Italian Raw Sienna is an iron oxide earth pigment that resembles yellow ocher. Unlike yellow ocher, which generally is opaque, Sienna is transparent. Italian Raw Sienna is from the clay soil of Tuscany near Monteriggioni.

The pigment now identified as Sienna is a yellow iron oxide earth pigment generally warmer and more transparent than other yellow earth. In the scientific study of historical pigments, it is not generally possible to distinguish yellow earth and Sienna earth. Even in the case of modern pigments, there is often not a 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 pigment since the Renaissance. The localities where Sienna earth is found are not confined to the original ones near Siena. Still, earth of similar quality is found throughout Tuscany, Sicily, Sardinia, the Luberon Mountains of France, and the Hartz Mountains of 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.

Iron oxide earth pigments, often called, are an important 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, secondary coloring ingredients, 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.

Iron oxide hydroxide is the principal color-producing ingredient in Sienna pigments. 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.

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

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 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 goethite and hematite, associated with varying mineral impurities.

Our Italian Raw Sienna is from natural Sienna earth extracted from the clay soil of Tuscany near Monteriggioni, which is rich in limonite, a term used for unidentified hydroxides and oxides of iron, with no visible crystals and a yellow-brown streak. Limonite is commonly composed of the mineral goethite but can also consist of varying proportions of the minerals lepidocrocite, hisingerite, jarosite, maghemite, hematite, etc.

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.

Sienna requires more oil to make it oil-based paint than other earth pigments except for umber. 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, Sienna would require 45–55 grams (by weight) of linseed oil to grind 100 grams (by weight) of pigment to form a stiff paste.

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

Rublev Colours Pigment: Italian Raw Sienna