A bronze coin of the Han Dynasty - circa 1st century BC with green patina. The Han Dynasty ( 206 BC–220 AD followed the Qin Dynasty and preceded the Three Kingdoms in China.

Patina is a coating of various chemical compounds such as oxides or carbonates formed on the surface of metal during exposure to weathering. An oxide is a Chemical compound containing at least one Oxygen atom as well as at least one other element In Chemistry, a carbonate is a salt or Ester of Carbonic acid. Weathering is the decomposition of earth rocks, Soils and their Minerals through direct contact with the planet's Atmosphere. The green patina that forms naturally on copper and bronze is known as verdigris and consists of copper carbonate. Verdigris is the common name for the Green coating or Patina formed when Copper, Brass or Bronze is weathered and exposed to 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. 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.

Patinas are restricted to exposed surfaces and can flake off. One reason bronze is so highly valued in statuary is that its patina protects or passivates it against further corrosion. Passivation is the process of making a material "passive" in relation to another material prior to using the materials together Corrosion means the breaking down of essential properties in a material due to Chemical reactions with its surroundings This natural patina 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.

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.

One example of a patina is a green surface texture created by slow chemical alteration of copper, producing a basic carbonate. Copper (ˈkɒpɚ is a Chemical element with the symbol Cu (cuprum and Atomic number 29 In Chemistry, a base is most commonly thought of as an aqueous substance that can accept Protons This refers to the Brønsted-Lowry theory of acids and It can form on pure copper objects as well as alloys which contain copper, such as bronze or brass. An alloy is a Solid solution or Homogeneous mixture of two or more elements, at least one of which is a Metal, which itself has Bronze is any of a broad range of Copper alloys, usually with Tin as the main additive but sometimes with other elements such as Phosphorus Brass is any Alloy of Copper and Zinc; the proportions of zinc and copper can be varied to create a range of brasses with varying properties

The Statue of Liberty gets its green color from the natural patina formed on its copper surface. Liberty Enlightening the World (La liberté éclairant le monde commonly known as the Statue of Liberty (Statue de la Liberté was presented

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. Exposure to chlorides leads to green, while sulfur compounds (such as "liver of sulfur") tend to brown. Potassium sulfide is the Chemical compound with the formula K2S For artworks patination is deliberately accelerated by heat. Colors range from matte sandstone yellow to deep blues, reds and various blacks, sometimes with the surface sheen enhanced by waxing for artwork displayed indoors.

Patina is also found on slip rings and commutators. A slip ring (in Electrical engineering terms is a method of making an electrical connection through a rotating assembly A commutator is an electrical Switch that periodically reverses the current direction in an Electric motor or Electrical generator 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, 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 could damage the patina and possibly allow the wok to rust.

In terms of antiques, "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

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In this cast bronze sculpture, Balance by David Ascalon, reactive chemicals were applied to the metallic surface to create an intentional surface patina (in this case, a marbleized blue). Bronze is the most popular metal for cast metal Sculptures; a cast bronze sculpture is often called simply a "bronze" David Ascalon is a contemporary sculptor and Stained glass artist and co-founder of Ascalon Studios.

The splash pattern from "instant" patination by highly reactive chemicals is visible on this brass surface by Stoller Metals. [1]

Footprint by artist Pam Glew [2] - The footprint shows the copper beneath a layer of copper carbonate patination. The effect was made by the application of chemicals to the copper canvas.

References

1. ^ History Detectives . Investigative Techniques . Geological Analysis | PBS

Further reading

Richard Hughes, Michael Rowe,. The Colouring, Bronzing and Patination of Metals. London: Thames & Hudson Ltd. ISBN 0-500-01501-5.   An excellent reference to recipes and techniques for patinas on non-ferrous metals.