Vat dyes are a class of high-quality dyes primarily used for cellulosic fibers such as cotton, linen, and viscose. They are insoluble in water and must be reduced to a soluble form in an alkaline bath using a reducing agent (such as sodium hydrosulfite) before dyeing. After application, they are re-oxidized to form insoluble pigments that firmly bond to the fiber. Vat dyes produce bright, vivid shades and offer excellent color fastness—making them ideal for high-end, performance-demanding textiles.