in other words, if coins made of tungsten alloy become a threat to people’s holding of coins made of gold alloy, is there a cheap technical way to filter them out?
There was no definitive answered provided in this thread.
thanks for pointing it out. Upon trudging through that slough of despondent hijack I came upon the following potentially relevant claim (from yourself ;)):
Note that for our purposes here we don’t really need to use automagic gizmos to ascertain gold purity - we can do that via density measurement. All that we need here is to answer a simple question “is any tungsten present in this coin”. To my understanding, and I am willing to be proven ignorant, once we have a foolproof way of ascertaining that, we can just say like this:
- if the coin apparently has traces of tungsten, the buyer refuses to take it because figuring out its gold content would impose too high a transaction cost
- if the coin has no traces of tungsten, the buyer proceeds to measure its density and infer the lower bound on the gold content
Agree? Disagree? Or is detecting tungsten noninvasively and cheaply not a feasible thing?
The non-invasive part is the problem for doing it economically I think. But except for rare coins, is a non-invasive approach necessary?
ok, maybe I should emphasize the “cheap” aspect rather than “non-invasive”. Also, it seems lame to have the coins routinely damaged during transfers.
From the other thread we learned that gold is diamagnetic while tungsten is paramagnetic. I’m pretty sure this difference could be exploited to create a non-invasive test.
good point, this bit was on the 2nd page that I never got to. Strike 2 against ignorance by TriPolar:
ok, so tungsten is a paramagnetic. Now, how about the metals used in legit gold alloys, like 22K gold or whatever? Are any of the metals now commonly used also paramagnetic?
Of course, regardless of the answer to that, if paramagnetic effect can be used to detect tungsten then this sort of “gold standard with the scanner” can work regardless. We should just change the policy from “coins with tungsten not allowed” to “coins with paramagnetic effect not allowed”. After all, we can manufacture coins from gold alloys that don’t have this paramagnetic quality.
So how much would the scanner for testing the paramagnetic issue for coins cost anyway?
The paramagnetism may not be detectable in alloys. For instance, some alloys of stainless steel lose much of the ferromagnetic property in certain alloys.
ok, so then the viability of this approach would depend on the density of the heaviest tungsten alloy that does not show paramagnetism. If we use the paramagnetism scanner to detect pure tungsten, the cheater will be forced to use a non paramagnetic alloy of tungsten with a lighter metal. So this will work or not work depending on whether the best such alloy would have the correct density.
Incidentally, on a different note, wikipedia articles indicate that gold and tungsten have different electrical conductivity measures. For gold electrical resistivity is 22.14 nΩ·m and for tungsten 52.8 nΩ·m. Whereas let’s say copper has resistivity slightly less than gold and could conceivably be added to tungsten to improve conductivity. This raises two questions:
- if tungsten is alloyed with copper to emulate the gold conductivity, would its density become clearly distinct from gold? My WAG would be yes, but who knows
- what if instead of alloying copper is added in some other form, like as several wires in the coin. Could a resistivity measuring device be constructed that would differentiate this setup from the alloy case?
The Young’s Modulus of Tungsten is almost 4x that of Gold. So, it should be possible to detect counterfeit coins or bars by striking them and observing their resonance.
do you actually need the macro “strike” event? Or can you sandwich the coin between a “microphone” and a “receiver membrane” and get a distinct transformation pattern that will hopefully differ for tungsten alloys and gold alloys?
I’d be willing to bet that you could tell the difference in sound by jingling a couple of them in your hand.
well, sounds great 
That suggests a cunning way to make money from selling stuff to people interested in gold - sell them prominently marked tungsten coins for use in comparative jingle tests. That way buyers would be able to compare the jingle of the suspect coins with the jingle of certified tungsten coin as well as with whatever gold coins that they may already have and see which pair sounds more similar.
I just thought of another way but was surprised to learn that the specific heats of gold and tungsten are very close to each other. A sensitive enough device could probably still distinguish them.
I also found that the thermal conductivity of gold is much higher than that of tungsten. This seems like an easy thing to measure cheaply and non-invasively.
All in all there’s a lot of ways in which gold and tungsten are different and I’m sure if there were tungsten coins out there they would be trivially easy to detect.
If you have pure metals, you can simply weigh them, drop them in water, and shout Eureka! But are the properties of thermal conductivity and resonance maintained in alloys?
X-ray fluorescence alloy analyser, like this one: http://www.directindustry.com/prod/bruker/hand-held-xrf-alloy-analyzers-60556-395504.html They’re pretty good except they can’t measure carbon levels in steels.
The isotopic pattern of Tungsten in mass spectrum would be trivially easy to recognize. You would not even need a quality instrument. There are small enough mass specs to carry around, but one could also have mass spec stations. People could swab their coins with tape and then put the tape in the machine located at the bank or something. The tape thing would only work in a central location, because there you would be looking for parts per billion of metal. A hand held method would probably need to use a spark gap to vaporize a small amount the coin.
Here a device that should do it. I’ve always wanted one
Nice! I was thinking Compton scattering might do the trick, but it looks like that’s how the X-ray fluorescence analyzers basically work.
Geologists routinely use Electron Microprobes* for precisely this sort of thing: