Sulfur dyes are prepared by reacting phenols, amines, or nitro compounds with sulfur and caustic soda under heat. They contain sulfur-sulfur bonds that make them insoluble in water but soluble in the presence of sodium sulfide and soda ash. Sulfur dyes are cheap and easy to apply to cotton, with good wet and light fastness. However, the exact chemical structures of sulfur dyes are unknown and they have poor fastness to chlorine.