Vat and sulphur dyes are insoluble in water. They are chemically reduced to a soluble form, applied to cellulose fibres, and then oxidised so the insoluble dye is trapped inside the fibre.
Vat dyes give some of the best all-round fastness on cotton. Sulphur dyes give deep, economical blacks, browns and navies, especially on denim and workwear.
Vat dyes
Vat dyes are reduced with sodium hydrosulphite and caustic soda to their soluble leuco form. After dyeing, air or an oxidising agent turns them back into the insoluble pigment inside the fibre.
Indigo is the best-known vat dye and the basis of denim. Anthraquinone vat dyes are used where very high light and wash fastness are needed, such as workwear, uniforms and some furnishing fabrics.
Vat dyes are more complex to apply than reactive dyes. The bath must stay reduced, temperature and alkali must be controlled, and oxidation and soaping must be done properly to reach full shade and fastness. Indigo is also sold in pre-reduced liquid form, which lets denim mills use fewer chemicals in the dye range.
Sulphur dyes
Sulphur dyes are reduced with sodium sulphide or with more modern reducing agents, then oxidised. C.I. Sulphur Black 1 is one of the largest-volume dyes in the world by tonnage. Sulphur dyes are also used to bottom or top indigo in denim.
- Sulphur dyes have good wash fastness but often weaker chlorine fastness.
- Some sulphur blacks can tender cotton in storage if the dyeing is not finished properly.
- Pre-reduced liquid forms reduce the use of sodium sulphide in the mill.
Buying points
- Ask for the form and concentration, since vat dyes come as powders, pastes and fine grains, and liquid sulphur dyes vary in strength.
- For indigo, ask for purity and for any limit on aniline, which customer restricted lists cover.
- Ask what reducing system the supplier assumes, so your mill can match it.
- Ask for fastness results to washing, rubbing and chlorine on your fabric.
- Check the hazard data and transport class, as some reduced liquid dyes are corrosive.
Safety and shipping
Both dye classes bring chemicals into the supply chain that need care. Sodium hydrosulphite is self-heating and is classed as dangerous goods for transport. Sodium sulphide is corrosive and releases toxic hydrogen sulphide with acid. You may not buy these from the dye maker, but your mill will need them, and your forwarder will ask about them if you do.
Read the safety data sheet for each product, and confirm the transport class with your forwarder before booking. The shipper is responsible for declaring dangerous goods correctly.
Next step
Gujarat's denim mills create strong local demand for indigo and sulphur dyes. We find and audit makers and check their certificates of analysis. To begin, book a free sourcing review.