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.
