Iron oxides earth pigments are an important group of inorganic pigments derived from natural minerals.

Iron oxide pigments are yellow, red and brown, but are also known as ochre, 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

Pigment Characteristics

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 hematite. 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.