1921 penny detector find with strange reverse!

[chris 0]

gooday to everyone

yesterday i went metal detecting on a local piece of farm land and unearthed a 1921 penny. i put it in my pocket and thought nothing of it until i started to clean it

on cleaning i noticed the date was pressed into the coin as was the whole of the reverse of the coin.

now i know pennys and this detail normally sticks out of the coin not into it

condition of te coin isnt the best but i have included a scan of it maybe someone here can shed some light on the subject

the obverse of the coin is a little better than the reverse but seems normal

[Peckris 579]

That's quite weird. I've tried to visualise all kinds of situations, such as a brockage or someone attempting to use a normal reverse as a kind of punch to impress into a sawn-in-half penny, but either way, the reverse would be 'mirrored', i.e. back to front. And in the case of a brockage, you should have a normal reverse on the other side.

So this is a bit of a mystery.

[davidrj 194]

So this is a bit of a mystery.

This looks similar to this 1880 penny

I haven't a clue what's going on either!

[Rob 3,002]

I've had a couple like this over the years as well. The common theme is that they are all pitted and corroded, so I think the answer lies there. I have a theory that puts it down to a weakness in the metal at points where there has been relatively significant metal flow when struck. When metal sheet or whatever form is made, it has a crystalline structure. When a coin is struck there is metal flow, particularly at the points of varying relief in the design. The use of a collar means that flan spread is minimal and the radial metal flow lines particularly associated with the legend (such as tails emanating from the serifs) as seen on old coins is not present. This flow must by definition destroy the original lattice structure resulting in weakness at that point which can be more easily attacked by a corrosive substance. I think it is this we are observing as the effect is usually associated with a change in relief.

Corrosion of a homogenous material would result in an even layer of metal being removed from the surface, but if the lattice structure has faults the corrosive agent could penetrate under the surface and so you would see greater pitting at this point. In extremis, this could result in metal being attacked from all sides to the point where a small detail feature could be detached. This may not be the complete or even the correct answer, but I think is a good starting point.

I also think this explains the frequently sold 'thin flan' pieces on ebay which have probably been left in acidic solution for a period of time. 1/4mm or more of metal removal all round leaves a coin which is not appreciably smaller in diameter, blindingly obviously thinner, and also has thinner legend even though everything else looks ok. The slight diminution in the bust size would not be obvious as it would be the same as the change in diameter.

I have never seen a coin with this effect without corrosion.

Edited August 23, 2009 by Rob

[Dg43 0]

An extremely erudite and entirely plausible explanation Rob.