In case you're wondering why tungsten would be used to fill a gutted (formerly solid) gold brick;....

How to Make Convincing Fake-Gold Bars



http://www.popsci.com/diy/article/20...fake-gold-bars

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Now, for me the more interesting question is, how do you make a fake gold bar that at least passes the pick-it-up test? The problem is that there are very few metals that are as dense as gold, and with only two exceptions they all cost as much or more than gold.
The first exception is depleted uranium, which is cheap if you're a government, but hard for individuals to get. It's also radioactive, which could be a bit of an issue.



The second exception is a real winner: tungsten. Tungsten is vastly cheaper than gold (maybe $30 dollars a pound compared to $12,000 a pound for gold right now). And remarkably, it has exactly the same density as gold, to three decimal places. The main differences are that it's the wrong color, and that it's much, much harder than gold. (Very pure gold is quite soft, you can dent it with a fingernail.)


A top-of-the-line fake gold bar should match the color, surface hardness, density, chemical, and nuclear properties of gold perfectly. To do this, you could could start with a tungsten slug about 1/8-inch smaller in each dimension than the gold bar you want, then cast a 1/16-inch layer of real pure gold all around it. This bar would feel right in the hand, it would have a dead ring when knocked as gold should, it would test right chemically, it would weigh *exactly* the right amount, and though I don't know this for sure, I think it would also pass an x-ray fluorescence scan, the 1/16" layer of pure gold being enough to stop the x-rays from reaching any tungsten. You'd pretty much have to drill it to find out it's fake. (Unless, of course, central bank gold inspectors are wise to this trick and have developed a test for it: Something involving speed of sound say, or more powerful x-rays, or perhaps neutron activation analysis. If bars like this are actually a common problem, you certainly could devise a quick, non-destructive test for them, and for all I know, they have. Except, apparently, in Ethiopia.)


Such a top-quality fake London good delivery bar would cost about $50,000 to produce because it's got a lot of real gold in it, but you'd still make a nice profit considering that a real one is worth closer to $400,000. A lower budget version could be made by using the same under-sized tungsten slug but casting lead-antimony alloy around it (to match the hardness, sound, and feel of gold), then electroplating on a heavy coating of gold. Such a bar would still feel and sound right and be only very slightly underweight, while costing less than $500 to produce in quantity. It would not pass x-ray fluorescence, and whether it passes a chemical test would depend on how thick the electroplating is.


This is the solution I recommended for Cobra Gold, because they only needed their fake gold to pass a field inspection, which is to say, someone picking it up and knowing what gold should feel like when you lift it. You may quibble for other aspects of the plot if you like, but I think the fake gold would have worked.