Patina
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Patinas are chemical compounds formed on the surface of metals. There is a wide range of chemicals both household and commercial that can give a variety of results. They are often used by artists as surface embellishments either for color, texture, or both. One example of a patina is a green surface texture created by slow chemical alteration of copper, so as to produce a basic carbonate. It can form on pure copper objects as well as alloys which contain copper, such as bronze. Many patinas form naturally, by weathering.
Patinas are restricted to exposed surfaces and can flake off. One reason that bronze is so highly valued in statuary is that its patina protects it against further corrosion. This natural patina seldom shows a tendency to flake. Brass would also be resistant to corrosion, but it would in the long run not be as attractive, as local pitting would show against a shiny background.
Figuratively, "patina" can refer to any fading, darkening or other signs of age, which are felt to be natural and/or unavoidable.