Blade fragment of winged axe - Tin Bronze - Middle 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. 8.

Analyses performed:

Metallography (etched with ferric chloride reagent), Vickers hardness testing, SEM/EDX, EPMA/WDS, Raman spectroscopy.

The remaining metal is a tin bronze (Table 1) with high porosity and grey copper sulphide inclusions (Figs. 4 and 5, Table 2). The etched metal has the typical dendritic structure of a cast tin bronze with an average hardness of HV1 135 (Fig. 5). The cored dendritic structure is surrounded by an alpha-delta eutectoid. The core of the dendrites is rich in Cu whereas the outer layers are rich in Sn.

 

Elements Cu Sn As Fe Ni Pb Sb Co Ag Au Zn Bi Si
mass% 85.14 11.95 1.54 0.49 0.39 0.18 0.14 0.13 0.02 0.02 < < n. d.

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

 

Elements

Cu S Fe Total
Dark-grey inclusion 66 24 10 100

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

A dark green corrosion crust with a thickness between 100 and 320µm covers the entire surface of the blade fragment (Fig. 6). It retains a metallic ghost structure (Sn-rich eutectoid alpha + delta phase). Under polarized light localized orange and red corrosion products can be seen at the metal - corrosion crust interface (Fig. 7). Interdendritic corrosion and corroded slip lines can be seen in the metal structure and near fissures (Fig. 8). Elemental mapping (Fig. 9) shows that the green layer is Sn-rich (probably SnO2) and depleted of Cu, whereas the orange and red corrosion particles are depleted of Sn and rich in Cu (Fig. 9, Table 3). Their Raman spectra indicate that they are mainly composed of cuprite (Fig. 10). The overall corrosion crust contains O, Si, C and Fe from the environment while S is concentrated around the cuprite particles (Fig. 9).

 

Elements

O Si P Fe Ni Cu As Sn Total
Surface CP1e 43 0.8 < 6.2 < 16 1 43 111
Middle CP1e 42 1.7 0.7 12 < 10 0.7 43 110
CP2i 41 0.9 < 4.4 < 36 < 22 104
Remnant metal phase 9 0.7 < 5 0.8 34 < 47 97

Table 3: Chemical composition (mass %) of the corrosion crust from Fig. 7. Method of analysis: SEM/EDX, Laboratory of Analytical Chemistry, Empa.

Corrected stratigraphic representation: none

Fragment of an unfinished cast bronze median-winged axe produced at the local workshop, Erlenbach ZH. The evenly corroded tin bronze contains numerous sulphide inclusions and shows signs of interdendritic corrosion penetrating the metal structure. The Sn enriched surface is decuprified and influenced by the environmental elements such as O, Si, Fe, C and Al and Cl. The corrosion crust is composed mainly of a dark green layer with local orange-red cuprite particles at the interface with the remaining metal. Both the remnant metallic phases and the Sn-rich corrosion layer can be interpreted as inferior markers, defining the limit of the original surface above them. For all these reasons, the corrosion is thought to be of type 1 according to 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, 168.

 

Reference sample

2. Northover, P. (1997) Metalworking waste from Erlenbach-Obstgartenstrasse. In: Fischer, C. Innovation und Tradition in der Mittel- und Spätbronzezeit. Monographien der Kantonsarchäologie Zürich 28, Zürich, 99-101.

References on analytic methods and interpretation

3. Bertholon, R. (2001) Characterization and location of the original surface of corroded archaeological objects. Surface Engineering, 17 (3), 241-245.
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.