Transformation of Goethite to Hematite: Dependence of Ochre Color on Temperature and Time of Heating

Ochres, natural pigments based on iron oxide and (oxy)hydroxide minerals, range in color from yellow to red to violet to brown. Ochres have been used and manipulated by humans for more than 100,000 years, and they continue to be valued because they are intensely colored, stable to light and heat, and cheap. Ochres offer critical evidence for the evolution of human cognition and symbolic behavior. Interpretation of the archeological evidence requires an understanding of ancient processing of ochre. Of particular importance in the Paleolithic is the heating of yellowish-brown goethite to convert it to red hematite. We heat natural ochres containing goethite in a furnace and characterize the colors of the resulting hematites. Within the temperatures achievable by early humans in simple wood fires (300 to 800 °C), the color is almost entirely independent of temperature and time of heating with λm ≥ 580 nm and reflectance at 780 nm varying by less than 5%. At higher temperatures (800 to 1100 °C) natural ochres may become a brighter red (increasing reflectance by 15%), purplish red, or various shades of brown depending on the accessory minerals in the ochre.