Non-Destructible Tests

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Mark Messa

Member
Joined
Nov 4, 2015
Messages
23
Oracle,

What are the non-destructible tests (NDT) that you could use when dealing with 999 gold coins/bars?

Regards
Mark Messa
 
Ken,

The problem with Nitric Acid is that you have to scratch the surface.
Unfortunately, most dealers don't let you do it.
 
upcyclist,

> "An XRF and a density measurement would get you pretty close [...]"
Ok, I understand your point.
With XRF you can be sure about the surface content but not about the interior.
To minimize the interior problem, you can do a density measurement.
Even so there is a chance of the interior be made of a material different than gold but with the same density.

Is there a way to avoid that error?

Regards,
Mark Messa
 
I would think not, but as you can see from my "join date", you'll want to wait for other opinions.

An XRF, as you know, only shows what's on the surface. By definition, you can't get below a surface without destruction. The density is the best thing I can think of to point to internal composition.

FWIW, the only elements denser than gold are among the Platinum Group Metals (Osmium, Iridium, Platinum), radioactive Actinides (Uranium, Americium), Rhenium, and Tungsten. Therefore an alloy as heavy as gold would have to include one of those, right?
 
upcyclist,

> "By definition, you can't get below a surface without destruction."
I'm not so sure about that.
Ultrasonic tests, for example, can measure the speed of sound inside the sample without destroying it.

> "[...] the only elements denser than gold are among [...]"
Ok, I agree. There are not so many elements denser than gold.

> "Therefore an alloy as heavy as gold would have to include one of those [...]"
Although this is the conventional wisdom disseminated all over the internet, I need more convincing before believing in it. The reason is the following:

There are some chemical compounds denser than their constituents. I don't remember their formulas, but there are some liquids that can dissolve several liters of gases. At the end, the final solution is considerable denser than the initial liquid. This is a well know phenomenon that even has industrial applications. Similar situations occur in crystallography: the atomic packing factor is changed and, consequently, the density also.

I've never heard about an alloy denser than gold that was made up of lighter elements. But this doesn't mean it is impossible. The principle of mass conservation of Lavoisier states only that the mass must remain constant, not the density. I would like to hear the comments of the chemists and physicists about that.


> "The density is the best thing I can think of to point to internal composition."
An additional ultrasonic test might solve any doubt regarding the internal composition?
 
Dangit--you're right about alloys, I missed that, and should know better. Just as an alloy of two higher-melting point metals can have a lower melting point than either of the constituent elements, I can't say for sure that an alloy of two metals will have a density between the two constituents. I too can say I've never heard of one, but that doesn't mean it isn't out there.

On the ultrasonic, I'll just beg forgiveness for my ignorance. There are indeed tests that can "see" underneath the outer microns of something (x-rays, too!)--I guess I was thinking that nothing will reveal it in the physical sense, that you can't "touch" it with anything.
 
Mark Messa said:
I've never heard about an alloy denser than gold that was made up of lighter elements. But this doesn't mean it is impossible. The principle of mass conservation of Lavoisier states only that the mass must remain constant, not the density. I would like to hear the comments of the chemists and physicists about that.
I think it's theoretically possible and I know about silicon with diffused lithium inside the crystal structure. What it does to the density I don't know.

... so I googled a bit and found
https://en.wikipedia.org/wiki/Atomic_packing_factor
... where it states
It can be proven mathematically that for one-component structures, the most dense arrangement of atoms has an APF of about 0.74 (see Kepler conjecture), obtained by the close-packed structures. For multiple-component structures, the APF can exceed 0.74.
APF = Atomic Packing Factor, basically an alloy can be tighter packed than a pure element.

If I think it could be a real threat? No, I've never heard about any alloy that's been used as fake gold based on density. What I've heard of (on this forum) is drilled bars with tungsten inside and tungsten with plated gold on the outside. Those should be easy to revealed with ultrasonic and density combined. Even an exotic alloy should be possible to detect with ultrasonic and measuring thickness and length. By sending a pulse through a bar and measuring the time as well as the length the pulse travels the speed of the pulse could be measured and it would be hard to match that for an alloy while keeping density equal.

Göran
 
Göran,

g_axelsson said:
If I think it could be a real threat? No, I've never heard about any alloy that's been used as fake gold based on density.
The problem is that if someone discover such alloy, nobody will hear about it until is too late. As happened with tungsten filled bars.

g_axelsson said:
Even an exotic alloy should be possible to detect with ultrasonic [...]
Yes, theoretically this is true. Unfortunately, there is a practical problem.
Most ultrasonic thickness devices don't give accurate results bellow certain thickness (<5mm roughly). Although this covers most of the gold bars, the precision for gold coins would be severely limited.

So, due to this limitation, how to test the interior of gold coins? Is it possible with NDT?

Regards,
Mark Messa
 
eddy current testing

ultrasonic

densityscale

xrf /lips

pick a combination of all 4 and you can be quiet sure . but some times the touch stone is much superior and faster .
 
On coins, I just look up stated dimensions and then measure with calipers as well as taking the mass and XRF. On pure gold coins, perhaps you can simply heat them up to red heat and look for spalling or deformation from a difference in thermal expansion (or even a difference in incandescence--tungsten doesn't conduct nearly as well as gold, so perhaps you'd see a dark spot!?). Try it on a junk maple leaf or something.

Truth be told, the gold-clad tungsten bars would invariably weigh light because the tungsten is not full dense, usually only 97-98.5% of theoretical with sintering. Sintered tungsten is an order of magnitude cheaper than electron beam melted, fully dense plate.

Now if they had done something similar to silver-infiltrated tungsten, like Elkonite, and then clad them...that would be more difficult to distinguish.

Lou
 
not an XRF fluorescence test but an actual X-ray, yes, but only if it were high enough resolution to distinguish the difference in the materials' density.
Same density...kind of useless.


Cheers!
 
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