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Umber [500 g bag] (17.6 oz.)

Item No: 460:410500
Category: 29


Price:  $22.50
Quantity: 





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Description:
Common Names: English: umber, raw umber; French: ombre, ocherous, terre d'ombre; Italian: terra d'ombra [literally, earth of shadow or shade], ombra; Spanish: umbra; Latin: umbra [literally means shadow, shade]
Color: Brown Colour Index: Pigment Brown 7 (77491) (77492) (77499) ASTM Lightfastness: Oil: I Acrylics: I Watercolor: I

A yellowish brown to greenish brown mineral pigment used in tempera, oil and watercolor mediums, obtained from certain natural clays variously colored by the oxides of iron and manganese. Just as with sienna, the chemical composition of umber is closely related to the iron oxide content of ochre. What makes umber different is the increased content of manganese. Other substances naturally occurring in umber include clay, talc, and calcium carbonate do not affect its color. Umbers with the highest tinting strength are those with the highest content of manganese and iron. Some of the finest umber comes from Cyprus and may contain up to 16% manganese oxide. German umber typically contains 1-2% manganese, English umber 7%, and umber from the Ukraine about 4% manganese. Our umber is from deposits found in the Moscow region of Russia. Our pigment is finely ground and exhibits a yellowish brown masstone with greenish undertones. It has excellent tinting strength and good covering power.
Tinting Strength: As demonstrated in experiments conducted by K. I. Tolstikhinoy [17], the chromacity of natural iron oxide and clay pigments are closely related to the content of iron oxide, and from a mineralogical point of view -- the content of goethite (yellow iron oxide). Thus, with the content of iron oxide less than 23%, luminosities of the tone of pigment compose 40-50%, the purity of tone 60-70%. With the content of the iron oxide from 22 to 74%, luminosities of tone varies in interval of 25-40%, the purity of tone from 70 to 85%, and with the content of the iron oxide more than 75%, luminosity is located in interval of 18-25%, the purity of tone 83-90%.
Pigment Characteristics: A characteristic of pigments containing goethite is their ability to change color when heated. Heating ochre and sienna causes the hydrated iron oxide to give up water, and with the resulting dehydration darken in shade while its tone intensifies. At temperatures exceeding 300C yellow pigments acquire red-brown tones. The most intensive red tones are obtained as a result of calcining goethite at a temperature between 500-600C. The change in color is directly related to the dehydration of goethite and its transformation into hematite (red iron oxide). Prolonged heating at high temperatures causes another change into a mineral of dark gray color -- magnetite [12]. Roasting umber gives a pigment of black-brown color known as burnt umber.
Alteration: Some manufacturers tint their ochre, sienna or umber pigments or paints with aniline red (synthetic organic pigment) to give them a more intense reddish hue. To determine whether a sample of pigment has been altered a simple test can be made with ethyl alcohol [9]. Iron oxide pigments are unaffected by alcohol. However, if alcohol exhibits a reddish tint after a small sample is immersed in it, then this may indicate the presence of aniline red in the pigment.
Origin: Historically, European sources of yellow earths (goethite) were mined near Leghorn or Siena (Tuscany, Italy). These are siennas containing roughly 50% iron oxide and less than 1% manganese dioxide. The dark red or brown umbers, containing 45% to 70% iron oxide and 5% to 20% manganese dioxide, were originally extracted from the Umbria region of Italy, but are now mined primarily in Cyprus.
Permanence and Compatibility: Umber does not react with other pigments and is effectively used in fresco, oil, tempera and watercolors. It is considered to be permanent with medium to excellent tinting strength and high opacity. It does not react with solvents, and is indifferent to alkalis, but is partially soluble in acids.
Absorption and Grinding: Umber moderately absorbs oil when dispersing it in this medium. It hastens the drying of oil, and forms a good, flexible film.
Toxicity: Umber is considered non-toxic, but care should always be exercised when handling the dry powder pigment so as not to inhale the dust.

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500 gram bag
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