Chris's Metal Detecting Page -
Chemistry and Cleaning of Detecting Finds

Chris's Mineral Collecting Page is mainly about [what else?] mineral collecting, but I thought I'd put a little bit of Treasure Hunting stuff up here as well... I may start enlarging this little sub-section if time allows.  Behold, Chris's Metal Detecting Page.  The following is something I've written in response to a few forum posts I've seen where people seemed confused.  I'm a chemist but not a professional conservator, so claim no responsibility if you use some method to ruin a coin, even if I said it would work.  I also claim no responsibility if you destroy property or hurt yourself or someone else.  Nobody is forcing you to work with chemicals.  If you want to live in a world free from all sources of harm, go live in a bubble.   (Even then you'll slowly degrade due to your body's own biochemical processes.)

A little Chemistry lecture

I'm going to tell you why certain things can't hurt metal artifacts (including coins), and why other things can.

The surface of a metal object can be damaged in only two ways: 
chemically or mechanically.

"Mechanically" is easy to explain.  That includes rubbing, scratching, buffing, grinding, and that sort of thing.  If you take a dirty coin and scrub it with a toothbrush, some of the dirt and rock particles will come loose and scratch the surface of the coin, just as if you'd put pumice on the toothbrush.  That is a mechanical type of damage.

"Chemically" is where a lot of people, sometimes even conservators who should know better, seem not to understand a few simple concepts.

The metal in a coin or other object has zero charge.  It is neutral (it is actually metal-bonded, meaning there's a community of electrons, but we needn't get into that. As far as we need be concerned, it's neutral).  Along comes a chemical compound that reacts with the metal.  How does it react?  Normally, the metal has to go from zero charge to having a positive charge.  To dissolve metal (other than throwing it in a pot of molten steel!) the metal has to become ionized.  To do this, a metal atom has to lose one or more electrons. 

So we first look at compounds that are capable of reacting with metal:

Acids (examples: muriatic acid, vinegar, lemon juice, tomato juice, nitric acid, etc.)
Bases (examples:  lye, sodium carbonate, baking soda, etc)
Ion-Forming Substances (example: table salt)
Impure Water  (because it has dissolved ions!)

Some compounds fit into sub-categories of the above.  For example: chlorine gas.  If you leave copper around chlorine gas, the copper will slowly corrode.   There is moisture in the air, so some molecules of chlorine will dissolve in some microscopic droplets of water vapor and go to work on the surface of the metal.  When that happens, you will have ions that can chemically attack the metal.   The same thing goes for sulfur compounds.  These can react with the coin because, when they are left in contact for long enough, there will form sulfide ions (S^2-) and copper ions (Cu⁺ and / or Cu²⁺), giving you copper sulfide... a kind of tarnish that is tough to remove without ruining the piece.  If the tarnish is thin, electrolysis might be OK.  If it's thick, you'd better get used to living with tarnish. 

Conservators like to guard their trade secrets, but remember this:  either they are touching the metal mechanically or chemically, or else they're just soaking it to remove loose dirt.  There is no magical, airy, hands-off way to remove tarnish and refrain 100% from degrading the metal surface in some (albeit microscopic) way.  If you're removing one kind of surface (tarnish, corrosion) but trying to leave another (the coin's original details), there are only ways to minimize the degradation.  Yes, there are ways to conserve certain coins so you won't notice the degradation without a powerful microscope, but the success depends largely on the amount and type of tarnish or corrosion that's on the coin (as well as what kind of metal it is).  There is no magic involved. 

Let's go back to the chemistry lecture.  Not all ions are equally reactive toward a given metal... but for our discussion let's just say that it's not a good idea to leave a coin in a solution containing a high ion concentration.  That includes salt water!

Electrolysis of a coin, where the coin is placed at the cathode, is still a semi-destructive "conservation" method, at least for badly pitted coins.  The reasons are that (1) the layers of corrosion that flake off will leave open the pits they had previously filled, sometimes even pulling away very tiny amounts of metal with them, (2) there will probably be some plating-out of metal ions from the solution, meaning you're chemically attaching something onto the coin's surface that wasn't there in the first place, and (3) the loss of patina from deeper areas can make the coin look uniformly shiny, a definite giveaway that it's been cleaned.  As I might have said before, you don't have much to lose if your coin is just a badly-corroded disc with no legible details.  Then again, you may run into relic collectors (not coin collectors, usually) who like those corroded discs.

Let's say you start with a copper coin and perform electrolysis on it.  First, its tarnish will flake off into the liquid;  a very small amount of that will go into solution in the electrolyte;  finally, these newly-released copper ions will plate back out onto the coin!  If you aren't able to control the conditions of that electrolysis cell perfectly, it will turn out not to be one of your magical, airy "no touching" conservation tricks.  It will be a mess.  At best, if no plating happens, you still run the risk of making the coin look spongy and otherwise messed-with.  It will not have the naturally-aged look.

I also want to mention Hydrogen Peroxide briefly.  I haven't yet studied this in-depth with regard to cleaning coins, but I can tell you that most of the bubbling you see when you use peroxide on a coin is from catalytic decomposition of the hydrogen peroxide itself.  You are seeing oxygen bubbles.  This decomposition is caused by the metal and the metal tarnish compounds.  It can also be caused by organic debris such as soil that may still cling in traces to the coin.  Hydrogen peroxide, if it does react with the metal to dissolve it or form new compounds, does so only slowly at room temperature and at the customary 3 to 6% concentration.  It may, however, accelerate the action of other impurities that could attack the metal.  Though I use 35% H₂O₂ for cleaning certain things, it must be treated with the utmost respect.  It can cause severe skin burns and must be kept away from certain compounds.  I may post some photos of 35% H₂O₂ treatment of dug coins (nothing valuable though), as time permits.  It will be necessary to use a container about 10 times too big, because the peroxide will bubble up vigorously when it touches the metal.

Acetone - This is an organic solvent that, for all practical purposes, doesn't ionize.  I could show you organic reaction mechanisms involving carbocation formation, the haloform reaction, etc-- don't worry about these, though.  As far as you need to know, acetone is a non-ionizing solvent when it's pure.  During extended soaking, impurities might react with your artifact.  However, fresh-out-of-the-bottle acetone is NOT going to hurt your coins if you soak them for a few hours or even overnight.  Just don't leave a coin in acetone for weeks or months unless you happen to know it is ultra-pure and has no dissolved water in it.

Detergent (certain types) - Theoretically, a Non-Ionic surfactant should help lift dirt without doing anything to the coin itself.  Then again, you must remember the water may have other impurities in it, like atmospheric carbon dioxide.  It all depends on how rare that coin is that you're trying to clean. 

Naphtha - This is an organic solvent that is even less likely to react, because it doesn't contain as much oxygen... and it doesn't mix readily with water, so it won't carry a bunch of dissolved, ionizable impurities. 
Ideally you want a degreasing agent that's made of just carbon and hydrogen, evaporates easily, and isn't toxic (therefore benzene is out).  If you could get your hands on petroleum ether or heptane, those would be excellent degreasing solvents for extended soaking / degreasing of coins.  Hexane, on the other hand, is toxic to humans because of what it becomes in the liver. 
Like acetone, any of these volatile solvents would be extremely flammable.  Methylene chloride, though it contains more than just carbon and hydrogen, is another excellent degreaser that should not react at all with a coin's surface, except perhaps during extremely long soaks such as months or years.

Olive Oil - This is a fat (actually, a combination of several fats).  The problem with fats is that there can be some free fatty acids in it.  These are not nearly as reactive toward metal as, say, vinegar.  Generally speaking, the longer the molecule, the less acidic is the fatty acid.   If you leave a copper object in the olive oil for months or years, you may notice some chemical attack has happened.  Some people complain of darkening-- I wouldn't doubt if there were sulfur compounds present in olive oil.  Olive oil is, after all, not a pure substance.

Mineral Oil - This makes more sense than vegetable oils from a chemical standpoint, because Mineral Oil does not have fatty acids in it.  That means pure mineral oil cannot attack metal, no matter how long you leave it in there.  That also means a follow-up soak with degreaser (such as acetone) can remove ALL traces of the oil.  I've heard a conservator say that putting oil on a coin is "adding something" extra to the surface.  That isn't chemically true;  mineral oil will cling to the surface but cannot actually combine with the metal.  An organic degreaser such as acetone, methylene chloride, or naphtha will take the oil away completely, provided you soak it long enough to get the oil molecules out of any pores that may be in the surface of the metal.  This would especially apply to dug coins.

Pure Water - that's right: distilled, degassed, deionized water.  Practically speaking, there are no ions in pure water.  As soon as atmospheric carbon dioxide dissolves in the water, it's not pure anymore-- it's now very weakly acidic.  It is not easy to maintain pure water because of this.   In fact, when your coins in water you're almost guaranteed to bring ions into solution because of the remnants of dirt and slightly-soluble metal tarnish compounds that cling to the coin's surface.  The more ions that go into solution, the more of a chemical attacker the water becomes.  It's a sort of positive feedback cycle.

You may be wondering:  if oils and inert solvents don't actually do anything to the metal, why use them?  The reason is that they tend to lift dirt and loose corrosion from the coin.  Dirt particles stuck to a coin have a reasonably high affinity for each other and for the coin's surface, otherwise they'd just fall off by themselves.  The penetrating action of oil makes the dirt particles have less of this affinity.  In a manner of speaking, the oil creeps in between the particles and isolates them from each other.  Because dirt is not a homogeneous (chemically uniform) substance, some of its components will be better soluble in water, while others will be more soluble in oil.  Either one can potentially do the trick, though, because one type of particle may provide the foundation for another particle to stick to the coin or to another particle.  Take away one type of particle, and the other may have nothing to hold onto... you'll see a lot of dirt dropping away.  Over time, the oil can gradually lift away flakes of what you thought was a hard surface of brown corrosion, revealing a green layer that's thinner and might show more detail.