These inclusions provide a great deal of information about the chemical history of the material. Recent studies carried out at the Pierre Süe Laboratory, Saclay, show that the analysis of these inclusions yields crucial information about iron manufacturing processes and the origin of the ore used. Thus, if the trace-element chemical signature of a region is well characterised, it can be found in the inclusions in objects. This allows their origin to be determined.
The analysis performed at the Pierre Süe Laboratory involved a representative number of ores and slags from various sites in the Pyrenees region (Ariège, Eastern Pyrenees), a major production centre in the Middle Ages, and a chemical signature for the region was identified. When the usual techniques (LA-ICP-MS: Laser Ablation Inductively Coupled Plasma Mass Spectrometry) do not work because of the small size of the inclusions, micro x-ray fluorescence is an especially useful technique to track this signature in the non-metallic inclusions present in ferrous objects made from this ore. It allows extremely valuable museum pieces, from which only a tiny sample can be taken, to undergo non-destructive testing.
| In order to calibrate the method, which will then be transposed to beamlines with a beam diameter of only a few micrometres, x-ray microfluorescence measurements (µXRF) were carried out under synchrotron radiation provided by the DIFFABS beamline at SOLEIL, with a 50 µm beam. These measurements were performed on a large number of old iron artefacts. μXRF allows localised measurements to be taken on the various inclusions and provides a mapping of the distribution of elements, whose exact location in the metal matrix can then be pinpointed. The tunability of the beam energy allows inclusions to be examined at high energy in order to detect a wide range of trace elements with a high atomic number: in particular, As, Rb, Sr, Y, Zr Nb, and Mo, whilst maintaining an acceptable photon flux.
The experiments were performed at an energy of 20 keV. The size of the inclusions ranges from the order of ten micrometres to the order of a hundred micrometres, requiring precise movement of the sample and a small beam to analyse the areas of interest. For this purpose, a pinhole was placed upstream from the sample to select a 50-µm diameter beam. The sample is placed at 45° to the incident beam, on two micro control travelling tables allowing the sample to be moved horizontally and vertically. The surface to be examined is observed by reflection using an optical microscope. The fluorescence spectra are collected by an SDD detector placed at 90° to the incident beam.
| 
Set-up for special sample environment
on the DIFFABS beamline |
The content of each trace element detected is quantified using the 'PyMCA' x-ray fluorescence spectrum processing software package developed at ESRF. Quantification tests were carried out on reference glasses whose composition was similar to that found in the non-metallic inclusions and on inclusions of known composition.
This methodology will later be applied to artefacts with smaller inclusions, in particular using a beam of the order of ten μm diameter, when the second KB mirror system will be operational on the DIFFABS beamline. This new mirror system, combined with the intense flux of a synchrotron beamline, will also allow data to be collected in a very short time for a statistical quantitative study which is required for the analysis of the circulation of metallurgical products.
| Conclusion
This µXRF work on very small inclusions, supplemented by other analytical methodologies on different scales, now makes it possible to establish the origin of archaeological ferrous materials by comparing trace element signatures—something that had not been possible until now. This new approach paves the way for an overall understanding of the exchange of these materials in the distant past. | 
X-ray fluorescence spectrum of an inclusion obtained
on the DIFFABS beamline |
References
A.-M. Desaulty, et al. Trace elements behaviour in direct- and indirect-iron metallurgy: the case of the Pays-De-Bray (France). Archaeometallurgy in Europe, 2007
Coustures, M.-P., D. Beziat, and F. Tollon, The use of trace element analysis of entrapped slag inclusions to establish ore-bar iron links: examples from two gallo-roman iron-making sites in France (Les martys, Montagne Noire, and Les Ferrys, Loiret). Archaeometry, 2003. 45(4): p. 599-613.
Sole, V.A., and al. PyMCA, A multiplatform code for the analysis of energy-dispersive X-ray spectra. Spectrochimica Acta Part B: Atomic Spectroscopy, 2006: p. 14
P. Dillmann, M. L'héritier, Slag inclusion analyses for studying ferrous alloys employed in French medieval buildings: supply of materials and diffusion of smelting processes. Journal of Archaeological Science (2007), 34(11):1810-1823.
