Patina (/ˈpætɨnə/ or /pəˈtiːnə/) is a thin layer that forms on the surface of stone; copper, bronze and similar metals (tarnish produced by oxidation or other chemical processes); a sheen on wooden furniture 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”.
In geology and geomorphology, the term “patina” is used to refer to discolored film or thin outer layer produced either on or within the surface of a rock or other material by either the development of a weathering rind within the surface of a rock, the formation of desert varnish on the surface of a rock, or combination of both. It also refers to development as the result of weathering of a case-hardened layer, called “cortex” by geologists, within the surface of either a flint or chert nodule.
The word “patina” comes from the Latin for “shallow dish”. Figuratively, patina can refer to any fading, darkening or other signs of age, which are felt to be natural or unavoidable (or both).
The chemical process by which a patina forms is called patination, and a work of art coated by a patina is said to be patinated.
The green patina that forms naturally on copper and bronze, sometimes called verdigris, usually consists of varying mixtures of copper chlorides, sulfides, sulfates and carbonates, depending upon environmental conditions such as sulfur-containing acid rain. In clean air rural environments, the patina is created by the slow chemical reaction of copper with carbon dioxide and water, producing a basic copper carbonate. In industrial and urban air environments containing sulfurous acid rain from coal-fired power plants or industrial processes, the final patina is primarily composed of sulphide or sulphate compounds.
A patina layer takes many years to develop under natural weathering. Buildings in damp coastal/marine environments will develop patina layers faster than ones in dry inland areas.
Facade cladding (copper cladding; copper wall cladding) with alloys of copper, e.g. brass or bronze, will weather differently than “pure” copper cladding. Even a lasting gold colour is possible with copper-alloy cladding, for example Colston Hall in Bristol, or the Novotel at Paddington Central, London.
Often, antique and well-used firearms will develop a patina on the steel after the bluing, parkerizing, or other finish has worn. Firearms in this state are generally considered more valuable than ones that have been re-blued or parkerized. The patina protects the firearm from more damaging rust that would occur were the patina to be polished off.
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