harlequin discolouration

[vitruvius 0]

Firstly, my qualifications ... None at all.

I have been given a plastic box full of metal discs.

Most of them are coins; most are U.K. or Commonwealth(pre and post decimal).

Some are foreign ... and some are filling station 'medallions'.

I'm not after a valuation, nor do I want to sell them or clean them.

It's just scientific curiosity

The mystery is this.

The 'silver' coins have become coated in copper and the 'copper' coins are coated in silver.

(Not all of them and not all completely; thus my harlequin allusion)

Can anyone explain how this happens?

[bagerap 706]

First question has to be where and how were they stored? It isn't unheard of for a form of electrolysis to occur naturally given the right circumstances. Salts may have been exchanged

[Peckris 579]

Firstly, my qualifications ... None at all.

I have been given a plastic box full of metal discs.

Most of them are coins; most are U.K. or Commonwealth(pre and post decimal).

Some are foreign ... and some are filling station 'medallions'.

I'm not after a valuation, nor do I want to sell them or clean them.

It's just scientific curiosity

The mystery is this.

The 'silver' coins have become coated in copper and the 'copper' coins are coated in silver.

(Not all of them and not all completely; thus my harlequin allusion)

Can anyone explain how this happens?

Can you post some pictures for us? It could have been done deliberately, but it's very unlikely to happen as a result of coins rubbing together in a tray, even over a period of time. For example, it could have been done by someone experimenting with chemicals or maybe an electrolysis kit, something like that.

You could just take one snapshot of a few examples side by side, and edit it down (you only have 150k per post to play with here), or post it to somewhere like Photobucket and post the link here.

[Rob 3,002]

Copper, silver and gold being in the same group of the periodic table share the same basic atomic structure. If you have one metal plated on a different metal of the same group, over time there will be some migration of the two materials into each other under the right conditions. You would not expect this to happen on a large scale and quickly however. If the area of contact was great enough, then it might happen, but coins are hardly a flat surface and so the actual contact points are minimal in relation to the surface areas of the two objects. A solution based explanation seems far more likely.

At a bimetallic junction, a voltage is generated - that's how thermocouples work. Voltage generated, circuit made and current will flow, allowing for the migration of atoms within the crystalline structure.

Edited July 3, 2012 by Rob

[Generic Lad 25]

It might be possible what you are seeing is PVC residue, many plastics contain PVC and it will damage coins, but it usually turns up "greenish" rather than silver/copper.

Perhaps it isn't PVC damage but the plastic could hold the key to what caused this.

[Peter 615]

It might be possible what you are seeing is PVC residue, many plastics contain PVC and it will damage coins, but it usually turns up "greenish" rather than silver/copper.

Perhaps it isn't PVC damage but the plastic could hold the key to what caused this.

A drop of olive oil and a quick acentone dip will sort that.