Patina (pron.: // or //) is a tarnish that forms on the surface of copper, bronze and similar metals (produced by oxidation or other chemical processes); stone; 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 layer 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). 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 study the patinas found on flint tools and ancient stone monuments. Stone develops a corticated layer over time that is due to a range of complex factors. 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".
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.
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/marine environments will weather and 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.
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.
Patina is also found on slip rings and commutators. This type of patina is formed by corrosion, what elements the air might hold, residue from the wear of the carbon brush and moisture; thus, the patina need special conditions to work as intended.
Patinas can also be found in woks or other metal baking dishes, which form when properly seasoned. The patina on a wok is a dark coating of oils that have been burned onto it to prevent food sticking and to enhance the flavor of the foods cooked in it. Steaming foods or using soap on a wok or other dishware could damage the patina and possibly allow rust.
In the case of antiques, several views are held on the value of patination and its replacement if damaged, known as repatination.
Preserving a piece's look and character is important and removal or reduction may dramatically reduce its value. If patination has flaked off, repatination may be recommended.
Appraiser Reyne Haines notes that "a repatinated metal piece will be worth more than one with major imperfections in the patina," but less than a piece still with its original finish.
See also 
- European Geosciences Union. General Assembly; Richard Přikryl; Bernard J. Smith; Geological Society Of London (2007). Building Stone Decay: From Diagnosis to Conservation. Geological Society. pp. 295–. ISBN 978-1-86239-218-2. Retrieved 26 September 2012.
- "Finish issues with regard to copper sinks". October 2010.
- Mark Edmonds (2 June 1997). Stone Tools and Society: Working Stone in Neolithic and Bronze Age Britain. Psychology Press. pp. 192–. ISBN 978-0-415-21449-0. Retrieved 26 September 2012.
- Nordic Embassies Berlin : Architecture Information, http://www.e-architect.co.uk/berlin/scandinavian_embassies.htm
- Oxidation Reaction: Why is the Statue of Liberty Blue-Green? Engage Students in Engineering; www.EngageEngineering.org; Chun Wu, Ph.D., Mount Marty College; Funded by the National Science Foundation (NSF) under Grant No. 083306. http://www.wepanknowledgecenter.org/c/document_library/get_file?folderId=517&name=DLFE-2454.pdf
- Yahoo! Answers - What is the patina of an oxidised copper coin made of? http://sg.answers.yahoo.com/question/index?qid=20090726064632AAiDf2k
- K.P. Fitzgerald, J. Nairn, A. Atrens (1998). The chemistry of copper patination; ScienceDirect.com - Corrosion Science - Volume 40, Issue 12, December 1998, Pages 2029–2050; http://www.sciencedirect.com/science/article/pii/S0010938X98000936
- Application Areas: Architecture - Finishes – patina; http://www.copper.org/applications/architecture/finishes.html
- Glossary of copper terms, Copper Development Association (UK): http://www.copperinfo.co.uk/resources/glossary.shtml
- Berg, Jan. "Why did we paint the library's roof?". Archived from the original on 2007-06-25. Retrieved 2007-09-20.
- Architectural considerations; Copper in Architecture Design Handbook, http://www.copper.org/applications/architecture/arch_dhb/fundamentals/arch_considerations.htm
- Peters, Larry E. (2004). Preventing corrosion on copper roofing systems; Professional Roofing, October 2004, http://www.professionalroofing.net
- Gaffney, Dennis (May 19, 2009). "What "Patina" Really Means".
Further reading 
- Angier, R.H. : Firearm Blueing and Browning, Onslow County 1936.
- Fishlock, David : Metal Colouring,Teddington 1962.
- Hiorns, Arthur (1907). Metal Colouring and Bronzing. London.
- Hughes, Richard; Rowe, Michael. The Colouring, Bronzing and Patination of Metals. London: Thames & Hudson Ltd. ISBN 0-500-01501-5.
- LaNiece, Susan; Craddock, Paul : Metal Plating and Patination: Cultural, Technical and Historical Developments, Boston 1993.
- Sugimori,E. : Japanese patinas, Brunswick 2004.
- W.A.Franke,M.Mircea Plutarch report on the blue patina of bronze statues at Delphi: A scientific explanation
- A. Giumlia-Mair Colouring treatments on ancient copper alloys
- Cockrell, B. Colourful Corrosion:Black Bronze and its Enigmatic Patina
- Patina on Bronze Sculpture From the Historical-Artistic Point of View
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