Earth colors are made from natural iron oxides that are found all over the earth in various shades of brown and muted shades of red, orange, yellow, and green. Other minerals deposited with iron oxides, such as calcium, manganese or carbon, effect their colors. Ochre is clay and silica colored with various kinds of iron oxides. It contains silicates and aluminates that increase the transparency of the pigment. Umber is found in sites where naturally occurring manganese dioxide combines with the iron.
Pigments containing manganese make quick drying oil colors.
Burnt Umber and Burnt Sienna are made by roasting earth pigments until the desired reddish colors are produced. In the studios of the Old Masters, painters pushed against the limitations of their colors. Sienna and Umber are key colors in creating effects of depth like Caravaggio's chiaroscuro or Leonardo's fumato with its almost imperceptible transitions from light to dark. A hundred years after the Masters, there was a revival in their techniques. Asphaltum glazes were used to make oil paintings look artificially old.