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 Replies: 12 / Views: 4,658 |
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Valued Member
Singapore
151 Posts |
 Hello all, Bellow is pics of the two coins:  The left is the genuine and the right is the fake. A hairline crack runs through the hair of the fake (what do you call that? die crack?).  Pitting on the reverse and "damage" edge for the fake coin. It is quite deceiving especially when it is among other same coins. The genuine on left weighs 5 gram and the right weighs 4.5 gram. I did a resonance test on Bullion test free app  The Peaks for the genuine are 8247Hz and 8613Hz and are very clean (less noise).  Whereas that for the fake is 10422Hz and 10379Hz and noisy. Base on George W. Rhodes Precious Metals Imaging http://www.lbma.org.uk/assets/event...2_Rhodes.pdf Resonance Frequencies depend on only 3 factors - density - dimensions - elastic properties and Mechanical first order resonance value is given by  While I am able to google the Shear Modulus of Silver, Copper and Nickel as 27.8 GPa, 46 GPa, 76 GPa respectively, I am unable to apply directly to the formula to get the theoretical resonance frequency. Does anyone knows the full formula that factor in dimension and density as well? I am only able to while assuming that the fake is make out of common Cupronickel 3:1 ratio (white copper) the resultant frequency is 1.275 times (thus 10517Hz) of the genuine frequency of 8247Hz using above formula. As you can see that the in the resonance test the result 10422Hz is very near. It will be very helpful if we have the full formula that factors in all the 3 parameters. Hope there is someone among us who has a doctorate in Physics. Is it a modern fake or contemporary casting fake? Also, please excuse me if I am unable to express well in English. Cheers! 
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Bedrock of the Community
Australia
21788 Posts |
Both coins have about the same amount of detail definition.
But the fake does not seem to be worn over the foggy high parts of the design;
the genuine has foggy detail, due to wear.
Ping tone test can be a good justification to be suspicious in this sort of situation, and to do follow up testing. Close examination in hand under a 10x loupe in most cases should confirm.
Ping tone testing is useful, because it doesn't need any special equipment, and is quick and easy to do.
Useful at coin shows, when you don't carry any special testing equipment with you, save for a 10 x loupe. Some dealers get very annoyed if the probable buyer happens to be clumsy, especially if the coin has to be taken out of a protective 2x2.
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Pillar of the Community
 United States
5191 Posts |
Simple weight and ping test should tell you all.
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Valued Member
 Singapore
151 Posts |
My point is if there is a formula where we can plug in the modulus of elasticity for a known silver coin composition together with the diameter and density (or mass). Then regardless of any coin size, density (mass) and finest of silver we can get the theoretical resonance frequency. Which we can then directly compare with actual result from using the App. I think it will be tremendously useful.
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Pillar of the Community
United States
5191 Posts |
Here is a scientific paper discussing the natural frequencies of American circulating coinage. https://me363.byu.edu/sites/me363.b...fication.pdfAnother very good article is "The music of gold: can gold counterfeited coins be detected by ear?". It deals with real and fake 20 Franc gold coins. Edited by NumisEd
10/27/2020 12:44 pm
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Pillar of the Community
United States
5191 Posts |
Frequencies of real 20 Franc gold coins:
Helvetia (1890) Au-Cu 5375 Hz 5379 Hz
Louis-Philippe (1839) Au-Cu 5531 Hz 5,563 Hz
Franz-Josef (1882) Au-Cu 5478 Hz 5495 Hz
Bonaparte (An XIII) Au-Ag 5344 Hz 5350 Hz Frequencies of fake 20 Franc gold coins: Napoleon III with naked head (1860) Pt + Au plated 6.45 g 6320 Hz / 6620 Hz
Genie (1839) Pt + Au plated 6.36 g 6630 Hz
Coq (1908) Au+Ag ? 6.46 g 5290 Hz / 6040 Hz
Louis-Philippe (1831) Au+Ag ? 6.47 g 5760 Hz |
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Pillar of the Community
United States
7953 Posts |
Quote:
Hope there is someone among us who has a doctorate in Physics. Not really required:  I am not sure this helps you much anyhow, since you would probably need the shear modulus of the specific coin alloys, not generic values downloaded from the internet. Also, measurements of density and dimensions are far less accurate than the measurement of mass. Edited by tdziemia
10/27/2020 5:33 pm
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Bedrock of the Community
Australia
21788 Posts |
Would the edge milling or thickening at the rims have any dampening effect on the resonance?
From a practical point of view, just compare the resonance of a suspect fake against a known genuine piece (if you have one).
Edited by sel_69l
10/27/2020 6:30 pm
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Valued Member
Singapore
151 Posts |
Use a Neodymium magnet a moment ago and confirmed the fake is indeed non silver.
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Pillar of the Community
Singapore
631 Posts |
If you figured out a quick sure-proof scientific way using ping to differentiate fakes from real coins, I would say it's a game changer.
I have some high grade counterfeits that I would like to use as test subject, see whether you're up to the challenge.
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Valued Member
Singapore
151 Posts |
@ Numister,
How does the resonance frequency signatures looks like for the high grade counterfeit? Would you like to share?
Regards
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Pillar of the Community
Singapore
631 Posts |
I don't know how to measure resonance frequency, frankly am a little skeptical.
But like I said but if it's found to be fool-proof then it is a game changer.
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Valued Member
United States
257 Posts |
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Replies: 12 / Views: 4,658 |
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Posted 10/27/2020 10:08 am
