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Indigo may be a very vague and unnecessary color, but it has an interesting history that involves some plants, turmoil, and Isaac Newton's interest in the number seven.

Hosted by: Hank Green
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Hank: Look down at your lap. You wearing jeans? Even if you’re currently in sweatpants, I’m willing to bet that you’ve got a few pairs of denim jeans nearby, right?

With so many jeans walking about today, it’s easy to forget that their classic indigo color is actually rare in nature. And while modern synthetic dyes have made this deep blue a common color, it wasn’t always so. In fact, for centuries, in part because of its rarity, people across cultures and religions went totally bonkers for indigo. Demand for this color sparked trade wars, fueled Trans-Atlantic slavery -- and for a time, the dye was actually used as straight-up currency.

The indigo craze probably even influenced Newton’s original color wheel. But most color specialists no longer include indigo in their rainbows, because the truth is: it isn’t much different from plain old blue. You can make natural blue dyes from a number of plants, but the deepest, richest, and most long-lasting bluer-than-blue indigo dye comes from the leaves of various tropical and subtropical shrubby plants in the Indigofera genus. Which probably explains where the genus gets its name.

To create the dye, people crushed the plant’s leaves and let them ferment in water over a period of time. Eventually the liquid was drained, mixed with lye, molded into cakes, dried, and pounded into powder. Dyers would then mix that powder with water and other substances in soaking vats to get the exact purpley-blue shade they wanted.

India probably cultivated the original commercial supply of Old World indigo that worked it’s way around the European trade routes during the Greco-Roman era. The word “indigo” itself comes from the Ancient Greek word indikon -- which means “Indian dye.” The so-called “blue gold” was culturally important and highly desired throughout Europe, Japan, and West Africa, before eventually making its way to the American colonies, where its cultivation and trade helped fund the Revolutionary War effort. Then, around the turn of 20th century synthetic indigo dye came on scene and decimated the natural dye market.

So, you might be asking yourself: what does any of this have to do with Newton. Well, back in the mid-17th century, apple-loving Isaac set out to prove that white light was made up of a spectrum of colors. Other scientists had experimented with prisms before, and seen light split into a rainbow, but they thought the rainbow came from flaws in the prism’s glass. Newton used a second prism to turn the rainbow back into a solid beam of light, which showed that the spectrum of light had nothing to do with flaws in the glass. White light did contain different colors. Then Newton did something that has haunted color charts and grade school mnemonics for generations. He divided that visible spectrum into seven known colors: red, orange, yellow, green, blue, indigo, and violet -- what you might know as ROY G. BIV.

The thing is, colors on the spectrum represent a continuum, blending together without a whole lot of distinction. Although people all over the world tend to perceive different colors in a similar way -- as long as they aren’t colorblind -- the way the colors in the spectrum are labeled and divided is often influenced by culture and history. So in a way, colors are kind of arbitrary. It turns out that Newton created the ROYGBIV designation because he thought the number seven had some cosmic significance, and he wanted to connect the principal colors to the seven musical notes, and other famous groups of seven.

The truth is, a lot of people have a hard time even registering spectral indigo as a unique hue, and many common color charts have dropped the indigo designation in favor of just the six main spectral colors. Some color specialists think that because the terminology for certain colors may have changed over time, it’s possible that what Newton referred to as “blue” was really more of a modern cyan, or blue-green, while his “indigo” was actually more what we’d call blue today.

Either way, the fact that when he chose to squeeze that seventh color in, he made it indigo and not say, yellowish-green, may speak to the power and importance of the indigo dye trade at the time. Just some food for thought next time you’re shopping for jeans.

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