Playing around with hydrochloric acid - cleaning my avatar coin

Some of you probably know that I like to restore ancient silver coins. Over the past few years, I've treated hundreds, both my own coins, but for other collectors as well. I prefer chemical cleaning above mechanical cleaning (which may result in scratches), though usually, some mechanical cleaning is required.

My usual process is as following:

1. Identify the material: coins with high silver content are easier than billion coins, as silver is relatively inert (i.e. not harmed) by most chemicals, while the copper of billion coins is easily dissolved.
2. Take a macro photograph of both obverse and reverse.
3. Identify the deposits: there are many types of deposits on coins, that can be sub-categorized as well. It is common to encounter more than one type. However, the most occurring depositions are:
   • Silver chlorides, or horn silver. A black to grey deposit
   • Silver sulphides. Black to grey deposits, sometimes difficult to tell apart from silver chlorides.
   • Copper oxides. Green deposit
   • Earthen encrustations
   • Iron oxides. Brown deposits
4. Define the optimal cleaning strategy
5. Predict how the coin will turn out (i.e. avoid pitting, unwanted colour differences, or other side effects).

Cleaning is relatively easy if you know what you're doing. For example, silver chlorides dissolve easily in sodium thiousulphate (anhydrate), without harming the underlying patina. Copper oxides dissolves easy in weak acids, such as synthetic citric acid.

Iron oxides are notoriously difficult however. I usually boiled the coin in concentrated solutions of synthetic citric acid, brushed it, poked it with a bamboo stick, and repeated this procedure until the deposits were gone. Depending on the amount of iron oxides, this procedure could take hours of cleaning. Also, even minor deposits required me to immerse the entire coin in the solution, resulting exposure of the entire surface to potentially harmful chemicals.

So I needed a stronger chemical that did not harm the patina, nor the silver. After reading around, I tried concentrated (30%) hydrochloric acid with dramatic (positive) results. And today, a friend of mine who works at the lab gifted me two mechanical pipettes (2-20 uL and 20-200 uL). Of course, I had to try it out, and looking for a silver coin with only minimal deposits, I remembered my Azes tetradrachm:

Note the iron oxides between the forelegs, and underneath the horse. I then aspirated a tiny amount of 10 ul (1/100th of a mL!), and placed this small droplet at the deposit:

Much to my frustration, the droplet quickly (i.e. this timelapse took about 30 seconds) 'flattened out' and did exactly what it not was supposed to do: expose the nice areas to the (quite aggressive) HCl. The iron oxides remained unchanged.

However, after rinsing the coin, and brushing it with a soft towel, the iron oxides were easily brushed away (that is, most of them).


So, some lessons learned:

• Smaller droplets are required. Next time, I'll try 2uL and see how it goes.
• Perhaps protecting the nicer areas, e.g. using vaseline

Curious to hear your replies.

 

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Just for fun, I did another, dropping only 2uL of HCl at the right side of the head, and another below the right wing:


And after.


Very subtle effect, but pleasing in hand.

 

And finally, Caracalla's ear had a minor deposit:

The HCl reacted with the silver, creating a layer of AgCl (i.e. horn silver):

... which was then removed by applying a droplet of sodium thiosulphate:

... showing that, if you know what happens, cleaning coins can be a fun and predictable experience.

 

For the iron oxides, why not just submerge in thiosulfate for an extended period? Works well enough for me.

Before:


After sodium hydroxie:


After sodium thiosulfate:

 

@Roerbakmix - WOW! this is awesome - thank you for the images and for explaining your process and especially for how to distinguish the types of deposits on the coins.

Do you have any similar process steps for billon or orichalum coins as well? I love the way you explain this.

Thanks again!

 

For billon, I just soak in thiosufulfate for several days for best results.

Before


After sodium thiosulfate


After Renaissance Wax



For orichalcum, I worked on a Trajan sestertius. I don't have a step-by-step, but it took, in order: sodium hydroxide, vinegar, lemon juice, and then sodium thiosulfate





Basically, if there is iron oxide then sodium thiosulfate will remove it with no real impact on the coin's surfaces.

 

Just swallow it and experience 'end game'........

 

@hotwheelsearl thank you!

 

Any ideas about how to tidy up this provincial of Trebonianus Gallus?


There's some gunk, probably sand that was glued on by someone in the past. I've tried distilled water, alcohol - but nothing seems to get rid of the gunk.

 

If it’s applied sand, then acetone may help.

extreme method is sodium hydroxide but I don’t recommend for anything worth anything.
I’ve had luck with simple distilled water soaks for LONG periods of time with daily attacks with metal picks.

 

I'm no expert in this type of thing, so take what I say, with a huge grain of salt. I've had some success, removing stubborn stuff, by putting a coin under a low powered microscope, and using a sharpened wooden toothpick (try this first), or using a very pointy, very sharp scalpel, to push or scrape the stuff away. However, it is very easy, to accidentally scratch the underlying green patina, exposing bare metal (I have had this happen). So you have to be careful. And you have to be lucky.

 

it is a truly great coin. Take no chances. A long soak in distilled water and picking with a wooden toothpick would be my choice.
Keep us posted on progress. I, for one, am very interested.

 

That looks more like a natural patina than one that was applied. Or whoever applied it put a big glob of gunk over his ear and made it look worse instead of "better ".

I hate putting nice bronze coins into water because water plus any chlorine equals bronze disease.

I think mechanical cleaning by an expert would be the way to go.