Fake American Silver Eagle Alert

[charliebrown]

Re: Fake American Silver Eagle Alert

I think you are right Astonas. The object acts like a faraday cage?? I forgot to connect the dots on this one. I will see if I can scare up some silver plated dinner ware and a sterling silver piece and see if i can detect the difference.

[astonas]

Re: Fake American Silver Eagle Alert

I think you are right Astonas. The object acts like a faraday cage?? I forgot to connect the dots on this one. I will see if I can scare up some silver plated dinner ware and a sterling silver piece and see if i can detect the difference.

Very close. Technically, it's more the reverse. A Faraday cage is a structure that acts (to electromagnetic waves) as though it were a solid metal. It can do this even while hollow precisely because the electron resonances set up in the metal fully cancel the fields close to the metal's outer surface, causing EM radiation to be specularly reflected rather than absorbed or scattered. The depth of penetration depends very heavily on the wavelength, but for a 10 GHz (microwave) signal, the depth of penetration is ~0.64 microns for silver. At (higher) optical frequencies, both silver mirrors and gold leaf reflect visible light very well, even at thicknesses of 10s of nanometers.

I'm not sure what frequency or range of frequencies commercial metal detectors cover. But if you wanted to check the differential conductivity as a function of depth, you'd have to span a pretty wide range of frequencies, since depth runs as one over the square root of frequency:

60 Hz	8470
10 kHz	660
100 kHz	210
1 MHz	66
10 MHz	21
100 MHz	6.6

Because of these numbers, I would expect that a multi-layer or plated metal object would be dominated in its reflectivity by the surface metal, even if another metal lies just beneath.

The real challenge is the precision of the measurement: Various metals aren't exceedingly different in the first place:

Aluminum	0.80
Copper	0.65
Gold	0.79
Silver	0.64

(The reason that gold and silver look more shiny is less dependent on conductivity than it is on the surface oxide that forms on base metals - something a metal detector would be relatively insensitive to.)

So unless you own a coin shop and are willing to invest in some decent test equipment (I'd guess about ~$20-50k to do it really well) depth profiling of conductivity isn't likely to be a great choice.


Still, if you try it and can see the difference, please let us know. It is always possible that your metal detector spans a wider (and lower) frequency range than I'm assuming. The design decision ultimately comes down to a tradeoff between how deep it can "see" into the ground, vs. precision in identifying the type of metal. Given that the main job of the tool is to find metal, I'm sort of guessing that they usually shortchange the precision of identification, and stick to thin skin depths (high frequencies).

[shiny!]

Re: Fake American Silver Eagle Alert

I just got home from shooting the breeze with a friend who owns a coin shop. He showed me a bunch of fake silver coins that he's collected. Fake Peace dollars, older silver dollars, all kinds of coins. Some were crude and obvious, others were pretty convincing at a casual glance, but there are ways to spot them. The first thing he does is drop them on the glass counter. Real silver makes a sweet, bell-like ring that fakes don't have. All but one of the fakes could be picked up with a magnet. The one that wasn't magnetic was the wrong weight.

In the last few months he has seen a rise in the number of silver fakes of all types. He's even seen fake Sunshine Mint and Northwest Territorial Mint 1-oz. rounds that came from Ebay. The Ebay ad clearly said the rounds were fake, and they sold for only a few dollars. But the rounds themselves were stamped ".999 Pure Silver". Nasty! He said they are easy to spot because their weight is off. He notified Sunshine mint and the page was soon gone from Ebay.

He doesn't like coins or rounds that come in hardshell plastic cases, because it's hard to get the case off and you must get the case off in order to weigh the coin.

He has very little silver. Says it's been somewhat scarce since it peaked two years ago. The rumor he's heard from other dealers is that a few buyers sucked up most of it and melted it down into bars, which they're sitting on. What's been coming in to his shop for the last two years has mostly been people selling scrap, and lately that has slowed down to a trickle. The US Mint is out of silver.

He told me a funny story:

A guy recently came in to the shop bragging that he had the motherload. He plunked down three silver dollars. Dealer told him they were fake. The man left, disappointed, taking his fakes with him. Fifteen minutes later, two guys walked in with the same three coins. Puzzled, the dealer asked them if they had just purchased them. "No", they said. They had received them as change when they bought a soda at the Circle K. They had been sure the cashier had made a stupid mistake and they were going to profit from his ignorance.

[newnewthing]

Re: Fake American Silver Eagle Alert

Silver has the highest electrical conductivity of any metal but it is still fairly close to copper. Of course copper is about 15% less dense. There is one attribute silver has that is unique and is far enough from other metals that you can test for it without a density check. Silver has a thermal diffusivity that is 50% higher than copper, the next closest metal. Thermal diffusivity is the speed with which a temp change propagates. If you hold a silver coin against an ice cube it will feel colder between your fingers significantly faster than a silver plated copper coin of the same size.