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anonguest

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 of non-precious metal objects??? 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?

Edited by anonguest

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Your mate is roughly correct.

The really big businesses with a massive throughput, which can justify the enormous cost, (such as the Assay Offices), use X-Ray technology - specifically X-Ray Fluorescence Spectrometry - to analyse most metals given to them. If they have any doubt, they scrape a minute amount off the article and test it in their laboratory before they hallmark the item. But I digress...

If you get an ultrasonic sound meter, (they are relatively cheap) you can set it to the speed of sound in Gold (for example) which is 3240 m/s. Once the meter is set to the expected velocity of sound in the test material, you then test your bullion bar. Our set of calipers gives, say, a measured thickness of 20 mm. The meter returns an expected thickness of the bar. If our bar returns a tested thickness of 20 mm and our caliper gauge returns a thickness of 20 mm, then the article is pure Gold.

However, if our ultrasound meter is set for Gold and our ultrasound meter returns a thickness of 12 mm, but our original measurement with our caliper gauge says the bar is 20 mm thick, then the speed of sound in the bullion bar is much faster than the speed of sound expected in Gold.

Perhaps our bar contains an insert of a substance with a speed of sound much faster than Gold, such as Tungsten....

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It's about 30 years since I earned my living as an ultrasonic technician testing metals and the technology has moved on massively so that you can now count your baby's teeth in the womb. But the fundamental physics hasn't changed.

I wouldn't risk the price of a large ingot of gold based on an ultrasonic test. Here's the options:

1. The gold is alloyed - weighs correctly but is lower density. Ultrasonic is not sensitive enough to spot this, you need XRF or chemical assay.

2. The gold is pure on the outside, tungsten inside. Ultrasonic might help if you are lucky, but anyone smart enough would put the interface layer in the dead zone under the probe.

3. X-Rays would help you spot any interfaces or slugs of foreign metal inside, but the kit at your local dentist isn't powerful enough.

I wouldn't trust an ultrasonic thickness meter to tell me with sound velocity measurement. There's too much variation due to grain structure, etc. The only secure way is to take a through-thickness drilling and analyse the contents. Even that doesn't ensure you have hit all of the tungsten that someone has put inside. But if soft gold suddenly turns into something the drill won't penetrate, you have bought a duffer.

Gold is expensive (and still massively overpriced) stuff. If you don't trust people, don't buy it.

Edited by Stainless Sam

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It's about 30 years since I earned my living as an ultrasonic technician testing metals and the technology has moved on massively so that you can now count your baby's teeth in the womb. But the fundamental physics hasn't changed.

I wouldn't risk the price of a large ingot of gold based on an ultrasonic test. Here's the options:

1. The gold is alloyed - weighs correctly but is lower density. Ultrasonic is not sensitive enough to spot this, you need XRF or chemical assay.

2. The gold is pure on the outside, tungsten inside. Ultrasonic might help if you are lucky, but anyone smart enough would put the interface layer in the dead zone under the probe.

3. X-Rays would help you spot any interfaces or slugs of foreign metal inside, but the kit at your local dentist isn't powerful enough.

I wouldn't trust an ultrasonic thickness meter to tell me with sound velocity measurement. There's too much variation due to grain structure, etc. The only secure way is to take a through-thickness drilling and analyse the contents. Even that doesn't ensure you have hit all of the tungsten that someone has put inside. But if soft gold suddenly turns into something the drill won't penetrate, you have bought a duffer.

Gold is expensive (and still massively overpriced) stuff. If you don't trust people, don't buy it.

You raise good points, but...

1. Fake marks are actually quite easy to spot. Moreover, to make it worthwhile, so much of the Gold would have to be replaced that the ultrasound meter would simply give readings incorrect enough to be suspicious - remember it doesn't have to give accuracy to "n" decimal places, if it's out of kilter by a small amount, then you don't purchase the bar. I also acid-test ingots, and any alloy would fizz like sherbet.

2. No gold-plated tungsten bar would fool an ultrasound tester for an instant. All the dodgy ingots I have seen, have had rods or a complete slug of tungsten inside. These would both show on the ultrasound tester. You would take measurements in at least three or four places.

3. I agree with this one; if I wanted it tested that badly, I would send it to be hallmarked at the Birmingham Assay office. That would be a cast-iron (cast-gold?) guarantee. They simply cannot be fooled.

4. If you are buying an actual "London Good Delivery" Bar (400 toz Gold or 1000 toz Silver), you'd send it to be melted, re-cast and re-assayed like the Germans, Indians and Chinese do. Sadly, no-one trusts the LBMA markings any more.

The bottom line (bold and underlined) is the best guarantee; completely agreed there, although I'd disagree with the overpriced bit.

Edited by Old Nis

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Always buy from a top-rate and trusted dealer. I've never had any problems with Coininvestdirect.

Yes, agreed. It's the buying that's important. When you sell, you know you are honest, and you convince the other party, then all well and good. There is nothing to stop him using these or any other techniques to satisfy himself as to the quality of the Gold/Silver/Palladium/Platinum.

Any decent bullion dealer has far more to lose in terms of their reputation than they would stand to gain by passing off a dodgy bar to one of us. They would be able to set it against their tax, anyway.

Edited by Old Nis

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  • 238 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% +
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      • Even
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      • up 5%



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