So you're growing Japanese indigo (or other indigo-producing plants) in your garden this year. You planted the seeds, cared for the plants... maybe you have even harvested some leaves and done a fresh indigo dye bath. But what do you do if you want to have homegrown botanic blue at your fingertips in the months of winter?
If you are familiar with traditional Japanese indigo processing, you may have heard that that the making of sukumo (composted indigo) generally requires 300 to 450lbs of dry indigo leaves, a composting floor, and three and a half months. This method (a variation of which is used in traditional European woad production), produces a precious, concentrated pigment in the form of compost, which can be stored indefinitely and added to fermentation vats throughout the year.
Making a hot compost pile in the traditional Japanese fashion reportedly requires growing, harvesting, and drying around 5,000 indigo plants... which is not really possible for those of us who grow indigo on a garden scale.
I grow Polygonum tinctorium in three places: my home garden, my parent's garden, and the garden where I work. Combined, this now amounts to between 200 and 500 plants, depending on the year. In order to gather enough leaves to make my own compost pile, I would need to harvest, dry, and store my indigo crop every year for the next several decades... that is, unless I move to somewhere where I can start the farm of my dreams! Anyway, you get the point...
While I would like to dispel the myth that this is the only way to make sukumo (it is actually possible to compost smaller quantities using a slightly different method), I prefer to use the process described below, which is straightforward and produces isolated indigo pigment instead of compost. Once dried, this pigment can be used in traditional indigo vats, or added to soaps, botanical inks, etc throughout the year.
Below I will show you how to isolate indigo pigment from your plants for storage and future use. The method I use is one adapted in various forms around the world--Southeast Asia, India, Central America, the Southern US... on a large scale, it is most appropriate for tropical climates (where hot weather aids in large vat fermentation), but on a small scale it can be used by anyone who has a sunny spot where summer temperatures rise above 70 degrees (F). Any species of indigo-producing plant can be used.
The following process is broken down into five basic steps:
5. Strain & Store
Tip: if you have only a few plants to work with, I would recommend using the Indigo Summer Dye Recipe I have put together, substituting hydrated lime for baking soda (see this post for more information on choosing alkaline agents). To save your pigment rather than do a dye bath, stop after aerating the bath and allow the pigment to settle... then you can follow steps 4 & 5 listed below.
What you will need:
-A large plastic bucket, storage tub, or livestock tank (preferably dark in color). For this demonstration I have used a 36-gallon, dark blue storage container.
-Freshly harvested indigo plants--enough to pack into your container with some space to spare at the top. Here my container is filled with the stems of seven healthy Polygonum plants, which were cut 4-6" from the base to allow for regrowth and a second harvest.
-Non-chlorinated water (this is a fermentation process, which can be negatively affected by chlorinated water!)
-Slaked lime (calcium hydroxide, also known as pickling lime)
-Weights and galvanized fencing, or some other way to hold the plants beneath the surface as they ferment.
-A tarp, lid, or clear plastic sheeting to cover
-A sunny spot (or greenhouse) and several warm, consecutive days.
-Citric acid (for neutralizing leftover lime water)
-Extra buckets for decanting and storing water
-A sturdy leaf rake that fits inside your container (if you are using a larger one)
Tip #1: if you want to keep track of pigment yield from year to year, consider weighing and recording the plants you harvest before starting.
Tip #2: if you live in a cooler climate, or are doing this process in the fall when temperatures are lower, consider using space in a greenhouse. Another trick is to lay out a long hose in the sun and pressurize it, then fill your fermentation container in the afternoon with the hot hose water to give fermentation a jump... then cover the container with a sheet of heavy-duty clear plastic to help it heat up during the day.
Tip # 3: while the first few days of this fermentation process produce a lovely, almost floral smell, things inevitably degenerate from there... as you may well know, there are a wide range of smells associated with the various stages of indigo processing and dyeing, and while us indigo lovers may come to adore them, our family members may not! Be forewarned that this process might need to be done entirely outside, away from the general public 🙂
Pack the plants (stems and all) tightly down into your bucket or tub and fill the container with water to cover the plants. Cut a piece of galvanized fencing to fit in your container, and place weights (ie clean bricks or large rocks) on top to keep the plants below the surface of the water. Here my piece of fencing was just large enough that I had to bend the corners in slightly to fit it in the tub--and the tight squeeze provided enough pressure to hold the plants under water.
Cover the container with a lid or tarp, and let it sit in a warm, sunny spot for 3-5 days. After a few days, the liquid will take on an almost neon cast. During this time, if any stems rise above the surface, tuck them back down beneath the fencing. When the water looks like antifreeze, it is ready for the next step (see photo).
Remove the plant matter, which can now be disposed of on the compost pile. The leaves should be mostly brown and a bit slimy by now--if a large portion of them are still bright green, you can move the plants to another container with fresh water and allow them to ferment a second time to extract additional pigment.*
*According to the research papers I have come across, indoxyl (indigo pigment's precursor)** is fairly unstable at this point, and it is better not to let it ferment too long or it will begin to break down... therefore, if the weather and water were not warm enough to fully break down the leaves within the initial 3-5 day period, I prefer to do a second ferment--this usually takes only a few days more, and insures I maximize pigment harvest).
**Fresh indigo leaves contain indican, which is broken down into indoxyl (indigo pigment's precursor) during the fermentation process. The bright-colored fermented liquid you have contains indoxyl... in order to turn it into indigo pigment (aka indigotin), you will first need to alkalize, then aerate the bath.
(See this post for more information on choosing alkaline agents)
After discarding the plant material, remove some of the liquid with a small bucket. Add approx 1Tbsp slaked lime per gallon of water to this bucket. My 36-gallon tub was about half-full after removing the plant material, so I added 1 cup + 2Tbs (18Tbs total) slaked lime powder. Stir vigorously to dissolve, then stir this liquid back into your large container.
The pH of the entire bath should now be between 9 and 11.
Use caution when adding slaked lime to water, as the solution will be highly alkaline. Avoid splashing the liquid, and if it gets on your hands or skin rinse immediately with fresh water.
Depending on the size of your container and how full it is, you can approach oxidation in several ways:
-pour the liquid between two containers of the same size (great for gallon jar or bucket-scale experiments)
-use a broom or rake to vigorously swish the water around (my favorite for larger tubs and livestock tanks)
-use a smaller bucket to pick up buckets of water and pour them back into the bath (may work for you, but I do a lot of bending and lifting already at work, so I prefer the rake method to save my back!).
-get creative... The point here is to introduce as much air as possible into the liquid, as oxygen will turn the indoxyl chemical compound into indigo.
In India, where this method is employed in large-scale production, they use a series of giant cement pools in which groups of workers physically kick the water around to aerate it. For more on this, I would recommend taking a few minutes to watch this video...
...though I don't know that I would recommend doing it that way, given the high alkalinity of the water.
You will notice as you work that the foam you create will turn blue with exposure to the air, and gradually you may see blue (or teal-colored) pigment precipitating out of the solution-- Very exciting! Focused agitation for 20-30 minutes should be plenty, unless you are working at a larger scale (the large cement tanks used in India are reportedly agitated for around two hours). A large livestock tank can be worked by several people at once using rakes to make things go faster.
Stop once the liquid turns reddish brown... you will need to stop agitating and let the pigment settle for a moment or two to check this (see photo). At this point the foam may have stopped turning blue, or disappeared altogether.
Now it is time to wait... after a few hours, the indigo pigment will have settled to the bottom and you can carefully decant the clear brownish liquid.
Save this liquid so you can properly dispose of it! Remember that it is highly alkaline, and it is very important to neutralize it with citric acid before dumping it outside or in a septic system. If you live in an area where drought is an issue, neutralized water can be saved and reused to ferment another round of fresh indigo, or for making indigo dye vats (if you are going to make an indigo vat out of it, there is no need to neutralize the water--it is already the perfect pH!) .
Be careful not to disturb the sediment at the bottom as you work--as soon as you see the pigment stirring up, take a break and walk away for a bit to let it settle back down before continuing to decant. As most of the liquid is decanted off, the slurry can be transferred into smaller buckets or jars and allowed to settle (this makes it easier to continue decanting liquid). If you like, a simple siphon can be made to make this process go faster.
Strain and Store
Once you have removed most of the liquid, it is time to strain the indigo pigment to remove bits of leaf and debris. Using a fine mesh colander/strainer, carefully transfer the pigment slurry (which may be more of a sludge at this point, depending on how meticulous you were in decanting) through the strainer. Pour some clear water through the leaf bits in the strainer at the end to wash any remaining pigment from the strainer.
Once this is done, I like to take the strained pigment and add it to a 5-gallon bucket full of fresh, hot water, stirring vigorously and then letting it settle and decanting one last time to dissolve bits of lime that remain undissolved... if you skip this step, that's fine--your dried pigment may be lighter blue in color (due to the presence of residual lime, and perhaps some disintegrated plant matter, neither will have a negative impact on your future vats).
Once strained, the liquid pigment can be stored in glass jars for up to a year, or dried in the sun for indefinite storage. Your pigment can now be used in future dye baths, homemade soaps, etc... congratulations!
PS. For drying, I have made a square frame out of wood, and stapled hardware cloth to one side. Placing the frame mesh-side down over a basin, I lay 2 wet pieces of muslin inside the frame, large enough that the edges flow up and over the sides. On top of this I lay a piece of wet 8mm habotai silk of the same size. The muslin protects the silk from the hardware cloth, and the silk catches the indigo pigment while allowing any remaining water to filter through into the basin. The pigment can be left on the silk in the sun to dry. At this point it is still pretty stinky, so you may also want to place a window screen or something similar over the top of the frame to keep bugs from drowning as the water filters out! When it is all dry, it can be peeled away from the silk and stored in chunks or ground into powder.
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