Italian Yellow Earth is a natural iron oxide hydroxide mineral (goethite) from quarries in northern Italy.
Origin and History
Known since the antiquity, iron oxide earth pigments occur naturally as yellow () or as red () earths. Yellow earths have been used as a pigment since prehistoric times and is perhaps the most widely used pigment for artists paints.
Italian yellow earth is a natural earth containing clay tinted by hydrated iron oxide and is composed of and traces of clay, gypsum or manganese carbonate. Historically, yellow earth was designated by the mineral name limonite. Limonite, however, is not a true mineral, but a general term used to describe all forms of hydrated iron oxide minerals (α-FeOOH) that occur as natural earth. Limonite includes the minerals , akaganeite and lepidocrocite. The names for yellow iron oxide pigments are confusing. The names earth and ocher are suitable when it is clear that the pigment comes from a natural source. Earth is a more general term, since ocher refers to a specific type of iron oxide deposit containing kaolin (clay) and quartz. To be considered an ocher, for example, the content of iron oxide must not be less than 12%. Depending upon the content of hydrated iron oxide, the color of ocher varies from light yellow to orange-red. Like red iron oxides (), they are found around the world and have been used as pigments since prehistory. French ocher, historically one of the best grades of limonite, contains about 20% iron oxide and is high in silica.
Ocher grades into sienna, a yellow-brown pigment containing a higher percentage of iron ore than ocher as well as some manganese dioxide. Sienna grades into umber, which is darker brown and contains a higher percentage of manganese dioxide. Burnt sienna is brown or bright red, burnt umber is a darker brown than umber.
Ocher is not naturally usable as a pigment and therefore needs to undergo several important processes, such as:
- Extract ore from the quarries.
- Separate ocher from sand using water and centripetal force.
- Blend different ocher extracted from diverse veins to obtain the selected colors.
- Grind to 50 microns and pack.
Permanence and Compatibility
Ocher is among the most permanent colors among the artist's palette. It is compatible with all other pigments, and can be used with good results in all mediums.
Oil Absorption and Grinding
Ocher absorbs a moderately high amount of oil. The oil absorption ratio is 30–45 parts by weight of linseed oil to 100 parts by weight of pigment . If the measurement were grams, Italian yellow earth would require 30–45 grams of linseed oil to grind 100 grams of pigment to form a stiff paste. It slows the drying of oil paint, but forms an excellent film.
Ocher is not considered toxic, but care should be used in handling the dry powder pigment to avoid inhaling the dust.
Pigment: Italian Yellow Earth