Dyes and Pigments
Organic And Inorganic Colorants
Organic colorants are made of carbon atoms and carbon-based molecules. Most organic colors are soluble dyes. If an organic soluble dye is to be used as a pigment, it must be made into particle form. Some dyes are insoluble and must be chemically treated to become soluble.
Vegetable-based organic colorants are produced by obtaining certain extracts from the plants. An example of a dye that is not water soluble is indigo. Indigo is derived from plants of the genus Indigofera. By an oxidation process where the plant is soaked and allowed to ferment, a blue-colored, insoluble solid is obtained. To get the indigo dye into solution, a reducing agent (usually an alkaline substance such as caustic soda) is used. The blue dye, after reduction, turns a pale yellow. Objects dyed with indigo react with the air, oxidize, and turn blue. The imparted color is not always that of the dye itself. Animals are another, rather interesting, source of organic colorants. Royal purple, once worn only by royalty as the name suggests, is obtained from the Murex snail. Sepia is obtained from cuttlefish, and Indian yellow is obtained from the urine of cows that have been force-fed mango leaves.
Organic sources of color often have bright, vivid hues, but are not particularly stable or durable. Dyes that are not affected by light exposure and washing are called colorfast, while those that are easily faded are called fugitive. Most organic natural dyes need a fixing agent (mordant) to impart their color.
Inorganic colorants are insoluble, so by definition, they are pigments. This group of colorants is of mineral origin—elements, oxides, gemstones, salts, and complex salts. The minerals are pulverized and mixed with a dispersing or spreading agent. Sometimes heating the minerals produces different hues.