Antica Green Earth is from the natural mineral glauconite, a greenish mineral of hydrated iron potassium silicate containing small amounts of aluminum, calcium, magnesium, sodium, and numerous trace elements. It is a grayish warm green mineral that looks like tiny flakes of the mineral mica, or small lumps of clay. The color of glauconite varies considerably from pale green, bright green, bluish-green, olive-green, and black-green, depending upon its constituent elements. Our Antica Green Earth is from basaltic tufa deposits near the ancient city of Prun, not far from Verona, in northern Italy and is known there as "terra di Prun."
Origin and History
The word glauconite is derived from the Greek word glaucos, originally meaning gleaming, later bluish green, and then silvery or gray. It has been in painting since ancient times. Restorers have proved through X-ray diffraction that the famous pigments of past centuries known as Veronese and Bohemian terre verte are in essence glauconite.
Green earth, also widely known as terre verte, is primarily composed of the minerals celadonite and glauconite. The presence of celadonite or glauconite, minerals of complex silicate composition, produces colors that vary from cold bluish greens to warmer yellow and olive hues. It may have been formed originally from biotite (a dark-colored mica), but other minerals, and even organic matter, may change it into glauconite. The mineral has a micaceous structure, that is, a silicate crystallized in monoclinic forms that readily separates into very thin leaves. It is characteristically formed on submarine elevations of ancient seabeds ranging in depth from 30 to 1,000 meters (100 to 3,300 feet) below sea level, and in the sedimentary rocks of different geological systems. Usually it forms small (from 0.8 to 1.5 mm in diameter) green round or angular grains.
The most famous deposit of green earth was found near Verona, Italy, and this mine was active until World War II. Other mines produced variations in color and texture of the pigment: Baltic states, Bohemia, Cypress, France, Hungary, Poland, Saxony, Tyrol, and the Mendip hills of England. Our green earth is from open mines near Mt. Baldo, Italy.
Preparation and Alteration
Glauconite is not found in nature as large accumulations, but rather in combination with clay or sand. Its content rarely exceeds 50% of the deposit, making it necessary to process the mineral for use as a pigment. The preparation of glauconite is usually done with levigation, sifting and sometimes electromagnetic separation. Sometimes clay or sandy minerals, tinted with aniline red or copper pigments, have been marketed under the guise of glauconite. Ethyl alcohol can be used to detect the presence of aniline red, while mixing a sample with ammonium hydroxide can reveal the presence of copper-based colors.
Use in Painting
Green earth pigments were often used by medieval artists for flesh undertones. This green underpainting neutralized the effect of the pinks and reds of the flesh colors. On the color wheel, the hue of green earth is the approximate complement to the pink used by medieval painters. Complementary colors neutralize each other, and this neutralization was important for medieval painters because of the materials they worked with. Paint pink and red tones of flesh directly onto white gesso would achieve a "sunburn" effect in the flesh of the figures. To neutralize the pink, painters painted a layer of green earth under the pink. The light would pierce the pink layer, then the green, and reach the white. White reflects all light colors, so the light is reflected back through the paint. Each color paint absorbed some colors, or wavelengths, of the light. Thus, as the light passed back through the paint to the viewer, the flesh tones appeared neutral.
Permanence and Compatibility
Earth pigments, and green earths in particular, are often described as the most permanent of colors. Earths are not affected by sunlight or by atmospheric conditions. They do not react with solvents, but according to Feller, green earth is partially soluble in acids and alkalis.
Oil Absorption and Grinding
Green earths have reasonable tinting strength and covering power, and the pigment is easily ground. Since it contains some clay, green earth absorbs oil at a moderate to high rate.
The pigment is considered non-toxic, but care should always be exercised when handling the dry powder pigment so as not to inhale the dust.
Rublev Colours Pigment: Antica Green Earth (Glauconite)