Headrest or horse bit - Leaded Bronze - Modern Times

Christian. Degrigny (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 object from visual macroscopic observation.

Fig. 5: Stratigraphic representation of the object in cross-section using the MiCorr application. This representation can be compared to Fig 11.

Analyses performed:

Metallography (etched with ferric chloride reagent), SEM/EDX and FTIR.

The remaining metal is a leaded bronze (Table 1) containing numerous copper sulphide and lead (Pb) inclusions (Figs. 5 and 6). The porosity is difficult to appreciate since pores seem to have similar dimensions than the inclusions that could have been removed during the polishing process of the sample (Fig. 5). After etching, the structure of the metal appears made mostly of dendrites, but a grain structure seems to have developed on the right side of the sample, with occasionnal twin lines (Figs. 7 and 8). The twinned grain structure could be the result of cold work and annealing after casting, possibly through the application of an artificial patina under heat.

 

Elements Cu Sn Pb
mass% 85 7 5

Table 1: Chemical composition of the metal. Method of analysis: SEM-EDX, Lab of Electronic Microscopy and Microanalysis, IMA (Néode), HEI Arc.

The remaining metal seems to have developed intergranular corrosion at the interface metal / corrosion layer (Fig. 9) slightly enriched in Sn (Fig. 11). The outer green corrosion product is matt, powdery and mixed with sediments. It looks regular on Figs. 9 and 10 (around 50µm). This corrosion layer is mainly composed of lead (or/ and sulphur), oxygen, silicon, chlorine and is depleted in Cu (Fig. 11 and table 2) except in its top part where it is Cl, Cu, O-rich (Fig. 11 and table 2). FTIR seems to indicate that it is constituted of atacamite – Cu2Cl(OH)3 (Fig. 12). This is confirmed on the EDX spectra of Figure 13 where Pb is clearly detected. The inner black corrosion product is a dark brown, matt layer (Figs. 2 and 10). It covers all the surface of the object, and forms a very thin layer. It is sulphur and oxygen-rich (Fig. 14). FTIR analysis could not reveal the presence of a specific corrosion product.

 

Elements proportions O Si Cl Pb/S Cu Sn
Green corrosion product (CP1e) ++ (+) + ++ + nd
Black corrosion product (CP2i) + ++ (+) ++ (+) nd
Remnant metal phase nd nd nd + ++ ++

Table 2: Chemical composition of the corrosion crust from Fig. 10. Method of analysis: SEM-EDX, Lab of Electronic Microscopy and Microanalysis, IMA (Néode), HEI Arc (+++: high concentration, ++ medium concentration, + low concentration, nd: not-detected).

Based on the analyses carried out, the schematic representation of the stratigraphy of corrosion layers has been corrected: a thin, black sulphur-rich layer covers the metal surface. A thick green layer has developed on top and seems to be constituted mainly of atacamite enriched in lead with sediments on top.

The artefact is a cast leaded bronze that has been partially annealed after cold working (surface finishing?). Strangely one of the rolled up ends of the middle bar is full of metal testifying that it was cast already rolled-up. Normally it would have been rolled up after inserting the plates.  The remaining metal seems to have developed intergranular corrosion limited to the interface metal / corrosion layers. The corrosion crust is constituted of an outer thick, green atacamite layer enriched in Pb and mixed with sediments while the inner thin, black corrosion layer is S, Cu and O-rich. This stratigraphy is unexpected for an archaeological artefact where we would expect chlorine to be at the interface metal / corrosion layer. Similarly Si should be on top layers although it was found deep in the outer green layer. Furthermore S is found next to the metal surface while it should be more present in top layers. Finally Sn appears in an irregular and interrupted layer on top of the remaining metal. In an archaeological bronze it should be found as a clearly defined enriched layer. The red spots that looked like cuprite turned out to be paint. Since chlorine and sulphur are commonly used for the artificial patination of bronzes, we tend to conclude that this object is probably a fake produced during the 20th century.

References on object and sample

Reference object

1. Houshang Mahboubian. Art of ancient Iran: Copper and Bronze. Philip Wilson, London, 1997.
2. Rickenbach, Judith. Magier mit Feuer und Erz. Museum Rietberg, Zürich, 1992.

References on analytic methods and interpretation

3. Craddock, P. (2009) Scientific investigation of copies, fakes and forgeries. Butterworth-Heinemann, Oxford.
4. Northover, P. (1997) “Appendix”. In Houshang Mahboubian. Art of ancient Iran: Copper and Bronze. Philip Wilson, London, 325-338.
5. Oudbashi, O. and al. (2013) Micro-stratigraphical investigation on corrosion layers in ancient Bronze artefacts by scanning electron microscopy energy dispersive spectrometry and optical microscopy. Heritage Science, 1, 21.
6. Oudbashi, O. and al. (2014) Bronze in Archaeology: A Review of the Archaeometallurgy of Bronze in Ancient Iran”. In INTECH [Online]. 2012 [consulted on february, the 21th 2014]. http://www.intechopen.com/books/copper-alloys-early-applications-and-current-performance-enhancing-processes/bronze-in-archaeology-a-review-on-archaeometallurgy-of-bronze-in-ancient-iran