Sword - P-rich iron - Iron Age - Switzerland

Marianne. Senn (EMPA, Dübendorf, Zurich, Switzerland) & Christian. Degrigny (HE-Arc CR, Neuchâtel, Neuchâtel, Switzerland)

Complementary information

Nothing to report.

Stratigraphic representation: none.

Fig. 4: Stratigraphic representation of the object 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. This representation can be compared to Fig. 9, credit MiCorr_HE-Arc CR.

Complementary information

Nothing to report.

Analyses performed:
Metallography (nital etched), Vickers hardness testing, LA-ICP-MS, SEM/EDS.

The remaining metal is a P-rich (0.2 mass%) iron (Table 1) with long parallel slag inclusions (Fig. 5) concentrated on one side of the blade. The slag inclusions are composed of wüstite/FeO dendrites and fayalite/Fe2SiO4 in a glassy matrix (Fig. 6 and Table 2). Their chemical composition is typical for iron produced by the bloomery process (dominated by iron oxides and silica). It is difficult to identify the ore type from the slag composition. Interestingly in one case the slag composition is very specific (alumina and P-rich material). After etching, the metal shows a ferritic structure (Fig. 7). The grain size is variable (between ASTM grain sizes of 4 to 7) and some grains include Neumann bands (Fig. 8). A large crack has developed through the metal section (Figs. 3 and 7). The average hardness of the metal (HV1 185) is quite high for a wrought iron. The level of hardness and Neumann bands are typical for a P-rich iron. Neumann bands are said to develop by cold working and shock deformation. According to Swiss and McDonnell 2003 they form when little cold work is carried out. Distortion (grain deformation) after cold working starts to be apparent in iron after a reduction in thickness of between 30-40%. Since no grain deformation is visible, the present reduction is probably a little less than this range.

Elements V Cr Mn P Co Ni Cu As Ag
Median mg/kg < 5 10 2200 140 270 500 260 <
Detection limit mg/kg 1 4 1 50 1 1 1 2 0.1
RSD % - 56 48 10 14 16 37 15 -

Table 1: Chemical composition of the metal. Method of analysis: LA-ICP-MS, Lab Analytical Chemistry, Empa (for details see Devos et al. 2000).

 

Location MgO Al2O3 SiO2 P2O5 K2O CaO TiO2 MnO FeO Total SiO2/Al2O3
Wüstite in glass 0.7 2.1 15 2.7 < 0.6 < < 87 109 7.2

Wüstite and fayalite in glass

< 4.8 24 0.7 1 1.4 < < 70 103 5.0

Wüstite and fayalite in glass

0.6 2.7 23 1.8 < 0.8 < < 69 98 8.4
Fayalite in glass < 5.6 26 2.8 0.8 1.4 < < 72 110 4.8
Fayalite phase 0.8 1.0 31 1.2 0.7 1.1 < 0.8 69 106 33
Fayalite in glass < 8.8 24 3.2 1.0 1.4 0.6 < 66 106 2.8

Table 2: Chemical composition of the slag inclusions (mass%) at the tip (pearlite) and the body (ferrite) of the knife. Method of analysis: SEM/EDS, Laboratory of Analytical Chemistry, Empa.

Complementary information

Nothing to report.

The remaining metal, including the large crack, is covered by a thin, fissured corrosion crust (Figs. 3, 5, 7 and 9). The corrosion crust is thicker on the side containing the long slag inclusions (Fig. 3). In bright field and in the BSE-mode of the SEM image only light and dark-grey areas can be distinguished (Figs. 5 and 10). Under polarised light the corrosion products appear yellow-orange near the metal surface and then become successively dark-red and black. The outer layer (CP1) is orange-yellow, as is the inner one (CP4, Fig. 9). There is a correlation between the level of grey in SEM/BSE-mode, the colours under polarised light, and the chemical composition of the corrosion layer (Table 3, Figs. 9 and 10). The lighter the grey of the SEM/BSE-mode, or the colours (light-brown and red) under polarised light, the richer the area is in Fe and the more depleted it is in O. Surprisingly the inner corrosion layers (CP3 and CP4) are contaminated with Si, Al and O.

 

Elements

O Si Fe Total
Light or dark-red corrosion products (CP3) 25 1.6 67 94
Light or dark-red corrosion products (CP3) 32 < 74 108
Dark or dark-red corrosion products (CP2) 41 < 67 109

Table 3: Chemical composition (mass %) of the corrosion layer (from Fig. 9). Method of analysis: SEM/EDS, Laboratory of Analytical Chemistry, Empa.

Complementary information

Nothing to report.

Corrected stratigraphic representation: none.

The sword blade is made of a hard, P-rich iron. It displays poor workmanship compared to other Celtic swords. The metal was first hot worked followed by a final cold working. The corrosion layer, typical of terrestrial context, has been partially removed by the conservation treatment. The possible use of air abrasive cleaning with glass beads and aluminium oxide, or the use of abrading tools, could explain the enrichment in Si and Al of the surface.

References on object and sample

References object

1. Senn Bischofberger, M. (2005) Das Schmiedehandwerk im nordalpinen Raum von der Eisenzeit bis ins frühe Mittelalter. Internationale Archäologie, Naturwissenschaft und Technologie Bd. 5, (Rahden/Westf.), 30.

References sample

2. Senn Bischofberger, M. (2005) Das Schmiedehandwerk im nordalpinen Raum von der Eisenzeit bis ins frühe Mittelalter. Internationale Archäologie, Naturwissenschaft und Technologie Bd. 5, (Rahden/Westf.), 240-242.

References on analytic methods and interpretation

3. Swiss, A. J. and McDonnell, J.G. (2003) Evidence and interpretation of cold working in ferritic iron. International Conference, Archaeometallurgy in Europe 2003, Proceedings, vol. 1, Milan, 209-217.

4. ASTM E112-13: Standard Test Methods for Determining Average Grain Size.