X-ray fluorescence-based methods to measure the thickness of protective organic coatings on ancient silver artefacts

Organic coatings are commonly applied to protect the surfaces of metal artwork when preventive control of microclimatic conditions is not fully effective. In particular, the protection of silver artefacts is a major issue in conservation, as uncoated surfaces of silver objects tend to turn dark after few months in museum environment. Among the parameters that are considered to compare different coating products and check homogeneity and protection performance over the time, the coating thickness is a most relevant one. Aim of this paper is evaluating the possibility of using X-ray fluorescence (XRF) methods for non-invasive assessment of the coating thickness. Measurements were carried out on both mock-up samples made by assembling several Mylar sheets on a silver substrate and silver plates treated with different organic polymers, according to common conservation practices. Three different quantification approaches, based on empirical and theoretical calibration curves, and on the Fundamental Parameters method were tested. The measured values of thickness were compared to those provided by the eddy current technique to evaluate the accuracy of different approaches. As expected, the empirical approach resulted the most accurate one, although its application is limited by the availability of suitable calibration standards. The results prove the suitability of XRF-based method as a non-invasive tool to measure or at least compare the thickness of organic coatings in different areas of a given artefact. The measurements performed on treated silver plates showed that the common practice of applying a protective product by brush actually produces coatings quite uneven in thickness, even on flat surfaces.