Patina (pronounced /ˈpætənə/ or /pəˈtiːnə/) is a film on the surface of bronze or similar metals (produced by oxidation over a long period); a sheen on wooden furniture 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 oxides or carbonates formed on the surface during exposure to the elements (weathering). 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.
Patinas are restricted to exposed surfaces and are fragile (that is, they can flake off). One reason bronze is so highly valued in statuary is that its patina protects or passivates it against further corrosion. This natural patina is solid and seldom shows a tendency to flake. Brass is also resistant to corrosion, but it is, in the long run, not as attractive since local pitting shows against the shiny background.
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 and/or unavoidable.
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 is often mis-named verdigris and usually consists of a mixture of chlorides, sulphides and carbonates. copper carbonate or copper chloride. Atacamite is another name for the patina compounds. Verdigris can be produced on copper by addition of vinegar (acetic acid) - such a verdigris is water-soluble and will not last on the outside of a building like a "true" patina.
One example of a patina is a green surface texture created by slow chemical alteration of copper, producing a basic carbonate. It can form on pure copper objects as well as alloys which contain copper, such as bronze or brass.
A patina layer takes many years to develop under natural weathering. A copper roof will patinate faster than a copper facade, due to the longer dwell time of water on the surface. Buildings in coastal / marine locations will weather and develop a patina layer faster than ones in inland areas. For example, a new copper facade in central London will most likely not develop a "typical" green patina until after 50 years.
Facade cladding (copper cladding) with alloys of copper, e.g. Brass or Bronze, will weather differently to "pure" copper cladding. Even a lasting gold colour is possible with copper-alloy cladding. Look at Colston Hall in Bristol, or the Novotel at Paddington Central, London. There you can see some colours that one might not have expected from copper / copper-alloy cladding.
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.
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.
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 dish ware could damage the patina and possibly allow rust.
Apart from the aesthetic appearance and practical protection of patination, antique experts confirm that an object's value increases when its patination is intact because it is an important effect of the aging process and this evidential history is reflected in the value of the piece.
In terms of antiques then,
Patina is everything that happens to an object over the course of time. The nick in the leg of a table, a scratch on a table top, the loss of moisture in the paint, the crackling of a finish or a glaze in ceramics, the gentle wear patterns on the edge of a plate. All these things add up to create a softer look, subtle color changes, a character. Patina is built from all the effects, natural and man-made, that create a true antique.—Israel Sack, 
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 though 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.
Footprint by artist Pam Glew  - The footprint shows the copper-alloy beneath a layer of copper carbonate patination. The effect was made by the application of chemicals to the metal.
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