Metal Detecting Articles:

Electrolysis of Dug Coins

njminerals.org is mainly about (what else?) mineral collecting, but it seemed fitting to put some metal detecting stuff up here as well... this little sub-section may expand if time allows. 

There is something about finding things that has an almost universal appeal.  This is especially so for people who already collect stuff!  A mineral collector, for example, can easily cross over into gold prospecting, metal detecting, relic hunting, and other hobbies.

Over the years I've done a little metal detecting, at least when I'm not searching for rocks and minerals.

Coin digging

Twenty-odd years ago, the detector brochures used to show people with a blanket laid out in front of them where they'd spread out like 2,000 silver coins.  Or you'd see the photo of some guy who found a gold sovereign his first time out, under some kid's swing set in a park that he somehow luckily guessed was 100 years older than everyone thought possible.

Well, in my first few years of detecting, I found mostly clad (modern) coins and a lot of junk.  Pulltabs, foil, nails, and bottle caps.  As of the time this article was first put up on the web, I'd ever found maybe 5 silver coins (junk silver, common dates), a couple Buffalo Nickels, and a badly corroded 1858 Flying Eagle Cent where you could barely, and I mean barely, read the date after squinting at it really hard.  I was pretty psyched about that find, though.

The fact that I hadn't found a whole lot of great stuff was probably due to (1) my going to sites that had been pounded thoroughly by other detectors, and (2) my lack of patience when I didn't find the 2,000 silver coins I saw in the brochures. 

I did find a single jack, though:  the toy kind that you pick up when you bounce a ball.  It was rusted to pieces and all by itself, missing the rest of the jacks.  It lay buried almost a foot deep at an old house site.  Have you ever played jacks?  This one wasn't much good for that.  It crumbled into dirty red flakes in the box where I stored it.  (An article on preserving iron relics may eventually appear on njminerals.org).   This latter find, made with a White's Eagle II SL, suggested to me that I simply wasn't looking in the right places.  The depth capability was there;  I began to suspect that operator skill had been the problem.

Of course, I found later on that it sure didn't hurt to have a newer machine!

Anyhow, digging up coins brings us to an important debate that still rages on the forums:

Should you clean your coins? 

You should not clean a coin if it's one of the valuable dates.  You should probably not clean any coin that's even somewhat collectible, except in special circumstances.  The most radical thing you should do to a good coin is soaking in distilled water or maybe an extended soak in oil (which can take a long time). 

Many beginners to the hobby have ruined a lot of good coins by taking metal polish, buffing wheels, sandpaper, or steel wool to them.  The scratches these cause will ALWAYS show up with sufficient magnification, and an experienced collector will often be able to spot such harsh treatments at a glance.  Buffing, polishing, or abrading a coin cuts the value in half, sometimes worse.  An honest dealer will always disclose this kind of treatment. 

Same thing with acids:  vinegar, muriatic acid, you name it;  these are destructive treatments and should be disclosed if for some reason they've been used on a coin.   They dissolve some of the metal and take away fine detail.  Lye will also attack most metals, though in another article perhaps we can explore the limited uses of lye solution for cleaning coins.

Some of the coins you dig up are either so badly corroded or are of such common date that they aren't worth much in the first place... so what about them?   I doubt anyone would care if you overcleaned a common-date Wheat Cent.   I'm not saying to turn your dug coin into a shiny disc, however.  Sometimes dug coins have character that goes beyond value, even if the coin "books" for only 25 cents.  If, however, you can read the date and the coin has a nice patina, leave it alone.

Most clad coins are not likely to have collector value in the foreseeable future, so you can go ahead and scrub 'em till your heart is content.  Just watch you aren't ruining any Doubled Die coins or anything like that.

One more thing:  If a coin is really dark or pitted BUT it's a rare date, don't clean it yourself.  Have a numismatic conservator look at it.  If it is something really valuable it could be worth the money to pay a professional restorer to work on it, or else just leave it alone completely.  A few coin dealers are a little sketchy and might tell you to clean a valuable coin with Tarn-X or Nevr-Dull or something like that.  DON'T do this to a coin unless it's already of little value the way it is.

One method advanced for cleaning dug coins and relics is electrolysis.  This works very well for iron relics, which are often extremely rusted to the point they are unrecognizable.  A little while ago, a couple guys posted some before-and-after pictures of dug Parrott shells from the War Between the States (link to their site is broken now).  The as-found state was so ugly, I doubt even the most obsessive purist would have wanted them.  A rusted blob, where you can't even tell what the object is, simply is not very appealing.   Electrolysis made the dug Parrot shells look quite nice.

Anyway, you use a car battery charger as the power source.  Sodium carbonate or sodium bicarbonate solution is going to be the electrolyte (WARNING:  it is YOUR responsibility to do it right.  I don't want to hear about it if you hurt yourself or cause a fire.)  Your setup should be in such a place that nothing can catch fire if the whole arrangement should fall over, short out, etc.   Read that last bit again... it's important.

Electrolysis:  further considerations... If you hook up the electrodes backwards, you will cause the coin to be eaten away.  Remember, Oxidation occurs at the Anode.  Hooking a metal coin to the Anode will cause the metal to dissolve into solution. If you hook up the electrodes properly but have dissolved metals in the solution, they could plate out on your coin and ruin it, too!  Stainless steel anodes will put some iron and chromium ions into solution, but normally there has to be very high current to get chromium to plate out on something and stay there.  UPDATE regarding iron (Fe):  having tried many times to get iron to "stick"... it doesn't seem to.  The alkaline solution seems also to turn Fe++ into rusty iron compounds that precipitate in short order. Though iron is sufficiently low on the reactivity series to allow plating-out (in theory), it evidently doesn't adhere well.  You still probably shouldn't run a piece of copper or silver in a solution that has iron compounds in it, though.  For all you know, you could have just discovered the one, magical combination of factors that enables plated iron to form a layer on your substrate... Certain other metals, such as nickel and zinc, will almost certainly plate out on the coin if they're present.  You will probably never get it off of there once it plates out on the coin.  Copper salts are the worst in this respect-- never use a copper wire as the anode.  Contrary to popular belief, you can use a copper wire at the cathode.  The copper will not go into solution when it's the cathode.

  
If you use a piece of carbon or graphite for the anode,  you won't have to worry about the whole issue.  Aluminum also cannot plate out onto your coin from aqueous solution, because aluminum is too active a metal.   (However, you should always test a setup on a few "junk" coins, just to be sure it isn't going to cause some unforeseen quirk that alters the surface appearance of the object.)

Anything with a reduction potential more negative than the following

2H₂O + 2e^- <-----> H₂ + 2OH^-   (E₀ = -0.8277)

is expected not to be able to plate out from water solution, because the decomposition of water into hydrogen will take precedence.  Thus, you cannot plate out titanium, aluminum, magnesium, sodium, etc. from aqueous solution.

All this talk of anode and cathode might be confusing to the non-chemistry person, so here's the deal: 

You want the coin hooked up to the NEGATIVE terminal (that's the cathode).  Just remember:  Coin, Cathode.  The cathode is the negative one (if you don't know the difference between anode and cathode, just think of "Black Cat", because the negative terminal of a battery charger should be the Black one.  The black cat gets the coin.   

The "red" terminal means "danger" to your coins and relics.

The red terminal (positive -  the anode) should be hooked to a CARBON or GRAPHITE rod.  A "pencil lead" will work OK;  these are carbon, not real lead.  You can cut open a wooden pencil very carefully down the middle, but you will probably break the graphite in the process.  If you can get your hands on a fatter piece of graphite, the cell will function better anyway.

Even a correctly-done setup may appear to cause some pitting of the coin.  Actually, what happens is that the corrosion or tarnish has already eaten into the surface of the coin long before you even dug it up, and now that you're removing this corrosionlayer, you will see the metal pitting left behind. Oxygen bubbles form at the cathode, and these bubbles will cause some of the corrosion or tarnish to flake away from the coin.  That's OK.  You are not actually dissolving the metal when the coin is at the cathode.  If the coin is pitted after you did proper electrolysis, that means the tarnish had already eaten into the surface.

I found a 1921 George V penny (British) at a friend's house in New Jersey, maybe 15 years ago.  It was buried about 11 or 12 inches deep, no exaggeration (I had just gotten a White's Eagle II SL, which in the opinion of some enthusiasts is still one of the best detectors ever made... though I was surely no expert with it!)  So anyway, this penny was very tarnished.  Thick, dark green tarnish coated it to the point where you couldn't see much detail.  Dirt was stuck so far into it that soaking did not take it out. 

The coin therefore volunteered itself to undergo a little experiment.  Should I have left it alone?  Probably.  The date was readable.  Oh well, too late now.

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