So I guess you would cast the tungsten first since it has a higher melting point, then cast the gold around it.
True.. this creates a simple problem to solve in suspending the tungsten in the casting while hardening. You could also try electroplating or hot dip plating. It does mean that the outside may appear different that is if it is an already held reserve (you would be recasting existing bars). On the other hand, if you are trying to create ‘fake’ gold from ground zero and then inject it into the exchange, it is very effective.
http://www.popsci.com/diy/article/2008-03/how-make-convincing-fake-gold-bars
Note that your link referred to ‘steel’ being inside.. which is a little obvious. It is no where close to the density of gold. If they can get away with using steel.. there is a real problem. I wonder if everybody is now busy checking the density of what they have.
I like the quote in your reference.. at the end:
And let me tell you, it’s a sad day for criminal masterminds when my fictional fake gold, designed only to trick a terrorist cell, is so much better than the real fake gold used to rip off a real government bank for millions of real dollars.
