When most people hear the words “indigo dyeing,” they imagine a simple process — fabric submerged in a vat of blue dye. In practice, what happens inside an indigo vat is closer to microbial fermentation than to dyeing. And it is precisely here that Japanese selvedge holds a kind of scarcity that cannot be transplanted.
What Sukumo Is
Indigo dyeing begins with the leaves of Polygonum tinctorium, the indigo plant. Harvested and dried, these leaves are fermented and matured for roughly three months. The result is sukumo — the foundational raw material of traditional Japanese indigo dyeing. The honbai (authentic indigo) of Tokushima Prefecture (formerly Awa Province) is the most internationally recognized.
The leaves of Polygonum tinctorium contain a colorless precursor called indican. Through fermentation, indican undergoes hydrolysis and oxidation, converting into indigo — the blue pigment. Put another way, “blue” is already being born during the drying and fermentation of the leaves themselves.
What Happens Inside the Indigo Vat
Sukumo cannot dye anything if simply dissolved in water. Indigo is insoluble in water; it cannot penetrate fabric fibers in this state. To dye, indigo must be reduced to a water-soluble form known as the leuco form.
In a traditional indigo vat, this reduction is achieved through microbial activity. Sukumo, lye (alkaline water made from wood ash), slaked lime, wheat bran (or starch), and sometimes sake are combined and “raised” over roughly ten days. During this period, specialized bacteria — capable of operating in the alkaline environment — go to work inside the vat.
Recent microbiological research has identified the principal genera as Amphibacillus and Alkalibacterium. These bacteria break down indigestible carbohydrates from the bran or starch, producing lactic acid. Further, electron transport from cell-surface proteins — mediated by carrier molecules such as anthraquinones — passes electrons to indigo particles. This is the biochemical mechanism of reduction (leuco-formation).
The indigo vat is therefore not merely dyeing equipment; it is a living microbial community. If fermentation stops, the dye does not take.
Why the Microbes Cannot Be Transplanted
Here is the crux. Sukumo, lye, slaked lime, and bran can be procured anywhere in the world as raw materials. Recipes for making lye can be passed down. Mixing ratios can be measured.
But the specific microbial community that lives stably inside any given vat cannot be replicated by recipe alone. Climate, water quality, ambient airborne flora, the artisan’s manual work (frequency of stirring, temperature management) — all of these combine to form a unique ecosystem in each vat. It is no accident that the world’s most detailed microbiological research on sukumo fermentation is conducted in Japan: the diversity exists in the field, in a complexity researchers find almost too rich to map.
The Relationship to Rope Dyeing
In industrial mass-production denim, the dominant method is chemical reduction (hydrosulfite reduction) of pure synthetic indigo powder. This is highly reproducible, scalable, and cost-effective. The large-scale rope-dye operations running in China, Türkiye, and Pakistan also use chemical reduction, by and large.
In Japan, by contrast, certain mills in Ihara and Kojima still operate hybrid setups — synthetic indigo combined with chemical reduction — and, in limited quantities, traditional fermentation indigo dyeing. The proportion is a small fraction of total regional output, but for limited collaborations with luxury houses and for vintage-faithful projects, the very fact that “fermentation indigo is selectable” becomes a decisive differentiator.
What This Means for ALLBLUE’s Projects
ALLBLUE is a textile converter that owns no looms and no indigo vats. What we offer is the choice of which dyeing site to partner with. Distinguishing which projects belong on the stable supply line of chemical reduction, and which belong on the fermentation line, according to the maison’s design intent — this is at the core of our role.
In an era when mills around the world can “dye with indigo,” what is scarce is no longer the color itself, but the provenance of how that color came to be. Fermentation indigo and the microbial science of the vat remain, today, in the hands of a few sites in the Japanese production region — quietly underwriting that provenance.
Key references: Aya Dyeing & Weaving Studio (sukumo fermentation process); ZIKURI (Japanese aizome); Ellis Textiles (biochemistry of sukumo fermentation); Japan Travel JNTO (history of indigo dyeing in Kojima and Ihara); Watanabe’s Brand (sukumo and ai-tate kits).