Scientific fact about the Halo effect of buried metals

[Astrodetect]

Hi everyone
Today I read this article, this is what the LRL can detect.......

The Halo Effect

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

Some topics engender controversy way out of proportion to their importance in the world. This is one such topic. There are actually people on detecting forums who have stopped talking to each other entirely, all thanks to this itty bitty subject that revolves around whether or not a buried coin produces an ion halo. It's not because the debates were especially animated, either.

I'm just going to try to present scientific principle and fact here. Take it or leave it. Really, this article is not meant to stoke the fires of debate; it's for the newbie who just bought a detector and thinks it's no good because it can't seem to find a quarter that was just buried in the front yard.

Some people believe the "halo effect" theory has been disproven. However, I have yet to see a peer-reviewed scientific journal article that even comes close to doing this. In fact, everything I have seen points the other way. I'm talking about scientific principles here, not opinion.

Many people understand that iron objects form a halo, but they go so far as to claim that copper and silver objects cannot form halos.

This claim is not at all true. The timetable required isn't even all that long; certainly it's less than a hundred years, unless you live in the Atacama Desert.

It 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 roughly synonymous with "non-reactive" or at least "not very reactive". The term is used in conjunction with metals, and gases. Xenon is a noble gas, for example, while palladium is a noble metal (though not one of the three traditional ones of antiquity).


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!



Have 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.

Recall also that 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 years.


III. 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, 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. 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.


IV. 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!

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.

If we know that gold alloys can leach base metal ions (and gold particles, once their matrix is gone...), then 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".

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.


V. Summary of the Halo Effect

The halo effect 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.

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.

So if you build a detector to detect these kind of signals you have an LRL.
Regards