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

Steyn, Ruan (2014-04)

Thesis (MSc)--Stellenbosch University, 2014.

Thesis

ENGLISH ABSTRACT: Non-destructive portable X-ray Fluorescence (pXRF) and Particle Induced X-ray Emission (PIXE) were used to measure the elemental concentration of rock art fragment paintings. For pXRF the Amptek Silicon Drift Detector (SDD) and Niton XL3t spectrometers were used to perform the measurements. These two spectrometers 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. The validation is important to establish if the pXRF technique is properly understood and used and to advance the investigation to more complex rock art paintings, with heterogeneous and layered properties. The elemental concentrations obtained for the Standard Reference Materials (SRMs), which were used for the validation, were in good agreement with that of the known concentration of the SRMs. The two rock art fragments which were analysed from the Mount Ayliff and Ha Khotso caves were part of larger rock art painting prior to it being naturally exfoliated from 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. The colour of the paint ranged from black, shades of brown and shades of red. The black paint is due to manganese or charcoal. The red colour is due to iron oxide and the red-brown colour is due to Hematite (a type of ferrous oxide) [1]. For the Ha Khotso fragment 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 spectrometers indicated that the elemental concentration was uniform across the fragment. This is due to the formation of a uniform layer of minerals such as silica and calcium introduced by the seepage of water through the cracks of the cave. Therefore no correlation could be established between the colour of the rock art paint and the elements detected, as was found with the work done by Peisach, Pineda and Jacobson [1]. For the Ha Khosto rock fragment a relation between the Ca composition and the cream colour of the rock art paint was established. Both the PIXE and pXRF techniques were used to identify the compound concentrations of the Ha Khotso rock art fragment. The comparison between the two techniques highlights the complexity of rock art paint analysis. The results from the PIXE elemental mapping indicated the non-uniform distribution of the elements in the analysed region. From the rock art fragment measuring the analysed points 5 times and obtaining the same results, indicated that the particle size and inhomogeneities did not have much effect on the compound compositions. In order to obtain high accuracy results with pXRF, sound scientific methodology with specific knowledge and expertise, not only about the XRF technique, but also about the sample under investigation is required. For alloy analysis pXRF is well suited, the analysis of geological material however more complex, since they are composed predominately of low atomic elements e.g. silicon, aluminium, magnesium, sodium, oxygen and carbon – all of which are excited with very low efficiencies.

AFRIKAANSE OPSOMMING: Nie-beskadigended X-straal Fluoresensie (pXRF) en Deeltjie Geinduseerde X-straal emmissie (PIXE) was gebruik om die elementêre konsentrasie van die rotstekeninge in hierdie studie te bepaal. Vir die pXRF-tegniek is die “Amptek Silicon Drift Detector (SDD)” en die “Thermo Scientific Niton XL3t” spektrometers gebruik gemaak om die metings uit te voer. Die twee spektrometers maak gebruik van verskillende spektrum analiseringsmetodes.Die “Peak Deconvolution (PD)” analiseringsmetode is gebruik vir die “Amptek SDD” en die “Inverse Overlap Matrix (IOM)” analiseringsmetode is gebruik vir die “Thermo Scientific Niton XL3t” spektrometer. Vir die validasie van die pXRF-metode is van allooie, muntstukke en rots standaarded gebruik gemaak. Die validasie is belangrik om vas te stel of die pXRF tegniek behoorlik verstaan en gebruik word en om die ondersoek te bevorder na meer komplekse rotstekeninge, met heterogene en lae eienskappe. Die element konsentrasies wat vir die “Standard Reference Material (SRM)” wat gebruik is vir die validasie, was in 'n goeie ooreenkoms met die van die konsentrasie van die SRM, wat bekend is. Die twee rotstekeninge wat ontleed is van die Mount Ayliff en Ha Khotso grotte en was deel van 'n groter rots kuns skildery voordat hul natuurlik afgebreek het. Vir die Mount Ayliff rotskuns, is sewe verf punte, twee ongeverfde rots (vernis) punte aangrensend aan die verf en die agterkant van die rots ontleed. Die kleur van die verf het gewissel van swart, skakerings van bruin en skakerings van rooi. Die swart verf kan toegeskryf word aan mangaan of houtskool. Die rooi kleur is as gevolg van ysteroksied en die rooi-bruin kleur is as gevolg van Hematiet ('n tipe van yster oksied) [1]. Vir die Ha Khotso rotskuns is die verf aan die voorkant van die rots en die rots substraat (agterkant van die rots) ontleed. Vir die Mount Ayliff rotstekening het die resultate vir beide pXRF spektrometers aangedui dat die elementele konsentrasie uniform oor die rotstekening is. Dit is as gevolg van die vorming van 'n uniforme lagie van silica en kalsium, wat deur die sypeling van water deur die krake van die grot na die oppervlak van die rotstekening beweeg het. Daarom kon geen korrelasie tussen die kleur van die rotstekening en die elemente wat gemeet is bepaal word nie, soos gevind deur die werk van Peisach, Pineda en Jacobson [1]. Vir die Ha Khotso rotstekening is ‘n verband tussen die room kleur van die rotstekening verf en Ca konsentrasie gevind. Beide die PIXE en pXRF tegnieke is gebruik om die konsentrasies van die Ha Khotso rotstekening te identifiseer. Die vergelyking tussen die twee tegnieke beklemtoon die kompleksiteit van rotstekening verf analise. Die resultate van die PIXE elementele karakterisering het aangedui die nie-eenvormige verspreiding van die elemente in die ontlede area. Deur die meting van die ontlede punte 5 keer te herhaal, en dieselfde resultate verkry, is ‘n aanduiding dat die deeltjie grootte en inhomogeniteite nie veel invloed op die elementele konsentrasies het nie. Ten einde 'n hoë akkuraatheid resultate te kry met pXRF, moet goeie wetenskaplike metode toegepas word met spesifieke kennis en kundigheid, nie net oor die XRF tegniek, maar ook oor die rotstekening wat ondersoek word vereis. pXRF is wel geskik vir die ontleding van allooie, die ontleding van geologiese materiaal is egter meer kompleks, aangesien die materiaal hoofsaaklik bestaan uit lae atoomgetal elemente bv silikon, aluminium, magnesium, natrium, suurstof en koolstof - wat almal met lae doeltreffentheid opgewek en baie afgerem word in die materiaal.

Please refer to this item in SUNScholar by using the following persistent URL: http://hdl.handle.net/10019.1/86541
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