Bracelet Lnr. 30557
Sabine. Brechbühl (archaeological service canton Bern, Bern, Bern, Switzerland) & Naima. Gutknecht (HE-Arc CR, Neuchâtel, Neuchâtel, Switzerland)
Round hollow bracelet with lunar section and flat end. The outside face has decorative lines on the outer surface. The inner part is raw as cast.
Jewellery
Möringen, Biel/Bienne, Bern, Switzerland
2015
Bronze Age
Lake
archaeological service canton Bern, Bern, Bern
archaeological service canton Bern, Bern, Bern
Lnr. 30557
Rinsed with deionised water
White crystals developed after rinsing the object.
Credit Archaeological Service Canton Bern / HE-Arc CR, N.Gutknecht.
The schematic representation below gives an overview of the corrosion structure encountered on the bracelet from a first visual macroscopic observation.
| Strata | Type of stratum | Principal characteristics |
| D1 | Deposit | Extra light grey, medium, botryoidal microstructure, scattered, compact, hard |
| D2 | Deposit | Grey, medium, botryoidal microstructure, scattered, compact, hard |
| D3 | Deposit | White, thin, drusy aggregate, scattered, compact, soft |
| CP1 | Corrosion product | Olive green, thin, discontinuous, compact, soft |
| CM1 | Corroded metal | Layer, average ratio (50/50) between CP1 and M1 |
| M1 | Metal | Yellow, metallic, soft, dendrites microstructures |
Table 1: Description of the principal characteristics of the stratum as observed under binocular and described according to Bertholon's method.
No sample has been taken. The observation and analysis were performed in a non-invasive way on the object.
Cu Alloy
Cast
None
None
None.
Analyses performed:
Non invasive approach
- XRF with handheld portable X-ray fluorescence spectrometer (NITON XL3t 950 Air GOLDD+, Thermo Fischer®). General Metal mode, acquisition time 60s (filters: Li20/Lo20/M20).
- Raman spectroscopy: it is performed on a Renishaw VIRSA Raman Analyser spectrometer equipped with a 785nm laser, the laser power employed is 1mW with 15 acquisition time of 1s.
The XRF analysis was carried out without sampling. All strata, from soil and corrosion products to metal, are analyzed at the same time. The metal is presumably a copper-tin alloy with some lead, while Si probably originates from the burial environment.
|
Elements (mass %) |
1 |
σ |
2 |
σ |
|
Cu |
81.9 |
0.5 |
83.1 |
0.7 |
|
Sn |
10.2 |
0.1 |
9.8 |
0.1 |
|
Pb |
1.9 |
0.03 |
1.4 |
0.03 |
|
Si |
1.9 |
0.1 |
1.7 |
0.2 |
|
Fe |
1.5 |
0.03 |
1.3 |
0.03 |
|
S |
0.2 |
0.03 |
/ |
/ |
Table 2: Chemical composition of the surface of the bracelet at two representative points shown in Fig.2. The results are rounded up to 1 number after the comma.
Raman spectroscopy was performed on different surface deposits (D1, D2 and D3). The hypothesis was that it could be a development of corrosion products since it was only observed after drying and storage. Nevertheless, the Raman spectra of the three crystal types (D1/Pt1, D2/Pt2 and D3/Pt3) correspond well to the reference spectra of calcite (Fig. 7). It appears to be a deposit from the burial environment (lake) with different levels of crystallisation.
The composition of the metal is assessed from XRF in table 2: it should be a leaded bronze.
Dendritic structure
Cu
Sn, Pb
None.
CP1 (data not shown) was matched with cuprite (CuO2) through Raman spectroscopy.
None
None
None.
None.
The bracelet is a tin bronze probably with some lead. It appears to be covered with copper oxide (cuprite) and various types of crystals attributed to calcite from the lake burial condition.
As this object comes from a lake environment, it was expected that a lake patina (copper iron sulphide - chalcopyrite) would be found and that the white products would be a sulphate deterioration of the chalcopyrite. However, the description of the corrosion structure does not correspond to any of the lake patinas documented in Schweizer's publication (Schweizer, 1994) or in the MiCorr database. We can therefore exclude the hypothesis of deterioration of the lake patina from observation alone.
The white crystals are deposits from the lake environment and were present prior to the drying of the object, but were only documented afterwards.
1. Schweizer, F. (1994) Bronze objects from Lake sites: from patina to bibliography. In: Ancient and historic metals, conservation and scientific research (eds. Scott, D.A., Podany, J. and Considine B.B.), The Getty Conservation Institute, 33-50.
