Silver from clad? The conductivity of the target surface and near sub-surface is not the only determinant of the Fe-Co values. If it were, life would be much easier. The basic problem requires a magnetic field to illuminate and drive either voltage (eddy currents) or align ferrous domains (iron) in the target, or both. A coin is non-ferrous, but the soil contains ferrous minerals (and salts), which alter the field, spreading it out horizontally. While you swing the coil the field strength at depth is rising and falling, the field dimensions are being distorted (but keeping the same volume), so that the intersection of the magnetic field lines may intersect the target at various angles, ideal being 90 degrees and worst case parallel to the target's surface. This means the induced voltage on your silver coin is rising and falling, depending upon the strength of the intersecting field, whose strength is rising and falling with the soil matrix.
When the voltage varies so does the secondary magnetic field induced in the coin's surface (and near sub-surface), which in turn weakens or strengthens the returned magnetic field, which then induces a voltage in the receive coil (weaker or stronger corresponding to the returned signal strength). While you are sweeping the coil, the machine is driving 4,000 time per second into the soil, but if you could see the illuminated area it would look like a lava lamp blob expanding and contracting. This is why you can easily miss a target and hit it the next time - a slight change in angle of attack allowed the field to penetrate to the target. This is why most people slow down - not that the detector needs the time (the transmit coil is working too fast for that to matter) but as you move about over the target there is an increased chance that a slight coil sweep offset will unknowingly bypass some soil blockage (this is why striking different angles is so important - especially for the deeper, weaker signals).
But, the soil minerals and salts aren't the only factor affecting your Fe-Co reading. If the target is angled away from the horizontal, then the maximum field strength (in the coil center-line, which is essentially perpendicular to the ground) will intersect the coin (or nail) at an undesirable angle. This may be mitigated by the strong field strength so that a strong eddy is generated anyway, but again, sometimes it is the field lines that are more parallel (to the ground) that will drive the voltage with an angled coin (hitting perpendicular to the coin's surface). Those skewed field lines are outside the center-line of the coil, where the field strength is weaker, but the induced voltage may be greater due to the angle of intersection. You still get a signal in the detector (perhaps) but the signal may be weak (and appear deeper than it really is - a coin on edge).
The phase offset between the returned signal and the transmitted signal will determine the ferrous/non-ferrous nature. The detector will do best when the signal is strong - the weaker field from an angled coin or warped field in the ground matrix may cause other competing signals (soil, nails) to alter the silver coin so that it looks like a clad dime/penny or drive it further into the ferrous region. An elongated ferrous target (nail) may act to gather field lines into itself depriving the nearby coin in a field void that masks the coin. You may only get an intermittent signal - jumping about from Fe12 to Fe35 as the two phases play tug-of-war in the ground.
The bottom line, in neutral soil the sliver coin will behave like a silver coin - unless very deep and at a large angle in respect to the field lines. In mixed soil matrixes, which is most common, the silver Fe-Co values can be affected greatly and vary greatly depending upon sweep angle, speed, soil moisture, iron and salt content, elongated iron nearby, and coin position and depth. A weak voltage in the coin means a weak return signal amongst strong garbage signals - the end result is a silver coin that can appear as non-silver. It happens all the time.
Most people just get used to the ambiguity of detecting and dig anything that appears somewhat repeatable in a Fe-Co zone that is known to produce good targets. Of course, if you only hunt the high Co values you will miss all the non-standard signals generated by silver coins (especially deeper coins), and you'll miss all the gold rings which lie along the 12Fe line (+/- 3) but near the 01-15Co area (low conductivity). In other words, you will be the 15% hunter - missing 85% of the good targets. The TID is nice and the Fe-Co values are helpful, but in the end it comes down to digging the target out. There is no secret formula to discriminating good from bad with much certainty, especially in difficult ground conditions. The next pull-tab you ignore or bothersome clad penny TID, might just be that diamond studded gold ring or old silver half-dollar. Done correctly, detecting is hard, dirty, tiring, and often frustrating work. You won't read that in a manufacturer's detecting brochure, but it's true.
Johnnyanglo
