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Bronze 1975 Png 20 Toea Coin

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 Posted 09/25/2022  12:47 pm Show Profile   Bookmark this topic Add marcpasquin to your friends list Get a Link to this Message
I was noodling through a 10kg lot of world coins and found 2 1975 PNG 20 Toea coins.

The one to the right is the usual silvery colour while the one to the left is uniformly bronze coloured (obverse, reverse and rim). I am 99.9% sure that the bronze is *not* a case of being minted on a wrong planchet but simply a case of the outer layer having being stripped.

What I'm wondering though is this: What would be required to strip *all* of the outer layer without damaging the inside ?

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 Posted 09/25/2022  5:07 pm  Show Profile   Bookmark this reply Add David Graham to your friends list Get a Link to this Reply

The 1975 20 Toea is a Cu-Ni alloy so there is no "outer layer" to strip. What you have is more likely a chemical reaction between an unknown reagent and the alloy. I have seen similar browning on Cu-Ni coins but not to this extent.
Edited by David Graham
09/25/2022 5:09 pm
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 Posted 09/25/2022  5:38 pm  Show Profile   Check NumisRob's eBay Listings Bookmark this reply Add NumisRob to your friends list Get a Link to this Reply
This coin is of exactly the same cupro-nickel alloy used for British shillings and florins and their equivalent decimal 5p and 10p coins from 1947 until 2008. I've found lots of those with my metal detector which were toned to exactly the same color as the OP's coin.
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 Posted 09/25/2022  5:44 pm  Show Profile   Bookmark this reply Add gxseries to your friends list Get a Link to this Reply
Have you checked the weight? If they are within expected range, it's a case of environmental damage of some sort
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 Posted 09/25/2022  6:25 pm  Show Profile   Bookmark this reply Add Sap to your friends list Get a Link to this Reply
As noted by others, these coins are solid cupronickel, not plated. More modern versions of the coin are plated steel, but these original 1975 versions are solid alloy, with no plating.

When a solid cupronickel coin is "coppery colour", the reason is almost always "environmental damage" - specifically, from burial in soil. Humic acids leach out of the soil and attach to the metal, giving it a soil-brown colour, which can look very similar to oxidized copper, as in the examples in NumisRob's picture. The exact colour it turns will depend on the chemical nature of the soil in question.

The "chemical stripping" thing isn't likely to happen, but might be possible: copper is a minority component of the alloy, but nickel is more chemically reactive than copper, so you'd need to find a very particular stripping agent that reacted with nickel but left copper alone, then leave the coin in that solution for long enough. I'm not sure the resultant coin would look as clean as this; I'd expect some pitting and roughness.
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 Posted 09/27/2022  1:19 pm  Show Profile   Bookmark this reply Add Princetane to your friends list Get a Link to this Reply
Also at the end of the day, most Cupronickel coinage is 75% copper and only 25% nickel, so environmental triggers can bring a change to a brownish colour.
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