Nicola Ricotta - Optimisation of the use of Pleco to locally and safely clean the tarnishing developing on sterling silver heritage artefacts
Host: Empa’s Joining Technologies and Corrosion Laboratory, Dübendorf, Switzerland
May 8 to 12, 2023
The results of this STSM concern the important development of scientific research on Pleco and its use in the field of cultural heritage. With the protocols studied and optimised Pleco currently represents one of the most valuable tools for the analysis and cleaning of silver-based artefacts. In particular, the following achievements have been reached:
- Protocol of use of Pleco electrolytic pencil for analytical purposes. Starting with the measurement of the GC reference electrode potential by OCP, to the setting of parameters to minimise current fluctuation, and following with the characterisation of tarnishing by LSV and up to the determination of the time of tarnish reduction by chronoamperometry. During chronoamperometry measurements, it is possible to evaluate the cleaning level that is achieved at different values of potentials characteristic of the silver tarnish present;
- Protocol for pre-removal of copper-based corrosion products using 5% EDTA at pH 10. For the same tarnish level and Na4-EDTA solution, copper-based corrosion products on a coupon immersed are removed faster than in gel application (15h for immersion and around double time with gel);
- Optimised protocols for producing reproducible coupons in terms of artificial tarnishing levels. By means of 80% pure albumen powder, it is possible to tarnish silver more slowly than boiled eggs and faster than liquid albumen. This solution is a middle ground to control the tarnishing processes and make them more reproducible than with boiled eggs but at the same time faster than with liquid albumen. In addition, with albumen, there is a slow tarnishing process, and the typical reduction peaks present on real objects (Cu2O, Ag2S and Cu2S) appear. With boiled eggs, Cu2O does not appear;
- Optimised protocols for pad production. By means of a hole puncher, it is possible to cut the pad while it is still wet to define its final diameter and height. The pad is then left to dry and shaped with abrasive paper according to the desired shape.
This STSM was an opportunity to strengthen the collaboration between several Italian and Swiss institutes: the University of Florence and the Opificio delle Pietre Dure in Florence, the Haute Ecole Arc Conservation-restauration in Neuchâtel, the Conservation lab of the Abbey of St-Maurice and the Empa’s Joining Technologies and Corrosion Laboratory in Dübendorf. These institutes are already collaborating within the ASTEC project which aims to develop protocols to safely electrolytically clean silver tarnishing and/or the Endless Metal Innovators Grant project which aims to disseminate the use of low-cost, portable and easily accessible analytical tools developed at HE-Arc CR such as Pleco.
These results form the basis for the development of the investigation and restoration phases of silver-based alloy artefacts. The first objects on which the protocols studied so far will be applied will be a Valadier cooler and a bas-relief with the Last Supper from the Tesoro dei Granduchi, Gallerie degli Uffizi, Florence.
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