Patina is the natural surface oxidation that develops on metal and on certain organic surfaces over decades of exposure to air, moisture and trace pollutants. On ancient coins, an even green or brown patina is positive, since it indicates an undisturbed surface and helps confirm authenticity. On bronze artefacts and antique brass, a continuous patina protects the underlying metal. On copper coins, an uneven or streaky patina is a negative, since it usually signals environmental damage rather than even ageing.

The collector rule across categories is to never clean a patinated surface. Cleaning destroys the original surface in a way that can be detected under magnification long after, and almost always reduces value substantially — even when the cleaned surface looks superficially brighter and more attractive.

For coins, patina describes the surface oxidation that develops on copper, silver and bronze over years of exposure to air and humidity. A natural patina protects the underlying metal and is preserved by collectors; cleaning a coin to remove patina is widely considered destructive and reduces value substantially. For watches, patina describes the ageing of dials, lume plots and hands. For comics, the term refers to the natural ageing of paper that produces a warm tone preferred over harsh white. See our tropical dial entry for an extreme form of valued ageing.