Sheet fragment with dark green / grey patina - Tin Bronze - 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. This representation can be compared to Fig. 7.

Complementary information

Nothing to report.

Analyses performed:
Metallography (etched with ferric chloride reagent), Vickers hardness testing, ICP-OES, SEM/EDS, Raman spectroscopy.

The remaining metal is a porous (red arrows on Fig. 5) tin bronze (Table 1). Five analyses were carried out. S was detected in the non-corroded part of the metal (2 measurements) while P was present only in the corroded metal (3 measurements). As no major difference in the composition was observed (comparison of relative standard deviation, RSD) all analyses were used to calculate the median value. Inter- and transgranular corrosion has developed so extensively that all grain boundaries and twin lines are outlined (Fig. 5). After etching, the metal shows annealed polygonal grains with a few twins and slip lines below the surface (Fig. 6). The slip lines are restricted to the right side of the sample where the metal is best preserved (Fig. 6). The grain size varies between 50 and 170µm, due to an excessively long or hot annealing procedure leading to a grain coarsening. Small copper sulphide inclusions appear in blue (Fig. 6). The average hardness of the metal is HV1 90.

 

Elements Cu Sn As S* P** Co Ni Pb Sb Ag Zn Fe Bi
mass% (median value of 5 measurements) 83.13 16 0.26 0.1 0.07 0.032 0.025 0.02 0.02 0.009 0.002 < 0.002
RSD % 2 9 13 25 45 3 5 41 15 12 6 < 43

Table 1: Chemical composition of the metal. Analytical method: LA-ICP-MS, Laboratory of Basic Aspects of Analytical Chemistry at the Faculty of Chemistry, University of Warsaw, PL. *S is only present in the metal, whereas **P indicates the presence of corrosion products in the analysed metal.

Complementary information

Nothing to report.

The corrosion crust varies in thickness between 60 and 150µm (Fig. 3). In bright field, it appears dark-grey (Fig. 5) and consists of two layers (CP1 and CP2). The inner layer is dark-grey and dense while the thin outer layer is slightly lighter coloured. Within the metal, the corrosion products are light-grey (CP3, Fig. 5). Under polarized light, the corrosion layer turns blue-green with dark-blue areas (Figs. 7 and 8) whereas corrosion products inside the metal are either light-blue or red-orange (Figs. 7 and 8). The red corrosion products (CP3) have the composition of cuprite/Cu2O while the orange compounds (also CP3) are enriched in Sn (Table 2). The blue-green corrosion products (CP2) both within the remaining metal and on the surface are even richer in Sn and O, and contain some P (Table 2 and Fig. 9). The thin, irregular dark-grey surface layer (CP1) is enriched in P, Fe, Si and Al (Table 2 and Fig. 9). XRD analyses of powdery particles sampled from the thin, dark surface corrosion layer (CP1) indicate the presence of tenorite/CuO and cassiterite/SnO2 (Museum report 1992). The Raman spectra of this layer (Fig. 10) confirmed the presence of tenorite.

 

Elements

O Cu Sn Si Fe P As Total
CP1, outer dark-grey corrosion layer. Fig 7 34 16 49 < 3.4 3.0 0.86 108
CP2, blue-green middle corrosion layer. Fig. 7 40 21 41 1.4 < 1.7 0.58 106
CP3, Red corrosion product (average of 2 similar analyses). Fig. 7 11 95 < < < < < 106
CP3, Orange corrosion product (average of 2 similar analyses). Fig. 7 24 54 30 0.6 < < 0.59 109
Blue-green corrosion product. Fig. 8 32 21 51 1.1 < < 1.0 106
Blue-green inner corrosion layer. Fig. 8 34 22 39 0.8 < 1.4 < 98

Table 2: Chemical composition (mass %) of the different corrosion products and layers from Figs. 7 and 8. Method of analysis: SEM/EDS, Laboratory of Analytical Chemistry, Empa.

Complementary information

Nothing to report.

Corrected stratigraphic representation: none.

The tin bronze sheet shows traces of cold working but has been exposed to an extended or excessively hot annealing process. According to Northover (Northover in preparation), the relative lack of twins and their large size confirm a prolonged annealing process. Furthermore large grains, large twins and extensive intergranular corrosion are characteristic of objects that have been exposed to a hot reducing flame either in a house fire or on a funeral pyre. All corrosion products except the cuprite are Sn enriched. The enrichment in P of the surface layer might be due to an environment rich in organic material (for example bones). Tenorite analysed by XRD and Raman spectroscopy is very rare in ancient Cu corrosion and must be interpreted as a further tracer for Cu corrosion in burning context. The original surface of the metal has been destroyed resulting in a type 2 corrosion layer after Robbiola et al. 1998. Only locally in the areas where tenorite is preserved does type 1 patina occur.

References on object and sample

Reference object

1. Publication in preparation (Biljana Schmid-Sikimic).

 

Reference sample

2. Degli Agosti, M., Santoro, I., Senn, M., Untersuchungen zur Brandpatina an Kupferlegierungen. In: Schmid-Sikimic, B. in preparation.
3. Northover, P., Untersuchungen an Fragmenten einiger Negauer-Helme. In Schmid-Sikimic, B. in preparation.
4. Rapport d'examen, Laboratoire Musées d'art et d'histoire, Genève 92-5-2 (1992).

References on analytic methods and interpretation

5. Robbiola, L., Blengino, J-M., Fiaud, C. (1998) Morphology and mechanisms of formation of natural patinas on archaeological Cu-Sn alloys, Corrosion Science, 40, 12, 2083-2111.