Chromium compounds

Chromium is a constituent of several green, yellow, orange and red pigments; its name (from the Greek chroma or "color") refers to the color span of its compounds noted when it was discovered in the mineral ore crocoite by Nicolas-Louis Vauquelin in 1797. Several warm hued pigments called "chrome" colors, first described by Vauquelin in 1804, were commonly used and highly valued during the 19th and early 20th centuries: chrome yellow (lead chromate, PY34, which in pure form has a middle yellow hue; the color is shifted toward lemon yellow by increasing admixture with lead sulfate), two shades of chrome orange (PO21 for the yellowish shade, and PO45 for the red shade), and chrome red (PR103). The many related pigments include barium chromate (PY31, too dull and opaque for use in watercolors), molybdenium chromate (molybdate orange, PO35), zinc yellow (the carcinogenic zinc potassium chromate, PY36, probably discovered in the 1820's and only occasionally used as a pigment after 1850), and strontium chromate (PY32, a bright light yellow that is too opaque for use in watercolors). Hue variations within each of these pigments can be produced through differences in particle size or other added compounds. Use of chrome colors began to decline steadily in the 20th century because most shades contain lead and the light yellow shades are impermanent (they fade in light and blacken when mixed with sulfur pigments), even after the introduction in the 1950's of more lightfast, silica encapsulated lead chromates. Most important, however, was the fact that the cadmium compounds were brighter, less toxic and much more lightfast (though more expensive) in the same hue range. Chrome titanium oxide (PBr24) is a moderately saturated medium yellow that is lightfast and very appealing. Among the greens, the hydrous (water containing) chromium oxide, commonly known as viridian , is a moderately saturated, weakly tinting, granular, transparent and moderately staining blue green, discovered and produced in limited quantities from a secret process by the Parisian colormen Pannetier and Binet in 1838. The process was publicly disclosed and commercially applied to the manufacture of artists' colors by Charles-Édouard Guignet (Guignet's green) around 1859. The anhydrous (water free) chromium oxide, usually sold as chromium oxide green is an unsaturated, smooth, highly opaque and staining yellow green — the primary pigment in camouflage paints — that was known since 1809 but produced as an artists' color by Pannetier only in 1862. Increasing the average particle size shifts the color from light yellowish green toward a darker bluish green. The pigment is manufactured by heating chromium salts in the presence of boric acid, soaking in water (to produce the hydrated form, viridian), then grinding and washing to remove residue salts. (The historical colors green cinnebar or chrome green were mixtures of strontium chromate or [url=http://www.lookchem.com/cas-775/7758-97-6.html]lead chromate [/url] with prussian (iron) blue.) Chromium is also a compound in cobalt compounds such as cobalt green deep and the green shades of cerulean blue and cobalt turquoise.

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