Fragments of oenochoe - Tin Bronze - Roman Times - Switzerland

Christian. Degrigny (HE-Arc CR, Neuchâtel, Neuchâtel, Switzerland) & S. Gillioz (HE-Arc CR, Neuchâtel, Neuchâtel, Switzerland) & Valentin. Boissonnas (HE-Arc CR, Neuchâtel, Neuchâtel, Switzerland)

The schematic representation below gives an overview of the corrosion layers encountered on the oenochoe base from visual macroscopic observation.

Fig. 5: Stratigraphic representation of the object in cross-section using the MiCorr application. This representation can be compared to Fig. 11.
Fig. 6: Stratigraphic representation of the object in cross-section using the MiCorr application. This presentation can be compared to Fig. 13.

Analyses performed:

Metallography (etched with ferric chloride reagent), SEM-EDS.

The remaining metal is a tin bronze (Table 1). The etched metal shows a structure of polygonal grains with twin and strain lines (Fig.6).

 

Elements Cu Sn
mass% 91 9

Table 1: Chemical composition of the metal. Method of analysis: SEM-EDX, Lab of Electronic Microscopy and Microanalysis, IMA (Néode), HEI Arc.

Intergranular corrosion is observed on the edges of the remaining metal (Fig. 7).The sample shows two forms of corrosion: multi-layered pustule corrosion at the left extremity of the sample (Fig. 8, area 1) and a corrosion crust covering the metal (Fig.8, area 2). The multi-layered pustule corrosion has an average thickness of about 1.1 mm (L) and 0.79 mm (W) (Fig.9). It is composed of a sandwich of 7 corrosion products, mainly green, grey, red and blue in dark field. Microscopic observation allows us to highlight new corrosion products that were not detected during the first visual examination (Fig. 9):

1. Light grey layer, containing mainly Sn, O, some Fe, P and a small amount of Pb (Fig. 10)

2. Dark green layer, containing mainly Cu, O and P (Fig. 10)

3. Red layer, containing mainly Cu and O (Fig. 10)

4. Blue layer, containing mainly Cu, Cl (Fig. 10)

5. Black layer containing mainly Sn, O (Fig. 10)

6. Brown layer containing mainly Cu, Sn, O (Fig. 10)

7. Dark grey layer containing mainly Cu, Sn, O (Fig. 10)

Superior markers such as contextual Si, Fe and P are present in several layers. Their penetration illustrates the cracking of the primary corrosion layer during the formation of the pustule. The P-enrichment in some corrosion layers may be due to an environment rich in organic material (for example bones). The multi-layered pustule corrosion type has developed similarly to the process presented by Formigli (1975, p.53) and Scott (2002, p.337).

The corrosion crust on the metal has an average thickness of about 70 μm (Fig. 11). It consists of two sub-layers. The inner corrosion layer is thin and brown in dark field or light grey in bright field. It has penetrated into the metal structure in some areas (Fig.7). In dark field, the outer corrosion layer is constituted of a heterogeneous light grey corrosion crust (Fig.11). The inner brown corrosion layer is enriched in Sn and O but also contains P, while the outer light grey corrosion layer is mainly composed of Pb and O but also contains P and Fe (Fig. 10). The outer light grey layer is probably due to the presence of a soft solder used to assemble the base to the body.

 

Elements

Cu Sn O P Fe Pb Cl
Grey layer nd +++ ++ ++ ++ + nd
Dark green layer ++ nd +++ +++ + + +
Red layer +++ nd ++ nd + nd nd
Blue layer +++ nd + nd nd nd +++
Black layer nd +++ +++ + + + nd
Brown layer ++ ++ ++ nd nd nd nd
Dark grey layer ++ ++ ++ nd nd nd nd
Dark brown layer nd +++ ++ + + + nd
Light grey layer nd nd ++ + + +++ nd

Table 2: Chemical composition of the multi-layered pustule corrosion from Figs. 9, 10 and 11. SEM-EDX, Lab of Electronic Microscopy and Microanalysis, IMA (Néode) (+++: high concentration, ++ medium concentration, + low concentration, nd: not-detected).

Based on the analyses carried out, the schematic representation of the stratigraphy of the multi-layered pustule form has been corrected.

The metal of the oenochoe’s base is a tin bronze. The polygonal and twinned grains with strain lines show that the base has been repeatedly cold worked and annealed with a final cold work. The metal is either well preserved or heavily corroded with the formation of pustules that go through the whole thickness of the metal. The limit of the original surface corresponds to the top surface of the dark brown layer. In the presence of a pustule it is highly deformed but discernible by the tin enriched surface. The corrosion is multiform. The well preserved and only lightly corroded areas are of Robbiola type 1 (Robbiola et al. 1998), the pustules however are of the Formigli type (Formigli 1975).

References on object and sample

Reference object

1. Gillioz S. (2012) Oenochoé GV132-01/US.26-obj.10, Genève, Place Simon-Goulart, rapport d’intervention. Haute Ecole ARC, Neuchâtel, 2013, non-publié.

 

Reference sample

2. Gillioz S. (2012) Oenochoé GV132-01/US.26-obj.10, Genève, Place Simon-Goulart, rapport d’intervention. Haute Ecole ARC, Neuchâtel, 2013, non-publié.

References on analytic methods and interpretation

3. Formigli, E. « Die Bildung von Schichtpocken auf antiken Bronzen ». Arbeitsblätter, Heft 1, 1975, p.51 à 74.
4. 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.
5. Scott, D. A. Copper and bronze in Art, corrosion, colorants, conservation. Getty publications, Los Angeles, 2002.