What is Tarnish?
[[File:Wolfram evaporated crystals and 1cm3 cube.jpg|thumb|[[Tungsten]] rods with evaporated crystals, partially [[Redox|oxidized]] with colorful tarnish]] [[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>]] '''Tarnish''' is a layer of [[corrosion]] similar to [[rust]]. It is caused by [[oxidisation]] - when semi - reactive [[metals]] such as [[copper]], [[brass]], [[silver]], or [[Aluminium|aluminum]] meet chemicals in the air. It will cause the metal to darken or go dull by making a thin film on the metal. It is caused by a chemical reaction, between a metal and a non metal (or compound), usually [[oxygen]] or [[sulphur dioxide]]. ==References== {{reflist}} [[Category:Metals]] [[Category:Chemical reactions]]