequence we all learned is taken from a book called Opticks, written by Isaac Newton in 1704. In this book he sets out his discoveries about the way light breaks up as it passes through a prism. Newton was a rather superstitious person, and he believed that the number seven is really important, so it seemed good to have seven colours. Here's the diagram he drew.

Diagram from Newton's "Opticks"

The colour Newton calls "blue" comes immediately after green. So it's a greenish blue-- what we might now call cyan, or turquoise. Indigo, then, must be blue-- and in fact it's the name of a dye with a deep and brilliant blue colour.

Blue has always been a difficult colour to produce. The Ancient Egyptians knew the art of making things blue, but with the fall of the Roman Empire their technology was lost. In the Middle Ages blue was so rare that it was worn only by the very rich. One of a very few places you could get blue dye was from the indigo plant, Indigofera tinctoria, a kind of bean. You take the plant's leaves, soak them in water, and wait for them to ferment. Then you drain off the water and mix the residue with a strong alkali, such as lye. Heaven knows how they discovered this.

The indigo plant

The indigo plant comes from India, as you may have guessed from the name. By the eighteenth century it was also grown in other hot parts of the world, such as Mexico and the southern United States. Predictably those who farmed the plants and extracted the dye were soon slaves; there was a major non-violent revolt in Bengal in March 1859, which was severely suppressed.

Must indigo be grown? Can it be produced in a lab instead? Yes, it can: Adolf von Baeyer discovered how, which won him the 1905 Nobel Prize for Chemistry. These days almost all indigo dye produced is artificial, and most of it goes on dyeing denim jeans.

The indigo plant can only grow in hot climates. But there's another plant with similar properties, which grows even in Britain: a kind of cabbage called woad (Isatis tinctoria). There is a story that the Picts used to dye their bodies with woad, and strip naked to scare invaders. It's probably untrue, and based on a misreading of Caesar's Commentarii de Bello Gallico. Which is a shame, because there aren't many things more likely to make you run away than naked blue people smelling of rotten leaves.