Cooking pot handle fragment - Leaded Bronze - Roman Times - Switzerland

Christian. Degrigny (HE-Arc CR, Neuchâtel, Neuchâtel, Switzerland) & Lucile. Ruynat (HE-Arc CR, None) & Valentin. Boissonnas (HE-Arc CR, Neuchâtel, Neuchâtel, Switzerland)

Complementary information

The soil of Windish is on the border between calcareous soil (Jura) and molasses-type (Swiss plateau). Calcareous soils tend to be alkaline whereas the molassic soil is generally more neutral. The Swiss climate is temperate, with the four marked seasons. It can have strong frosts in winter.    

The stratigraphy below gives an overview of the corrosion layers encountered on the object from visual macroscopic observation. The stratigraphy was created before treatment by observation under a binocular microscope and modified during the mechanical removal of corrosion products.

Fig. 4: Stratigraphic representation of the object in cross-section using the MiCorr application (to be compared to Fig. 3). The stratum is missing because it is integrated in CP2 and it was not possible to represent it in MiCorr while POM1 and POM2 are merged as POM1, credit MiCorr_HE-Arc CR, L.Ruynat.

XRF on the object with portable X-ray fluorescence spectrometer (NITON XL3t 950 Air GOLDD+ analyser, Thermo Fischer®).

XRD of powder samples using Stoe Mark II-Imaging Plate Diffractometer System (Stoe & Cie, 2015) equipped with a graphite-monochromator. Mo-K𝛼 radiation (𝛾 = 0.71073Å, beam diameter 0.5 mm, exposure time: 10 min).

SEM/EDS on the object.

Raman on powder samples of corrosion products.

SEM on sample of organic remains, as well as X-ray picture not presented in this report.  

 

The mechanical removal of corrosion products stopped at the limitos, so the metal was not directly observed. XRF analysis carried out after the cleaning process (Fig. 2, blue dot 3) showed that the metal is a leaded bronze (Table 1). Theoretically the metal is as-cast and should present a dendritic structure.

Elements mass %

Cu

Sn

Pb

Sb

Si

P

Ti

Fe

 M1

61.1

21.1

14.2

0.9

0.9

0.4

0.2

0.1

 

 

 

 

Table 1: Chemical composition of the metal. Method of analysis: XRF, mode General metals, 60s (filters M20/Lo20/Li20). Located at point 3 Figure 2, credit MiCorr_HE-Arc CR, C.Degrigny.

Above the well-preserved metal core, CP4 is discontinuously interlocked in CP5. The XRD analysis done on a sample (Fig. 2, yellow square 2) of both corrosion products, shows that they are mainly constituted of cuprite (Cu2O), (Fig. 5). The colour difference can be explained by a tin enrichment in the darker CP4, as measured by XRF. XRF analysis of CP4 (Fig. 2, blue dot 5) and CP5 (Fig. 2, blue dot 4) is given in Table 2. The EDS analysis of the light green CP3 (yellow square 1 Fig. 2) layer did not reveal the presence of chlorides (Fig. 6). Complementary XRD analysis validated the absence of nantokite. Indeed the XRD and Raman indexing was not successful probably because the compound has a large amorphous part. The other layers were not analysed, we expect CP1 and CP2 are malachite (carbonate hydroxyde) because of the green colour and absence of chlorides. Organic remains were mineralised and preserved by the corrosion process.    

Elements mass %

Cu

Sn

Pb

Sb

As

Ag

Fe

P

Cl

S

BAL

Dark-red layer (CP4)

24.9

18.2

11.2

0.7

0.1

0.09

0.1

0.3

0.4

7.2

36.1

Red area layer (CP5)

33.5

14.5

16

0.5

0.8

0.1

0.05

1.3

0.6

1.5

30.5

 

 

 

 

 

 

 

Table 2: Chemical compositions of the dark-red and the red layers. Method of analysis: XRF, mode mining Cu/Zn, 180s (filters M30/Lo30/H60/Li60). BAL corresponds to the elements not analysed: O and C, credit MiCorr_HE-Arc CR, C.Degrigny.

 

The object is a leaded bronze with a well-preserved metal core. Covering the metal are two strata (CP4 and CP5) composed of cuprite, the darker (CP4) appears enriched in tin. The superior interface of these layers represents the limitos. In areas the limitos, CP4 and CP5 have been replaced by a light green porous corrosion product (CP3). The following layers CP2 and CP1 are probably malachite.

The powdery green corrosion layer CP3 has frequently been observed on bronzes of Vindonissa where it can be located within cuprite or malachite layers. It is typically developed below the limitos and renders the latter extremely fragile. The nature of this corrosion product has not yet been determined. However, the absence of chlorine indicates that it is not a chlorinated corrosion product such as atacamite or paratacamite. The aggressive urban soil could be a reason for the transformation of these naturally grown and stable corrosion layers.

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