Situla - Tin Bronze - Iron Age - Switzerland

Christian. Degrigny (HE-Arc CR, Neuchâtel, Neuchâtel, Switzerland) & Marie-Jeanne. Scholl (HE-Arc CR, Neuchâtel, Neuchâtel, Switzerland) & Valentin. Boissonnas (HE-Arc CR, Neuchâtel, Neuchâtel, Switzerland)

Complementary information

The size and quality of the object makes it likely that is was reserved for important occasions or for ritual use. Secondary use could be a votive offering. Burial condition: inside a 3 meters deep hole in upside-down position.

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

Fig. 6: Stratigraphic representation of the object in cross-section using the MiCorr application. This representation shows the interior face and can be compared to Fig. 10 (top square).
Fig. 5: Stratigraphic representation of the object in cross-section using the MiCorr application. This representation shows the exterior face and can be compared to Fig. 10 (bottom square)

Analyses performed:

Metallography (etched with ferric chloride reagent), SEM-EDS, FTIR, Raman spectroscopy and XRD.

The remaining metal is a dense tin bronze (Fig. 5, Table 1), showing no inclusion. In bright field, the etched alloy shows a structure principally consisting of polygonal alpha phase grains (Fig. 6). Some of the grains include twin lines (Fig. 7). The presence of strain lines (slip lines) indicates a final cold work without annealing (Fig. 7).

 

Elements Cu Sn
mass% 90 10

Table 1: Chemical composition of the metal. Method of analysis: SEM-EDX, Lab of Electronic Microscopy and Microanalysis, IMA (Néode), HEI Arc.

The corrosion crust is heterogeneous and has in places completely replaced the metal. The metal – corrosion products interface is irregular due to transgranular corrosion (Figs 5-6). In most cases, the corrosion crust can be divided in three main layers: an inner compact blue layer directly on the metal core. In areas of extensive corrosion this blue layer coexists with a friable green layer (Figs. 8 and 9). Depending on the area either a very fine dark green or red corrosion layer marks the limit of the original surface. It is followed by an external fourth layer, consisting of friable and sometimes curly pale green corrosion products intermingled with soil products (Fig. 8 & 11). In heavily worked and fragile parts of the object a cleavage between the inner layers of blue and green corrosion is present, rendering the green corrosion vulnerable to loss (Fig. 9).

The inner layers are Sn and O-rich and depleted in Cu (Table 2). The outer layers are Cu and O-rich, contain no Sn, and are contaminated with Ca, Si, Al and Fe coming from the soil (Fig. 10). The colour of the corrosion crust varies according to the content of Sn (the more Sn, the darker green or blue the corrosion). FTIR and Raman spectra on the inner blue (Fig. 12) and outer green (Fig. 13) layers were difficult to interpret. Only malachite could be identified in both cases. XRD spectra could not be interpreted because of the deficiency of peaks.

The limit of the original surface is well defined (interface of CP1i and CP3i) but due to the fragility of the inner corrosion layers it was difficult to uncover.

 

Elements

O Cu Sn
Outer green layer (CP1e) +++ +++ nd
Inner green layer (CP4i) ++ + +++
Inner blue layer (CP5i) ++ + +++
Remnant metal phase nd +++ +

Table 2: Chemical composition of the corrosion crust from Fig. 11. SEM-EDX, Lab of Electronic Microscopy and Microanalysis, IMA (Néode) (+++: high concentration, ++ medium concentration, + low concentration, nd: not-detected).

Based on the analyses carried out, the schematic representation of the stratigraphy of corrosion layers (Fig. 3) was corrected.

The metal structure of this low-tin bronze shows extensive cold work and multiple annealing cycles with a final cold work. The total absence of inclusions highlights a highly developed knowledge in bronze metallurgy. The metal is much corroded. Transgranular corrosion is visible. The majority of the internal corrosion products are composed of copper carbonates that have replaced much of the metal. Curly malachite has developed in clusters on the outside of the surface. This pattern is characteristic of a long-term burial period. The presence of inner enriched Sn layers shows a decuprification phenomenon (dissolution of Cu). Because of the friable nature of the inner green layer that supports the limit of the original surface, as well as its cleavage with the blue layer underneath, the original surface has become very fragile. The corrosion is thought to be of type 1 according to Robbiola et al. 1998.

References on object and sample

Reference object

1. Archeodunum (Gollion) (2009). Le Mormont : un sanctuaire des Helvètes en terre vaudoise vers 100 avant J.-C. Section de l'archéologie cantonale, Lausanne.

 

Reference sample

2. Scholl, M.-J (2013) Situle en bronze et anse en fer, EMT09/554.665, vers 100 av. J.-C., La Sarraz/Eclépens, Le Mormont (VD), Musée cantonal d’archéologie et d’histoire, Lausanne. Rapport d’intervention, Haute Ecole Arc de Conservation-restauration, Neuchâtel [not published].
3. Eggert, G. (2007) Pseudomorph or corrosion? The enigma of the curly malachite, in Metal07 - Proceedings of the International Conference on Metals Conservation, Degrigny, C., Van Langh, R., Ankersmit, B. and Joosten, I. (eds), Rijksmuseum, Amsterdam (2007), 1, 57-60.

References on analytic methods and interpretation

4. Robbiola, L., Blengino, J-M., Fiaud, C. (1998) Morphology and mechanisms of formation of natural patinas on archaeological Cu-Sn alloys, Corrosion Science, 40, 12, 2083-2111.