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Step-by-Step Indigo Pigment Extraction

The full indigo pigment extraction process using Japanese indigo. It begins with freshly harvested indigo leaves that you ferment in water. Next, you extract the blue pigment from the liquid by increasing the pH and oxygen levels. At the end of the process, you're left with a vibrant blue slurry that is dried out and then ground into powder. All in all, the process can take several weeks to complete, and each attempt may take a different period of time. However, your patience pays off with valuable blue indigo dye at the end. Also, you can scale this method up or down in size, depending on how much indigo plant material you have. Instead of a bucket, you could use a quart-sized jar for a smaller batch.
Prep Time 1 hour
Cook Time 5 hours
Waiting time 20 days
Total Time 20 days 6 hours
Servings 20 g
Author Tanya Anderson
Cost $20

Equipment

Ingredients

Instructions

Fermenting the Indigo Leaves

  • Begin with harvesting enough indigo to fit halfway to three-quarters the way in your clear bucket. Cram it all, leaves and stems inside. Indigo dye is only in the leaves but it takes time to strip each stem so you don't have to bother.
  • Take the bucket outside and go through it all, making sure that you remove any insects, other creatures, and any obvious impurities. You can rinse the leaves, too, but it's not necessary. In the photo I show an immature moth that I rescued.
  • Pack the indigo leaves back in the bucket and weigh them down with something clean and heavy. I used two clean bricks, but large stones work well, too.
  • Fill the bucket with clean, cold water¹, covering the tops of the leaves by at least an inch. If you have heavy or highly chlorinated water in your region, it would be best to use rainwater.
  • Cover the bucket with a lid, and leave it in a warm place for several days.

Indoxyl Solution

  • After three days, kept at room temperature (20C/68F) or above, have a look inside the bucket. You're trying to ferment the leaves just enough² to give you a beautiful indoxyl solution, also called mermaid liquid. The water that you added to the indigo leaves should now have a stunning coppery sheen on the surface with vivid turquoise-green (some say antifreeze color) liquid underneath. The fermented leaves and liquid should have bubbles around the edges and smell faintly to sickly sweet.
  • When you've achieved this, take the brick(s) and leaves out of the liquid, squeezing all of the liquid from the leaves. Gloves are optional and the leaves can be composted.
  • Next, strain the liquid through a fine mesh sieve a few times to get any small pieces of leaves or debris out.
  • The clean indoxyl solution needs to be aerated now³. Pour it from one bucket to the next twenty or more times⁵. Try to pour from as high as you can without spilling. This adds more force and more air to the liquid.
  • You'll see the color become darker during this time to a greeny-teal with pale green froth or foam.

Adding the Alkali

  • Next, add about three teaspoons of pickling lime to the bucket. It's also known by its formula name, calcium hydroxide, and is a safe chemical made from limestone. Stir the mixture well with a clean stick or other implement.
  • When you think the pickling lime is dissolved, take the solution's pH. You can use a clean pH strip or a pH meter. You're trying to get the solution up to pH 9 to 10, and if it's lower than that, add another teaspoon of pickling lime and repeat.

Aerating the Indoxyl Solution

  • The indoxyl solution now needs more aeration to help the indoxyl to convert to indigotin, the blue dye pigment that we're aiming for. So that means more pouring of the liquid back and forth between buckets. Do it twenty or more times.
  • You stop when the liquid is deep blue in color and the froth on the top is thick and blue, too. The more you pour and aerate, the more indigo pigment you're likely to get.

Settling the Indigo Pigment

  • Put the lid on again and leave the liquid to settle in a safe place where it won't be disturbed. Now it's time to wait.
  • You can move to the next step when a thin dark layer settles to the bottom of the bucket and the liquid above is a clear tea color. Japanese indigo is infamous for taking more time to settle than true indigo, so you'll need to be patient. It can take a week or sometimes much longer.

Siphoning

  • The aim now is to isolate the dark layer of indigo pigment and remove the liquid and any crusty bits that form at the surface (if present) from the bucket. I think the best way to do this, when using a safe alkali such as carbon hydroxide, is to siphon it off with a small winemaking hose. Siphon it into a second bucket, trying not to disturb the indigo layer at the bottom, and pour the liquid somewhere outside⁶. See notes below on this.
  • You are left with a dark layer of indigo sludge at the bottom of the bucket, along with a little tea-colored water above.

Cleaning the Indigo Pigment

  • Although optional⁷, you can clean the indigo pigment now. I think this is best done by using a clear container that's smaller than the bucket now, which is why I've recommended a clear pitcher or jug in the materials list.
  • Fill the jug halfway with clean water, then gently pour it into the bucket containing the indigo sludge. Gently swirl it around to pick up the indigo, then pour the dark liquid back into the smaller container. Cover so that nothing can fall in.
  • Leave the container to settle again, before siphoning off the liquid again. Japanese indigo takes a long time to settle, another several days to a week or more, so it may be that the liquid that you siphon off may be green before you continue to the next step. Siphon it off and leave it to settle on it's own and you'll eventually get a small amount of indigo pigment at the bottom.

Drying the Indigo Pigment

  • After the last siphoning of liquid from the sludge of indigo, it's time to dry it out for storage⁸. Line a dish or tray with raised edges with greaseproof paper. Basically waxed kitchen paper that won't allow water through.
  • Pour the indigo sludge onto the paper. Use a rubber spatula to get as much as possible from the container. If you've reserved the liquid siphoned off the indigo previously, please read the note below⁹.
  • Leave the indigo sludge to dry out in a warm place until its bone dry. It takes about a week in my warm greenhouse and is finished when there is no tackiness and the indigo is pulling away from the paper.

Storing the Indigo Pigment

  • With a spoon, gently loosen the ring of indigo that formed around the edge of where the sludge was poured.
  • Then, fold the paper up, remove it from the dish, and gently roll over it several times with a rolling pin to break up and crush the indigo.
  • Pass the indigo through a fine metal sieve over a bowl, to create a fine powder. Use a spoon to push and grind it through.
  • Then, pour the powder from the paper and into a jar for storage. The powder keeps indefinitely, but its dye potential is said to decrease after five years.

Video

Notes

1. There's also a warm water method, which can speed up the process drastically. These instructions don't cover that, though.
2. If you think you might be almost there but not all the way, don't wait any longer. Over-fermented Japanese indigo will give less than a yield and is unpleasant to work with. You'll know it's over-fermented if the liquid looks or feels slimy, is green rather than blue, and/or smells terrible. If the liquid stinks, then it's over-fermented.
3. Calcium hydroxide, as an alkali, needs carbonic acid to help convert indican into indigotin. Carbonic acid is added to the liquid through adding air to it.
4. Plumping it with carbonic acid helps with indigo extraction later, when using calcium hydroxide as an alkali.
5. For really large batches, some people use electric mixers or a device that pushes bubbles of air into the liquid.
6. The liquid will be very alkaline, so you can either neutralize it by adding vinegar or citric acid or dilute it with a lot more water before pouring it on the soil. If you have plants that like alkaline soil, you could pour the undiluted solution around them.
7. Cleaning indigo helps to remove remaining calcium hydroxide (and calcium carbonate) from the indigo slurry. It helps make the indigo purer and potentially of better quality for soapmaking and other applications.
8. We dry the indigo out in these instructions because it's easier and helps maximize yield. It does take more time than filtering it, though.
9. After pouring the indigo sludge on the greaseproof paper, rinse the rubber spatula off in the liquid you previously reserved from siphoning. Then pour that liquid into the jug that just held the indigo sludge. This will help you settle that indigo pigment out and maximizes your yield.