Monday, August 5, 2019

Where did my indigo? (part 1)

I answered a question on Quora recently. I am so proud of my astute answer that I decided to expound on it to make it into a blog post.

The question: What is the true color of indigo?

In today's blog post, I provide the first answer to that question.

Dyeing is a pigment of my imagination

The words dye and pigment are often used interchangeably. Just don't try that in the presence of any colorist. At best, they will roll their eyes and/or laugh at you.

Cool pic of dyes from Alliance Industries

The general term is colorants, something that is used to impart color. Dyes and pigments are two types of colorants.

Dyes are molecules (often organic) that are incorporated into fibers. As a rule of thumb, they are soluble, and we work with them typically in solution, so they are individual molecules. Also generally they require a binder molecule to attach them to whatever we are dyeing. Dyes are most commonly used for dyeing fabric. Dyes need to penetrate into the substrate (e.g. the cloth fibers) in order to become permanent.

Pigments are made from grinding stuff up -- usually non-organic compounds. Since they are ground up, they come in the form of solid particles with lots of molecules clumped together. when used, they are generally suspended in a vehicle that is evaporated when the paint or ink dries. The vehicle might be water (as in latex paints and inkjet inks) or oil (oil-based paints and lithographic ink) or a solvent such as alcohol or toluene (sometimes used in gravure printing ink). Pigments generally coat the substrate rather than becoming incorporated into the substrate.

I expect everyone to use these terms correctly from now on. This will be on the final exam.

Mordant or less?

When I was young, I remember my sister showing me a neat trick. She showed me how to crush the flower buds of a certain flower between my fingers. The flower excreted a gorgeous rich blue fluid. My sister told me that this was a dye used by Native Americans.

Naturally, I was scolded. Maybe my mother was angry because the fluid stained my hands. More likely, she was angry that I stained my clothes. Then again, maybe she scolded me because I was destroying her irises. But, my memory is hazy. Maybe it was a wildflower. And maybe the Native American dye that my sister told me about was beets. I dunno which version is true, but one fact is perfectly clear. It was Nancy's fault. She forced me to get into trouble. This last part will be on the final exam.

This all left me with the impression that dyeing was pretty easy. You find some natural color in the woods or in a field. You crush the plant, dissolve it in water, and then soak your cloth in it.

Indigo dye is almost that easy to use. You just soak the cloth in a solution of the dye, rinse it, and then dry the fabric. Well, maybe that's a simplification, but I think I have the basics of it.

But making the dye is a bit more involved. The indigo plant has pretty pink flowers, but that's not where the dye comes from. Oddly enough, the dye is made from the leaves. These leaves are mashed and then fermented. The gunk left over is then dried and beaten to aerate it, since the process needs oxygen. Then it is left to dry into cakes.

When I first heard this, I was surprised. Who'da ever thunk to ferment leaves? Who'da ever thunk to ferment something and then not drink it? That part boggled my mind. (I'll get back to that in just a bit.)

Getting back to the dye, indigo is odd in the world of dyes. Indigo is called a non-mordant dye, because it does not require a mordant.

Ok, so what's a mordant?

Most dyes do not have a natural affinity for cloth. That is, they don't stick. Cloth is treated with mordants (like alum or tannic acid) so that the dye molecules will stick. The mordant molecules have an affinity for both the cloth and the dye, so they bind dye molecule to cloth by linking the left arm to one and the right arm to the other.

Here is the lesson that will appear on the final exam: Indigo was one of the earliest dyes because of two properties. First, the dye is created naturally in a way that humans could readily see and imitate. Second, the dye did not require pretreatment of the cloth to fix the dye.

Who invented indigo?

Indigo comes from the the indigo plant, specifically from the species of the indigo plants named indigofera tinctoria and indigofera suffruticosa. I called up the ancient Roman historian, Vitruvius, to help answer the question of who invented indigo dye. He told me that "indigo comes from India... where it attaches itself as mud to the foam of the reeds." Hence the name, indigo, which is derived from the Latin word indicum, meaning substance from India.

Now I can understand how someone would have come up with the idea of fermenting indigo leaves. I surmise that someone noticed that the muddy foam on the indigo plant stained fingers and cloth. They then sought to duplicate the natural process, and likely found that they needed a bit of this sticky foam as a starter. Whatever animicula was in the foam -- maybe it was yeast? -- would greedily ingest something in the leaves and go through some sort of chemical reaction that liberates the indigo dye molecules.

A bit of historical pedanticness -- just which Roman historian I spoke with is a matter of disagreement among historians. The quote above is from Ball, p. 201. Another account (Phipps) refers to Pliny, without a fancy quote. Another author, DeBonnet, attributes a similar quote to Dioscorides in 23 BC. I am not really sure who I talked to on the phone. I had spent the whole night sampling my fermented indigo, and ... well...

Marco Polo brought this dye to Europe from his travels in India. Indigo dye became a much desired item for import from India to Europe, first along the Great Silk Road, and then around the Cape of Good Hope. For example, Dutch ships carted nearly 170 tons of the dye from India to Europe in 1631. This factoid will not appear on the final exam.

Cool picture of the indigofera plant, from Phipps, 1832

I pause here to admit to a gap in my personal recollection of world history. I'm a bit surprised that the ancient Greeks had trade routes going with India. Here I thought Marco Polo was the guy who first connected European commerce with Indian commerce. Did I sleep through that day in history class?

So far, the story goes like this: The recipe for making indigo dye from the indigofera plant was devised in India, which became the source of indigo dye in Europe. There are reports of indigo dye in Harappan Civilization in the Indus valley somewhere around the time 2,000 BCE, so this all makes sense.

But that's only part of the story. A clay cuneiform tablet, dating back to 600 or 500 BCE, was found in southern Iraq with the recipe for creating indigo dye.

This is a bit of a conundrum having to do with the history of indigo. Suppose you were transporting indigo from India to Babylonia. Would you load your camels or rafts or what-have-you with bundles of leaves, or would you load up with cakes of the processed dye? I'm thinking I would go with the more compressed form. Keep up with me now... so if we have an ancient Babylonian tablet, written in ancient Babylonian cuneiform, that was found in the area that was ancient Babylon... and that tablet gives a recipe for turning indigo leaves into indigo dye...

Do you see where I'm going here? Why would a Babylonian go to the trouble of describing he process to manufacture indigo dye to a Babylonian when Babylonians are only receiving shipments of the rocessed dye???!?

I think that the indigo plant may have been cultivated in ancient Babylon. I think that indigo seeds were transported from India to Mesopotamia at some point before 500 BCE. Then again, maybe the seeds went the other direction at some time before 2,000 BCE?

Here's another indigo sighting from about that same time period. Herodotus (a Greek historian from about 450 BCE) wrote that in Caucasus "They have trees whose leaves possess a most singular property: they beat them to a powder, and then steep them in water; this forms a dye which they paint figures of animals on their garments." Herodotus doesn't actually use the word indigo, but it sure sounds like he was talking about how to make indigo dye. He also doesn't mention India. Strange.

So, now I'm confused about whether the process to manufacture indigo dye actually came from India.

Let's muddy the waters a bit more. In ancient Egypt, mummies were wrapped in linen. The strips of linen were dyed with indigo. This puts the invention of indigo back to around 2,400 BCE, and perhaps earlier. Did the very ancient Egyptians get their indigo from India? Or did the Indians get their indigo from Egypt? Were there even trade routes between these civilizations at that time? Tell me, just where did the indigo?!!??

There are also early reports of indigo dyes being used in the Xinjiang province of China around 1,000 BCE. Were there trade routes between China and India? Here my meager world history completely falls on its face. I done got edicated in 'murica. We don't need no stinking urapean history there.

Earliest occurrences of indigo dye in various regions of the Old World

I look at the map above, and it seems likely to me that indigo dye was developed independently in Egypt and India, and possibly also in China.

But I omitted one last piece of the puzzle. Indigo dyed fabric was found in scraps of cloth in Huaca Prieta, Peru that date back as far as 5800 BCE! I hope you're as excited about that as I am.

My conclusion: a process to extract indigo dye from the indigofera plant was developed independently in multiple places around the globe. That will be on the final exam.

Stay tuned for part 2 of this series of blog posts, where I investigate Isaac Newton and the indigo that he put in our rainbow!

Want to know more about pigments and dyes?
Have a look at a blog post about mauve, Tyrian purple, and magenta.
Or, check out the blog post about the invention of Klein blue.
Or better yet, have a quick read about vermilion and cinnabar.


Ball, Philip, The Bright Earth, Art and the Invention of Color, University of Chicago Press, 2001

Beloe, William, Herodotus, Translated from the Greek, 1814, p. 254

DeBonnet, Maurice, Origin of Paint Pigments, Varnishes, Vehicles, National Painters Magazine, Vol 48, Jan. 1921, page 26.

Finlay, Victoria, The Brilliant History of Color in Art, Getty Publications, 2014

Mattson, Anne, History of Indigo in the Early Modern World,

Nassau, Kurt, The Physics and Chemistry of Color, The Fifteen Causes of Color, John Wiley and Sons, 1983, p. 285

Phipps, John, A series of treatises on the principal products of Bengal, 1832

Splitstoser, Jeffery C., Tom D. Dillehay, Jan Wouters, and Ana Claro, Early pre-Hispanic use of indigo blue in Peru, Science Advances  14 Sep 2016: Vol. 2, no. 9,

St Clair, Kassia, The Secret Lives of Colour, pps. 189 - 192

Wikipedia, Indigo

Wild Color, History of Indigo & Indigo Dyeing,

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