<skos:note xml:lang="en">Paint pigments prepared by grinding minerals containing containing iron oxides and other materials such as clay, chalk, and silica. Each deposit has a unique composition, working characteristics, and color, in varying ranges of yellow, orange, red, brown, and black. Earth colors have been used since prehistoric times and are permanent in all techniques. Synthetic iron oxides, called Mars colors, have been produced since the late 18th century.</skos:note>