Support the “gold” bar on each end. Press hard in the middle. The solid gold bar will bend without too much effort. A bar of mostly tungsten probably not so much (I haven’t bothered to look it up, but tungsten doesn’t strike me as particularly soft).
Not so long ago that is how a thief stole a gold bar from the Mel Fisher museum in Florida. You could reach in a cage/box thing and pick up a real honest to goodness gold bar. But the geometry was such that you “couldn’t” get the bar out of the box.
But one guy pulled like hell, the bar went u shaped and off he went with it.
Slugs can be made of any metal at all. In practice, steel and aluminum are probably most common. But neither of them will have the same conductivity as gold.
The Chinese fake gold bar supplier says that their product will “pass an x-ray fluorescence scan, the 1/16” layer of pure gold being enough to stop the x-rays from reaching any tungsten", I guess with handhelds since they’re so common.
Which gets back to what I said about there being many tests, and any given counterfeit will pass some tests and fail others. Obviously the seller of a counterfeit is going to emphasize the tests which they pass, and will omit mentioning the tests which they fail.
But the counterfeiters cannot predict what tests an inspector will use. For all a counterfeiter may know, the inspector may use every test the counterfeiter never heard of and ignore the tests they DID know. It’s too much of a gamble.
This is true. However, the seller of counterfeit items only needs to convince the buyer the item is authentic. If a bar has the right density of gold and the XRF instrument shows that it is not plated, it would convince a lot of people that it is genuine. At 1/16" thick, it might also pass a hardness test.
No gold dealer is buying a large amount of gold without destructive testing and a confirmed identity of the seller. Counterfeiters don’t try to fool the experts, they try to fool fools, or don’t even try to fool someone else who thinks they can resell the goods to fools. There are plenty of people who buy filled gold jewelry just because their friends won’t be able to tell it’s not real.
I don’t think anybody bothers making gold plated slugs either, there can’t be very many vending machines that accept gold coins. I used to service vending machines and slugs would show up from time to time. The losses were too small for me to care much about and while I can understand picking up a round piece of metal and seeing if works in a vending machine it’s a little difficult to see why anyone exerts any effort into producing slugs. I got one that was a penny stuck into the middle of a steel washer, good enough at the time to pass for a quarter in that machine, someone with such an enterprising spirit could have easily earned more than 24 cents or less that they gained from the effort.
Ever hear of the “Trade Dollar”? The U. S. Mint made them in the 19th Century to circulate abroad. (An act of Congress demonetized them so they could not be used in the U. S.) The coin book I read said that many of those trade dollars still circulate in the Far East–mutilated with “chop marks” made to check their content. A fake gold bar with plating 1/16" thick wouldn’t pass THAT test.
Except in this case, the counterfeiters aren’t actually trying to pass the fakes themselves; they’re admitting that they’re fake and trying to convince someone else that they’ll pass. Which is a lot safer for them.
There’s a trivial, cheap and portable way to distinguish a gold-encased tungsten bar from a solid gold bar: ultrasound.
An ultrasonic transducer can be used to measure the speed of sound in any given object. The speed of sound in any particular material is related to its stiffness. Gold has a stiffness of about 77 GPa, while tungsten has a stiffness of about 400 GPa, or about 518% that of gold.
The speed of sound is correlated to stiffness via the Newton-LaPlace equation: c=sqrt(K/rho) where:
c is the speed of sound;
K is the stiffness of the material (bulk modulus); and
rho is the density of the material.
Since the densities of gold and tungsten are nearly identical, we can treat them as 1 (unit density). Similarly, we can treat the stiffness of gold as 1 and the stiffness of tungsten as 5.18. This means that, proportionally, the speed of sound in tungsten is sqrt(518/1) or ~2.28 times the speed of sound in gold.
This sort of difference is dead easy to pick up with an ultrasonic transducer and oscilloscope. But you wouldn’t even need to do that, as the transitions between gold and tungsten would cause all sorts of crazy reflections. It would be obvious that a doctored bar wasn’t solid gold.
The equipment to do this sort of test can be had for something like $1000 or less. Similar ultrasound inspections are performed to find cracks in aircraft components. (The cracks cause unusual reflections just like the gold/tungsten transition would). I would be shocked if ultrasound wasn’t in regular use on the gold-bar-assay circuit.
For what it’s worth, I’m a mechanical engineer. Material moduli are from matweb.com, and the Newton-LaPlace equation is from, you know, Newton and LaPlace.
You can probably test conductivity even more cheaply. And I suspect that it’s easier to monkey around with the bulk modulus via alloying than it is to do so with conductivity.
Um, are you suggesting alloying tungsten with gold? Melting tungsten is theoretically possible, except that you have to find something to melt it in. And tungsten’s melting point is so high (3422 °C, 6192 °F) that it’s hard to find anything to contain it.
As a result, you’re looking at sintering tungsten powder together with gold filling in the gaps between particles. But the proportion of tungsten in the mix would still raise the apparent bulk modulus of the bar considerably.
A sintered tungsten/gold bar would fail your conductivity test, yes, but it would also fail an ultrasound test.
I recognize, Chronos, that you’re an éminance grise around these parts, so maybe you’re imagining alloying with another material with a density and modulus nearly identical to gold. If so, what is it?
I don’t have any idea of specific suggestions. But I do know that an alloy of two materials can have a bulk modulus vastly different than either of the two original materials. Compare steel, for instance, to either iron or carbon. Given that, it seems plausible that there might be some alloy of mostly tungsten plus a little of something else that would have a density very close to tungsten’s, but be much less stiff.
It’s funny that you mention comparing iron to steel. Iron has a bulk modulus of 166 GPa. 4130, aka “chrome-moly,” has a carbon content of about 0.33% and a bulk modulus of 160 GPa. 52100, a steel used for high-quality ball bearings, has a carbon content of about 1% and a bulk modulus of 160 GPa.
In fact, all steels have about the same modulus. The yield strengths and hardness can vary wildly, but the stiffness of steel is pretty much the same no matter the alloy.
What’s more, gold boils at 2970 °C (5378 °F), almost 500 °C before tungsten melts. This difference is a vague hint that tungsten may not be especially soluble in gold, and you’d end up with a simple mixture of the two metals (much like a metal-matrix composite) rather than an alloy.
A quick google search returned this paper, which assesses tungsten’s solubility in gold. The authors assert that it’s negligible, and that the eutectic composition for an Au-W alloy is 0.137% tungsten.
The authors didn’t report a bulk modulus for their alloy, but there’s so little dissolved tungsten in it that even if the alloy had the stiffness of Camembert, the stiffness of the undissolved tungsten in our fake gold bar would swamp the softer alloy. Our fake gold bar would still be nearly as stiff as a sintered tungsten bar.
So I can’t say I agree that it’s plausible to put together a tungsten alloy that’s much less stiff than tungsten itself. Again, hardness, reactivity, and strength vary widely from alloy to alloy. Stiffness, generally speaking, does not.
No, you didn’t. You said that you thought it would be easier to develop an alloy that would match both gold’s stiffness and its density than to match gold’s conductivity and density.
My only point was that it’s nontrivial to do what you suggest, and so ultrasound is a pretty good way to screen for fake gold bars. In reality, it’s inexpensive to gauge both conductivity and stiffness, so both are useful tools. Can we at least agree on that?
