Hydrogen cleaning of coins???

[Gallienus]

I was wondering if anyone ever tried reducing oxidation on old coins using hydrogen? As you know this is a very powerful reductant and unlike various "silver dips" does NOT contact the coin with any liquid. Oxides of silver or copper will be turned into water vapor and I imagine even sulfides will be turned into a gas. Of course the hydrogen will have to be heated, possibly to 150F or so.

 

Of course the procedure requires more setup and care than the traditional cleaning methods which just require a bowl of Tarn-X. However given the increased value of some old coins (compared with values 100 years ago) this method may be worthwhile.

[MTHead-migration]

Yes. It failed horribly!

 

[Gallienus]

Seriously however - this seems like it would be a good concept. Note that coin cleaning is a different area than air travel & a comparison is inaccurate.

 

Of course H2 can be dangerous when used improperly, however, the procedure here (used in hundreds of chemistry classes) is to have a system in which the H2 is kept free of oxygen. The coin is inserted into a pyrex chamber & the system is purged with nitrogen before adding the H2.

 

The advantage here is that unlike the regular dipping, absolutely no metal is removed from the coin: only the oxides/sulfides/etc. are reduced and the metal is returned back to its original form.

 

N.B. I have degrees in chemistry & chemical engineering & I was hoping for a serious reply one of these years....

[ronday]

How would you hold the coin so that all parts were equally exposed? Any unexposed area would have holder marks?

[Jim Seaman-migration]

How would you hold the coin so that all parts were equally exposed? Any unexposed area would have holder marks?

 

A tractor beam?

 

But seriously ... Although you are talking only a few atoms of thickness those few atoms are the visible surface of the coin. The conversion of the coin's metals to their oxides or sulphides has disturbed the crystalline structure of the metal. Treatment with Hydrogen may well remove the Oxygen and/or Sulphur atoms but it will not restore the metal's structure. I don't think the appearance would be that of a new, untreated coin. Of course this depends on the original level of oxidation.

 

But even so, I would think it less damaging than "dipping"!

[WaxonWaxov]

wait a minute... just put the coin in Hydrogen?

 

do you need an electric current or something?

 

If all you need is H2, then you just need to put zinc into Sulfuric acid.... lost of H2.... kaboom!

[Gallienus]

Holders: well I imagine some sort of holder which would support the coin by the edges should work. I don't think something as complex as a tractor beam is needed. The coin holder can be similar to ones currently being used to dip the coin in tarnex.

 

H2 generation. I would not generate it from acid & an active metal. The reason is that there might be acid vapors which would damage the coin. One can buy cylinders of compressed H2. They look the same as those cylinders used to fill helium ballons in the stores.

 

Reaction The H2 would need to be at higher temperature to increase reactivity. Also all oxygen [air] would need to be removed from the container to prevent coin re-oxidation. Note that H2, even heated, will not explode nor burn. It takes H2 + oxygen [air] + a catalyst to have an explosion. Remove the air & there'll be no explosion.

 

Surface damage of the coin. This is the ONLY serious issue posed. If the oxidation was deep enough to disturb the surface metal then this cannot be fully restored. If only the surface was tarnished then presumably this can be restored. Still it can't be worse than sucessive stripping away of the coin's metal which is done by dipping. Even Unc (US) Seated & Bust dollars are being repeatingly dipped and this is no good for them in the long term.

[WaxonWaxov]

H2 generation. I would not generate it from acid & an active metal. The reason is that there might be acid vapors which would damage the coin. One can buy cylinders of compressed H2. They look the same as those cylinders used to fill helium ballons in the stores.

 

Yea, acid not good... you can buy cylinders of hydrogen!?! holy poop! you'd think there would be security issues with that

[Narmuriel]

This thread piqued my interest. A good friend of mine teaches chemistry at the local college. Im gonna work with him in a controlled lab environment to find out what the result is. I'll make sure to post pictures and full details.

[Narmuriel]

Bah! He wont help me, just wants to talk about it.

[Heavymetal]

My wife has been saying that my coin collection is starting to take up a lot of space. I wonder what she is going to think about tanks of bottled gases and evacuation chambers?

[MrSpud-migration]

Here is a link to a discussion on oxidation and reduction of copper using hydrogen. It even has a movie showing an oxidized piece of copper turning into red copper after treatment with hydrogen Link

[Conder101]

Movie seems to be gone. Or at least my computer doesn't want to run it.

 

I think you will find the hydrogen will need to be heated to considerably more than 150 degrees F. Probably more like 150 Degrees C or higher. I have seen the movie before and in that case the heat was supplied by the hot copper. If you are wanting to do it with coins you'll have to heat the hydrogen.

 

And while this will reduce the tarnish it will not restore lost luster, and may actually result in a duller appearance than dipping would because it will leave the reduced silver on the surfaces in a more disorganised state that will scatter light in a more random manner than the underlying surface would, and which would have been visible if dipping was used.

 

As to the problem of what to use to hold the coin to provide even and complete exposure, that isn't a problem. You could let the coin lay flat if you like and treat one side and the edge, then flip the coin over and treat the other side. Unlike dipping, prolonged exposure to hydrogen is not damaging. Once the oxidation is reduced no further reaction occurs. So as long as you expose all parts of the coin long enough to remove the toning you're fine. It doesn't matter if some areas get exposed multiple times and another just once, it won't have a mottled appearance. The only downside that I can see is that with prolonged exposure the coin will get hot and you will have to take steps to prevent it from reoxidizing when the hydrogen is turned off. This might be accomplished by having a second gas source available that can flood the coin with Argon to give it a protective inert atmosphere while it cools down. (Nitrogen might work as well. It is pretty much inert and won't react with the silver. Plus it's cheaper.)