Pigments are generally classified according to their origin. Those produced by processing colored earths are referred to as EARTH COLORS , while those produced by chemical processes from inorganic raw materials that are not in themselves coloring matter are called artificial inorganic pigments. The finest grades of earth colors occur in specific areas, e.g., French yellow ochre, Italian raw and burnt sienna, etc. Early pigments included the native earths, e.g., red iron oxides and yellow ochre, as well as manufactured pigments, such as crimson lake, lampblack, and white lead.
Pigments should be chemically inert, so as not to react in an undesirable manner with other pigments or liquids with which they may be mixed, and they should also be lightfast and not fade or darken when exposed to typical indoor lighting, such as indirect sunlight, artificial light of normal intensity, and controlled temperature and humidity variations. All pigments of very high quality will endure indefinitely under proper conditions, although some may undergo loss of color if exposed to direct sunlight. Cf: DYE . (17 , 195 , 233 )