Sulfur dyes are applied to cotton from an alkaline reducing
bath with sodium sulfide as the reducing agent. These dyes are
water-insoluble but they are soluble in their reduced form and exhibit
affinity for cellulose. They dye by adsorption; but on exposure to air, they
are oxidized to reform the original insoluble dye inside the fiber. The cost
is low and they have good fastness to light, washings, and acids.
