Application of coatings on silver studied with punctual and imaging techniques: from specimens to real cases

The protection of silver artifacts with a coating of organic material isolating the surface from oxygen and S-bearing gases is still one of the most used techniques for the preservation of silver surfaces. A variety of materials suitable to this purpose are being used in the conservation practice. Among them there are acrylic polymers, nitrocellulose lacquer, microcrystalline wax and combination of them in superimposing layers. They are applied either by brush or, in some cases, by spraying. The first stage in a conservation project is often the cleaning of residuals of old protectives, aimed at making the silver surface free from remnants of yellowish materials and enabling the application of new coatings. Unfortunately, removing aged coatings and applying new ones are "blind" processes, since the techniques available for not invasively monitoring the surface of silver artifacts are punctual and do not allow an overall overview of the surface. This is the case of reflectance FTIR and thickness measurement with the Eddy Current technique, which, apart from being punctual, are tricky to apply to uneven surfaces. In this paper, we aim to show the benefits of using an imaging technique, UV induced VIS luminescence, for monitoring the presence and the distribution of protective coatings on silver surfaces and of combining it with reflectance FTIR and thickness gauge. At first, our study was focused on specimens treated with nitrocellulose, acrylic emulsion and microcrystalline wax. The application of the treatments was studied with UV-induced VIS luminescence as well as with the abovementioned punctual techniques to cross-validate the outcome of each method and to assess their suitability for application on organic protective layers on silver artifacts. In a further stage, UV induced VIS luminescence was applied to a real case, a silver chalice that was treated with nitrocellulose lacquer after cleaning.