Bust of an applique - Quarternary bronze alloy - Roman Times

Christian. Degrigny (HE-Arc CR, Neuchâtel, Neuchâtel, Switzerland) & Marie. Arnautou (HE-Arc CR, Neuchâtel, Neuchâtel, Switzerland) & Valentin. Boissonnas (HE-Arc CR, Neuchâtel, Neuchâtel, Switzerland)

The schematic representation below gives an overview of the corrosion layers encountered on the bust from a first visual macroscopic observation.

Fig. 6: Stratigraphic representation of the object in cross-section using the MiCorr application. This representation can be compared to Fig. 5.
Fig. 7: Stratigraphic representation of the object in cross-section using the MiCorr application. This representation can be compared to Fig. 5.

XRF, SEM/EDX. XRF is carried out with portable X-ray fluorescence spectrometer (NITON XL3t 950 Air GOLDD+ analyser, Thermo-Fischer®, mode "General metal", acquisition time: 20/20/20s).

 The metal has not been examinated.

The entire surface of the bust is covered by a dark grey layer directly located on the remaining metal (Fig. 3). Three XRF analyses (Table 1) performed on the surface of the bust have revealed a high amount of Cu, as well as Pb, Sn and Zn which can be constituants of the alloy, and elements in minor amount: Al, Si and Fe which certainly come from the environment. The qualitative analysis carried out on a scale of this dark patina by SEM/EDS (Fig. 6) confirms the XRF results (Table 1) showing the same elements and the presence of O, which probably correspond to copper oxide (cuprite Cu2O or tenorite CuO). The dark patina is covered by an adherent green corrosion layer which has developed in scattered clusters (probably copper carbonate). The clusters have a surface area of 2 square millimeters to 2 square centimeters, and have a thickness of 0.5 to 5 millimeters. In some place, the green corrosion layer has formed in the porous blisters of the dark layer (Fig. 4). 

 

Table 1: Chemical composition of the dark-grey patina of the selected areas of Fig.2 (red squares). Method of analysis: He-Arc portable XRF.

 

Elements

Cu

Pb

Sn

Zn

Si

Al

Fe

Spot 1

mass%

43

24

13

8

5

3

3

Spot 2

mass%

52

23

11

6

4

2

2

Spot 3

mass%

69

17

5

4

2

2

1

 No modification.

The metal is a quaternary bronze (Cu-Pb-Sn-Zn) according to the qualitative XRF analysis performed on the surface of the bust. The high amount of lead is probably due to its diffusion towards the outside of the metal because of the exposure to high temperatures. The dark patina has developed from a smooth layer to voluminous green crusts corresponding to type 1 corrosin type according to Robbiola and al. 1998. The artefact has been excavated from a burnt ground stratigraphy, which could explain the formation of the black patina (tenorite formed above 300/400°C). A green corrosion has developed in the porous blisters of the dark layer. The limit of the original surface is located at the interface of the dark smooth corrosion and the green one. In areas the limit of the original surface has been lifted up.

 

References on object and sample

References object

1.     B. Pfäffli : Ausgrabungen in Augst im Jahre, 2005.

2.     E. Künzl, S. Künzl, Das römische Prunkportal von Ladenburg, Stuttgart, 2003.

References on analytic methods and interpretation

3.     L. Robbiola, J.M.Blengino and C. Fiaud, Morphology and mecanisms of formation of natural patinas on archeological Cu-Sn alloys, in Corrosion science. Vol. 40, n° 12, pp. 2083-2111, 1998.

4.     D. A. Scott, Copper and bronze in art: corrosion, colorants, conservation, Los Angeles, 2002.