The Mirrors of Villa della Regina in Turin: Study of Manufacturing and Deterioration Process

A study has been carried out in order to investigate the manufacturing and the deterioration processes of tin-mercury mirrors, coming from the Villa della Regina, in Turin, a prestigious Savoy residence built in the seventeenth century and currently under restoration. The mirrors are backed with a tin-mercury amalgam as a reflective coating which is a two-phase system (a tin-mercury compound surrounded by a liquid phase rich in mercury); this layer is not stable, because the crystals grow and the mercury slowly evaporates, moreover the corrosion phenomena lead to the formation of SnO and SnO2 and to the release of liquid mercury from the solid phase. Microchemical and microstructural analyses have been performed by means of scanning electron microscopy equipped with X-ray spectrometry (SEM+EDS) and by means of X-ray diffraction (XRD) on fragments of the tin-mercury mirrors removed from the looking-glasses for restoration purposes and on experimental mirrors produced for comparison purposes following the ancient manufacturing receipt. The experimental mirrors have been made by laying down a thin sheet of tin on a flat marble surface, by pouring successively mercury on the tin and finally by pressing a glass sheet on the top. The reaction starts immediately and lasts within one or two days. The experimental mirrors have been submitted to ageing tests in a climatic chamber, in order to simulate the deterioration process. The corrosion of tin-mercury mirrors is an extremely slow process, as a matter of fact, on all the old mirrors fragments analyzed a certain amount of liquid mercury is still present. The old mirrors show several dark areas, where the brightness is disappeared. In the corroded areas cassiterite and romarchite, two typical atmospheric-corrosion products of the tin-rich phases, have been identified by XRD analysis. There is no way to prevent the deterioration process of the tin-mercury mirrors; it must be avoided the contact with materials that may react with mercury; furthermore, the amalgam reflecting layer is very soft, thus any restoration is difficult to perform and may lead to a complete destruction of the artefact. A restoration method is proposed here based on an amalgam inlay produced in-situ on the corroded zone of the mirrors.