Patina is a tarnish
Tarnish is a thin layer of corrosion that forms over copper, brass, silver, aluminum, and other similar metals as their outermost layer undergoes a chemical reaction. Tarnish does not always result from the sole effects of oxygen in the air. For example, silver needs hydrogen sulfide to tarnish; it...

 that forms on the surface of bronze
Bronze is a metal alloy consisting primarily of copper, usually with tin as the main additive. It is hard and brittle, and it was particularly significant in antiquity, so much so that the Bronze Age was named after the metal...

 and similar metals (produced by oxidation or other chemical processes); a sheen on wooden furniture
Furniture is the mass noun for the movable objects intended to support various human activities such as seating and sleeping in beds, to hold objects at a convenient height for work using horizontal surfaces above the ground, or to store things...

 produced by age, wear, and polishing; or any such acquired change of a surface through age and exposure. On metal, patina is a coating of various chemical compounds such as oxide
An oxide is a chemical compound that contains at least one oxygen atom in its chemical formula. Metal oxides typically contain an anion of oxygen in the oxidation state of −2....

s or carbonate
In chemistry, a carbonate is a salt of carbonic acid, characterized by the presence of the carbonate ion, . The name may also mean an ester of carbonic acid, an organic compound containing the carbonate group C2....

s formed on the surface during exposure to the elements (weathering
Weathering is the breaking down of rocks, soils and minerals as well as artificial materials through contact with the Earth's atmosphere, biota and waters...