Curved pin or tang - 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. 7.

Analyses performed:
Metallography (etched with ferric chloride reagent), Vickers hardness testing, EPMA/WDS, SEM/EDX.

The remaining metal is a tin bronze (Table 1) with copper sulphide inclusions that contain some Fe (Figs. 4-5, Table 2). After etching, the tin bronze shows polygonal grains with few twins (Fig. 5). The grain size varies between 70 and 180µm indicating grain growth due to an extended or excessively hot annealing process. The copper sulphide inclusions appear in blue. The average hardness of the metal is HV1 70.

 

Elements Cu Sn Pb As Sb Fe Zn Ag Au Co Bi Ni S
mass% 89.94 8.11 0.63 0.41 0.29 0.24 0.13 0.1 0.1 0.04 0.01 < n. d.

Table 1: Chemical composition of the metal. Method of analysis: EPMA/WDS, Lab Department of Materials, University of Oxford.

 

Elements

S Fe Cu Total
Dark-blue inclusion 21 2.8 77 102

Table 2: Chemical composition (mass %) of the dark-blue inclusions on Fig. 4. Method of analysis: SEM/EDX, Laboratory of Analytical Chemistry, Empa.

The corrosion crust has an average thickness of 500µm but can in areas be much thicker (Fig. 3). It is divided in two layers. The inner layer is itself divided in two sub-layers: a thin light-grey sub-layer at the interface with the remaining metal surface (in bright field) which appears red-orange in polarised light (Fig. 6) topped by a medium-grey sub-layer (in bright field) that contains remnant metal. It turns dark-blue in polarised light (Fig. 6). The outer corrosion layer can also be divided into two sub-layers: a porous sub-layer followed by a dark-grey sub-layer in which crystals are outlined by cracks (in bright field). Under polarized light, the latter turns blue-green while on top it appears olive and brown (Fig. 6). Chemically the corrosion crust is Sn enriched and Cu-depleted (Table 3, Figs. 7-8). The maximum of the Sn enrichment occurs on the outer blue-green sub-layer. The corrosion layer also contains P, Si, C and O. Inclusions containing Fe or Ag can be found in the corrosion crust (Figs. 7 and 8).

 

Elements

O Cu Sn Si P Fe Pb S Cl Total
CP1e, outer corrosion layer 27 17 57 1.1 1.9 0.9 1.0 < < 105
CP3i, inner corrosion layer 39 27 26 1.1 2.5 < < < < 97
CP3i, Mi, remnant metal < 88 8.3 < < < < < < 97

Table 3: Chemical composition (mass %) of the corrosion crust from Fig. 6. Method of analysis: SEM/EDX, Laboratory of Analytical Chemistry, Empa.

Corrected stratigraphic representation: none

The tin bronze was exposed to an extended or excessively hot annealing process. This, combined with the extreme thickness of the corrosion crust and the dark surface, confirms that the object originates from a fire burial context. At the metal - corrosion crust interface some copper oxide (cuprite?) occurs. On top copper carbonates (azurite or malachite?) are mixed with tin oxide (cassiterite/SnO2?). Tin oxide dominates in the brown-black extremely Sn-rich outer layer. The P-enrichment in the whole corrosion layer may be due to an environment rich in organic material (for example bones). The original surface of the metal has been destroyed, presenting a type 2 corrosion layer after Robbiola et al. 1998.

References on object and sample

Reference object

1. Fischer, C. (1997) Innovation und Tradition in der Mittel- und Spätbronzezeit. Monographien der Kantonsarchäologie Zürich 28 (Zürich und Egg), 181 and plate 43.

 

Reference sample

2. Fischer, C. (1997) Innovation und Tradition in der Mittel- und Spätbronzezeit. Monographien der Kantonsarchäologie Zürich 28 (Zürich und Egg), 96.
3. Rapport d'examen 92-5-4 (Schweizer, F. and degli Agosti, M.), Laboratoire Musées d'art et d'histoire, Geneva GE (1992).

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.