The form of corrosion occurring on a historic metal should first be thoroughly analysed prior to any conservation treatment, as this information reveals much about the composition of the base material but also allows the treatment options to be more optimally defined.

The electrolytic pencil Pleco designed by HE-Arc is a valuable tool for diagnosing corrosion products but also allows their transformation by reduction to stabilize or clean metal surfaces. In Vienna, a few years ago, a somewhat small electrochemical pen was built along the lines of Pleco. Like Pleco, this slightly smaller and simpler pen should fulfil both the diagnostic and treatment functions of metal surfaces.

The aim of this STSM was training in the technical construction, functioning and application of Pleco electrolytic pen on the one hand, and on the other hand, the comparison of Pleco with the self-built electrolytic pen from Vienna. In addition to the different auxiliary electrodes in the two pencils (platinum wire in Pleco and gold wire in the Vienna pencil), cheaper graphite leads from the Faber-Castell company of hardness grade 3B were installed in the Vienna pencil instead of the GC electrode used in Pleco. The electrochemical performance of both pencils was compared by OCP and LSV measurements in 1%(w/v) KNO3. The result was stable and comparable for both pens and electrodes. Since the graphite leads from Faber-Castell are considerably cheaper, a further test and the possible use of low-cost graphite electrodes will make it possible to offer the Pleco at an even lower price to the user in the future.

As part of the STSM, Pleco was first tested on artificially patinated pure silver and sterling silver coupons and then on historical objects made of various silver alloys. It was found that the size and severity of the reduction peaks depended on the thickness of the tarnish layer. Reduction reactions on tarnished Ag999 showed reduction of Ag2S only, and on tarnished sterling silver two reduction peaks were measured of both Ag2S and Cu2S. AgCl was also reduced on utilitarian objects, indicating contact of the objects with bare hands.

This STSM also strengthened the cooperation between Haute École Arc Conservation-Restauration in Neuchâtel, Switzerland, the University of Vienna, where the Vienna pencil was constructed, and the TU Wien, where the Pleco and Vienna pencil will be further used and developed as part of the Heritage Science project PHELETYPIA of the Austrian Academy of Sciences. In September 2023 a training school about electrochemical / electrolytic techniques in metal conservation together with Christian Degrigny will take place in Vienna. Through this training, great number of conservators in Austria will learn about Pleco and its fields of application.

Full report

Illustration of STSM tasks