As a follow-up to my
brief demo on measuring specific gravity, I decided to pursue a bit more research after a
random test suggested several Swiss coins had a
notably lower SG than
expected for the .835 silver alloy. This got me wondering if many coins are underweight for silver content...and could I
accurately tell?
To verify my methods were
accurate, I first tested a
control group of 9 bronze or cupro-nickel coins where
composition probably varies little, given less economic motive. 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
homemade testing methods against 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 then surprised by the degree SG varied for those silver coins I tested. While some were
very close to minting 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. Whether those coins were intentionally debased is unknown, but such a practice would certainly add to seigniorage revenues already enjoyed by the issuing mint.
Similar to the control group chart, the second depicts the measured SG against calculations based on mint specifications. Note: the vertical scale between both charts differs. I have also noted baselines for .925, .800, and .500 silver content.
Results for individual silver coins. (all were clean, undamaged and relatively free of tarnish)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 copperHere are a few specific gravity values calculated for silver content with the remainder copper:.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
Please let me know if you spot any mistakes.
Posted 12/28/2008 9:26 pm
