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 cams. The cams are fastened to one another by welding points of a lead-tin alloy on both side of the panel.
Schaffhausen, Schaffhausen, Switzerland
16th century AD
Outdoor to indoor atmosphere
Museum zu Allerheiligen, Schaffhausen
Museum zu Allerheiligen, Schaffhausen
Inv. Nr. 52583
Restauration in 2009 by Urs Wohlgemuth (Boniswil)
The study of this object is based on a simple problematic: 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 cam 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 cams. Thus, while the object is dated to the 16th century, the replaced cams were new when they entered the display case. In 2018, they were completely corroded. In comparison, the other stained glass panel, with older lead cams, was placed in the same display case at the same time. For this second panel, the cams 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.
Visual description of the corrosion: Voluminous forms of corrosion, white powdery efflorescence can be observed.
The schematic representation (Fig. 6) gives an overview of the corrosion layers encountered on the object from a first visual macroscopic observation.
The sample is a piece of metal taken from the corroded lead cam of the stained glass. This cam was replaced during the 2009 restoration. The purpose was to understand why this new metal has been completely corroded in a few years, while the historical metal next to it is not affected at all.
Haute École Arc Neuchâtel, Neuchâtel
For this lead cam, the alloy used is a lead alloy with about 1.5% antimony. Under a microscope, the alloy is not homogeneous. The antimony has formed nodules in the alloy, which are visible both under an optical microscope and under SEM.
The cross-section shows that corrosion does not attack the antimony nodules in the alloy, it progresses by mineralizing the lead around these nodules.
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).
Lead alloy with approximatly 1.5% antimony. Heterogeneous alloy with a lead-rich main phase and antimony-rich nodules. Grains visible under differential interference contrast.
Polygonal grains with inclusions
CP1 seems to be an heterogeneous compound as indicated on Fig. 10. But 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.
Corrosion develops specificilly on cams 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).
The limit of the original surface lies somewhere in the corrosion layers. Antimony nodules are inferior markers of the limit of the original surface.
Uniform - transgranular
Active corrosion of lead
Heterogeneous active corrosion on lead cams. The tin content of the alloy indicates whether or not a cam will corrode ; with a low tin content lead is likely to corrode more. In a lead-antimony alloy, corrosion preferentially attacks the lead.
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.
References on object and sample
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.