Ochre

Synonym of :
Ferritungstite, Tungstite

The principal use of ochre is as tinting colours. It is used for colour washes, distemper and oil paints. It is also used in making coloured paper.

The History Says

In Middle English it is known as oker, from Middle French ocre, from Latin ochra, from Greek Ochra, from feminine of Ochros yellow.

The Present Scenario

Ochre is among the most permanent colors among the artist's palette. It is compatible with all other pigments, and can be used with good results in all mediums.

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OCHRES are the natural mineral pigments occuring in various shades and colours, generally ranging from yellow to red to brown. The colouring power is mainly due to oxides of iron.

Note : They are also called 'coloured earths'. Ochre is essentially a clay stained with colouring minerals. The hydrous iron oxide imparts yellow colour and the unhydrous red colour. The amount of iron oxide (Fe₂O₃) in ochre is quite variable. It may be as low as about 20%, going up to 70%.

The use of natural mineral pigments is known to man from his historical past. In ancient times it had been used in colouring earthen-ware and household utensils and for decorative purposes. In modern times also it has not lost this utility. It dominates in the market because of its cheapness, abundance in occurrence and good pigmentary quality, despite a number of manufactured pigments presently in vogue.

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.

Red oxide usually contains about 70% Fe₂O₃ . Sienna is a brownish yellow containing about 60% Fe₂O₃ with some quantity of manganese oxide. It is named after sthe town of Sienna in Italy where a large deposit is located. Sienna is marketed in the raw and burnt (calcined) states. The colour of the latter is brownish red.

Umber is a greenish brown containing some 45% Fe₂O₃ and 15% MnO₂. Umber is named after a department in Central Italy where it was first tested. Umber of good quality, called Turkish Umber, is found on the island of Cyprus. Numerous deposits of colouring earths occur in various parts of the world.

Preparation of pigments from red oxide requires an elaborate process. Since it is hard, it has to undergo milling and finally separation of the coarse particles by elutrition.

Ochres being friable are crushed and lixivated, just like china-clay, to obtain extremely fine-textured material of uniform colour. Refining by elutriation followed by drying often improves both iron content and colour.

Uses

The principal use of ochre is as tinting colours. It is used for colour washes, distemper and oil paints. It is also used in making coloured paper. For this particular use ochre and china-clay or soapstone, after dissolving in water in paste form, are acreened and added in the beater for the prepareation of coloured paper pulp. Red oxide pigments are widely used as primers for painting structural steel, automobile bodies, ship bottoms, etc.

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.

Occurrence in Persian Gulf and Spain

The highest red oxides are found along the Persian Gulf and in Spain. The Persian deposit is mined on the Omur island. A brown oxide found in Pnnsylvannia, USA, is known as 'metallic brown'.

Pigment Information

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. French ochre, historically one of the best grades of limonite, contains about 20% iron oxide and is high in silica. In Russia, high quality ochres can be obtained from the Izyumskyy deposit in Ukraine, and the Zhuravskoye, Skarnovskoye and Dubovikovskoye deposits in the Voronezh region, and the Lyubytinskoye deposit near Novgorod.

Oil Absorption and Grinding

Ochre absorbs a medium amount of oil. It slows the drying of oil paint, but forms an excellent film.

Toxicity

Ochre is not considered toxic, but care should be used in handling the dry powder pigment to avoid inhaling the dust.

Ochre, Gold

Gold ochre is a natural earth containing clay tinted by hydrated iron oxide, such as yellow-brown limonite or brown-yellow to green-yellow goethite.

Ochre, Warm Red

Warm red ochre is a natural earth containing clay tinted by hydrated iron oxide that gives an exceptionally warm orange red hue.