Verifying Gold Coins Are Real?

[Tortoise]

I am not a metallurgist, but in the process of obtaining my engineering degree I had to take a fair number of chemistry and physics courses. What you are proposing, WildAboutHarry, would violate the definition of the word density.

Density is defined simply as mass divided by volume. So if you create an alloy that has exactly the same density as the alloy in a real coin, and you then create a coin out of it that weighs exactly the same as the real coin, then by definition the volume of the fake and real coins will be exactly the same as well.

[Tortoise]

I wonder if a "ring test" might be useful for detecting coin fakes?

Canadian quarters minted in 1967 are either 80% silver or 50% silver, and one way to determine the difference (that seems to work very well) is to record the sounds of a dropped coin and examine the peaks on an audio frequency histogram.

See the video:

http://www.youtube.com/watch?v=qZdxf655fe4

All you need are some free software and a microphone.

Very interesting! If this method would also distinguish between 1-ounce gold bullion coins and tungsten (I'm guessing there's a good chance it would), that would be extremely useful! Fast, cheap, and portable.

[WildAboutHarry]

Density is defined simply as mass divided by volume.

Right, but the crystalline structure of gold plus copper may result in a different density than tungsten plus copper, since the atoms would not necessarily align the same in the two alloys.  One may pack better than the other and that would then throw off the weight-diameter-thickness relationship.

If this method would also distinguish between 1-ounce gold bullion coins and tungsten (I'm guessing there's a good chance it would), that would be extremely useful!

I thought it was a pretty slick method.  Does anyone have any tungsten coins to test?




   

[TripleB]

I'd be willing to buy a tungsten coin if the price was right. Would that be legal? Or would I be arrested for owning counterfeit items? Is it only illegal if I try to sell it fraudulently? I know owning counterfeit paper money is definitely illegal. I guess since American Eagles are considered currency with a $10 or so price, it would be illegal just to own a fake tungsten Eagle. However kruggerands should be OK

[Tortoise]

I don't have any tungsten coins, but I think it would be interesting to use the free Audacity software to perform a ring test on both American Eagles and Krugerrands and compare the peaks in their audio spectra. I may stop by my safety deposit box this week and pick up a couple of my coins to do that test.

The only metal difference between Eagles and Krugs is that Eagles contain 3% silver and Krugs use more copper instead of that 3% silver. So if comparing audio spectral peaks can reliably detect a difference between those two coins--which are very similar in metal content--that would give me a lot of confidence that any tungsten fakes would stick out like a sore thumb using that method. On the other hand, if the audio spectral peak method can't detect the Eagle/Krug difference, then probably the only thing that would give me confidence in the method would be to test an actual tungsten fake.

I was surprised to read the following in the gold coin entry on Wikipedia:

The majority of bullion counterfeits (of all types) are rare, and fairly easy to detect when comparing their weights, colors and sizes to authentic pieces. This is because the cost of reproducing any given coin precisely can easily exceed the market value of the originals.

That would be interesting if it's true. But as Craig pointed out, as the price of gold continues to climb, the cost of producing high-quality counterfeits relative to the price of gold will decrease and attract more counterfeiters.

[smurff]

To add confusion to the issue, there are a couple of companies that sell "replicas" of popular bullion gold coins like the American Eagle.  You see them advertised on TV, usually for $19.99. (But wait!  Call now and we'll double your offer--now get two for the same low price of $19.99.)  Some even sell replicas of American Eagle Silver coins, replicas made of 99.99% silver, but in a 4 oz (1/4 pound) size, like this one
http://www.littletoncoin.com/webapp/wcs ... 9318?nav=1

The term "fake" is never used, but I have seen the term "tribute" applied to such coins. They are perfectly legal, as long as the word "replica" or "copy" is stamped on the coin.  But that word may be hard to see, or it may be scratched off, or it never was stamped on in the first place. 

Here's a bunch of them from one of Amazon's Affiliates:

http://www.amazon.com/s?ie=UTF8&search- ... %20Company

The price would be a give away to intelligent people functioning under common sense, but I have ceased to believe that everyone is like this.  When a gold rush is on, weird things will start to happen. 

To an experienced bullion coin owner, those replicas are easy to spot.  They're a different weight and make a different sound from real bullion coins when dropped on a hard surface.  The stamped-on or imprinted design will not be as distinct as that of the real coins--or it will be excessively so.  But I suspect that as the gold bubble develops and people with no experience (holding real gold coins in their hands) buy coins on eBay and other miscellaneous venues, some of the replicas will be sold to inexperienced, unsuspecting buyers as real bullion.

[Tortoise]

I used the free Audacity software just now to perform a few quick ring tests on a gold Krugerrand and a gold Eagle (recorded the sound they make when dropped on a hard surface, then looked at the audio spectrum of the recordings). Surprisingly, the audio spectra of those two coins have peaks at distinctly different frequencies! Take a look:



Krugerrand peak frequencies in kHz: 9.0, 10.9, 18.5, 18.9
Eagle peak frequencies in kHz: 8.1, 10.4, 17.1, 17.4, 18.0

For each of the two coins, I took three recordings. The spectrum for each of the two coins was very repeatable; the peaks were at exactly the same frequencies each time. It's like a signature. Unfortunately, this is a limited sample set since those are the only two gold coins in my possession at the moment (the others are in a safe deposit box).

I plan to perform this same ring test on other Krugerrands and Eagles to see if the peak frequencies are the same for all specimens I try. I also encourage anyone interested to independently verify my results above. (It's a very easy test.) If the test can so clearly differentiate between Krugerrands and Eagles--coins that are virtually identical in size, weight, and gold content, with the only difference being a 3% non-gold metal content--imagine how clearly it should differentiate between a tungsten fake and the real thing!

This quick, easy test that anyone with a laptop and microphone can do is making me feel much better about being able to avoid counterfeit bullion coins that can fool standard size/weight tests. Many thanks for bringing it to our attention, WildAboutHarry!

[Gumby]

Surprisingly, the audio spectra of those two coins have peaks at distinctly different frequencies!

That doesn't surprise me. The American Gold Eagles are alloyed with 3% Silver and 5.33% Copper. The Krugerrands are only alloyed with 8.33% Copper. You'll also notice that the color is ever so slightly different between the two coins.

[Tortoise]

Surprisingly, the audio spectra of those two coins have peaks at distinctly different frequencies!

That doesn't surprise me. The American Gold Eagles are alloyed with 3% Silver and 5.33% Copper. The Krugerrands are only alloyed with 8.33% Copper. You'll also notice that the color is ever so slightly different between the two coins.

Yes, and that is why I specifically mentioned the 3% metal difference in my post. What was surprising to me was that a mere 3% difference in metal content can lead to a difference in peak frequencies of much more than 3%.

[WildAboutHarry]

Many thanks for bringing it to our attention, WildAboutHarry!

My pleasure.  I was very skeptical when I saw the YouTube video demonstrating the technique for separating Canadian quarters, but it worked! 

While I think weight-diameter-thickness will detect most of the fakes out there, having another confirming test independent of those three is quite valuable.  Did you export the audio spectrum to Excel to do your graphs?  It would be nice if a library of audio signatures could be developed for various bullion coins, but that might require standardization of the surface, height of drop, etc.

Could you imagine trying to do this test in the pre-computer days?  Microphone, audio amplifier, oscilloscope, etc.

Since gold has greater thermal and electrical conductivity than tungsten, presumably you could use these as well for detecting fakes.  The electrical resistance measurements might be difficult, but for thermal conductivity I'm thinking about a hot plate and one of those cheap non-contact infrared thermometers...

[Tortoise]

While I think weight-diameter-thickness will detect most of the fakes out there, having another confirming test independent of those three is quite valuable.  Did you export the audio spectrum to Excel to do your graphs?

Yes, after you record the sound and go to Analyze --> Plot Spectrum, clicking the "Export" button allows you to save the spectrum as a text file. You can then cut-and-paste the text into a spreadsheet. I used a Google Docs spreadsheet so that I could share an image of the plot with everyone.

It would be nice if a library of audio signatures could be developed for various bullion coins, but that might require standardization of the surface, height of drop, etc.

Definitely. Luckily I think just about anyone with a laptop and mic can experiment a little bit with different surfaces, drop heights, etc. to see how the spectral peaks vary and thereby start developing their own library of audio signatures (if they are so inclined).

The results I showed above were using a granite countertop (the hardest and heaviest surface I could find in my home) and a drop height of about 1 inch. On one of the tests I spun the coin gently instead of dropping it, but the peaks in the resulting spectrum were the same.

I tried the test again just now with a Gold Eagle using the top of a light wooden table. The spectral peaks at 8.1 and 10.4 kHz are still very prominent, but it's harder to reproduce the peaks at the higher frequencies; sometimes they're there, and sometimes they're swamped by the noise floor. My guess is that the top of a light wooden table is inferior to a granite countertop for this test since it isn't as hard and makes its own unique sound when a hard object is dropped on it.

[Lonestar]

How about this instead of the $170 fisch tool?

http://www.goldcoinbalance.com/

That is very cool. Looks like a simplified version of the Fisch tool. I ordered one and will post a review. Thanks for the link.

Craig,

Any review for us yet?