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Related to Pigments: photosynthesis, Photosynthetic Pigments
white and variously colored natural minerals, rocks, ores, and other mineral formations used as dyes. The coloring components (chromophores) in pigments are primarily iron, chromium, manganese, copper, and nickel in the form of oxides or hydroxides, as well as carbonaceous substances.
The following chemical types of white or colored pigments are distinguished: carbonate pigments (chalk, malachite green, azure blue); silicate pigments—clay and others (for example, kaolins, brown-ochre clays, volchonskoites, glauconites, garnierites, chrysocollas, and lazurite); oxides and hydroxides (brown iron ore, hematites, hydrohematites, manganese oxide ores); sulfate pigments (barite, jarosite); and carbonaceous pigments (natural coals, soot, schungiter, and asphalt tars). Sulfides (cinnabar) and phosphates (vivianite) are sometimes used as pigments.
According to their color properties pigments are divided into white (chalk, kaolin, barite), yellow and yellow-brown (brown iron ore, clay ochre), red and reddish brown (colcothar, red lead [hematites and hydrohematites], cinnabar), brown (sienna, burnt umber), green (volchonskoite green, malachite green, glauconite green), dark blue (azurite [copper], vivianite, lazurite [lapislázuli]), and gray and black (carbonaceous pigments, pyrolusite black).
Natural pigments are highly resistant to the effects of light and air. This quality and the wide geographical distribution and simplicity of production make natural pigments superior to synthetic ones. The most widely used pigments are the silicate, oxide, and carbonaceous pigments, which are used in printing, artwork, and the production of wallpaper; they are also used in the paint and varnish industry and in the construction industry.
REFERENCESZalesskii, B. V., and Iu. A. Rozanov. “Opyt klassifikatsii mestorozhdenii mineral’nykh krasok.” In the book Voprosy mineralogii, geokhimii i petrografii. Moscow-Leningrad, 1946.
Champetier, G., and H. Rabaté. Khimiia lakov, krasok i pigmentov, vol. 2. Moscow, 1962. (Translated from French.)
(in chemistry), fine powders of various colors used for coloring plastic, rubber, and paper and for the production of printing inks and other dyes. Pigments-differ from soluble dyes in that they are insoluble in water and the materials being colored. Pigments not only impart color but in some cases improve the properties of the paint films, which protect the material from corrosion.
Organic pigments are synthetic coloring substances of various chemical structures. Azo and diazo pigments are very important. They are products of the coupling of diazotized aromatic monoamines and diamines with arylides of acetoacetic or 3-hydroxy-2-naphthoic acids, 2-naphthol, or N-arylpyrazolones. Their colors range from greenish yellow to Bordeaux. Phthalocyanine pigments are bright blue complexes of copper with phthalocyanine. The chlorination of phthalocyanine copper gives a bright green pigment; the simultaneous introduction of chlorine and bromine gives a yellowish green pigment. Phthalocyanine pigments are distinguished by their high stability. Highly stable polycyclic pigments, which have a broad color range (from yellow to green), are also important.
Inorganic pigments are natural minerals with a high content of iron oxides, synthetic products obtained by chemical precipitation and by calcination of natural materials, and several sulfides, selenides, oxides, and chromates. The oxides are more resistant than the sulfides, particularly to the atmospheric conditions. Inorganic pigments are opaque and have less coloring capacity than organic pigments but are more resistant to light and have greater density. Inorganic pigments are especially widely used in the paint and varnish industry.
The practical value of pigments is determined by their purity of color, their resistance to light, high temperatures, solvents, and various reagents, their lack of tendency to migrate from the treated material, their covering power, and their tendency to disperse in the pigmented media and to impart certain technological properties to paints and varnishes. The more uniform the size of the pigment particles, the better the optical and technological properties of the pigment.
The importance of pigments is steadily increasing. Pigments are finding increasingly wide use in the dyeing of artificial and synthetic fibers during production and for coloring artificial leathers and fabrics by pigment printing.
REFERENCESChampetier, G., and H. Rabaté. Khimiia lakov, krasok i pigmentov, vol. 2. Moscow, 1962. (Translated from French.)
Vvedenie v fizicheskuiu khimiiu pigmentov. Edited by D. Patterson. Leningrad, 1971. (Translated from English.)
Z. I. SERGEEVA