Tarnishing of silver is a well-known problem in the silver industry, especially concerning sterling silver. A typical sterling silver alloy consists of 92.5% (in weight) silver (Ag) and 7.5% copper (Cu). This specific alloy has been known since the fourteenth century. Adding Cu to the alloy gives it strength without losing ductility, but it also becomes more susceptible to tarnishing. Fine silver, with at least 99.5% Ag, has a lower tarnishing rate but is still easily tarnished under severe conditions. Tarnishing is predominantly caused by sulfur, which reacts with the surface of silver and creates silver sulfide (Ag2S). Tarnishing can be seen as a cosmetic discoloration (i.e., black areas on silver surface). Tarnishing is a major problem for jewelry, collector coins and mirrors, since the visual appearance and optical properties of silver are important for these products. Furthermore, silver mirrors are widely used in solar mirrors, optical devices and lighting. This paper presents an efficient thin film method to prevent tarnishing and white spot formation on silver. The method involves creating nanometer-scale thin films on silver by atomic layer deposition (ALD). The anti-tarnishing properties of ALD thin films have been tested extensively and compared with other anti-tarnishing methods.

Author(s): Nora Isomäki, Sami Sneck