Why Does Silver Slowly Turn Black?

Silver does not usually darken because dirt settles on it. Its surface is gradually transformed into a different material.

Small amounts of sulfur containing gases exist in ordinary air. Hydrogen sulfide may come from pollution, decaying materials, food, rubber, wool, and other sources. When these compounds reach silver in the presence of moisture and oxygen, they react with its surface.

The result is mainly silver sulfide, a dark solid that forms a thin coating over the metal. At first, an extremely thin layer may appear yellow, blue, or purple because light reflects from both its upper and lower surfaces. As the coating thickens, it becomes brown and eventually black.

Silver objects tarnish at different speeds depending on humidity, air quality, handling, storage materials, and the other metals mixed into the silver. Eggs can accelerate tarnishing because they release sulfur compounds, especially when cooked.

Polishing removes the tarnished layer, but it also removes a minute amount of silver. Repeated aggressive polishing can gradually soften engraved details. Some cleaning methods instead use an electrochemical reaction involving aluminum to convert the tarnish while removing less metal.

The dark surface is not silver losing its shine. It is silver reacting with nearly invisible traces of sulfur in the surrounding air.