Pin without head and light-brown patina - Tin Bronze - Late Bronze Age - Switzerland

Marianne. Senn (EMPA, Dübendorf, Zurich, Switzerland) & Christian. Degrigny (HE-Arc CR, Neuchâtel, Neuchâtel, Switzerland)

Stratigraphic representation: none

Fig. 4: Stratigraphic representation of the object in cross-section using the MiCorr application. This representation can be compared to Fig. 10.

Analyses performed:
Metallography (etched with ferric chloride reagent), Vickers hardness testing, XRF, ICP-OES, SEM/EDX, XRD, Raman spectroscopy.

The remaining metal is a tin bronze and contains copper sulphide as well as heavy metal (Pb-rich) inclusions (Table 1, Figs. 4 and 5). Close to the surface of the remaining metal, copper sulphide inclusions are elongated and form rows (Fig. 4). The etched structure of the tin bronze shows polygonal grains; some of them are twinned (Fig. 6). In the centre of the sample and on the side, the grains are smaller. The copper sulphide inclusions are located at the grain boundaries and in the grains. The average hardness of the metal is about HV1 110.

 

Elements Cu Sn Pb Sb As Ag Fe Ni Co Zn
mass% 89.22 9.57 0.34 0.26 0.19 0.15 0.09 0.05 0.06 0.05

Table 1: Chemical composition of the metal. Method of analysis: ICP-OES, Laboratory of Analytical Chemistry, Empa.

The corrosion crust is regular in thickness (100µm). One third is missing (Fig. 3). At the metal - corrosion crust interface, there is a crack showing that the latter has separated from the metal core along its whole length (Figs. 7 and 8). The corrosion crust can be divided into three distinct layers. Directly above the crack is a first dense but cracked and irregular inner layer (Figs. 7 and 8). In bright field it appears brown (Fig. 9), in polarised light dark (Fig. 10). It is separated from the adjacent layer by a clear line (Figs. 8 and 9). The second layer is dense with little porosity (Figs. 7 and 8). In bright field it appears light-brown (fig. 9), in polarised light-black (Fig. 10). The third and outermost layer (light-brown in bright field, Fig. 9) contains particles (Fig. 8) and is very porous (appearing as golden reflections under polarized light, Fig. 10). The elemental chemical distribution of the SEM image selected reveals that the inner layer is depleted in Cu but Sn,O and Si-rich (Fig. 11 and Table 2) and its interface with the intermediate layer could represent the limit of the original surface (Figs. 7 and 11). The second and third layers are Fe,Cu and S-rich (Fig. 11) and have a composition similar to chalcopyrite/CuFeS2 (Table 2). This was confirmed by XRD. The particles (inclusions) have a composition similar to covelline/CuS (Table 2). Both chalcopyrite and covelline have been identified by Raman spectroscopy (Figs. 12 and 13).

 

Elements

S Fe Cu O Si Sn Total
CP1e and CP2i 35 30 34 < < < 99
Particles in CP1e 26 4.1 68 < < < 98
CP3i 5.8 5.0 13 32 2 41 99

Table 2: Chemical composition (mass %) of the corrosion layer from Fig. 9. Method of analysis: SEM/EDX, Laboratory of Analytical Chemistry, Empa.

Corrected stratigraphic representation: none

The pin is made from tin bronze and has been annealed after cold working. It is covered with a regular, light-brown patina typical of lake context (Schweizer 1994). The inner, thin Sn-rich corrosion layer contains soil elements such as Si. The light-brown, thick intermediate and outer corrosion layers have the composition of chalcopyrite. This object was certainly abandoned rather quickly in an anaerobic, humid and S and Fe-rich environment, favouring then the formation of chalcopyrite. The limit of the original surface can be located between the chalcopyrite and the Cu depleted but Sn-rich inner corrosion layer. Thus, the corrosion is a type 1 according to Robbiola et al. 1998.

References on object and sample

References object

1. Rychner-Faraggi A-M. (1993) Hauterive – Champréveyres 9. Métal et parure au Bronze final. Archéologie neuchâteloise, 17 (Neuchâtel).
2. Hochuli, S. et al. (1988) SPM III Bronzezeit , Verlag Schweizerische Gesellschaft für Ur- und Frühgschichte Basel, 76-77, 379.

 

References sample

3. Empa Report 137 695/1991, P.O. Boll.
4. Rapport d'examen, Lab. Musées d'Art et d'Histoire, Geneva GE, 87-194 à 87-197.
5. Schweizer, F. (1994) Objets en bronze provenant de sites lacustre: de leur patine à leur biographie. In: L'œuvre d'art sous le regard des sciences (éd. Rinuy, A. and Schweizer, F.), 143-157.

References on analytic methods and interpretation

6. 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.