Proof of principle: removing iron oxides from silver coins

Iron oxides can be extremely difficult to remove. Whereas silver chlorides (AgCl), also known as horn silver dissolves easily in sodium thiosulphate, iron oxides (i.e. rust) can be rock-hard. Usually, mechanical cleaning (e.g. abrasion, scraping etc.) can do the trick, but this comes with the risk of damaging the coin. Previously, I've dissolved iron oxides with synthetic citric acid, a weak organic acid. It takes ages though, and additinal mechanical cleaning is often required.

I purchased this pitiful denarius of Severus Alexander for the sum of €8.

The coin shows three types of encrustations: iron oxides (the brown deposits), copper oxides (the green deposits) and silver chlorides (the black / dark grey deposits). At the start of the treatment, this already corroded coin weighed 1.92g.

I immersed the coin in 30% hydrochloric acid. This is a strong solution, and you should know what you're doing. Hydrochloric acid will dissolve iron- and copperoxides, but it's relatively harmless for silver of high purity.

Immediately after submersing the coin, the solution turned yellow:

This is a result of dissolved iron- and copper oxides. Note that the coin turned black, which is a reaction of the silver with the chlorides (Ag + HCl > AgCl + H+, i.e. horn silver). At regular intervals (1-2 min), I removed the coin from the solution and dropped it in a glass of tap water. Then, I rinsed the coin with tap water. This is important: if you rinse the coin under flowing water, acidic droplets may cause burns, even worse, eye damage.

A very thin layer of AgCl covered the entire coin, slowing the reaction (as the iron oxides were now unexposed, protected to the acid by a thin layer of relatively inert material). This thin layer was disolved using sodium thiosulphate. The entire procedure was repeated five times in total:


And some close-ups:"






The weight dropped from 1.92 g to 1.71 g: 0.21 g of copper, ironoxides and silverchlorides had been dissolved. In hindsight, it would have been better to first remove most of the deposits by immersing the entire coin, followed by local application of HCl at the more sturdy deposits.

Yet, it's nice to have another chemical tool in my toolbox!

 

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Wow! Dramatic difference!

 

Really spectacular results. Nice work. Thanks for sharing.

 

Great job! Congrats!

 

Great job on the clean up!
I wish I had the guts to use these kinds of chemicals.

 

Great work!

Do you think the same results could have been achieved using lemon juice instead of HCL with just longer soak times?

I think the sodium thiosulfate is relatively safe to work with, what do you think?

John

 

Excellent results, and thank you for the instructions!

 

Thanks for the comments!

In the end, lemon juice might have worked just as fine. However, it can take ages (for this coin, probably more than 2h of cleaning, compared to c. 10 minutes with HCl).

Sodium thiosulphate is perfectly safe. It requires some experience though to pick coins that might benefit versus those that don't, i.e. there is no 'undo-button' with coin cleaning.

 

Indeed! Stunning results -- congrats!

Question: Would you suggest using the same hydrochloric acid solution on bronze coins to remove deposits? I'm thinking, as for example, this Domitia from Ephesus below. (Be sure to click the image to enlarge it.)


(I'm thinking in terms of local application on the reddish deposits only.)

 

Great info.

 

One question comes to mind (I'm sure I've got others).
Would a TPG (NGC or PCGS) detect the cleaning?
One or both of them have some sort of "sniffer" for the purpose of detecting cleaned coins.

 

acids on bronze can give you a pinkish patina

 

I'm not sure what ancient coin conservators are willing to put up with, but speaking from a chemical perspective, it seems like a bad idea.

Hydrochloric acid would rapidly leach out tin and other more active metals. It would also take off the oxide layer that gives copper-based alloys their "natural" patina. So, you'd be left with a porous surface that's got more copper -- as @Victor_Clark said while I was typing , "pinkish".

 

You achieved a stunning result there--congratulations!

 

It can also entirely strip that waxy green patina sometimes seen in nice bronzes. Vinegar turned a nice smooth green tetrassarion into an ugly, rough, brown slag

 

My Ephesian Domitia and I both thank you three gentlemen!

I've unsuccessfully tried repeated LONG soaks in distilled water, followed by carefully gentle mechanical cleaning attempts under the scope. The acid approach was a clutching-at-straws idea. But I'm starting to love the coin "as is".

Although... do you think Domitia may look pretty in pink? It worked for Molly Ringwald. Of course Molly had more going for her aesthetically than did Domitia who almost rivals Cleopatra for her beauty -- if you know what I mean. ;-) (Sometimes Domitia's numismatic image looks like Domitian wearing her wig.) :-o

I digress.

Thanks again! I look forward to trying the O.P.'s process on a few silver coins with those types of encrustation.

 

I'm curious what the source of the iron might be and how it binds to the surface of the coin. Also, what test(s) can used to determine the presence of iron oxide?

 

I would try a base before an acid. Usually won’t be as destructive against patina. Then again, sometimes it’ll turn the coin into powder so....

 

I know some people use olive oil.
It's mildly acidic so nothing happens quickly but apparently gets the job done over time.
"Over time" means a month or more.
Don't know what is used to clean the olive oil off when it's done.
Acetone?
Distilled water?

 

Cleaning is not considered a sin for ancient coins.