Bracelet with uniform inclined rib - Leaded Bronze - Late Bronze Age - Switzerland

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

Complementary information

Nothing to report.

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.

Complementary information

Nothing to report.

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

The remaining metal is a porous leaded bronze (Fig. 5 and Table 1). In bright field dark-blue copper sulphide (Fig. 5, Table 2) and tiny dark-grey Pb inclusions (Fig. 5) can be seen. The Sn-rich eutectoid alpha + delta phase appears in light-blue (Fig. 5) and incorporates Pb-rich inclusions. The etched leaded bronze has the dendritic structure of an as-cast metal (Fig. 6) with an average hardness HV1 90. After etching the inclusions have turned darker (Fig. 6) while the eutectoid phase appears whiter.

 

Elements Cu Sn Pb Sb Ag Ni As Co Zn Fe Bi
mass% 90.93 6.43 1.40 0.52 0.26 0.20 0.18 0.04 0.02 0.02 <0.01

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

 

Elements

S Cu Total
Dark-blue inclusion 21 80 101

Table 2: Chemical composition (mass %) of dark-blue inclusions on Fig. 5. Method of analysis: SEM/EDS, Laboratory of Analytical Chemistry, Empa.

Complementary information

Nothing to report.

The interface between the metal and corrosion products is irregular (Fig. 5). The corrosion crust has an average thickness of 70µm and is composed of two layers separated by a large fissure (Fig. 5).  In bright field, the inner layer includes remnant metal (Sn-rich eutectoid phase, Figs. 5 and 7) and is dark-grey (Fig. 5) while in polarised light it is orange-brownish (CP3, Fig. 8). This Cu depleted layer is rich in Sn, Fe, Si and O (Table 3 and Fig. 8). At the metal - inner layer interface a corrosion product (CP4, light-grey in bright field, greenish in polarised light) shows a slight increase in Cu and Sn content but a decrease of the Fe content (Table 3). In bright field, the outer dense layer is light-grey (CP2, Fig. 5) while in polarised light it appears black with superimposed red to orange areas (CP1, Fig. 8). It is depleted of Cu and richer in Fe. The Sn content is variable but increases in the top brown areas (Table 3 and Fig. 9).

 

Elements

O Cu Sn Pb Fe Si S Total
CP1, outer brown area 36 6.7 24 2.5 27 4.3 < 102
CP2, outer black layer (average of 2 similar analyses) 34 9.0 14 2.8 34 4.3 < 99
CP3, inner orange-brown layer (average of 2 similar analyses) 31 16 18 2.4 21 5.2 < 95
CP4, inner greenish layer (average of 2 similar analyses) 36 29 19 1.5 14 6.1 < 107

Table 3: Chemical composition (mass %) of corrosion layers from Fig. 8. Method of analysis: SEM/EDS, Laboratory of Analytical Chemistry, Empa.

Complementary information

Nothing to report.

Corrected stratigraphic representation: none.

The leaded bronze shows an as-cast structure. The metal surface is selectively corroded, showing a remnant Sn-rich phase in the inner corrosion layer. Because of this remnant metallic structure, the corrosion type is similar to a type 2 corrosion after Robbiola et al. 1998. In this case, two corrosion processes have occurred in parallel: a typical Cu depletion and Sn enrichment, but at the same time a surface enrichment with Fe and Si that could be explained by an Fe-rich lake environment.

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 137.

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.