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<p>[QUOTE=&quot;John Anthony, post: 2375437, member: 42773&quot;]My understanding is basic, so take it with a grain of salt. A <i>bona fide</i> patina, in the way collectors of ancients use the term, is a layer of various compounds that accrue on top of the surface of a bronze coin, ie., oxides, carbonates, etc. This layer forms very gradually, and evenly, due to various atmospheric conditions. It doesn&#039;t start out with green spots appearing suddenly. It starts as a very thin film that gradually thickens, over hundreds of years.</p><p><br /></p><p>Green spots, especially flaky, bright green spots, are a type of corrosion we call bronze disease. BD occurs from the excretion of various acids in the alloy that interact with moisture in the air, and generally occur in a spotty sort of way, although they can cover an entire coin if left unchecked.</p><p><br /></p><p>So BD is a type of corrosion, but a real patina is not. A real patina actually protects the metal underneath, as opposed to destroying it. I&#039;m sure a chemist could explain it better, paging [USER=15199]@desertgem[/USER] ! Please pick up the white courtesy phone. <img src="styles/default/xenforo/clear.png" class="mceSmilieSprite mceSmilie1" alt=":)" unselectable="on" unselectable="on" />[/QUOTE]</p><p><br /></p>

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