Iron sheet - Fe Alloy - Modern Times - Switzerland

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

Stratigraphic representation: none

Fig. 4: Stratigraphic representation of the object in cross-section using the MiCorr application. This representation can be compared to Figs. 7 and 8.

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

The remaining metal is an iron containing elevated (more than 1g/kg) concentrations of Co and Ni (Table 1), small round slag inclusions and cracks resulting from corrosion and deformation (Fig. 4). The composition of the round, elongated slag inclusions is similar to wüstite-FeO (Table 2). Iron reduced in the direct smelting process never contains such pure compounds and only FeO inclusions. In modern steels FeO occurs in low carbon alloys and Armco-iron (Schumann 1991, 474). After etching, the cross-section shows a heavily deformed ferritic microstructure from cold working (Fig. 5). The cracks have the same orientation as the deformation. The average hardness of the metal is HV1 195.

 

Elements Al Ti V Cr Mn P Co Ni Cu As Mo Ag Sn Sb W Ni/Co
Median, mg/kg < < < < 20 300 1100 1200 800 200 < < 60 10 < 1.1
RSD % - - - - 3 17 6 5 35 15 13 - 35 45 - 4
Detection limit mg/kg 6 9 19 17 2 77 1 4 2 3 3 1 1 1 3 -

Table 1: Chemical composition of the iron. Method of analysis: LA-ICP-MS, Lab of Inorganic Chemistry, ETH.

 

Elements

O Fe Total
Round inclusion 1 21 75 96
Round inclusion 2 20 74 95
Round inclusion 3 20 75 96
Round inclusion 4 20 73 93

Table 2: Chemical composition (mass %) of some of the round inclusions of Fig. 4. Method of analysis: SEM/EDX, Laboratory of Analytical Chemistry, Empa.

The corrosion is massive and large parts of the outer corrosion layers have been lost during the sample preparation. The heavily cracked corrosion crust represents about one third of the thickness of the sample (Fig. 3) and is located on one side of the metal (the rest having been lost during the cutting process). It does not show well defined layers. Despite this, three areas can be distinguished (Figs. 6 and 7). Adhering to the metal (area 1), we find an orange-red-brown layer enriched in Cl (Fig. 8 and Table 3). The dark or violet middle area 2 is richer in Fe. The outer area 3 is red-brown and strongly contaminated by soil material (rock fragment inclusion and elements like Si, Al, Mg, P etc.). This layer is enriched in O (Fig. 8).

 

Elements

Location O Na Al Si P Cl Fe Cu Mo Total
Area 1 Dark-brown 31 < < < < < 67 < < 99
Dark-brown 35 < < < < 0.7 64 < < 100
Red-brown 38 < < < < 2.2 64 < 0.6 104
Red-brown 36 < < < < 3.3 67 < < 106
Red-brown 39 < < < < 1.3 63 < < 104
Orange-brown 35 1.3 < < < < 61 < < 98
Area 2 Orange-brown 31 < < < < 0.7 63 < < 95
Violet-brown 30 < < < < < 75 0.8 1.0 108
Brown 30 < < < < < 77 < 0.6 108
Violet-brown 35 < < < < < 68 < < 103
Yellow 33 < < < < < 67 < < 100
Violet-brown 32 < < < < < 74 < < 106
Area 3 Quartz inclusion 54 < < 52 < < < < < 106
Mixture brown 42 < < 1.6 0.7 < 58 < < 102
Mixture brown 39 < 1.9 4.3 < < 56 < < 102
Mixture brown 35 < < 0.9 < < 59 < < 95

Table 3: Chemical composition (mass %) of the corrosion products from near the metal (area 1) to the outer surface (area 3). Method of analysis: SEM/EDX, Laboratory of Analytical Chemistry, Empa.

Corrected stratigraphic representation: none

This iron (and steel, not considered here) sheet is entirely cold worked and hard compared to the annealed metal. The corrosion is massive and masks the shape of the object. The presence of Cl adjacent to the metal surface indicates that the corrosion front is potentially active. The very thick top corrosion layers may have slowed down the corrosion. The object was mentioned as being Roman in Senn Bischofberger 2005, however the chemical composition of the slag inclusions shows that it is a product of the 20th century AD which has been introduced accidentally into a Roman layer.

References on object and sample

References object

1. Rychener, J. (1999) Der römische Gutshof in Neftenbach. Katalog, Tafeln und Tabellen. Monographien der Kantonsarchäologie Zürich 31/2 (Zürich und Egg), 138.

 

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.), 137-138.

References on analytic methods and interpretation

3. Schumann, H. (1991) Metallographie. Leipzig.