Old coin which fluoresces under short wave UV.

[sjastro]

Since copper seems to be the key issue here I was fortunate to find a 10 Yen Japanese coin which also exhibits fluorescence under shortwave UV.
The composition is 95% copper, 3.5% zinc and 1.5% tin.

Cleaning with concentrated sodium chloride had very little effect in removing whatever causes the fluorescence.
The next step involved the effect of electrolysis.
Using a battery charger and the concentrated sodium chloride solution, the coin was used as the cathode and partially submerged in the solution.
The anode was stainless steel.

The chemistry of the electrolysis of sodium chloride is shown.

The fluorescence no longer occurs where the coin was submerged.

The theory behind cleaning coins using electrolysis is when the coin is the cathode, the production of hydrogen gas results in a mechanical action of lifting the patina off the surface.
Since the mechanical action of cleaning of the coin after soaking with sodium chloride solution had no effect I’m not convinced hydrogen gas removed whatever is causing the fluorescence.

It’s possible this mysterious coating was dissolved in NaOH (sodium hydroxide) produced during electrolysis or was plated over.
Whatever it is I’m not any closer to identifying it.

 

[SelfSim]

This material seems to be in the depressed areas of the pattern, yes?
I wonder whether it might have some closer relationship with whatever is being done with the dies used to create the imprint?

I think dies are typically made from hardened steel, which apparently become decarbonised when reannealed (see decarburization). This process alters the chemistry of the steel, too:

Decarburization is the result of an improper mix of metals and chemicals in the annealing furnace where the dies are sent to soften. If there is not enough ambient carbon in the furnace, the steel will give up some of its carbon and will become soft. The result is a thin surface of inadequate metal whose chemical composition has been altered and is below standard.

I see the dies are also polished every so often (maybe using the mysterious substance .. which then becomes embedded in the coin, once minted?)

I realise this may not help much .. but perhaps the substance has more to do with the die ageing (and carbon chemistry) than with the coin's?

 

[sjastro]

This material seems to be in the depressed areas of the pattern, yes?
I wonder whether it might have some closer relationship with whatever is being done with the dies used to create the imprint?

I think dies are typically made from hardened steel, which apparently become decarbonised when reannealed (see decarburization). This process alters the chemistry of the steel, too:

I see the dies are also polished every so often (maybe using the mysterious substance .. which then becomes embedded in the coin, once minted?)

I realise this may not help much .. but perhaps the substance has more to do with the die ageing (and carbon chemistry) than with the coin's?

I agree with you the build up of the material in the depressed areas is related to the manufacturing process.
The question is in what part of the process and what is it?

 

[sjastro]

I agree with you the build up of the material in the depressed areas is related to the manufacturing process.
The question is in what part of the process and what is it?

The 2000 year old Roman coin is the outlier here.
Coin production was probably no different to the ancient Greeks.

I wonder why fluorescence occurs mainly along the edge of the coin.