Cleaning of corroded silvered copper plates from the reliquary shrine of Abbot Nantelm (AD 1225) using EDDS in agar gel

The Abbey of Saint-Maurice d’Agaune in Switzerland houses an important treasure of goldsmith’s work, including the reliquary shrine of Abbot Nantelm. Dated to 1225 the shrine is made of larch wood covered with finely engraved copper plates, silvered and partially gilded. Portable XRF analyses and SEM-EDS on cross-sections revealed the use of mercury silvering, a technique rarely identified on large medieval objects. This discovery enhances the shrine’s significance, both as a religious and historical symbol and as an important technological artifact from its time. Covered with copper corrosion due to the porosity of the silvering, electrolytic reduction methods were excluded from the restoration process. Instead, the team employed chemical cleaning methods using a chelating agent applied in agar gel. This approach confines the treatment to the visible side and reduces the risk of penetration into the pores of the silvering. Multiple tests were conducted to determine the most effective agar gel, application method, concentration, and best chelating agent. Initial trials on artificially corroded samples compared the performance of several chelating agents, including tri-ammonium citrate (TAC), ethylenediamine-N,N’-disuccinic acid (EDDS), and two forms of ethylenediaminetetraacetic acid (EDTA) Na₂ and Na₄. The tests involved visual observations and electrochemical measurements (LSV) to assess treatment effectiveness. Ultimately, a 4 percent concentration of Agarart gel, applied warm at 70°C, was selected. EDDS (0.1 M, pH 10) was chosen as an alternative to EDTA Na₄ for its comparable effectiveness and lower environmental impact. Multiple applications were necessary, especially over the engraved areas, to ensure uniform removal of copper corrosion. This method achieved a satisfactory and even cleaning, restoring the brightness and legibility of the silvered surface. Following restoration, the shrine of abbot Nantelm has regained its intended appearance and luminosity, allowing it to reclaim its place within the abbey’s treasure as both a sacred object and a precious witness to medieval craftsmanship.