They must have to drive tungsten rods into the drilled out cavities. There is no way to pour molten tungsten into a gold bar. The melting point of tungsten is far above the melting point of gold.
Quite right.
To use original gold bars (as the story I saw claimed), they would have to hollow out the bar somehow and replace the void with a preformed tungsten plug, which would have to fit very snugly in order to avoid rattling and to pass the density test. Then they would have to cover the opening with real gold convincingly. Sounds like a very difficult process to me.
It would seem much easier to coat tungsten bars with gold and stamp them with counterfeit markings.
Here is a disturbing story about tungsten fakes turning up in Hong Kong:
When the news of the tungsten gold bars surfaced in Hong Kong it was first assumed they were manufactured there. After all China has the reputation of being the biggest knock-off capital of the world. However the amount of gold bars in question was allegedly between 5,600 and 5,700 – 400 or around 60 metric tonnes. Such a sizable amount would have to be organised by a well financed and resourceful organisation.
Within hours of this scam being identified, the Chinese officials had most of the perpetrators in custody and apparently uncovered the following:
Roughly 15 years ago - during the Clinton Administration [think Robert Rubin, Sir Alan Greenspan and Lawrence Summers] – between 1.3 and 1.5 million 400 oz tungsten blanks were allegedly manufactured by a very high-end, sophisticated refiner in the USA [more than 16 Thousand metric tonnes]. Subsequently, 640,000 of these tungsten blanks received their gold plating and WERE shipped to Ft. Knox and remain there to this day. I know folks who have copies of the original shipping docs with dates and exact weights of "tungsten" bars shipped to Ft. Knox."The balance of this 1.3 million – 1.5 million 400 oz tungsten cache was also plated and then allegedly "sold" into the international market. So the global market is literally "stuffed full of 400 oz tungsten coated bars".
Rob Kirby, Gold Seek
Now, the two biggest gold exchange traded funds, StreetTracks and SPDR, both have their gold bars held by the LBMA Banks. When you consider there are well over a billion dollars in deposits with these two trusts, the GLD itself holds over 1117 metric tonnes of gold bullion than the central banks of China, Switzerland, Japan, Europe and India, then it becomes a big matter that so much of the gold is likely not really gold at all but tungsten with a coat of gold over it.
Fortunately, ultrasound scanning makes it easy to detect fake bars.