Removing verdigris on silver
Barberian
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I recently purchased a 1846-O tall date seated half in VF35-EF40. The coin is handsomely darkly toned, VERY crusty, and nice in hand. The problem is that the coin is TOO CRUSTY, and has thick accumulations of crud and vertigris around the lettering on the lower half of the reverse. When I examine the coin under a good stereoscope, I can see some tiny patches where the crust has come off the coin (perhaps removed), revealing shiney surfaces underneath (probably removing the patina). The coin also shows slight porosity in some areas at 10X. Is there any way of removing this vertigris without affecting the toning at all? I have read that soaking copper coins in olive oil seems to work well. I am reluctant to try this on this coin because of its value. I am also reluctant to send the coin to NCS out of fear that they will alter the thick, crusty appearance of the coin.
People generally value a darkly toned coin with "crust". My concern is that the tarnish and corrosion process has reached the point where the coin is becoming degraded. Again, in hand, it is a sharp, handsomely toned coin. Should I just keep it the way it is, or should I try olive oil treatment or consult with coin preservation specialists?
Edited to add photos. The verdigris is visible around "half Dol."
People generally value a darkly toned coin with "crust". My concern is that the tarnish and corrosion process has reached the point where the coin is becoming degraded. Again, in hand, it is a sharp, handsomely toned coin. Should I just keep it the way it is, or should I try olive oil treatment or consult with coin preservation specialists?
Edited to add photos. The verdigris is visible around "half Dol."
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Please don't try cleaning in any form.
What is on the coin looks exactly like verdigris. It is not PVC.
<< <i>All I can say it LEAVE IT ALONE!!!!!!
Please don't try cleaning in any form. >>
That is what I have chosen to do for the time being. I may take the coin to a show and get some consultation on whether the coin's problems need to be addressed to prevent further degradation. Even then, I'm reluctant to do anything to it because it is very handsome as it is right now.
One the one hand, the coin looks nice as it is right now. On the other hand, the vertigris probably will prevent it from being slabbed by TPG and will hurt its resale value. But I fear olive oil treatment may create new problems.
Here are ebay photos of the coin. The vertigris is just barely visible in the bottom reverse photo with a 2X lens
Are those who state "leave it alone" with no explanation just parroting what we tell novices when they inherit a collection? There is a difference between a new owner polishing off that ugly tarnish with a cleaning cloth and a professional or advanced amateur removing gunk with proper conservation techniques, when appropriate, not that I'm conviced it is appropriate in this case.
--Jerry
I won't allow anybody to dip this coin because the toning is really dark and attractive now.
Found in various sources....
The fact is that, as metals age those which are alloyed, or made of a combination of two or more metals, may separate slowly into their individual metal 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 wear. 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 separate itself from the silver. The technical name for this is "precipitation of copper at the grain boundaries", which means copper coming out of the alloy at the edges of the natural crystals of the metal. This is known as crystallization of the metal, to coin collectors, though it is really just the crystals of the metal becoming visible as the copper comes out of the alloy and begins to corrode, thus weakening the metal. To clarify this point, some alloys are not stable, and, over hundreds or thousands of years, they will begin to separate back into more stable natural states.
Since the coin shown is obviously not 300 to 500 years old, and since the composition of the coin is .900 silver and .100 copper, there is not enough copper available to form verdigris which is copper acetate.
If it's green, it's more likely than not, PVC damage which is altogether different than verdigris. PVC (Polyvinyl chloride) used in older plastic coin flips, breaks down and can release Hydrochloric Acid. The plasticizers and stabilizers in the plastic react with the surface of the coin producing copper pthalates and that is what looks green on a silver coin like the one shown.
So that people learn the correct terminology, I offered the preceeding information as verdigris is simply not the right term for the discoloration on the coin.
Thanks,
Bill
myurl http://www.foundinrolls.com
<< <i> I won't allow anybody to dip this coin because the toning is really dark and attractive now. >>
Just to be clear, I agree with you.
Worry is the interest you pay on a debt you may not owe.
"Paper money eventually returns to its intrinsic value---zero."----Voltaire
"Everything you say should be true, but not everything true should be said."----Voltaire
Hoard the keys.
Now I have to sweat out trying to soak some nice coins in acetone without destroying them. Bust halfs, bust quarters, rare half dimes, large cents, old unc lincolns, key lincolns, an expensive 46-O tall date....this is just so discouraging! Are there any web sites with instructions on treating coins with acetone?
Sheesh, coins are a pain in the ass! Who was the idiot who invented the PVC coin flip?