Question About Environmental Damage

With the exception of gold most coins under the wrong conditons will suffer from corrosion. Copper and bronze coins are particulary prone to damage from moisture and form a greenish covering called bronze desease. If left unchecked it will destroy the coin.

G'day, I've heard the "greenish covering called bronze desease" called verdigris, but I have read that it is "carbonate of copper, and should not be confounded with true verdigris". Other sources say that it is copper sulphate or copper chloride. One article called it "coin cancer".

Some people describe it as a "patina", but it is in fact corrosion, that eats holes into coins.

I gather that Bronze coins are much more resistant than copper coins. The British changed from copper to bronze in 1860. It's a lot easier to find well-conserved British pennies of the 1860s than of the 1850s, which bears out the theory. The French made this transition several decades earlier. I don't know about other countries.

I always remove green coins from all others. There is a theory that one green coin can infect a clean coin, by contact. Even if that's not true, it's my operative assumption.

I live in the monsoonal tropics: 12 degrees from the equator; temperatures between 24 and 33 Celsius all year; and humidity above 90% for most of the "wet season", of five months duration. Apparently, we are high in Ozone, because the incidence of lightning strikes is high. And our plentiful rain (1660mm annually) is weakly acidic, of the nitric variety. I mention all of that, because even in this environment, I've never had coins that I've "put aside" go green on me.

But, I've seen a few green spots on circulating coins, including on CuNi. And, I've been given plastic bags of mixed old coins by friends who know that I'm a collector, which seems the best source for green coins.

I haven't found any especially valuable coins that are afflicted. So, I keep them, in order to do experiments on. I believe that green coins have truly zero value, because no one else would want to have them around. I suppose I should add that I'm not talking about antiquities. So, if you've wondered about a possible exception to the "no cleaning" rule, I reckon that green coins are it.

There is a most informative article by a collector who tried to arrest the deterioration of an Australian penny. I commend it to the forum:
"Cleaning and preservation of coins"
http://triton.vg/cleaning.html

There is a book, which I've not read, but the title interests me: "Coin Chemistry, Including Preservation and Cleaning"
by Weimar W White, 2004.

Perhaps another forum member has a copy ...

Peter in Oz