A year or so I suggested in post that gold coated tungsten bar could be spotted by neutron activation of it as the gamma rays emitted would be different (and the induced radioactivity was not very long lived), but perhaps there is a much simpler way you can do at home with very little expense: Between 0 and 100 C the specific heat of gold can be assumed to be 0.0301 but that of tungsten is 0.0321 cal/gram. Thus if 100 grams of tungsten is placed in boiling water (100C) long enough to be at 100C and then placed in small amount of 0C water, say 5 grams (5 cc) held in a well insulated container slightly larger than the bar or gold being tested - it cools down to a lower temperature, T. It will heat that ice water up more than a pure gold bar or one with part of the "gold" bar being actually tungsten. I.e. Tt > Tg. Use distilled water. A one liter bottle will let you test all of your 200 bars of gold. Please Register or Log in to view the hidden image! First put precisely 5 grams of it in your home made (more below) "well insulated container" (except for the open top) and then into your freezer. For stability, the flat bottom of the container has an area at least a four times greater than the cross section of the bar. That container is about 50% taller than the bar is long with very slightly greater cross section. Then put metal bar into boiling water. Periodically checking freezer's water for a thin ice layer on top. Break it and mix the ice chips and water - without mixing, the bottom water would be at 4 degrees C, water's densest state. Use a fine glass "swissle stick" and tap / wipe it on the inside wall of the container, to be sure not to remove any of the 5 grams of water. While your "gold' brick continues submerged in gently boiling water, wait for the tiny bits of broken ice floating on top to melt. As soon as the last one melts, remove bar from boiling water (via the fine fishing line loop attached to the top - more below) and quickly wipe dry with only an adsorbent paper towel lightly touching the hot bar as it hangs in the air via the fishing line loop from your finger. When dry, so that water mass remains exactly 5 grams, lower metal bar into the 0 C water. Wait for water and bar to come to same temperature - about 4 minutes of wait before you first measure the temperature - more below. Measure again every minute and use the highest water temperature observed. (Keep measuring until you see the temperature is falling.) For a numerical example, If bar were pure gold the heat gained by the five grams of water would be same as that lost by the gold bar and when this heat transfer is over the water and bar are at temperature Tg. Raising 5 grams of 0C water to temperature Tg requires 5Tg calories. Cooling 100grams of gold down from 100C to Tg give up 100x(100-Tg)x0.0301 calories. Set these two heats equal and solve for Tg = 37.578 C. Do the same for a pure tungsten bar and solve 5Tt = (100-Tt)x3.21 for Tt. Or Tt = 100 / [1 + (5 / 3.21)] = 100 / 2.5576 = 39.099 C If the bar were 60% W and 40% gold, then the expected final temperature of the cold water would be: 0.4 x 37.578 + 0.6 x 39.099 = 38.437C The temperature of boiling water may not be exactly 100 C but a simple approximate correction for altitude (or better by the measured air pressure) is adequate as the final cold water temperature does not depend strongly on the value of “100 C” used. What it does depend upon strongly is the precision that the mass of the ice water to be heated is precisely known. (5 grams, not 5.1 grams, etc. To show this, is why I gave more of the algebra in the calculation of Tt.) This is relatively easy to know precisely by careful volume measurement (or precises weight scale measurement is better as no correction for density as function of temperature is needed. For example, small hypodermic syringes with capacity of only 2.5 cc exist and they can be read to ~1% accuracy. I.e. with one you can know your water mass Volume, V, is 5.01 > V > 4.99 cc. Many clinics will give you one so long as you make it clear you have no interest in the needle. Measuring the temperature of the final cold water to two decimal points is not easy but very feasible. A fine platinum wire with 1000.0 ohms when at 0 C will have a resistance of 1385 ohms when at 100 degree C* and essentially none of the heat released by the cooling “gold bar” will be used to heat it to only a fraction of 50 degree C as it will have mass at least 10,000 times less than the bar and like tungsten and gold also has a very low specific heat. The change in the resistance of almost any wire (if you don't want platinum expense even for a fine wire) is nearly linear between 0 and 100 C, so one could probably use an old tungsten filament light bulb (after removing the glass bulb) ** that you have calibrated the temperature change – measured resistance at both 0 and 100 C and one point between to check the linearity. For example equal masses of 100 and 0 C water when mixed are nearly exactly at 50 C. * Data from table 4 here: http://www.edn.com/design/analog/43...call-for-PRTDs-and-precision-delta-sigma-ADCs Please Register or Log in to view the hidden image! ** Only the tips of the more massive “rods” supporting the tungsten filament should be immersed in the final cold water. Use a low wattage bulb, like a 110V refrigerator light bulb, to get a small, very fine, tungsten filament. To avoid breaking the filament, slowly crush the glass bulb in a vice. To make only 5 grams of water cover the “gold bar” with layer of water for the temperature immersion probe on top the bar, the “well insulated container” must snugly fit the bar. That "enough water covering top of bar for temperature measurement" is the first thing to confirm is possible. If you must, use 10 grams of water, redo the math, but you accuracy will be less. Perhaps there must be 10 or more % tungsten in the "gold bar" to detect it with confidence. To be sure the tungsten wire does not touch the highly conductive “gold bar” coat top of bar with film of plastic that can survive the boiling – for example “Duco cement” - What I called “air plane glue” when building flying rubber band model airplanes as kid. I have not done a careful sensitivity analysis, but am sure this approach, if carefully done, would detect the presence of Greater than 5% tungsten in the "gold bar" and any one going to the time and effort to make a false gold bar, using a tungsten core would surely put more than 5% W in it. PS make your "snug fitting insulating container" by gluing flat sections of thick foam (sheet isopour)*** around the gold bar which has some thin cardboard sheet spacers on it surfaces, except the top. (There under the duco cement, is a thin fishing line loop that makes lifting the bar out of the boiling water easy.) Your only significant expense will be to get or borrow a digital ohm meter that can measure to at least one decimal place resistance of ~ 1000.x ohms. (You could make that too in the form of an analog bridge with one leg your temperature probe and the balancing leg linear stretch of nichrome wire on a yard stick.) Nichrome wire is very cheap as used in electric heaters. If you do the temperature measurement with a home made resistance bridge, or a borrowed precise digital ohm meter, the cost of you equipment should be less than $5 and your consumables per bar tested less than 5 cents. *** Zero cost as many items now come in box with thick isopour filling space between the item and the inner walls of the box. - I. e. have at least one side flat.