Votive figure of a bat - Gilded brass

Christian. Degrigny (HE-Arc CR, Neuchâtel, Neuchâtel, Switzerland) & Naima. Gutknecht (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: MiCorr stratigraphy 1 from Fig. 4, credit MiCorr_HE-Arc CR, N.Gutknecht.
Fig. 6: MiCorr stratigraphy 2 from Fig. 4, credit MiCorr_HE-Arc CR, N.Gutknecht.

XRF (on the object) with portable X-ray fluorescence spectrometer (NITON XL3t 950 Air GOLDD+ analyser, Thermo Fischer®), FTIR-ATR (on corrosion products), SEM/EDS (on the object). 

The metal is a gilded brass (Table 1) containing traces of Fe and Pb. Brass was not used by the pre-Columbian metalsmiths (Scott, D. (2000) and Hosler, D. (1994)) who worked with copper, arsenic copper or bronze. If ancient, the only possibility would be the use of brass imported by the Spanish conquistadors. Until 1500 the manufacture of brass with more than 28% zinc was not feasible (Craddock, P. (2009) p. 148). As regards pre-Columbian surface enriching technology, the following gilding techniques can be expected (Scott, D. (2000)):

- depletion of copper on a copper/gold alloy (tumbaga)

- gold foil, fusion gilding, electrochemical plating 

Elements mass %

Cu

Zn

Au

Fe

Pb

Ag

Sn

Ni

Cr

 1 (back)

64.1

28.5

7.1

0.06

0.02

0.03

0.02

0.02

0.07

 2 (back)

64.5

31.4

4.0

0.05

0.04

0.02

0.02

0.02

 3 (back)

65.3

29.3

5.1

0.08

0.1

0.04

0.03

0.02

0.04

 4 (front)

64.8

28.6

6.4

0.13

0.04

0.03

0.03

 5 (front)

66.3

26.9

6.4

0.16

0.3

0.03

0.03

0.03

Table 1: Chemical composition of the metal. Method of analysis: hand held XRF, mode precious metal, 60s. 1 and 2 were done in the chamber of the XRF but not 3, 4 and 5, credit MiCorr_HE-Arc CR, C.Degrigny. 

 

The metal seems to have been heavily attacked (acid dissolution). There are several perforations on the lower part of the object, and where the gold layer is absent the surface is unevenly pitted by corrosion (Figs. 1, 2 and 7). The submetallic core is not visible from the surface.

There are two corrosion products (Figs. 7, 8 and 9). The red one (CP2) has a submetallic brightness and is opaque, non-magnetic, compact, hard and adherent. It is very thin (less than a few microns thick) and only appears where the gold layer has been lost. The EDX spectrum (Fig. 10) shows that it is a Cu- and O rich product, probably cuprite (Cu2O).

The light green to white corrosion product (CP1) has a waxy consistency and is thin, translucent, non-magnetic, powdery and very soft. It is very loosely bound. It is distributed on the whole object (front and back) as disparate clumps. It is mainly located on the cavities and irregularities of the surface. Its FTIR analysis (ATR mode) (Fig. 11) was performed on green and white areas of CP1. The two compounds have similar spectra and were identified as copper nitrate (Fig. 11b).

Complementary information

 

For a long time the only brass used in the Americas was imported from Europe after the Spanish reached its shores in 1492. Pre-Columbian metalsmiths only produced copper, arsenic copper, bronze and copper-silver-gold alloys (tumbaga) (Hosler, D. (1994). p. 171). Brass with a zinc content superior to 28% (as is the case here) is an alloy that could only be achieved from 1500 and onwards (Cradock, 2009, p.148). We can then rule out that the artefact was made from early recycled European brass.

FTIR analysis of the green/white corrosion product suggests that it is a copper nitrate, a highly water soluble corrosion product that cannot be found in an archaeological context. However, it has been recorded on artificially patinated copper alloys (Scott, D. (2000) p. 250-251 ; Craddock, P. (2009) p. 365).

The dissolution of the metal and the subsequent formation of the patina is most likely the result of a forced attack with concentrated nitric acid. The holes and pits in the metal are unnatural for a natural corrosion process which would produce a stratified, adherent and thick corrosion crust (Craddock, P. (2009) p. 349-351). The copper nitrates could have formed over time from residual acidic solution that was left on the surface.

The lack of an archaeological context, the modern alloy composition and the presence of copper nitrates on the unnaturally pitted surface suggest the object being a forgery made in modern times.

References on object and sample    

1. Scott, D. (2000). A review of gilding techniques in ancient South America. In: T. Drayman-Weisser (ed.) Gilded Metals: History, Technology and Conservation. London, Archetype Publications, p. 203-222.

2. Hosler, D. (1994). The sounds and colors of power – the sacred metallurgical technology of ancient west mexico. Massachusetts Institute of technology.    

References on analytic methods and interpretation    

3. Craddock, P. (2009) Scientific Investigation of Copies, Fakes and Forgeries. Butterworth-Heinemann, London.

4. McEwan, C (ed.) (2000) Precolumbian Gold – Technology, style and Iconography. British Museum, London.