In the case of some compounds the addition of an auxochrome group also changes a colorless compound into a colored one. For this reason synthetic dyestuffs are often popularly known as coal-tar dyes. From the basic materials, intermediates are manufactured by a number of chemical processes that in general involve the substitution of specific elements or chemical radicals for one or more of the hydrogen atom s in the basic substance. Dyestuffs can be classified according to their use or by their chemical structure. The chemical classification is generally made according to the nucleus of the compound.
Indigo is a vat dye, occurring in nature in the crystalline glucoside indican.
An important new group of dyes is the phthalocyanine dyes, which are blue or green in color and resemble chlorophyll in chemical structure. Of all the groups of dyes the azo dyes are the most generally useful and widely employed. See Diazo Compounds . A stain is a chemical dye or pigment used to color glass, paper, textiles, or wood. The staining substance, which uses alcohol , oil, or water as a vehicle, is transparent and thinner than paint or coating, and it penetrates into the grain of material being dyed.