Indigo belongs to the group known as vat dyes. These dyes are typified by the fact that they do not form a chemical compound with the yarn, instead adhering to its surface as a surrounding pigment. Hydrosulphite (dithionite) is used as an auxiliary material when dying cotton with indigo (e.g. denim). It turns the water-insoluble blue colour indigo into a water soluble light-yellow leucoindigo (indigo white). This process is referred to as vatting.