We're not talking about a gold plated tungsten bar. If 10 oz of tungsten is inside your gold bar, how does that change the thermal or electrical properties of the gold around it? Can you really detect a hard slug inside a softer bar?
And if there is any question, a 1/8 simple drill bit will solve the conundrum in a few seconds.
Which part of the 400 oz bar is the tungsten hidden in? Or are you turning the bar into swiss cheese?
I’ll just clean up those drill shavings for you there...
Put the drill through the middle, TP. The fraud will be exposed after a few holes. Much easier than detecting Korean “super bills,” for example. And whoever provided the fake bars will be arrested or at least put out of the business. Once the rumor of a tungsten fraud has gotten around (and it has) then detecting fake tungsten cored bars will be easy.
The GLD ETF may have a big problem with their credibility, if folks wonder about their physical gold. One of the many reasons I never touched PM ETFs. PM ETF just scream fraud potential in a dozen different ways. The only real gold is physical gold that you own.
I'm pretty sure a high-intensity X-ray would detect it.
Look up "Industrial radiography".