Analysis of geological ochre: its geochemistry, use, and exchange in the US Northern Great Plains

Samples of pigments indigenous to the US Northern Great Plains were collected in association with the conservation of a buffalo hide tanned and painted by a Crow Indian(s) in the 19th century, which is now in the collection of the National Museum of American Indian. The pigments were characterised using a series of analytical techniques – some common and others uncommon to the conservation science field, including portable X-ray fluorescence (pXRF), X-ray diffraction (XRD), and instrumental neutron activation analysis (INAA). XRF is not capable of differentiating between various ochre samples due to high detection limits. XRD can detect some matrix minerals in each sample, but these data cannot characterise pigments by original source location. INAA is capable of characterizing ochres from different sources based on trace element geochemistry; however, the large sample size it requires (approximately 100 mg), makes sampling from objects challenging and therefore makes it difficult to use for technical art history studies that focus on museum objects. INAA is useful if applied to reference materials, such as historic pigments or known sources for historic artistic materials.