In contrast, vat or reactive dyes are applied on cottons to achieve excellent washing fastness. The dyes have excellent light and dark fastness. These dyes have good all round fastness except to chlorine bleaches. The vat dyes have high color fastness, which is uncommon in other dye classes. It provides pigments with light fastness and prevents clear polymers from darkening in sunlight. Similarly inorganic pigments tend not to be salts, because insolubility is required for fastness. It has good colour fastness in silk and wool. The dye, however, may have poor fastness to rubbing. They have poor (basic dyes) to good (metal complex based) light fastness. Longevity, fastness and hand properties are important. Furthermore, many lacked stability through washing or exposure to sunlight, or fastness. These were used for economical high color fastness dark shades such as black and navy. Overall wash fastness is poor although light fastness is quite good. Furthermore, all tested items must have a skin-friendly pH value and good color fastness. The strength (fastness) of this bond reflects the strength of this ionic interaction.