Sacrificial knife (tumi) - Cu Alloy

Christian. Degrigny (HE-Arc CR, Neuchâtel, Neuchâtel, Switzerland) & Gerber. Alice (HE-Arc CR, Neuchâtel, Neuchâtel, Switzerland) & Valentin. Boissonnas (HE-Arc CR, Neuchâtel, Neuchâtel, Switzerland)

Complementary information

The original archaeological context is unknown, so there is very little information about the culture of origin and no exact dating. The object entered through a purchase and donation to the museum.

The schematic representation below gives an overview of the corrosion layers encountered on the sacrificial knife from a first visual macroscopic observation

Fig. 10: Stratigraphic representation of fragment b of the military tag in cross-section 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, A.Gerber.

Complementary information

Nothing to report.

Analyses performed:
X-ray radiography and tomography*, XRF** and SEM-EDS***.

* Conditions were not documented.
** Directly on the metal with a portable X-ray fluorescence spectrometer NITON XL3t 950 Air GOLDD+ analyser from Thermo Fischer®.
*** On a few samples with a Jeol JSM-6400 device, HEI Arc, La Chaux-de-Fonds, Switzerland

The metal is an arsenical copper alloy with 2-3% (in mass) As (Table 1 and Fig. 11). As can be seen from the stratigraphy in Fig. 9, other materials make up the figure of the decorated handle: a whitish-yellowish opaque material as well as a crystalline material with golden reflections in a resinous material (amber?).

Element Cu As Cl S P Ca BAL
mass % 75 2-3 11.5 0.2 0.2 0.1 10-11

Table 1: Chemical composition of the partly-cleaned metal surface. Method of analysis: XRF. Mining mode Cu/Zn, acquisition time 120s (filters: M30/Lo30/H30/Li30). BAL = non-analysed elements (probably O and C).

Complementary information

The metallographic structure is unknown, but it is likely that the object was cast and then reworked cold.
Depending on the area, there should be cored grains with remaining dendritic structure and, if the metal was annealed, grains with twin lines.

Table 1 shows a high amount of the chlorine element on the surface of the front face of the sacrificial knife.

As shown in the stratigraphy of Fig. 9, the metal of the sacrificial knife is covered with a double layer of orange-brown / dark grey corrosion products (CP5 / CP6). In some areas, the powdery orange-brown sub-layer is interspersed with a green corrosion layer (CP3) (pustules) or covered with hard blue corrosion products (CP4). A whitish, transparent corrosion product of waxy consistency (CP7) was found under the pustules. Locally and on the surface, a light blue corrosion product (CP2) was found with a whitish layer (CP1) on top.

EDS analyses of these different corrosion products showed that CP7 contains Cu and Cl and is identified as nantokite, CP5 and CP6 could be cuprite since they contain exclusively Cu and O, CP3 and CP4 containing both Cu, O and C could be malachite and azurite respectively. The eye-like aspect of CP3 seems to validate this hypothesis. CP2 is identified as atacamite or paratacamite since it contains both Cu, O and Cl. CP1 could not be identified. Most of the corrosion products contained phosphorus (P), suggesting that the sacrificial knife was probably buried in a grave, or near any significant amount of decaying organic matter.

Complementary information

The surface of the object is corroded in a very heterogeneous way. Some zones correspond to Type I corrosion models according to Robbiola, and others to Type II with the formation of pustules (Formigli 1975). Parts of the object that have been cold worked and / or deformed are those with the most corroded surfaces, sometimes with complete lost of the limitos. The edge of the blade, for example, was hammered after casting and was bent during its use: it is one of the most corroded areas.

The schematic representation of corrosion layers of Fig. 9 integrating additional information based on the analyses carried out is given in Fig. 14. The limit of the original surface was identified as still present and is located at the interface between CP5 and CP6.

The metal of the sacrificial knife is an arsenical copper alloy. The object was cast and then cold-worked, probably hammered. The metal is heterogeneously corroded with Robbiola types I and II and pustules. In the analyzes carried out, the chlore element appeared, indicating a so-called "active" corrosion of the metal. The limit of the original surface is at the CP5 and CP6 interface.

References on object and sample

Reference object

1. Billot, M. 2018 MiCorr file of a Tumi from Peru.

References on analytic methods and interpretation

2. Formigli, E. (1975) Die Bildung von Schlichtpocken auf antiken bronzen, Arbeisblätter, Heft 1,  51-74.
3. Gutknecht, N. (2018) La corrosion active sur les alliages cuivreux archéologiques - Evaluation de la stabilisation par biopassivation fongique. Mémoire de Master, Haute École Arc Necuhâtel, Conservation-Restauration, non-publié.
4. Robbiola, L. (1990) 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.
5. Scott, D. (2002) Copper and Bronze in Art: Corrosion, Colorants, Conservation. Getty Conservation Institute, Los Angeles.
6. Scott, D. (1991) Metallography and Microstructure of Ancient and Historic Metals. Getty Conservation Institute, Los Angeles.