The use of plasma-chemical extraction in radiocarbon dating of rock paintings is discussed. Radiocarbon dating of rock art allows an inventory of images to be studied along with other cultural remains of a given archaeological time period. Assigning painted images to a particular prehistoric culture allows archaeologists to gain information on artistic, cultural, and religious aspects of a people. Most dates on pictographs worldwide have been accomplished through measuring radiocarbon in either charcoal pigments or beeswax, the latter occurring only in a limited area of Australia.
Because the samples are small and contaminated, we chose to use the plasma oxidation process to prepare the samples for accelerator mass spectrometric radiocarbon dating. The plasma-chemical oxidation (PCO) process was initially developed for selectively reacting with organic binding media in rock paintings, allowing the resulting carbon dioxide to be dated with accelerator mass spectrometry (Rowe 2009; Rowe and Steelman 2002). While there is still some question about the nature of such binding media, the method is also applicable to charcoal or soot, which is much more readily characterized through microscopy or Fourier transform infrared spectroscopy (Bonneau et al. 2011; Bonneau et al. 2017).
Oxygen gas was research-grade (99.999+%). The oxygen plasma has been shown to react with organic carbon in the paint samples at a sufficiently low temperature (~150°C) that the inorganic oxalates and carbonates present are unaffected (Rowe and Steelman 2002; Russ et al. 1992).