Our Blue Ridge Burnt Umber is a natural mineral from Virginia, USA that has been roasted and is a dark warm-brown pigment used in tempera, oil and watercolor mediums, obtained from certain natural clays colored by the oxides of iron and manganese.
Origin and History
Brown earths have been known since prehistoric times, and were mentioned in the earliest painting treatises. However, the name umber did not appear until the 16th century. Sixteenth century Italian painting treatises used the term terra d'ombra, which was translated as umber for a type of brown earth. The name is likely derived from ombra, Italian for "shadow," because the pigment was useful for dark or shadow areas. Writers of the 17th century mentioned calcining umber before use and the term burnt umber entered literature around this time. The term raw umber was not commonly used in English treatises before the 19th century. The name umber was frequently used for organic brown pigments in the 18th and 19th centuries, while brown iron oxides were referred to as brown ocher. Today, the name is most often associated with brown earths composed primarily of iron and manganese oxides. In the 17th century, Turquet de Mayerne made the first association of umber with manganese containing iron oxide pigments by describing the drying properties of umber and the fact the color became brown-red by burning.
Historically, European sources of brown earths (goethite) were mined throughout many regions of Europe. The dark brown umbers, containing 45% to 70% iron oxide and 5% to 20% manganese dioxide, were originally extracted from the northern Italy, but are now mined primarily in Cyprus. Other sources of umber were found in England, France, Germany and later in the United States.
Iron oxides earth pigments are an important group of inorganic pigments derived from natural minerals. Iron oxide pigments are yellow, red and brown, but artists know them as ocher, sienna, red oxide and umber. Unlike manufactured pigments, the color of natural iron oxide pigments varies with the composition of the particular segment of earth from which they come. The color of these pigments is derived from three constituents: the principal coloring ingredient, secondary coloring ingredients and a base. The combination of these ingredients produces the particular color of the earth. The innumerable forms and variations in which these ingredients can combine result in the wide range of possible yellows, reds and browns. Principal coloring ingredient: Iron oxide is the principal color producing ingredient in the earth. The properties of the particular iron oxide present in the earth determines its color. The nature of the iron oxide found in the deposit, rather than its percentage, is critical to the resulting earth color. Most rock contains some iron oxide. Those bearing the least amounts are limestone. white clay and colorless kaolin. Those containing the highest amounts are the rocks from which metallic iron is extracted. Secondary coloring ingredients: Calcite (calcium carbonate), pyrolusite (manganese oxide) and quartz (silica) are some common accessory minerals that affect the specific color of natural iron oxides. Manganese oxide, for example, enriches the brown in umbers. Base: Nearly all iron oxides have a clay base. Clay is the weathered product of silicate rocks and is extremely varied in composition. As a result, it has numerous effects on the earth's color.
Umber is a yellowish brown to greenish brown mineral pigment used in tempera, oil and watercolor, obtained from natural earths colored by the oxides of iron and manganese. Just as with sienna, the chemical composition of umber is closely related to its iron oxide content. 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 greatly. Umbers with the highest tinting strength are those with the highest content of iron and manganese. 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 Russian and Ukraine about 4% manganese.
Our burnt umber from deposits in the Blue Ridge Mountains of Virginia, USA is finely ground and exhibits a warm brown masstone with reddish brown undertones. It has excellent tinting strength and good covering power.
As demonstrated in experiments conducted by K.I. Tolstikhinoy, the chromacity of natural iron oxide and clay pigments are closely related to the iron oxide content, and from a mineralogical point of view -- the content of . 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%.
A characteristic of pigments containing goethite is their ability to change color when heated. Heating umber causes the hydrated iron oxide to give up water and with the resulting dehydration darken in shade while its tone intensifies. At temperatures exceeding 300° C yellow and brown pigments acquire red-brown tones. The most intensive red tones are obtained as a result of calcining goethite at a temperature between 500-600° C. The change in color is directly related to the dehydration of goethite and its transformation into . Prolonged heating at high temperatures causes another change into a mineral of dark gray color -- magnetite. Roasting umber gives a pigment of black-brown color known as burnt umber.
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.
Oil Absorption and Grinding
Umber moderately absorbs oil when dispersing it in this medium. The oil absorption ratio is 45–55 parts by weight of linseed oil to 100 parts by weight of pigment. If the measurement were grams, umber would require 45 to 55 grams of linseed oil to grind 100 grams of pigment to form a stiff paste. Due to its manganese content, umber hastens the drying of oil, and forms a good, flexible film.
Umber is considered non-toxic, however, manganese, a constituent of umber, is considered moderately toxic, so care should be exercised when handling the dry powder pigment so as not to inhale the dust.
Rublev Colours Pigment: Blue Ridge Burnt Umber