Circular base - Cu Alloy - Modern Times - 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 Fig. 8.

Analyses performed:
Metallography (etched with ferric chloride reagent), Vickers hardness testing, LA-ICP-MS, SEM/EDX, Raman spectroscopy.

Table 1 indicates that the metal is a copper alloy. The high relative standard deviations (RSD) result from the heterogeneity of the metal (inclusions). The alloy differs from the quaternary alloy given in the report for the circular base (Benoît 1994, p.18). It contains numerous inclusions of copper oxides (Figs. 4, 5, 7 and 8, Table 2) or heavy metals (Fig. 8 and Table 2) that are residues from the manufacturing process. In polarized light, the copper oxide inclusions appear red (Figs. 4 and 7) while with SEM under BSE-mode they look light-grey (Fig. 8). Heavy metals inclusions appear white with SEM under BSE-mode (Fig. 8). Due to the rolling process the copper oxide inclusions are parallel to the sample shape (Figs. 4 and 5). They were identified as cuprite by Raman spectroscopy (Fig. 6). The etched alloy shows a structure of polygonal grains with annealing twins (Fig. 5). The grain size is variable. The average hardness of the alloy is HV1 80, which is quite high compared to the average hardness of a pure annealed copper of around HV1 40-50 (Schumann 1991, 627).

 

Elements Cu Sb Pb Sb Ag Bi Sn Zn Ni As S
mass% 99.6 0.2 0.12 0.2 0.1 0.004 < < < < <
RSD % 0.3 112 132 112 1 112          

Table 1: Chemical composition of the metal. Method of analysis: LA-ICP-MS, Laboratory of Basic Aspects of Analytical Chemistry at the Faculty of Chemistry, University of Warsaw, PL.

 

Elements

O Cu Fe Ni Sn Sb Pb Total
Light-grey inclusion 10 86 < < < < < 96
White inclusion 1 18 48 17 3 11 0.9 0.8 105
White inclusion 2 13 8 < < < 34 44 99

Table 2: Chemical composition (mass %) of the inclusions (light-grey and white in Fig. 8) in the metal. Method of analysis: SEM/EDX, Laboratory of Analytical Chemistry, Empa.

The average thickness of the corrosion crust is about 90µm but may be thinner or thicker depending on the area of origin (Fig. 4). Under polarized light, we observe a multilayer system with a dense thin inner red corrosion layer (probably cuprite/Cu2O) on the metal (Fig. 7) containing chlorides (Fig. 8 and Table 3). This is followed by bands of heterogeneous green corrosion products (copper chloride / copper sulphate / copper carbonate?). This outer corrosion layer is contaminated with atmospheric components (Si, Al, Cl, C and O and perhaps gypsum particles (CaSO4)) (Fig. 8).

 

Elements

O Si Cl Cu Total
CP2i, inner red layer 11 < 23 60 94

Table 3: Chemical composition (mass %) of the inner corrosion layer (CP2i). Method of analysis: SEM/EDX, Laboratory of Analytical Chemistry, Empa.

Corrected stratigraphic representation: none

The circular base of the door knocker consists of a rolled copper alloy that was annealed after cold working. The metal is rich in copper oxide inclusions. The corrosion most likely is composed of cuprite in proximity to the metal surface followed by green copper corrosion products (chlorides, carbonates or sulphates ?) the surface of which is contaminated with gypsum and airborne particles. The presence of chlorides in the inner corrosion layers could be explained by contamination through handling. Soil elements are probably airborne dust, whereas sulphur could have come from urban SO2 pollution. No trace of gilding was observed on the sample studied. In this case the corrosion is mostly of type 1 but can be locally of type 2 after Robbiola et al. 1998).

References on object and sample

1. Benoît, C. (1994) Cathédrale de Lausanne: conservation de deux appliques en bronze à tête de lion avec anneau mobile et encadrement circulaire. Rapport de travail, non publié.
2. Rapport d'examen, Laboratoire Musées d’Art et d’Histoire, Genève No 94-156-1/2 (1998).

References on analytic methods and interpretation

3. 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.
4. Schumann, H. (1991) Metallographie, Leipzig.