Why does copper turn green - an explanation of the patina process

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Published: 11th September 2009
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Most people know that pennies are made of copper, but are you aware that the Statue of Liberty is also constructed from this timeless and durable metal?



Even if this comes as no surprise to you, you may be confused by why pennies are such a shiny bronze while the Statue has an almost aqua hue. This is due to the unique chemical composition of copper which results in oxidation, patina formation, and eventual color change.



* Oxidation: Oxidation refers to the chemical changes that a substance undergoes when it is exposed to the elements. Iron, for example, forms rust when it is oxidized. When copper is oxidized, however, it forms a unique patina layer that, in addition to the color change you can see, provides a protective barrier that you cannot see. While oxidation leads to the corrosion and corruption of some materials, it does not affect the stability of copper.







* Patina: While patina can refer to the layer that forms as a result of oxidation over any metal, the patina of copper has unique properties. The patina that forms on copper protects the copper and results in color change. The patina is responsible for the dramatic difference in appearance between a brand new penny and the century-old Statue of Liberty.







* Color Change: The color change that results from copper's oxidation, patina formation, and aging first tends to be a darkening to a deeper brown or rust color. Only after three to four years does that brownish hue begin to fade and eventually the lighter, greener hues emerge. This color change process can be stopped by treating copper before it oxidizes, and it can also be hastened by treating copper in a different manner which will speed up the oxidation and result in an aged look much faster. For an idea of how your copper additions will look over the next few years, check out this color chart: (Insert picture or link)







Perhaps the most interesting part of this process is that it changes over times and takes years to fully develop. During the first 5-10 years of exposure copper will change colors gradually, but that process speeds up after approximately 10 years eventually resulting in the bluish-green color many people recognize.



Crescent City Copper manufactures copper products such as awnings, finials, cupolas, gutters and more. To learn more about why a href= http://crescentcitycopper.com/home/index.php?option=com_content&task=view&id=24&Itemid=77>copper turns green or see a copper weathering chart please visit our website. Article by Andre' Savoie.

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