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Using Ultrasound To Test Bullion Metals

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Am in a split mind as to whether this question might get a better/more technically knowledgeable audience in the off-topic board and so am posting this there too.....

Those interested in the precious metals markets (predominantly gold I guess) will likely know even better than me that, given the rise in precious metals prices, fake/counterfeit bullion products are becoming a more serious problem?!

Unsurprisingly to laymen such as myself it is now not uncommon to test larger, and hence very high value, bullion products such as bars with ultrasound to detect non-precious metal content.

An acquaintance of more advanced, but not professional, technical academic background than myself recently said (if I understood correctly) that he believed that beyond just simple testing for presence of foreign bodies within a metal bar/coin, that ultrasound could in principle be used to also determine the 'grade'/quality of the metal making up a bullion coin/bar??? that ultrasound can be used to identify alloy compositions in non-precious metals??? that the process involves something to do with the way the ultrasound pulse fired into the sample is 'coded'?? some sort of spectroscopy?? (I know about light spectroscopy, etc).

I was intrigued. Presumably if technically correct then it should, in principle, be possible to not just conduct quick tests of bullion for obvious foreign body inclusions but also test the 'grade' of genuine bullion products.

Or is my friend himself confused/misunderstanding what ultrasound can and cannot be made to do?

Anyone with a physics background able to answer this one?

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A quick google would seem to indicate that ultrasound testing of precious metals is intended to to reveal porosity, flaws and micro inclusions (i.e. small holes) in the metal rather than its actual chemical composition. I doubt that it'd be possible to detect, say, a small amount of silver mixed with gold by ultrasound. I could be wrong though. :)

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Ultrasound is most useful for analysis of "heterogeneity" - i.e. foreign bodies, voids, sudden changes from one material to another, etc.

A pure, defect free lump of metal, fluid, etc. is completely transparent to ultrasound and produces no echoes (what the probe detects). A flaw, a change from one material to another (with different stiffness, or more precisely impedance) results in a reflection.

The way ultrasound testing works is to place a transducer on the surface, send a pulse, and check when the echo arrives. If it arrives at the time you'd expect for the sound to travel through to the other end, and reflect back off the other side, and return, then the part is legit. If it comes back early, then there is a problem.

Testing alloy grade is much more difficult, and I'm not sure how this would work. There are harmonic distortion methods which are used in the medical ultrasound field, which work by detecting distorted echoes (from non-linear deformation of tissues, which gives 2nd harmonic distortion, or non-linear deformation of injectable gas-filled microbubbles, which give 3rd harmonic distortion). I find it difficult to believe that a metal would give much distortion at all, except at very high ultrasound energies, but I suppose it's possible to measure the hardness or resistance to plastic deformation this way. I doubt very much that it would be practical.

Potentially, other ways are techniques that measure change in velocity of sound, which is sensntive to material stiffness. With transducers placed a known differenec apart, this is potentially doable.

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A quick google would seem to indicate that ultrasound testing of precious metals is intended to to reveal porosity, flaws and micro inclusions (i.e. small holes) in the metal rather than its actual chemical composition. I doubt that it'd be possible to detect, say, a small amount of silver mixed with gold by ultrasound. I could be wrong though. :)

I did some Googling too on this but, if you aint a scientist (which I aint) then one can easily spend hours looking at loads of websites and still be none the wiser.

Agreed on the point that it is ordinarily used to test for flaws, foreign objects, etc. That much I do know.

But, similar to what my friend alluded to, I did see some stuff on some websites that seemed to suggest (to me at least, from the titles) that identification of alloy composition is also possible?? But as I say, it was very technical and way over my head.

The point being that IF ultrasound can be used for material identification then surely that makes it superior to all the various surface only testing methods (e.g acid tests, XRF tests) ???

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Ultrasound is most useful for analysis of "heterogeneity" - i.e. foreign bodies, voids, sudden changes from one material to another, etc.

A pure, defect free lump of metal, fluid, etc. is completely transparent to ultrasound and produces no echoes (what the probe detects). A flaw, a change from one material to another (with different stiffness, or more precisely impedance) results in a reflection.

The way ultrasound testing works is to place a transducer on the surface, send a pulse, and check when the echo arrives. If it arrives at the time you'd expect for the sound to travel through to the other end, and reflect back off the other side, and return, then the part is legit. If it comes back early, then there is a problem.

Testing alloy grade is much more difficult, and I'm not sure how this would work. There are harmonic distortion methods which are used in the medical ultrasound field, which work by detecting distorted echoes (from non-linear deformation of tissues, which gives 2nd harmonic distortion, or non-linear deformation of injectable gas-filled microbubbles, which give 3rd harmonic distortion). I find it difficult to believe that a metal would give much distortion at all, except at very high ultrasound energies, but I suppose it's possible to measure the hardness or resistance to plastic deformation this way. I doubt very much that it would be practical.

Potentially, other ways are techniques that measure change in velocity of sound, which is sensntive to material stiffness. With transducers placed a known differenec apart, this is potentially doable.

I wonder what area of physics is involved in these guys claimed proprietary method of testing?

http://www.bullionanalysis.com/index.html

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Way of the world....nobody knows what they are buying anymore, nobody trusts another because governments, banks, corporations, rating agencies etc etc have been seen and proven to be untrustworthy. ;)

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Archimedes + Ultrasound should give a pretty reliable indication of the purity of gold, but if there are impurities of any significance, I would thing that various different impurities in different quantities could have the same 'signature' (i.e. change in Young's Modulus, which effectively is the value for the relative speed of sound in any medium).

To put it another way, Gold with X% of impurity A might be hard to distinguish from Gold with Y% of impurity B, if the values result in the same change in the speed of sound in the material. But Archimedes' principle of calculating density would then show them apart.

Another problem might be that the impurities might not be heterogenous (e.g. a bar of tungsten inside a bar of gold) causing multiple echoes within the bar.

While Chumpus' is correct in saying that a defect-free bar would be transparent to ultrasound, it is the boundary (the edge of the bar) that reflects, as the boundary with (say) air changes the refractive index (gold and air do not transmit sound at the same speed). Just like looking into a swimming pool, there is refraction and reflection at the boundary. If the bar thickness is known, then the purity can be assessed.

I understand it is quite hard to coat small tungsten items with gold, though some Chinese companies claim to produce such 'novelty coins'.

If you buy gold on the open market, it probably is gold. All the tungsten is stacked away in J. P. Morgan's basements.

I am waiting for Chinese counterfeit ultrasound testers to come on the market that detect tungsten as gold....

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Am in a split mind as to whether this question might get a better/more technically knowledgeable audience in the off-topic board and so am posting this there too.....

Those interested in the precious metals markets (predominantly gold I guess) will likely know even better than me that, given the rise in precious metals prices, fake/counterfeit bullion products are becoming a more serious problem?!

Unsurprisingly to laymen such as myself it is now not uncommon to test larger, and hence very high value, bullion products such as bars with ultrasound to detect non-precious metal content.

An acquaintance of more advanced, but not professional, technical academic background than myself recently said (if I understood correctly) that he believed that beyond just simple testing for presence of foreign bodies within a metal bar/coin, that ultrasound could in principle be used to also determine the 'grade'/quality of the metal making up a bullion coin/bar??? that ultrasound can be used to identify alloy compositions in non-precious metals??? that the process involves something to do with the way the ultrasound pulse fired into the sample is 'coded'?? some sort of spectroscopy?? (I know about light spectroscopy, etc).

I was intrigued. Presumably if technically correct then it should, in principle, be possible to not just conduct quick tests of bullion for obvious foreign body inclusions but also test the 'grade' of genuine bullion products.

Or is my friend himself confused/misunderstanding what ultrasound can and cannot be made to do?

Anyone with a physics background able to answer this one?

It'snot a physicist you need, it's a metallurgist. Or better still a non destructive testing (NDT) technician. That was me in a previous career. I did about 5 years as a quality assurance tester in a steelworks, and used ultrasonic detection quite a lot on steel bars etc. It was testing for inclusions (usually bits of refractory crap) and porosity that had been rolled or drawn into the wire/bar, If it had been a composite material, such as tungsten plated with gold, then that would have been very detectable (due to the interface between Au and W). Chemical analysis is not what ultrasonic testing does though - spectroscopic or wet analysis does that (both are destructive though).

We did need a smooth surface to apply the probe to though - you need good probe to metal contact.

Archimedes and ultrasonic testing could be a pretty reliable way of assessing a smooth ingot.

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If you buy gold on the open market, it probably is gold. All the tungsten is stacked away in J. P. Morgan's basements.

There's still probably some in Carrock Mine (along with rather a lot of arsenic IIRC).

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Archimedes + Ultrasound should give a pretty reliable indication of the purity of gold, but if there are impurities of any significance, I would thing that various different impurities in different quantities could have the same 'signature' (i.e. change in Young's Modulus, which effectively is the value for the relative speed of sound in any medium).

To put it another way, Gold with X% of impurity A might be hard to distinguish from Gold with Y% of impurity B, if the values result in the same change in the speed of sound in the material. But Archimedes' principle of calculating density would then show them apart.

Another problem might be that the impurities might not be heterogenous (e.g. a bar of tungsten inside a bar of gold) causing multiple echoes within the bar.

While Chumpus' is correct in saying that a defect-free bar would be transparent to ultrasound, it is the boundary (the edge of the bar) that reflects, as the boundary with (say) air changes the refractive index (gold and air do not transmit sound at the same speed). Just like looking into a swimming pool, there is refraction and reflection at the boundary. If the bar thickness is known, then the purity can be assessed.

I understand it is quite hard to coat small tungsten items with gold, though some Chinese companies claim to produce such 'novelty coins'.

If you buy gold on the open market, it probably is gold. All the tungsten is stacked away in J. P. Morgan's basements.

I am waiting for Chinese counterfeit ultrasound testers to come on the market that detect tungsten as gold....

Agreed. Making gold/tungsten bars is only worthwhile for massive bars.

For dealers, my guess their reputation is worth more than any amount they may save and the risks they take if they make fake material. It would destroy their business. If you want to be sure just buy direct from a long established quality dealer.

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If you're looking to make sure you don't get ripped off buying gold, stick to coins and buy a Fisch

Not cheap, but does the job.

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  • 239 Brexit, House prices and Summer 2020

    1. 1. Including the effects Brexit, where do you think average UK house prices will be relative to now in June 2020?


      • down 5% +
      • down 2.5%
      • Even
      • up 2.5%
      • up 5%



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