Dyeing is normally done in a neutral or slightly alkaline dye bath, at or near the boil, with the addition of either sodium chloride (NaCl) or sodium sulphate (Na2SO4). Also known as hot-water dye, this type can be used with hot tap water and requires no leveling or exhausting agents. It is convenient but lacking in light-fastness and wash-fastness. Direct dyes are used on cotton, paper, leather, wool, silk and nylon. They are also used as ph indicators and as biological stains.
Direct dyes are another class of dyes, one of the two types of dyes that are mixed in 'all purpose' dyes such as rit. (the other type in the mixture is an acid dye, which will not stay in any cellulose fiber for long.) the colors of direct dyes are duller than those provided by fiber reactive dyes, and the wash fastness is poor - except anything dyed with them to 'bleed' forever. The one advantage is that direct dyes may be more light fast, that is, resistant to fading in the light, than fiber reactive dyes. The "direct dye" classification in the color index system refers to various planar, highly conjugated molecular structures that also contain one or more anionic sulfonate group. It is because of these sulfonate groups that the molecules are soluble in water. Though most direct dyes still can be obtained in powder form, it is increasingly popular to receive them as liquid concentrates. The advantage of concentrates is that they are easy to handle and meter. The disadvantage is that the surfactants and co-solvents needed to keep the dye concentrates stable may interfere with retention and sizing in the case of very deeply colored grades.