METAL 2025

Course 3 (Afternoon): Workshop (limited capacity – max. 35 delegates)
 

Portable X-ray fluorescence spectrometry (pXRF) has become well established for many tasks in archaeological, historical, and cultural heritage fields, but is it always used wisely and well, and how well are the results understood? Tasks range from detailed, standardized analyses of bronzes in a museum context to identification of as-excavated materials in the field. To make the most of the information that pXRF can provide in any context requires two things, an understanding of the system being analysed and, also, of the X-ray spectrum and how it is presented by the instrument and its software. Experience suggests that appreciation of this has not kept pace with the expanding use of pXRF instruments. This workshop is designed to look at these issues, discussing first what exactly pXRF is analysing when applied to corroded metalwork, and the second demonstrating this with some practical examples from which well characterized samples have been taken for comparison.

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A cross-section through the surface of a corroded metal artefact can reveal quite a complex stratigraphy for which a section cut in an excavation provides a useful analogy. At the base of the section is uncorroded metal, if any survives; above this is a zone where metal and corrosion product are intermixed, but nonetheless appropriate analysis can still reconstruct much of the alloy composition and impurity pattern. The next layers are composed of corrosion products, and these may have their own complex stratigraphy, such as the remains of surface coatings, pseudomorphs of inclusions, and layers marking the original surface. At the top of the section is a layer where the object is in direct contact with the environment, which can show a marked change of composition.

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Because the X-rays excited by the instrument’s beam are soft (<10 keV) with a range of a few micrometres at most, it may be only from this thin surface layer that analytical data are obtained. further obscuring the original composition of the object. In terms of an excavation it is like making a site plan after only the turf and the first few centimetres of topsoil have been removed – with luck, some structures may be visible. The first part of the workshop will show what compositional and microstructural information is contained within each layer and the best way of extracting it. The examples in the second part will come from both terrestrial and marine environments with different depths of corrosion. For each there will be at least one metallographic sample where both electron microprobe analyses and optical metallography are available for comparison with pXRF analyses carried out live in the demonstration.