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 Replies: 32 / Views: 4,543 |
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Pillar of the Community
United States
3343 Posts |
I carry junk silver coins in my pocket. After a short period of time the coins start forming ugly black spots on the faces. They can be rubbed or scraped clean with a fingernail, but start forming spots again once back in the pocket.
I carried circulating silver coins back in the 1960's and never saw this. I wonder whether the coin silver alloys break down with age. "Two minutes ago I would have sold my chances for a tired dime." Fred Astaire
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Pillar of the Community
Canada
1747 Posts |
I would think more likely the issue is what you have in your pockets with those silver coins, along with what chemicals have been used in the making of your pants.
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Pillar of the Community
United States
1601 Posts |
 with Ace. I don't carry a lot of pocket change with me but do have a pocket piece (or six) that I carry in my left pocket. When I have other things in that pocket, material invariably "rubs off" on the coin. That's why you can use a nail to remove the spots. Tarnish tends to happen when the silver is in the right heat/humidity conditions in a box or album. That's my experience, anyway.  |
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CCF Advertiser
United States
1533 Posts |
I can think of no better high humidity, high heat environment for toning than in somebodies pocket during the summer. I think the toning would compete with the wear in that situation.
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Pillar of the Community
 United States
3343 Posts |
I took the pocket variable out by putting the coins in a leather pouch. No difference, the spotting still occurs. I threw a couple junk gold coins in with the silver and spots appeared on the gold. I took the silver out and the spotting on the gold disappeared. Here's what it looks like. As you can see this is not toning - more like flattened tarnish balls. This quarter developed its tarnishy glob/spot buildup in a day of pocket contact with other old silver. And on well-worn smooth surfaces. It doesn't matter what country of origin. Spanish, French, US, UK, Netherlands are all the same.  The age of the coins appears to be a major factor, which is why I suspect metallurgy. Maybe microcrystallization or slow reduction reactions with sulfur over a century weaken the coin surfaces. I'll try it with fresh bullion rounds and see if there's a difference. "Two minutes ago I would have sold my chances for a tired dime." Fred Astaire
Edited by thq
07/21/2016 8:12 pm
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Pillar of the Community
United States
7390 Posts |
Are you an auto mechanic? This is strange
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Pillar of the Community
United States
3343 Posts |
Not recently @cascade. Not many people carry 200 year old silver coins in their pockets, and I don't have anyone to compare notes with. It's more irritating than anything, because the coins get so ugly. I also collect old spoons and see a similar tendency of coin silver spoons to tarnish rapidly.
"Two minutes ago I would have sold my chances for a tired dime." Fred Astaire
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Pillar of the Community
United States
7390 Posts |
I don't think it's tarnish. It would wear off in the pocket. Looks more like gunk to me but the repeatable nature of what you're describing is strange. Other people don't get that happening with their pocket pieces or we would have past threads about it. So the variable is you imo. It's gotta be something you're doing or maybe your local atmoshoere. I don't know but I'm curious. Send badthad a message with a link to this thread.let's get his scientific opinion
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CCF Advertiser
United States
1533 Posts |
I hope you're not breathing whatever it is that is depositing on that coin.
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Pillar of the Community
United States
3343 Posts |
Here's what may be going on. The second important thing that happens as metals age is that alloys may separate. In other words, those metals that are alloyed, or made of a combination of two or more metals, may separate slowly into their original components. An example of this is ancient silver coins, which become brittle. Silver used in coins is almost always a combination of silver with about 1.5 to 15% copper. Adding a little bit of copper to silver makes the normally soft silver harder, and more resistant to wearing down. Silver and copper don't really mix all that well, however, and over time (300-500 years or more), at normal temperatures, the copper will sometimes begin to again separate itself from the silver. The technical name for this process is low-temperature migration or precipitation of copper at crystal grain boundaries. The 'grain boundaries' part refers tot he fact that copper coming out of the alloy tends to migrate to and accumulate at the edges of the natural crystals within the metal. This is known, to coin collectors, as crystallization of the metal, although it is really just the crystals of the metal becoming increasingly visible over time as the copper and corrosion products accumulate. The effect of all of this is that the metals becomes weak or brittle.http://robertbeauford.net/cleaning_ancient_coinsWhen this gunk is rubbed off it blackens my fingers the way tarnish does when silver is polished. What is remarkable to me is the ability of the coins to continue generating the gunk. If the copper in the alloy is separating this could provide a source for plenty of fresh gunk. XRF of the deposits would confirm what it is. In other sources I read about serious embrittlement in 400 year old coins, to the point that the coins broke like glass. I expect that my dirty coins would show low ductility compared to fresh metal, but I would have to subject them to destructive MOE testing to find out. "Two minutes ago I would have sold my chances for a tired dime." Fred Astaire
Edited by thq
07/23/2016 08:39 am
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Valued Member
United States
403 Posts |
You're conducting an experiment few have tried or want to try. It's certainly possible that the microcrystalline nature of the silver alloy has slowly changed to the point that it's more susceptible to environmental reaction. Have they been subjected to any heat or stress in the past? What did you do with the coins on laundry day?
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Bedrock of the Community
United States
20753 Posts |
I would suspect it's the material of the pants long before the coins. Way back in the 50's and 60's clothing material did not go through all the stuff they do today. And also, you may want to check where your pants were made. If in China, you could have material made from almost anything.
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Bedrock of the Community
United States
19935 Posts |
Strange, clean it off and leave it at home for a day exposed to open air. More than likely, it's something in your pocket. The migration of atoms occurs VERY slowly, we're talking in terms of years. That's not what this is.
Lincoln Cent Lover!VERDI-CARE™ INVENTOR https://verdi.care/
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Pillar of the Community
United States
3343 Posts |
I've done that multiple times @badthad. The spots come right back. I even switched to a brand new leather coin purse and got the same effect. In every case some of the coins in my pocket mix are over 200 years old. 300 year old Spanish cobs and Dutch rijders, a 400 year old Charles I half crown, they show the same thing as the 1807 quarter in the photo. Gold coins don't do this, but old copper is as bad as the silver.
@carl my pants are made in the USA. They're getting harder to find, especially the jeans. The leather coin purses were made in China though...and for $5 each they're not made of rare water buffalo hide...got them to save the pocket linings which get worn through by coins and keys...
All the old silver coins are out of the pouch, replaced by shiny AU silver: a French 1965 5F and a 1964 Canada dollar.
"Two minutes ago I would have sold my chances for a tired dime." Fred Astaire
Edited by thq
07/23/2016 3:30 pm
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Pillar of the Community
United States
3343 Posts |
The composition of tarnish on old silver coins has been studied by conservators. Here's one study on 1200 year old Islamic silver.
Corrosion%20behavior%20and%20preservation%20of%20Islamic%20Silver%20Alloy%20Coins.pdf (this won't open as a link - Al-saad is the author)
The SEM/EDAX and XRF of the corrosion surprised me. Silver is the main component, but after that oxygen, silica and chlorine. Copper and sulfur are relatively minor components. There is also a lot of surface microcracking.
After 2 days the shiny silver coins in my pocket are still bright and shiny with no deposits whatsoever. 50 year old coins don't seem to be a problem.
"Two minutes ago I would have sold my chances for a tired dime." Fred Astaire
Edited by thq
07/25/2016 11:46 am
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CCF Advertiser
United States
1533 Posts |
Do you have any of the material you scrape off of the surface? If you can send it to me, I'll run an SEM/EDAX on it.
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Replies: 32 / Views: 4,543 |
Posted 07/21/2016 11:23 am
