tarnished

Tarnished is an adjective describing a surface that has lost its luster or brightness due to a chemical reaction with the environment, often oxidation or corrosion, and it is also used metaphorically to indicate a damaged reputation or honor. It can also be used as a verb: tarnish.

In metals, tarnish arises when the surface reacts with oxygen, sulfur compounds, moisture, or other pollutants. The resulting layer of oxides, sulfides, or carbonates can dull appearance and alter surface properties without deeply corroding the metal. Some metals resist tarnish, while others form visible films readily.

Silver tarnish forms silver sulfide (Ag2S) when silver encounters sulfur-containing compounds in air or hydrogen sulfide, producing a dark layer on the surface. Copper and brass tarnish with copper oxide and copper carbonate-based patinas due to exposure to oxygen, carbon dioxide, and moisture. Aluminum rapidly forms a thin oxide layer (aluminum oxide) that can give a dull finish but also protects the metal; gold is relatively resistant to tarnish, making it less prone to visible discoloration.

Prevention includes storing metals in low-humidity environments, applying protective coatings or passivation, and avoiding contact with sulfur- or chloride-containing substances. Tarnished surfaces can sometimes be cleaned with gentle cleaners; aggressive polishing or improper chemicals can remove protective layers or damage underlying finishes.

In common usage, tarnished describes a diminished reputation or value, as in a tarnished reputation, and can also describe objects whose appearance has darkened or dulled due to wear or aging.