There is a thermal and electrical method to detect tungsten inserts which were found in 2.5 million ounces of 400 ounce gold bars in Hong Tong Kong. as well, but it has difficulties due to sensitivity, see below.
It would seem to me that gold bars would yield to both sonic ulrasound and MRI much differently than tungsten. Tungsten is paramagnetic and gold is not so. Colour ultrasound developed in the seventies will distinguish between materials of slightly different densities. The difference in density of pure gold and tungsten is 0.27%.
Tungsten is the same SG as gold, so it defeats SG tests which eliminate large amounts of all other metals. Tungsten itself is an expensive metal, and the idea of gilding it, milling the exterior gold and forging the stamps is not an easy one to entertain. Even a 1/8 inch thick melt coating of gold would be trop cher to do lightly. That is almost 13 cubic inches of gold covering your 100 troy lb brick. (of 1200 ounces) (12 inches by 4 by 3 by 2.875 inches.) --> 133 ounces of gold! Granted there is a payoff but it is a high tech expensive forgery. The tungsten is alone worth 1400 dollars, perhaps, and difficult to mill as it is harder than the hubs of hell, then you need to pour molten gold around it, probably by cire perdu then mill that and or stamp it with a forged stamping. Not for your average amateur.
This is the internet rumour. Caveat all emptance.
"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."
Thermal and electrical purity test - patent.
"Another example of a nondestructive test method is described in U.S. Pat. No. 3,981,175, which was issued to Ogden H. Hammond III and Francis I. Baratta (hereby expressly incorporated by reference). In accordance with that system, the device is a nondestructive counterfeit gold bar and silver bar detection system based upon heat transfer principles. The principle entails the application of identical finite suddenly applied controlled heat pulses at a first region of a gold or silver bar of known purity, used as a standard, and the test bar of the same dimensions. The system is enclosed in an insulating medium. The temperatures are measured at a second region of each bar, which are not only dependent upon the thermal properties of each bar, but upon the time. Specifically, those thermal properties which are tested by this devise are specific heat, thermal conductivity, and density; and the combination of these properties known as diffusivity. Since these properties in gold and silver are unique, the temperature at the second region, specifically the end opposite from that which is suddenly pulsed by a quantity of heat will be at a higher temperature in a given time than that of any bar less pure than the standard gold or silver bar. Because of the large differences in thermal properties of gold and an alloyed gold sample, temperature measurements will reveal differences. However, the thermal properties of Grade 8 and 8.2 bolts are very similar and temperature determinations at their far ends will not guarantee discrimination.
Yet another example of a nondestructive test method to detect fraudulent precious metal bars is revealed in U.S. Pat. No. 4,381,154, issued to Ogden H. Hammond, III. It was found that of all possible forgeries, a non-alloyed tungsten forgery of gold, i.e., an insert of tungsten within the gold bar, is the most difficult to detect because the density and heat-capacity of tungsten and gold are virtually identical (a less difficult forgery to detect is an alloyed forgery wherein its composition is generally uniform throughout). Thus, an improvement in accuracy over the previous U.S. Pat. No. 3,981,175 was required. This improvement consists mainly of increasing the accuracy of the detection system by providing and controlling heat into the test chamber resulting in equilibrium, termed dynamic insulation; accurate heater control and using a compensated infrared sensor to measure the temperature at the far end opposite the heated end of the sample.
Although the improved techniques adopted in U.S. Pat. No. 4,381,154 will enhance the sensitivity of this test method it requires additional temperature sensors, controls and electronic instrumentation as compared to the method prescribed in U.S. Pat. No. 3,981,175.
Because the thermal properties of ferrous materials are not as unique as gold or silver, simple improvements of U.S. Pat. No. 3,981,175 are required to nondestructively detect counterfeit, mismarked, and substandard steel fasteners in a viable manner. "
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