Ferrous, Nails, Coins, Metal Objects and your coil.

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Filed under: Articles, Machines — Norfolk Wolf

I find it amazing the number of times that I read on different forums around the world about detectorists claiming that they can find coins under nails. I read one recently where he altered the number of filters on his machine and hey presto, it was as if the iron wasn’t there!
Let’s start right here by telling you that your detector’s coil doesn’t have eyes like you and me; it doesn’t “see” a nail or a coin, in the ground, or even the ground underneath it. Whether widescan or concentric, they both have a send coil and a receive coil, the send coil induces a current in the ground creating an electromagnetic field, any metallic object within this field distorts it and the receive coil analyses the distortion. (Look Ma, I can do it with my eyes shut!)
The “I changed my filters” guy is a non –runner straight away; manual filters ala Whites DFX don’t work on iron, their job is for the differing ground mineralisation.
Next up is the guy who swears blind his machine found a coin under a nail. “Step this way if you will Sir and take a seat as this will be quite a lengthy explanation and pay attention as I will be asking questions later”.
Iron has one property that all other metals don’t, it’s magnetic and what sort of field is produced? Yep, an Electromagnetic field, give that guy a coconut.
Next to mineralisation, iron is the second biggest pain in the butt for the manufacturers and when iron is wet, the problems become a helluva lot worse!
Wet =water= conductive, try throwing an electric fire in the bath, now fill the bath with water and have another go! Get the picture? (Yeah, in a flash).
So we gather that as far as detecting is concerned, Iron is the muscle-man around here, but what about this here rotary knob called Discrimination?
Set the control to Discriminate iron, (notice I said “to Discriminate” and not “to ignore”). Although you can Discriminate against a certain size of iron, that doesn’t mean that it isn’t there anymore and that it doesn’t have an effect on the electromagnetic field.
You’re detector is optimised to find coin sized objects, so how do you think it would perform on any iron that was larger than that? It would overload the machine and anything good in the near vicinity is lost.
Okay, now let’s go smaller iron, say a two inch nail. Go over it with the coil in an east to west sweep, (minimum Disc.) one beep. Now go over it at 90 degrees (north to south), A double beep. Why?
The answer is that when the coil is travelling over the length of the nail it is spending more time on it and the machines analysis is saying, “this is longer than a coin”.(A machine with a very slow recovery speed would give one elongated beep). The 90 degree sweep goes over the width of the nail, giving just a short beep.
See how easy it is to suss out nails, even in all- metal?
Right, the next step is to introduce a silver coin (better conductivity), say a Roman Denarius, 20mm diameter under the nail.
Which direction should we sweep to have the best chance of pulling the coin; east west or north south? North to south, you’ve won another coconut!
There is one proviso and this is the important bit; the coin’s mass must be greater than that of the iron nail.
Remember that the coil doesn’t “see” the coin and nail, it can only analyse the disturbance in the electromagnetic field. When you Discriminate the iron this gives a negative signal, the coin still gives a positive one.
What do you get when you add a positive to a negative? Yessirree, a negative! (Bung that geezer another coconut).
So you might be able to pull the coin with the nail “short-ways” on, but not “long-ways” and this is above ground. Let’s see what happens when we go underground.
The first thing we notice is that other pain in the butt, mineralisation, the worse it is, the worse the loss of signal definition! Let’s make it even worse by going out when the ground is damp.
All ferrous objects oxidise it’s called rusting; the more damp the conditions, the more it rusts. Rust holds water, water= conductivity, an electromagnetic field goes at it like a rat up a drain pipe, creating an even larger disturbance!
If an iron object lies in the ground, over a period of time the rust crumbles and spreads out around the object, developing what we call a halo effect. This is big league now, what chance of finding that silver coin? Slim to none, and as we all know, Slim has just left the building.
Think of a coin in the ground and a nail is lying at the same depth but a couple of inches to the east of it. Sweep over them from east to west and you miss the coin, sweep slowly west to east and you could pick it up, depending on sweep speed and depth. If the nail is above the coin, no chance in either direction, if well beneath the coin, your chances have improved. (But I wouldn’t risk your day job looking for these).
So far all we’ve talked about are small nails, let’s go bigger and talk horseshoes. Do you know if you went over one on the ground with your coil, you wouldn’t pull a coin for at least a foot after it in any direction? Whatever must happen when they are about a foot down in the ground?
Okay let’s get back to nails and smaller ferrous items.
I love nails. Why? Because they represent some form of habitation, habitation =losses, the more nails and other ferrous, the bigger the habitation. They also represent to me two other things as well, loss of depth and smaller coils. When you are in a “naily” area, the last thing you will get is depth, so why have you got your Sensitivity set as though you are still working in a clear area? Turn it way down, (if you don’t know the capabilities of your detector at the lower settings, find out, or take up kite flying). The smaller coil will enable you to get between some of the nails, if larger pieces of iron are close to the surface detect with the coil a couple of inches or so above the ground (less effect from mineralisation as well). Working slowly and diligently it is surprising just how much you can wheedle out of these areas. Like they said in the 60’s “this is where it’s all happening man”. So make it “happen” for you.
One thing I haven’t touched on is what effect there will be using different frequency machines. As a rule of thumb the lower frequencies (1.5 KHz-7 KHz) will handle mineralisation and recognise iron better, the recovery speed is slower though and they are better suited to the bigger thicker coins and objects which can be found at good depths. The higher frequencies (15 KHz-19 KHz) will find the smaller thin sectioned items, have a faster recovery speed and benefit from using wide-scan coils. So when the area is saturated with nails it should be obvious to use the higher frequency with a small coil and take advantage of the faster recovery speed; as I said depth will be the last thing you get in this type of conditions.
How about this one. “Man, I got this good signal and dug down over 15 inches and pulled this coin, and there was this lump of iron in the hole below it.”
Is this guy trying to say that he’s a crack detectorist or that his machine really outperforms all others (because it’s his)? Let’s leave this ego tripper and look at what really happens.
To find any single object will be depend on the amount of mineralisation / compactness of the soil / dampness of soil / size and shape of object / the metal of the object / how long it has been in the ground (halo and leaching) /frequency of detector and finally coil size.
Remember me saying about Iron being a pain in the butt for the manufacturers?
On or near the surface it is easy for the detector to recognise it, the deeper it is, (depending on the size and shape, obviously) the harder the recognition becomes; until eventually the detector can and will give a good signal and it is no good relying on the meter for a coin sized object beyond 8 to 10 inches. No matter what the manufacturers tell you, deep iron can fool your detector.
Shall we go back to the man with the “15 incher”? I don’t think that I have to tell you whether it was the coin or the iron that made him dig, do I?
How many times have you had what you thought was a good loud signal and on digging down and getting the coil closer the signal changed and you realised it was iron?
Most big iron that is deep can be recognised by using the all-Metal mode and the broad signal it gives. Another way is back off the Sensitivity and sweep across it making a mental note of where the signal is, then sweeping at 90 degrees and noting where that signal is; if the two signals don’t match up, walk on its iron.
How does this work? If the iron is a complete circle (a washer) no matter what direction you approached it with the coil, the two signals would be in the same place on the ground. However all other shapes of iron will give two different signal positions because the disturbance in the electromagnetic field will be greater when approached from one side than the other,
Okay so what about an absolutely square piece of iron? For this to give the two signals in the identical spot it would have to be absolutely dead flat and the coil would have to be swept and dead 90 degrees to all the sides. You that good?
Steel washers fool all of us, but here is something to help you with those old rusty iron washers. A signal (on a good machine) will have a start, middle and an end; you need to listen to the end. Try this at home in your garden, run the coil over a coin at a slower than normal sweep speed; a sweet rounded sound right the way across. Now try it with a rusty washer and listen carefully, starts good, middle is good, but the end sounds chopped and abrupt. Go back to the coin and listen again and notice the difference. In future if you are in an area with a rusty washers, it will only be the first one to fool you.
For as long as I can remember the nail and coin “trick” has been shown off and on over the years in detecting shops (along with look how deep it goes)and talked about by people/clowns convinced that they have discovered a new and amazing property of their detector. When a bit of basic knowledge how a detector works allied with a dollop of common sense, you will recognise it for the utter rubbish it is. You’re detector is meant to be used in the real world, out in the fields and open spaces; finding coins and artefacts in the ground. If you want to waste your detecting time convincing yourself that it can do tricks why not go the whole hog, paint your face, stick on a red nose and join a circus.
If this gives you the impression that I think these “look how clever I am people” are a complete waste of time and space, you’re damn right. These “armchair detectorists” wouldn’t know what a good days detecting was, even if it came up and bit them on the arse! John Lynn