Cécile Vacquié, Jean-Michel Blengino, and Christian Degrigny
Abstract
Silver, unlike gold, tarnishes rapidly when exposed to air. In gilt silver objects, the silver is protected by a thin layer of gold. However, this layer is so thin that, on the one hand, it can be easily removed by wear and, on the other hand, it is poorly protective for silver, which can diffuse through it. Corrosion products of silver, mainly sulphur compounds but also chlorides and oxyhydroxides, can then form and denature the metal surface.
The present study demonstrates that it is possible to easily remove this tarnishing from gilt silver surfaces in a very simple way, easily feasible in restoration workshops. The proposed electrolytic treatment involves a two-stage process. The object is first made the cathode of an electrolytic cell and then the anode. The relative potentials are kept constant and are determined in advance by plotting current-potential graphs. During the first stage (cathodic polarization), the corrosion products of silver are reduced. This intermediate product is then eliminated during the second stage (anodic polarization).
This rapid, reliable and reproducible method, suitable for workpieces which can be immersed in aqueous solution, has the additional advantage of being adequate to the cleaning of very small surfaces of complex geometry. When the artifact cannot be immersed in the treatment solution (for example when some components like wood, etc… cannot be brought in contact with water), it is possible to carry out locally the electrolytic treatment with an adequate pencil under a drop of solution. In some instances it may prove also necessary to repeat the polarization cycle several times.
This technique has been successfully applied to fringes of flag and sacred goldsmith works (chalices, oilers, monstrances, crucifixes, a.s.o). It gives to the gilding its former appearance, revealing something that has always been present but was masked, without harming any parts where it had vanished.