[expat2]

They are looking for a non-destructive way of checking, and melting the gold bar does not qualify. Sawing the gold bar would also work, but does not qualify, either. Ultrasonics is probably the most likely n-d procedure, I think. I don't have the numbers, but the sound velocity in gold is probably quite a bit different from tungsten, which is a very hard material.

[chrisser]

Just curious...

If you struck a gold bar vs a tungsten bar, would they “ring” differently? That might be an easy way to tell without a lot of sophisticated equipment, presuming gold bars share a common set of dimensions.

[zagger]

You are correct. Ultrasonic thickness gauges can easily find bars containing a high percentage of tungsten since the speed of sound in tungsten is nearly twice as fast as in gold. Ultrasonic gauges are calibrated for the speed of sound in the material - you must know the material for the gauge to work. If a gauge calibrated for gold looks at a gold plated tungsten bar, the measured bar thickness will be roughly half of the actual thickness. This method gets less accurate if only a small piece of tungsten is embedded in a larger piece of gold. But if the tungsten piece is small then the profit/loss from the fake is small also.