Zinc sulfate is a chemical in dyes and pigments. Chromium salts were until recently extensively used in dyeing wool with synthetic mordant dyes. It is used in the production of rubber chemicals, sulfur dyes and other chemical compounds. Nitration gives 1,4-dichloronitrobenzene, a precursor to commercial dyes and pigments. These processes afford detergents, dyes, and pharmaceuticals. Teeth blackening was the custom of dyeing one's teeth black with various tannin-rich plant dyes. This is probably due to the perennial difficulty of making good blue dyes and pigments. Primary aromatic amines are used as a starting material for the manufacture of azo dyes. Heterocycles are commonly found in a wide range of products including aniline dyes and medicines. It dyes cellulose fabrics a brilliant red color, but is not very fast. A variety of other synthetic dyes were invented thereafter. Many thousands of synthetic dyes have since been prepared. Examples are voltage sensitive dyes and fluorescing proteins. This is consistent with the generalized strategy for preparing azo dyes. Colors can darken if attacked by dust or by certain dyes and mordants.