Tang fragment of a knife with brown patina - Leaded 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. 9.

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

The remaining metal is a leaded bronze (Table 1) containing numerous copper sulphide and tiny Pb inclusions (Figs. 4-6, 9 and Table 2). The porosity within the metal is high, particularly along a band through the middle of the sample (Figs. 3 and 4). The etched structure of the leaded bronze shows small, regular polygonal grains, some with twinning (Fig. 5). Strain lines appear in grains close to the metal surface (Fig. 6). The average hardness of the metal is HV1 120.

 

Elements Cu Sn Pb Ni Sb As Co Ag Fe Zn
mass% 87.52 8.02 1.46 1.04 0.81 0.60 0.24 0.21 0.05 0.03

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

 

Elements O S Fe Cu Total
mass% 1.5 20 1.0 71 93

Table 2: Chemical composition of dark-grey inclusions. Method of analysis: SEM/EDX, Laboratory of Analytical Chemistry, Empa.

The metal has lost most of its original corrosion crust, the remainder having an average thickness between 60 and 190µm (Fig. 3). In some areas up to three corrosion layers are visible (Fig. 7). In polarised light (Fig. 8), the corrosion stratigraphy appears more clearly: it is composed of a dense black inner layer, an intermediate thick brown layer with bright spots (indicating porosity) and an outer red layer with white particles. The elemental chemical distribution of the SEM image reveals that the black inner layer is Sn-rich but contains Cu, O, Ca, S and Fe (Table 3, Fig. 9). The brown layer contains S, Fe, and Cu and has a composition similar to chalcopyrite/CuFeS2 (Table 3, Fig. 9). The red layer is an iron oxide (main elements Fe and O) and is contaminated with calcite/CaCO3 particles (Table 3, Fig. 9). XRD analyses of the brown layer by Schweizer confirms the presence of chalcopyrite (1994, museum report (1987)).

 

Elements

O Fe Ni Cu Si P S Ca As Sn Pb Total
CP1e, red layer 37 51 1.8 < < < < 1.5 0.8 < < 93
CP2i, brown layer < 30 < 42 < < 35 < < < < 107
CP2i, white particles 50 < < 0.6 < < < 39 < < < 90
CP3i, black layer 39 4.8 1.2 5.2 3.9 3.7 < < 0.7 37 3.7 100

Table 3: Chemical composition (mass %) of the corrosion layers (from Figs. 7 and 8). Method of analysis: SEM/EDX, Laboratory of Analytical Chemistry, Empa.

Corrected stratigraphic representation: none

The tang fragment is made from a leaded bronze and has been cold worked on the top surface after annealing. The SEM/EDX examination and past XRD analyses indicate the presence of chalcopyrite in the corrosion crust, typical of lake context (Schweizer 1994), enriched with Sn close to the metal surface and depleted of Cu on the outer surface. 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 most probably lies between the Sn-rich inner layer and the Fe and S-rich outer layers. The presence of iron oxides on top of the copper corrosion crust has not yet been explained. The corrosion is a type 1 according 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).

 

References sample

2. Rapport d'examen, Laboratoire Musées d'art et d'histoire, Geneva GE (1987), 87-194 à 197
3. 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

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.