Testing specific gravity of coins: a simple demo

[KurtS]

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.

As I just received a fascinating coin in a gift 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.