World silver coins and specific gravity test results

Has anyone ever performed specific gravity tests on their coins? A while ago, I did a slew of tests and found the results very interesting, particularly for silver coins.

Control group tests

The methodology for testing specific gravity is very simple and can be performed accurately using a good jeweler's scale. To get a baseline of accuracy, I first tested a control group of 9 bronze or cupro-nickel coins where composition probably varies little, given less economic motive for debasing. In the chart below, the darker vertical bar shows the actual specific gravity for each coin, and the lighter top bar indicates any disparity between measured values and specs for each coin. Interestingly, the discrepancy only varies from 0-1.6%, which I consider remarkably low considering my 'home lab' results against any possible variances in coin alloys. Each value below represents an average of 3 tests on each coin.

Coins represented by the control sample.
1. US Lincoln bronze cent, average of 3 coins: 1958, 1968, 1972.
2. Canada bronze cent, average of 3 coins: 1943, 1962, 1965.
3. Australia 1989 bronze 2 cent
4. Australia 1943-m bronze halfpenny
5. Finland 1 Markka (cupro-nickel)
6. US 2006 CuNi 5c
7. Iceland Bronze 1956 1 Eyrir--a very small coin, and predictably the greatest discrepancy.
8. Poland CuNi 1995 1 Zloty
9. Canada 1940 Ni 5c

Graph of control group results:



Results for testing silver coins

Given the apparently close tolerances of the control group, I was surprised by the greater discrepancy for the silver coins I tested. While some were very close to mint specifications, others were under density by as much as 6.8%, as in the case of one 1943 Newfoundland 10c. This suggests to me the actual silver content was often lower than official mint specifications. If this is due to errors introduced by my methodology, it’s interesting to note that the discrepancies, with one exception, were all on the lower end.

Test results for sampled silver coins—and corresponding mint specs. Samples were clean, undamaged and relatively free of tarnish.
1. Australia 1916 Shilling, SG test approx. .800 silver / .200 copper ; mint spec: .925 silver
2. Australia 1924 Shilling (this tested slightly higher than sterling .925) mint spec: .925 silver
3. Australia 1959 Shilling, SG tested below .300 silver / .700 copper; mint spec: .500 silver
4. Australia 1921 Sixpence, SG tested ~.750 silver / .250 copper; mint spec: .925 silver
5. Australia 1934 Sixpence, SG tested ~.925; mint spec: .925 silver
6. Australia 1942 Sixpence, SG test approx. .600 silver / .400 copper; mint spec: .925 silver
7. Australia 1954 Sixpence, SG test approx. .350 silver / .650 copper; mint spec: .500 silver
8. India 1913 1 Rupee, SG tested close to .835 silver / .165 copper; mint spec: .917 silver
9. US Kennedy 50c 1964, SG tested approx. .800 silver / .200 copper; mint spec: .900 silver
10. Switzerland ½ Frank. Two different coins, exact same SG results as .500 silver / .500 copper : mint spec: .835 silver
11. Canada 1947 25c, SG ~ .750 silver / .250 copper; mint spec: .800 silver
12. Canada 1913 10c, SG ~ .700 silver / .300 copper; mint spec: .925 silver
13. Newfoundland 1943 10c, ~ .500 silver / .500 copper; mint spec: .925 silver!

Graph of SG results for silver coins. Note: I provided baselines for .500, .800, and .925 silver content.




Finally, a short table of SG values for various Ag/Cu alloys:
.999 = 10.49
.925 = 10.36
.900 = 10.31
.835 = 10.23
.800 = 10.17
.750 = 10.08
.600 = 9.84
.500 = 9.68
.400 = 9.53
.300 = 9.38
.200 = 9.23

 

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Very interesting finds. Have you found discrepancies on any modern .999 AG?
I don't think tarnish would affect the SG would it?

 

Tarnish wouldn't affect it by much if any fraction of a %, but I strove for standard results. All told, the discrepancies for silver coins were unexpected. A friend also tested a bunch of control sample bronze/Al/CuNi coins, getting expected results. Then he tested an 2914 ASE, and got a reading suggesting close to .999 purity. Perhaps he will test more silver coins and share his results here?

For my results above, I should note the vertical scales on the graphs for the control group and the silver group are different. Below is the silver group adjusted to the same vertical scale as the control. It's pretty clear there's something going on when silver coins are involved.

 

Glad to see you are alive, Kurt! And as you know, we both have noted lower silver content in some US coins, coins which should be .900 seem less!! Well, more testing is required, I do have more silver to test, in time. Recall that my 12 page chart is always available, at:

www.Biblical-data.org/GSDykes_Specific_Gravity_tests.pdf

Gary in Washington

 

Gary, yes--you also tested that proof silver, 2014 Great Sand Dunes quarter. The SG result was only 10.16 , which suggests a silver content closer to .800, The mint spec should be: .900 Ag, .100 Cu.

 

Testing your sample coins with an XRF scanner would be interesting.

 

Agreed--that is a higher tech solution and you would know the exact metals/percentages involved. I will do a search and see if any collector/numismatist has done such tests.

 

Update: here are some XRF results for ancient and modern silver coins--source here. There is one US coin included, which has a significantly lower silver content than expected. I'll keep this thread going as I find new data.

 

could we have more details of your home lab please ?

 

Well...'home lab' is my way of saying it's DIY--with some precision. That said, I used a calibrated, gemological-grade scale to calculate the SG (margin of error is +/- 0.15gr--or +/- .3% for a US nickel's weight. Most of my other equipment is related to microscopy and optical measurements for minerals. Wish I could afford a handheld XRF unit; I'm guessing they'll be cheaper in the future...

 

Kurt,
"Bench top" model XRFs are a lot cheaper than the hand helds. I really valued your chart you just posted above! It reënforces what we have been saying, the silver content of various US silver coins MAY not be up to specs. This matters, especially if hundreds of ounces of Ag is being used for financial or investment purposes, et cetera. I will keep looking here for updates! Great idea....I sure hope someone else can add, certainly somebody has such testing tools, you would suspect that "DesertGem" ought to.

 

Well...the XRF will have to wait--I'm sure it still costs more than my fancy camera setup. I haven't sold any bulk silver coin to big outfits, but I'm certain they're aware of the variances in purity. Perhaps they'll pay the 90% melt value to everyone because they don't want the hassle of assaying every coin?

 

This is the kind of experimentalism that I would love to see done by others to corroborate these findings, including "laboratories" with finer measures and controlled environments.

Also, I don't see this kind of survey being done by any publisher of numismatic literature (that I'm aware of). It should be...

 

If you're calculating differences in expected/reported data, you should use the specific gravity figures, and not the interpolated silver percentages (secondary data at best). In other words, sample nr. 13 is supposed to be 10.36, but the calculated SG is only 9.66. That is 93.4% of mint specs or a 6.6% discrepancy. Just by eyeballing the bars on the silver graph, you can see the discrepancy isn't that much, and possibly within their margin of error--but something we may never know. When you calculate the possible silver weight involved, the discrepancies reported aren't that large.

And yes, I do not trust my results outright, which is why I did a large control group. For every coin, the end result is an average of three tests. While I don't consider my tests proof of debasing, the differences between base metal coins and silver coins are rather intriguing--and Gary arrived at similar findings in his report, linked above. Also take note of the XRF analysis of the 1943 quarter, posted above.

That said, I'm not satisfied with my own SG tests when there are better ways to analyze the elemental composition, such as by XRF. This is why I'll be focusing on such tests in the future.

 

While I think SG testing is fairly straightforward (if you have an accurate scale and good methodology), instruments which can measure actual elemental content are far more conclusive than interpreting SG results. As XRF analyzers get more prevalent, we'll know the composition of coins with far greater accuracy. So in the future, we'll have composition data for specific coins and mintage dates to compare against a coin in question. No doubt, small impurities will matter as much as silver or gold content and should help in forgery detection.

 

That table on silver fineness, and how it relates to SG, is well-known in the metallurgical/jewelry industry. These are consistent, scientifically-calculated figures for Ag/Cu alloys and they also match the measured data for reference samples. You can do the math yourself, or find the source.

But in regards to sample nr. 13, we're talking about a mint spec of .925, not .999...so I find your point rather confusing.

I would respectfully suggest there's a big difference between doubting my findings, and using your suspicion to declare that our methodology is inherently faulty--due to a belief that all mints, for every year, are "within spec"--an unknown variable after all. Respectfully, such unsupported conclusions aren't science; the burden of proof is upon you to provide some data backing that conclusion. And in this case we're discussing data for 5 different countries; we should admit there is potential for these discrepancies.

As I said before--SG tests are not elemental compositional tests. Let me be very clear: I am not making some final judgment on the motives/integrity of these respective mints. For my own benefit, I have conducted enough of these SG tests to be intrigued by the results--that's all, make your own conclusions. Regarding assays, this is done now by XRF. As I said before--I will be posting more results as my research uncovers new data. Take note of that last chart--which are XRF elemental assays of various silver coins. So for the future, I'll be focusing on such analyses exclusively.

 

Recall that Kurt is not the only person who has tested these and found lower percentages! Hence, there exists a stronger possibility that the coins were actually (intentionally) debased. IF indeed, there exists only "2 possibilities" as you suggest.
Gary in Washington

 

Additionally, is there not some soul on this site who can test the primary chemical compositions of US silver coins (pre 64 stuff, and the modern silver sets)? I mean the XRF test is only one of several. Hopefully non-destructive. Along with Kurt, the costs of some of this equipment is beyond my reasonable reach (sure I could go into debt). But I suspect someone already owns such equipment out right. We posted sufficient reasons and data to truly question numerous silver coin compositions! Perhaps another can now take the reins!
Gary in Washington