Achieving High Spatial Resolution in Elemental Mapping of Metal Samples from Archaeological Contexts
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Achieving High Spatial Resolution in Elemental Mapping of Metal Samples from Archaeological Contexts Scott Lea1, Don Baer1, Ernesto Paparazzo2, Peter Northover3 and Chris Salter4, W.R. Wiley Environmental Molecular Sciences Laboratory, Pacific Northwest National Laboratory, 902 Batelle Boulevard, Richland WA 99352, USA 2 Istituto di Struttura della Materia, Area della Ricerca di Roma-Tor Vergata CNR, Via del Fosso del Cavaliere, I-00133 Roma, Italy 3 Department of Materials, University of Oxford, Begbroke Business and Science Park, Sandy Lane, Yarnton, Oxford, OX5 1PF, UK 1
ABSTRACT Improving the characterisation of archaeological artifacts brings a need to understand better the relationships between composition, structure and properties. With archaeological material there is also a requirement to consider the effects of ageing and environmental interactions in altering the original structure and composition, both in the bulk and at the surface. However, curatorial constraints and, frequently, the condition of the objects preclude the sampling methods required for the most powerful means of structural analysis of materials, the high resolution transmission electron microscope. The samples normally available are small bulk samples and we must find other means of maximising spatial resolution in microchemical and microstructural analysis of both bulk and surface regions of the samples. This paper describes ways in which this is being achieved using the scanning proton microprobe (SPM) with both particle induced X-ray emission (PIXE) and Rutherford back scattered proton (RBS) spectra at resolutions down to ca. 1:m, electron probe microanalysis (EPMA) at 250-300nm, and scanning Auger microscopy (SAM) at resolutions of 10-20nm, but only from the surface layers of atoms in a sample. Examples will be given which demonstrate the contribution that each instrument can make, and that new and useful information is obtained each time resolution is increased. They will also show that structural features can be identified which are invisible to other microscopies. It will also be shown how modern PC-based software has greatly enhanced the mapping capability of all instruments. INTRODUCTION Historically, the approach to the characterisation of ancient metalwork and the associated development of instrumentation has varied very much according to the material. With copper based alloys one major strand of research has been dominated by questions of provenance based on trace element and stable isotope analysis with rather little attention paid to structure and to the contribution of individual elements to material properties. Here analytical developments have focussed on increased spectral resolution, sensitivity and reproducibility, with inductively coupled plasma mass spectrometry (ICP-MS) the new fashion. However, interpreting the data has its pitfalls; as one of us pointed out many years ago key elements may be sequestered as non-metallic inclusions with a diminished contribution to the properties of the metal [1]. At the
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