Lead cames of a stained glass window 52583
Alice Gerber. (Haute École Arc Neuchâtel, Neuchâtel, Neuchâtel, Switzerland)
Stained Glass panel from the 16th century representing prominent families of Schaffhausen (CH) through coat-of-arms. Object typically swiss called "Wappenscheiben".
The object is made of incolor and colored glass (light blue, pink and red) decored with grisaille, silver yellow stain and blue enamel. The glass parts are crimpted with lead-alloy cames. The cames are fastened to one another by welding points of a lead-tin alloy on both side of the panel (Fig. 1).
Supporting structure
Schaffhausen, Schaffhausen, Switzerland
Unknown
Unknown
16th century AD
Outdoor to indoor atmosphere
Museum zu Allerheiligen, Schaffhausen
Museum zu Allerheiligen, Schaffhausen
52583
Restauration in 2009 by Urs Wohlgemuth (Boniswil)
In 2009 two stained glass panels were placed in a showcase for a permanent exhibition, the lead cames were then in a good conservation condition. In 2018, the lead showed voluminous efflorescence of white, powdery corrosion products.
The environment of this showcase contained a high level of acetic acid. Lead being sensitive to organic acids, it corroded strongly. This is not new, but what is interesting here is that the two objects corroded in a very heterogeneous way. One lead came may be deformed and completely covered with bulky white efflorescences, and the one next to it shows no corrosion.
The "stokar" window, which is of interest here, was restored in 2008, just before it was put on display. The restoration consisted of replacing some of the lead cames. Thus, while the object is dated to the 16th century, the replaced cames were new when they entered the display case. In 2018, they were completely corroded. In comparison, the other stained glass panel, with older lead cames, was placed in the same display case at the same time. For this second panel, the cames are slightly corroded in 2018, but not as badly as those of the 'Stokar' panel. The metal chosen by the restorer reacted more strongly with the corrosive environment (the display case) than the historical metal.
Credit Museum zu Allerheiligen.
None.
The sample is a piece of metal taken from the corroded lead came of the stained glass (Fig. 2). This came was replaced during the 2009 restoration. The purpose was to understand why this new metal has been completely corroded in a few years (Fig. 2, 3 and 5), while the historical metal next to it is not affected at all.
Pb Sb alloy
Cast and cold worked
None
HE-Arc CR, Neuchâtel, Neuchâtel
HE-Arc CR, Neuchâtel, Neuchâtel
None
None.
Metallography: hand polishing (grit sizes 200, 500, 1000, 1200, 2500, with water), then machine polishing Struers® LaboForce-3 with diamond oil solution (grit sizes 3 µm and 1 µm). Finally, chemical and mechanical polishing (same machine), with Struers® OP-S solution (0.04 µm grit size) with 10% H2O2.
X-ray Fluorescence, in General Metals mode, acquisition time 60s (filters: M20/Lo20/Li20).
Fourier transform IR spectroscopy (FTIR) to identify the various corrosion products found on the object.
Scanning electron microscope/Energy-dispersive X-ray spectroscopy (SEM/EDX).
XRF analyses of the different lead cames of the "Stokar" stained glass panel showed that the tin (Sn) content is very variable from one came to another (Fig. 6). Those with a very low Sn but a high Sb (1.4%) contents are much more corroded.
The lead alloy contains approximatly 1.5% antimony. The grains of the metal are slightly visible, even without etching. The light-coloured dots in the alloy are Sb-rich inclusions (Fig. 7).
Polygonal grains with inclusions
Pb
Sb
None.
Corrosion develops specificilly on cames made of a lead-antimony alloy, where antimony inclusions constitute the cathode (0.150 V/SHE) versus lead (-0.125 V/SHE) which corrodes as an anode in presence of lead acetate (the electrolyte). Fig. 8 shows that corrosion does not attack the antimony nodules in the alloy, it progresses by mineralizing the lead around these nodules.
CP1 seems to be an heterogeneous compound as indicated on Figs. 8 and 9. It is heavily cracked (Fig. 9). EDX analyses do not indicate a significant difference in composition. The attack of lead by organic acids causes the formation of salts, such as lead acetate which are then transformed into basic lead carbonates by the action of CO2 from the environment as indicated by FTIR analysis of CP1.
Credit HE-Arc CR, A.Gerber.
Uniform - transgranular
Active corrosion of lead
None.
Fig. 11 shows an optimised stratigraphic representation of the corrosion structure. It is to be noted that CP1, CP2 and CP3 although visible on some cross-sections (Fig. 9) could not be differentiated by analysis.
The limit of the original surface lies somewhere in the corrosion layers. Antimony nodules are inferior markers of the limit of the original surface.
The lead came is a lead-antimony alloy. Lead is a metal that is very sensitive to organic acids. But it can be alloyed with elements that can either increase its resistance to corrosion (tin) or make it more sensitive to organic acids. Antimony (Sb) is such an element that enhances active corrosion due to the galvanic effect between this element and lead.
The lead came is heavily corroded and its original surface contained in the thick and very fragile corrosion layer might get lost easily in case of any physical shock.
References on object and sample
References object
1. Hasler (2010). Die Schaffhauser Glasmalerei : des 16. bis 18. Jahrhunderts. Corpus Vitrearum, Vitrocentre Romont, Peter Lang, 2010.
References sample
2. Gerber (2018). Corrosion du sertissage en plomb de vitraux - Recherches autour de la dégradation de deux objets dans leur vitrine au Museum zu Allerheiligen de Schaffhouse. Haute Ecole Arc Neuchâtel, travail de diplôme de Bachelor, non-publié, 2018.
References on analytic methods and interpretation
3. Costa and Urban (2005). Lead and its alloys: metallurgy, deterioration and conservation. In Studies in Conservation, 50:sup1, 2005, 48-62.
4. Tétreault et al. (2003). Corrosion of copper and lead by formaldehyde, formic and acetic acid vapours, 4, Studies in conservation, 48, 4, 2003, 237-250.
5. Degrigny and Le Gall (1999). Conservation of ancient lead artifacts corroded in organic acid environments: electrolytic stabilisation / consolidation, Studies in Conservation, 44, 3, 1999, 157-169.
