3.2.1 XRF goes portable
The first truly portable, hand-held instrument on the market was produced in 1994 by Thermo Scientific/Niton1. The downsizing of instruments from bench-top to hand-held was thanks to the development of smaller components, such as the Peltier cooled Si-PiN X-ray detector (Pantazis et al., 2010). Early instruments contained a radioactive source, which has the advantage of being highly compact, but the obvious disadvantage of being radioactive and therefore needing periodic replacement and special licensing (Liritzis and Zacharias, 2011). The development of miniature X-ray tubes resolved this issue, as well as increasing the potential kV, thus improving the accuracy of results and range of elements that could be measured. Alongside this advancement, the improvement in detectors (e.g. the silicon drift detector, SDD), automatic filter selection, and fundamental parameters (FP) software, have transformed the size, ease of use and accuracy of XRF technology (Liritzis and Zacharias, 2011). Ease of use and better software calibration does not eliminate the need for empirical calibration and the use of recognised international standards (Shackley, 2011a), but takes away the exclusivity of the technology from highly specialised laboratories (Frahm and Doonan, 2013). Essentially, without empirical calibration the analysis is qualitative and incomparable, although the results may be internally consistent (Speakman and Shackley, 2013, Frahm, 2013). For some studies this may, of course, be more than acceptable and suitable to the research goal (Frahm, 2013). The idea of using the manufacturer’s settings and the suitability of using internally consistent results only, in order to answer archaeological research questions, is tested in regard to the data sets presented within this thesis. The definition of portable in this chapter is not in the purist sense of the word, and some instruments in papers cited require a power supply, whilst others are handheld. This theme is discussed further below.
1 See http://www.niton.com/en/portable-xrf-technology.
37 3.2.2 Obsidian
The use of pXRF in archaeology has grown exponentially in the past decade or so, as have the variety of applications. The focus has been predominantly on lithic provenance, initially through the vocal support and expertise of M. Steven Shackley, Robert Speakman and colleagues who have all been part of the Geoarchaeological XRF laboratory in, Berkley, U.S.A at some point in time. New specialist laboratories have also appeared over the last decades, such as at the University of Missouri Research Reactor (MURR), and at The Center for Applied Isotope Studies at the University of Georgia, amongst others. For published examples see Phillips and Speakman (2009), Craig et al. (2010) Shackley (2010), Speakman et al. (2011), Shackley (2011a), Speakman and Shackley (2013). This has now an echo on this side of the Atlantic from Sheffield University through the works of Frahm (2013), Frahm et al. (2013), Frahm and Doonan (2013) and Frahm and Feinberg (2013). Similar studies increasingly abound worldwide, and although some do not adhere strictly to purely portable instrumentation, they can be considered similar in aim and scientific method (Jia et al., 2010, Nazaroff et al., 2010, Burley et al., 2011, Millhauser et al., 2011, Sheppard et al., 2011, Forster and Grave, 2012, McCoy et al., 2011, Kellett et al., 2013, Neri et al., 2015). These all are concerned with obsidian, which lends itself especially well to the technique. Obsidian is a geological sample which is not subject to the plethora of taphonomic processes that would otherwise be detrimental to non-destructive analysis, and in some senses the focus on obsidian by the most vocal, has stifled the critical debate over the method and application of pXRF in archaeology. The success of pXRF with lithic sourcing is due to it being potentially accurate to ppm (mg/kg) for mid-weight elements (from Ti, Z=22 to Au, Z=79, although few studies use elements heavier than Z=58 as the signal to noise ratio declines) for the majority of recent (i.e. since 2008) pXRF instruments. The combination of a proportion of a small group of elements present as major and minor traces within the artefact, which are statistically treated and compared to likely sources, thus providing a potential provenance. This plays to the strengths of the instrument, which is non-destructive and allows rapid analysis without complex sample preparation, therefore allowing a large quantity of artefacts to be analysed, giving a statistically viable and relevant data set. In a more recent article in this obsidian trend, Frahm et al. (2014) demonstrate that by teaching the portable XRF the chemical
‘fingerprint’ of selected obsidian types, the analytical time required to provenance the artefact is minimised. They suggest identification can be achieved in ten seconds using pXRF allowing hundreds of artefacts to be analysed and sourced, per day. Again, obsidian is ideal, as the chemical signature can often be constrained to a singular volcanic event or geographic area.
There are issues with the application of pXRF that can, generally speaking, be safely ignored in obsidian sourcing. These are the elemental range required, moisture content, the surface of the
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sample, the corrosion/contaminate layers on artefacts from burial or conservation, the homogeneity and therefore representativeness of the sample area and the detector resolution (overlap between emitted wavelength/energies). These issues are discussed below.
3.2.3 Beyond obsidian
Aside from obsidian, pXRF is also readily applied to ceramics, glass, paints and metal artefacts, and a range of lithics types. The appeal of the instrument, with its flexibility and ease of use, means it is rapidly becoming a feature in research and conservation of a wide range of materials.
For example, pXRF has been applied to carbonate rocks, more specifically limestone in Sicily, to source the materials used for construction materials and sculpture (Barbera et al., 2013). Away from geology, Uda et al. (2002) and Abe et al. (2012) considered the composition of pigments and paints on Egyptian glass and ceramics, and Bonizzoni et al. (2011) looked at pigments in a sarcophagus. Glass from the far-east have also been studied using pXRF (Liu et al., 2011, 2012, Tantrakarn et al., 2012) and Europe (Oikonomou et al., 2008), and the study of ceramics (Forster et al., 2011, Frankel and Webb, 2012) and glazes (Pappalardo et al., 2004) is also becoming frequent. In common for many studies is the purpose, which is generally provenance. This can be the provenance of the raw material itself, or that of the pigments, dyes or paints used. This is not to say other aspects are not considered, such as the manufacture techniques of the objects or issues connected to preservation and conservation. XRF and pXRF are also an established technique for testing the alloy composition of artefacts, although it is not always sensitive enough for more than coarse compositional analysis of objects (Gliozzo et al., 2011, Heginbotham et al., 2011, Martinón-Torres et al., 2012, Karydas et al., 2004), however, it is often more than sufficient for historic and prehistoric artefacts where the manufactured composition contains considerable variation.
Once away from the dominant field of provenance, the amount of published studies relating to archaeology falls dramatically. Indeed, few studies have been published in the field of archaeology using truly portable XRF (i.e. hand held) that do not relate to the provenance and/or composition of an artefact in a museum or laboratory. Few studies use the instrument on site, and of those that do, many are in a stable field station rather than outside exposed to the elements. For example Carter (2009) used the hand-held Bruker Tracer III-V at Catalhöyük, although the analysis of pigments on skulls was done at the local museum rather than in situ. As Phillips and Speakman (2009) point out, the portability of the instrument alleviates the need to export artefacts to a foreign laboratory, something that can be problematic, and in some cases impossible. In addition, analysis on artefacts in situ, i.e. outdoors on-site is rather pointless. The only cases where this is perhaps more relevant is when considering phenomena on upstanding monuments and buildings, and soils and sediments. There is huge range and potential in using
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pXRF on archaeological and historical sites for the purposes of in situ conservation, researching past activities that do not leave clear artefact or other physical evidence, or refining the understanding of those that do, such as the function of oven constructions (Cook et al., 2009).
The debate over what defines ‘portable’ is well covered by Frahm et al. (2014), who chooses to divide instruments in the portable category into hand-held (HH), field-portable and lab-based.
The first division is useful, as it differentiates between instruments that in theory can be moved to locations and those that can be easily carried and used without mains electricity. The second is vague and less useful, as many field-portable instruments are light and flexible but require a power supply and stand to operate (e.g. Outstex used by Liu et al. (2012)) and many are portable in the sense that can be moved but weigh 8 kilos (see Frahm 2013). As many prospective surveys are carried out over large areas and/or in remote locations, the division in this sense is between those instruments that allow on site, in situ results without extensive resources, and those that do not. A reassessment of the definition is not offered here as to choose which instruments fit into which category, and what is suitable for in situ analysis varies with the physical constraints and resources of the fieldwork. In addition, with a rapid technological development in this area, the range of lightweight, battery powered instruments is moving steadily forward, meaning soon the debate over what qualifies as portable, will soon be redundant.
Lack of published studies using in situ pXRF and pXRF on archaeological sediments does not reflect the potential. Portable technology is demand-led innovation from industry, such as mining/mineral extraction, metal sorting and quality control, and environmental monitoring. In common for these industries is the need for robust, weatherproof design and flexibility, which the major manufactures have long since provided2. Robustness is not the only advantage to pXRF design. They allow for non-specialist spaces to be utilised, and the instrument to be applied in a more reflexive, intuitive manner. Perhaps the lack of published studies is the traditional divide between laboratory science in archaeology and fieldwork, and specialists in instrumental techniques in archaeology have a tendency to focus upon portable objects rather than the constraints or application of portable instruments. There are limitations to pXRF in terms of specialist calibration and accuracy over a wide elemental range, however, the question is whether these are offset by the flexibility and more fundamentally, what is the level of precision
2 Major manufacturers referred to are: Olympus (http://www.olympus-ims.com/en/xrf-xrd/delta-handheld/), Themo Scientific/Niton (http://www.niton.com/en/portable-xrf-technology),
Bruker (http://www.bruker.com/products/x-ray-diffraction-and-elemental-analysis/handheld-xrf.html), Oxford Instruments (http://www.oxford-instruments.com/products/analysers/handheld-xrf-analyser-x-met7000-series),
and Spectro/Ametek (http://www.spectro.com/pages/e/p010602_spectro_xsort_overview.htm).
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required in archaeology to provide reliable, applicable data? These issues will be addressed through the application to geochemistry in archaeology.