Mr T

Well that's interesting. I remember quite recently seeing some Morgan dollars with toning like that and the asking prices were really high for otherwise ordinary coins.

It was a pretty badly corroded coin and I didn't dilute the stuff (some descaler) so that could have been the issue.

I think in Australia the grades for proofs are FDC, aFDC and then anything else is just impaired, though I've never seen an impaired proof in a 2x2 to see what actual grade it has.

So it shouldn't noticeably ruin the rest of the coin? I've tried cleaning some worthless coppers before but have ended up with bright orange coins with entirely pitted surfaces. With olive oil, all sorts of things given that olive oil seems to be made up of all sorts of acids. If you were take the simpler example of vinegar which is acetic acid, acetic acid will react with the verdigris (copper carbonate) to give water, carbon dioxide and copper acetate. The copper acetate is soluble (dissolves in water) so if you give your coin a rinse afterwards it should be fine. I don't know if the copper acetate would affect a coin or not but you should probably rinse it off to be sure. If you generalise the above reaction to the acids in olive oil, the same thing should happen anyway i.e. the acid will react with the verdigris to give water, carbon dioxide and a soluble copper compound.

Not sure. Maybe the acids react with air and the solution becomes non-acidic. I'm sort of guessing here. I believe http://en.wikipedia.org/wiki/Polyethylene_terephthalate is what the plastic in flips/2x2s and if you do a bit of quick googling you'll see that PET a.k.a polyester takes forever to break down, but acid can hasten the process. Does it leave copper coins with a pitted surface?

I wouldn't think olive oil would react with plastic (my knowledge of chemistry isn't great but I didn't think acids affected plastic). I thought olive oil expired or went off though but I don't know if that changes anything.

My 2013 McDonald's Guide to Australian Coins and Banknotes gives the value of $48 AUD for a VF example, but half that would be what you'd expect to pay. Yours isn't quite VF so it'd be less than that. They're not a low mintage year either so it's not exceedingly difficult to find a nice one in a bulk lot if you're persistent.

According to this article (search for convex), some of the dies used for striking some Australian sixpences were concave or convex which biased which side of the coins suffered more wear. It says that it happened for coins struck in London so maybe it didn't just happen on Australian sixpences.

Can you post a bigger picture and a picture of the other side?

Thanks again for the advice. I'm interested in 1816 - 1970 so I'll probably look into both books. Too many books isn't a bad thing (much like coins).

Thank you, I'll look into that. For 19th century stuff is British Silver Coins Since 1816 a good reference? Thanks for the information.

Ah, thanks. Is there an up-to-date reference book that covers this? I have a copy of English Silver Coinage but it doesn't seem to go into that sort of detail.

No, probably not. I'm not familiar with all of the bronze/plated issues you referred to, but the Solomon Islands as of last year no longer has any copper coins, plated or otherwise; as of 2006 Papua New Guinea no longer had its low denomination copper plated coins and Australia hasn't had any circulating bronze since 1991 (though the bronze coins were remade for mint/proof sets in 2006 and 2010). New Zealand has had its copper plated 10c only since 2004 or 2005 though, so it might linger for a bit yet. I guess copper-nickel is the new bronze.

What obverse do the the circulation coins have?

I've never really considered it but you make a good point. With modern day mint sets the situation could be worse I guess, but looking at some bulk bronze coins I have (most of which have at least some verdigris), it could be better. Also, I think Fiji, New Zealand and the Solomon Islands at least used to have bronze coins before switching to plated.

By the look of your pictures the coin is in reasonably good condition, maybe UNC. It's a common date though so it would only be worth bullion value plus whatever premium is added for its high grade.