It does make sense that corrosion or pitting of the surface of a coin would cause a lowered Co reading. If the FBS understanding is correct, the Co value is primarily the opposition resultant to the primary magnetic field by the secondary field in the coin's surface. The secondary field will be strongest in a high conductor and weakened by odd shaped targets or rough surfaces that do no permit eddy currents to flow freely. You could think of the secondary magnetic field that is created by those circulating free electrons on the coin's surface as a blocking force that neutralizes by having the opposite polarity, of the primary field of the detector. Think of it as a mini-force field that shifts the primary field off center of the coin. That shifting, to follow the analogy, causes an imbalance in an induction-balanced coil, which drives a current, which causes a signal on the non-ferrous target. The stronger the secondary field strength the more effective the primary is cancelled and the greater the shift and stronger the signal, all things being equal.
Interestingly, ferrous targets may have a weak eddy formation but their main driving force is that the electrons in the iron atoms can shift to match the polarity of the primary field, somewhat. As the primary A/C field switches polarity at the coil, the magnetic field drops to zero, which causes the magnetic domains in the ferrous target to return to their original orientation. That turning of the field to and away causes a secondary magnetic field in the ferrous target which does not oppose the primary field. It is the nature of ferrous targets to attract the primary field through it, unlike the non-ferrous target that repulses the field away. Bending into ferrous targets causes an imbalance in the I-B coil and drives the signal too.
But, ferrous targets that are slightly conductive find that the bending inward is greater, giving a ferrous signal but if you change the coil orientation a bit the conductive opposition causes bending away of the primary field - this back and forth battle will often cause a jumpy Fe value while the overall Co value will not move as much. If you lift the coil higher over a ferrous target, the primary field strength rapidly drops, the weak eddy currents in the rusted surface quickly die but the ferromagnetic nature of the target remains stronger. Thus, on raising the coil the Co values drop down and the Fe values rise higher, giving away the nature of the target (bottle top rather than coin).
I am surprised at times when a clear quarter coin target signal reveals to be a copper coin, which will register lower Co values when out of the ground. It may be that the soil conditions are better than average to create those eddy currents: perhaps wet soil and some conductive mineral salts help raise the Co value on a clean penny. Likely, those same salts and wet conditions will shortly ruin that same penny in the ground, and its Co value will drop a bit as eddy formation is hindered by corrosion. Certainly, when there are much ferrous minerals in the ground, the total volume under the coil of ferrous signal swamps the non-ferrous signal of the coin, and the Fe values will drop significantly, down to 29-30Fe. But if the coin is still in good shape, the Co value should stay in the coin neighborhood, depending upon depth of the coin and the angle of incidence of the magnetic field to the coin surface, with perpendicular field lines to the coin surface being ideal for keeping the Co values higher.
But it is still always disappointing to expect a shiny quarter and get a copper penny.
john
