A Test Of US Coins For Silver Content

After a test of world silver coins indicated lower SGs than expected for official silver content, I began to wonder if this same disparity applied to US silver coins?

To verify my methods were accurate, I first tested a control group of 9 bronze and cupro-nickel coins where composition probably varies little, given fewer economic motives. 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%--remarkably low tolerances considering my homemade testing methods and any possible variances in coin alloys.



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 (to see accuracy for smaller coins)
8. Poland CuNi 1995 1 Zloty
9. Canada Ni 5c

Given the apparently close tolerances of the control group, I was a bit surprised by the degree SG results varied for US silver coins against expected values. The results below are the average of 3 separate tests. Being rather skeptical and a stickler for details, I checked another "control group" of 4 bronze and CuNi coins between each silver test. For the control group, the variance between test and mint specs were only 0.2-1.8% (all on the low end, an average of 3 tests for each coin).

And here are the new results for US silver coins
1954 Franklin 50c = 10.17, SG test .800 silver / .200 copper (-1.7%, within margin of error)
1932 25c = 10.0, SG test approx. .750 silver / .250 copper (-3.3%)
1853 25c = 10.11, SG test approx. .775 silver / .225 copper (-2.2%)
1964 50c = 10.24, SG test approx. .850 silver / .165 copper (-1%, within margin of error)
1964 50c (second test coin) = 10.22, SG test approx. .835 silver / .165 copper (-1.1%, within margin of error)
1966 Mexico Peso = 9.06, SG test approx. .100 silver / .900 copper --this is as expected!

Interesting how the Mexico 1 Peso is very close to specification, while several US coins are somewhat low.


On the other hand, other world coins fared somewhat worse for silver content, as shown below by SG results against mint specifications.
There I have noted baselines for .925, .800, and .500 silver content.



Calculated actual silver content, based on SG:
1. Australia 1916 Shilling, SG test approx. .800 silver / .200 copper
2. Australia 1924 Shilling (this tested slightly higher than sterling .925)
3. Australia 1959 Shilling, SG tested below .300 silver / .700 copper
4. Australia 1921 Sixpence, SG tested close to .750 silver / .250 copper
5. Australia 1934 Sixpence, SG tested same as .925
6. Australia 1942 Sixpence, SG test approx. .600 silver / .400 copper
7. Australia 1954 Sixpence, SG test approx. .350 silver / .650 copper
8. India 1913 1 Rupee, SG tested close to .835 silver / .165 copper
9. US Kennedy 50c 1964, SG tested approx. .800 silver / .200 copper
10. Switzerland ½ Frank. Two different coins, exact same SG results as .500 silver / .500 copper (it should be .835)
11. Canada 1947 25c, SG tests closest to .750 silver / .250 copper
12. Canada 1913 10c, SG test approx. .700 silver / .300 copper
13. Newfoundland 1943 10c, Spec is .925, coin SG is closest to .500 silver / .500 copper


Here are a few specific gravity values calculated for silver content with the remainder copper:

.999 = 10.49 (ASEs, silver rounds and bullion should test to this)
.935 = 10.41 (Some Spanish colonials)
.925 = 10.39 (Sterling silver, ie Canada to 1919, Australia to 1945)
.917 = 10.38 (Some Spanish colonials)
.900 = 10.34 (US Coin silver)
.850 = 10.23 (Swiss silver coinage)
.800 = 10.17 (Canada to 1967)
.750 = 10.08
.600 = 9.84
.500 = 9.68 (most Australian pre-decimal silver after 1945)
.400 = 9.53
.300 = 9.38
.200 = 9.23
.100 = 9.09 (Mexico 1 Peso 1957-67)