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one of the most important groups of dyes. They are insoluble in water. Vat dyes contain carbonyl groups, which are reduced to the enol form by hydrosulfite, Na2S2O4, during the dyeing process. The resulting soluble leuco compounds of the vat dyes are absorbed by the substrate (for example, cellulose fiber) and converted into the original insoluble vat dyes during subsequent oxidation upon exposure to air. Vat dyes are classified according to chemical structure as polycyclic, indigoid, and thioindigoid. They are manufactured in all colors except bright red. Polycyclic vat dyes are very durable and are widely used in the textile industry for dyeing cellulose fibers; they also serve as pigments.