Blue Ridge Burnt Sienna Pigment

Burnt sienna is an iron oxide earth pigment that resembles a red ocher. Unlike ochers, which generally are opaque, siennas are more translucent and warmer. When sienna earth is calcined (roasted) at high temperatures, it loses its water content (hydration) and becomes burnt.

Origin and History

The pigment, now identified as burnt sienna, is a calcined yellow iron oxide earth pigment that is generally warmer and more transparent than other yellow earth. In the scientific study of historical pigments, distinguishing yellow earth and siennas is not generally possible. Even in the case of modern pigments, there is often no clear dividing line between yellow earth and raw sienna.

The pigment sienna owes its name to the Italian city, Siena, in the Tuscany region. An area of rolling hills, it has been famous for mining and producing sienna since the Renaissance. The localities where Sienna is found are not confined to the original ones near Siena. However, an earth of similar quality is found throughout Tuscany, Sicily, Sardinia, the Luberon Mountains in France, and the Hartz Mountains in Germany. In the United States, in Pennsylvania, deposits of sienna earth are found, and rich deposits have been developed in the mountain ranges of Virginia. In the European part of Russia, the most widely known sources of sienna earth are the Kudinovskoye deposit in the Moscow region and the Ukholovskoye deposit of the Ryazansk province.

Sienna earth has been used as a pigment since prehistoric times, although the name terra di Siena for yellow earth did not appear in painting treatises until the second half of the 18th century.

Source

Iron oxide earth pigments, often called, are an essential group of inorganic pigments derived from 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 iron oxide earth pigments varies with the composition of the particular segment of earth from which they come. The color of sienna earth pigments is derived from three constituents: the principal coloring ingredient, a secondary coloring ingredient, and a base. The combination of these ingredients produces the particular color of the earth. The innumerable forms and variations these ingredients can combine result in a wide range of yellows, reds, and browns.

Principal Coloring Ingredient

Iron oxide hydroxide is the principal color-producing ingredient in sienna earth. The properties of the particular iron oxide hydroxide in the earth determine 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

Calcium, manganese oxide, carbonic materials, silica, and limestone are some common modifiers that affect the specific color of natural Sienna earth pigments. Manganese oxide, for example, enriches the brown in umbers.

Base

Nearly all iron oxides have an alumina-silicate (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.

Natural sienna earth is a yellow pigment with a more or less brownish-red tinge in the mass tone and a more or less yellowish undertone. It differs from the yellow ocher by having a much deeper color, more than twice the tinting power, containing only two-fifths as much silica, only one-third as much alumina, and from two and one-half to three times as much iron oxide hydroxide, and in addition to this from one to one and one-half percent of manganese oxide, to the presence of which is due the color difference.

Natural sienna earth closely resembles yellow ocher by its composition. The content of iron oxide hydroxide in sienna varies from 40 to 70%, while in ocher, it may be less than 20%. A. H. Church reports the typical constituents of sienna based upon three analyses given by M. G. Hurst: Hygroscopic water 8.2 to 17.5%; combined water which includes traces of organic matter 9.0 to 12.4%; manganese dioxide 0.6 to 1.5%; iron oxide 45.8 to 59.7%; and silica 5.0 to 17.4% [The Chemistry of Paints and Painting, p. 230]. The differences in color between ocher and sienna earth pigments are most likely due to the degree of hydration or quantity of water bonded to its ferric oxide content. These pigments are composed of goethite and hematite, associated with varying proportions of mineral impurities.

Roasting natural sienna produces a significant change in its hue and the depth of its color. The iron oxide hydroxide becomes converted to iron oxide, accompanied by a significant increase in the transparency and depth of the color. Our Blue Ridge burnt sienna is roasted in furnaces from natural sienna earth extracted from deposits in the Blue Ridge mountains of West Virginia, United States.

Permanence and Compatibility

Burnt sienna does not react with other pigments and is effectively used in fresco, oil, tempera, and watercolors. It is considered permanent, with medium tinting strength and excellent transparency. It does not react with solvents and is indifferent to alkalis but is partially soluble in acids.

Oil Absorption and Grinding

Burnt sienna requires a large proportion of oil to make it into a good oil-based paint. The oil absorption ratio is 45–55 parts by oil weight to 100 parts by weight of pigment. If the measurement were grams, burnt sienna would require 45–55 grams (by weight) of linseed oil to grind 100 grams (by weight) of pigment to form a stiff paste.

Toxicity

Burnt Sienna is considered non-toxic, but care should always be exercised when handling the dry powder pigment to avoid inhaling the dust.