Polo Mint error bi-metal pound coins!

[Coinery]

If these are genuine mint errors, are they still worth the silly money that was being paid for them 8 years ago?

Any recent sales for comparison? Best to slab if genuine?

[Paddy]

As I understand it, none of these are genuine mint errors.

The two parts of the coin are put together as blanks, before they have the design pressed into them. The minting machines would not be able to process separated pieces, so there is no way the patterns could be minted onto incomplete blanks.

It is far too easy to knock the centre out of a coin and then claim "mint error" for a premium. Even worse are the ones where they have re-inserted the centre rotated or even back to front - again all impossible in the minting process.

[Rob]

If you stick the bimetallic coins into liquid nitrogen or whatever at these typical temperatures, the centres fall out due to different rates of expansion for the two metal alloys. It would work the other way round as well, by increasing the temperature to a few hundred degrees, but would increase the risk of surface oxidation and give the game away.

[Coinery]

Many thanks, Gents, much appreciated…will inform my soon-to-be-disappointed friend accordingly 👍

[Rob]

Sorry, incorrect answer. Should read it could happen the other way round, all dependent on the metal alloys used. Whatever, the key to having a solidly held central section depends on tight tolerance machining of the two blanks and sufficient temperature divergence from those normally expected to drop the middles into the outer ring, before putting the design on. Nobody is going to align two pieces consistently to make it a plausible mint error. This was demonstrated soon after they came into circulation, with videos to prove so.

Edited October 8 by Rob
grammer (sic)