silver content of US coins

Discussion in 'US Coins Forum' started by Gallienus, Oct 23, 2017.

  1. Gallienus


    I was perusing a thread regarding the silver debasement in other coins when I saw a link discussing specific gravity tests on US coinage.

    According to the tester, some US coins show from his specific gravity tests to be less than 90% silver. Specifically he reported 82.5% silver for a 1964-P Kennedy half and 82% silver for a 2014-D commemorative Kennedy silver half. Both coins are supposed to be 90% silver.

    The link is here:

    The usual caveats: I've done weighing of coins but never s.g. Even for simple weighing I've found it necessary to buy a Ohaus Analytical scale. The usual $99 digital jewelers' scales are simply not accurate. They will show a small coin as being lighter and a heavy coin as weighing more than they should. Thus it's not just calibrating to a single value to get the correct weight. I have an ASTM weight set for calibration.

    Even a small weighing error will throw the s.g. off by a large % and I can't find any evidence of scientific method in this paper.

    Nonetheless if true this is disconcerting. I wonder if anyone else has tested coinage via s.g. or other methods? Of course the type I trimes are 75% silver I think and pre 1837? silver is ~89.26. A google search is fruitless as nobody is concerned with the purity of earlier silver coins in South Florida these days...
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  3. -jeffB

    -jeffB Greshams LEO Supporter

    Specific gravity tests are really delicate, especially on an item as small as a coin.

    The US government made 90% silver coins for over a hundred years, with the composition regularly tested. Refiners, whose livelihood depends on actual silver content, routinely process these 90% silver coins. Anyone with access to an XRF scanner can non-destructively check as many 90% coins as they like; anyone with access to a silver testing kit can do destructive tests.

    Anyone in any of these positions would raise the rafters if they found lots of "90%" coins deficient in silver.

    On the other side of the dispute, we have an amateur with a string, a cup, and a scale.

    I have great respect for amateur scientists and investigators, and I want to do everything I can to encourage them. But if your test setup frequently indicates that US coins have the wrong composition, there's most likely a problem with your test setup, not the coins.
  4. abuckmaster147

    abuckmaster147 Well-Known Member

    Know anyone?
    Last edited by a moderator: Oct 24, 2017
  5. -jeffB

    -jeffB Greshams LEO Supporter

    I only wish... :(
  6. Kentucky

    Kentucky Supporter! Supporter

    I think many coin dealers have hand-held units, or send it to @BadThad he's got some KILLER stuff.
  7. -jeffB

    -jeffB Greshams LEO Supporter

    I'm finding it hard to pull myself away from this. :rolleyes:

    The paper's author says he expected to see a SG of 10.31 for US 90% coinage, and instead was seeing 10.18 or 10.19. That's just over a 1% discrepancy.

    He estimates his own weighing accuracy (without any supporting evidence) at +/- 0.05 g, a hair under 0.5%.

    The density of water decreases by 0.2% or more as it warms from 10 C (50 F, cold tap water) to 20 C (68 F, "standard room temperature"). Colder water will yield lower SG measurements; warm water will yield higher measurements. The author doesn't say anything about controlling for this. (This also opens the possibility that the scale itself drifts as its temperature changes.)

    The author doesn't say anything about the dimensions of the nylon thread he used for suspending coins. This might be negligible, but I'd expect it to have more of an effect as coins got smaller -- i.e. ASEs and silver dollars would show a higher SG, quarters and dimes would show lower, since nylon is less dense than coin metal.

    The author doesn't say anything about the stability over time of his scale. I know that mine shifts predictably over time (I'll see if I can dig up the post where I talk about it). If he wanted to control for that, he should measure that ASE (which he said read a little high -- see above for large vs. small coins!) twice, once before measuring the suspicious half-dollar and once after, and see if those measurements are consistent.

    Between all these sources of error, I'm going to say that the author's measurements are easily within the expected range for true 90% silver.

    I don't mean any of this to cast aspersions on the author. Like I said, I admire and want to encourage people who actually Go Forth And Experiment. But again, if your measurements contradict a huge body of existing evidence, you really need to check your measurements before announcing that breakthrough (or scandal).
  8. Conder101

    Conder101 Numismatist

    I have done several coins over the years and my results have been in line with the standard specs for SG.

    GDJMSP Numismatist Moderator

    Some of the points being made about the possibility of error and the results of error are valid. For example, a 1% error in the weight of the coin can result in up to a 10% error of the specific gravity. Given this -

    All by itself that could account for roughly half of the discrepancy when it comes the reported specific gravity.

    Then you also have the legal tolerances to consider. For example, the gross weight of the coin, in this case a 90% silver half dollar, is specified to be 192.90 grains. But the tolerance is 1.5 grains which means the coin may weigh only 191.40 grains and be perfectly legal. And the fineness of the silver is specified at .900 but the tolerance is 0.006, which means the actual fineness may only be .894 and be perfectly legal.

    Throw either one or both of those into the calculations, along with the estimated error in weight 0.5%, and I think it would be pretty easy to come up with the specific gravity numbers being reported.
    Lemme Caution likes this.
  10. Dougmeister

    Dougmeister Well-Known Member

    Reading this thread makes my head hurt.

    Cool stuff though.
    LA_Geezer and abuckmaster147 like this.
  11. Lemme Caution

    Lemme Caution Well-Known Member

    You too, eh? :wacky:
    LA_Geezer likes this.
  12. Kentucky

    Kentucky Supporter! Supporter

    I've played with doing specific gravity or density, but the specialized equipment required to do good work is prohibitive.
  13. Lemme Caution

    Lemme Caution Well-Known Member

    I'm too dense to appreciate the gravity of this. :yack:
    Kentucky likes this.
  14. Kentucky

    Kentucky Supporter! Supporter

    Scientific humor is relatively rarely appreciated.
    Lemme Caution likes this.
  15. Lemme Caution

    Lemme Caution Well-Known Member

    But this is one of those times. Thanks, Kentucky! :happy:
    Kentucky likes this.
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