discolored gold plating

[kurtak]

Ok - the other day I went in to a local coin dealers shop that I do business with to buy any gold plated sterling he had & part of what I got was these gold plated 925 medallions - as you can see some of them have this discoloration like you see on the one on the right - at first I thought maybe it was because the plating was a very thin "flash" plating but as you can see from the foil in the beaker its not & in fact the plating is heavy enough to hold the full shape of the medallion after dissolving away the silver --- in fact it took quit a long time before the "hot" nitric even worked through the plating & started reacting with the silver - like an hour to hour & a half

So - does anyone know what "causes" this discoloration of the gold --- I asked him but he had no idea what causes it - only that in the coin dealer business they call it "carbon spots"

Not that I really care because I bought it as sterling for processing & the gold is a bonus anyway so I am just wondering what is causing the discoloring as its the first I have seen of it like this

Kurt

 

[resabed01]

Looks like someone took a propane torch to it. Maybe as a test to see if it was pot metal?

 

[goldsilverpro]

No nickel diffusion barrier maybe - on the rainbow one?

 

[Grelko]

Looks like someone took a propane torch to it. Maybe as a test to see if it was pot metal?

It does look like someone tried to burn it, maybe trying to melt it or even temper it.

 

[FrugalRefiner]

No nickel diffusion barrier maybe - on the rainbow one?

That's what I was thinking. Without a diffusion barrier, the gold will migrate into the silver, and the silver will migrate into the gold. It looks like classic rainbow toning on silver coins. So the silver may have migrated to the surface where it toned. But that's just a big guess.

Dave

 

[kurtak]

I can say for sure that they did not have a torch put them as they were still sealed in there "original" packaging

Kurt

 

[goldsilverpro]

I've seen that same looking discoloration several times on coins sealed in plastic. I'm thinking the coins were silver.

Maybe your coin is from a combination of Ag migration and plastic outgassing.

The gold does look thick. Vermeil is 2.5 microns (100 micro") gold plated on sterling. If so, and the medallion is 2" dia, that's ~$7 in Au, 0.2g.

 

[Grelko]

Without a diffusion barrier, the gold will migrate into the silver, and the silver will migrate into the gold.

I've read about this, but haven't seen it. Learning something new everyday.

 

[jason_recliner]

I've seen that same looking discoloration several times on coins sealed in plastic. I'm thinking the coins were silver.

Maybe your coin is from a combination of Ag migration and plastic outgassing.

The gold does look thick. Vermeil is 2.5 microns (100 micro") gold plated on sterling. If so, and the medallion is 2" dia, that's ~$7 in Au, 0.2g.

Not gold, but I can attest to this in aluminium bronze Australian 'uncirculated' coins, shipped from the mint, and kept in their original PVC sleeves. While not as dramatic as the example shown, after 20 years they have taken on uneven pink/bluish colours and are quite considerably less lustrous than circulated coins that were merely cleaned for finger grease and tossed into a box.

Ironically it appears that the sleeves intended to protect them has caused the damage.

 

[FrugalRefiner]

PVC + coins = green slime.

Dave

 

[macfixer01]

Even solid (non-plated) .900 or .917 gold coins stored in protective sleeves can get spots or toning which is generally attributed to their copper content. In theory toning shouldn't be able to occur with .999 coins but the older I get the less inclined I am to say never?

https://www.cointalk.com/threads/do-gold-coins-tone.68524/

Macfixer01

 

[Geo]

It looks like it has been heated. As stated before, someone may have tried to melt them thinking a small propane torch would do it. I have seen those colors on a lot of different metals and all had been heated.

 

[kurtak]

Thanks for the replies guys

I got 16 of them & they weigh 40 grams each (925) & they are now in a beaker of nitric - had to score them with a knife so the acid can better get to the silver

Geo - they were definitely not heated - they were still in the original packaging

Chris (GSP) they are 1.75 inch diameter --- looks like its going to be a nice little gold bonus - he (the coin dealer) said he has more but needs to find them - I am getting them for 90% of the 925 silver content so with the gold plate not a bad deal

macfixer - thanks for the link it was an interesting read

I guess the part I don't get is why no Ni barrier to prevent this problem on this type of product ? --- does Ni not plate well to silver ? --- & its kind of interesting that the problem seems to be more prevalent on coins/medallions that are housed in plastic cases ----- Edit to add; can only assume that is caused by off gassing of chloride from the PVC

Kurt

 

[goldsilverpro]

Even solid (non-plated) .900 or .917 gold coins stored in protective sleeves can get spots or toning which is generally attributed to their copper content. In theory toning shouldn't be able to occur with .999 coins but the older I get the less inclined I am to say never?

https://www.cointalk.com/threads/do-gold-coins-tone.68524/

Macfixer01

I think I've seen this on Kruggerrands, which are .917.

 

[g_axelsson]

I think pure silver can easily be toned by sulfur containing components.

I have seen coins sitting in a case getting toned like this on only one side.

What the toning consists of mostly is a thin oxide layer, but other surface films can produce the same effect. It just have to be close to a quarter of a light wavelength to create this effect. One part of the light is reflected in the surface of the film, the other between the film and the underlying material. When it mixes it creates a destructive interference if it's off by a half wavelength. This effect is most commonly seen as the rainbow colors of thin oil on water.

For this medal I guess it's either diffusion or remnants from a bad plating that have reacted with the packaging.

Göran

 

[macfixer01]

I think pure silver can easily be toned by sulfur containing components.

I have seen coins sitting in a case getting toned like this on only one side.

What the toning consists of mostly is a thin oxide layer, but other surface films can produce the same effect. It just have to be close to a quarter of a light wavelength to create this effect. One part of the light is reflected in the surface of the film, the other between the film and the underlying material. When it mixes it creates a destructive interference if it's off by a half wavelength. This effect is most commonly seen as the rainbow colors of thin oil on water.

For this medal I guess it's either diffusion or remnants from a bad plating that have reacted with the packaging.

Göran

Yes the color effect looks much like Dichroic filters do, which are made from layers of different metallic oxides deposited on glass.

 

[kurtak]

Thought I would go ahead & post some pics of the foils after dissolving the silver in nitric - I am going to wait on processing the foils in the hope that the coin dealer I got them from will find some more of them for me to add to this first

Kurt

 

[g_axelsson]

Make a snow globe! 8)

When I was young and started my gold collection I used nitric to dissolve the base metal out of some contacts. I save the foils in a bottle together with water. For 20 years I had that bottle in my shelf and I went there now and then to give it a shake.

Göran

 

[kurtak]

Make a snow globe! 8)

Göran

These foils are to large (size wise) & to heavy (thickness wise) for that :mrgreen:

Kurt

 

[jason_recliner]

Make a snow globe! 8)

Göran

These foils are to large (size wise) & to heavy (thickness wise) for that :mrgreen:

Kurt

Don't you hate it when your gold is just too big and heavy?