Pigments are materials that change the color of reflected or transmitted light due to wavelength selective absorption. This physical process is different from the emission of fluorescence, phosphorescence, in which the material emits the light. Materials used as pigments have special properties that make them useful for coloring other materials. The pigment must have a high tinting strength relative to its coloring material and be stable in solid form at ambient temperature.
Inorganic pigments include white opaque pigments that provide opacity and lighten other colors. Inorganic pigments can be made by relatively simple chemical reactions, in particular oxidation reactions. White extender pigments are added to the paint to lower costs or improve properties. This class includes calcium carbonate, calcium sulfate, and china clay. Black pigments are mainly produced from carbon particles. For example, carbon black is used to impart black color to printing inks. Iron oxide earth pigment yields ochres, siennas, and haze. Some of the chromium compounds are used to provide chrome yellow, orange and green, while various cadmium compounds produce bright yellow, orange and red. And ultramarine blue is the most widely used inorganic blue pigment.
Organic pigments are currently synthesized from aromatics in most cases. They are compounds containing a carbon atom structure in which hydrogen atoms are linked to form a closed ring. Organic pigments include azo pigments containing nitrogen groups; they make up most organic red, orange, and yellow pigments. Copper phthalocyanine offers intense blue and green colors. Some pigments, such as fluorescent dyes, are simple dyes that are made insoluble by chemical reaction.
Pigments are used in paints, inks, plastics, fabrics, cosmetics, food and other materials. Most of the pigments used in the manufacturing and visual arts are dry colorants and usually ground to a fine powder. Pigments can also be used in biochemistry analysis and clinical practice. For example, Alizarin red is used for biochemical analysis to determine the presence of calcific deposition in cells. In clinical practice, it is used to stain synovial fluid to evaluate alkaline calcium phosphate crystals. Alizarin is also used in studies involving bone growth, bone marrow, vascular system gene expression, tissue engineering, and mesenchymal stem cells.