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Iron archaeological artefacts were studied to understand long-term corrosion by the atmosphere. Indeed, these samples collected on the construction elements of ancient monuments present ancient rust layers formed during their exposure over centuries to the indoor atmosphere. Thanks to Raman spectroscopy and the acquisition of hyperspectral images of the corrosion scales, several zones of the samples observed on cross sections could be characterised. It has been shown on six dated samples that the main phase is goethite (a-FeOOH). Lepidocrocite and akaganeite (g-FeOOH and b-FeOOH) occur locally in the corroded products, often correlated with cracks. A less crystallised phase, a hydrated oxy-hydroxide, has been identified abundantly in more or less extended zones inside the layer. This phase could play an important role in atmospheric corrosion mechanisms.

The description and identification of corrosion products formed on archaeological iron artefacts need various approaches at different observation scales. For this study, samples from five sites were prepared using two techniques. The first consists in cutting cross-sections perpendicular to corrosion layers. This allows local observations and analysis of the corrosion layer stratigraphy at different levels. The second consists in performing manual grinding or abrading of the corrosion layers starting from the current surface of the excavated artefact to the metal core. It allows the description of the successive layers and is well adapted for the analysis on a larger scale. In addition to these two observation scales, the identification of the iron oxides formed needs the coupling of several complementary techniques. Elementary compositions were determined by scanning electron microscopy-energy-dispersive x-ray (SEM-EDX) analysis and electron probe microanalysis (EPMA). Structural identification was performed by x-ray micro-diffraction under synchrotron radiation (µXRD) and micro-Raman spectroscopy. These analyses were performed on the same samples with both x-ray diffraction and Raman spectroscopy in order to ensure a reliable characterization. In some cases there are some ambiguities or overlapping between signatures of different phases by µXRD (such as maghaemite-magnetite) or Raman spectroscopy (such as goethite-magnetite) which can be overcome by the association of the two methods. The final aim is to set up an analytical methodology that will be optimal for the study of ancient iron corrosion products. It is the first step in the study of long-term mechanisms of iron in soil.

The description and identification of corrosion products formed on archaeological iron artefacts need various approaches at different observation scales. Among analytical techniques available to document phase structure at the microscopic range, Raman spectroscopy offers sensitivity and discrimination between iron corrosion products with an easy implementation. Results obtained for iron artefacts corrosion in soils and atmosphere are presented. Corrosion forms observed for anoxic and aerated soils on one hand and indoor atmosphere on the other are documented. Beyond the identification and organisation of corrosion products through hyperspectral imaging, Raman micro-spectroscopy could also provide quantitative phase proportions which will be needed in the proposition of reactivity diagnosis indicators.

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