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.
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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| Right: My preliminary
electrolysis
setup. Anode (+): hooked to a carbon or graphite
electrode. This is the only material you should use, because
metals such as steel or copper will go into solution and deposit out on
your coins. The graphite electrode must NOT touch the coin or the aluminum foil. Cathode (-): hooked to the item to be cleaned. For a coin I used aluminum foil, but I found later it's best to minimize the surface area of aluminum exposed to the liquid, otherwise the coin won't be cleaned up as much. For best results, use an insulated wire with as little exposed metal as needed to make contact with the coin. |
 |
Considering the equation V=IR, where V is voltage, I is current, and R is resistance, we would expect 12 volts to yield more current than would 6 volts across a given cell.
My baking soda solution allowed something like 300 milliamps to flow. Too much current is undesirable; it shouldn't go much above 500 mA, especially for a coin-sized object. Add just enough baking soda to the water so it will conduct a couple hundred mA, no more. A decent battery charger should have an Amperes readout on it so you know how much current is being drawn. Never let the positive and negative electrodes touch each other. (You knew that already, but in case you forgot...)
More often than not, the copper will have a very ugly, artificially-cleaned look to it. If the patination is covering pits, it's usually better to leave it there.
Brass comes out looking as if it's been copper-plated. Perhaps some copper metal comes out of the alloy and re-coats the surface, or else some of the zinc disappears from the alloy surface and leaves copper behind. Since the brass object is hooked to the cathode, it is not clear how such a thing could happen (off the top of my head: a galvanic cell-within-a-galvanic cell?). Though the mechanism is a mystery to me, I have seen this strange thing happen many times. I found a brass trunk lock from the 1890's and finally, after having "copperized" too many brass items already, I decided just to use #0000 steel wool on it.
If a coin is really badly corroded and nearly worthless anyway, and you just want to bring out some details so you can recognize it, then abrasives are actually better than electrolysis. I find that #0000 steel wool works well on these far-gone copper coins.
 |
 |
| Above (Left):
Before electrolysis. Even soaking didn't take away the
dirt, but then again it was only for a few days. The 1921 George
V penny will fetch no more than about $5 if
the hair detail is gone and King George's head looks like one big, bald
cranium. Some collectors really like the green tones, even if they're thick like this. That's why you shouldn't mess with the rare ones; leave them as-found or soak in olive oil for a few weeks or months. In this case I figured I'd never sell the coin, it wasn't worth much, and... it was unlucky enough to "volunteer" for my experiment. If you are lucky enough to dig something rare, I'd advise against electrolysis until you get a professional opinion. Copper coins often look nasty after electrolysis anyway. |
Above
(Right): After electrolysis. The coin had a very
dull appearance when it came out of the bath. I dried it off and
rubbed it in olive oil for a
while between my cleaned fingers (being careful not to have any grit on
them). Then I washed off the oil with soap and water and rubbed
the coin between my cleaned, dry fingers for a while. This helped
the highlights stand out a bit, yet it
left not even a trace of scratching at 10x magnification (though there
were of course 100's of micro-pits remaining where the verdigris had
been...). Rubbing a coin with your thumb is called "thumbing" and is considered a deceptive practice when used to hide blemishes. In this case, it's assumed you're doing it to a low-value coin that you don't plan to sell, so you can "thumb" it if you want to. Some argue that "thumbing" is really no different from the ordinary processes that already happen to a coin in circulation. No comment here; you make the decision. |
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