Indigo is the blue dye that colours denim. It is a vat dye, insoluble in water, so it is reduced to a soluble form, applied to the yarn, and then oxidised in air to fix the blue colour onto the fibre.

Why does indigo denim fade?

Indigo sits mainly on the surface of the yarn rather than penetrating it, leaving a white core, and it bonds only loosely. Friction and washing knock surface indigo off the high points, revealing the white core, which creates the characteristic fades.

Is denim dyed with natural or synthetic indigo?

Most modern denim uses synthetic indigo, which is chemically identical to the natural dye but cheaper and more consistent at scale. Some artisanal and premium denim still uses natural plant indigo from Indigofera tinctoria for its organic character.

Ring-dyeing is when indigo coats only the outer surface of each yarn, leaving the core white. Brief dips and surface-bound indigo create this ring effect, which is essential for high-contrast fading.

Does indigo bond permanently to denim?

No, and that is intentional. Indigo bonds loosely to the surface of the fibre, so it gradually wears away with friction and washing. A permanent bond would prevent the fading that denim is loved for.

Indigo Dye Explained

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Indigo Dye Explained

The vat dye that sits on the surface so denim can fade.

OD's Designer Clothing - St Helens - Updated June 2026

In brief: Indigo is the dye that gives denim its blue. It is a vat dye that does not dissolve in water, so it is chemically reduced to a soluble form, applied to the yarn, then oxidised in air to fix the blue. Because indigo bonds only loosely to the surface of the fibre, it abrades and fades over time, which is the whole basis of denim fading. Natural indigo comes from plants; most modern denim uses synthetic indigo.

What is indigo dye?

Indigo is the blue dye responsible for the classic colour of denim. Chemically it is a vat dye, meaning it is not soluble in water in its normal state. To use it, the indigo is first reduced into a soluble, yellow-green form called leuco-indigo. Yarn is dipped into this solution and then lifted out into the air, where exposure to oxygen converts the dye back into its insoluble blue form, locking the colour onto the yarn. This dip-and-oxidise cycle can be repeated many times to build up depth of colour.

Why does indigo fade?

The key to denim fading is that indigo does not penetrate deeply or bond strongly to the cotton. It sits mostly on the surface of the fibre, which is why denim yarn is described as ring-dyed, with a blue surface and a white core. Because the bond is loose and surface-bound, ordinary friction and washing gradually knock indigo particles off the high points of the fabric, revealing the white core beneath. That is what produces the whiskers, honeycombs and high-contrast fades prized in raw and selvedge denim. A dye that bonded deeply and permanently would not fade in this characterful way.

Natural and synthetic indigo

Historically, indigo was extracted from plants, chiefly Indigofera tinctoria, in a labour-intensive process. Today the vast majority of denim is dyed with synthetic indigo, which is chemically identical to the natural dye but far cheaper and more consistent to produce at scale. Synthetic indigo made mass-market denim possible, while small quantities of natural indigo are still used in artisanal and premium denim for their slightly varied, organic character.

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Indigo Dye Explained

What is indigo dye?

Why does indigo fade?

Natural and synthetic indigo