Bracelet with a dense, black lake patina - 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. The characteristics of the strata are only accessible by clicking on the drawing that redirects you to the search tool by stratigraphy representation. This representation can be compared to Fig. 7, credit MiCorr_HE-Arc CR.

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 leaded bronze (Table 1) with low porosity, light and dark-grey inclusions (Fig. 5). In bright field the unetched alpha-delta eutectoid appears light-blue (Fig. 5). Etching reveals the dendritic structure of an as-cast metal (Fig. 6) with an average hardness of HV1 100. The inclusions appear as dark-grey (Pb-rich) and light-grey (copper sulphide) (Fig. 6 and Table 2) while the alpha-delta eutectoid is white (Fig. 6). The pink alpha phase is cored.

 

Elements Cu Sn Pb Sb As Ni Ag Zn Fe Co Bi
mass% 87.62 6.98 4.36 0.42 0.23 0.18 0.13 0.04 0.03 0.02 0.02

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

 

Elements

S Cu Pb Total
Light-grey inclusion 21 82 < 103
Dark-grey inclusion < 2 92 94

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

Complementary information

Nothing to report.

The corrosion crust has an average thickness of 80µm. It is composed of two layers (Fig. 7). The inner layer (CP2), which appears grey in bright field (Fig. 5), retains a Sn-rich dendritic ghost structure (Table 3 and Fig. 8). In polarized light, this layer is a mixture of reddish and yellow-brown corrosion products with some green areas (Fig. 7). The reddish parts have a composition similar to cuprite/Cu2O (Table 3). The adjacent dense, cracked layer, which appears dark-grey in bright field (CP1), is mainly composed of Fe,O and Sn with Sn,O and Fe-rich and Fe and O-rich zones contaminated with Si while being depleted of Cu (Fig. 8). In polarised light, it is dark, almost black with some red areas. In areas it contains Ag, Fe and Sn-rich inclusions with traces of Pb and Cu (Fig. 8 and Table 3).

 

Elements

O Pb Fe Cu Si Sn Ag Total
CP1, light-grey in Fig. 8 40 5.7 8.1 21 3.5 < < 106
CP1, dark-grey in Fig. 8 30 5.3 19 3.7 3.1 < < 99
CP1, bright inclusion 4.5 9.8 45 2.4 < 28 28 98
CP2, dendritic ghost structure 39 5.5 17 32 3 < < 109
CP2, reddish part 14 < 77 3 < < < 96

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

Complementary information

Nothing to report.

Corrected stratigraphic representation: none.

The surface of the cast leaded bronze has been replaced by a Sn-rich corrosion that retains a dendritic ghost structure. It is composed of a mixture of copper oxides (cuprite?) and a Sn-rich corrosion product (cassiterite?). The outer corrosion layer is composed of Fe-O and Sn-O-Fe areas depleted of Cu, but contaminated with Si. The enrichment in Fe seems to be the same as for the formation of patinas from lake contexts. However the outer corrosion layer was not formed in anaerobic conditions (Fig. 7). Since the original surface is absent (destroyed) we refer to type corrosion 2 after Robbiola et al. 1998.

References on object and sample

1. Boll, P. (1991) Empa-Bericht n° 137'695/1991, not published.
2. Mottier, Y., Schweizer, F. (1977, 1991) Rapport du Laboratoire de recherche des musées d'art et d'histoire, not published.
3. Paszthory, K. (1985) Der bronzezeitliche Arm- und Beinschmuck in der Schweiz. Prähistorische Bronzefunde X-Bd. 3, München 1985, 164, Tafel 82.

References on analytic methods and interpretation

4. Mottier, Y., Schweizer, F. (1977, 1991) Rapport du Laboratoire de recherche des musées d'art et d'histoire, not published.
5. 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.