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Earth pigments are naturally occurring minerals containing metal oxides, principally iron oxides and manganese oxides, that have been used since prehistoric times as pigments. The primary types are ochre, sienna and umber.
After mining, the mineral used for making a pigment is ground to a very fine powder (if not already in the form of clay), washed to remove water-soluble components, dried, and ground again to powder. For some pigments, notably sienna and umber, the color can be deepened by heating (calcination) in a process known as "burning", although it does not involve oxidation but instead dehydration.
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