Portable X-Ray Fluorescence Analysis of Rock Art Pigments

Article excerpt

Portable x-ray fluorescence technology has proved to be a very useful tool for identifying the mineral constituents of rock art paintings. This paper describes its use to identify chromium, whose likely source was the mineral fuchsite, as the green coloring agent in Montana and Wyoming pictographs. High levels of lead were also detected in a series of pictographs from Weatherman Draw in south central Montana that were reported to have been repainted or retouched to enhance their visibility.

Keywords: x-ray fluorescence, pigment, rock art

Developed over the last century, x-ray fluorescence (XRF) spectrometry provides elemental chemical analysis. As the atoms of an element are struck by high energy photons from an x-ray source, electrons from the inner shells are knocked from their orbits around the nuclei of the atoms, causing the latter to become unstable ions. To reestablish stability, electrons from the next higher shell move to the vacant inner orbits, thereby emitting energy, a phenomenon referred to as fluorescence. Because each element maintains a different electron shell configuration, the spectrum produced by an episode of fluorescence is unique, thereby allowing the element to be identified (U.S. Department of Energy 1998:1 ). X-ray fluorescence has been used on a variety of materials in disciplines ranging from art conservation (Ferrero et al. 2002), medicine (Geraki 2002), geology (Boyle 2000), archaeology (Tykot 1996:116), and environmental sciences (Melquiades et al. 2004), to the silicon wafer industry (Istratov 2000).

Standard operating procedure for XRF has required transportation of the sample to be analyzed, or the material recovered from it, to a laboratory. Relocation can be problematic in some cases, depending on the sample's size or value, or the potential damage that can occur during sample recovery. Recently, a compact, light-weight, handheld, and field-portable XRF unit has been developed that offers new opportunities for nondestructive, on-site sample analysis (Piorek 1997:317).

During the past three years, our research has focused on determining the composition of paints recovered from rock paintings and completing an experimental study involving the creation and application of our own paint (Newman and Loendorf 2003, 2004a, 2004b). We used several analytical techniques in these studies, including inductively coupled plasma mass spectrometry, x-ray diffraction, x-ray fluorescence, Raman microscopy, and scanning electron microscopy. The most persistent problems we encountered were the presence of excessive background rock in the samples and an insufficient amount of paint necessary for successful analyses. Using the portable XRF unit, we hoped to overcome some of the difficulties associated with sample recovery and identification.

Field portable XRF spectrometers are available from a number of manufacturers. We used the Innov-X Systems XT series Model 700. The unit uses an x-ray tube instead of radioactive isotopes, is approximately 1x1 foot, and weighs less than five pounds. It uses a Windows CE software platform with a Compac iPAQ Pocket PC that can be removed for data download. It comes equipped with a minimum element package, but the user can request additional elements. Concentrations of detected elements measured in parts per million are displayed on the analyzer's LCD and can also be viewed as spectra. Collected data are exported to a Microsoft Excel spreadsheet (Innov-X Systems, Inc. 2003).

We established three goals for our initial research with the portable XRF unit: to identify the color-producing agent in green paint in Montana and Wyoming rock paintings; to explore the possibility of historic retouching or repainting of some Montana pictographs; and to determine the utility of the field portable XRF spectrometer in the study of rock art pigments.

GREEN PAINT

The research to determine the composition of green paint was conducted at the Valley of the Shields and the Musselshell sites in Montana, and at the Castle Gardens site in Wyoming (Figure 1). …