New technique to preserve brightness of silver discovered

The technique keeps silver surfaces shiny for longer than traditional methods and ensures that historically important artifacts are preserved for future generations.

A team of researchers led by Ray Phaneuf from the University of Maryland, is investigating the less labour-intensive ways to protect silver artifacts from tarnishing.

They are studying a technique that could overcome some of the shortcomings of current preservation methods.

Called atomic layer deposition (ALD), the process gives scientists atomic-level control over the thickness of a transparent oxide film that they grow on the surface of silver objects.

By running a series of surface-limited chemical reactions, researchers can build the protective film one atom-thick layer at a time. The films are under 100 nanometres thick, less than 1/1000th the thickness of a human hair.

Researchers are currently experimenting by applying ALD films to highly uniform silver test wafers. The uniformity of the wafers allows them to control variables, such as the composition of the silver, in order to create a model of the tarnishing kinetics as sulphur diffuses through the ALD film.

"This is when we get to put on our physicists' hats," Phaneuf said.

Results showed two components to the concentration profile, indicating a faster rate of sulphur diffusion through tiny pinholes in the protective oxide film.

The researchers are now experimenting with multilayer films that plug these pinholes.

ALD films may have another advantage over conventional nitrocellulose lacquer, which can become yellow with age.

"Untreated silver beautifully reflects white light," Phaneuf explains.

Silver tarnishes when hydrogen sulfide in the air reacts with it, forming an unsightly black layer of silver sulfide on the surface of the artifact.