Random thought about some of my coins

You went waaay random, but I like your train of thought.

 

Log in or Sign up to hide this ad.

My question is how did patinas start? Green spots on a buried LRB?

 

Doesn't the acidity of the soil have alot to do with whether a coin will aquire a patina or melt away? I know I've seen a coin buried for one year that was nearly unidentifiable, yet others buried in another part of the world, last for thousands of years.

 

Yes, soil composition is very relevant.

 

My understanding is basic, so take it with a grain of salt. A bona fide 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't start out with green spots appearing suddenly. It starts as a very thin film that gradually thickens, over hundreds of years.

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.

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'm sure a chemist could explain it better, paging @desertgem ! Please pick up the white courtesy phone.