Funeral mask - Cu/Ag, with surface enrichment (Ag)

The schematic representation below gives an overview of the corrosion layers encountered on the object from visual macroscopic observation. The first stratigraphy (Fig. 3a) is for the front, while the second (Fig. 3b) represents the back.

Fig. 5a: Schematic interpretation of the cross-section from 3a (face of the object), credit MiCorr_HE-Arc CR, N. Gutknecht.
Fig. 5b: Schematic interpretation of the cross-section from 3b (back of the object), credit MiCorr_HE-Arc CR, N. Gutknecht.

XRF with portable X-ray fluorescence spectrometer (NITON XL3t 950 Air GOLDD+ analyser, Thermo Fischer®, SEM/EDS (with an acceleration voltage of 20 kV) and Raman spectroscopy.   

The metal is a copper-silver alloy with traces of Pb and As. The surface has been decuprified according to the tumbaga making process (Scott, D. (2000) and McEwan, C (ed.) (2000)). This pre-Columbian surface enriching technique results in a silver looking object, even though the general silver content is low (about 30%). The metalsmith would repeatedly hammer and anneal the metal, which would create a copper oxide scale on the surface. The latter was then dissolved in acidic plant juices (Scott, D. (2000)). This process was repeated until the surface was enriched with silver, giving it the appearance of a silver artefact.

It is impossible to know the original alloy composition, as the proportions have changed through corrosion and migration of elements. Nevertheless, the inner metal is reddish, which could indicate a 30% silver content for 70% copper. The silver-rich surface has been further enriched by the migration of copper ions that have formed the thick corrosion crust on top of the original surface.

The XRF analysis is a surface analysis that was done without any removal of the copper corrosion products. The proportions are given for the oxide on the surface. The SEM/EDS analysis (Fig. 6) shows that there are Cu-rich phases alternating with Ag-rich phases. There are inclusions of Pb and As. Cl has been detected throughout the entire corroded metal, most likely in the form of Cu and Ag corrosion products.

A layer of mercury can be seen with EDS (Figs. 7 and 8). Mercury was not used by the pre-Columbians in metallic form (Scott, D. (2000) and McEwan, C (ed.) (2000)) but was common as a pigment in form of mercury sulfide (cinnabar).    

The remaining metal shows a preferential corrosion of copper and a presence of chlorides (Fig. 9, Table 1). There is a preferential corrosion of copper that has fragilized the structure of the sheet metal.

The silver enriched surface is entirely covered with copper corrosion products. The outer green layer (CP1) is a copper carbonate (Fig. 12, Table 1), while the red layers (CP2, CP4 and CP5) are consisting of a copper oxide, most likely cuprite (Fig. 10). Below the cuprite and above the silver enriched surface a thin black layer is present (appears grey under polarized light). SEM-EDS analysis shows that in that stratum the mercury of the pigment has been transformed into a mercury-silver compound (Figs. 8 and 11a and b) which according to Raman spectrometry (Fig. 15) is not cinnabar anymore. In areas this silver- and mercury-rich surface has been pulled off by raising the corrosion layers, leaving structural voids that were subsequently filled by secondary corrosion products, most likely copper carbonates (Figs. 7 and 8)

The silver sulphide (HgS, cinnabar, Fig. 16), present as a red pigment on the silver surface, could have been reduced through an electrochemical process in the presence of chlorides (Keune, K. (2005)). The released sulfur recombined with the silver to form black silver sulphide. Above that layer a porous mercury- and silver rich stratum has formed (see Hg & Ag on cartography Fig. 9). It remains unclear if the limitos is located in the silver enriched surface or within this silver-mercury compound (Fig. 8).

 

Elements

Cu

Ag

O

Cl

Hg

S

As

Pb

Interpretation

 Red layer (CP2)

+++

 

+

         

Copper oxide

 Grey layer (CP3)

 

+++

   

+++

++

   

Pigment (HgS) and silver

 Green layer (CP1)

+++

 

+

++

       

Copper carbonate

 Black layer

+

++

     

++

   

Silver and copper sulfides

 Nodules in the   metal

 

+++

 

+++

 

 

 

 

Silver chlorides

 Metal phases 1

+

+++

 

 

 

 

 

 

Silver-rich phases

 Metal phases 2

+++

++

+

+

 

 

 

 

Copper-rich phase

 Inclusion in metal

 

 

+

 

 

 

++

+++

AsPb impurities

Table 1: Chemical composition of the corrosion crust from Fig. 4. Method of analysis: SEM-EDS, Lab of Electronic Microscopy and Microanalysis, IMA (Néode), HEI Arc (+++: high concentration, ++ medium concentration, + low concentration), credit MiCorr_HEI Arc_S. Ramseyer.

The artefact is a repeatedly hammered, annealed and pickled Cu-Ag tumbaga. The cuprite and hydroxicarbonate layers and the presence of chlorides are typical for the corrosion of tumbaga alloys in an archaeological context. Cinnabar has been identified as pigment applied to the surface before burial. Over the centuries, it was partially incorporated into the growing copper corrosion layers.

Close to the enriched silver surface and below the cuprite layer a silver- and mercury-rich stratum has formed. Only one publication mentions the reaction of cinnabar with a gold-enriched tumbaga surface but does not go into details about the possible mechanism involved (Masuda, S. (ed.) (1997)).

The presence of a silver-sulphur compound below this silver-mercury stratum could indicate that the cinnabar was reduced by electrochemical processes to mercury, releasing sulphur that reacted with the silver enriched surface. The mercury itself formed a silver-mercury layer above the latter. More research is needed to fully comprehend this phenomenon.

1. Keune, K. (2005) «Analytical Imaging Studies Clarifying the Process of the Darkening of Vermilion in Paintings». Analytical Chemistry, n° 77, 2005. p. 4742-4750.

2. 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.

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

4. Masuda, S. (ed.) (1997). Sicàn - ein Fürstengrab in Alt-Peru: Eine Ausstellung in Zusammenarbeit mit dem peruanischen Kulturministerium. Museum Rietberg, Zurich.