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KurtS
Pillar Of The Community
5316 Posts |
 Posted 12/25/2008 10:22 pm
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After a fellow collector expressed interest in determining specific gravity for his coin, I wanted to do a brief demo on how I perform SG tests on small objects including minerals and coins. The credit for this test goes to Swamperbob, who describes his method in detail here.
Yesterday, when I received a fascinating coin in the Secret Santa Exchange, I thought it would be an excellent subject for this demo, as well as suggest composition for 8 Skilling coins during this period.
The most important element of this test is an accurate scale, followed by correspondingly accurate methods. Here I use a pocket jeweler's scale which I have calibrated against other instruments to an accuracy of +/- .04 grams. Previous SG tests with known materials have confirmed reasonably accurate SG results from .7 grams and upward. So at 2.90 grams, this 8 Skilling coin is an excellent candidate for this test.
Since calculating SG involves suspending the coin in water, at first glance a simple electronic scale appears unfit for the task. Yet, it is quite possible to get good results with the setup as pictured, where the scale is positioned over the edge of the table to allow suspension.
For this method to work well, you will create a harness to hold the coin securely but not interfere with the scale or surrounding supports. Pictured below is a light harness made from a bamboo rod and copper wire that holds the coin securely without damage. I should note the wire was not wrapped around the coin, but consists of a few loops which loosely stabilize the coin.
Once your setup is complete, you are ready to conduct the SG test. See above for Swamperbob's detailed method.
• First, weigh the coin without the harness, shown below as W
• Weigh the harness without the coin in air, shown below as Sa
• Weigh the coin suspended in air with the harness, Wa
• Weigh the coin suspended in distilled water* with the harness, Ww
(Note: coin should be completely submerged in water)
• Weigh the harness alone in water, Sw
(Note: if the harness is wet from a previous measurement, dry off completely before this test).
*Note: add a trace of detergent to break surface tension.
Now you have 5 distinct measurements, from which you can calculate SG as shown:
W
_________________ = Specific Gravity
(Wa-Sa) - (Ww-Sw)
Applying my measurements to the coin pictured above, I arrive at the following equation:
W..................................................2.90
_________________ = _______________________________ = 9.35
(Wa-Sa) - (Ww-Sw)..............(3.65 - 0.75) - (3.34 - 0.75)
Given a SG result of 9.35, I have a better idea of the silver content of this coin.
As the SG for .999 silver is 10.49 and .999 copper is 8.96, the SG may suggest a composition of .250 silver and .750 copper.
Does that sound correct?
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CoinHunter53562
Pillar Of The Community
USA
1086 Posts |
Posted 12/25/2008 10:32 pm
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That is quite helpful. I always wondered exactly what specific gravity meant but never bothered to research it. The timing is good since I was just reading an article in Coin World about a counterfeiter from China that mass produces fake US coins. Coin World did an analysis including diameter, weight, and specific gravity of the fake coins vs the authentic ones so this info is helpful in fully understanding what it means. Thanks for the post.
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MrCanada
Valued Member
Canada
168 Posts |
Posted 12/25/2008 11:14 pm
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Great science,I can't wait to try it out on a couple pieces I have been wondering about.
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KurtS
Pillar Of The Community
5316 Posts |
Posted 12/26/2008 3:31 pm
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Thanks for your feedback! While I have no doubts as to the authenticity of this coin, I was curious about the silver content.
It was also fun to devise a way to measure SG with materials on hand. 
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t360
Pillar Of The Community
USA
1198 Posts |
Posted 12/29/2008 9:18 pm
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Great setup, Kurt! Thanks for explaining the calculation.
Does that agree with the composition for the coin listed in Krause?
(I don't have my books handy)
I am a little surprised that it would be so debased.
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_t360__________________
ANA # R-3139699
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KurtS
Pillar Of The Community
5316 Posts |
Posted 12/29/2008 9:44 pm
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Tom,
I did find an online reference which gives the silver composition as .562, which is a bit different than my result--I'm going to re-test to confirm. I also did a "control group" test of bronze coins, comparing to some modern silver coinage with interesting results! But if these 8 Skillings were debased at all, I wouldn't be too surprised since Denmark was engaged in war at the time. Great coin--thanks again!
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tepritts
Valued Member
United States
218 Posts |
Posted 12/30/2008 1:44 pm
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Thank you for the information. I've been looking for a method to determine the minting dates for the Ethiopian fifty cent coin. It has a frozen date and there are 80% and 70% silver coins depending on the when it was minted.
One question about wear: Does anyone know how much weight a coin looses with wear such as a Good grade vs. Uncirculated?
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KurtS
Pillar Of The Community
5316 Posts |
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QuickSilver
Pillar Of The Community
United Kingdom
1073 Posts |
Posted 01/28/2009 12:30 pm
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Can I use De-ionised water in place of distilled?
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biokemist6
Pillar Of The Community
USA
4797 Posts |
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QuickSilver
Pillar Of The Community
United Kingdom
1073 Posts |
Posted 01/29/2009 11:15 am
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Thanks for that biokemist6. I can get de-ionised at the supermarket, but not distilled.
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nod2003
Pillar Of The Community
United States
1296 Posts |
Posted 01/29/2009 12:33 pm
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Wouldn't this be easier if you measured the volume of the coin directly, using a plastic coin tube with water of a measured depth, then placing the coin into to tube and measuring the new depth? You then find the diameter of the inside of the coin tube, and you find the volume of the coin. Combined with an accurate weight measurement and you have the density of the coin.
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just carl
Pillar Of The Community
USA
3844 Posts |
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QuickSilver
Pillar Of The Community
United Kingdom
1073 Posts |
Posted 01/30/2009 02:27 am
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I also thought that a different harness might be better and wondered if it was ok. I thought about a rubber band double crossed around the coin and attached to thread.
The de-ionised water I have seen is from a reputable, very large company and there are laws in the UK about mis description, I don't personally think this company would take the chance of calling a product something it's not, but I see your point.
By the same token, if this is the case, how do you know distilled water has been distilled?
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josie
Valued Member
Ireland
497 Posts |
Posted 05/14/2009 1:28 pm
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Very nice info.
Eureka.
Takes time to absorb all the info very nice thread.
weight,mass,specific gravity,metal content,troy ounce.
other are reading the lower part of distilled water in test tube.
if may apply in coins when they change the metal content like the GB predecimal silver and predecimal copper that ay goes also to other comonwealth coins struck in GB and their metal compositons.
shipwreckcoins is there specific gravity also change? seen some on TV that a metal support in oil rigs change its shape and metal composition maybe due to force and pressure perhaps there specific gravity also change.
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Edited by josie - 05/14/2009 5:05 pm |
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josie
Valued Member
Ireland
497 Posts |
Posted 05/14/2009 7:58 pm
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Physics,chemistry.
elctricity,electronics,much more difficult to find the conductibility of a coin if it is high conductable metal content,just like a homemade tester with several side by side light emiting device in circuit form for measuring a poor conductor to high conductibilty,to many variables to pyx,it is a start homemade device to test the metal content other are using x ray flourencence in universities for knowing the metal content or composition.
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