Everyday Mysteries Chemistry No background needed ~7 min read

Why does indigo-dyed cloth turn from green to blue when you pull it from the vat?
― The artisan is getting the air to "bring out" the colour

Cloth pulled from an indigo vat isn't blue — it's yellow-green. Then, right before your eyes, exposure to air turns it blue. The indigo in the vat has actually been deliberately put into a "non-blue" form. Blue indigo won't dissolve in water, so it can't get into the cloth as it is.

Published: 2026.10.05 Difficulty: ★☆☆ (no background needed) Formulas appear only in the final collapsible section

First, picture this scene

In an indigo-dyeing workshop, an artisan lowers a white cloth into a large vat. The liquid in the vat is less blue than brownish yellow-green. On the surface floats a mound of purplish-tinged foam.

After a while, the artisan lifts the cloth out and wrings it hard. Unfolded, the cloth is a dull yellow-green. But as the artisan shakes it out into the air, the colour shifts from the edges — first to green, then to blue.

In under a minute, the cloth turns a clear blue. The artisan dips it back into the vat and repeats the same process, again and again. Why not dye it deep in one go?

There are only two reasons the colour changes

The indigo in the vat has been turned into a "soluble form"

Blue indigo barely dissolves in water. So the artisan uses the power of microbes to attach hydrogen to the indigo, turning it into a yellowish form that dissolves in water. This is called "building the vat."

Oxygen in the air turns it back to blue

The soluble indigo that has soaked into the gaps between threads loses its hydrogen to oxygen on contact with air. It then reverts to the original blue indigo, forming water-insoluble particles that stay trapped inside the thread.

In other words, indigo dyeing is a two-stage process: "dissolve it to carry it in, then harden it with air." It isn't the vat that brings out the colour — it's the air.

Why won't blue indigo dye cloth as it is?

The source of indigo's blue colour is a substance called indigotin. Its molecules pull strongly on each other, like linked hands. So when put in water, it barely dissolves — the powder just sinks. Anything insoluble can't get into the fine gaps between threads. Mix blue powder into water and dip cloth in, and it only leaves a faint coating on the surface, which washes off.

So artisans of old found a way to turn indigo into a "soluble form." In the traditional Japanese method, "sukumo" — fermented leaves of Japanese indigo — is put into alkaline water (lye) made from wood ash. Food for the microbes, such as wheat bran or sake, is added, and the temperature is kept steady while it sits. Over several days to about two weeks, microbes in the liquid are thought to pass hydrogen to the indigo, turning it into its soluble form.

This soluble indigo isn't blue — it's yellowish. The vat liquid looks yellow-green because a little blue indigo is still mixed in with the dissolved yellow form. The purplish tinge on the surface foam comes from the foam's film touching air and starting to turn back to blue indigo right there. Artisans call this foam the "indigo flower," and use it to gauge the vat's condition.

Number of dip-and-air cycles 8 times Move the slider to change how deep the cloth colour on the right looks

Figure 1: How indigo dyeing works. Indigo that has taken on a soluble (yellow-green) form in the vat on the left meets oxygen in the air at the centre and reverts to blue. The solid arrows in the bottom row show the change inside the vat; the dashed arrows show the change in air. The square on the right shows the colour for roughly each number of repeated cycles — moving the slider deepens the colour (the names and the number of cycles they match vary by workshop).

What actually happens when the cloth hits the air?

The gaps between threads in the lifted cloth are soaked with soluble indigo. As shown in the centre of Figure 1, when this meets air, oxygen strips hydrogen from the indigo. The indigo then reverts to its original blue form. Since the blue form doesn't dissolve in water, it instantly becomes tiny particles that get caught in the gaps between threads and can't escape.

The amount of oxygen needed for this is smaller than you might think. Work it out in the "Check with a formula" section below, and even enough indigo to dye one T-shirt a deep colour needs only about as much air as an adult breathes in in a single breath. Shaking and spreading the cloth out is to let air reach deep into the gaps.

So why not dye it deep in just one pass? Soluble indigo doesn't stick to thread very strongly. Even soaking it for a long time, the amount that gets into the thread is thought to plateau quickly. But once air converts it into blue particles, dipping again lets a new layer build on top. That's why artisans repeat "dip, wring, expose to air" over and over. From the pale light blue of "kamenozoki" to the deep "kon" navy, the colour names are also said to track the number of repeats.

At the end, the layered cloth is washed well with water. This removes particles merely sitting on the surface, leaving only the indigo locked inside the thread.

💡 Jeans blue is the same indigo

Jeans are mostly dyed blue with indigotin too. Today, factory-made indigo is turned soluble with chemicals instead of microbes. Indigotin tends to stay near the surface of the thread, so colour wears away where the fabric rubs, revealing the white core. That's what gives jeans their characteristic "fade."

💡 Indigo vats have long been treated as "living things"

Because the workers in the vat are microbes, if it gets too cold or their food runs out, the indigo stops dissolving. So artisans keep tending the vat — warming it, adding more food. The skill of reading the vat's condition from the liquid's colour, the foam, and the smell has been passed down through long experience.

Summary

Blue indigo doesn't dissolve in water, so it can't get into thread as it is. Artisans use microbes to pass hydrogen to indigo, turning it into a soluble yellow-green form that soaks into the thread. When the lifted cloth meets air, oxygen strips the hydrogen away, turning it back into blue and leaving it behind as insoluble particles trapped inside the thread.

Indigo is dissolved in the vat, then dyed by air.
That instant from green to blue is the signal that colour has been locked into the thread.

There are other everyday examples where oxygen changes colour. The story of copper slowly turning green can be read in "Why was the green patina on copper once called 'poison'?", and the story of leaf pigments switching over in "Why do autumn leaves turn red and yellow?".

🧪 Try it at home: see that indigo sits "on the surface"

Rub the inside hem of a new, dark pair of jeans firmly against a white cloth or paper. If blue colour transfers, that's proof the indigo sits near the surface of the thread.
Take a loose thread from the hem, tease it apart with your fingers, and look at the cross-section. You may find threads that are blue on the outside but whitish at the core.
If you get the chance to visit an indigo-dyeing workshop, time how long the lifted cloth takes to change colour. Compare shaking it versus not shaking it to see how many seconds it takes to go from yellow-green to blue — that shows you the role of the air.

The liquid in an indigo vat is alkaline, so follow the workshop's instructions during a hands-on experience, and wear gloves and keep it away from your eyes.

Want to know more? ― Terms, formulas, and links to the curriculumWe mark which level each topic belongs to, from middle-school science to university specialist courses

How to read the labels that follow

MSCovered in middle-school science
HSCovered in high-school "Chemistry"
HS+High-school advanced content, or textbook sidebar material
UnivNot covered in high school — university specialist courses (organic chemistry, dye chemistry, microbiology)
ResearchNot yet taught as settled fact even at university — something researchers are still investigating

MSTerms: this phenomenon has names

Oxidation and reduction: a substance combining with oxygen or losing hydrogen is oxidation; the reverse is reduction. Inside the vat is reduction; in the air is oxidation.
Indigotin: the substance behind indigo's blue colour. It barely dissolves in water.
Building the vat: using fermentation to turn indigo into a soluble form and make a liquid ready for dyeing. An artisan's term.
Sukumo: the raw material for indigo dyeing, made by fermenting Japanese indigo leaves over several months.

MSHSCheck with a formula: how much air does it take to turn cloth blue?

Let's estimate, from the reaction equation, how much oxygen is used when soluble indigo reverts to blue indigo. Here we'll assume 2 g of indigo goes into the cloth (roughly enough for one deeply dyed T-shirt).

⓪ The base equation

In symbols	2 C₁₆H₁₂N₂O₂ + O₂ → 2 C₁₆H₁₀N₂O₂ + 2 H₂O
In words	2 units of soluble indigo + 1 unit of oxygen → 2 units of blue indigo + 2 units of water. Volume of air needed = (weight of indigo ÷ weight of 1 mole of indigo) ÷ 2 × volume of 1 mole of gas ÷ fraction of oxygen in air
Where this equation comes from	It's a redox reaction equation. Soluble indigo has 2 more hydrogen atoms than blue indigo, and one oxygen atom carries those 2 away as water. Because atom counts are conserved before and after the reaction, the ratio of indigo to oxygen is fixed at 2 to 1.

① Starting figures

Weight of indigo going into the cloth (assumed)	2 g
Weight of 1 mole of blue indigo	about 262 g
Volume of 1 mole of gas at room temperature	about 24 L
Fraction of oxygen in air	about 0.21 (21%)

② Running the numbers

Amount of indigo (moles)	2 ÷ 262 ≒ 0.0076
Oxygen needed (moles)	0.0076 ÷ 2 = 0.0038
Oxygen volume (L)	0.0038 × 24 ≒ 0.091
Air volume needed (L)	0.091 ÷ 0.21 ≒ 0.43

The answer is about 0.43 L — less than the roughly 0.5 L an adult breathes in a single quiet breath. The air in a room is tens of thousands of times more than that, so there's never a shortage of oxygen. The reason it takes tens of seconds to turn blue is thought to be not the amount of oxygen, but the time it takes oxygen to reach deep into the gaps between threads.

HSHS+Redox seen as an exchange of hydrogen

HSIn high-school chemistry, redox is taught as an exchange of electrons. Soluble indigo is the form blue indigo takes after gaining 2 electrons and 2 hydrogen ions. In air, oxygen takes those electrons, forming water. The symbols stand for atoms: C for carbon, H for hydrogen, N for nitrogen, O for oxygen.

HS+The soluble form of indigo is called "leuco" (meaning "white body"). In alkaline liquid, it gives up a hydrogen ion and becomes a negative ion, which is why it dissolves in water. That's why the liquid is kept alkaline with lye or lime. Dyeing by turning a dye into a soluble form and then restoring it with air is called "vat dyeing."

UnivWhy blue, and why insoluble

The indigotin molecule is flat and forms hydrogen bonds both within itself and between molecules. Because the molecules stack and bind strongly to each other, the substance is thought to resist dissolving in both water and most organic solvents. It looks blue because the molecule strongly absorbs orange light around 600 nm, leaving the remaining light to reach our eyes as blue. When reduced, the conjugated system — the pathway of electrons spread across the whole molecule — is broken partway through, shifting absorption toward shorter wavelengths, which is why the colour becomes a pale yellow. In fermentation-based vat building, alkali-tolerant bacteria are thought to break down sugars and other substances and pass the resulting electrons on to indigotin.

📖 For the derivation of the equations and further reading: Redox reaction (Japanese Wikipedia) ／ Indigo dyeing (Japanese Wikipedia)

ResearchWhat's still not fully understood

Which microbes are doing what inside the vat. Several bacteria that reduce indigo have been found in fermentation-built vats. But which bacteria take the lead at which stage, and how they pass electrons to indigo, is still being investigated.
Can an artisan's "feel for the vat" be turned into numbers? Research is underway linking the skill of judging a vat's condition from colour, foam, and smell to measurable acidity and microbial composition.
Dyeing methods that are lighter on the environment. The reducing chemicals used in factories burden wastewater. Methods using electricity or microbes to dissolve indigo are being tried.

In other words, even this article's content is "the explanation as currently understood." Within the centuries-old artisan's craft, there are still parts that science hasn't caught up with.

Links to the curriculum (by level)

Level	Subject / unit	Where in this article
MS	Science (chemical change, oxidation and reduction)	Reduction in the vat, oxidation in the air
HS	Chemistry (redox, amount of substance)	The reaction equation and the air calculation
HS+	Chemistry (organic compounds, dyes)	Leuco form and vat dyeing
Univ	Organic chemistry, dye chemistry, microbiology	Hydrogen bonds and conjugated systems, fermentation bacteria
Research	Applied microbiology, environmental chemistry	Vat microbes, lower-impact reduction methods
―	Everyday connections	Jeans fading, indigo-dyeing experiences

References and sources

Wikipedia (Japanese), "Indigo dyeing" (藍染め)
Wikipedia (Japanese), "Redox reaction" (酸化還元反応)
Jenny Balfour-Paul, Indigo: Egyptian Mummies to Blue Jeans, British Museum Press
Yoshioka Sachio (吉岡幸雄), Nihon no Iro Jiten (日本の色辞典, "Dictionary of Japanese Colours"), Shiko-sha, 2000
Yumoto I. et al., Alkalibacterium psychrotolerans sp. nov., International Journal of Systematic and Evolutionary Microbiology, 2004 (a report on bacteria found in fermented indigo liquid)

※This article is a general-audience science explainer. The figures given are approximations meant to help you understand the mechanism. The liquid in an indigo vat is alkaline. Please follow workshop instructions during any hands-on experience.