Sciencemadness Discussion Board

Clean ancient coins?

Xanax - 6-10-2016 at 05:19

I collect ancient coins, most roman, greek and byzantine. The most of them is from the year 200-300 A.D. My oldest coin is from the year 241 B.C.

I know that these coins is not ment to be to be cleaned, except wipping of dust and so. The bronze-coins is coated with a dark patina. And that should be there.

But I will try for one or to coins, to remove the patina, even if they are so old, they arn't wort much, maybe $2... And I have about 300 of them. But if the emperor on the coin is identified, it will be worth more. I have some from fameous emperors like Caligula and Nero, they are pretty expensive.

How do I do this on the best way? I will not use an expensive coin to this.

Then I was so stupid, I also have some silver-coins to, and try to cleaned some of them with hydrochloric acid... :( They just get blacked... They have no patina, but I'll managed to save them. They aren't wort som much either, except they are in silver, maybe $20 each.

Sulaiman - 6-10-2016 at 07:10

a common procedure is Al foil & bicarbonate electrolysis
https://www.youtube.com/results?search_query=clean+coins

diddi - 6-10-2016 at 15:27

great! I will give it a try also.

ziqquratu - 6-10-2016 at 15:52

You could try simple mechanical polishing - I did this to some old (decades, rather than centuries, mind you!) but essentially value-less bronze coins that I wanted to use for a decorative purpose. I just stuck the coin to a piece of scrap wood with double-sided tape to hold it in place, then hit it with some metal polish (I used Mother's brand on a recommendation) via a felt wheel in my knock-off Dremel. Came up remarkably well within just a couple of minutes.

diddi - 6-10-2016 at 22:52

abrasives wear the detail off the coin, so it is fine for non collectable coins, but a no-no otherwise

zed - 8-10-2016 at 16:26

Isn't this fun?

Fidelmios - 12-10-2016 at 14:57

I would love to see pics!

diddi - 12-10-2016 at 16:49

I might try some Northern Song dynasty coins. I have some spares
they are copper 64%, lead 27%, tin 9% alloy

AJKOER - 16-10-2016 at 12:48

First, try washing the coin in a dilute household solution of ammonia. Copper and copper salts will form soluble complexes with NH3.

Also, you may wish to try adding one of the inexpensive coins containing Cu and Sn to a mix of dilute household ammonia, dilute H2O2 and some sea salt. In place of H2O2, one can bubble air into the solution using an air pump (mine is from a fish tank).

Even the slow dissolution of a tiny amount of copper will form a strong royal blue solution.

Caution, a large foaming reaction and gas release may appear in the first 10 minutes so no closed vessels (will burst).

The initial solution may form some toxic NH4NO2, which should not be allowed skin contact. Wear gloves and eye protection from the ammonia. Perform outdoors.

[Edited on 16-10-2016 by AJKOER]

unionised - 16-10-2016 at 20:40

As far as I'm aware, the best advice for how to clean rare coins is "don't.
For some reason AJKOER has proposed a method for dissolving them.

(I'm a long way from sure that any significant NH4NO2 will be formed- since it's not very stable.)
Whether it's there or not, don't get chemicals on your hands if you can help it.

AJKOER - 17-10-2016 at 12:53

Here is a reference on patina surfaces with suggested mixes. To clean, very dilute H2SO4, which makes sense as the copper surfaces are coated with basic copper carbonate. To manage color, the employment of ammonium sulfate with a small dose of ammonia water and CuSO4. Link: https://www.copper.org/resources/properties/protection/finis...

Repeat my comment on the need to test these mixtures first on cheap coins.

[Edit] If you need more mixes to test, I would change my original recipe of Cu/NH3/H2O/H2O2/NaCl by replacing the NaCl with MgSO4 and filtering off the white precipitate of Mg(OH)2.

[Edited on 17-10-2016 by AJKOER]

diddi - 17-10-2016 at 13:41

perhaps you can recrystallise the coins from the royal blue solution?

unionised - 18-10-2016 at 02:36

Quote: Originally posted by AJKOER  
Here is a reference on patina surfaces with suggested mixes. To clean, very dilute H2SO4, which makes sense as the copper surfaces are coated with basic copper carbonate. To manage color, the employment of ammonium sulfate with a small dose of ammonia water and CuSO4. Link: https://www.copper.org/resources/properties/protection/finis...

Repeat my comment on the need to test these mixtures first on cheap coins.

[Edit] If you need more mixes to test, I would change my original recipe of Cu/NH3/H2O/H2O2/NaCl by replacing the NaCl with MgSO4 and filtering off the white precipitate of Mg(OH)2.

[Edited on 17-10-2016 by AJKOER]


The Op is trying to clean coins.
Patination (as described in that link) is pretty much the opposite process.
Adding an oxidant (air or H2O2) will increase corrosion of metal.

AJKOER - 18-10-2016 at 04:19

To quote from the referenced source:

"Oxide film on the copper will cause poor adherence of the patina. Copper roofs which have weathered for six months or more should have the oxide film removed before the coloring operations start. This is done by swabbing the surface with a cold, 5%-10% solution of sulfuric acid."

My understanding of the above is that prior to engaging in an application of artificial patina to a copper object, removing the existing basic carbonate layer is required for adhesion. The cleaning process can be accomplished by the application of cold dilute H2SO4, which is the point of this thread.

Next step, if deemed appropriate for the ancient coins in question, is a coloring exercise. This apparently involves the application of ammonium sulfate, (NH₄)₂SO₄ plus a little ammonia and CuSO4.
--------------------------------------------

My original idea was to react the patina coated coins in ammonia or NH3/H2O2/NaCl. However, the use of basic ammonia water or methods for forming the tetraamminecopper(II) hydroxide or slightly more acidic tetraamminecopper(II) sulfate, is not likely to work efficiently in dissolving the patina. Use of a cold dilute acid for a short time should work followed by a thorough rinsing. Pre-test the acid wash to limit damaging any coin of value.

[Edited on 18-10-2016 by AJKOER]

unionised - 18-10-2016 at 05:27

Quote: Originally posted by AJKOER  
To quote from the referenced source:
Copper ... should have the oxide film removed ... This is done by swabbing the surface with a cold, 5%-10% solution of sulfuric acid."

... The cleaning process can be accomplished by the application of cold dilute H2SO4, which is the point of this thread.

[Edited on 18-10-2016 by AJKOER]


Yes, the reference talks about dissolving the patina with dilute acid. That's a well known way to strip the oxide off copper.
The clever bit is that, with plain copper, the oxide is soluble in dilute acid- but the metal isn't because the oxidation potential is too high. So (in principle) you only dissolve the metal, but not the oxide.
On the other hand, you talked about treating them with air and ammonia.

Did you somehow think that was the same thing?
Well, it isn't.
The addition of an oxidant like air or H2O2 will enable the mixture to etch the copper.


None of this matters much, because you should't chemically clean old coins.

With old coins the oxidation is often so deep that there's not much metal left under it. If you strip away the oxide you will get a smooth blob of clean metal which is worthless.
The only remaining details on old coins are actually made of oxide (in the shape of the original metal) rather than the metal itself.

It's a bit like trying to wash the stone away from a fossil to try to get the original animal's remains.

AJKOER - 18-10-2016 at 14:46

If the problem is to bring more clarity to reading a date on an old coin, thereby greatly increasing its value, could a partial dissolution of the patina aid in this quest?

This woud be a more careful slow process that doesn't involve largely dissolving the coin.

j_sum1 - 18-10-2016 at 15:13

That which cleans it also degrades it. This is always the problem with restoration of anything ancient. Sometimes it is better not to fix your antiques at all.

If your goal is to preserve or enhance value at all, I would talk with the experts in the field -- ie, those who deal with old coins in this case. because, although we might be pretty good at the chemistry here we cannot really advise much on the specifics of looking after your investment.

Melgar - 21-10-2016 at 15:00

Actually, your goal is to plate some of the partially-oxidized metal back onto the coin, creating a gap between the coin and the patina so that the patina can come loose easier. Sodium bisulfate is what I've used, and I probably wouldn't go looking for some better electrolyte.