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FilthyBroke · January 23, 2009 4:18AM

1jester · January 23, 2009 5:07AM

That's just old crust starting to come off. Really, it looks like the old (nearly 1000 years?) patina is simply lifting off at that point. Nice coin halves!

IloiloKano · January 23, 2009 5:13AM

<< patina is simply lifting off >>

I agree. What you see under it is, in my opinion, original coin surface. But that doesn't mean I'd advise trying to flake more off.

FilthyBroke · January 23, 2009 5:17AM

<< 

<< patina is simply lifting off >>

I agree. What you see under it is, in my opinion, original coin surface. But that doesn't mean I'd advise trying to flake more off. >>

Thanks, I was wondering about that. I'll have to stop handling it.

I've had my fun, now it's time for it to rest in a 2x2 with the others.

laurentyvan · January 23, 2009 6:50AM

I yield to all previous opinions, but what wouldn't make this a planchet flaw, perhaps delamination?

FilthyBroke · January 23, 2009 7:33AM

<< I yield to all previous opinions, but what wouldn't make this a planchet flaw, perhaps delamination? >>

I don't know, I think I'm buying the peeling crust theory. And just think what all could be contained inside a thousand years of crust. Hmm....that was a little early for the Black Death, I guess.

1jester · January 23, 2009 8:54AM

Laurent, I don't think lamination defects are related to what you see here. The planchet itself is intact. As we know, the crust starts with toning, and gets darker and thicker with age, in fact isolating the underlying metal planchet from contact with the outside, and thus protecting the metal. Here you can see how clean looking the silver is after the crust has come off. Pretty neat, if you ask me. I mean also that finding such old patina is not exactly common, and to me is intriguing evidence of its originality and history. This coin in fact exhibits very little wear. I wonder what a TPG would grade it?!

IloiloKano · January 24, 2009 4:31AM

<< what wouldn't make this a planchet flaw, perhaps delamination? >>

I believe orginality of the coin surface, as well as indication this halfpenny was cut a long looong time ago, can be seen in the clean straight surface of the cut where some of the patina has flaked off the cut edge.

Presumably, the original cut was clean and straight.

Of course, I could make a better determination with the coin in hand, but take a close look below, and see if you agree with me. I have reduce the original picture by 50%, and I increased it by 50% (easier on old eyes) for the enlarged inset.

FilthyBroke · January 24, 2009 4:48AM

<< 

<< what wouldn't make this a planchet flaw, perhaps delamination? >>

I believe orginality of the coin surface, as well as indication this halfpenny was cut a long looong time ago, can be seen in the clean straight surface of the cut where some of the patina has flaked off the cut edge.

Presumably, the original cut was clean and straight.

Of course, I could make a better determination with the coin in hand, but take a close look below, and see if you agree with me. I have reduce the original picture by 50%, and I increased it by 50% (easier on old eyes) for the enlarged inset. >>

Here's another shot of the edge-

FilthyBroke · January 24, 2009 4:49AM

From this picture, you can actually see the crusty surface extend around the cut edge in places.

GDJMSP · January 24, 2009 5:33AM

I would agree that it is the coin's crust flaking off. But what has not been mentioned so far is that that crust is also a layer of metal flaking away. For that is what the crust is - oxidized metal.

Yes, toning or a coin's patina protects the underlying layers. But it is at the same time destroying the outer layer for eventually toning, if left unchecked, turns into corrosion. And that is what is happening with this coin. The outer layer of silver that has become completely corroded is flaking off.

IloiloKano · January 24, 2009 6:07AM

<< a layer of metal flaking away >>

Not as much metal as you might think.

Here an example to illustrate my point. Say you place a piece of iron outside exposed to the environment for a while, but you protect half of it from exposure. When you see heavy rust on the exposed side, it will be raised above the surface of the protected metal, and the heavier the rust, the higher it will be raised. That is because rust is iron oxide, and when the oxygen atoms combine with the iron atoms to form rust, the resulting molecule is much larger, taking way more space, than the original iron atom. I chose iron instead of silver for my illustration, because it is cheap and easy for anyone to perform such an experiment and see it occur for himself. However, silver oxidation similarly forms the silver oxide molecule, and the resulting molecule is also much larger than the original silver atom that combined with oxygen from the air to form it. There will also be a limit to how much oxide is formed, if the coin was left untouched, at some point the oxide itself will prevent further oxidation, since no more uncombined silver atoms are reachable through the crust that was formed, except in the case of the oxide flaking off of course.

It is due to the fact of an oxide being "protective" to remaining metal that a passivation technique is often applied to metals that may become exposed to a salty sea environment. Passivation is a technique to form just such an oxidized surface. In the context of corrosion, passivation is the spontaneous formation of a hard non-reactive surface film that inhibits further corrosion. This layer is usually an oxide or nitride that is a few atoms thick.

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