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

Funeral mask

Christian. Degrigny (HE-Arc CR, Neuchâtel, Neuchâtel, Switzerland) & Naima. Gutknecht (HE-Arc CR, Neuchâtel, Neuchâtel, Switzerland) & Valentin. Boissonnas (HE-Arc CR, Neuchâtel, Neuchâtel, Switzerland)

Complementary information

None.

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.

Complementary information

None.

Analyses performed:
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 XRF analysis is a surface analysis that was done without any removal of the copper corrosion products. It seems that the metal is a copper-silver alloy.

The SEM/EDS analysis of the core metal shows that there are Cu-rich phases alternating with Ag-rich phases. There are inclusions of Pb and As (Fig. 5). The surface of the metal has been decuprified (Fig. 6) according to the tumbaga making process (Scott 2000 and McEwan 2000). This pre-Columbian repeatedly hammered and annealed the metal, which would create a copper oxide scale on the surface. The latter was then dissolved in acidic plant juices (Scott 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. For this artefact, 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.
A layer of mercury can be seen with EDS (Figs. 6 and 7). Mercury was not used by the pre-Columbians in metallic form (Scott 2000 and McEwan 2000) but was common as a pigment in form of mercury sulfide (cinnabar).

Complementary information

None.

The remaining metal shows a preferential corrosion of copper and the presence of chlorides throughout the entier corroded metal, most likely in the form of Cu and Ag corrosion products (Fig. 8, 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. 11, Table 1), while the red layers (CP2 in Fig. 15 ; CP4 and CP5 in Fig. 16) are consisting of a copper oxide, most likely cuprite (Fig. 9). Below the cuprite and above the silver enriched surface on the front face a thin black layer is present (appears grey under polarized light). SEM-EDS analysis shows that in that stratum the mercury of the pigment (Fig. 14) has been transformed into a mercury-silver compound (Figs. 7 and 10) which according to Raman spectrometry (Fig. 14) 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. 6 and 7).
The silver sulphide (HgS, cinnabar, Fig. 13), present as a red pigment on the silver surface, could have been reduced through an electrochemical process in the presence of chlorides (Keune 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. 8). It remains unclear if the limitos is located in the silver enriched surface or within this silver-mercury compound (Fig. 7).

 

Elements

Cu

Ag

O

Cl

Hg

S

As

Pb

Interpretation

 Red layer (CP2, front & CP4-CP5, back)

+++

 

+

         

Copper oxide

 Grey layer (CP3, front & CP2, back)

 

+++

   

+++

++

   

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.

Complementary information

None.

Fig. 15: Stratigraphic representation of the cross-section (dark field) from Figs. 3a or 4 (front of the object) 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, credit MiCorr_HE-Arc CR, N. Gutknecht.
Fig. 16: Stratigraphic representation of the cross-section (dark field) from Figs. 3b and 4 (back of the object) using the MiCorr application. CP2 is CP3 while CP3 is CP4 and CP4 is CP2 in Fig. 3b. The characteristics of the strata are only accessible by clicking on the drawing that redirects you to the search tool by stratigraphy representation, credit MiCorr_HE-Arc CR, N. Gutknecht.

The schematic representation of corrosion layers of Fig. 3 integrating additional information based on the analyses carried out is given in Fig. 17.

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 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.

References on object and sample

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