Portable X-ray fluorescence and nuclear microscopy techniques applied to the characterisation of southern African rock art paintings

[摘要] ENGLISH ABSTRACT: Non-destructive portable X-ray Fluorescence (pXRF) and Particle Induced X-rayEmission (PIXE) were used to measure the elemental concentration of rock artfragment paintings. For pXRF the Amptek Silicon Drift Detector (SDD) and NitonXL3t spectrometers were used to perform the measurements. These twospectrometers use different spectrum analysis methods. The Peak Deconvolution(PD) analysis method is used for the Amptek SDD and an Inverse Overlap Matrix(IOM) method is used for the Niton XL3t spectrometer.The pXRF methods were validated by using alloys, coins and rock standards. Thevalidation is important to establish if the pXRF technique is properly understood andused and to advance the investigation to more complex rock art paintings, withheterogeneous and layered properties. The elemental concentrations obtained forthe Standard Reference Materials (SRMs), which were used for the validation, werein good agreement with that of the known concentration of the SRMs.The two rock art fragments which were analysed from the Mount Ayliff and HaKhotso caves were part of larger rock art painting prior to it being naturally exfoliatedfrom the rock. For the Mount Ayliff rock art, seven paint points, two unpainted rock(varnish) point adjacent to the paint and the back of the rock were analysed. Thecolour of the paint ranged from black, shades of brown and shades of red. The blackpaint is due to manganese or charcoal. The red colour is due to iron oxide and thered-brown colour is due to Hematite (a type of ferrous oxide) [1]. For the Ha Khotsofragment the paint on the front of the rock and the rock substrate (back of the rock)were analysed.For the Mount Ayliff rock art fragment the results for both pXRF spectrometersindicated that the elemental concentration was uniform across the fragment. This isdue to the formation of a uniform layer of minerals such as silica and calciumintroduced by the seepage of water through the cracks of the cave. Therefore nocorrelation could be established between the colour of the rock art paint and theelements detected, as was found with the work done by Peisach, Pineda andJacobson [1]. For the Ha Khosto rock fragment a relation between the Cacomposition and the cream colour of the rock art paint was established. Both thePIXE and pXRF techniques were used to identify the compound concentrations ofthe Ha Khotso rock art fragment. The comparison between the two techniqueshighlights the complexity of rock art paint analysis. The results from the PIXEelemental mapping indicated the non-uniform distribution of the elements in theanalysed region. From the rock art fragment measuring the analysed points 5 times and obtaining thesame results, indicated that the particle size and inhomogeneities did not have mucheffect on the compound compositions.In order to obtain high accuracy results with pXRF, sound scientific methodology withspecific knowledge and expertise, not only about the XRF technique, but also aboutthe sample under investigation is required. For alloy analysis pXRF is well suited, theanalysis of geological material however more complex, since they are composedpredominately of low atomic elements e.g. silicon, aluminium, magnesium, sodium,oxygen and carbon – all of which are excited with very low efficiencies.

[发布机构] Stellenbosch University