Iron Oxide Pigments in Archaeology
Ochre (also spelled ocher) and hematite (sometimes spelled haematite) are names archaeologists use to refer to several forms of iron oxide, that is to say a type of clay or sandy clay mineral that is found in natural deposits in many different regions of the world, and used for pigments for various purposes.
Ochre comes in a variety of colors, from brown to red to yellow, and it was and is a popular pigment choice for artists beginning right with the first art of the Middle Stone Age. MSA sites such as Blombos Cave and Klein Kliphuis in South Africa have examples of engraved ochre.
Red ochre (called hematite) is associated with sesquioxide of iron (i.e., it forms in areas where the soil is well-drained), while the yellow ochre (called limonite or goethite) is hydrated iron oxide (i.e., where iron was allowed to freely combine with water).
The colors are not necessarily stable over time; in certain circumstances yellow or brown turns to red as the minerals themselves convert to hematite.
Recent studies researching the potential to source ochre--that is, to determine where the ochre came from at a particular site or instance--have been attempted in the Tucson basin of Arizona. Using instrumental neutron activation analysis (INAA), researchers were able to identify specific geochemical characteristics of three sources, based on percentages of their metals and rare earth elements.
Sources
Popelka-Filcoff, Rachel S. et al. 2008. Elemental analysis and characterization of ochre sources from Southern Arizona. Journal of Archaeological Science 35(3):752-762.
Pough, Frederick H. 1993 Hematite. Lapidary Journal(January 1993):16, 112, 116.
This glossary entry is part of the Dictionary of Archaeology. Any mistakes are the responsibility of Kris Hirst.
