Editing Tarnish

{{Short description|Corrosion on outer layer of some metals}}
{{For|1924 silent film|Tarnish (film)}}
[[File:Wolfram evaporated crystals and 1cm3 cube.jpg|thumb|upright=1.3|[[Tungsten]] rods with evaporated crystals, partially [[Redox|oxidized]] with colorful tarnish]]
'''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.

Tarnish preserves the underlying metal in outdoor use, and in this form is called chemical [[patina]],<ref name=":0">{{Cite web |date=2019-08-14 |title=Guide to Tarnish |url=https://www.riogrande.com/knowledge-hub/guide-to-tarnish/ |access-date=2024-08-17 |website=Rio Grande}}</ref> an example of which is the green or blue-green form of [[copper(II) carbonate]] known as [[verdigris]]. Unlike patina advantageous in applications such as copper roofing and copper, bronze, and brass statues and fittings exposed to the elements, a chemical patina may be considered undesirable, as on silverware,<ref name=":0" /> or a matter of taste or convention, as in [[Toning (coin)|toning]] on coins.

==Chemistry==
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.

==Prevention and removal==
[[File:1899_reverse.jpg|thumb|A [[Morgan dollar]] demonstrating a colorful form of [[Toning (coin)|toning]] on its reverse. While tarnish 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 preserved.<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>]]

Heavy tarnish can be mechanically removed by using tools such as a [[File (tool)|file]] or abrasive materials such as [[steel wool]], [[sandpaper]], [[emery paper]], and heavy [[polishing]] compounds.  Lighter tarnish may be abrasively removed with lighter polishing compounds or chemicals such as [[Sodium bicarbonate|baking soda]]. Gentler abrasives, such as [[calcium carbonate]], are often used by museums to clean tarnished [[silver]], which will not scratch it or leave unwanted residues.<ref>{{cite web |url= http://ohiohistory.wordpress.com/2011/06/02/making-it-shine/ |title=Making it Shine |publisher= [[Ohio Historical Society]] | website= ohiohistory.wordpress.com |date=2011-06-02 |access-date=2012-10-19 |url-status=dead |archive-url= https://web.archive.org/web/20120323201653/http://ohiohistory.wordpress.com/2011/06/02/making-it-shine/ |archive-date=2012-03-23 }}</ref> Objects such as silverware may have their tarnish non-destructively reversed electrochemically by resting them on a piece of aluminium foil in a pot of boiling water with a small amount of salt or baking soda.<ref>{{cite web| url= http://www.philamuseum.org/booklets/7_44_85_1.html?page=2 |title= Finishing techniques in Metalwork | website= philamuseum.org| publisher= [[Philadelphia Museum of Art]]|url-status=dead | archive-url=https://web.archive.org/web/20160331180638/http://www.philamuseum.org/booklets/7_44_85_1.html?page=2|archive-date=2016-03-31}}</ref><ref>{{cite web |url= http://scifun.chem.wisc.edu/homeexpts/TARNISH.html |title=Remove Tarnish from Silver |website= scifun.chem.wisc.edu | publisher= Department of Chemistry, University of Wisconsin-Madison |access-date= 2012-10-19 |url-status=dead | archive-url=https://web.archive.org/web/20180401064415/http://scifun.chem.wisc.edu/homeexpts/TARNISH.html|archive-date=2018-04-01}}</ref>

==References==
{{Reflist}}


[[Category:Chemical reactions]]
[[Category:Metals]]
[[Category:Metalworking terminology]]
[[Category:Corrosion]]