Dyes can be classified according to the chemical structure of the dye molecule. In practice, they are classified according to the way in which the dye is applied or is held on the substrate. Originally, they were applied from an acidic bath. Metallized dyes are forms of acid dyes in which the negative ion contains a chelated metal atom. They are used for acrylic fibres and also for wool and silk, although they have only moderate fastness with these materials.
Direct dyes are dyes that have a high affinity for cotton, rayon, and other cellulose fibres.
They are applied directly from a neutral bath containing sodium chloride or sodium sulphate. Vat dyes are insoluble substances used for cotton dyeing. The dye is applied in this form, then oxidized by air or oxidizing agents to precipitate the pigment in the fibres. Indigo and anthroquinone dyes are examples of vat dyes. Sulphur dyes are dyes applied by this technique using sodium sulphide solution to reduce and dissolve the dye.