From the Blog
Coin Masking – A Metal Detector Dilemma
There are “NO” hunted-out metal detecting sites! In 39 years of hunting coins, relics and jewelry with metal detectors, I have continued to find valuable items on sites that I have hunted dozens of times. But, I am totally convinced that no area will be completed hunted and all good targets removed with the present limited capabilities of metal detectors. The major obstacle or dilemma in hunting-out a site is masking.
Masking is not a new concept in the metal detecting field, but probably one of the least known or least understood topics in the hobby. I discovered masking early in the 1970’s when discriminate was added to detectors and I built my first test garden to practice on the varying signal types introduced through the medium of discrimination. Using high discrimination to get rid of those pesky pop tops and pull tabs, caused me to notice that some signals were mere blips but turned out to be good finds. By placing good targets next to junk targets, I noticed that standard and larger coils produced unusual signals or none at all, and I was missing the good targets. I took out my Fisher 441 all metal unit and could get readings on most of the good targets that were being masked by the tabs, pencil erasers, gum wrappers and buck shot that I had in my test garden. I decided to test out masking by going to a favorite site that always produced a few old keepers. The Fisher 441 was a great all metal detector with exceptional depth for the time period. I marked-off an area and hunted it using three different patterns and found piles of junk items and about 40 coins. I cleaned that area so well, that I called it my hunted out site. I did not go back there again for nearly 15 years, believing I got all the goodies that had been lost there. I so cleaned it out that none of my top line detectors received signals of any kind. I thought that I had licked the masking problem. I was recommending to all of our club members to use little or no discrimination and/or utilize a smaller coil to get around the masking.
During the mid 80’s, I discovered the second type of masking. This menace is known as “silent” masking. Silent masking works this way. Bury a good target such as a silver dime, at three to eight inches deep. Use both all medal mode and discrimination to confirm the signal. Now take a very small staple or lead or steel buckshot and bury it on top of the dime at about one inch deep. This item will mask the dime in discrimination mode and even moving the item as much as four inches from center of the dime, will produce masking or cause the dime to read as a pull tab. Then, bury the staple or buck shot to three inches deep over the dime and something amazing happens. There will be complete silence and even changing to all metal mode will give only a chatter sound like ground minerals. What is taking place here is called inductive coupling. The eletromagnetic energy from the coil hits the small junk item producing an electromagntic halo around it. The signal strength is greatly reduced and in all probability will not reach the dime. Even if it does the signal will fail on the return path when it hits the halo again.
What does this really mean to the detectorist? Probably only about 25% of all coin targets in schools, stadiums, and ball fields have been recovered because of masking, silent masking and deep depths. No metal detector on the market today can see through iron. I further proved my postulate that no site is completely hunted-out. I took my Fisher Impluse, a pulse induction detector, and went back for a visit to my hunted out spot. I dug up over 300 targets including five old coins that were masked by this high amount of hidden trash that none of my conventional locators could even get a signal. Please do not go buy a PI detector to use as a coin shooter. Sixty to one odds of trash to coins will be greatly increased in most places to more than 100 to 1 odds. Remember, I had cleaned or sanitized this area with my conventional detectors more than 15 years earlier. These great detectors are not affected by mineralization or the medium between the detector and the target. Performance does not change whether detecting through air, water, silt, sand, or solid coral. However, using a PI as a coin shooter might shoot you down for a very long time as you will dig very deep and every few inches. Here’s to “diggin it”! Larry