Forgery remain - Fe Alloy - Roman Times

Naima. Gutknecht (HE-Arc CR, Neuchâtel, Neuchâtel, Switzerland) & Valentina. Valbi (Laboratoire Métallurgie et Culture (LMC), Belfort, Franche-Comté, France) & Neff. Delphine (LAPA, None) & Philippe Dillmann. (LAPA, Gif-sur-Yvette, Île-de-France, France)

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Excavation "Les Terres de Diane". Rural site exploited form the neolithic age to the gallo-roman age. The site was dedicated to multiple activities (artisanal and agricultural) among which forging activity is documented dating from the second half of the first century and the second century.

The schematic representation below gives an overview of the corrosion layers encountered on the object from a first visual macroscopic observation

Analyses performed:

Metallography (etched with Nital reagent 3%), SEM-EDS, Raman spectroscopy.

After Nital etching, the microstructure shows that the metal is only constituted of iron equiaxe grains (Figure 7) characteristics of a ferritic structure. Ghost structures next to the weld reveal anomalies in the repartition of phosphorous content and a possible migration of P from one metal sheet to another during the welding process (Figure 8). The SEM-EDS analysis confirmed the composition of the metal being almost pure iron (Fe > 99%) with low percentages of phosphorous (P<1%) varying over the sample. The central metal sheet presents elongated slag inclusions composed of glassy matrix with wustite dendrites (%?). One weld is well executed (Figure 8) while the other one was not completed and the space within the two metal sheets is filled with residual slag inclusions from the welding process and corrosion products (Figure 9).

The metal is covered with a thick layer of corrosion products of total thickness of 4-6 mm.

The CP1 layer is  500-1000 µm thick and it is composed of a mixture of goethite and lepidocrocite with external markers such as quartz grains (transformed medium).

The CP2 is composed of iron oxides produced by oxidation at high-temperature during the forging process. It is 50 µm thick and three sub-layers can be identified. An thin upper layer of hematite (1-2 µm), an intermediate layer of magnetite (5-6 µm) and a bigger bottom layer of wustite.

The CP3 has a marbling structure alternating bands of dark goethite bands and light not-crystallized iron oxides (light bands) (bands thickness of 10-50 µm).

The CP4 is composed of alternating bands of goethite and magnetite (bands thickness of  200-300 µm) .

The CP5 is composed of goethite and acicular akaganeite, showing an active corrosion next to the metal interface.

Complementary information

Higher concentrations of P were detected in the corrosion products next to the structural voids.

Raman analysis conditions : laser 532 nm....

The object is composed of three metal sheets welded together. The metal of the three sheets has a ferritic structure.

The object is covered with a thick layer of corrosion products. The identified corrosion products are mainly goethite, magnetite, akaganeite and iron oxides not crystallized.

The original surface is identified by the three-layered hematite/magnetite/wüstite scale structure.

Aerated soil environment

D. Neff et al. Corrosion of iron archaeological artefacts in soil : characterization of the corrosion system. Corrosion Science 47 (2005) 515–535

Chen, R., Yeun, W. Review of the High-Temperature Oxidation of Iron and Carbon Steels in Air or Oxygen. Oxidation of Metals 59 (2003) 433–468