Patina
Overview
 
Patina is a tarnish
Tarnish
Tarnish is a thin layer of corrosion that forms over copper, brass, silver, aluminum, and other similar metals as their outermost layer undergoes a chemical reaction. Tarnish does not always result from the sole effects of oxygen in the air. For example, silver needs hydrogen sulfide to tarnish; it...

 that forms on the surface of bronze
Bronze
Bronze is a metal alloy consisting primarily of copper, usually with tin as the main additive. It is hard and brittle, and it was particularly significant in antiquity, so much so that the Bronze Age was named after the metal...

 and similar metals (produced by oxidation or other chemical processes); a sheen on wooden furniture
Furniture
Furniture is the mass noun for the movable objects intended to support various human activities such as seating and sleeping in beds, to hold objects at a convenient height for work using horizontal surfaces above the ground, or to store things...

 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 oxide
Oxide
An oxide is a chemical compound that contains at least one oxygen atom in its chemical formula. Metal oxides typically contain an anion of oxygen in the oxidation state of −2....

s or carbonate
Carbonate
In chemistry, a carbonate is a salt of carbonic acid, characterized by the presence of the carbonate ion, . The name may also mean an ester of carbonic acid, an organic compound containing the carbonate group C2....

s formed on the surface during exposure to the elements (weathering
Weathering
Weathering is the breaking down of rocks, soils and minerals as well as artificial materials through contact with the Earth's atmosphere, biota and waters...

).
Encyclopedia
Patina is a tarnish
Tarnish
Tarnish is a thin layer of corrosion that forms over copper, brass, silver, aluminum, and other similar metals as their outermost layer undergoes a chemical reaction. Tarnish does not always result from the sole effects of oxygen in the air. For example, silver needs hydrogen sulfide to tarnish; it...

 that forms on the surface of bronze
Bronze
Bronze is a metal alloy consisting primarily of copper, usually with tin as the main additive. It is hard and brittle, and it was particularly significant in antiquity, so much so that the Bronze Age was named after the metal...

 and similar metals (produced by oxidation or other chemical processes); a sheen on wooden furniture
Furniture
Furniture is the mass noun for the movable objects intended to support various human activities such as seating and sleeping in beds, to hold objects at a convenient height for work using horizontal surfaces above the ground, or to store things...

 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 oxide
Oxide
An oxide is a chemical compound that contains at least one oxygen atom in its chemical formula. Metal oxides typically contain an anion of oxygen in the oxidation state of −2....

s or carbonate
Carbonate
In chemistry, a carbonate is a salt of carbonic acid, characterized by the presence of the carbonate ion, . The name may also mean an ester of carbonic acid, an organic compound containing the carbonate group C2....

s formed on the surface during exposure to the elements (weathering
Weathering
Weathering is the breaking down of rocks, soils and minerals as well as artificial materials through contact with the Earth's atmosphere, biota and waters...

). Patina also refers to accumulated changes in surface texture and colour that result from normal use of an object such as a coin
Coin
A coin is a piece of hard material that is standardized in weight, is produced in large quantities in order to facilitate trade, and primarily can be used as a legal tender token for commerce in the designated country, region, or territory....

 or a piece of furniture over time.

Advantages

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
Passivation
Passivation is the process of making a material "passive", and thus less reactive with surrounding air, water, or other gases or liquids. The goal is to inhibit corrosion, whether for structural or cosmetic reasons. Passivation of metals is usually achieved by the deposition of a layer of oxide...

 it against further corrosion
Corrosion
Corrosion is the disintegration of an engineered material into its constituent atoms due to chemical reactions with its surroundings. In the most common use of the word, this means electrochemical oxidation of metals in reaction with an oxidant such as oxygen...

. 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.

Etymology

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
Copper
Copper is a chemical element with the symbol Cu and atomic number 29. It is a ductile metal with very high thermal and electrical conductivity. Pure copper is soft and malleable; an exposed surface has a reddish-orange tarnish...

 and bronze, sometimes called verdigris
Verdigris
Verdigris is the common name for a green pigment obtained through the application of acetic acid to copper plates or the natural patina formed when copper, brass or bronze is weathered and exposed to air or seawater over a period of time. It is usually a basic copper carbonate, but near the sea...

, usually consists of a mixture of chlorides, sulphides and carbonates (copper carbonate
Copper(II) carbonate
Copper carbonate is a blue-green compound forming part of the verdigris patina that is found on weathered brass, bronze, and copper. The colour can vary from bright blue to green, because there may be a mixture of both copper carbonate and basic copper carbonate in various stages of hydration...

, copper chloride
Copper(II) chloride
Copper chloride is the chemical compound with the formula CuCl2. This is a light brown solid, which slowly absorbs moisture to form a blue-green dihydrate. The copper chlorides are some of the most common copper compounds, after copper sulfate....

 or copper sulphide). Atacamite is another name for the patina compounds. Verdigris can be produced on copper by the addition of vinegar (acetic acid
Acetic acid
Acetic acid is an organic compound with the chemical formula CH3CO2H . It is a colourless liquid that when undiluted is also called glacial acetic acid. Acetic acid is the main component of vinegar , and has a distinctive sour taste and pungent smell...

). Such a verdigris is water-soluble and will not last on the outside of a building like a "true" patina. It is usually used as pigment.

One example of a patina is a green surface texture created by slow chemical alteration of copper, producing a basic
Base (chemistry)
For the term in genetics, see base A base in chemistry is a substance that can accept hydrogen ions or more generally, donate electron pairs. A soluble base is referred to as an alkali if it contains and releases hydroxide ions quantitatively...

 carbonate. It can form on pure copper objects as well as alloy
Alloy
An alloy is a mixture or metallic solid solution composed of two or more elements. Complete solid solution alloys give single solid phase microstructure, while partial solutions give two or more phases that may or may not be homogeneous in distribution, depending on thermal history...

s which contain copper, such as bronze or brass
Brass
Brass is an alloy of copper and zinc; the proportions of zinc and copper can be varied to create a range of brasses with varying properties.In comparison, bronze is principally an alloy of copper and tin...

.

Often, antique and well used firearms will develop a patina on the steel after the bluing
Bluing (steel)
Bluing is a passivation process in which steel is partially protected against rust, and is named after the blue-black appearance of the resulting protective finish. True gun bluing is an electrochemical conversion coating resulting from an oxidizing chemical reaction with iron on the surface...

, parkerizing
Parkerizing
Parkerizing is a method of protecting a steel surface from corrosion and increasing its resistance to wear through the application of an electrochemical phosphate conversion coating...

, 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.

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
Copper cladding
There are four main techniques used today in the UK and mainland Europe for copper cladding a building:* Seamed-cladding : max 600mm by 4000mm 'seam centres'....

) 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
Colston Hall
The Colston Hall is a concert hall and grade II listed building situated on Colston Street, Bristol, England. A popular venue catering for a variety of different entertainers, it seats approximately 2,075 and provides licensed bars, a café and restaurant....

 in Bristol, or the Novotel
Novotel
Novotel is a mid-scale hotel brand within the Accor group. Novotel has close to 400 hotels and resorts in 60 countries, situated in the business districts and tourist destinations of major international cities.-History:...

 at Paddington Central, London.

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
Distressing
Distressing in the decorative arts is the activity of making a piece of furniture or object appear aged and older, and there are many methods to produce an appearance of age and wear. Distressing is viewed as a refinishing technique although it is the opposite of finishing in a traditional sense...

.

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
Liver of sulfur
Sulfurated potash is a poorly defined mixture of potassium sulfide, potassium polysulfide, potassium thiosulfate, and probably potassium bisulfide...

") tend to brown. The basic palette for patinas on copper alloys includes chemicals like ammonium sulfide
Ammonium sulfide
Ammonium hydrosulfide is the chemical compound with the formula SH. It is the salt derived from the ammonium cation and the hydrosulfide anion. The salt exists as colourless, water soluble, micaceous crystals. The compound is encountered mainly as a solution, not as the solid...

 (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
Auguste Rodin
François-Auguste-René Rodin , known as Auguste Rodin , was a French sculptor. Although Rodin is generally considered the progenitor of modern sculpture, he did not set out to rebel against the past...

 used to instruct assistants at his studio to urinate over bronzes stored in the outside yard.

Patina is also found on slip ring
Slip ring
A slip ring is a method of making an electrical connection through a rotating assembly. Slip rings, also called rotary electrical interfaces, rotating electrical connectors, collectors, swivels, or electrical rotary joints, are commonly found in electric motors, electrical generators for AC...

s and commutator
Commutator (electric)
A commutator is a rotary electrical switch in certain types of electric motors or electrical generators that periodically reverses the current direction between the rotor and the external circuit. In a motor, it applies power to the best location on the rotor, and in a generator, picks off power...

s. 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.

Repatination

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.

Further reading

  • Angier, R.H. : Firearm Blueing and Browning, Onslow County 1936.
  • Fishlock, David : Metal Colouring,Teddington 1962.
  • LaNiece, Susan; Craddock, Paul : Metal Plating and Patination: Cultural, Technical and Historical Developments, Boston 1993.
  • Sugimori,E. : Japanese patinas, Brunswick 2004.

External links

The source of this article is wikipedia, the free encyclopedia.  The text of this article is licensed under the GFDL.
 
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