The team employed a combination of plasma oxidation and accelerator mass spectrometry, which has been used to date rock paintings worldwide (e.g., Russ et al., 1990; Steelman et al., 2005; Rowe, 2009). In contrast to traditional combustion methods, plasma oxidation occurs below the decomposition temperature of carbon-containing minerals such as carbonates and oxalates; therefore, their inclusion in the measured AMS graphite target is avoided for samples with high mineral content. Thus, extensive acid washes used in conjunction with combustion are not necessary and can be avoided, minimizing the loss of organic material during wet chemical pre-treatment steps.
The plasma oxidation method together with AMS radiocarbon measurement is a direct technique for dating both charcoal and inorganic-pigmented pictographs. The Steelman laboratory uses a custom-built plasma oxidation apparatus to extract organic material from ancient paint samples for AMS radiocarbon dating (Russ et al., 1990; Rowe, 2001a; Rowe, 2009; Steelman and Rowe, 2012). Plasma oxidation negates the use of extensive acid pretreatments because plasma temperatures (<150 C) are below the decomposition temperatures of carbonates and oxalate minerals.
Results from the first intensive dating program for pigment art in the Australian arid zone: Insights into recent social complexity
Worldwide, the first direct dates for rock art were obtained by Van der Merwe et al. (1987) on charcoal pigment from a South African rock painting. Since then, several laboratories have radiocarbon-dated carbon-based pigmented rock paintings using acid-base-acid (ABA) pretreatment, combustion, and AMS measurement (see Aubert, 2012; Langley and Taçon, 2010; Steelman and Rowe, 2012; Rowe, 2012 for review articles). A unique application of ABA pretreatment was employed to directly date beeswax rock art, occurring in northern Australia.