Shingle of a roof - Cu 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 Fig. 7.

Analyses performed:
Metallography (etched with ferric chloride reagent), Vickers hardness testing, LA-ICP-MS, SEM/EDX, XRD, Raman spectroscopy.

The remaining metal is a copper alloy (Table 1). The evenly distributed inclusions observed under SEM, SE-mode, are either light-grey or white (Fig. 4). The oval shape of the light-grey inclusions is due to deformation, probably hot rolling (common in the 18th century). Under polarised light they look red (Fig. 6) and their analysis reveals a composition similar to cuprite/Cu2O (Table 2). The white inclusions are rich in Pb and are remnants of the refining process (Table 2). The etched copper shows a structure of polygonal and twinned grains (Fig. 5). The grain size is variable. The average hardness of the metal is about HV1 70.

 

Elements Cu Pb As Sb Ag Bi Sn Zn Ni Fe Co
mass% 99 0.7 0.1 0.1 0.05 < < < < < <
RSD % 0.3 25 20 7 4            

Table 1: Chemical composition of the metal. Method of analysis: LA-ICP-MS, Laboratory of Basic Aspects of Analytical Chemistry at the Faculty of Chemistry, University of Warsaw, PL.

 

Elements

O Cu Pb As Sb Total
Light-grey inclusion 9.8 86 < < < 96
White inclusion 9 9.1 68 5.1 2.6 94

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

The corrosion crusts of the external and internal sides are distinctively different (Fig. 6). On the internal side an irregular red-orange corrosion layer has developed and pitting corrosion has occurred. The more uniform corrosion layers on the external side are composed of a red-orange layer, followed by a thicker green outer layer. In some areas, dark-red corrosion products can be observed between the green and red-orange sub-layers. The same dark-red sub-layer can be seen in areas on the internal side covering the red-orange corrosion products. The red-orange corrosion layer has a chemical composition similar to cuprite/Cu2O, while the green layer contains Cu, S and O and is enriched on its upper surface with Si (Table 3 and Fig. 7). XRD analysis of the corrosion products on the external side of another shingle fragment from the same roof identified brochantite/Cu4SO4(OH)6 and cuprite as corrosion products (Rapport d'analyse  no MAH 98-257). These results are confirmed by Raman spectroscopy of the external side of this sample where the same compounds were clearly identified (Figs. 8 and 9).

 

Elements

O Cu S Total
CP1e 20 59 6.2 85
CP2i 11 86 < 97

Table 3: Chemical composition (mass %) of the corrosion layers of the external side. Method of analysis: SEM/EDX, Laboratory of Analytical Chemistry, Empa.

Corrected stratigraphic representation: none

The copper shingle was rolled (probably hot rolling) and annealed to recover the ductility of the original material. The metal is covered on its external side by a typical “urban outdoor” patina consisting of copper sulphate (brochantite/Cu4(OH)6SO4) formed on top of a cuprite/Cu2O layer. The surface of the internal side, protected from the diluted sulphuric acid present in urban rain water, has developed only a cuprite layer. The silica present in the brochantite on the external side is due to airborne particle pollution. The corrosion is probably of type 1 after Robbiola et al. 1998.

References on object and sample

1. Rapport d'analysen° MAH 98-257. Laboratoire Musées d'art et d'histoire, Genève. The report describes a sample from another shingle.

References on analytic methods and interpretation

2. 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.
3. Selwyn, L. (2004) Metals and Corrosion: A Handbook for the Conservation Professional, Ottawa, ON: Canadian Conservation Institute, 68-70.
4. Stöckle, B., Mach, M. and Krätschmer, A. (1997) La durabilité des couvertures en cuivre selon les conditions environnementales. Résultat de l’UN/ECE-Programme d’exposition climatique, Les couvertures métalliques, matériaux et techniques, Les cahiers de la section française de l’ICOMOS, Paris, 129-135.
5. Welter, J-M. (2007) La couverture en cuivre en France: une promenade à travers les siècles, Le métal dans l’architecture, Monumental, 104-112.