Sukumo indigo holds a special place as a time-honored dyeing technique. Derived from the leaves of the Polygonum tinctorium plant, Sukumo indigo has been employed for centuries to produce rich, vibrant hues that adorn textiles rich with history and culture.
Sukumo indigo has deep roots in Japanese culture. The technique is closely associated with Awa, the fertile region in Tokushima Prefecture, where indigo cultivation and dyeing have been practiced for generations. The name "Sukumo" すくも - sukumo - means dye made of fermented indigo leaves. Aizome 藍染め means Indigo Dye.
The Indigo Plant
Polygonum tinctorium, commonly known as Japanese indigo, Persicaria, or dyer's knotweed, is the plant used for Sukumo. It is a plant species native to East Asia, particularly Japan and China. Still, from my experience, it grows very well almost everywhere, even on my extremely hot balcony, or in moderate rainy Dutch backyards. In France, there are some commercial crops grown in the southern regions.
In the picture, you see the blue indican pigment in the dried leaves.
The Sukumo process
In Japan, the Sukumo is made in the Tokushima prefecture by expert craftsmen.
The creation of Sukumo indigo involves a labor-intensive process that demands patience and expertise. The journey begins with the cultivation of indigo plants, which are harvested and left to ferment in a process called "sukumo-zukuri."
The process begins early September by carefully separating the leaves from the stems, and finely chopping and drying them. These leaves are then laid out in layers (about 1 meter thick) on an earthen floor, undergoing meticulous composting and fermentation. This generates ammonia with a distinctive odor at a temperature of 70°C. The leaves are watered, turned every five days, and exposed to air, for a period lasting over 100 days during the autumn and winter
The Sukumo becomes black, and then gets covered by a white mold, a sign the Sukumo turned mature.
The Sukumo gets packed and stored in straw bags that retain the moisture of the indigo well. The Sukumo that you receive at your home should be moist, earthy and smelling faintly of ammonia. Some of your Sukumo may still contain bits of mold, and that's a sign of great quality 'Ai'.
The Sukumo dye vat
For the dye vat, the sukumo is combined with water, lye, and aeration in a process called "hana-ai," resulting in the creation of a dye bath known as "sukumo-ai."
The dyeing process itself is an art form, requiring skilled artisans to dip fabrics into the sukumo-ai repeatedly. Each immersion allows the fabric to absorb the indigo, and the exposure to air causes the characteristic oxidation that turns the material into the deep, iconic blue associated with indigo.
Importance of water
Our tap water has generous levels of chlorine, meant to kill bacteria. Since we want bacteria to start our fermentation process, we need to either use rainwater (hard to come by technically for me) or process your tap water as follows: Boil the water for at least ten minutes and leave it standing for another 24 hours in an open vessel.
Finding a Sukumo vat recipe
Preparing the natural indigo dye vat is called Ai-date. Like woad balls, the Sukumo contains bacteria and enzymes to help reduce the vat. Also like woad balls, the Sukumo itself does not have a high concentration of indigotin and is often used as a 'starter' in combination with regular indigo paste or powder.
The bacteria found in woad balls is called Clostridium isatidis. The bacteria in Sukumo vats are of the genus Halomonas and Amphibacillus
The more I have been diving into the recipes available for Sukumo vats, the more similarities I have found with traditional 18th-century woad-indigo vats of the south of France, many of them including bran, lye, and madder roots for good measure. In Japanese, the vats that use bacterial/fermentation reduction are called Hakkō-date (発酵建て). (A regular vat using sodium hydrosulfite is called Kagaku-date).
One traditional indigo vat starter is done as follows:
In a container, mix Sukumo with a mixture of wood ash and water, (prepared at a temperature of 80ºC). The result is a clay-like substance that is then well-kneaded, combined with a small amount of Japanese rice wine, and left to settle overnight at room temperature.
The next day, more hot wood ash extract is incorporated, constituting up to one-third of the final volume. In this mix, reduction of indigo takes place, and more fermentation liquid is introduced, comprising up to two-thirds of the final volume. The fermentation mixture with reduced indigo is maintained at a pH10–11 by incorporating lime hydrate (Ca(OH)2) and stirring once daily. Test the reduction with small pieces of cotton cloth.
For me, this is way too complicated so I start with the recipe below:
This is a recipe that you can scale according to the size of your vat.
1 liter hot water (70ºC)
100 grams Sukumo
50 grams of wood ash (ashes are from the Japanese blue oak, Japanese zelkova, or sawtooth oak)
In a sizable container, combine your Sukumo with your oak tree ash.
Add 1 liters of hot water (70-80°C) to the mixture and use a stick to stir for 5 minutes.
During this process, the liquid will foam up, and its color will transition to brown.
Continue stirring the liquid daily for a week, and over time, the liquid will gradually shift towards a blue hue. Check your pH, it should be around 10.
Once you observe large bubbles forming on the liquid's surface, it is ready for use in dyeing. The prepared liquid maintains its efficacy for up to six months, provided you stir regularly.
Second vat:
It's the same recipe with an addition of 5 grams of indigo powder from India.
(to check if I can combine the indigo powder and use the Sukumo as the reducing agent).
I will follow up in a next blog with detailed steps and pictures of my vat experimenting.
For your viewing pleasure: this video with some perfect Japanese indigo eye candy.