Sacrificial knife (tumi) IVc 24745 - Cu Alloy - Peru

Artefact name

Sacrificial knife (tumi) IVc 24745

Authors

Christian. Degrigny (HE-Arc CR, Neuchâtel, Neuchâtel, Switzerland) & Marion. Billot (HE-Arc CR, Neuchâtel, Neuchâtel, Switzerland) & Valentin. Boissonnas (HE-Arc CR, Neuchâtel, Neuchâtel, Switzerland)

Url

/artefacts/348/

The object

Description and visual observation

                                            Description of the artefact
                                        
Sacrificial knife made of an arsenical copper alloy. Semi circular flattened cutting edge. Trapezoidal handle with rectangular section. 
   L = 105 mm; W handle = 25 mm; W blade = 55 mm; T = 1 mm; WT = 18,7 g    

                                            Type of artefact
                                        
Sacrificial knife (tumi in vernacular language)

                                            Origin
                                        
Probably Cajamarca (Peru), Cajamarca, Cajamarca, Peru

                                            Recovering date
                                        
Unknown

                                            Chronology category
                                        
None

                                            chronology tpq
                                        
                                            chronology taq
                                        
                                            Chronology comment
                                        
Between 200 at 1300 AD

                                            Burial conditions / environment
                                        
Soil

                                            Artefact location
                                        
Museum der Kulturen, Basel

                                            Owner
                                        
Museum der Kulturen, Basel

                                            Inv. number
                                        
IVc 24745

                                            Recorded conservation data
                                        
Not conserved
Complementary information
  Little information on the origin of the artifact. Most likely not documented before being donated to the museum.    

                                        Study area(s)
                                
                                            Credit HE-Arc CR, M.Billot.
                                        
                                            Fig. 2: Location of Fig. 3, XRF analyses (white squares) and the sampling area (yellow square), 
                                            
                                            Credit HE-Arc CR, M.Billot.
                                        
                                            Fig. 3: Detail of the corrosion stratigraphy of the fractured cutting edge. To be related to the schematic stratigraphy in Fig. 6, 
                                            
                                            Credit HE-Arc CR, M.Billot.
                                        
                                            Fig. 4: Detail of the sampling area for XRF and XRD analysis, 
                                            
                                        Binocular observation and representation of the corrosion structure
                                
The schematic representation below gives an overview of the corrosion layers encountered on the tumi from a first visual macroscopic observation under binocular microscope.

                                            Credit HE-Arc CR, M.Billot.
                                        
                                            Fig. 5: Stratigraphic representation of the sacrificial knife in cross-section by macroscopic observation, 
                                            
                                        MiCorr stratigraphy(ies) – Bi
                                
            Fig. 6. Stratigraphic representation of the tumi in cross-section using the MiCorr application, Credit HE-Arc CR, M.Billot.

Sample(s)

Analyses and results

X-ray radiography[1], XRF[2], SEM-EDS, and XRD.
Conditions of the XRD analysis: Stoe Mark II-Imaging Plate Diffractometer System (Stoe & Cie, 2015) equipped with a graphite-monochromator. Data collection was performed using Mo-Kα radiation (λ = 0.71073Å, beam diameter 0.5mm). Two-dimensional diffraction images (10min. per exposure) were obtained at an image plate distance of 200mm with a continued sample rotation. The resolution was Dmin - Dmax 24.00 - 1.04Å and intensity integration has been performed over the entire image (360°).
[1] The conditions are unknown.
[2] On the object with portable X-ray fluorescence spectrometer (NITON XL3t 950 Air GOLDD+ analyser, Thermo Fischer®.

                                        Non invasive analysis
                                
                                        Metal
                                
The metal is an arsenical copper alloy with low concentration of iron and traces of silver (Table 1).
XRF analyses have been conducted on eight areas, on the two sides of the non-cleaned artifact. Almost all analyses are the same except for the area “e” (Fig. 2, Table 1) where a slightly higher concentration of arsenic was measured.

Elements mass%

Cu

As

Ag

Fe

Cl

Al

Si

K

P

Ca

S

BAL

a

61.9

0.5

0.05

0.4

0.3

3.6

12.6

0.7

0.2

1.1

0.2

18.3

b

51.9

0.3

0.04

0.4

0.3

1.4

11.9

0.7

0.07

0.8

0.1

31.8

c

51.4

0.3

0.04

0.5

0.3

2.5

14.4

0.8

0.1

0.9

0.1

28.2

d

52.7

0.3

0.04

0.5

0.4

2.6

16.6

0.8

0.06

1.1

0.2

24.5

e

39.8

1.1

0.03

0.9

2.2

3.1

13.6

0.8

0.07

1.0

0.4

36.7

f

52.6

0.3

0.03

0.3

0.2

1.8

11.7

0.7

0.04

1.5

0.4

30.3

g

52.5

0.3

0.04

0.3

0.3

2.1

11.5

0.6

0.2

1.6

0.3

30.1

h

38.4

0.2

0.03

0.8

0.1

3.0

16.5

0.8

0.1

0.9

0.2

38.8

Table 1: Chemical composition of the non-cleaned metal surface. Method of analysis: XRF. Mining mode Cu/Zn, acquisition time 180s (filters: M30/Lo30/H60/Li60), in the support. BAL corresponds to the elements not analysed: O and C. The exogenous elements are indicated in light-yellow while remarkable concentrations are highlighted in yellow, credit MiCorr_HE-Arc CR, C.Degrigny.

                                            Credit Centre d'Imagerie Médicale, La-Chaux-de-Fonds.
                                        
                                            Fig. 7: X-ray radiography of the artifact. The base of the blade is thicker and is probably as-cast (red arrow). The blue arrows indicate casting flaws or localized corrosion, 
                                            
                                            Microstructure
                                        
None

                                            First metal element
                                        
Cu

                                            Other metal elements
                                        
As
Complementary information
Complementary information given by X-radiography (Fig. 7) shows that the object is not totally mineralized and still has a lot of remaining metal. Irregularities and different thicknesses are visible and parallel lines show that the artifact was hammered. The whiter the area, the thicker the metal is.

                                        Corrosion layers
                                
Results from XRF analysis of the non-cleaned metal surface (Table 1) indicate an elevated concentration of chlorine. 
   The exogenous elements come from the landfill are mainly Si and Al, certainly associated to aluminosilicates.    
  
A sample was taken in the powdered light-green isolated layer (Figs. 2, 4 and 5). Chlorine, copper and oxygen was analysed by SEM-EDS. The presence of high concentration of Cl as well as Cu and O seems to indicate active corrosion (Fig. 8). XRD analysis confirmed the presence of paratacamite often associated to active corrosion (Fig. 9).

                                            Credit HEI-Arc, S.Ramseyer.
                                        
                                            Fig. 8: EDS spectrum of a sample of the powdered light-green isolated layer (Fig. 2), Lab of Electronic Microscopy and Microanalysis (Néode), 
                                            
                                            Credit  Empa, A.Neels.
                                        
                                            Fig. 9: XRD spectrum of a sample of the powdered light-green isolated layer (Fig. 2), Laboratory of Analytical Chemistry, Empa, HE-Arc, 
                                            
                                            Corrosion form
                                        
Multiform - transgranular

                                            Corrosion type
                                        
Type II (Robbiola)

                                        MiCorr stratigraphy(ies) – CS

Synthesis of the binocular / cross-section examination of the corrosion structure

  
Based on the analysis carried out, the schematic representation of the stratigraphy of corrosion layers (Fig. 5) was corrected. The limit of the original surface (represented by the dotted line on the figure below) was identified as still present and is located at the interface between CP6 and CP7.

                                            Credit HE-Arc CR, M.Billot.
                                        
                                            Fig. 10: Improved stratigraphic representation of the tumi from visual observations and analyses,

Conclusion

This tumi is an arsenic copper alloy with a low percentage of iron, as well as traces of silver. This alloy was common in pre-Columbian South America[1]. The X-ray radiography shows that this object was formed from a metal sheet and cold hammered.

Chlorine is found locally in the form of paratacamite.

The limit of the original surface has been altered by the corrosion products but can be found at the interface of CP6 and CP7. This tumi follows the type II corrosion model of L. Robbiola.

[1] Pillsbury, 2001, p. 97.

References

References object

1. Pillsbury, J. Moche art and archeology in ancient Peru. National Gallery of Art, Washington, 2001.

References on analytic methods and interpretation

2. Scott, D. Copper and Bronze in Art: Corrosion, Colorants, Conservation. Getty Conservation Institute, Los Angeles, 2002.

3. Robbiola, L. Caracterisation de l'altération de bronzes archéologiques enfouis à partir d'un corpus d'objets de l'âge du bronze. Mécanismes de corrosion. Université Pierre et Marie Curie - Paris VI, 1990.