AIC's 46th Annual Meeting

The Conservation of Silver and Copper Alloy Objects project is part of a multiphase institutional initiative to treat objects in Winterthur Museum’s diverse collections prioritized by greatest conservation need. Winterthur Museum, Garden & Library received IMLS funding to support a two-year project to remove aged or failed lacquer coatings, polish, and coat approximately 500 of its most vulnerable silver objects, to continue analytical research related to silver surface corrosion, and to commence new research on corrosion and coating issues. Most of the time tarnish is easily removed from a silver surface with a mild abrasive; however, polishing the silver in this way permanently removes the most superficial layer of the precious metal. This is commonly considered acceptable if the silver in question were to be lacquered, which would protect the object for up to 30 years by making aggressive polishing campaigns unnecessary. However, unlike silver, silver-plated objects have an extremely thin display surface of silver and a more substantial substrate of a base metal. Objects that have been electroplated are particularly prone to mistreatment as the precious metal display surface is merely atoms thick and can be removed during a single polishing campaign. For delicate electroplated surfaces, technicians at Winterthur have been using a newly developed acidified thiourea gel cleaning system. However, uncertainty regarding the efficacy of thiourea on a copper substrate has encouraged the IMLS team to research other methods for cleaning electroplated silver. Initial treatment results will be presented here. The silver-plating method employed on an object will likely determine the treatment of the surface. As a result, the Scientific Research and Analysis Laboratory at Winterthur Museum has been conducting parallel research into ways of differentiating plate methods. ED-XRF (energy-dispersive x-ray fluorescence) spectroscopy was performed to determine object elemental composition. Alpha and beta x-ray emission lines are absorbed differently by surrounding elements after emission: the deeper the element or the thicker the layer on top, the more the ratios are affected. In order to semi-quantify the thickness of the plate, Cu-K ratios were used to measure the absorption of these emission lines by the silver plate. For example, during cleaning tests performed on an Argand lampshade, the thinness of the silver film and the fact that it was easily removed by abrasive cleaning all indicated the presence of an electroplated film. By analyzing the XRF data in the aforementioned way, the shade showed a silver plate thickness of much less than 1 µm, which was clearly different in plating technique from the rim, the shade support and the Argand lamp body, which was already confirmed as Sheffield Plate. Electroplate is generally found to be much less than 5 µm, although can be built up to more. It was therefore found that XRF can be used to quickly and non-destructively gauge possible plating techniques. As more plated objects are analyzed as part of this project, a database will be accumulated in order to inform thickness variations across plating methods.