FERRIC OXIDEThe red iron oxide, Fe2O3, also called gamma ferric oxide, found in abundance as the ore hematite, or made by calcining the sulfate. It has a dark-red color and comes in powder or lumps. The specific gravity is 5.20 and melting point about 2822°F (1550°C). It is used as a paint pigment under such names as Indian red, Persian red, and Persian Gulf oxide. In cosmetics and in polishing compounds it is called rouge. The Persian red oxide from the Island of Hormuz contains from 60 to 90% Fe2O3 and is marketed on a 75% basis. Brown iron oxide is made from ferrous sulfate and sodium carbonate and is not a pure oxide, though its chemical formula is given as Fe2O3. It is also called iron subcarbonate and is used in making green glass, paints, and rubber.
The names metallic red and metallic brown are applied to pigments from Pennsylvania ores containing a high percentage of red iron oxide. Venetian red is a name for red iron oxide pigments mixed with various fillers, most commonly an equal proportion of the pigment extender calcium sulfate. Commercially it is made by heating ferrous sulfate with quicklime in a furnace. Venetian red is a permanent and inert pigment that is generally used on wood. It cannot be used on many metals, including iron, because the calcium sulfate can cause corrosion. The Tuscan red pigments are red iron oxide blended with up to 75% of lakes, but may also be barium sulfate with lakes. Ferric oxide pigments make low-priced paints and are much used as base coats for structural steel work. The natural oxides come chiefly from Alabama, Tennessee, Pennsylvania, Iran, and Spain.
The Mapico colors of Binney & Smith Co. are iron oxide pigments refined under controlled conditions to give uniformity free of other mineral impurities. Mapico red and Mapico crimson contain 98% Fe2O3, the balance being almost entirely material lost on ignition or water-soluble impurity. The red oxide has a spheroidal particle shape, while the crimson has an acicular, or needle-shaped, particle. Mapico lemon yellow contains 87% Fe2O3, with 11.85% ignition loss. The particles are acicular and are only half the size of the crimson particles, being only 3.9 to 31.5 (xin (0.1 to 0.8 (xm). Mapico brown contains 93.1% Fe2O3 and 5 FeO. Its particles are cubic and of sizes from 7.9 to 15.7 (xin (0.2 to 0.4 (xm). Mapico black contains 76.3% Fe2O3 and 22.5 FeO, with a cubic particle shape. The Auric brown of Du Pont, used for giving light-fast shades to paper, is a hydrated ferric oxide ground to an extremely fine particle size.
Yellow iron oxide, known also as ferrite yellow and Mars yellow, used as a paint pigment, is Fe2O3 • 3H2O plus from 2 to 12% calcium sulfate. It is made by precipitating ferrous hydroxide from iron sulfate and lime and then oxidizing to the yellow oxide. Black ferric oxide, ferroferric oxide, or magnetic iron oxide is a reddish-black amorphous powder, FeO • Fe2O3 • H2O. It is used as a paint pigment, for polishing compounds, and for decarbonizing steel. The finely ground material used as a pigment is called magnetic black, and when used for polishing, it is called black rouge. Hammer scale is the iron oxide Fe3O4, formed in the hot rolling or forging of steel, and is used for decarbonizing steel by packing the steel articles in the scale and raising to a high temperature. It is very hard, Mohs 5.5 to 6.5, and is used as an abrasive.
Acicular gamma iron oxide is representative of a typical magnetic particle used in magnetic storage media. It is made from goethite, which is used in the paint industry as yellow ocher. When goethite is reduced in hydrogen, it forms magnetite, a strongly ferri-magnetic particle. It can be used as is in magnetic recording, but is usually reconverted to the acicular form by gradual oxidation in air to maghemite, because it is possible to orientate all the particles in a longitudinal direction by an external magnetic field. Recent trends are to dope the iron oxide with cobalt to get higher magnetic coercivity Acicular chromium oxide from Du Pont Co. contains no iron. Hardened polyurethane, epoxy, and phenoxy resins are used to bind the iron oxide particles for high-grade tapes, such as for video and computer disks. For audio tapes, vinyl-based copolymers or tripoly-mers are employed. Hercules Inc. makes an iron oxide called NP, for nonpolar, that consists of smooth and regularly shaped particles. These remain suspended in the binder longer and therefore are evenly dispersed when the binder cures.
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