Bracelet Lnr. 30557 - Cu Alloy - Bronze Age - Switzerland

Bracelet Lnr. 30557 - Cu Alloy - Bronze Age - Switzerland

Bracelet Lnr. 30557

Sabine. Brechbühl (archaeological service canton Bern, Bern, Bern, Switzerland) & Naima. Gutknecht (HE-Arc CR, Neuchâtel, Neuchâtel, Switzerland)

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Credit Archaeological Service Canton Bern.

Fig. 1: Bracelet,

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

Complementary information

White crystals developed after rinsing the object. 

Credit Archaeological Service Canton Bern / HE-Arc CR, N.Gutknecht.

Fig. 2: Location of XRF analysis (red circles) and Fig. 3,

Credit HE-Arc CR, N.Gutknecht.

Fig. 3: Detail of the corrosion structure showing the strata documented in Fig. 4,

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.

Credit HE-Arc CR, N.Gutknecht.

Fig. 4: Stratigraphic representation of the corrosion structure of the bracelet by macroscopic and binocular observation,

Fig. 5: Stratigraphic representation of the corrosion structure of the bracelet observed macroscopically under binocular microscope using the MiCorr application with reference to the whole Fig. 4. The characteristics of the strata are only accessible by clicking on the drawing that redirects you to the search tool by stratigraphy representation, Credit HE-Arc CR, N.Gutknecht.

No sample has been taken. The observation and analysis were performed in a non-invasive way on the object.

Cu Alloy

Cast

None

None

Complementary information

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. Method of analysis: XRF, UR Arc CR.

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.

Credit HE-Arc CR, N.Gutknecht.

Fig. 6: Location of the Raman analysis,

Credit HE-Arc CR, N.Gutknecht.

Fig. 7: Raman spectra reference for calcite (RRUFF ID=R040070.1) and points 1, 2 and 3 located in Fig. 6,

The composition of the metal is assessed from XRF in table 2: it should be a leaded bronze.

Dendritic structure

Cu

Sn, Pb

Complementary information

None.

CP1 (data not shown) was matched with cuprite (CuO2) through Raman spectroscopy. 

None

None

Complementary information

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.