Ochre

Ochre (ˈoʊkər ; , ), or ocher in American English, is a natural clay earth pigment, a mixture of ferric oxide and varying amounts of clay and sand. It ranges in colour from yellow to deep orange or brown. It is also the name of the colours produced by this pigment, especially a light brownish-yellow. A variant of ochre containing a large amount of hematite, or dehydrated iron oxide, has a reddish tint known as "red ochre" (or, in some dialects, ruddle). The word ochre also describes clays coloured with iron oxide derived during the extraction of tin and copper. Ochre is a family of earth pigments, which includes yellow ochre, red ochre, purple ochre, sienna, and umber. The major ingredient of all the ochres is iron(III) oxide-hydroxide, known as limonite, which gives them a yellow colour. Yellow ochre, FeO(OH)·nH2O, is a hydrated iron hydroxide (limonite) also called gold ochre. Red ochre, Fe2O3, takes its reddish colour from the mineral hematite, which is an anhydrous iron oxide. Purple ochre is identical to red ochre chemically but of a different hue caused by different light diffraction properties associated with a greater average particle size. Brown ochre, also FeO(OH), (goethite), is a partly hydrated iron oxide. Sienna contains both limonite and a small amount of manganese oxide (less than 5%), which makes it darker than ochre. Umber pigments contain a larger proportion of manganese (5-20%), which makes them a dark brown. When natural sienna and umber pigments are heated, they are dehydrated and some of the limonite is transformed into hematite, giving them more reddish colours, called burnt sienna and burnt umber. Ochres are non-toxic and can be used to make an oil paint that dries quickly and covers surfaces thoroughly. Modern ochre pigments often are made using synthetic iron oxide. Pigments which use natural ochre pigments indicate it with the name PY-43 (Pigment yellow 43) on the label, following the Colour Index International system. File:LimoniteUSGOV.