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Verdigris

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I have some Indianhead cents that are not real valuable (thus not worth sending to NCS) that have some verdigris on them. What is the general view of verdigris? I have read that left on a coin, verdigris can continue to damage a coin's surface.

 

I have also read that olive oil is used by some to remove verdigris. Can this harm a copper or nickel coin?

 

By the way, some of the spots appear more rust-like in color.

 

Any advice would be appreciated. I know the real answer is to not mess with the coins, but I also don't want to leave them alone and let further corosion occur.

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Verdigris is a copper salt - usually copper acetate, but can be a copper hydroxide. Neither of these would come off with any kind of oil. Some forms are soluble in water so it might be possible to use hot water and rub them with your thumb to remove some of the material. If that does not work the only other option is an acid dip. Chances are the coins are pitted from the corrosion and removing the verdigris will expose the pits and they may look worse after dipping than before.

 

P.S. the copper salts will flake off with just rubbing - and that is likely why some people think olive oil, or cooking oil will remove it. In fact it's just the mechanical friction of rubbing that causes it to come off.

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Interesting post, Stev32k. I don't know much about chemistry, but what you say about verdigris makes sense in light of what I've discovered in practice. I've used olive oil and a toothbrush to remove grime and early (not hard encrusted) verdigris from coins, but I'm sure the friction had a lot to do with the success. On the same token, with the harder deposits of verdigris, it was nearly impossible to remove everything on every coin, and often if removed, the resulting pitting was evident.

 

What about electrolysis? Wouldn't that remove verdigris?

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I've never tried electrolysis, but it might do more harm that good. You would have to put the copper ions in solution which means an acid bath and that is the same as dipping. So I don't think you would gain anything and might make it a lot worse.

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I think I posed a similar question years ago and wish someone from NCS could make some comments and educate us about verdigris and whether they've had success removing it without damaging the coin and under what conditions.

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Does anyone remember that tarnish remover they sold by the ga-billions on TV in one of those get two for one at $9.95 ? Remember how it looked like a flat plate with the sides like a casserol dish , or maybe it was just a flat plate laying in a casserol dish and then they would drop an old dirty tarnished fork on it and voila! while you watched they pulled it out and the part touching the plate got all shiny looking? Did those things really work? You know, like most of that stuff never really works , but it looked like it was probably coated with some kind of reactant that when exposed to both water(it worked when it was submerged in a container of water) and a different type of metal like silver , it would cause some kind of reaction that would cause the tarnish to just fly away.

Anyone seen that thing and remember getting one to test out on an old cull or something?

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What is called verdigris is often caused by several different things.

 

Some green encrustation on a copper coin is protecting the surface of the coin. Although this could be removed, it will leave the coin vunerable to further deterioration quickly. When the encrustation appears to be of this type, NCS conservators will most likely not remove the encrustion.

 

Other green on copper coins is corrosion which can and should be removed. The coin will be more stable in the long-term and more attractive as well. Usually the coin will still be deemed "corroded" by a major third party grading service such as NGC in this case.

 

Other green can be PVC residue. This can usually be removed safely. Assuming the PVC has not begun to eat into the surface of the coin and it does not have other irreversible surface problems, the coin should be gradeable by a third party grading service.

 

Chris, NCS

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Virgin olive oil is slightly acidic, so it's the same process that makes EZ-est work, just much more slowly (and safely.) And green PVC residue is corrosion, it just never "stuck" to the coin because of the oil used in the flip making process. If you take the coin out of the flip and let the oil evaporate acetone won't work, as what acetone actually removes is the oil the corrosion is suspended in.

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To be clear, the "shower curtain" smell produced by PVC is hydrogen chloride gas. This combines with moisture in the air to produce hydrochloric acid. That, in turn, mixes with mineral oil residue used to lubricate the flip making machine. Before this concoction reacts with the coin it looks like white slime. After it reacts it looks like green slime. Mystery solved....

 

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To be clear, the "shower curtain" smell produced by PVC is hydrogen chloride gas. This combines with moisture in the air to produce hydrochloric acid. That, in turn, mixes with mineral oil residue used to lubricate the flip making machine. Before this concoction reacts with the coin it looks like white slime. After it reacts it looks like green slime. Mystery solved....

Close. The smell is not HCl gas, it is the phenol compounds that are added to the oil used to soften the PVC and make it flexible. The phenol has an aromatic carbon structure (carbon ring) that gives it the odor. The oil does absorb the HCl gas an is also hydroscopic meaning it absorbs water vapor. The HCl combines with the water vapor in the oil to create the hydrochloric acid and then the oil holds that acid in contact with the coin. The acid tends to react with the copper in the alloy producing Copper Chloride which is green in color That's why the oil turn into "green slime"

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Close. The smell is not HCl gas, it is the phenol compounds that are added to the oil used to soften the PVC and make it flexible. The phenol has an aromatic carbon structure (carbon ring) that gives it the odor. The oil does absorb the HCl gas an is also hydroscopic meaning it absorbs water vapor. The HCl combines with the water vapor in the oil to create the hydrochloric acid and then the oil holds that acid in contact with the coin. The acid tends to react with the copper in the alloy producing Copper Chloride which is green in color That's why the oil turn into "green slime"

 

Almost right.

 

Having spoken to a person that actually works on flip making machines, he assures me that oil is only used as a lubricant. In fact, even the non-PVC "safe flips" you get straight from the TPGS' have a small amount of oil in them. You can see it with the naked eye.

 

Plasticizers are alcohol-like, not oil-like (esters to be exact.) They evaporate very quickly and are what create that "new car" smell some people seem to like. Anyone who has cut hard PVC pipe will recognize the "shower curtain" smell, which is extremely noxious in comparison. The two are completely unrelated odors.

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