Tarnish: Difference between revisions
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{{Short description|Corrosion on outer layer of some metals}} |
{{Short description|Corrosion on outer layer of some metals}} |
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{{For|1924 silent film|Tarnish (film)}} |
{{For|1924 silent film|Tarnish (film)}} |
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{{more references|date=January 2024}} |
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[[File:Wolfram evaporated crystals and 1cm3 cube.jpg|thumb|[[Tungsten]] rods with evaporated crystals, partially [[Redox|oxidized]] with colorful tarnish]] |
[[File:Wolfram evaporated crystals and 1cm3 cube.jpg|thumb|[[Tungsten]] rods with evaporated crystals, partially [[Redox|oxidized]] with colorful tarnish]] |
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'''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. The layer of tarnish seals and protects the underlying layers from reacting. |
'''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.<ref>{{cite book |last1=Revie |first1=R. Winston |last2=Uhlig |first2=Herbert Henry |title=Corrosion and corrosion control: an introduction to corrosion science and engineering |date=2008 |publisher=Wiley-Interscience |location=Hoboken, N.J |isbn=978-0-471-73279-2 |edition=4th}}</ref> 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. The layer of tarnish seals and protects the underlying layers from reacting. |
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Tarnish |
Tarnish preserves the underlying metal in outdoor use, and in this form is called [[patina]]. The formation of patina is necessary in applications such as copper roofing, outdoor copper, bronze, and brass statues and fittings. Patina is the name given to tarnish on copper-based metals, while [[Toning (coin)|toning]] is a term for the type of tarnish which forms on coins. |
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==Chemistry== |
==Chemistry== |
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[[File:1899_reverse.jpg|thumb|right|A [[Morgan dollar]] demonstrating a colourful form of [[Toning (coin)|toning]] on its reverse. While tarnishing on other metal objects is generally cleaned off, some toning on coins may be considered aesthetically pleasing or beneficial to the coin's value, and is therefore kept ''in situ''.<ref>{{cite web |last1=Bucki |first1=James |title=Coin Toning: Definition, Discussion and Effect on Value |url=https://www.thesprucecrafts.com/coin-toning-definition-768303 |website=The Spruce Crafts |access-date=1 April 2019}}</ref>]] |
[[File:1899_reverse.jpg|thumb|right|A [[Morgan dollar]] demonstrating a colourful form of [[Toning (coin)|toning]] on its reverse. While tarnishing on other metal objects is generally cleaned off, some toning on coins may be considered aesthetically pleasing or beneficial to the coin's value, and is therefore kept ''in situ''.<ref>{{cite web |last1=Bucki |first1=James |title=Coin Toning: Definition, Discussion and Effect on Value |url=https://www.thesprucecrafts.com/coin-toning-definition-768303 |website=The Spruce Crafts |access-date=1 April 2019}}</ref>]] |
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Tarnish is a product of a [[chemical reaction]] between a [[metal]] and a [[Nonmetal (chemistry)|nonmetal]] [[chemical compound|compound]], especially [[oxygen]] and [[sulfur dioxide]]. It is usually a metal [[oxide]], the product of [[oxidation]] |
Tarnish is a product of a [[chemical reaction]] between a [[metal]] and a [[Nonmetal (chemistry)|nonmetal]] [[chemical compound|compound]], especially [[oxygen]] and [[sulfur dioxide]]. It is usually a metal [[oxide]], the product of [[oxidation]]; sometimes it is a metal sulfide. The metal oxide sometimes reacts with water to make the hydroxide, or with carbon dioxide to make the carbonate. It is a chemical change. There are various methods to prevent metals from tarnishing. |
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==Prevention and removal== |
==Prevention and removal== |
Revision as of 22:29, 12 July 2024
This article needs additional citations for verification. (January 2024) |
![](http://upload.wikimedia.org/wikipedia/commons/thumb/1/1a/Wolfram_evaporated_crystals_and_1cm3_cube.jpg/220px-Wolfram_evaporated_crystals_and_1cm3_cube.jpg)
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.[1] 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. The layer of tarnish seals and protects the underlying layers from reacting.
Tarnish preserves the underlying metal in outdoor use, and in this form is called patina. The formation of patina is necessary in applications such as copper roofing, outdoor copper, bronze, and brass statues and fittings. Patina is the name given to tarnish on copper-based metals, while toning is a term for the type of tarnish which forms on coins.
Chemistry
![](http://upload.wikimedia.org/wikipedia/commons/thumb/3/30/1899_reverse.jpg/220px-1899_reverse.jpg)
Tarnish is a product of a chemical reaction between a metal and a nonmetal compound, especially oxygen and sulfur dioxide. It is usually a metal oxide, the product of oxidation; sometimes it is a metal sulfide. The metal oxide sometimes reacts with water to make the hydroxide, or with carbon dioxide to make the carbonate. It is a chemical change. There are various methods to prevent metals from tarnishing.
Prevention and removal
Using a thin coat of polish can prevent tarnish from forming over these metals. Tarnish can be removed by using steel wool, sandpaper, emery paper, baking soda or a file to rub or polish the metal's dull surface. Fine objects (such as silverware) may have the tarnish electrochemically reversed (non-destructively) by resting the objects on a piece of aluminium foil in a pot of boiling water with a small amount of salt or baking soda,[3][4] or it may be removed with a special polishing compound and a soft cloth. Gentler abrasives, like calcium carbonate, are often used by museums to clean tarnished silver as they cannot damage or scratch the silver and will not leave unwanted residues.[5]
References
- ^ Revie, R. Winston; Uhlig, Herbert Henry (2008). Corrosion and corrosion control: an introduction to corrosion science and engineering (4th ed.). Hoboken, N.J: Wiley-Interscience. ISBN 978-0-471-73279-2.
- ^ Bucki, James. "Coin Toning: Definition, Discussion and Effect on Value". The Spruce Crafts. Retrieved 1 April 2019.
- ^ "Finishing techniques in Metalwork". philamuseum.org. Philadelphia Museum of Art. Archived from the original on 2016-03-31.
- ^ "Remove Tarnish from Silver". scifun.chem.wisc.edu. Department of Chemistry, University of Wisconsin-Madison. Archived from the original on 2018-04-01. Retrieved 2012-10-19.
- ^ "Making it Shine". ohiohistory.wordpress.com. Ohio Historical Society. 2011-06-02. Archived from the original on 2012-03-23. Retrieved 2012-10-19.