Patina ( pə-TEE-nə, or PAT-ih-nə) is a thin layer that variously forms on the surface of copper, brass, bronze, and similar metals and alloys (tarnish produced by oxidation or other chemical processes), or certain stones and wooden furniture (sheen produced by age, wear, and polishing), or any similar acquired change of a surface through age and exposure. Additionally, the term is used to describe the aging of high-quality leather. The patinas on leather goods are unique to the type of leather, frequency of use, 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.
Usage 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). Patina also refers to accumulated changes in surface texture and color 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 corticateed 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.
Etymology The word patina comes from the Italian patina (shallow layer of deposit on a surface), derived from the Latin patĭna (pan, 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 or is deliberately induced is called patination, and a work of art coated by a patina is said to be patinated.
Acquired patina
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 or marine environments will develop patina layers faster than ones in dry inland areas.
Façade cladding (copper cladding; copper wall cladding) with alloys of copper, like brass or bronze, will weather differently from "pure" copper cladding. Even a lasting gold colour is possible with copper-alloy cladding, for example Bristol Beacon in Bristol, or the Novotel at Paddington Central, London. Antique and well-used firearms will often develop a layer of rust on the action, barrel, or other steel parts after the original finish has worn. On this subject gunsmith Mark Novak says "... This is what everybody calls patina, I call it a nice thick coat of rust..." The removal of such rust is often necessary for a firearm conservation to prevent further decay of the firearm.
Applied patina
Artists and metalworkers often deliberately add patinas as a part of the original design and decoration of art and furniture, or to simulate antiquity in newly made objects. The process is often called distressing. A wide range of chemicals, both household and commercial, can give a variety of patinas. They are often used by artists as surface embellishments either for color, texture, or both. Patination composition varies with the reacted elements and these will determine the color of the patina. For copper alloys, such as bronze, exposure to chlorides leads to green, while sulfur compounds (such as "liver of sulfur") tend to brown. The basic palette for patinas on copper alloys includes chemicals like ammonium sulfide (blue-black), liver of sulfur (brown-black), cupric nitrate (blue-green), and ferric nitrate (yellow-brown). For artworks, patination is often deliberately accelerated by applying chemicals with heat. Colors range from matte sandstone yellow to deep blues, greens, whites, reds, and various blacks. Some patina colors are achieved by the mixing of colors from the reaction with the metal surface with pigments added to the chemicals. Sometimes the surface is enhanced by waxing, oiling, or other types of lacquers or clear-coats. More simply, the French sculptor Auguste Rodin used to instruct assistants at his studio to urinate over bronzes stored in the outside yard. A patina can be produced on copper by the application of vinegar (acetic acid). This patina is water-soluble and will not last on the outside of a building like a "true" patina. It is usually used as pigment.
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