Vat dyes:

These dyes are insoluble in water and cannot be helpful directly. These dyes on decline along with sodium hydrosulphite (NaHSO₃) in a vat make a soluble compound that has great affinity for cotton and other cellulose fibers. The cloth is saturated in the solution of a decreased dye and after that hung in air or treated with oxidants such as perboric acid. The result of it is the colourless compound is oxidized to insoluble dye which is now bound to the fabric. The colourless and decreased state of the dye is called the Leuco base. The common types of vat dyes are indigo and tyrian purple. These are generally used on cotton.