Understanding the Halo Effect

[Dan Hughes]

When a coin has been in the ground for a long time, it is attacked by outside elements. The result is that ions of metal from the coin are leached out into the surrounding ground.

This is called the Halo Effect, and it causes the coin to look larger to your metal detector. That's why you can find older coins easier than you can find newer coins at the same depth.

My newest podcast (internet radio show you can listen to on your computer) gives you the facts about the Halo Effect, and tells you how you can take advantage of it.

Go to http://thetreasurecorner.com and click the pink button to listen to this short (about 4 minutes) program.

---Dan Hughes, http://danhughesbooks.com

 

Amazon Forum Fav 👍

How to Research for Treasure Hunting and Metal Detecting From Lead Generation to Vetting - Grab it through Amazon!

[U.K. Brian]

Which is why you can dig two thousand year old silver and gold coins out in perfect condition ?

Even copper and bronze hundreds of years old may well not suffering from any chemical or acid attack and don't provide a halo. If there is a halo your digging a coin that's damaged and not collectable. Far better to move to an area of none aggressive soil. If you are on the type of soil that will cause a halo you then have to contend with the one metal that does always have a halo if there's any moisture in the ground which is iron. The halo from the ferrous will then mask out surrounding and deeper targets (though you could select a type of detector that doesn't respond to iron halo).

 

[Charlie P. (NY)]

Oh Boy! A good debatable topic.

Is there such thing as a "Halo Effect". It has never been proven and is as widely accepted as it is dismissed.

I'm neutral - here's why. Certain metals can trigger chemical reactions without being degraded by the process. Google "Catalytic reaction". Soil contains salts and minerals that, when watted down, have ions that will react with metal and each other.

Also, you cannot observe molecular changes with the naked eye. To say a very old or ancient coin hasn't lost molecules or ions just because it appears unchanged is not true. Granted, it is not a significant portion of the total - but how many molecules of silver are in a silver coin? The molecular weight of silver is 107.87 g/mole. Avogadro's constant (6.0221 x 10 to the 23rd power) gives us the molecules per gram, so a silver U.S. dime contains of 0.07234 oz which equals about two grams of silver - 1,200,000,000,000,000,000,000,000 atoms of silver. So billions of molecules could leach off the surface and only represent 0.000000001% of the total, give or take. And there are also 10% additional metals in a silver dime. Then again, are our detectors capable of sensing that small of a molecular halo? The soil conducts better because of it, so perhaps.

BUT - I believe an older coin is easier to spot simply because the soil is stabilized around it. A fresh coin, or a test garden coin, is surrounded by jumbled up soil that messes up the detector's ground balance. Just by having been repeatedly rained on, snowed on, etc., will cause the soil above the coin to blend in with the sorrounding soil.

Personally, I can accept the conductivity or inductance of the soil is effected by long-term exposure to a buried coin. But I can't prove it and I can't prove it is enough to effect a metal detector . . . but I can see how it might.

 

[extractor]

I believe there is a Halo Effect.

 

[U.K. Brian]

No one says there isn't a halo effect. It has most effect on iron and next most marked effect is on copper coins that have a percentage of poor metal mixed in so the breakdown of the coin is accelerated.

But "breakdown" is the important word. The coin even if it doesn't look like a bit of rubbish is underweight and can often be snapped like a carrot between two fingers. In other words not worth the digging.

 

[gregl01]

On silver and gold NO- on iron copper aluminum sure. Rates of oxidation vary on many accounts, but on silver and gold I don't think its enough that we could ever really know.

greg

 

[AUDuke]

A noticable halo effect exist on ferrous metal. It may exist to a certain degree on SOME non ferrous metals, but not enough to be noticed or measured. JMO

 

[LuckyLarry]

There is no debate.

The "halo effect" in a practical way only applies to Iron, steel, or FeO, or more explicitely, Fe2O3.

It is the Oxidation which forms when iron with it's loose covalent bonding easily combines with Oxygen. It means there is an attraction between some molecules in the coin and some of the molecules in the soil. It is not the same thing as the slow deteriorating of iron or steel, etc that is falling apart and separating into little chunks of rusty red or brick-colored steel. Oxidation is a term used to describe and show that a certain metal or alloy or mineral is combining with Oxygen. ONLY with irons Fe's is it measureable with our current detector technology.

When you see a rusty piece of iron you see the results of the "halo effect". When you see fire you see the same effect - although it is oxidating at a very rapid rate. Yes, iron rusting is the same thing as fire burning, but at a much slower rate. It's all the same, just... oxidation.

Now with other elements or metals the rate of oxidation is slower, such as with brass, copper, lead, bronze, brass, aluminum, etc, but with copper it's a bit faster than with brass, and considerably faster than with silver.

With GOLD though, the rate of oxidation is so little and so slow that it can stay the same atomic weight (not losing any molecules at all, nor any electrons) for thousands and even millions of years, and with silver it still takes many, many decades or hundreds of years. This is why silver is so often still silver colored, and at the worst, tarnished a bit after it's very few free electrons have combined with other elements in the surrounding soil. However, a silver coin can be in the ground for several hundred years before there is even a hint of a "halo".

There are a lot of old-timers here who still claim that the "halo effect" is the same as the slow deterioration and breaking up of and scattering of the steel (or often aluminum), but that's because they never have taken a chemistry class in their life - and they don't plan to either. They just like to talk.

There are still others who don't understand that gold or silver lose so few of their free electrons that there is no device ever made that can detect or weigh the loss. It has to be done mathematically and at a sub-atomic level, something that a metal detector is incapable of. This means that unless someone pours acid in the soil surrounding the coin - gold, silver, and most semi-precious metals never do create a "halo-effect" unless it's hundreds to thousands of years later. Copper though can lose it's free electrons (the halo effect) at a rate of measureability of sometimes as few as 50-100 years in some few types of high-acid soils, but there is still a marginal loss of free electrons and insufficient to cause a "halo effect".

For all practical purposses though, the halo efect applies to only iron or iron oxides, never gold or silver, and only marginally, copper. It's a matter of science; chemistry, and physics (quantum physics, to be exact).

LL
AKA "EasyMoney"

 

[Coinsteamer]

I had this problem with my bounty hunter years ago.

 

[lastleg]

Coinsteamer, what problem?

 

[ClonedSIM]

BUT - I believe an older coin is easier to spot simply because the soil is stabilized around it. A fresh coin, or a test garden coin, is surrounded by jumbled up soil that messes up the detector's ground balance. Just by having been repeatedly rained on, snowed on, etc., will cause the soil above the coin to blend in with the sorrounding soil.

I believe this is going to be closest to the truth when it comes to finding those older, deeper coins. I've seen many instances while out digging when I'll get a great signal from 7 or 8 inches down, dig my hole, and then have a tough time finding the target in my diggings. I'll usually end up waving handfuls of soil under my coil till I hear a tone. Don't know exactly why, but this happens most often with wheats and buffalos.