From Green to Brown: Characterization of the Fast Alteration of Modern Greenish Enamels in Glass Windows

Spots and stains appeared on the modern green enamel from different stained-glass windows as a consequence of environmental alteration. The multianalytical characterization showed a 3-step alteration mechanism divided into Pb²⁺ lixiviation from the enamel, formation of hydrocerussite (2PbCO₃·Pb(OH)₂), and transformation into scrutinyite (α-PbO₂) and plattnerite (β-PbO₂).

In 2009–2014, new glass panels were produced by the Stained-Glass Studio of Canterbury Cathedral (UK) and installed in St Peter's Church (Little Barrington, Burford), St Lawrence's Church (Mereworth, Kent), and Canterbury Cathedral. After a few years, some spots and stains have appeared on the greenish areas of the panels. The common factor is that the panels were produced using the same green enamel. The present work reports on the studies of the observed alterations in green enamels to propose possible degradation mechanisms. These studies are based on using different analytical techniques such as optical microscopy, scanning electron microscopy energy dispersive X-ray spectroscopy, inductively coupled plasma mass spectrometry, laser-induced breakdown spectroscopy, X-ray diffraction, µ-Raman spectroscopy, profilometry, X-ray photoelectron spectroscopy, and UV–vis spectroscopy. The characterization of the fragments from the original panels showed that a brown layer of dark lead compounds was formed on the green enamel. The alteration occurred in a 3-step alteration mechanism divided into Pb²⁺ lixiviation from the enamel, formation of hydrocerussite (2PbCO₃·Pb(OH)₂), and transformation into scrutinyite (α-PbO₂) and plattnerite (β-PbO₂).

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