Nature’s magic is in full force when we watch plant-based indigo turn from clear yellow to light green to a spectrum of teals to deep blue within seconds after taking a textile out of an indigo vat. This year I am taking a dive deep into the science and art of indigo and other natural dyes by taking three intensive workshops with the Maiwa School of Textiles. After learning that over 99% of indigo used today is synthetic and derived from fossil fuels and discovering the huge carbon footprint of fast fashion, I decided to focus on the science of plant-based indigo. In this post, I share what I’ve learned about the ecology and science behind plant-based indigo.
Jump to:
- The ecology behind why I switched from synthetic to plant-based indigo
- From pee to persimmons: the diversity of indigo reducers
- The chemistry behind plant-based indigo dyeing
The ecology behind why I switched from synthetic to plant-based indigo: from the blue in my jeans to the blue in my genes
My first forays into indigo dyeing used a quick, simple, synthetic chemical-based $12 indigo dye kit, which you can easily find online and in art stores. I still think this kit is a great inexpensive way to try out indigo dyeing and build a vat quickly for an indigo dyeing party or workshop. These synthetic chemical-based vats reduce indigo super-fast for same-day dyeing with dark blues but can also crash (stop dyeing) sooner than fruit vats. Chemical indigo reducers such as sodium hydrosulfite and thiourea dioxide also release sulfur dioxide, which smells awful, like rotten eggs, and can lead to health problems. Make sure to use these synthetic chemical vats outdoors or in well-ventilated spaces and wear a mask and gloves when handling the ingredients.
In contrast, fruit-based organic indigo vats smell like cooked fruit (much better than the farty, rotten-egg smell of sulfur!), take a week to really get going and require much *much* more time, patience and TLC… but can last for multiple years. It’s a long-term investment for myself, art and the earth. Plus, because plant sugars break down much more slowly to reduce the indigo, you get a wider range of lighter to medium blues. I decided that for my health and sustainability of indigo dyeing, I would commit to making and maintaining a plant-based indigo vat, like my Hakka ancestors did. This is an extra challenge for me as I learn to shed hustle culture and my hyper-drive productivity mindset. But the direction is clear: Slow is the way to go.
On top of the health and olfactory benefits, there are strong ecological reasons to use plant-based indigo and ingredients. Did you know that synthetic indigo is made from crude oil? Yep, over 99% of the denim we wear today is made with indigo synthesized from fossil fuels mined from our planet that took millions of years to form. You can see the chemistry of how crude oil is turned into synthetic indigo using the Pfleger process on page 7 of this very interesting “True Blue” series booklet (from Fibershed: Indigo: Sources, processes and possibilities for bioregional blue (PDF). You can read more about the fascinating and fraught history of indigo in my previous post.
Again in stark contrast to synthetic indigo, plant-based indigo is not only easily regenerative, indigofera plants species are legumes, which fix atmospheric nitrogen to enrich the soil between other crops. Growing indigo in rotation can help farmers diversify soil nutrition, ward off pests, and also produce another cash crop. Our ancestors knew how to do this, but in our craze to produce more faster in the past century, we lost our way and now make fast fashion clothes from fossil fuels that are barely worn more than once. There are now some attempts in the fashion world to use more plant-based denim, and to make the textile industry more sustainable, we need to do more of this and use what we have longer.
If you can afford it, there are plant-based indigo kits available. Here are some suppliers I know of: Maiwa (I have the bigger version of this kit), Kristin Arzt (I’ve gifted this kit), Botanical Colors (I use their indigo), and Stony Creek Colors grows and processes their indigo in the US (I haven’t tried their indigo but plan to).
From pee to persimmons: the diversity of indigo reducers
Indigo requires a reduction and oxidation reaction for the color to dissolve in water and stick to fabrics. Reduction can be achieved quickly and strongly with synthetic chemicals, moderately slowly with minerals or urine (!!), or much more slowly with plant fermentation. Oxidation occurs when the reduced indigo contacts oxygen.
I imagine that thousands of years ago, our ancestors noticed that when the leaves of certain wild plants fell into pools of urine in their hot tropical climates, the resulting stinky plant stew produced a clear yellow-green that magically transformed into a lovely deep blue color, which they then tried to use as a dye. These ancient scientists may have deduced through trial and error that the plant needed to be fermented (reduced) and then oxidized to stay on the fibers. Urine is a good ecological choice for a reducer: abundantly available and fermenting into ammonia/urea through naturally-occurring environmental bacteria to create the alkaline and low-oxygen environment indigo needs to reduce. Urine was often used in European and North African woad indigo vats before the introduction of synthetic reducers. But, boy, it gets stinky. Apparently indigo workers in India had to wear masks to tolerate working with traditionally urine-reduced indigo vats.
For you paleo dye-hards out there, here are a couple of practical (and hilarious) DIY guides on making your very own indigo urine vat: Wild Colours Indigo Urine Vat and Kevin Dunn’s Caveman Chemistry.
Perhaps some indigo producers, turned off by the stench, experimented with different ways to create a “cake” of dye from fermented plants and re-reduce it using locally available plants to dye textiles later in water-based vats. Such experiments found that the carbohydrates in plants such as wheat bran, used in East Asian indigo vats, and fructose sugars in overripe and dried fruit to be effective (and less smelly) reducers for indigo dye vats. Modern-day natural dye chemist and artist Michel Garcia has experimented with fruit-based reducers and has inspired much wider use of organic fruit-based indigo vats.
The chemistry behind plant-based indigo dyeing
So, how does plant-based reduction work?
This infographic, from a scientific review on traditional Japanese plant-based indigo fermentation, illustrates the following chemical processes that transform indigo plants into textile dye:
(1) Plants containing indican, a colorless compound and precursor to indigo, are fermented or undergo enzymatic action with β-D-glucosidase to split into β-D-glucose and indoxyl.
(2) Oxidative degradation: The indoxyl is oxidized by exposure to air to form indigotin (Japanese sukumo in the image), which is the indigo dye that we typically purchase in cake or powder form.
(3) Fermentation/reduction: Indigo needs to be reduced to leucoindigo (“white indigo”) to become soluble in water and absorbed by fibers. The reduction process breaks the double bond to oxygen by adding a hydrogen electron. A high pH alkaline environment is necessary to stabilize the reducing agent and keep leucoindigo soluble. Traditional plant-based indigo vats use straw or wood ash to create an alkaline environment and organic matter (roots, leaves and fruits) as electron-donating reducers.
Learn more about plant-based indigo dye making in these excellent resources:
- See small-scale plant-based indigo dye making in Thailand (photos)
- See large-scale plant-based indigo dye making in India in Maiwa’s World of Blue documentary
(4) Oxidation: To transform the leucoindigo now absorbed by fibers back to indigo blue, the dyed fibers are exposed to the oxygen in air. The oxidation process traps the indigo mechanically into the fiber’s structure. That’s why indigo gradually gets lighter with washing and wearing, like in denim jean fading. Longer dips and more dips trap more indigo molecules, making longer-lasting color.
My video at the top of this post shows how the leucoindigo in a vat reduced using citrus peels transforms a white piece of linen into yellow, then green, then teal and finally deep blue indigo. This is the science behind indigo’s magic!
In my next post, I will focus on how I’ve built and maintained an indigo vat with plant-based indigo and fruit reducers from my family’s persimmon and citrus tree discards.