Chris's Metal Detecting Page -
Soaking in Olive Oil or WD40 is mainly about [what else?] mineral collecting, but Treasure Hunting is somewhat related.

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.  If you're a metal detectorist out there who's never gotten into mineral hunting, you might want to try it sometime!

Cleaning Dug Coins with Oil Soaking

Some people, especially conservators, might tell you never to put anything on a coin, even oil.  Read this for a little chemistry lecture.  Your author happens to know a few things in this regard, so it's not just some random guy pontificating on a forum.

Anyway, let's get on to the story.

A White's Eagle II SL had been sitting in a closet for a few years, untouched.  At first the thing wouldn't even detect a dime or penny on DISC mode, which was weird.  There was something wrong with it.  I took it apart, blew off any dust inside & dumped out any grit that found its way into the case, made sure nothing was shorted out, and put it back together.  For some reason that did the trick-- it worked just fine from that point.

I carted the detector to a thoroughly-pounded area where I'd never found anything good.  That sounds really encouraging, doesn't it?    Well, this time I just felt lucky.  After digging up a couple of crumpled soda cans, 3 or 4 pulltabs, and a piece of aluminum sheet, I got a pretty solid signal and dug up a penny-sized coin at maybe 3 inches depth.  (I put that in my pocket-- if you can't see the details on a coin because there's too much dirt, DON'T rub it off while you're in the field.  You might scratch the coin permanently.) 

I went back to swinging the coil.  About two feet from the first coin, I got another good signal and dug up a big coin that was too big to be a quarter.  It was about 3 1/2 inches deep.   It was too dirty to see what it was, so of course I kept it.  Judging from the amount of corrosion, it was old.
I rinsed them off when I got back.  The first one turned out to be an Indian Head penny.  As of the time this article was originally written, I'd never found one of these before (believe it or not).  The second coin was even better... it turned out to be an early large cent (which I'd later learn was a Liberty Cap or Draped Bust, made between 1793-1807).  Even though badly corroded and probably worthless, it's still noteworthy as far as I'm concerned.  What were the odds that all those people in the past had never put their coils on those two spots and dug those fairly shallow coins?   I guess it was in the cards to find them and write this little article.  The discovery of those coins, in fact, was what prompted the graduation to a newer (2000's) metal detector.  If there were still a couple coins at only 3 or 4 inches depth, what was there at deeper levels?  Only time would tell.

I could not read the Large Cent's date, but at least I could narrow it to a range based on the design on the coin's reverse. 

The large cent had dirt stuck to the front.  I didn't want to rub it off and risk scratching the face badly.  I decided not to do electrolysis;  this time it seemed better to have thick green patina than freshly-cleaned pitting.  So here begins an experiment with oil soaking.  A prolonged soak in oil is considered the only widely-acceptable coin cleaning method, besides soaking in water.  There are some minor qualifications, of course  (Read my article on the chemistry of cleaning to find out more.)  Olive oil, WD-40, and Liquid Wrench are popular types for this procedure.  If you want the most chemically inert oil for soaking, then mineral oil (U.S.P.) is probably the best choice.

Above: Both coins, Reverse, prior to oil soaking.  At least the backs were vaguely readable.  Even a badly worn-out or corroded Large Cent isn't something I'd subject to electrolysis, unless it's so bad that no details are legible in its current condition.   (The 1921 George V penny wasn't quite rare enough in my estimation to escape electrolysis)

Below:  Both coins, Obverse, prior to oil soaking.  Look how much dirt is stuck to the front of the Large Cent, even after rinsing with water.  I used a hose nozzle with a jet, too.   I didn't want to rub the coins with my fingers for fear of obliterating what little detail might remain.  As you can see in the photographs, copper doesn't age well in the damp, highly-mineralized soil of the Eastern states.

The Indian Head cent has a semi-legible date that says 1870-something.  I don't think that last digit is a 7, but it would be nice  Hopefully the oil soak will bring it out a bit.

The first phase of oil soaking was in WD-40.  I just sprayed some into the container and saw dirt lifting away in just a few minutes.  I let the coins sit for about 8 hours.  I removed the large cent and looked at it under a microscope while I used a toothpick GENTLY to tease away the cakes of dirt.  I put the coin back in the WD40.  After about another 8 hours I did the toothpick treatment again, followed by a degrease and a prolonged soak in olive oil that hasn't ended yet.   The final treatment will be a degreasing and a brief soak in distilled water.

Left:  the obverse of the Large Cent after soaking in WD 40 for 8 hours and very carefully lifting away dirt with a sharpened toothpick under a microscope.  There is still a layer of corrosion on the coin, but most of the soil is gone now.  I compared the coin to one in a book;   it's either a Liberty Cap or a Draped Bust cent, made between 1793 and 1807!  This is my oldest coin find yet!  The date on this specimen isn't readable.  Sometimes you just have to accept that the detail is gone;  we'll find out whether a longer soak helped this coin, if only I can find that little beaker where I put it.

As you can see in the photo below, the eight-hour soaking removed caked-on soil from the reverse as well.  The photo looks like two dug coins lain side by side, but it's just a single one:  Before and After.  Sometimes dirt is the only thing that lets you see the contrast of badly-corroded or worn features.

Even though it looks as if we've lost detail in the large cent's reverse after soaking, at this point all that's really been done has been to remove stuck-on surface dirt.  There has been no abrasion or chemical etching of the actual coin surface thus far.

The Bust cent and the Indian Head cent are now both soaking in olive oil (separate containers).  I may leave them for at least a few months before I try anything more on them.   I have noticed that, underneath the brown corrosion on the Indian Head cent, there lies a layer of green corrosion that is starting to show.  This green layer is very thin and might qualify as "patina", if only I could expose the whole thing.  Two or three small flakes of brown corrosion have lifted to reveal the green.

Sometimes the corrosion goes so deep on a copper coin that the detail is actually made of corrosion.  

UPDATE:  These projects were put aside for a while.  The oil-immersed coins were still sitting around in tiny beakers (or possibly plastic cups... can't remember), last anybody saw them.  When they turn up, we'll take a look at what happened after a couple years in oil. 

The most important thing to remember is patience;  oil soaking can take a year or more.  The subsequent de-greasing can take many changes of acetone, each lasting at least several days.

If the coins for some reason never do turn up again-- how silly, to find them with a metal detector and then lose them again!-- at least consider this.  You're not going to turn a badly-worn or badly-corroded coin into something it can't be.  If the metal is gone, don't expect to bring out much with an oil soak.  The reasoning behind the procedure is simply to loosen dirt. 

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