Modelling an XRF method for determining gilding thickness in silver cultural heritage objects without sample collection: case studies on 16th century Portuguese silverware

In situ X-ray fluorescence is a non-invasive technique that is widely used in historical objects, namely in pieces of gilded silver, to determine the composition of the alloy and gilding. In the case of gilded silver, fire gilding was a mercury-based historical technique that is no longer practiced. Moreover, with traditional XRF analysis is possible to determine the thickness of the gilding by using given intensity ratios of the characteristic lines of silver. However, this requires the analysis of the substrate, for the calculation of the intensity ratio without gilding, which is not always accessible. This study presents and validates a methodology for the calculation of the thickness of fire gilding silver pieces using XRF analysis with a commercial spectrometer and without the need to analyse the isolated substrate. Six silver alloy mock-up samples were produced following historical techniques and generic intensity ratio for Kα and Kβ lines of silver in the alloy was calculated (6.35 ± 0.05), to be used in any alloy with a silver composition over 75 %. Since attenuation of the silver's characteristic lines depends on the gilding composition, different Hg concentrations (5 %–20 %) were tested. The results obtained with this approach for the mock-ups was compared with SEM-EDS measurements for gauging uncertainty and the methodology was then applied to three pieces of 16th century Portuguese silverware. This adaptation of previously established principles, proved to be effective to calculate the thickness of fire gilding on silver and was validated to be applied, in situ, in real museum artworks without the need for sample collection.