Indigo fibers
Nahaufnahme eines Blue Jeans Stoffes. Closeup of a Blue Jeans. Mark Michaelis

All up in

Denim fibers that were likely dyed with synthetic indigo.

This post has been updated.

Yesterday, a blog by Slate’s Jude Stewart, author of ROY G. BIV: An Exceedingly Surprising Book About Color, revealed the history of the blue in blue jeans. The story makes a nice, if somewhat predictable, narrative, but one detail stood out to me. “Unlike most natural dyes that, when heated, penetrate cloth fibers directly,” wrote Stewart, “indigo [as it is commonly used] binds externally to the cloth’s threads, coaxed by a chemical agent called a mordant.” (Important note: This is only true for the synthetic version. The natural version that was used by many indigenous weavers isn’t typically paired with a binding agent.)

I speak a couple of languages extremely remedially, so I know enough to know that the root “mor-” almost always means “death” in some capacity. In this case it actually translates to “to bite,” meaning it helps synthetic indigo dye that much of the fashion industry uses to bite onto the fabric with its VICIOUS JAWS OF DEATH. Just kidding. But seriously, these things are really toxic. Some mordants are acidic, but clothing companies most commonly use mordants made from metals like chromium or aluminum. The latter is slightly safer than chrome, but both kill off plants exposed to factory waste water, destroy ecosystems, poison drinking water, and generally are awful. They are, though, why your jeans fade so perfectly while your t-shirts just eventually all equalize to the same shade of puce or mauve or some other tone reached after five-plus years of the sweat-sun-wash cycle.

Aside from the dreaded mordants, synthetic indigo dye isn’t great for the planet either. It’s extra slow to decompose, it darkens river water so flora and fauna are starved of sunlight and oxygen, and, due to our love for artfully pre-faded jeans, an excessive amount of it is sent out into the world from factories. Last year, Greenpeace released an overwhelming study titled “Toxic Threads: The Big Fashion Stitch-Up,” which outlined the true dangers of dye pollution and jeans from Levi’s and Tommy Hilfiger popped up as top offenders of toxic dye use (it doesn’t specifically call out indigo). A few textile conglomerates, like DBL Group in Bangladesh, have upgraded their factories to use and waste less water, but most are still putting out nauseating amounts of dangerous dye waste water a year.

This story isn’t going to be a completely depressing denim guilt-trip, though. There’s hope! You can have your jeans and…eat…them…too? While I was waiting for a better idiom to come to me, I called up Juan Hinestroza, associate professor of fiber science at Cornell University and director the college’s textiles nanotechnology laboratory, to tell me about his department’s efforts to clean up our fashion mess.



Natural fibers embedded with nanoparticles clean up indigo-polluted water in Cornell’s labs.

“In some areas of China, you can tell which colors are fashionable in New York and Paris by the color of the rivers,” Hinestroza told me. His department handles much sexier applications of nanotechnology in clothing—look for a post on his anti-counterfeiting research here soonish—but for Hinestroza, reducing water pollution from textile dyes is the ne plus ultra. His approach: prevention and filtration. In September, Hinestroza and his students published research on the latter in the journal Green Chemistry. They found that natural fibers made from local plants—well, local to Hinestroza’s home country of Colombia—can be “immersed in a solution of sodium permanganate and then treated with ultrasound; as a result, manganese oxide molecules grow in the tiny cellulose cavities [which naturally occur in fibers]. Manganese oxides in the fibers react with the dyes and break them down into non-colored forms.” It took only a few minutes to remove 99% of the dye chemicals from the water. (See above photo of indigo-infused water during treatment.) “This is the first evidence of the effectiveness of this simple technique,” Hinestroza told the Cornell Chronicle last month. “It uses water-based chemistry, and it is easily transferable to real-world situations.”

But prevention is a trickier problem, because, guys, we really like wearing jeans. For that, Hinestroza’s gang engineered something he calls “structural coloration,” meaning nanoparticles of color are built directly into cloth fibers. In other words, the same technology that enables natural fibers to absorb dyes and toxins in water can also make fabric dyes—and mordants—obsolete. It should be noted here that this nanotechnology hasn’t been tested thoroughly enough yet to say whether it’s entirely environmentally safe, but it’s certainly not as massively damaging to the environment and human health as the current dye problem. Structural coloration also has some bonuses for fashion lovers. “The colors can be richer and never fade because they’re not affected by UV rays,” said Hinestroza. Rad.

The future of structural coloration, though, actually comes from its past. The term was coined by Isaac Newton in reference to natural iridescence like that in peacock feathers and butterfly wings. It’s natural, adaptive camouflage. If you’ve read Chuck Klosterman’s The Visible Man you’ll recognize this technique. A suit, or any clothing, embedded with nanoparticles can refract light so that when someone looks at you they only see what is behind you. When I asked if that was like an invisibility, Hinestroza straight-up laughed at me. “No, I don’t call it that. It’s just sophisticated camouflage.” So anyway, get out your Marauder’s Map and plot the mischief you’ll make under your invisibility cloaks, everyone.

But that Harry-Potter-esque future (and the fancy name) of textile nanotechnology is proving the biggest barrier to cleaning up dye pollution. “There is a perceived high cost,” said Hinestroza, who pointed out at that moment that he is a scientist, not a businessman. “People are thinking of it like a fabric coating, but in reality nanoparticles are so small and the process is so relatively simple that I don’t think price will be a factor.”

Correction: This story and its headline have been updated to note that industry-grade indigo is generally synthetic and bound with mordant. Natural indigo is not. We’ve also clarified that some of the sources cited do not specifically mention indigo or any pollution resulting from it.

Read Greenpeace’s report

[Read more about Cornell University’s study]( waters)