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Making sukumo on a small scale

Japanese indigo (Persicaria tinctoria) is quite easy to grow. These plants want what many garden annuals do: rich, well drained soil, adequate water, full sun, moderate temperatures. With these aspects covered, and a little extra nutrition after harvest, they'll thrive and bounce back with vigor after frequent cuttings, even in planters. The real challenge comes once you've grown the plants and need to decide what to do with them. I like the way Graham Keegan outlines the options for obtaining indigo in this post: I Grew Some Indigo, Now What?!

For some time I've been interested in making a composted indigo product for use in a vat. It is my understanding, via Michel Garcia, that composting the dried leaves of various indigo-producing plants was likely common in antiquity among various cultures that used indigo, but that these processes may have gone out of practice when the more easily transported, more highly concentrated indigo pigment from aqueous (water) extractions made indigo a traded, highly valued commodity. I love getting the rich, pure indigo pigment from a water extraction, and this form makes much more sense for those interested in creating other art materials from their indigo--paints, inks, pastels, etc. However, the water extraction is more input intensive--it takes a lot of water to ferment the leaves and then rinse the pigment. According to this excellent Fibershed publication, the water extraction method is also significantly less efficient than a composting method (meaning less indigo is collected from the plants). The composting process is simpler, in that there are less inputs, and once the compost is ready, a vat can be made with just the composted indigo and lye water (made with hardwood ashes).

Where it all begins, Japanese indigo (Persicaria tinctoria) seedlings

However, this process does take a lot of time, a fair amount of physical labor, and results in a less predictable, very much living creature. (Anyone who knows me will likely recognize that these potential draw backs do not phase me and may be, in fact, part of the attraction.)

planting the indigo in a thick layer of leaf mulch
Indigo harvest

Sukumo is the name for traditional Japanese composted indigo and, since I grow Japanese indigo, this seemed like the logical place to start. When I first searched for information on making sukumo, however, I came up with very little. The traditional process seemed inaccessible, requiring at least 440 lbs of dried leaves to start a pile and 100 days to process. This Fibershed article, though super informative, doesn't address making sukumo on a smaller scale in detail. In his book Singing the Blues, John Marshall suggests making sukumo in a backyard composter. I was looking for a little more guidance before I put all of my homegrown indigo into the compost.

Since those first internet searches, I've been lucky enough to join artist Debra Jircik of Circle of Life Studios in a series of zoom classes she held in late 2021 about making sukumo on a small scale. Debbie traveled to Japan to study traditional indigo processes, which she writes about in this article, and the small scale sukumo process she taught us was based on that of Takayuki Ishii of Awonoyoh. Every 3-4 days for 60 days we joined Debbie virtually to observe the turning of the sukumo, observing temperature, texture, smell, and any other changes. This class was extremely helpful in developing an understanding of the process, but that first year I didn't quite have the amount of dried leaf material to begin making sukumo.

Finally, this past fall, I felt I had the time--and plenty of dried indigo leaves!--to attempt making sukumo. I began with nearly 50 lbs (22.7 kg) of dried leaves, collected over multiple seasons. Below photos show drying and processing indigo.

Japanese indigo leaves (still on stems) drying in greenhouse (ideally with shade cloth!)
leaves turn blue as they dry

On a sunny day (when the dry leaves are nice and crispy), the leaves are bundled up in canvas cloth and stomped. The crushed leaves fall away from the stems.

dried Japanese indigo leaves, these get stored in large burlap bags and hung in my attic

Setting up the sukumo:

I began by consolidating all the dried leaves into a large kiddie pool (I was shocked it all fit!). You can see there are still some small stem pieces in the mix (some of my dried leaf bags were better sifted than others). I'm not sure how this may have impacted the composting process, but I did continue to remove the stems (which decompose much slower than the leaves) every time I mixed the sukumo. I then added water (collected from a local spring), about 3.5 gallons (13.25 L) for my 50 lb (22.7 kg) of dried leaves. This is significantly less than the guidelines in The Way of Indigo book, which recommend roughly 1 gallon of water for every 7.3 lbs of dried leaves (I used roughly half that), but I felt like I was getting the consistency right (evenly damp, but not wet or dripping). Traditionally, sake is added to the sukumo. I chose to add my own cider, 750 mL which was also a bit less than recommended in the book.

Once this was all thoroughly mixed, it was packed into its composting chamber. I chose to use a large, old shipping box that I found in my basement, since I couldn't fit it into a big wooden barrel. A wooden container is ideal for the first 30 days of decomposition, as the mixture produces a lot of heat and will be too hot in something less breathable.

loading the moistened leaves into the container

The set up looked like this inside the box:

The leaves are wrapped in canvas and sandwiched between layers of straw and rice hulls on either side. The straw and rice hulls are used both to keep the heat consistent during fermentation, as well as assisting in drainage of moisture from the compost. I used a probe thermometer to keep an eye on the temperature every day. Once closed, the box was placed upright.

Once set up, I took temperature readings of the compost every day. During the first period of time, the sukumo should get hot quickly. The key is to keep the temperature as stable as possible over the 60 days; according to The Way of Indigo, the temperature should not fall below 40 °C. In order to do this, the sukumo should be transferred to a plastic container with some kind of heating element after 30 days, or when the temperature goes below 40 °C (104 °F). The sukumo is turned every 3 days for the first 30 days, then every 4 days for the next 30 days. During this time, additional water can be added (sparingly) to replace moisture lost from evaporation. Clumps should be broken up in an attempt to gain a more soil-like consistency. White mold is a good sign! The straw should be replaced every time the sukumo is turned (it will be wet). Each time I took out the straw, I let it dry out and used it to replace the wet straw the next time. The rice hulls were replaced after the first 30 days.

I pretty much kept to this process for 60 days. Occasionally, a turning day would fall when I was away, so I'd turn it the next day. I had to get a bit creative to keep up the heat for the last portion of the composting. Below I summarize some of my notes and observations during this process, in the hope that is will be informative to others!

Days 1- 15 (9/14-9/26):

low: 113°F (44.4 °C) high: 156 °C (68.8 °C)

For the first couple of weeks, the sukumo temperature rose dramatically. At some points I was concerned it was too hot (see the high temps, it was very steamy and created a lot of moisture and runoff). Every time I mixed the compost, I'd get a slight rise in temperature the next day. The leaves broke down very quickly!

While initially filling the whole box, the sukumo quickly decreased in volume, by day 30 the volume should halve

By the second turning (day 6), a thick layer of white mold had formed on the surface
This white mold is a good sign, and indicates a healthy decomposition process

As the sukumo heated up, it created a lot of steam, making the inside of the box completely wet. There was some leakage at the bottom as well.

One of the most dynamic aspects of my sukumo process was the smell. This is not something that I remember being as intense during the class with Debbie, though of course virtual sessions didn't leave an olfactory impression. The sukumo smelled quite sweet and grassy for the first couple of turnings, but by day 6 I'd already noted the building ammonia. By day 9, the ammonia fumes burned my eyes as I leaned over the steaming box, and by day 15 I began wearing a respirator and sometimes eye protection while turning the sukumo. I quickly realized that the basement was not an ideal place for this experiment! The heating of the sukumo created a lot of steam. I added some water to replace anything that felt super dry, but generally tried to keep the moisture even and moderate so as not to create too much clumping and avoid anaerobic fermentation.

Days 18-30 (9/29-10/11):

High temp: 59.9 °C (139 °F)

low temp: 33.6 °C (92.5 °F)

By this point the sukumo has broken down a lot, and is very dark, almost black with a little bit of white mold

During this period of time, the sukumo began losing heat pretty dramatically. On day 22 it was about 40 °C (104 °F), and I moved the setup into a plastic trash can to conserve the heat, keeping a layer of both straw and rice hulls on top and bottom. I wrapped an electric blanket around the trash can, but still had a really hard time keeping up the heat. The heat began dropping below 40 °C after I moved it into plastic. It hit 33.6 °C (92.5 °F) on day 25, so I moved the entire plastic trash can setup into a large 50 gallon barrel filled with water kept warm by a bucket warmer. This hot water bath worked to stabilize the temperature, though it tended to hover just below 40 °C for the rest of the process.

hot water bath setup

The sukumo smell remained ammonia-y, but also shifted a bit manure-y for a few days. This was concerning to me, but as the temperature continued to rise, the smell balanced. Again, do not attempt sukumo making inside! The sukumo began to stain the canvas a very dark, almost indigo color. This was exciting! The white mold remained, though just a sprinkling.

Volume has decreased a lot!

Days 34- 60 (10/15- 11/10)

High temp: 39.2 °C (102.6 °F)

low temp: 35.5 °C (95.9 °F)

During this period of time, the sukumo temperature generally raised very slowly until it almost got back up to 40 °C. I felt happy that it hovered just below 40 °C, and generally the fumes lessened. I kept the bucket heater on all the time heating the hot water bath. Generally the white mold remained in tiny spots, but had diminished a lot. The sukumo did not need additional water during this time. I continued to break up clumps when turning, and the consistency became more and more soil-like.

sukumo on day 60, very dark, even consistency, lost much volume

Once we reached day 60, I'll admit, I was relieved. Turning the sukumo was a labor intensive process (though I probably got it down to 20 minutes by the end), and a quite unpleasant one to operate in my basement. Future sukumo making will be kept strictly to a barn! Once the active composting process is complete, the sukumo still needs to continue composting slowly for 6 months. I packed the mass into a burlap bag which is currently hanging from the rafters in my basement until May (at which point it should be ready to make a vat)!

In the chart above you can see the change in sukumo temperature over the composting process. Blue dots indicate a day in which the sukumo was turned, red dots are normal days, the yellow dot is the day the sukumo was moved to a plastic trash can with electric blanket for heat, and the green dot is when the plastic trash can was moved into the hot water bath. You can see that the temperature usually rises slightly the day after it's turned, but not always.


Of course, the biggest question still remains: how will the sukumo vat dye? This question will be addressed in a few months when it has finished curing, and for now I'm left with some data and a bunch of photos of compost. Here are a few thoughts.

First of all, this process should not be attempted inside! I wonder if my sukumo ran too hot too quickly, creating volatile fumes and making it a challenge to maintain the heat in the second portion of the process. In that case, is there a way to moderate the temperature, perhaps by starting the pile in a cooler environment such as an outdoor pile covered with a cloth? It did not seem like the sukumo Debbie made during class was quite At least, they weren't wearing respirators!

This sukumo process, while technically less input-intensive than a water extraction, is more labor and time intensive. I know I will continue to do water extractions for indigo pigment, especially since I use the pigment in other art applications (paints, ink, etc), regardless of how much I like dyeing with the sukumo. There could be ways to make the process easier on my body: I could get help with turning the sukumo from a group of friends/community members, or maybe some part of the turning process could be mechanized.

I'm struck by the dynamic process of sukumo making, that even with my guides so much of my decisions felt intuitive, reacting to the ever-changing fermentation. The process felt so different from other ways of getting indigo from the plant. I felt very much like I was observing and caring for a living thing, much like I do when I make a fermentation vat. The results may be less predictable, less stable than indigo pigment, but the process was fully engaging and fascinating. And, of course, I'm eagerly awaiting to see how this creature dyes!

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