Bracelet with round diameter - 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. 8.

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

The remaining metal is a porous leaded bronze (Table 1). Under bright field light Pb and dark-grey (copper sulphide) inclusions can be seen (Fig. 4, Table 2). The copper sulphide inclusions are rather small with variable forms, the Pb inclusions are larger and rounded. The etched leaded bronze has the dendritic structure of an as-cast metal (Fig. 5) with an average hardness HV1 80. After etching the inclusions have turned dark-grey (Pb-based) and light-grey (copper sulphide) (Fig. 5).

 

Elements Cu Sn Pb Sb As Ni Ag Co Zn Fe Bi
mass% 89.67 6.40 2.62 0.52 0.27 0.22 0.13 0.07 0.03 0.04 0.03

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

 

Elements

S Cu Total
Dark-grey inclusion 21 76 97

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

The corrosion crust has an average thickness of 60µm and is composed of two layers (Fig. 4). In bright field, the inner layer has a slight blue hue (Fig. 4). The corrosion products are stratified and the outer part is porous (Fig. 6). In polarised light the inner layer is composed of a mixture of reddish and orange corrosion products (Fig. 7). This layer is Cu and O-rich with some Sn, Fe and Si in the porous zone (Table 3, Fig. 8). In bright field, the outer cracked and stratified layer is dark-grey (Fig. 4). It is depleted of Cu and rich in Sn and Fe with important amounts of O and Si (Fig. 8). In polarised light, large red angular crystals (cuprite) appear clearly in this outer corrosion layer (Fig. 7). The surface of the outer layer is Sn and C enriched (Fig. 8).

 

Elements

O Fe Cu Sn Pb Si S As Total
CP1e 40 30 < 25 3.1 5 < 0.6 104
CP2i 33 42 3.7 8.8 2.6 4.4 < 0.6 95
CP4i 17 4.3 69 9.9 0.8 1.5 < < 103

Table 3: Chemical composition (mass %) of the corrosion layers from Figs. 6 and 7. Method of analysis: SEM/EDX, Laboratory of Analytical Chemistry, Empa.

Corrected stratigraphic representation: none

The leaded bronze artefact shows an as-cast structure. The outer layer is typical for a lake patina (though in this case formed under aerobic conditions), containing principally Fe as well as other contextual elements (Si). The absence of Cu in the corrosion layer could be due to its re-crystallisation in large cuprite crystals. Surprisingly the top of the outer layer is enriched in Sn which was not detected by Schweizer (Schweizer 1994). The additional presence of C in this top layer could indicate a secondary, terrestrial patina formation phase. The corrosion is a type 1 according to Robbiola et al. 1998.

References on object and sample

Reference object

1. Paszthory, K. (1985) Der bronzezeitliche Arm- und Beinschmuck in der Schweiz. PrähistorischeBronzefunde X-Bd. 3, München, 243, Tafel 171.

 

Reference sample

2. Empa report 137'695/1991, P. Boll.
3. Rapport d'examen, Laboratoire Musées d'art et d'histoire, Genève (1977-110), 1977 and 1991.

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.
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.