Natural inorganic pigments

Natural inorganic pigments are among the oldest used by humans. They first appear in the funeral preparation of human remains from 60,000 years ago, and in polychrome cave art from about 20,000 years ago. Mined in prehistory from surface deposits of clay and rock, many inorganic pigments have shown extraordinary permanence over long periods of time.

With few exceptions, inorganic pigments are combinations of a mineral element with oxygen and other elements (most often sulfur, silicon or carbon) that fall in the chemical classes known as oxides, oxide hydroxides, sulfides, sulfates, silicates and carbonates.

The pigments in this category that have been of importance to watercolor painting include:

Red earths. A large and diverse category of pigments, all made from earths (mostly clays) containing large proportions of iron oxide (the dark violet to light red hematite or everyday rust, the orange to yellow lepidocrocite, or the dark brown maghemite) that is processed and sold as natural iron oxide. The pigment color may range from a dull yellow through a dull deep yellow, dull orange, dull red or dark brown (listed either as PR102 or PBr7) to near black (PBr6). The color depends on the average particle size, the presence of manganese or other elements (which darkens and dulls the color), and whether water is chemically bonded within the iron oxide crystals. (The dull red orange to yellow hydrous oxides contain water, the maroon to dull red anhydrous oxides do not.) Although red iron oxides occur in all parts of the world and have been used as pigments since antiquity, rich deposits are currently found near Malaga, Spain (Spanish red, which has a characteristic brownish undertone) and in Ormuz, in the Persian Gulf (Persian red). Historically, European sources of yellow brown earths were mined near Leghorn or Siena (in Tuscany, Italy); these are the siennas, containing roughly 50% iron oxide and less than 1% manganese dioxide. The dark red or brown earths or umbers, containing 45%-70% iron oxide and 5% to 20% manganese dioxide, were originally imported to Europe from Turkey (via Venice), but are now mined primarily in Cyprus. (The name probably derives not from the Italian region of Umbria but from the Latin ombra or shadow, referring to the original use of dark iron oxides as shadow colors.) These earths are often “burned” (calcinated or roasted at a dull red heat) to darken them (burnt sienna, burnt umber), a technique that was probably suggested around 2000 BCE by the visible reddening or darkening of pottery after it had been fired or glazed. And natural manganese ores have sometimes been added to red earths create darker red, violet or black colors in pottery clays or glazes. (Due to growing scarcity of high quality natural deposits of iron oxides, most artists’ colors are now made from synthetic iron oxides, for example the same pigments manufactured for wood stains.)

Yellow earths. Natural earths containing silica and clay, hydrous forms of iron oxide (yellow brown limonite or the brown yellow to green yellow goethite), and traces of gypsum or manganese carbonate. Like the red iron oxides, they are found around the world and have been used as pigments since prehistory. French ochre, historically one of the best grades of limonite, contains about 20% iron oxide and is high in silica. Currently workable deposits for yellow oxides are located in the Republic of South Africa and France. Most often sold as yellow ochre (PY43) or brown ochre. Most yellow clays are normally not “burnt” as heat does relatively little to alter their color.

Green earths. Clays containing large amounts of silica and the green minerals glauconite and celadonite, consisting essentially of hydrous iron, magnesium, and aluminum potassium silicates. Color varies from a dark, grayish blue green to a dark, dull yellowish green. Completely lightfast and chemically inert, green earth or terre verte has been used around the world since ancient times. In Europe, the first documented use in paintings is in Roman frescos; it was also commonly used in the Middle Ages as an underpainting for flesh tones and shadows. Originally extracted from deposits in central Europe (today’s Czech Republic), near Verona (Italy), or in France, modern supplies come from high quality deposits in Cyprus. Most of these pigment deposits originated as marine clays. In watercolors the typical color is light valued (diluted), and paints made from the genuine pigment tend to be thin and gummy. The label terre verte is often applied to paints mixed from other inorganic pigments, typically chromium oxide green (PG17) or viridian (PG18) mixed with a red iron oxide. (See also the section on chromium compounds.)

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