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Ochres -


T he Ochre is a natural earth containing silica and clay tinted by hydrous forms of iron oxide, such as yellow-brown limonite or brown-yellow to green-yellow goethite, and traces of gypsum or manganese carbonate. Limonite is a general term used to describe all forms of hydrated iron oxide minerals (FeO(OH)) that occur as natural clay or earth. Limonite includes the minerals goethite, akaganeite and lepidocrocite. To be considered an ochre, the content of iron oxide must not be less than 12%. Depending upon the content of hydrated iron oxide, the color of ochre varies from light yellow to golden to orange. The higher the content of iron oxide in an ochre the greater its tinting strength and hiding power. Most yellow ochres are normally not calcined as heat does relatively little to alter their color. Like red iron oxides (hematite), they are found around the world and have been used as pigments since prehistory. Depending upon the colour, the ochres are called red ochre, yellow ochre, green earths, sienna, umber and by various other names. In addition to red ochre, the red oxide of iron, commonly called 'red oxide', is an important natural pigment. It results from the alteration of hematite and ferruginous laterite and consists essentially of Fe2O3 having pigmentary quality .

Properties

The staining power, brilliance and fineness of texture are the main properties by which an ochre is judged for its quality and value in industry. The tint should be wholly of the inorganic ingredient.
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