Tarnishing of Silver by Organic Sulfur Vapors: Rates and Film Characteristics

The rate of formation of tarnish films on silver coupons by vapors generated from organic sulfur compounds, especially dimethyldisulfide, has been measured at room temperature at both high and low relative humidity. The results for freshly cleaned silver coupons, as determined by cathodic reduction methods, indicate that atmospheres generated by organic sulfides rapidly tarnish silver. The rate increases with sulfur chain length and with the flux of u.v. light of wavelength greater than 2900Å. A mechanism involving the formation of free radicals is proposed. The tarnish composition is essentially identical to the sulfide films formed by hydrogen sulfide or by vapors from flowers of sulfur. Sulfide film morphology, as analyzed by SEM, tends to be more like field tarnished films than the highly structured tarnish films generated by flowers of sulfur vapor. Dimethyldisulfide has been used as a rapid, convenient life test gas for room temperature studies.