"If you can weigh the bar accurately, then you can test for purity by essentially dropping it in a bucket of water and seeing how much the water level rises: a gold-covered tungsten bar will displace more water than a pure gold bar. "
I'm afraid that either myself or the author doesn't fully understand fluid displacement.
Given two gold bars, one adulterated with Tungsten, and one not, either the weight will differ or the volume will differ. So, given equal weights (i.e. standard 400oz bars), you can check the displacement to establish purity.
If you use a container that is filled up to a spout, and you measure the displaced water by collecting it in a measuring cup, you can just directly calculate density, and then compare it to gold.
you can't just check density like that, come on. platinum (which apparently trades around the same price as gold) - has a density of 21.45 [1] which more than compensates. If you want 100 grams of a tungsten-platinum mix to match the weight/volume of 100 grams of gold, this is how many grams you get to use of tungsten:
19.3 * 100 = 19.25 * x + 21.4(100-x)
gold is left, mix is right (a sum of the tungsten and platinum parts). I solve for 100 grams because voila, it's now a percentage. That solves to x = 97.7273 (you get to use 97.7273 grams of tungsten) or you just need 2.2727% platinum. (which recall costs about the same as gold.) so if this were a great test people could just use an alloy, which apparently exist:
Yeah this looks viable. This would make the best fake bar. I don't think the metal even need to be alloyed, you can just pour in one first, then the other.
I didn't mean it to be a viable suggestion - I don't think it is one. If you're testing density to within 2.7% you are doing other tests (suggested elsewhere by others in this thread). All I was saying is that although I do think the author is wrong (there is no problem of fake gold bars), the existence of a simple density test isn't why...
The first sentence of the original article states that XRF won't tell you if the core of the bar has been replaced with a different metal: "You don’t need to be a conspiracy theorist to find this worrying: a 1kg gold bar, certified as 99.98% pure by XRF (X-ray fluorescence) tests, turns out to have been drilled out and largely replaced with tungsten"
The article also says that a micro-ohm meter can detect such replacement.
I don't know anything about this case, but it could be there's a bunch of air trapped inside the bar with the tungsten, assuming the forgers didn't alter the outside dimensions of the bar.
Not if you start with a gold bar, then hollow the core out like a Twinkie and fill it with tungsten and the plug the end. Which I believe is what gbhn was implying.
Then the bar is not heavy enough, because you've run out of room to add tungsten on the inside, and adding it on the outside would change the volume or shape of the bar.
Tungsten is less dense than gold. Not as much a difference as actual twinkie filling, but either way you're stuck with a similar issue.
I'm afraid that either myself or the author doesn't fully understand fluid displacement.