Written by the TreasureGuide for the exclusive use of the Treasure Beaches Report.
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| Three Half-Dollar Coins Used in Metal Detector Experiment. |
I started what will become a series of metal detector tests. This first test employed an Equinox 600 using the factory presets.
The ground was previously swept numerous times to make sure it was clear of other metal targets. It is an area where there can be ambient electrical interference from various sources, mostly underground cables. Electrical interference wasn't obvious during the tests described below but could have been present to some degree.
Three half-dollar coins were used, for not particular reason than they were handy and provided what I thought would be a decent starting point. One was a 1934 (90% silver, 10% copper), 1964 (90% silver, 10% copper), 1971 (75% copper, 25% nickel).
The coins were laid flat on the dry white sand ground. Each coin was detected at about 11 inches without any optimizing. The 1943 appeared to be detected at a very slightly greater depth than the 1971, but the difference was small. The difference appeared to be so small that it was probably of little practical significance, especially considering the inconsistency and all the other factors that could affect results in the field.
I recommend reading the following post for background. (The Treasure Beaches Report Direct From Florida's Treasure Coast.: 9/13/14 Report - Problems With Air Tests. How To Test Your Metal Detector. Buried Objects and Disappearing Signals. Tropical Storm Edouard.)
This test involved coins that were of almost the same size and mass, although there were some very small differences. The differences did not appear to be very significant, and it turned out that the 1964 was so similar to the 1943 half dollar that it didn't add much, if anything, to the tests. I intended to use a 40% silver half instead, but as it turns out, it looks like it would not add much to the test anyhow.
The conductivity numbers for the partly silver coins when laid flat were mostly around 33 and 34. The clad coin seemed to produce conductivity numbers that were only slightly different than the 90% silver coins, but only by about one point. The clad coin conductivity numbers centered more around 33, while the silver coins were something more like 34. There was a good bit of inconsistency or variance or overlap in the distribution of the numbers, so I doubt anyone could reliably tell the difference between the silver and clad halves in the field based upon the numbers alone anyhow.
Here is what I found interesting. When the coins were turned on end, the numbers varied a lot depending upon the direction the coil was moved relative to the edge of the coin. When the coil was swept in the same direction as the edge of the coin (along the edge of the coin) the numbers on both the clad and silver coins were most often something like 14 - 16. That is a lot different than the numbers obtained from the same coins lying flat on the surface. It appears that the position of the coin relative to the direction of the sweep affected the conductivity of the coins much more than the differences in composition. The surface area and mass of the coins was almost the same for the different coins. although there were very slight differences. The small differences in mass or composition had very little or no affect. The only affect, already noted, is the maybe one point average difference between the 90% silver and clad coins.
There is more though. When the coin was swept perpendicular to the edge of the coin, (across the edge rather than along with the edge), the numbers obtained from the clad coin centered mostly around 33 to 36, while the numbers of the silver coins was more like 36 - 38. These numbers were not much different than the numbers obtained from the coins when they were lying flat. Again, a very small difference was observed between the silver and clad coins.
Of course, the depth obtained from the coins was less than when they were lying flat. I didn't attempt to measure that in this experiment. When the coil was swept along the same direction as the edge of the coin, the depth was reduced a lot - some of the time, the signal virtually disappeared.
When the coin was moved at about a 45 degree angle to the edge of the standing coin, the numbers were very similar to those obtained by the perpendicular sweep, but maybe a touch higher.
The main point that I take away from this version of the experiment is that the silver coins seemed to produce numbers that were maybe slightly higher than the clad, but the difference was so small and inconsistent that I doubt it is of any practical significance. The difference was probably not large enough or consistent enough to be useful in the field.
The more important point, perhaps, is the importance of the sweep relative to the coin. The numbers obtained by sweeping over a nearly flat coin were very different from those obtained by a standing coin when the direction of the sweep was in the same direction as the edge of the coin.
The results showed that if you are digging such a coin and the signal disappears, you might be able to find it again by changing the direction of your sweep by ninety degrees.
The difference in composition for the silver and copper/nickel coins on the conductivity numbers were so small, I doubt the difference will be of no real value in the field.
This is only the first of a number of tests I have planned, which will involve different types of coins and different metal detectors.
Only one detector was used and the particular nature of the site undoubtedly affected the results to some extent, so additional experiments are needed to be able to generalize with confidence. I'm sure that I'll learn more as I conduct additional experiments.
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I just followed up using the Garrett Ace 150, a very inexpensive metal detector. I used the same 1943 and 1971 halves in the same location.
The Ace did not have nearly the depth of the Equinox. It detected the coins at only about six or seven inches.
The Ace target ID was totally accurate. The Ace does not give conductivity numbers, but does have a pictoral graph for various coins and a few other objects. The target ID was totally accurate and consistent on these coins. The Ace consistently gave a half dollar target ID and never anything else. It didn't matter if it was the 90% silver half or the clad half. The depth was much less than the Equinox but the target ID on these coins was perfect in both All Metals and Coin modes.
All Metals mode seemed to give about one more inch of depth than coin mode.
Once again, the differences in the actual metallic composition of these coins did not seem to make any difference.
You might find the Ace useful if you want to hunt junky areas and are primarily interested in picking out coins. From previous experience, I know that it is not good in wet salt sand.
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Below are some of the details on the coins used in this experiment. You'll notice the small differences.
Kennedy Half Dollar Values and Prices (1964-2015) (thesprucecrafts.com)
In 1964 Kennedy half dollars were made from 90% silver and 10% copper. Half dollars made from 1965 through 1970 are composed of two outer layers containing 80% silver and 20% copper with an inner core of 20.9% silver and 79.1% copper (net composition: 40% silver and 60% copper).
Walking Liberty Half Dollar (1916-1947) Value | JM Bullion™
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| Source: MagicSeaWeed.com. |
Late today expect a wind switch. Looks like we'll get five to eight feet. The tides aren't big now.
Good hunting,
TreasureGuide@comcast.net