The oldest blue in the world

Natural indigo is a blue pigment extracted from the leaves of indigo-bearing plants, and it behaves unlike any other dye on earth. It will not dissolve in water. To use it at all, a dyer has to strip the oxygen out of it in an alkaline vat, at which point the liquid turns a murky yellow-green. Cloth dipped into that vat comes out green. Then, in the open air, over about a minute, it turns blue in front of you.

Every dyer remembers the first time they watched it happen. It does not look like chemistry. It looks like the cloth is deciding something.

Why indigo is difficult, and why that matters

Most natural colour is straightforward in principle. You simmer the plant, you get coloured water, you put the cloth in. Turmeric, madder, onion skin, all of them work roughly that way.

Indigo does not. The pigment is insoluble, so simmering it achieves nothing at all. It has to be reduced, meaning the oxygen is chemically removed from it, and it has to happen in a vat that is alkaline enough to hold the reaction. In that state the pigment becomes soluble and the vat turns yellow-green with a coppery film on the surface that dyers call the flower.

The cloth goes in green and comes out green. Then oxygen from the air finds it again, the reaction reverses, and the blue arrives.

Depth comes from patience, not from strength

Here is the thing that surprises people. You cannot get a deep indigo by making a stronger vat. A stronger vat gives you a muddy, rubbing-off surface and nothing more.

Depth comes from repetition. Dip, oxidise, rest. Dip, oxidise, rest. A pale sky blue might be two dips. The deep midnight of an old Japanese work jacket can be twenty or thirty, each one laid over the last, with drying time in between.

This is the whole reason indigo work is slow, and the reason a genuinely deep natural indigo costs what it costs. You are not paying for pigment. You are paying for the same pair of hands returning to the same piece of cloth thirty times.

Which plants make it

• Indigofera tinctoria and its relatives, the tropical indigos, grown across India, Africa and Southeast Asia. The richest source, and the one behind most historical indigo trade.
• Persicaria tinctoria, Japanese indigo, which tolerates cooler climates and underpins the Japanese aizome tradition.
• Isatis tinctoria, woad, the European plant that supplied the continent's blue for centuries before tropical indigo arrived and undercut it.

Different plants, same molecule at the end of it. The blue is chemically identical wherever it comes from. What differs is the concentration in the leaf and the process a given tradition built around it.

Indonesia's indigo, which is what we work near

Indigo has been grown and used across the Indonesian archipelago for a very long time, under the name tarum. It appears in Javanese batik, in Sumbanese and Balinese weaving, and in the ikat tradition where the threads are bound and dipped before a single one is woven.

That is worth sitting with. In ikat, the dyer is colouring thread according to a pattern that will only exist once it is woven, weeks later. The blue has to be right before anyone can see what it is for. We work with hand-woven ikat in the Myrah Gown, and every time a piece arrives I think about the order those decisions had to be made in.

What happened in the 1880s

For most of recorded history indigo was one of the most valuable trade goods in the world. Whole economies were built on it, and a good deal of suffering with them, because indigo cultivation under colonial systems was brutal work.

Then the German chemist Adolf von Baeyer worked out how to synthesise the molecule in the laboratory, and by the end of the 1890s BASF had it in commercial production. Within a couple of decades the natural indigo trade had almost entirely collapsed.

Nearly all blue denim made today is dyed with synthetic indigo. It is the same molecule. What it does not carry is a plant, a vat, or a person tending it.

Why your jeans fade at the knees

This is indigo's other peculiarity, and once you know it you cannot unsee it.

Indigo does not penetrate a fibre the way most dyes do. It sits on the outside in thin layers, wrapped around the yarn rather than soaked through it. Break a dyed thread and the core is often still pale.

So when denim wears at the knee, the hip, the edge of a pocket, what is actually happening is that layers of surface indigo are being abraded away to reveal the paler thread underneath. The fade pattern on a well-worn pair of jeans is a physical record of how a particular person moved. No other dye does that.

How to tell natural indigo from synthetic

Honestly, not easily, and anyone who tells you it is obvious is overstating it. The molecule is the same. But there are signals.

• Variation. Natural vats shift between batches and even through a single day's dyeing. Perfect consistency across every unit of a product is a synthetic signal.
• The other colours in the cloth. A natural vat carries plant matter with it, so natural indigo often reads slightly green, purple or grey in the shadows rather than flat blue.
• The smell, at first. A fermented vat leaves a faint earthy, sweetish smell in new cloth that washes out.
• What the brand will tell you. The reliable test is not the fabric, it is whether anyone can describe the vat. Ask what reduced it. A brand doing this by hand will know.

Living with it

Natural indigo crocks, which is the technical word for rubbing off onto other things, particularly when new and particularly on pale upholstery. It settles after the first few washes.

• Wash it cold, alone, for the first few washes.
• Skip detergents with optical brighteners, which flatten the depth.
• Dry it out of direct sun. Sun is what fades indigo fastest.
• Expect it to change. Indigo lightens where it is worn and deepens where it is left alone, and that is the point of it rather than a fault.

The same is true of everything we dye botanically, and we have written the full care method out in how to care for naturally dyed clothing. For what a botanical dye run actually looks like from the inside, over days rather than hours, there is The Sun Is the Metronome.

Frequently asked questions

What is natural indigo dye?

A blue pigment extracted from the leaves of indigo-bearing plants such as Indigofera tinctoria, Japanese indigo and woad. It is insoluble in water, so it has to be chemically reduced in an alkaline vat before cloth can take it up, and the blue only appears when the cloth meets the air again.

Why does indigo turn blue in the air?

In the vat the pigment has been reduced, meaning its oxygen has been stripped away, which makes it soluble and yellow-green. When the wet cloth is lifted out, oxygen from the air reverses that reaction and the pigment returns to its insoluble blue form, locked into the fibre. The change takes roughly a minute.

How do you get a dark indigo?

By dipping repeatedly, not by making a stronger vat. Each dip lays a further layer over the last, with oxidising and drying time in between. A pale blue may be two dips. A deep midnight can be twenty or thirty.

Is natural indigo different from synthetic indigo?

The molecule is identical. What differs is everything around it: natural indigo comes from a plant and a tended vat, varies between batches, and usually carries other plant tones in its shadows. Synthetic indigo, commercialised in the 1890s, is consistent, cheap, and used for almost all denim made today.

Why does indigo denim fade at the knees?

Because indigo sits on the surface of the yarn in layers rather than penetrating the fibre. Abrasion removes those layers and reveals the paler thread underneath, so the fade pattern is a physical record of how the wearer moved.

Does natural indigo rub off?

It can when new, which dyers call crocking. Wash it cold and alone for the first few washes and it settles. Keep it out of direct sun, which fades indigo faster than washing does.

With love from Bali,
Myrah