Pin or needle fragment with green-blue patina - Tin 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 Figs. 7 and 8.

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

The remaining metal is a tin bronze and contains small copper sulphide and Pb-rich inclusions evenly distributed throughout the metal (Fig. 4, Tables 1 and 2). The Pb-rich inclusions are only visible with SEM appearing as white particles. The etched structure of the tin bronze shows re-crystallised and angular grains, some of them with twins (Fig. 5). Strain or slip lines are visible, especially near the metal surface. They indicate a final cold working. Copper sulphide inclusions are found both at the grain boundaries and inside the grains (Fig. 5). The average hardness of the metal is about HV1 120.

 

Elements Cu Sn Sb Ni As Pb Ag Co Zn Fe
mass% 91.29 5.65 1.00 0.69 0.55 0.51 0.22 0.06 0.01 0.02

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

 

Elements S Cu Total
mass% 21 85 106

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

The corrosion crust has an average thickness of about 50µm (Fig. 6). In polarised light (Fig. 7), the corrosion stratigraphy is more clearly visible: it is composed of an inner orange-red corrosion layer (an agglomerate of nanoscale stannic oxides with cuprite) directly on the metal core (Table 3 and Fig. 8, already studied by Piccardo et al. 2007), an intermediate multi-layered black band and an outer turquoise-green layer analysed with XRD by Schweizer as malachite/CuCO3Cu(OH)2 (Schweizer 1994, 150). In some areas the orange-red layers can also be found in between the black band and the malachite. Elemental chemical distribution of the SEM image of Fig. 8 shows that the black layers are enriched in Sn but also contain Fe (Fig. 8). Superior markers such as contextual Al and Si are present in the outer malachite layers. S is present both on the rim of the outer black layer and in the malachite (Fig. 8, Table 3).

 

Elements

O Cu Sn S Cl Fe As Ag Total
CP3i ext. 20 40 12 15 < 5 < 1.9 94
CP3i int. 20 53 16 < 0.9 < 0.6 < 91

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

Corrected stratigraphic representation: none

The pin is made from a tin bronze and has been repeatedly cold worked and annealed. After the last annealing there has been some cold work, as can be seen from the strain lines visible after etching the metal. Due to the presence in the corrosion crust of an outer malachite layer, the corrosion was described as terrestrial by Schweizer (Schweizer 1994). The elemental chemical distribution of the corrosion crust shows a more complex situation: as expected for an object buried in a terrestrial site, a typical enrichment of Sn is observed in the inner and intermediate layers covering the surviving metal surface. However it is combined with Fe and S which are often present in lake patinas. According to Schweizer, these layers were formed in anaerobic conditions and developed later on into malachite in an aerated soil through partial dehydration (Schweizer 1994, Schwartz 1934). Since the original surface is absent (destroyed), we refer to type 2 corrosion after 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), planche 74.11.

 

References sample

2. Empa Report 137 695/1991, P.O. Boll.
3. Rapport d'examen, Laboratoire Musées d'art et d'Histoire, Geneva GE (1987), 87-194 à 197.
4. Schwartz, G.M. (1934) Paragenesis of oxidised ores of copper, Economic Geology, 29, 55-75.
5. Schweizer, F. (1994) Objets en bronze provenant de sites lacustres: 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

6. Interpretation of orange corrosion products, see: Piccardo P., Mille B., Robbiola L. Tin and copper oxide in corroded archaeological bronzes, In: Corrosion of metallic heritage artefacts, European Federation of Corrosion Publication n°48, 2007, ed. Dillmann et al, 239-262.
7. 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.