Thread: My Understanding (Flawed?) of Cleaning a Coin by Dipping

Okay, please poke holes in my understanding and correct me.

Toning of silver coins is most often because of sulfur.
Sulfur and silver combine rather easily to form silver sulfide; that's the black we've used to seeing on silver.
When a silver dip is used to remove the toning it removes the silver sulfide.
And that's where the problem is.
Note I said "it removes the silver sulfide", NOT "it removes the sulfide".
So some of the silver at the surface of a coin is removed.
And the removed silver comes from the microscopic flow lines formed when a coin is struck.
It's those microscopic flow lines that gives a coin with original surfaces its hourglass reflectivity.
That's why a coin dulls down after being dipped.
The pros that have practiced a lot use a quick dip in a diluted solution to clean their coins.
Then a total bath in distilled water to stop the cleaning process.
Many times that was undetectable by the TPGs (or ignored in the case of many 19th century coins since a high percentage have an old cleaning that has retoned naturally).

But with the new PCGS system I'm not sure that a light dip is undetectable.

The thing you have to understand is that dipping a coin, correctly, is 100% acceptable in the coin market. There is nothing wrong with it !

For some reason that is the concept that a lot of people just can't seem to grasp. There are literally millions and millions of dipped coins out there and they are in NGC and PCGS slabs. The TPGs have no problem with a coin being dipped - as long as it is done properly. Even NCS dips coins. The only time there is a problem is when the dipping is done improperly !

I simple terms a coin dip is nothing more than an acid solution. And if you dip your coin in that acid correctly then the acid will wash away whatever is on the very top layer of the metal. But that is on a microscopic basis. There will be no detectable weight difference. There will be no detectable difference in the luster of a dipped coin and a freshly minted coin.

Luster is the entire issue when you are talking about dipping a coin. Your goal in dipping the coin is to make that luster visible again because the luster is being hidden by heavy toning. But you have to realize something, there are 2 things that can destroy the luster you are trying to make visible again. The toning itself, and the acid you are dipping the coin into.

You see, only some coins are even capable of being dipped correctly. Those are the coins where the toning has not destroyed the luster yet. If the toning has destroyed the luster then the damage is already done and there is nothing you can do about it. The problem is, you don't know if the damage has been done or not. To find out you have to dip the coin.

If the luster has not been destroyed, then by correctly dipping the coin the acid will wash away the toning and make the luster underneath the toning visible again.

But even if the luster was not destroyed by the toning, and you leave the coin in the acid solution for even 1 second too long, then the acid itself will destroy the luster that was still there underneath the toning. It's a double edged sword, you can lose either way.

That's why people think that dipping is a bad thing. Dipping isn't bad, only dipping for too long is bad.

And toning isn't always good, toning for too long is just as bad as dipping for too long is bad. That's because both things will destroy the luster on a coin.


edit -

As for the new PCGS system. No, it cannot detect if a coin has been dipped or not. What the PCGS coin sniffer does is detect substances that are physically on the surface of the coin. Dipping a coin properly does not leave any substances on the surface of a coin. So there is nothing to detect.

Originally Posted by kanga

When a silver dip is used to remove the toning it removes the silver sulfide.
And that's where the problem is.
Note I said "it removes the silver sulfide", NOT "it removes the sulfide".
So some of the silver at the surface of a coin is removed.

I'm not following you. Silver sulfide is a compound made by combining silver and sulphur (two parts silver to one part sulphur - Ag₂S). If you take the silver out of silver sulfide you are left with sulphur. Am I missing something here?

Tarnish (toning) forms when sulfur combines with silver on the surface of a coin. When you dip the coin, it removes both -- the sulfur, and the silver that was chemically combined with it.

Doug indicates that the dip will also remove uncombined silver. I'm a little fuzzy on that, based on a quick skim of a relevant section in Coin Chemistry; it seems reasonable, but it also seems reasonable that an agent could attack silver sulfide without attacking the silver. It'll be easy enough to check -- take a lustrous coin without toning, let it soak in dip for a while, and see what you get (comparing appearance and/or weight). I just haven't been able to bring myself to do that to a lustrous silver coin, because I'm pretty sure Doug's right.

I've wondered about using low-pressure hydrogen plasma to remove the sulfur and leave the silver behind, but (a) the hydrogen might adsorb on the metal, damaging its finish, (b) the reduced silver might not look anything like the original finish, and (c) I don't want to blow up my garage.

Do it with any lustrous coin you can get from pocket change Jeff, you'll see I am right soon enough.

And as I stated, yes, you can remove toning without having the dip destroy the luster underneath the toning. It's merely a question of the amount of time that the coin is exposed to the acid. Make it a short enough period of time and the acid removes the outer layer and leaves the underneath layer alone.

But leave the coin in the dip long enough, and the dip will completely dissolve the coin.

With a good neutralization, dipping is undetectable to the sniffer -- there's nothing left to sniff.

Otherwise, I think your post is dead on accurate.

I've wondered about using low-pressure hydrogen plasma to remove the sulfur and leave the silver behind, but (a) the hydrogen might adsorb on the metal, damaging its finish, (b) the reduced silver might not look anything like the original finish, and (c) I don't want to blow up my garage.

This has been done in semiconductor manufacturing with copper to eliminate CuO and other oxides, but I think that sulfide removal would require too high a energy activation that would require quite a bit of damaging heat. In fact the thin layers they typically work with seem too thin to be considered even on copper cents. IMO.

Thiourea-based "dip" like E-Z-Est, or its former name JewelLuster, works by breaking up tarnish (silver sulfide) and leaving the silver on the coin while the sulfide goes into the air as hydrogen sulfide (rotten egg smell) The product does not work by the "acid eating away tarnish and metal" as often repeated on coin discussion boards. A very mild acid is used simply because the chemical reaction works only in an acid environment. Repeated tarnish-dip-tarnish-dip cycles changes the arrangement of the outer atoms of silver, dulling the luster a bit with each cycle (even though no net silver is REMOVED) thus the eventual deeming of the coin as OVERDIPPED, white but lifeless.

We shall have to agree to disagree Frank.

W White has run tests that show that repeated dipping does eventually result in a detectable reduction in the weight so some of the silver is carried off.

Also when the silver sulfide is disassociated the sulfur does combine with the H+ to form Hydrogen Sulfide, but some of the Ag+ ions also go into solution. That's why if you dp a bunch of silver coins and then dip a copper coin in the same solution it is possible to deposit a thin silver wash on the copper coin.

Originally Posted by GDJMSP

We shall have to agree to disagree Frank.

Those are two different chemical reactions you two are discussing, although in Franks case, thiourea is a sulfide compound as well. I'd need to see what the exact reaction was, but a sulfide can't remove another sulfide. It is possible that a redox reaction is happening.

Originally Posted by Frankcoins.com

Thiourea-based "dip" like E-Z-Est, or its former name JewelLuster, works by breaking up tarnish (silver sulfide) and leaving the silver on the coin while the sulfide goes into the air as hydrogen sulfide (rotten egg smell) The product does not work by the "acid eating away tarnish and metal" as often repeated on coin discussion boards. A very mild acid is used simply because the chemical reaction works only in an acid environment. Repeated tarnish-dip-tarnish-dip cycles changes the arrangement of the outer atoms of silver, dulling the luster a bit with each cycle (even though no net silver is REMOVED) thus the eventual deeming of the coin as OVERDIPPED, white but lifeless.

BTW - any acid will remove some of the silver either as a silver nitate or silver chloride or silver sulfdie. That silver is disolving baby.....

Originally Posted by mrbrklyn

BTW - any acid will remove some of the silver either as a silver nitate or silver chloride or silver sulfdie. That silver is disolving baby.....

Silver dissolves readily in nitric acid, but does not react with sulfuric acid. In any case, coin dip has about 5% sulfuric acid, about as strong as vinegar.

Silver ions in the dip are not from the reaction of removing tarnish, but from reactions by foreign ions.

From Metallurgical Transactions B, Volume 21, Issue 3, pp.419-427

The rotating disk technique was used to study silver dissolution with thiourea as a function of sulfuric acid, ferric sulfate, and thiourea concentrations. The effect of many foreign ions (Mn, Cu, Co, Ca, Na, etc.) and various additives was also examined. The dissolution of silver was zero order with sulfuric acid, first order with ferric sulfate, and second order with thiourea. Among the foreign ions, copper had a dramatically negative effect. The strong oxidants such as hydrogen peroxide and manganese dioxide were also detrimental for silver dissolution. According to the temperature effect studied (5 °C to 35 °C), the activation energy was 22.6 kJ/ mole. Silver does not dissolve with thiourea in the absence of ferric ions. Sulfuric acid does not participate in the dissolution reaction. The most important parameter for silver dissolution is the ferric sulfate/thiourea ratio. In excess ferric sulfate, a solid silver-thiourea complex is formed, which precludes transfer of silver into solution. In excess thiourea, the free thiourea reacts with formed solid silver-thiourea complex, and silver goes into the solution, predominantly as the dimers of AgTU+ 3 complexes. The solid silver-thiourea complex in question was characterized by various spectroscopic, microscopic, and chemical analysis techniques. According to chemical composition, it corresponds to Ag2SO2·3TUH2O compound.


It would appear that dipping silver war nickels and "golden" dollar coins, which contain manganese, contaminates the dip causing silver ions to be in solution

Originally Posted by GDJMSP

We shall have to agree to disagree Frank.

Disagreeing doesn't make the chemistry work any different.

Silver dissolves readily in nitric acid, but does not react with sulfuric acid. In any case, coin dip has about 5% sulfuric acid, about as strong as vinegar.

Silver ions in the dip are not from the reaction of removing tarnish, but from reactions by foreign ions.

Silver does react with HCl and H2So4 acid...

and Vinegar is Acetone...and I have a PhD in this stuff. And Thio groups of Sulfide groups with some oxidation capability which is often part of drugs, especially older drugs.

Ruben