Thickness mapping of organic layers applied on sterling silver by means of X-ray fluorescence scanning

The application of organic protective coatings is the most commonly used strategy to avoid tarnishing of silver artefacts. Non-invasive mapping of coating thickness is a key point to evaluate the consistency and completeness of coating application and to monitor protective effectiveness over time. This work is aimed at evaluating the feasibility of acquiring thickness maps of coated silver surfaces by means of X-ray fluorescence scanning. Experimental calibration curves were built by using organic films of known thickness made of several commonly used protective coatings, (Zapon nitrocellulose lacquer, and acrylic resins Paraloid B72 and Paraloid B44). X-ray fluorescence scans were performed on coated silver sterling coupons treated by brushing or spraying polymer solutions prepared with different solvents and concentrations. Thickness maps pointed out different average thickness values among selected polymers and highlighted that almost all the coatings were unevenly spread on the surface. Application by brush provided particularly uneven coatings and brush strokes were clearly distinguishable. The accuracy of thickness measurements was calculated by means of a semi-empirical approach, and results suggest that thickness can be predicted with an uncertainty of about +/- 5%.