Patina

Patina (/ˈpætᵻnə/ or /pəˈtiːnə/) is a thin layer that variously forms on the surface of stone; on copper, bronze and similar metals (tarnish produced by oxidation or other chemical processes); on wooden furniture (sheen produced by age, wear, and polishing); or any such acquired change of a surface through age and exposure. Patinas can provide a protective covering to materials that would otherwise be damaged by corrosion or weathering. They may also be aesthetically appealing.

On metal, patina is a coating of various chemical compounds such as oxides, carbonates, sulfides, or sulfates formed on the surface during exposure to atmospheric elements (oxygen, rain, acid rain, carbon dioxide, sulfur-bearing compounds), a common example of which is rust which forms on iron or steel when exposed to oxygen. Patina also refers to accumulated changes in surface texture and colour that result from normal use of an object such as a coin or a piece of furniture over time.

Archaeologists also use the term "patina" to refer to a corticated layer that develops over time that is due to a range of complex factors on flint tools and ancient stone monuments. This has led stone tool analysts in recent times to generally prefer the term "cortification" as a better term to describe the process than "patination".

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