Tarnishing is the most common alteration process observed on silver museum objects. Prevention strategies are currently based on the application of polymer coatings by brushing or spraying them in single or multiple layers. The result of such treatments is usually an uneven distribution of the polymer over the metal surface. On the other hand, the key role of coating thickness in the protection of silver from tarnishing is well known and is described by the diffusion law of H2S through the film barrier. Therefore, in order to compare the performance of different protective coatings, the film thickness distribution should be carefully evaluated. In this study, sterling silver coupons were treated with the two most common polymers for silver protection, Zapon and Paraloid B72. The coatings were applied by drop casting polymer solutions with various concentrations to obtain films with different average thickness and uneven distribution. The coupons were exposed to about 5 ppm H2S for several days. Polymer distribution on the surface and tarnishing were mapped by using micro-X-Rays Fluorescence (XRF) and digital photographs under visible (VIS) and ultraviolet (UV) illumination, respectively. In the central area of each coupon, where the maps showed a more uniform film, coating thickness and colour were measured by means of an Eddy Current device and a colorimeter. In this way, it was possible to compare a punctual and a mapping approach and to achieve more detailed data about colour change-thickness relationship. The results pointed out a strong correlation between tarnishing rate and coating’s thickness. Achieving a quantitative information about the protective film allows to compare the effectiveness of different silver protective products at equal thickness. Imaging approach, both through scanning XRF and UV fluorescence analysis, will provide a more comprehensive understanding of the film behaviour, comparing materials performance at any given thickness.
Mapping of coating thickness: a useful tool to set up anti-tarnishing strategies for silver / Brancolini, Teresa; Mugnaini, Sonia; Caruso, Ottaviano; Galeotti, Monica; Porcinai, Simone. - (2025), pp. 135-142. (Intervento presentato al convegno METAL 2025 tenutosi a Cardiff University).
Mapping of coating thickness: a useful tool to set up anti-tarnishing strategies for silver
Teresa Brancolini
;
2025
Abstract
Tarnishing is the most common alteration process observed on silver museum objects. Prevention strategies are currently based on the application of polymer coatings by brushing or spraying them in single or multiple layers. The result of such treatments is usually an uneven distribution of the polymer over the metal surface. On the other hand, the key role of coating thickness in the protection of silver from tarnishing is well known and is described by the diffusion law of H2S through the film barrier. Therefore, in order to compare the performance of different protective coatings, the film thickness distribution should be carefully evaluated. In this study, sterling silver coupons were treated with the two most common polymers for silver protection, Zapon and Paraloid B72. The coatings were applied by drop casting polymer solutions with various concentrations to obtain films with different average thickness and uneven distribution. The coupons were exposed to about 5 ppm H2S for several days. Polymer distribution on the surface and tarnishing were mapped by using micro-X-Rays Fluorescence (XRF) and digital photographs under visible (VIS) and ultraviolet (UV) illumination, respectively. In the central area of each coupon, where the maps showed a more uniform film, coating thickness and colour were measured by means of an Eddy Current device and a colorimeter. In this way, it was possible to compare a punctual and a mapping approach and to achieve more detailed data about colour change-thickness relationship. The results pointed out a strong correlation between tarnishing rate and coating’s thickness. Achieving a quantitative information about the protective film allows to compare the effectiveness of different silver protective products at equal thickness. Imaging approach, both through scanning XRF and UV fluorescence analysis, will provide a more comprehensive understanding of the film behaviour, comparing materials performance at any given thickness.I documenti in IRIS sono protetti da copyright e tutti i diritti sono riservati, salvo diversa indicazione.
