An imaging approach for a contactless monitoring of the conservation state of metallic works of art

An easy-to-use and cheap diagnostic tool based on digital photography and 2D-FFT imaging processing is described that can be used to monitor the corrosion process occurring over time on the surface of metallic works of art in indoor and outdoor environments and to assess the stability of the materials employed for restoration. The proposed solution has the advantage of not requiring standard lighting and artifacts manipulation, that are not easily achievable in real applications. The imaging algorithm is capable of highlighting the changes in the surface uniformity due to the corrosion process and can be used to put in evidence the beginning of dangerous localized corrosion phenomena onto the metallic surface. Although this technique does not allow a quantitative measurement of the corrosion rate, it has proved to be an effective approach to assess the stability of protective coatings. The proposed processing has been tested in laboratory to asses the stability of SiO2-like protective coatings deposited by PECVD on a set of silver reference alloys submitted to a tarnishing test in the presence of H2S vapors.