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A silver coin showing hairline rub scratches.
A topic as old as the ages. Here's the simple statement that people read too late or don't believe. Cleaned old coins are 99% of the time worth LESS than uncleaned coins.

Let's do silver coins first, as this is where I see the most damage being done: If you find a silver coin (dated 1964 or earlier for most US coins) with your metal detector, with soil stuck on the surface of the coin, rubbing it in the field is as bad as cleaning it.  The soil is VERY abrasive and leaves hairline scratches, the same as many harsh cleaning methods.  Regardless of the condition of the coin, poor to mint state, soil rubbed across the surface will leave scratches that downgrade any potential value the coin may have.  Don't rub silver coins fresh from the ground.

So you're saying to yourself, so then, Mr. Smarty Pants, how do I get the dirt off this coin so I
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I am saddened by the demise of the local detector dealer.  The cheery faces shown beaming behind fully stocked multi-line counters in the old detecting books are gone.  Some now retired, some deceased, and for those left it's been harder and harder for them to make ends meet. 

All of us in the metal detecting community bear some of the blame for the end of the local shop. Manufacturers have squeezed the little guys, requiring larger minimums and greater sales volumes - offering greater discounts to mass merchandisers.  Consumers have been driven to the internet looking for the lowest price, and relying on online forums for information.

Let's take stock of what the local dealer offers us:
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A Code of Honor 06/24/2009
 

The Federation of Metal Detecting and Archaeological Club (FMDAC) Code of Ethics can be found here: http://www.fmdac.org/about/codeofethics.htm

But beyond this, I feel there is a Code of Honor.  Honor is trustworthiness, one's character, and how one reflects honesty, respect, integrity and fairness.  This is more than a Code of Ethics.

I try to be an honorable metal detectorist.  I'm not perfect, but this is what honor means to me.  I always try to know the law.  I do not knowingly trespass or hunt sites without permission.  I respect locked gates and fences.

I honor my metal detecting friends by not hunting sites they have discovered or shared with me (public or private) without their knowledge or consent.  I try to teach and help others learn the hobby and pass along what I have learned.

I use the best retrieval techniques I can.  I try to dig neatly and leave little evidence I have metal detected a place.  I take my trash with me and fill my holes, even on the beach.  If I can do something simple to make it better than when I arrived, I will.

I am polite and courteous.  If I am asked to leave a site, I will do so without quarrel, even if I know I am doing no wrong.  If I have inadvertently trespassed, I apologize and do what I can to make things right.

Public perception is powerful.  If we are seen as looters, thieves, liars, grave robbers and trespassers we will lose access to the sites we depend on for our hobby.   Negative stereotypes, once formed are difficult to overcome.

Honorable detecting and good etiquette is what will make it possible for us to continue to enjoy our hobby.  Dishonorable detecting will close sites to all of us.  I urge you to do the best you can, be an honorable detectorist and a credit to the hobby.

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YES!  If you are serious about finding old coins in hunted out sites the Minelab Explorer or E-Trac are what you need.  I've been detecting for 19 years, so I'm old school (though not as old as some) and new school.  My first machine was a White's Classic II.  I was swinging Fisher CZs for years and finding a few silver coins - maybe 10-20 per year.  Last year with the Explorer, I got 51 silver.  This year I have 127 silver coins in about 3 months of digging.  The majority of these are from the same sites I have been hitting for years.  

If you look at the people who are serious about silver and finding 100+ silver coins in hunted out
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Cleaning modern coins to make them "spendable" again is best done with a rock tumbler.  Water, soap, gravel and coins go in for a few hours and come out presentable enough to be acceptable.  Harbor Freight offers an inexpensive and effective rock tumbler that works well, provided you take some common-sense care in using it.  The weak point in the design of this tumbler is the belt, but following these tips below will help your tumbler run better and your motor and belt last longer.

The original belt should last quite a while if it is adjusted properly and you are not overloading the tumbler.  In a pinch "standard" size 1/4" wide rubber bands work as belts too, but long term use of them can damage the plastic pulleys.

Make sure you adjust the spacing of the two pulleys (yes, this is adjustable) to be just enough that the belt is not too loose (i.e. it just catches).  This keeps the belt at the right tension and not too tight.  If it is too tight it will work, but it will stress the belt.

Don't overload the tumbler.  It is rated for 3#.  If you are filling it to the brim with coins, rocks and water it is too full and too heavy.  This stresses the motor and the belt.  Ideally the tumbler should be no more than 1/2 full of rocks and coins and no more than 3/4 full when you add water.  Besides saving the motor and the belt life, a properly loaded (not overloaded) tumbler works better and faster at cleaning your coins as the material has room to move around in the drum.

Replacement belts can be located cheaply on eBay and are typically as good or better than the original belt.  Many rock shops and lapidary stores sell them in retail stores and online too.

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Secret #1 - Minelab Explorer or E-Trac.  I have never found as much silver as I have with Minelab machines.  It's unreal what I've literally walked over for years with other machines.  If you look at the serious hunters who find tons of old coins in hunted out parks 9 times out of 10 they have Explorers.  In my first 15 or so years of detecting I found 200 silver coins.  In the past two years with Minelab machines I have found 150, from the SAME sites I was hitting years ago.

Secret #2 - Go slow, be patient, take a deep breath, keep your overlap tight and listen for every beep.

Secret #3 - Dig anything non-iron that registers deep on the depth meter.  Around here a pulltab

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Air tests or depth tests don't mean much in evaluating a detector.  I think where the rubber meets the road is not depth, but ID accuracy.  Getting a deep signal means nothing if the signal doesn't sound like it's worth digging.  Most of us are not digging everything under the coil, but trying to be selective and dig the "good" targets.

I'd rather have a detector that only had 6" depth but would nail the ID on old coins and trash in almost any hunting condition.  I'd be bringing home handfuls of silver and other obsolete coins every hunt.  Even better would be a detector that had less accurate ID but surface blanking - ignore the top 3-4" and only signal on targets over 4" deep.

All the evidence I need as to what detector is the best is in the online metal detecting forums.  Look at the various finds posted on the different forums.  Look what machines are making the best finds, the oldest finds, the deepest finds.  Consistently, you'll see the same few machines.  Look at the machines the tot-lot hunters prefer, look at what machines people who consider themselves "pros" prefer.

A non-biased scientific "Consumer Reports" style air test comparison of detectors would be of little use to me, but on the other hand, seeing someone pull a seated dime and a shield nickel out of a park - right in front of me - in an area I had hunted hard and repeatedly speaks volumes.

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I wrote the following response to the post below on the TreasureNet forum.

"Well ive noticed latly when you call a detector company like fisher minelab tesoro.They usually have one guy runing the teck support wich is pathetic. And by the time you get a hold of him for a question about your detector he doesnt now the answer.Most of the time there out on call wich makes no sense.How can companys like Minelab & fisher have one guy for teck support thats never there? And buy the time you get a hold of him its like your speaken fricken chinese to the guy? Seems like these companys manny times just sell there product and God forbid you ever have a question are problem with the detector. There like some company in China one guy answering the phone selling the product and runing the company all at the same time. Common companys get your sht together. Sorry guys just teed off when companys that make money pull these cheap labor tactics."

Running my own business I can tell you people want lots of service and the lowest price.  It's not

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Just thought folks would like to see these E-Trac numbers. I tested 33 war nickels and the results varied quite a bit.
1 read 12-14
5 read 12-15
12 read 12-16
10 read 12-17
4 read 12-18
1 read 12-19

I know air testing is biased, but knowing the range these vary by may be helpful. Unfortunately these numbers run directly into pulltab territory.

As long as I was setup to test, I grabbed some other coins too:

Dimes
13-45 -- 7 coins (Barbers and teens Mercs)
13-46 -- 30 coins
13-47 -- 87 coins (only one Barber)
14-45 -- 1 coin (Barber)
14-46 -- 33 coins
14-47 -- 4 coins

Quarters
06-48 -- 1 coin (Washington)
07-48 -- 8 coins
08-48 -- 16 coins
09-48 -- 4 coins (SLQ and Barber)
10-48 -- 1 coin (SLQ)

Halves
03-48 -- 5 coins
05-48 -- 2 coins
07-48 -- 2 coin (Barber)
05-49 -- 1 coin (Barber)

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Green Pennies 05/20/2009
 
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I am curious as to why dug indian heads and early wheats come out of the ground with such nice green patina.  It's too much for me to believe this is just chance that coins after the mid-1920's do not exhibit this patination.

I believe the answer lies in the "5% tin and zinc" content of bronze cents.  I cannot find a reference to the specific percentages of zinc and tin used.  Sometime in the early 1920s, the percentage of zinc must have been increased and tin decreased.  I can find no reference to this change, but I would be very interested in finding one.

Tin is a key element in bronze.  The properties of tin state that it is not easily oxidized, prevents corrosion, resists corrosion from water, but can be attacked by acids, alkalis and acid salts.  These are exactly the properties that would make the early cents better preserved than the later ones.

Anyone who has dug an eaten up modern penny knows how zinc weathers in the ground.  So, this also indicates that an increase in zinc would make the surfaces more easily corroded.

Follow up #1
The composition is the "same" only to the extent that it is "5% tin and zinc" this is a vague reference.  I can find no references that specify the specific percentages of tin and zinc.

My theory is the older "green" cents are maybe 3% tin and 2% zinc and the non-green are maybe 1% tin and 4% zinc.

It has little to do with the time in the ground, as these earlier cents even read differently on the meter of many metal detectors, which says to me that it is clear there is some difference in metallic content.  The conductive value of older cents on the Minelab E-Trac is 33-36, the conductive value of later cents is 42-43.  And this can be shown in the ground and in air tests with dug and non-dug coins, so the time in the ground is not significant.

Follow up #2
Well, the minerals or ground effect only prevent the green patination from forming or eat it away.  If the soil is "acid, alkali or contains acid salts" then the tin will be attacked and the coin will have a rough corroded surface.  

Where the soil is fairly neutral, the tin provides some protection against corrosion.  There is some oxidation/corrosion occurring in the ground as that is what makes the cent turn green.  

My hypothesis is a greater amount of tin in the mix helps keep the surfaces smooth and even, preventing (in more neutral soils) the rough or heavy corrosion as seen in many copper coins recovered from fertilized farm fields.

I find that in my soil, which is fairly stable and neutral that older small cents tend to have this smooth patination.  Coins that have been fertilized or are in clay type soil do not.  Even modern coins in clay type soil are heavy corroded.

Overall I guess my question is not, "Why do they turn green?" but rather, why are the early cents so much better preserved than later cents when supposedly the metals are the same.  I am saying is the metals must not really be the same, and the ratios of tin to zinc within that 5% have been altered.

Another random thought is wondering if the addition of tin to the copper coating on modern zinc pennies would improve their corrosion resistance. Hmm...

Follow up #3
Wikipedia shed some light on some things:

Brass is any alloy of copper and zinc; the proportions of zinc and copper can be varied to create a range of brasses with varying properties. In comparison, bronze is principally an alloy of copper and tin.

Aluminium makes brass stronger and more corrosion resistant. Aluminium also causes a highly beneficial hard layer of aluminium oxide (Al2O3) to be formed on the surface that is thin, transparent and self healing. Tin has a similar effect and finds its use especially in sea water applications (naval brasses). Combinations of iron, aluminium, silicon and manganese make brass wear and tear resistant. A well known alloy used in the automotive industry is 'LDM C673', where the combination of manganese and silicon leads to a strong and resistant brass.

Selective leaching, also called dealloying, demetalification, parting and selective corrosion, is a corrosion type in some solid solution alloys, when in suitable conditions a component of the alloys is preferentially leached from the material. The less noble metal is removed from the alloy by microscopic-scale galvanic corrosion mechanism. The most susceptible alloys are the ones containing metals with high distance between each other in the galvanic series, eg. copper and zinc in brass.

Follow up #4
I discovered this text from Jevon's 1875 book, Money and the Mechanism of Exchange:"

Bronze Coin.
XI.11  It was known, even in prehistoric times, that a small quantity of tin communicated hardness to copper, and the ancient nations were familiar with the use of bronze thus manufactured. The French Revolutionary Government melted up the bells of the churches seized by them, and the sous de cloche,as they were called, made from the bell metal, were superior to coins of pure copper. Yet curiously enough no modern government thought of employing a well-chosen bronze for small money, until the government of the late Emperor of the French undertook the recoinage of the old sous in 1852. This recoinage was carried out with great success.

XI.12  Between the years 1853 and 1867 coins to the nominal value of about two millions sterling, consisting of 800 millions of pieces, and weighing eleven millions of kilograms (10,826 tons) were struck, in addition to a subsequent issue of about 200 millions of pieces. The experiment was in almost every way successful. The ten and five-centime pieces now circulating in France are models of good minting, with a low but sharp and clear impression. They were readily accepted by the people, although only weighing as much as the sous rejected in the time of the Revolution, namely, one gram per centime, and they are wearing well.

XI.13  The bronze used consists of 95 parts of copper, four of tin, and one of zinc. It is much harder than copper, yet so tough and impressible that it takes a fine impression from the dies, and retains it for a long time. It cannot be struck except by a press of some power, and thus counterfeiting is rendered almost impossible. It can hardly be said to corrode by exposure to air or damp, and merely acquires a natural patina, or thin dark film of copper oxide, which throws the worn parts of the design into relief, and increases the beauty of the coin.

XI.14  Bronze has since been coined by the governments ofEngland, the United States, Italy, and Sweden, and it seems probable that it will entirely take the place of copper. The German government is now using bronze for the one-pfennig pieces."

My conclusions drawn from this: Bronze for coins was a new idea in 1852.  The specific formula is listed as containing 4% tin and 1% zinc.  This is most likely the formula used in the first Indian head cents in 1864, as this new bronze alloy would have proven itself with use in Europe.  This supports my contention that the earlier cents have a higher (4%) tin content, and that is why they corrode less and maintain smooth green surfaces.  We know that the tin was removed from cents completely in 1962, but I contend the amount of tin was reduced before 1962.

Domestic and foreign coins manufactured by mints of the United States (1965) states: "Alloy of 95% copper and 5% zinc coined 1944-46 by order of Secretary of the Treasury of Dec. 16, 1943, pursuant to act of Dec. 18, 1942."

According to the same document above, the US mint was producing bronze coins for other countries.  In 1917-1919 the US mint produced coins for containing 95% copper, 3% tin and 2% zinc for Peru. In the early twenties, coins were minted containing 95% copper, 1% tin and 4% zinc as early as 1921 (Indo-China) and1922 (Costa Rica).  In 1942, coins made for Surinam were 95% copper, 1% tin and 4% zinc.   It would seem odd that they would create a unique alloy for these small runs of foreign coins rather than use the "stock" US bronze alloy.

This points to the late teens as a target timeframe that tin was first reduced from the bronze cent, giving a speculative timetable that looks like this:
1864-1916   95% copper, 4% tin, 1% zinc
1917-1920   95% copper, 3% tin, 2% zinc
1921-1942   95% copper, 1% tin, 4% zinc*
1944-1946   95% copper, 5% zinc
1947-1962   95% copper, 1% tin, 4% zinc
1963-1982   95% copper, 5% zinc

*In testing wheat cents with a metal detector, the majority of coins having a arbitrary numerical CO value of 36-41 or were minted before 1942.  Almost all coins minted 1944 and after had CO values of 42-44.  From this interpretation the early 1920s coins may have been somewhat experimental or perhaps normal variations in batch consistency, and the reduction to 1% tin did not occur until 1947.

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