Vat dye
Also searched as: vat dyes, vat dyeing, indigo, leuco dye, leuco form, anthraquinone vat dye, indanthrene, vatting
A vat dye is an insoluble colorant that is chemically reduced to a soluble leuco form for application, then reoxidized inside the fiber to its insoluble colored form.
In detail
Vat dyes are pigments in their final state, so they cannot be applied directly. In the dyebath a reducing agent, commonly sodium dithionite under alkaline conditions, converts the dye to its leuco form, which is water-soluble and has affinity for cellulose. After the fiber absorbs the leuco dye, exposure to air or an oxidant regenerates the insoluble dye, trapping it within the fiber. Indigo is the best-known example and remains the dominant dye for denim.
The science and numbers
Indigo is a planar, poorly water-soluble molecule; reduction of its two carbonyl groups yields the leuco enolate, which is soluble at high pH. Leuco indigo has relatively low affinity for cotton, so rope and slasher dyeing use many short dips with oxidation between them, producing the ring-dyed yarn used for fading effects. Research to avoid sodium dithionite includes cyclodextrin inclusion complexes that raise indigo's solubility several-fold without chemical reduction. Industrial synthesis has historically relied on aniline, formaldehyde and hydrogen cyanide, and biosynthesis routes using flavin-containing monooxygenases are under development. Vat dyes also resist degradation in wastewater treatment, and one study found vat dyes more recalcitrant than reactive dyes to photocatalytic treatment. Beyond indigo, vat dyes can be added to cellulose dopes for spin dyeing.
A practical example
A denim warp passes through an indigo leuco bath and emerges yellow-green, then turns blue within a minute or two in air as oxidation proceeds. Repeating dip and air cycles builds depth in the outer layers of each yarn, leaving the core paler. Later abrasion wears away the colored surface to reveal the lighter core, which is why washed denim fades in characteristic patterns.
What to distinguish
Vat dyes are a chemical class, not a natural-dye claim: indigo can come from plants, chemical synthesis or microbial biosynthesis, and the molecule is the same. The reducing step generates sulfite, sulfate and sulfide byproducts. Indigo on denim has limited rubbing fastness by design, so crocking onto light fabrics is expected. Other vat dyes can be very fast.
Origins and history
Indigo is among the oldest dyes, obtained historically from plants such as Indigofera species, and it remains central to cotton dyeing. The class name refers to the vessel, or vat, in which the reduced dyebath was traditionally prepared. Plant extraction gave way to chemical synthesis, which carries its own worker-safety and environmental concerns, and microbial production is now being explored as an alternative.
Related terms
Sources & further reading
- Bioresources and Bioprocessing: Supramolecular encapsulation of indigo with cyclodextrins for reducing-agent-free dyeing (Devi et al., 2026)
- Biotechnology Advances: Overview of indigo biosynthesis by flavin-containing monooxygenases (Fan et al., 2024)
- Carbohydrate Polymers: Spin-dyeing of cellulose fibres with vat dyes using the Ioncell process (Nygren et al., 2024)
Technical references reviewed 2026-09-22. Examples are illustrative. Industry organizations and manufacturers describe their own fields; their references are not independent product endorsements. Figures are approximate and depend on the stated test conditions.