Bust of an applique - Quarternary bronze alloy - Roman Times - Switzerland

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

Complementary information

Nothing to report.

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. The characteristics of the strata are only accessible by clicking on the drawing that redirects you to the search tool by stratigraphy representation. This representation can be compared to Fig. 5, credit MiCorr_HE-Arc CR, C.Degrigny.
Fig. 7: Stratigraphic representation of the object in cross-section using the MiCorr application. The characteristics of the strata are only accessible by clicking on the drawing that redirects you to the search tool by stratigraphy representation. This representation can be compared to Fig. 5, credit MiCorr_HE-Arc CR, C.Degrigny.

Complementary information

Nothing to report.

Analyses performed:
XRF, SEM/EDS. XRF was 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.

Complementary information

Nothing to report.

The entire surface of the bust is covered by a dark grey layer directly attached to the remaining metal (CP2) (Fig. 3). The surface analyses (Table 1) performed on the bust have revealed a high amount of Cu, as well as Pb, Sn and Zn. These are elements which can be constituents of the alloy, while elements in minor amount such as Al, Si and Fe are likely to originate from the environment. The qualitative analysis carried out on the 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 (CP1) which has developed in scattered clusters (probably copper carbonate). The clusters have a surface area of 2 mm2 to 2 cm2, and have a thickness of 0.5 to 5 millimeters. In some places, the green corrosion layer has formed in the porous blisters of the dark layer (Fig. 5). Charcoal might be found locally (POM1) as well as different sediments: S1 (discontinuous and brown), S2 (mixed with fine and coarse sand grains) and S3 (scattered homogeneous brown layer).

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

 

Complementary information

Nothing to report.

 No modification.

The metal is probably 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 metal surface caused by exposure to high temperatures. The dark patina (CP2) has developed from a smooth layer to voluminous green crusts (CP1) corresponding to a type 1 corrosion according to Robbiola and al. 1998. The artefact has been excavated from a burial context characterized by burnt soil, which could explain the formation of the black patina (tenorite will form at temperatures 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 adherent corrosion product. In certain areas the limit of the original surface has been elevated from its original position.

 

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.