Verifying authenticity of gold via striking it like a bell, and mobile apps to support that

[MythicQuale]

Hi all, first post to this forum. I'm looking to invest a small amount in coins for all four types of metal shown at the top of this forum. Problem is, I'm a bit paranoid and want to test for authenticity.

Platinum is the easiest to check for and can be done in minutes by anyone. It's about 11% heavier than gold, and the only heavier metals that exist are Osmium and Iridium which are insanely expensive metals, so it's impossible to fake platinum and I LOVE that about it.

Gold is also very heavy, but as most of you probably know, it has a rival in its weight class known as tungsten which can be used as cheap filler inside the gold. So on top of measuring the density with some string like this, another idea is to use a "ping test", and here we come to the crux of my post: Tapping the gold coin like a bell to test for harmonic frequencies.

By testing for sound, you can test for not just fake coins, but various other types of coin like sterling versus pure silver, or bigger/smaller coins. Here's one example of how this works. As you can see the real gold coin has a nice resonant high pitched metallic sound, while the tungsten fake sounds dull.

So my main question is this: Is this a fool proof way of testing for tungsten inside 24K gold coins? Will tungsten ALWAYS make it sound dull? And will 24K gold ALWAYS have a resonant bell sound to it?

There are various mobile apps which directly measure the frequencies of a coin including CoinPing, Bullion Test, PingCoin and Precious Coin Tester (here's an example video with one of the aforementioned programs). They are then compared with known frequencies to see if they match. Are these reliable? Has anyone ever found 24K gold coins to unfortunately not match, or for fakes to ever unfortunately match the known real gold frequencies?

[Pete]

 I fear you are going to have some sleepless nights testing for authenticity as mentioned.
Buy your coins from a reputable source and don't try to verify by these methods unless you have a better understanding of the physics and especially the errors - like researching Dr. Google when you have a stomach ache.

I was especially disappointed in the video showing how to make a SG test.
Lots of boiler plate waffle in the dialogue.
Shows the basic method but is done like a primary school project.
Note he uses a piece of string rather than fine thread introducing too much of an error.
He dips the string in the water and the numbers are fluctuating too much but he ignores.
Water temperature is important but that doesn't get a mention either.
Yes he bought a calculator from XYZ store so who gives a F* and he suggests higher accuracy scales - doubt that would make any difference as there are larger errors at play.
Sorry for the rant but I find videos like this so irritating, at least the dog wasn't barking or he had to answer his phone unlike so may Youtube videos these days.
Why did he not just tare the scales with the string dipped and avoid a serious bit of mathematics of deducting B from A which he managed to do without using a calculator ?
He then shows his ASE has a measured SG of 10.60 which he also shows as 0.11 higher than fine silver.
This is an error of 5% and he would accept almost double this error !!!
SG is a precise measurement and has to be done properly and most of us might be happy with a 2 % error.
If as he has written down fine silver is 10.49 whilst sterling silver is 10.36 and he is happy to accept an error up to 0.2 !!
If he dipped a sterling silver 0.925 commemorative coin in his water and measured 10.36 + 0.11 = 10.47 he would be convinced that the coin was fine silver.

As for ping testing gold coins - good luck with that method having myself spent many hours trying out the technique with some success but for well known coins, weight and dimensions are enough. I very much doubt anyone will have tried to fake a gold coin with a tungsten core but a bar - possibly -but more like a lead/ base alloy filling so it will be measurably thicker but accurately measuring bars with tapered edges and rounded corners would be a problem so back to the beginning - know your source.

 

[MythicQuale]

Haha yeah like you, there were a few accuracy flaws I picked up too, but I linked it just to show the basic idea as you might have guessed. What's that saying, "measure with micrometer, mark with chalk, cut with axe" kinda thing. I almost linked this one instead actually, but I liked how the experiment can be done without using special purpose equipment.

[fearlessjs]

Hi MythicQuale,

Disclaimer: I'm the developer of one of the coin testing apps you mention: Pingcoin. That said, I also have a degree in physics specializing in materials science.

Quote

Is this a fool proof way of testing for tungsten inside 24K gold coins? Will tungsten ALWAYS make it sound dull? And will 24K gold ALWAYS have a resonant bell sound to it?

I was able to procure two tungsten filled gold-plated Krugerrands from a dealer in China a few years ago. A couple of observations I made at the time:

- Diameter/thickness was matching perfectly to the point that they passed the Fisch test (which only passes if weight, diameter and thickness are within an allowed range)

- The inscription on both sides was very well done, but would not fool the trained eye. The lustre was off, the color was a bit off and there wasn't the same degree of smoothness as with a real Krugerrand

- There was no "ping" to speak of, it sounded like a dud. 

 

Tungsten is typically sintered (not melted), which results in dud-like sounds

One difficulty counterfeiters have with tungsten is that it requires much higher temperatures to melt than gold. These temperatures are so high that all processing of tungsten happens through sintering rather than melting. This means they squeeze grains of tungsten together at high temperature and pressure rather than the metal ever passing through a liquid state. One consequence of this different production process is that the final product, at a microscopic scale, doesn't have the clean crystalline quality a metal will have that went through a liquid phase. A rough analogy would be a snowball (which you compress by squeezing together solid water molecules) vs. an ice cube. An ice cube is much denser of course than a snow ball, but imagine squeezing so hard you get to the same density. At a microscopic level the ice cube will have very neatly arranged atoms, because it was allowed to crystallize from the liquid state. The snowball on the other hand was crystallized already, and the crystal grains (microscopic scale remember) were simply smashed together. One consequence of this neater crystal structure (gold/ice cube) is that sound (caused by vibrations through the crystal) has an easier time propagating through the material. A vibration travelling through a messy crystal structure (like sintered tungsten) is going to attenuate much quicker. This leads to ping sounds that get dampened more quickly and sound like duds.

 

Even if you would be able to melt tungsten, it won't sound anything like gold

The sound of a coin is determined by a small number of variables:

Geometric variables: Thickness/diameter

Material variables: Density, elasticity, Poisson ratio

Counterfeit gold coins with a tungsten core might get close in terms of thickness/diameter and density, but they cannot simultaneously get close in terms of elasticity (E) and Poisson ratio. Here's an overview of the relevant material parameters compared:

	Gold	Tungsten	Silver	Platinum
Density [kg/m^3]	19300	19350	10500	21450
Elasticity modulus [Pa]	7.00E+10	4.11E+11	8.30E+10	1.68E+11
Poisson ratio	0.44	0.28	0.37	0.38

This turns out to be a huge difference. Platinum, gold and silver turn out to be much closer related in terms of elasticity and Poisson ratio than tungsten. Here's a graph of these materials plotted in terms of these two dimensions to give  you a better feeling for that.

How does that translate into the resonance frequencies that get recorded by these apps? Gold and silver coins actually produce very comparable resonance frequencies. In both cases there are 3 that can reliably be captured by a smartphone microphone. For a 1 oz gold maple leaf for instance, these are: 4700-5000 kHz, 11 kHz and 18 kHz.

If you put the parameters I listed above into a physics model (Finite Element Analysis) for a coin and calculate the theoretical frequencies (which I've done), you get something similar for the 1 oz gold maple leaf. If you calculate the theoretical frequencies for a tungsten coin, the frequencies blow up. The first frequency starts at 11kHz for tungsten, and the second and third won't even get recorded by your smartphone, because they'll be outside its range (of 22 kHz). We can understand this intuitively based on the parameters above: tungsten is much, much "harder" (i.e. resistant to elastic deformation) and much less "compressible" than gold.

Gold 1 oz Maple Leaf		Tungsten
1 (c0d2)	4846	11474
2 (c0d3)	11054	(25964)
3 (c0d4)	18885	(44187)

That said, if the coin is sintered tungsten (which it probably will be), you'll have a hard time registering any frequency with one of the apps you mentioned, because the sintered microstructure will prevent the coin from generating a nice ping sound.

 

The ping test is also sensitive to inhomogeneities in the coin

A lesser known benefit of the digital version of the ping test is that the resonance frequencies of a coin are sensitive to the homogeneity of the material. If you create a coin that consists out of two materials instead of one, it will vibrate differently (different peaks) and less well generally (shorter ping). Although no app will help you analyze the ping length, a change in peak location should get flagged by any quality ping testing app (that's the point!)

 

---

So to circle back answer your questions:

Is this a fool proof way of testing for tungsten inside 24K gold coins?

No. I think thinking in terms of a fool proof method is the wrong way to go about it. I find it helpful to think in terms of what level of risk you're willing to accept and to act accordingly.

Buy from a reputable dealer.

Get accustomed with the type of coin you're buying. Develop an intuition for their details, lustre, inscription. All the fakes I've seen had a lustre which was notably off.

In terms of testing, you can lower your risk by combining multiple tests which — and this is key — test for different, uncorrelated aspects of the coin.

1. Thickness/diameter (tests for coin dimensions)

2. Weight (tests for weight, in combination with the above, implicitly tests density)

3. Visual inspection (tests for inscription details, and lustre. Requires some experience)

5. Digital ping test (binary verdict on: geometry, density, elasticity and poisson ratio + homogeneity grouped together)

With specific gravity you're measuring the density. If you're already measuring the density through other means (e.g. implicitly as above), then you're not decreasing your risk by adding it on as an additional test.

 

Will tungsten ALWAYS make it sound dull?

Practically speaking, yes. Tungsten is sintered not melted, and sintered tungsten won't sound like much. And even if it's melted, the frequencies shift up by so much, that only 1 peak will appear in the apps (so they should flag it), and it won't sound right. In fact, tungsten fakes will sound so "off" that it will barely make sense to test them in the app. 

 

And will 24K gold ALWAYS have a resonant bell sound to it?

Most gold coins have a nice resonant bell to them, but some (due to a combination of composition and their production idiosyncracies) have very dull sounding pings. For instance many gold maple leafs have very dull, short sounding pings. But they're still sustained enough to show up in ping testing apps. Pure gold coins like maple leaf seem to be more prone to this, whereas gold-copper alloy coins like the gold Krugerrand sound beautiful and due to the copper aren't as easily dented/damaged.

 

[MythicQuale]

Fearlessjs, what a brilliant response - thank you! I have a few comments in response to your insight into all this...

Any chance of adding the 1oz Britannia 2021 (and 2014) to your pingcoin app? I'd love to see how that compares against the results from the Bullion Test app.

Quote

 

Is this a fool proof way of testing for tungsten inside 24K gold coins?

No. I think thinking in terms of a fool proof method is the wrong way to go about it.

 

If adding tungsten will ALWAYS results in dull thud as you say, then surely, the ping test IS fool proof, since tungsten is the only filler they can put into gold to make it a remotely convincing counterfeit coin (testing for weight or density is easy otherwise).

I know that more testing never hurts, but I'm just curious how reliable the ping test is for detecting tungsten in gold (which from everything you said seems to be VERY reliable, and yes, practically fool proof, since it'll produce no real tonal frequencies).

Perhaps also you can help with my other thread. Testing with the "Bullion Test" app, my gold coin has all the frequencies, except the 18-19k one, which it doesn't have at all. Any idea why that may be the case? Perhaps 1oz Britannia 2021 coins are made differently to the 2014 (which is what I selected in the Bullion Test app since it didn't support the 2021). My coin also has a short relatively muted ring sound, so maybe like you say, the gold is formed differently somehow? What sort of characteristics would that difference take? Work hardening or something?

[fearlessjs]

51 minutes ago, MythicQuale said:

I know that more testing never hurts, but I'm just curious how reliable the ping test is for detecting tungsten in gold (which from everything you said seems to be VERY reliable, and yes, practically fool proof, since it'll produce no real tonal frequencies).

Yeah, it won't produce a tone unless the counterfeiters have figured out how to melt tungsten, in which case the tone will be ridiculously high frequency and should show up in the app as such. I agree that's a pretty good test against counterfeits.

The only small caveat I would have is that, as I mentioned, the maple leaf (and probably some others) can also sound like a dud. Typically the dud for a maple leaf is enough to register as a ping in the app, and the tungsten dud isn't. But without an app, they might both sound equally alarming (or not) to the untrained ear / collector.