The centuries-old mystery of why silver tarnishes has been solved by a team of scientists at TCD.

It has been known since ancient times that if a piece of silver is left outside it will turn black and tarnish. And, since the beginning of the 20^th century, it has been known that sulphur is the chemical in air that causes the tarnishing.

However, until this new research it was a mystery exactly how – at the level of atoms – sulphur interacted with silver to create a black layer of tarnish.

“This was a question first posed in 1930, but nobody answered until now,” said Stefano Sanvito, Professor of Condensed Matter Theory at TCD who conducted with research with Dr Gabriele Salah in collaboration with Nokia Bell Labs.

“It’s cool to answer a question that was posed 100 years before,” added Prof Sanvito.

The mystery was solved during a TCD and Nokia Bell Labs research project aimed at finding out how silver corrodes as part of a wider research effort to figure out how metals commonly used in electronic equipment corrode.

The corrosion of metals in electronic equipment has become a big issue, said Prof Sanvito because people are increasingly using their equipment outdoors, and it’s even more of a problem near coastlines with corrosive salt air.

The researchers decided to investigate how silver corrodes, as a example of how metals in general undergo corrosion.

Atoms

The researchers knew that sulphur was the chemical that corroded silver, but they needed a way to examine precisely how sulphur and silver atoms behaved when they collided to create a tarnish layer in real-time.

They did this by building atom-simulation software that had all the information required to display, in video format, how sulphur and silver atoms interact.

Prof Sanvito described what the simulation software revealed.

“When a sulphur molecule arrives on a silver surface the molecule breaks right away and the individual sulphur atoms attach to the surface.”

“The other thing is that when sulphur atoms arrive on a silver surface, silver-sulphur compounds are formed.”

The simulation also showed that as sulphur collides with surface of silver, silver atoms begin to move the inside of the silver surface up to the surface.

As sulphur atoms are hitting the silver surface, said Prof Sanvito, free silver atoms are available there to bind with sulphur and build a layer of tarnish.

The research also revealed that higher quality silver is less prone to corrosion.

This chemical penetration of silver by sulphur, which causes tarnishing, happens at a faster rate when silver is defective, explained Prof Sanvito.

“High quality silver tends to corrode less than silver which might have small cracks or things like that.”

It would be interesting to take the findings and develop an anti-silver corrosion material, said Prof Sanvito, but there are no current plans to do so.