Indigo Pigment Extraction

The magic of indigo never gets old! It bogles the mind when I think about how the first humans figured out how to extract a blue dye from a green plant. What’s also amazing is that indigo comes from so many different plants! It’s the exact same chemical molecule in each different plant family and variety. Three common plants used by dyers include: Indigofera tinctoria (originally from India), Persicaria tinctoria (Japanese indigo), and Isatis tinctoria (woad). 

 
Indigofera tinctoria (originally from India)

  
Persicaria tinctoria (Japanese indigo)

 
Isatis tinctoria (woad)

As explained in earlier blog posts, I grow the Japanese variety (the 2nd image is in my garden with some extracted indigo pigment.

You Gotta Want It!
Growing indigo is easy – it’s a pretty hardy plant, if you get the species and variety suited to your growing zone. It’s the extraction and then making the vat that makes indigo soluble such a challenge (more on that next week).

There are many different ways to extract indigo from the plant. I use an aqueous extraction method (which is fancy-pants talk for using water!). Here’s the process:

1. Harvest the leaves and stems - I cut the plants down to a couple inches above the ground. At this stage, the plant contains indican, which is a soluble precursor to the indigo dye we want to use.

2. Soak the leaves and stems in water until they ferment. I use geodes (plentiful in Southern Indiana) and a baking rack (thrift store score!) to weigh down the leaves so they stay under the water. It takes 1-5 days to ferment, depending on the temperature outside and the amount of sun. During the hot summer months, usually it takes a day. In the fall, it can take up to 5 days, especially if it gets cold at night.

You know when it's done when you reach this crazy color that seems like it shouldn't occur in nature. Some call it anti-freeze green, others call it mermaid green. Apparently not everyone's anti-freeze is green. Who knew?

3. Once fermented, empty all the leaves out and strain the juice into another container. I strain several times throughout the process, but this one gets most of the big bits out. I use tulle or mosquito netting. 

Here's all the pretty fermented juice. At this stage we have indoxyl (still not indigo).

4. The next step is to aerate and increase the pH. This will cause the pigment to solidify and then precipitate to the bottom of the container. 

I use a paint stirrer attachment for my drill and pickling lime (calcium hydroxide). Another option is to pour the liquid from one bucket to another. As I'm using 35 gallon trash cans, I opted for the paint mixer. This process is not for the faint of heart. It takes about 20 minutes of aerating to get to where you want to be.

The liquid will turn a deep dark blue! From here, I let it rest until the pigment drops to the bottom. The calcium hydroxide does double-duty here - it raises the pH and acts as a flocculant - something that causes the molecules in a liquid to bond together. This makes the particles heavy enough to sink to the bottom so we can collect them and use them for dye!

Here's one of my tester jars after the pigment has precipitated to the bottom. You can just see a thin layer of the blue particles at the bottom.

Once the pigment has settled, I can start straining the liquid off the top. In the trash can, I use a bucket to bail out the liquid. It goes into my madder plants, which love a high pH soil. 

When the liquid gets low enough in the trash can, I will strain the rest. Here's one of my early systems - a couple layers of fabric clamped onto the side of a giant plastic tub. The particles are super fine (and not just pretty, but really really small), and it was difficult to make sure I captured the pigment without any slipping through.

After a couple years of trying various types of cloth for straining, I learned about these strainers (25 micron size) that fit right inside a 5-gallon bucket! Amazing inventions! Look at all that gorgeous pigment!!!

At this point, I give the indigo a really good rinse and strain. To do this, I clamp on some curtain sheer material to a bucket, then I scoop all the blue goodness onto it, and rinse with a garden hose. The pressure pushes the indigo through the fabric while leaving behind all the other impurities, like plant matter and sand and other crud.

Rinse and repeat, as the saying goes! I don't actually repeat the rinse part, but I do repeat the straining process. Once the pigment has settled to the bottom of the new bucket, I'll run it through the 25 micron strainer to recapture the particles. 

Once it gets to paste or pudding consistency, I transfer it to a jar and store in the fridge until I'm ready to use it in a vat. The other option is to spread a thin layer of the paste onto a cookie sheet or other smooth, flat surface, and let it dry into a powder. 

The dried powder will last indefinitely. I grind up the big chunks before using them in a vat. This helps the process along.

There you have it! The simple process of getting indigo pigment out of indigo plants! It is actually quite simple; it just takes time and patience and a fair amount of labor. 

If you want to try this yourself, check out Fiber Shed's most excellent PDF on extracting pigment. They describe both this aqueous method and also a composting method, which I plan to try this summer!

Next time I'll share how to get to some beautifully dyed fibers from the paste and powder I extracted from the plants.

Until then, I wish you abundant love and colors!