Natural Iron Oxide Pigments

About 60% of the natural iron oxide pigments is used to color cement and other building materials (qv). About 30% is consumed in the production of paints. For coloring plastics and mbber, synthetic iron oxide pigments are preferred. The main advantage of the natural iron oxide pigments, as compared to the synthetic ones, is cost. However, the quaHty is inferior, and in most cases, they are consumed in close proximity to the mines. As colorants, the natural iron oxides are about 50% weaker than synthetically produced iron oxides. 

Until the beginning of this century, the needs of the pigment industry were supplied by natural iron oxides. Pigments produced from ochres in Southern France... 

Since then, these materials have been increasingly supplanted by their synthetic analogues. The latter can be produced in very pure form with extremely consistent properties. Today, natural iron oxide pigments account for only around 20% of world consumption. The main producers of natural iron oxide pigments are France, India, Cyprus, Iran, Italy and Australia (Buxbaum Printzen, 1993). 

The natural iron oxide pigments are termed the ochres which are yellow and contain goethite (10-50%) as the Fe oxide constituent, the reds, with a high content of hematite, the medium to dark yellow siennas, the umbers and the blacks, which consist of magnetite (Benbow, 1989 Buxbaum Printzen, 1993). 

Like the natural iron oxide pigments, the synthetics are used for colouring concrete, bitumen, asphalt, tiles, bricks, ceramics and glass. They are also used extensively in house and marine paints. Because the shapes of the particles can be accurately controlled and the particle size distribution is narrow, synthetic iron oxides have a greater tinting strength than the natural ones and so, are chosen where paint colour is important, i. e., for top coats. Red iron oxides are used in primers for automobiles and steel structures. 

Buxbaum, G. Printzen, H. (1993) Natural iron oxide pigments. In Buxbaum, G. (ed.) Industrial inorganic pigments. VCH, Weinheim, 85-96... 

Plate 19.IV a) Processing natural iron oxide pigments at Luberon, France (Courtesy J. Burlot). 

Plate 19.IV b) Natural iron oxide pigments (Courtesy M. Sauvete). 

The processing of natural iron oxide pigments depends on their composition. They are either washed, slurried, dried, ground, or dried immediately and then ground in ball mills, or more often in disintegrators or impact mills. 

Natural iron oxide pigments are mostly used as inexpensive marine coatings or in coatings with a glue, oil, or lime base. They are also employed to color cement, artificial stone, and wallpaper. Ocher and sienna pigments are used in the production of crayons, drawing pastels, and chalks [3.5]. 

The economic importance of the natural iron oxide pigments has decreased in recent years in comparison with the synthetic materials. 

J. L. W. Jolly, C. T. Collins Natural Iron Oxide Pigments, Iron Oxide Pigments, part 2, Information Circular-Bureau of Mines 8813, Washington 1980. 

Natural Iron Oxides. The earth s crust contains about 7 wt % iroii oxides, but only a few deposits are rich enough in iron to be suitable for mining pigmentary-quality iron oxides. Deposits that are a suitable source of natural iron oxide pigments are usually hydrated aluminum silicates that contain various amounts and forms of iron oxide. 

Iron oxides are supplied to the market as red, ocher, sienna, and umber natural pigments. The hue of the natural iron oxide pigments is determined by raw material composition and processing. 

Natural iron oxide pigments are believed to have been used as colorants in cave paintings in prehistoric times. They are available in different shades and quality, depending on geographical location and the specific deposit. They occur naturally with other inorganic minerals such as clay or other metal oxides and therefore they vary significantly in their iron oxide content. 

Natural iron oxide pigments have a dull color and low tinting strength, but they are used in inexpensive protective coatings, lime-based paints, primers, fillers and sometimes wood stains. 

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