Research topic: ocher

ocher , mixture of varying proportions of iron oxide and clay, used as a pigment . It occurs naturally as yellow ocher (yellow or yellow-brown in color), the iron oxide being limonite, or as red ocher, the iron oxide being hematite. 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 produced in the United States, in France... Read more