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Scanning Laser Acoustic Microscopy for Assessing Gold Bars

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wwswimming

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SLAM is a tool i was introduced to as a way of non-destructively assessing welds and bonds in electronic packaging.

5 or 6 papers on the subject reveal a range of measurement size capabilities and tissue capabilities.

approx. description of technique
http://scitation.aip.org/getabs/ser...000710000S1000S80000003&idtype=cvips&gifs=yes

"For biological materials, the microscope's resolution is around 20 micrometers"
http://www.brl.uiuc.edu/Facilities/scanning_laser_acoustic_microscope_facility.php

"Scanning laser acoustic microscopy (SLAM) was used to visualize creep damage distribution in the gas-pressure-sintered silicon nitride after creep at 1300°C in 4-point bending."
http://www.springerlink.com/content/nw5x2q7u54271040/

"A preliminary non-destructive analysis of the welded area between ceramic samples (monolithic ZrO2 or Si3N4) and metals (stainless steel or alluminum alloys) by Scanning Laser Acoustic Microscope (SLAM) was carried out." (metal-ceramic joints are very common in the high-power radar tube industry.)
http://cat.inist.fr/?aModele=afficheN&cpsidt=1811279

"Amphibian limb regeneration curves generated by the scanning laser acoustic microscope"
http://www.jhc.org/cgi/content/abstract/34/1/53

so, the question is - could SLAM be used to non-destructively assess 400 ounce gold bars ?

it's a very relevant question right now because the Precious Metals world is buzzing about some assays that were done by China, in Hong Kong, on 400 ounce bars received from the US. the RUMOR is that 60 tons are so affected - so far.

tungsten is one of the few materials that has a density similar to gold; tungsten is $10 a pound.

my bet is that a 400 ounce gold-plated tungsten bar will "ring" differently than a solid gold bar under SLAM examination.

what does the Brain Trust at ATHT thing ?

the background article on the Rumor about gold-plated tungsten bars. this web page is loaded with precious metal ads; i would prefer to see an article from Bloomberg. but it does look like something is going on - so far there is a NYMex investigation, and an arrest in Hong Kong.
http://www.marketoracle.co.uk/Article14996.html

side-stepping the outstanding question of possible counterfeiting of US 400 ounce gold bars ... is SLAM a viable tool for assessing 2 different 28 pound metal bars ?

tungsten is very brittle, the opposite of gold. you can't make "tungsten leaf" like you can with gold. i think the difference in their acoustic properties MIGHT make SLAM a viable exam. technique.

Second Question - what other non-destructive exam. techniques, acoustic or otherwise, might be suitable for assessing metal samples of this size ? roughly 20 cm (8 inches) x 80 cm (3 inches) x 45 cm (1 1/2+ inches).
 
I went to a lecture last year that discussed using acoustic and electrical frequency sweeps to monitor structural wear and damage in wings, beams, and stuff. The issue with these methods was that you needed a good baseline beforehand, and they appeared to be extremely sensitive. I'm not sure if the shape and size of the bars is consistent enough for any baseline to produce meaningful data across a large number of lots and casting quality....but it seems like an idea that should work.

Wouldn't an xray work just fine? And you could probably scan a whole pallet at a time too.
 
drinkmorejava said:
I went to a lecture last year that discussed using acoustic and electrical frequency sweeps to monitor structural wear and damage in wings, beams, and stuff. The issue with these methods was that you needed a good baseline beforehand, and they appeared to be extremely sensitive. I'm not sure if the shape and size of the bars is consistent enough for any baseline to produce meaningful data across a large number of lots and casting quality....but it seems like an idea that should work.

Wouldn't an xray work just fine? And you could probably scan a whole pallet at a time too.
Click to expand...

the 400 ounce bar is a standard. for sure the vault managers have access to 400 ounce gold bricks to establish a baseline.

i'm not sure about X-rays. i had the impression that a dense brick of a certain size will all show up similarly in an X-ray.

did they describe the test set-up they used for the "acoustic and electrical frequency sweeps" ?

i talked to a local coin dealer on Friday. i asked him about the 400 ounce bars. he said he was at a conference about 2 weeks ago and that was one of the topics of discussion - they were warned that counterfeit 400 ounce bars had recently been discovered, and that 'the authorities were investigating'.
 
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I heard about the tungsten encased in gold plating scandal as well. I guess most of it is traced back to the Clinton presidency time period. I'm sure it has persisted from there on out since the US has been bleeding gold as a means to pay off debts. It is a government/international level scandal, that's for sure.

But the easy way to tell without destruction is to just analyze the elements that the object contains. There are machines you can put an object in, and it reads the energy emitted from that object. (like a whole body counter) The name of the machine is drawing a blank on me right now, but most science labs have them. Usually you see them on TV when asteroids are being analyzed to see if they are real and what elements they contain inside.
 
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wiretap said:
I heard about the tungsten encased in gold plating scandal as well. I guess most of it is traced back to the Clinton presidency time period. I'm sure it has persisted from there on out since the US has been bleeding gold as a means to pay off debts. It is a government/international level scandal, that's for sure.

But the easy way to tell without destruction is to just analyze the elements that the object contains. There are machines you can put an object in, and it reads the energy emitted from that object. (like a whole body counter) The name of the machine is drawing a blank on me right now, but most science labs have them. Usually you see them on TV when asteroids are being analyzed to see if they are real and what elements they contain inside.
Click to expand...

i had discarded IR imaging, thinking that a gold brick at room temperature would produce a near-identical reading to a gold-plated tungsten brick at room temperature.

but ... "Emissivity is a term representing a material's ability to emit thermal radiation. Each material has a different emissivity and it can be quite a task to determine the appropriate emissivity for a subject. A material's emissivity can range from 0.00 (completely not-emitting) to 1.00 (completely emitting); the emissivity often varies with temperature."

Emissivity = .03 for tungsten, .018-.035 for gold - on one scale.
 
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Kitco ftw. I banked some good money on silver through them. I bought at $10/oz. Paid for the down payment on my car. 😀
 
Density and thermal capacity of Au and W are similar, but thermal conductance are widely different.

http://www.engineersedge.com/properties_of_metals.htm

So, I believe it's possible to use a known heat source at one end, then monitor using a thermography or even surface temperature thermometers taped on and measure the heat conductance.

Also, maybe using a sonic energy to compare expected resonance vs actual. The hardness between tungsten and gold are so different that I would think that they'll ring differently enough to be heard by a sensor, just like a high carbon steel won't sound the same as a similar density soft steel.

Another thing is comparing the mechanical properties based on dimensions using a computer program, then measuring distortion. i.e. if a bar of gold is placed between saw horses, then a known force is applied on the center, its possible to predict the amount of flexing that will occur based on published materials properties. With gold and tungsten being so different in hardness, I predict that this will detect it.

Depending on how its constructed and how thick the gold is on surface, even a simpler surface hardness tester might work.

Doing all the above takes time, but they're not particularly capital equipment intensive compared to some high tech microscope.

Also, thermal expansion coefficients are wildly different between Au and W, with W being 4.3ppm/C and gold 14ppm/C. I don't think you can easily hack this one. Bake it in oven to 120C and expect tungsten to expand 430ppm and gold to expand 1400ppm and with this much difference, I think you can measure the size difference using a reasonable cost instrument.
http://hyperphysics.phy-astr.gsu.edu/Hbase/Tables/thexp.html
 
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wwswimming said:
the 400 ounce bar is a standard. for sure the vault managers have access to 400 ounce gold bricks to establish a baseline.

i'm not sure about X-rays. i had the impression that a dense brick of a certain size will all show up similarly in an X-ray.

did they describe the test set-up they used for the "acoustic and electrical frequency sweeps" ?

i talked to a local coin dealer on Friday. i asked him about the 400 ounce bars. he said he was at a conference about 2 weeks ago and that was one of the topics of discussion - they were warned that counterfeit 400 ounce bars had recently been discovered, and that 'the authorities were investigating'.
Click to expand...


Test setup was embedded or rigidly attached surface probes.

By sensitive, I mean most of the output waveform changed due to the presence of a micron sized crack. Although, I'm sure there are certain frequency ranges, probably lower, that are less responsive.

I like the gamma ray idea. But neutron scattering might be easier to implement. I'd imagine accurately detecting gamma ray energy doesn't lend itself to cheap industrial setups. I'll ask my NE roommate later.
 
What about electrical resistance?

W and Au have significantly different resistivity:
W: 56 nΩm
Au: 24 nΩm

Giving electrical resistance across the ends of a 400 oz bar of 6 and 3 μΩ respectively - this should be within the measurement range of a high-quality microohm-meter.
 
Mark R said:
What about electrical resistance?

W and Au have significantly different resistivity:
W: 56 nΩm
Au: 24 nΩm

Giving electrical resistance across the ends of a 400 oz bar of 6 and 3 μΩ respectively - this should be within the measurement range of a high-quality microohm-meter.
Click to expand...

Would it be possible to use the skineffect with ac current ?

If you would put a high frequency current through a metal and use a sweep generator would it not show different results when comparing a gold plated tungsten bar and a massive gold bar ? The point is how to detect the skineffect results when doing such a test.

How about using a magnetic field and a pickup coil ?

If you would generate a magnetic field with multiple coils driven from an in frequency tunable oscillator and start sweeping would there not be a measurable difference in resulting phase and amplitude ?
I mean , if i understand correctly, the absorption of a magnetic field seems to be depended on the material and the frequency/shape of the signal.
In essence it's just a very fancy metal detector. Would that not work ?
 
Originally Posted by Mark R View Post
What about electrical resistance?

W and Au have significantly different resistivity:
W: 56 nΩm
Au: 24 nΩm

Giving electrical resistance across the ends of a 400 oz bar of 6 and 3 μΩ respectively - this should be within the measurement range of a high-quality microohm-meter.
Click to expand...

William Gaatjes said:
Would it be possible to use the skineffect with ac current ?
Click to expand...

both great suggestions.

Thanks for the replies !

just trying to determine the most reliable & least expensive techniques.
 
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