Lead cames of a stained glass window - Switzerland

Alice Gerber. (Haute École Arc Neuchâtel, Neuchâtel, Neuchâtel, Switzerland)

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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 state of conservation. In 2018, the lead showed strong efflorescence of white, powdery and voluminous corrosion.

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" stained glass panel, which interests us here, was restored in 2008, just before it was put on display. These restorations involved replacing part of the lead cames. So if the object is dated to the 16th century, the metal is a modern alloy. The replaced cames were therefore new when they entered the showcase. Then in 2018 they were completely corroded. In comparison, the other stained glass panel, with older lead cames, was placed at the same time in the same showcase. For this second panel, the cames are slightly corroded in 2018, but less strongly than those of the "Stokar" panel. The metal chosen by the restorer reacted more strongly with the corrosive environment (the showcase) than the historical metal.

Visual description of the corrosion: Voluminous forms of corrosion, white powdery efflorescence can be observed.

The schematic representation (Fig. 5) gives an overview of the corrosion layers encountered on the object from a first visual macroscopic observation

X-ray fluorescence (XRF) was used to differentiate the different elemental compositions of lead cames. Two things seem to emerge from these analyses: first, the tin (Sn) content is very variable from one came to another. Those with a very low Sn content are much more corroded. Second, the most corroded areas, notably the border-came of the panel, presents, in addition to a near-zero Sn rate, an antimony (Sb) value of about 1.5%.

Fig. 6 Stratigraphic representation(s) of the object in cross-section using the MiCorr application

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For this lead came, 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): to differentiate the elemental composition of the different lead cames. Each came is made of a different alloy, with varying levels of lead, tin or antimony. This is probably due to the repairs made to the object during its existence, and to the recycling of old cames from other stained-glass windows.

Fourier transform IR spectroscopy (FTIR): to identify the various corrosion products found on the object. These are mainly lead carbonates, sometimes lead acetates.

Scanning electron microscope/Energy-dispersive X-ray spectroscopy (SEM/EDX): These analyses made it possible to observe how corrosion develops, how in the lead-antimony alloy, corrosion preferentially attacks the lead and bypasses the antimony.

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.

Under SEM the corrosion seems to have several formes, with different densities under the SEM (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 for example, which are then transformed into basic lead carbonates by the action of CO2 from the environment. This transformation leads to the release of the organic acids, which continue the attack on the metal. These are therefore autocatalytic reactions. What we observe under SEM is perhaps simply these different stages of the attack of the metal by acetic acid.

The most corrosion occurred on cams made of a lead-antimony alloy. As corrosion progresses by avoiding antimony nodules in the alloy, it would appear that the antimony is protected from this corrosion. The electrochemical potential difference between lead (-0.125 V/SHE) and antimony (0.150 V/SHE) could explain this phenomenon. As the alloy is made of two very distinct phases, there could be a galvanic effect between the two phases, which implies an accelerated and stronger corrosion of the lead phase, which has a lower potential.

The limit of the original surface lies somewhere in the corrosion layers. Antimony nodules are inferior indicators.

Heterogeneous corrosion on lead cames. The tin content of the alloy indicates whether or not a came is attempting to 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). Galvanic corrosion can occur at a microscopic level, between two different phases in the alloy.

Costa and Urban (2005). Lead and its alloys: metallurgy, deterioration and conservation. In Studies in Conservation, 50:sup1, 2005, p. 48-62.

Tétreault et al. (2003). Corrosion of Copper and Lead by Formaldehyde, Formic and Acetic Acid Vapours. In Studies in Conservation, vol. 48, n°4, 2003, p. 237-250.

Degrigny and Le Gall (1999). Conservation of Ancient Lead Artifacts Corroded in Organic Acid Environments: Electrolytic Stabilization/Consolidation. In Studies in Conservation, vol. 44, n°3, 1999, p. 157-169.

Hasler (2010). Die Schaffhauser Glasmalerei : des 16. bis 18. Jahrhunderts. Corpus Vitrearum, Vitrocentre Romont, Peter Lang, 2010.

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 École Arc Neuchâtel, travail de diplôme de Bachelor, non-publié, 2018.