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)

Complementary information

Nothing to report.

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. 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. 11, credit MiCorr_HE-Arc CR, C.Degrigny.
Fig. 6: 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 presentation can be compared to Fig. 13, credit MiCorr_HE-Arc CR, C.Degrigny.

Complementary information

Nothing to report.

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 twinned and strain lines (Fig.8).

 

Elements Cu Sn
mass% 91 9

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

Complementary information

Nothing to report.

Intergranular corrosion is observed on the edges of the remaining metal (Fig. 9).The sample shows two forms of corrosion: multi-layered pustule corrosion at the left extremity of the sample (Fig. 10, area 1) and a corrosion crust covering the metal (Fig.10, area 2). The multi-layered pustule corrosion has an average thickness of about 1.1 mm (L) and 0.79 mm (W) (Fig.11). 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. 11):

CP1. Light grey layer, containing mainly Sn, O, some Fe, P and a small amount of Pb combined with dark green layer, containing mainly Cu, O and P (Fig. 12, Fig. 5 and table 2)

CP2. Blue layer, containing mainly Cu, Cl (Fig. 12, Fig. 5 and table 2)

CP3. Red layer, containing mainly Cu and O combined with black layer containing mainly Sn and O (Fig. 12, Fig. 5 and table 2)

CP4. Brown layer containing mainly Cu, Sn and O (Fig. 12, Fig. 5 and table 2)

CP5. Dark grey layer containing mainly Cu, Sn and O (Fig. 12, Fig. 5 and table 2)

Superior markers such as contextual 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. 13). It consists of two sub-layers. The inner corrosion layer (CP2) is thin and dark brown in dark field or light grey in bright field. It has penetrated into the metal structure in some areas (Fig. 9). In dark field, the outer corrosion layer (CP1) is constituted of a heterogeneous light grey corrosion crust (Fig.13). 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. 12). 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 +++ +++ + + +
Blue layer +++ nd + nd nd nd +++
Red layer +++ nd ++ nd + nd nd
Black layer nd +++ +++ + + + nd
Brown layer ++ ++ ++ nd nd nd nd
Dark grey layer ++ ++ ++ nd nd nd nd
Light grey layer (crust) nd nd ++ + + +++ nd
Dark brown layer (crust) nd +++ ++ + + + nd

Table 2: Chemical composition of the multi-layered pustule corrosion from Figs. 10, 11 and 12 in dark field. SEM-EDS, Lab of Electronic Microscopy and Microanalysis, IMA (Néode) (+++: high concentration, ++ medium concentration, + low concentration, nd: not-detected).

Complementary information

Nothing to report.

Based on the analyses carried out, the stratigraphy of the multi-layered pustule has been corrected. The addition of  "e" and "i" within the coding refers to the location of the strata which are either internal ("i") or in contact with the atmosphere ("e").

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.