Chris's Metal Detecting Page:

The Halo Effect is mainly about [what else?] mineral collecting, but metal detecting is at least somewhat related.... so here we go.

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I've updated this article to organize it better, and also to add some ideas that further reinforce my primary thesis:  namely, that relatively "noble" metals can and do form "halos" in the soil.   There is some fairly advanced science involved (including biogeochemistry), but my aim is to distill this into material that a layperson can understand.

I.  The Trouble With Test Gardens

The fresh-burial test for coins has led many a detector owner to conclude his new machine is "no good".  That's because this test is nothing more than a worsened version of the Air Test.

You bury a coin six inches deep... and ten minutes later, you see if your machine can detect it.  Perhaps you get no signal whatsoever.  Maybe you even start to become discouraged, thinking you bought a lemon. 

This is like doing an air test, where the air is made of soil.  It fails to take into account an extremely important factor, which we'll discuss shortly. 

II.  The Controversy

The "halo effect" theory holds that buried metal objects have an ionization "halo" that increases their apparent size to a metal detector searchcoil.  This makes the objects detectable at greater depth.  In other words, according to this theory, long-buried metal objects have something you cannot duplicate with a recently-created test garden.

Many people understand that iron objects form a halo, but they go so far as to claim that copper, silver, and gold cannot form halos.   (This is where most of the controversy lies.)  Their reasoning is that copper, silver, and especially gold are too "inert" to react in just a few years of soil contact.

A popular sub-argument is that these metals cannot be forming halos in the soil, because they're not pitted or corroded enough when dug up.  This specious argument is especially common when talking about silver coins. 

The skepticism is understandable, of course, since silver and copper have traditionally been thought of as "noble" metals.  They're just not as noble as platinum, palladium, or gold.  In chemistry, the term "noble" is generally synonymous with "low reactivity".  The term is used in conjunction with metals, and gases.  Xenon is a noble gas, for example, while palladium is a noble metal (though it's not one of the three traditional ones of antiquity). 

We're going to see why the halo theory holds true for copper and silver.   There may even be a halo effect for gold objects, and we'll see why that is, too.  As we'll see, the timetable required isn't even all that long;  certainly it's less than a hundred years, unless you live in the Atacama Desert.

1920 Mercury dime in as-dug condition
Does silver ionize in the ground? 

Nahhh....   ;-)

This 1918 Merc was found in the woods, a few inches deep.  Pine needles and oak leaves covered the forest floor.  Organic acids, anyone?   It's pretty clear there was sulfur at work here, too. 

Silver will give an "ion halo" wherever groundwater or moisture can work on it for decades, especially if that water is even slightly acidic (which it nearly always is). 

When the black coating was removed electrolytically, it revealed only minor pitting of the surface.  Surprise!  The volume of sulfide corrosion was relatively large, compared to the apparent loss of metal.  This is an area that many seem not to understand very well.  Corrosion compounds can occupy a great deal of volume compared to the original metals.

III.  Let's Think About This

ave you ever dug up an old copper coin and found that it was green with corrosion? 

What do you think that means? 

How about a 90-year-old silver coin that emerged from the soil all blackened with tarnish?   Instead, perhaps, it emerged with just little traces of black, brown, or even other colors.  That's OK, too.

There doesn't have to be noticeable blackening to mean that ions have formed.  Silver sulfide is not the only possible compound that can form.  Some of the compounds are at least partly water-soluble and can leach out a couple inches into the surrounding soil.   Furthermore, it doesn't take much ionization to make the surrounding area conductive.

Some water-insoluble compounds can become appreciably soluble when that water is slightly acidic (e.g., because of some H2S, H2SO3, HNO2, tannic acid, etc.)

In the ground, ions are mobilized from the surface of a coin by soil acids and dissolved salts.  Rain picks up atmospheric NO2 and especially SO2, as well as tannic acid (etc) leached from pine needles, oak leaves, and other materials on the surface.  There is also decaying pyrite in many soils;  this yields H2SO3, H2S, etc.  Pyrite is one of the commonest minerals there is;  it can be present in all rock environments, from igneous to sedimentary.

Anyway, so you buried a coin 6 inches deep yesterday.   Maybe you can't detect it.  If you come back in a year, or perhaps five years, this could change.  It will not change in just a few days (or weeks), unless the object is made out of something reactive like magnesium or zinc.

With a very slow-reacting metal such as silver, you really ought to come back in thirty or forty years to notice a difference.  Or, if you want to do a useful experiment, you could come back every week and see how long it takes for your favorite detector(s) to get a faint signal over the coin.  Perhaps someone could speed the process by burying silver coins with egg yolks, and copper coins with vinegar.  I've been wondering how 5% HNO3 would work here, instead.

Since salt water is notorious for promoting corrosion (i.e., promoting ionization of metal), it might work instead of acids.  You might be able to get a good test garden without waiting many years.  (Although... see below.  Because of microbial involvement, you might be better off not using salt or acids.)

IV.  Now It Gets Really Interesting

Originally, I wrote this article from the perspective of classical (non-biological) chemistry. 

Since having done research in the field of bioremediation, I've realized the ubiquity of certain classes of microbes that I'd once thought were a geochemical curiosity.  For example, organisms that oxidize or reduce sulfur species are very widespread.  More widespread than I ever thought.  The same holds true, apparently, for metal-oxidizers and reducers.

Most notably, the activities of these organisms can be induced rapidly in the presence of their favored substrates.  In other words, there might only be a few metal-oxidizing microbes in a certain volume of sand or dirt, but if the conditions are right and the substrates are present, they can ramp up quickly in a very short timespan.

Certain archaea and bacteria can accelerate the oxidation of otherwise-noble metals... including gold. 

Furthermore, it just so happens that a major topic of my researches has been a common (in nature) ion that can solubilize both silver and gold... mobilizing them as complexes.   All that is necessary for this ion to form is sulfur or sulfate (common in organic matter), and the microbes will do the rest. 

Refer again to that picture of the heavily-blackened Mercury dime.

I think one reason why people doubt the halo effect is because they are expecting the behavior of metals in the soil to be governed exclusively by inorganic or "classical" chemistry.  At one time I expected that, too.  Based on my experimental observations, I've had to revise that idea.  Biochemical processes can facilitate reactions which would otherwise not occur at ordinary temperatures.  Indeed, that's how we are able to live.

It has long been suggested that some ore deposits (perhaps many of them) are the work of ancient microbes. 

It is highly relevant that many of these organisms are motile.  That means they can move of their own accord.  They transport metal ions throughout damp or moist soil, in effect creating a "halo" faster than ordinary diffusion would achieve. 

Most noteworthy of all is the possibility that the "halo" may actually include re-precipitated metal particles.  Through a series of reactions the microbes can oxidize and re-precipitate the metal elsewhere.   Remember what I said:  there are common, naturally-occurring ions that can dissolve silver and gold.  Remember what else I said:  it doesn't take much. 

The Ag+ion, like any of the other "noble" metal ions, easily undergoes spontaneous reduction in the presence of organic materials.  In other words, it doesn't take much to end up with finely-divided silver particles.

There's your halo. 

That's my hypothesis, and until I see definitive scientific proof to the contrary, I'm stickin' to it. 

V.  What About Gold?

Gold jewelry and coins are not pure gold, unless they are made of 24kt gold.   In the USA, most gold jewelry is only 14kt.  It is uncommon to go as high as 18kt, but even that is not pure. Thus, gold items are actually made of gold alloys.   American coin gold was 90% gold, 10% copper.  Once again:  alloys, not pure gold.

There are indeed some alloys that are famously good at resisting corrosion.  In practical terms, these alloys resist leaching of their component metals.  Two of the best-known alloys of this type are stainless steel and phosphor bronze.   Even these can give up ions under the right conditions. 

However, many (if not most) common alloys do not resist leaching very much, if at all.  It is a common myth that an alloy's most-noble metal will always protect its baser metals.  It really depends on the alloy.  Judging from the brass objects I have found, brass corrodes pretty quickly in the soil.  Bell metal and bronze, on the other hand, seem more durable.  However, they do still have patination... meaning ion formation.  Trace impurities in an alloy can also have a remarkable effect either way.  

You may wonder, what has this to do with gold, or silver? 

People often believe that gold coins and rings cannot form ionization halos.  Gold itself certainly does not form a halo by ordinary chemical reactions, because Au doesn't appreciably ionize in those conditions;  however, let us not forget that gold coins are not 100% pure gold.  If there's even a few tenths of a percent worth of baser metals (in reality, it's much more), there is a source of ions that can escape the surface chemically.  This halo will probably be slower-forming and smaller in extent than with a silver or copper coin, because there is less metal that can ionize.  It can still form.  Low-karat gold jewelry (e.g., a 10kt gold ring) is even more prone to this.  10 karats means the object is only 10/24ths pure gold.  What did you think the other 14/24ths were made of?  That's right, not gold.  Not plastic, either.  It's metal that can leach out of the alloy as ions, creating a conductive halo... and an electrochemical cell.

I once had an old ring sitting on a shelf in the lab.  Well, I picked up this ring and was going to throw it away, because I assumed it was brass. It was very dull-looking and even had traces of green on it.  I thought I saw "14K" on it, but I went back and looked at it closely.  It was, in fact, 10-karat gold. 

I also recovered another 10kt gold ring from the bottom of a lake with a Tiger Shark.  Sure enough, it was pitted and showed corrosion in some spots.  Guess what color the corrosion was?  Yes, it was green.  There was obviously some copper in that alloy.

If you've unearthed a 10kt or even 14kt gold item that shows mild corrosion, that's not out of the ordinary, especially if it was in a medium that favored ionization (e.g., at the beach, or in the bottom of a lake).  If, on the other hand, you have something marked 24kt that's even a little green around the edges... then suspect fakery.

UPDATE:  As mentioned earlier, with gold and silver there is of course the possibility of biological / biochemical reactions involving complex sulfur ions (etc).  These ions could effectively mobilize the metals out into the surrounding soil and deposit them as fine particles.  In other words, it is quite conceivable that a "halo effect" can happen to items made of pure gold, especially if the soil contains available sulfur which can be transformed by microbes.  As evidenced by the heavily blackened 1918 Merc dime, there doesn't have to be a visually-obvious amount of metal lost from the surface. 

VI.  Once Again, the Science Doesn't Lie

A metal object need not be visibly pitted in order to have leached ions into the surrounding soil or water.  To see this in action, you might take a mildly tarnished penny-- not a valuable one!-- and drop it into a solution of 2-propanol, water, and a little Murphy's Oil Soap.  Leave it for three or four days.  If you mixed it up right, you will find two things have happened:

1.  The solution is full of copper ions, which give it a blue-green color.

2.  The copper penny is still not pitted! 

There's your sufficient level of ions for a halo, and you can't even see a loss of metal from the coin.

As said before, even noble-metal alloys can give up ions to their surroundings.  If you take the time to read the scientific literature, you will find that corrosion of gold alloys is well known and has been for a very long time.   Factor in the effects of biologically-generated complex sulfur ions, and perhaps even the gold can mobilize.  Not even "perhaps".   Using complex sulfur ions to leach gold is pretty hot stuff in the mining industry.  (Far as I know, this is not taught in middle-school earth science, which is where a lot of people get their understanding of metals, ores, and chemistry.) 

We already know that gold alloys can leach base metal ions (and gold particles, once their matrix is gone).  It should be a "no-brainer" to understand that silver and copper coins will produce ion halos to an even greater extent.  Silver is more reactive than gold;  furthermore, silver coins aren't even pure silver anyway.  Coin silver is only 90% Ag.  Have you ever dug a silver coin that had a little verdigris on it?   I have.

It gets better, though. 

A 1964 article by Sveshnikov and Ryss in Volume 1 of Geochemistry International has a nice little diagram which, to the detectorist, might look like the field emitted by a search coil.  In fact, it is a diagram of the electromagnetic lines associated with a natural battery.  That battery has formed all by itself, around a sulfide ore deposit in the ground.

Why is this significant?

I thought you'd never ask.

Look once again at that silver coin, pictured at the top of this web page.  The black tarnish is made primarily of... silver sulfide. 

A tarnished coin in the ground is actually the center of an electrochemical cell.   All you need is a little moisture.  The more there is, the better.  Metal detector operators have long understood that detection depth improves when the ground is wet.  That's because wet ground is not only more conductive, but it also allows ions to move more easily.  This movement of ions produces a magnetic field.

Microgalvanic cells will form in the vicinity of a buried coin.  These will drive metal ions out into the soil through a complex series of processes where the anodic and cathodic regions are not constant. 

It may come as a surprise that this phenomenon is pretty well-established in geochemistry.  While hobbyists might know the term "ion halo", geochemists and geologists call these "dispersion aureoles".   How much of this is biological in origin we might not know for sure, but as I mentioned earlier, microbes probably play a role.

While a natural sulfide deposit might have taken a long time to set up a good-sized "earth battery", the alteration zones surrounding the main deposit are commensurately enormous (tens or even hundreds of feet).  In the coin situation, we're talking about a much smaller scale with shorter distances.  A few decades in the ground seems to do the job just fine, thank you. 

VII.  Summary of the Halo Effect

The halo effect is real, not just on iron.  It can happen by (A.) classical chemical mechanisms, (B.) biological-biochemical mechanisms, or (C.) both.

Once a halo forms, it would be expected to work in one or more of the following ways:

1.)  By increasing the size of the conductive area centered on the metal object.  Only a small amount of dampness is expected to be necessary for this, but more has a greater effect.  When the entire ground becomes conductive (as on a wet salt beach), notice how a typical VLF detector reacts.

2.)  By generating an electric field due to a potential difference.  This does not require moving current.  Regions of potential difference (i.e., voltage) are expected wherever a metal and its alteration products exist together in the soil.  Interactions can be complex due to soil chemistry (e.g., charged groups on organic molecules in soil).  What's important is that a potential difference gives rise to a DC electric field.  The required amount of soil moisture is probably low. 

The larger the extent of an electric field, the more it's going to interact with the coil of a metal detector.

3.)  By generating a magnetic field due to the movement of charge.  There has to be some dampness for ions to move, though really not that much;  consider a so-called "dry cell" battery.  More water helps, of course, such as after a couple days of rain.

A detector works by inducing a current in a metal object.  This induced current causes the object to emit its own magnetic field.  This in turn causes a back-induction in the search coil.  If the target is already emitting a field of its own, this is going to make it easier to detect.  I have found buried plastic pipes with a metal detector, but they always had water moving through them.  Moving ions = magnetic field.

VIII.  About That Detector

So.... don't fret about your brand-new detector.  It wasn't designed to detect coins that you buried five minutes ago beneath six inches of soil.   In fact, if your soil is highly mineralized or has teensy bits of metal junk in it, this "soil air-test" can fail with just two or three inches of soil.

To the doubters:  if you know where there are some old coins that are buried 6 to 12 inches deep, and they've been there at least 40 or 50 years, don't bother metal detecting them.  I wouldn't want you to waste your time, because you've already proven that your detector can't find them.  Just show me where the site is, and I'll take good care of it for you... hey, you can even lend me your detector for safe-keeping. 

Listen, I didn't make this article to step on anyone's toes.  It's just that there's a lot of non-scientific information out there.  Something can be well-intentioned and interesting, but that doesn't make it science.  

Some of the stuff out on the 'net is really about making people feel better about their favorite brands, especially the expensive ones.  That's to be expected.  Let's face it.  If you go out and buy $2,000 worth of fishing equipment, of course it's going to grind your gears a little bit when some kid shows up with a cheap bamboo fishing pole and catches more fish than you do.   Will he always catch more fish than you?  No, but it's going to happen sometimes.  Test gardens and air tests provide one kind of information, but they can become more of a self-assurance tool than anything. 

The way I see it, pretty much any of the big-name brands are worth having;  you just have to pick the one that works best for your detecting style and your soil conditions.  

Be that as it may, the fact remains that metals such as copper and silver probably do form halos in the soil, and those halos have an effect on how easily you can detect the coins.  Once you "break" the ionization halo (or perhaps the halo of biologically-redeposited metal particles too small to be seen individually), you may well find that the only way to locate the coin is with a pinpointer.  That's why it pays to get a good one.

One more thing.  I had to go to pretty great lengths here (formulating my own scientific hypotheses and all) to offer something that's not found on every other detecting site.  You can help me out, too.  If you found this article helpful, please show your support for this site by purchasing your stuff through the sponsored links.  Doesn't matter if it's metal detectors or car stereos or whatever you're looking for... it all helps me keep this site running and adding free content.  I like to be able to put quality information in front of you, to help you make informed decisions and sort through all the myths out there.

Thanks again for visiting.  Happy 'tectin!


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