Patina

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Copper roof on the Minneapolis City Hall, coated with patina Minneapolis City Hall.jpg
Copper roof on the Minneapolis City Hall, coated with patina
The Dresden Frauenkirche. The church was destroyed during the bombing of Dresden in 1945 and then rebuilt from 1993 to 2005 with new material; the stones with the black patina are the parts that survived the firebombing from the original 18th-century church. FraunkircheSouth.jpg
The Dresden Frauenkirche. The church was destroyed during the bombing of Dresden in 1945 and then rebuilt from 1993 to 2005 with new material; the stones with the black patina are the parts that survived the firebombing from the original 18th-century church.
Pre-colonial copper coin formerly used in the Copper Belt (Democratic Republic of the Congo and Zambia). The external layer has been weathered by moisture and rain, leading to the oxidation of copper. Ancienne unite de monnaie en cuivre.jpg
Pre-colonial copper coin formerly used in the Copper Belt (Democratic Republic of the Congo and Zambia). The external layer has been weathered by moisture and rain, leading to the oxidation of copper.

Patina ( /pəˈtnə/ or /ˈpætɪnə/ ) is a thin layer that variously forms on the surface of copper, brass, bronze and similar metals (tarnish produced by oxidation or other chemical processes), or certain stones, [1] and wooden furniture (sheen produced by age, wear, and polishing), or any similar acquired change of a surface through age and exposure.

Contents

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). [2] In common parlance, weathering rust on steel is often mistakenly [3] referred to as patina. 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. [4]

Archaeologists also use the term patina to refer to a corticated layer that develops over time that is due to a range of complex factors on flint tools and ancient stone monuments. [1] 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. [5]

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. [6] [7]

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.

Copper weather vane with verdigris patina Chickenvane.jpg
Copper weather vane with verdigris patina

Acquired patina

The Statue of Liberty gets its famous green color from the natural patina formed on its copper surface. Statofliberty.jpg
The Statue of Liberty gets its famous green color from the natural patina formed on its copper surface.

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. [8] [9] [10] [11] 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. [12] [13] [14]

A patina layer takes many years to develop under natural weathering. Buildings in damp coastal/marine environments will develop patina layers faster than ones in dry inland areas.

Natural copper patina at the Nordic Embassies (Berlin) Nordische Botschaften Berlin 1.jpg
Natural copper patina at the Nordic Embassies (Berlin)

Facade cladding (copper cladding; copper wall cladding) with alloys of copper, e.g. brass or bronze, will weather differently from "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.

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. This layer of rust is often erroneously [16] called a patina. [ unreliable source? ]

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.

Lisboa November 2011-13.jpg
The admiralty brass statue of Joseph I of Portugal in Commerce Square, Lisbon, with a surface layer of green patina caused by advanced oxidation (2011)
Lisboa January 2015-22.jpg
The same statue in 2015, after removal of the patina (2012–13), showing the original 1775 finish. The dark color of the statue made English sailors call the square that houses it "Black Horse Square".

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 needs 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 from sticking. Steaming foods or using soap on a wok or other dishware could damage the patina and possibly allow rust.

Knife collectors that own carbon steel blades sometimes force a patina onto the blade to help protect it and give it a more personalized look. This can be done using various chemicals and substances such as muriatic acid, apple cider vinegar, or mustard. It can also be done by sticking the blade into any acidic vegetable or fruit such as an orange or an apple.

Repatination

In the case of antiques, a range of 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. [17] 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. [17]

See also

Related Research Articles

Rust Type of iron oxide

Rust is an iron oxide, a usually reddish-brown oxide formed by the reaction of iron and oxygen in the catalytic presence of water or air moisture. Rust consists of hydrous iron(III) oxides (Fe2O3·nH2O) and iron(III) oxide-hydroxide (FeO(OH), Fe(OH)3), and is typically associated with the corrosion of refined iron.

Stainless steel steel alloy resistant to corrosion

Stainless steel is a group of ferrous alloys that contain a minimum of approximately 11% chromium, a composition that prevents the iron from rusting and also provides heat-resistant properties. Different types of stainless steel include the elements carbon, nitrogen, aluminium, silicon, sulfur, titanium, nickel, copper, selenium, niobium, and molybdenum. Specific types of stainless steel are often designated by their AISI three-digit number, e.g., 304 stainless. The ISO 15510 standard lists the chemical compositions of stainless steels of the specifications in existing ISO, ASTM, EN, JIS, and GB (Chinese) standards in a useful interchange table.

Smelting Use of heat and a reducing agent to extract metal from ore

Smelting is a process of applying heat to ore in order to extract a base metal. It is a form of extractive metallurgy. It is used to extract many metals from their ores, including silver, iron, copper, and other base metals. Smelting uses heat and a chemical reducing agent to decompose the ore, driving off other elements as gases or slag and leaving the metal base behind. The reducing agent is commonly a fossil fuel source of carbon, such as coke—or, in earlier times, charcoal. The oxygen in the ore binds to carbon at high temperatures due to the lower potential energy of the bonds in carbon dioxide. Smelting most prominently takes place in a blast furnace to produce pig iron, which is converted into steel.

Copper Chemical element, symbol Cu and atomic number 29

Copper is a chemical element with the symbol Cu and atomic number 29. It is a soft, malleable, and ductile metal with very high thermal and electrical conductivity. A freshly exposed surface of pure copper has a pinkish-orange color. Copper is used as a conductor of heat and electricity, as a building material, and as a constituent of various metal alloys, such as sterling silver used in jewelry, cupronickel used to make marine hardware and coins, and constantan used in strain gauges and thermocouples for temperature measurement.

Corrosion Gradual destruction of materials by chemical reaction with its environment

Corrosion is a natural process that converts a refined metal into a more chemically stable form such as oxide, hydroxide, carbonate or sulfide. It is the gradual destruction of materials by chemical and/or electrochemical reaction with their environment. Corrosion engineering is the field dedicated to controlling and preventing corrosion.

Passivation, in physical chemistry and engineering, refers to coating a material so it becomes "passive," that is, less readily affected or corroded by the environment. Passivation involves creation of an outer layer of shield material that is applied as a microcoating, created by chemical reaction with the base material, or allowed to build by spontaneous oxidation in the air. As a technique, passivation is the use of a light coat of a protective material, such as metal oxide, to create a shield against corrosion. Passivation of silicon is used during fabrication microelectronic devices. In electrochemical treatment of water, passivation reduces the effectiveness of the treatment by increasing the circuit resistance, and active measures are typically used to overcome this effect, the most common being polarity reversal, which results in limited rejection of the fouling layer.

Cookware and bakeware Food preparation containers

Cookware and bakeware is food preparation equipment, such as cooking pots, pans, baking sheets etc. used in kitchens. Cookware is used on a stove or range cooktop, while bakeware is used in an oven. Some utensils are considered both cookware and bakeware.

Tarnish Corrosion on outer layer of some metals

Tarnish is a thin layer of corrosion that forms over copper, brass, aluminum, magnesium, neodymium 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, although it may tarnish with oxygen over time. It often appears as a dull, gray or black film or coating over metal. Tarnish is a surface phenomenon that is self-limiting, unlike rust. Only the top few layers of the metal react, and the layer of tarnish seals protects the underlying layers from reacting.

Anodizing Electrolytic passivation process used to increase the thickness of the natural oxide layer on the surface of metal parts

Anodizing is an electrolytic passivation process used to increase the thickness of the natural oxide layer on the surface of metal parts.

Plating is a surface covering in which a metal is deposited on a conductive surface. Plating has been done for hundreds of years; it is also critical for modern technology. Plating is used to decorate objects, for corrosion inhibition, to improve solderability, to harden, to improve wearability, to reduce friction, to improve paint adhesion, to alter conductivity, to improve IR reflectivity, for radiation shielding, and for other purposes. Jewelry typically uses plating to give a silver or gold finish.

Rokushō (緑青) is a traditional Japanese chemical compound used in the niiro process for artificially inducing patination in decorative non-ferrous metals, especially several copper alloys, with the results being metals of the irogane class. These "colour metals," virtually unknown outside Japan until the late 19th century, have achieved some popularity in craft circles in other parts of the world since then.

Weathering steel Group of steel alloys designed to form a rust-like finish when exposed to weather

Weathering steel, often referred to by the genericised trademark COR-TEN steel and sometimes written without the hyphen as corten steel, is a group of steel alloys which were developed to eliminate the need for painting, and form a stable rust-like appearance after several years' exposure to weather.

Pickling is a metal surface treatment used to remove impurities, such as stains, inorganic contaminants, and rust or scale from ferrous metals, copper, precious metals and aluminum alloys. A solution called pickle liquor, which usually contains acid, is used to remove the surface impurities. It is commonly used to descale or clean steel in various steelmaking processes.

Architectural metals

Metals used for architectural purposes include lead, for water pipes, roofing, and windows; tin, formed into tinplate; zinc, copper and aluminium, in a range of applications including roofing and decoration; and iron, which has structural and other uses in the form of cast iron or wrought iron, or made into steel. Metal alloys used in building include bronze ; brass ; monel metal and nickel silver, mainly consisting of nickel and copper; and stainless steel, with important components of nickel and chromium.

Liver of sulfur

Liver of sulfur is a poorly defined mixture of potassium sulfide, potassium polysulfide, potassium thiosulfate, and likely potassium bisulfide. Synonyms include hepar sulfuris, sulfur, sulfurated potash and sulfurated potassa. There are two distinct varieties: "potassic liver of sulfur" and "ammoniacal liver of sulfur".

Cobalt extraction

Cobalt extraction refers to the techniques used to extract cobalt from its ores and other compound ores. Several methods exist for the separation of cobalt from copper and nickel. They depend on the concentration of cobalt and the exact composition of the used ore.

Conservation and restoration of silver objects

The conservation and restoration of silver objects is an activity dedicated to the preservation and protection of objects of historical and personal value made from silver. When applied to cultural heritage this activity is generally undertaken by a conservator-restorer.

Copper in architecture

Copper has earned a respected place in the related fields of architecture, building construction, and interior design. From cathedrals to castles and from homes to offices, copper is used for a variety of architectural elements, including roofs, flashings, gutters, downspouts, domes, spires, vaults, wall cladding, and building expansion joints.

Sulfidation is a process of installing sulfide ions in a material or molecule. The process is widely used to convert oxides to sulfides but is also related to corrosion and surface modification.

Chemical coloring of metals Process of changing the color of metal surfaces with different chemical solutions

Chemical coloring of metals is the process of changing the color of metal surfaces with different chemical solutions.

References

  1. 1 2 European Geosciences Union. General Assembly; Přikryl, Richard; Smith, Bernard J.; 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.
  2. What is Patina MarthaStewart.com
  3. Novak, Mark. "Anvil 093: 1891 Mosin/Nagant , Dragoon and Rifle". YouTube. Retrieved 15 January 2021.
  4. "Finish issues with regard to copper sinks". October 2010.
  5. Edmonds, Mark (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.
  6. Neuendorf, K.K.E., J.P. Mehl, Jr., and J.A. Jackson, J.A., eds. (2005) Glossary of Geology (5th ed.). Alexandria, Virginia, American Geological Institute. 779 pp. ISBN   0-922152-76-4
  7. Twidale, C.R., and J.R. Vidal Romani (2005) Landforms and Geology of Granite Terrains. A.A. Balkema Publishers Leiden, The Netherlands. 330 pp. ISBN   0-415-36435-3
  8. 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. https://web.archive.org/web/20131025094519/http://www.wepanknowledgecenter.org/c/document_library/get_file?folderId=517&name=DLFE-2454.pdf
  9. Fitzgerald, K.P.; Nairn, J.; Atrens, A. (1998). "The chemistry of copper patination". Corrosion Science. 40 (12): 2029–2050. doi:10.1016/S0010-938X(98)00093-6.
  10. Application Areas: Architecture - Finishes – patina; http://www.copper.org/applications/architecture/finishes.html
  11. Glossary of copper terms, Copper Development Association (UK): "Archived copy". Archived from the original on August 20, 2012. Retrieved September 14, 2012.CS1 maint: archived copy as title (link)
  12. Berg, Jan. "Why did we paint the library's roof?". Archived from the original on 2007-06-25. Retrieved 2007-09-20.
  13. Architectural considerations; Copper in Architecture Design Handbook, http://www.copper.org/applications/architecture/arch_dhb/fundamentals/arch_considerations.htm
  14. Peters, Larry E. (2004). Preventing corrosion on copper roofing systems; Professional Roofing, October 2004, http://www.professionalroofing.net
  15. Nordic Embassies Berlin : Architecture Information, http://www.e-architect.co.uk/berlin/scandinavian_embassies.htm
  16. Novak, Mark. "Anvil 093: 1891 Mosin/Nagant , Dragoon and Rifle". YouTube. Retrieved 15 January 2021.
  17. 1 2 Gaffney, Dennis (May 19, 2009). "What "Patina" Really Means".

Further reading