What is target masking? Target masking occurs when one or more metal objects hide a desirable item from being detecting or alter the metal detector id response as a result of being above, below, or adjacent to a coin, relic or piece of jewelry.
As a result, old coins or valuable pieces of jewelry are often missed by detectorists even when their coil did in fact pass over the desirable item. This happens more than one might think, especially in trashy locations. Even supposedly clean parks generally contain a massive amount of iron bits and junk metal that may not be detected due to discrimination or size but that can still affect identification.
Several small flakes of rusty iron from a disintegrating staple may be too small to register on the id, but they may cause the id of highly conductive coin to drop down into the mid-conductive range where pull-tabs and other junk items are typically found. When this happens, the coin is less likely to be dug as many detectorists rely heavily on discrimination features.
If those rust flakes were a larger piece of scrap iron, the ID may drop into the foil or even upper iron range where the signal from the trash item can overpower the coin signal. If iron is discriminated out, then the coin may not produce a response as the metal detector has analyzed the coin and surrounding debris as one piece of iron.
I recently experienced this last form of target masking while metal detecting a playground with the Garrett Ace 250. I was running the relic mode which accepts all metal items with the exception of the iron notch. In this mode, iron does not produce a sound.
After receiving a clear quarter signal that registered as shallow, I scraped about two inches of sand away with my shoe. When I swept the coil over the area, the Ace 250 did not make a sound. The coin had completely disappeared. On the surface I noticed a tiny metal spring and picked it up thinking that it may have been a false signal.
I always make a second pass with the coil to check for additional items. Again I heard a strong quarter signal in the exact spot where I had seen the spring. There was in fact a quarter 1 inch below the spring. When I scraped the sand with my foot, I moved the spring above the coin. Because the Ace 250 was set to ignore iron it determined that the spring was iron and ignored it as well as the quarter.
The takeaway from this experience is that many sites that are described as hunted out may still contain many valuables if one were to take the time to first remove some of the iron that may be masking old coins or jewelry.