by U.S. Geological Survey Wednesday, June 20, 2018
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Michael J. Potter, mineral commodity specialist for the U.S. Geological Survey, has compiled the following information on iron oxide pigments.
Iron oxide pigments, which may be natural or synthetic, have been used as colorants since early humans began painting on cave walls. Natural pigments are derived from several iron oxide minerals: Red pigments are derived from hematite. Yellow and brown pigments — ochres, sierras and umbers — are derived from limonite. Magnetite provides a black iron oxide pigment. Micaceous iron oxide is a special form of hematite that occurs in thin metallic gray platelets or flakes. Synthetic pigments are manufactured under controlled conditions such that particle size, distribution and shape can be accurately replicated, resulting in superior uniformity, color quality and chemical purity.
Iron oxide pigments are relatively low-cost materials that resist color change due to exposure to sunlight, have good chemical resistance and are stable under normal ambient conditions. The leading uses of the pigments are in paints, coatings and construction materials such as in concrete products, mortar, paving stones and roofing tiles. Natural pigments are used in primers and undercoats, where color consistency is less critical, whereas synthetic pigments are used in topcoat paints where color consistency is important.
Micaceous iron oxide imparts unique properties to paints and coatings because the flaky particles align in such a way as to resist penetration by moisture and gases. These coatings can prevent corrosion and rusting of metals and also resist blistering, cracking and peeling.
Deposits of iron oxide pigment occur in many countries, but have been significantly developed in only a few. Countries known for production of iron oxide pigments historically include Cyprus, France, Iran, Italy and Spain. Countries with recent significant production include India, Spain and Honduras.
Iron oxide pigments are also created through steelmaking. When steel is treated with hydrochloric acid to remove surface oxides, the acid is regenerated to be recycled and iron oxide is produced. Regenerated iron oxides are used in a variety of filters, inductors and transformers in electronic home appliances and industrial equipment, as well as in flexible magnets, generators, loudspeakers and electric car motors.
New developments in the synthetic iron oxide pigment industry in recent years include granular forms of iron oxides and new versions of nano-sized materials, which are being used in computer disk drives and high-performance loud speakers, and in biology and medicine, including nuclear magnetic resonance imaging.
Visit minerals.usgs.gov/minerals for more information on iron oxide pigments.
Total world production of iron oxide pigments (natural: 13 percent and synthetic: 87 percent) in 2006 was approximately 1.4 million metric tons.
China was the world’s leading producer in 2006, with 49 percent of the world’s total production.
In 2007, total estimated U.S. production was 50,000 metric tons valued at about $50 million.
In 2006, the estimated global market for iron oxide pigments was $1.1 billion.
Natural iron oxide pigments have been used in art for tens of thousands of years, since humans created the 32,000-year-old cave paintings at Lascaux, France.
Iron oxide pigments are used as colorants for ceramic glazes, glass, paper, plastic, rubber and textiles as well as in cosmetics and magnetic ink and toner.
Micaceous iron oxide coatings have been used for heavy duty applications in harsh environments, including industrial tanks, refineries, chemical plants, drilling rigs and bridges, and even on the Eiffel Tower.
Natural iron oxide pigments or iron oxide earth pigments, collectively referred to as iron oxides, comprise both oxides and oxide hydroxides of iron. Hematite (α-Fe2O3) is the most common iron oxide in red earth pigments, and the iron oxide hydroxide goethite (α-FeOOH) is the most frequently found iron compound in yellow earth pigments. Umbers are brown earth pigments containing both oxides of iron and manganese.
Names for natural iron oxide pigments are complex and inconsistent. Yellow pigments based on iron oxide hydroxides are referred to by many names, including goethite, limonite, raw sienna, Mars yellow, yellow earth, yellow ocher, yellow iron oxide, yellow hydrated oxide, and iron hydroxide pigment. Similarly, natural red pigments based on anhydrous iron oxide are referred to as hematite, red earth, red ocher, burnt sienna, Mars red, and red iron oxide.
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The problem is compounded further when manufacturers label synthetic iron oxide pigments with names typically associated only with earth pigments, such as a synthetic yellow iron oxide labeled yellow ocher.
Iron oxide pigment is an appropriate name for natural and synthetic materials that may contain mixtures of iron oxides and iron oxide hydroxides. The names earth and ocher are suitable when it is clear that the pigment comes from a natural source. Earth is a more general term since ocher refers to a specific type of iron oxide deposit containing kaolinite (clay) and quartz. Hematite is used to differentiate the crushing of mineral hematite, which contains a high proportion of iron oxide in a finely divided form and is usually associated with clay or other minerals. Umber refers to a chemically distinct class of brown earth pigment containing both manganese and iron oxides.
The Colour Index (1982, 1:16–17, 120–122, 64–65) lists natural red iron oxide pigments as CI Pigment Red 102 (nos. 77015, 77491, and 77538). Synthetic red iron oxide pigments are listed as CI Pigment Red 101 (nos. 77015, 77491, and 77538). The natural and synthetic yellow iron oxide pigments are listed as CI Pigment Yellow 43 and 42 (no. 77492), respectively. CI Pigment Brown 6 and 7 (nos. 77491, 77492, and 77499) designate synthetic and natural brown iron oxide pigments, including umber.
Generally, we can summarize the various iron oxide pigment names and their Colour Index designations as follows:
Earth Pigment Types
Colour Index
Common Names
Yellow Earth Pigments Pigment Yellow 43 Limonite, goethite, jarosite, yellow ocher, yellow earth, raw sienna, yellow iron oxide Red Earth Pigment Pigment Red 102 Hematite, red ocher, red earth, burnt sienna, red iron oxide Brown Earth Pigments Pigment Brown 7 Pyrolusite, brown ocher, umber, burnt umber
To determine whether a pigment is from a natural source, such as an earth iron oxide pigment, look for the Colour Index designation, such as Pigment Yellow 43 (PY43), Pigment Red 102 (PR102), or Pigment Brown 7 (PBr7).
The labels on paint tubes indicate whether they contain natural or synthetic pigments. The Colour Index Generic Name (CIGN) Pigment Yellow (PY 23) indicates natural raw sienna, while Pigment Red 102 (PR 102) indicates natural burnt sienna. Aspiring artists should be aware of these distinctions and choose their materials accordingly to achieve the desired effects in their artwork. The Colour Index for natural sienna can also be written as Pigment Brown 7 (PBr 7), as noted in Table 1 Suitable Pigment List of the ASTM D 4302 Standard Specification for Artists’ Oil, Resin-Oil, and Alkyd Paints.
Color
Mineral
Chemical Formula
Structure
Crystal System
Yellow Goethite FeOOH Ramsdellite / hcp oxygen Orthorhombic Akagneite Hollandite/bcc oxygen Tetragonal Lepidocrocite Boehmite / ccp oxygen Orthorhombic – Cdl2 Hexagonal Red Hematite Fe2O3 Corundum Hexagonal Maghemite Defect spinel Cubic Black Magnetite Fe3O4 Inverse spinel Cubic
Iron oxide occurs naturally in various forms and colors. Indeed, iron is the second most abundant element on Earth, representing over 5% of the Earth’s crust. These natural products tend to consist of coarse particles and contain impurities, making them appear less chromatic. The chemistry of iron is a very complex field where, under different natural geological conditions, several different crystal forms of the same compound, differing due to the arrangements of molecules within the unit cells of the crystalline lattice of each crystal form, are formed (see table above).
The different crystal forms and accessory minerals associated with each deposit of iron oxide earth create a nearly unlimited variety of color, texture, and rheological behavior when dispersed in paint. This variety gives artists ore choices of expression and is one of the main advantages of natural earth iron oxide pigments in art.
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