Why do silver articles become black over a period time? Is there a way to prevent this from happening?
Silver plates or any silver based materials become black/ brown over time simply because of the oxidation process, and also become a sulphide through reaction with atmospheric pollutants like sulphur compounds, in particular sulphur-dioxide from burning of vehicle fuels.
Through oxidation, this metal gets converted to silver oxide layer by layer [~ in micro-metre (10 mt) scale], the process akin to rusting of iron. Because of this discoloration, the metallic lustre and sheen are lost on storage.
Oxidation of silver or any metal is simply by losing an electron or two and forming a compound with oxygen which is ubiquitous in the ambient atmosphere.
There are several processes, which cause this oxidation viz., reaction with atmospheric gases, photo-chemical process, effect of temperature and so on.
Of these, the first two processes are mostly responsible for the oxidation leading to discoloration of metals and also we have only a little control on these processes.
In order to lose an electron to the surrounding media through oxidation, it only costs a few electron volts in energy, which is available in the sun-light in long UV to visible region (~ 350 to 700 nm in wavelength correspond to an energy of 3.5 - 1.5 eV) .
The process of discoloration of silver can be minimized if not totally avoided by proper shielding of this metal from atmosphere and light. It is usual to see silver ornaments, vessels covered with non-porous paper sheets like butter paper maintain their gloss longer than those kept open. R. JAGANNATHAN
Luminescence Group, CECRI
Karaikudi, Tamil Nadu