Specific Gravity is rather easy, but it requires a good balance scale, a glass case so no air currents disturb your readings and patience. You will need a balance scale that can weigh accurately to at least 1/100 gram but a scale that weighs accurately to 1/1000 th of a gram is preferable for alloy evaluations.
Caution - DO NOT DO THIS TO HIGH GRADE COINS. This form of testing is only for suspicious coins or coins that have poor surfaces (not original). Most coins I test have holes , big scratches and test cuts anyway. I do not want to impare original silver surfaces.
1. Weigh the coin in air - make sure there are no deposits on the coin
I wash coins in pure acetone to disolve plastics and in warm water
with a simple detergent to remove oils. Remember NO SCRUBBING -
just dip and swish at most. I will oil a coin after completion.
2. Make a small harness out of fine wire that will hold the coin.
3. Weigh the coin again suspended with the wire.
4. Immerse the coin in room temperature water and weigh the coin again.
A. Make sure the water is pure (demineralized if possible or at least boiled.)
B. Add a few drops of a liquid detergent to break the surface tension.
C. Make sure no air bubbles are trapped or clinging to the coin or harness.
5. After taking the weight in water you remove the coin from the wire.
6. Weigh just the wire in air (dry).
7. Weigh the wire alone suspended in the water.
You need to do this to fine tune the SG.
This gives you a series of weights:
Coin in air with no wire suspender - call this W
Coin in air with wire suspender. Wa
Coin in water with wire suspender. Ww
Suspender in air. Sa
Suspender in water. Sw
Here is the math you do:
_________________ = Specific Gravity
(Wa-Sa) - (Ww-Sw)
For a real example here are the numbers from an actual 8R test I ran.
W = 26.922 g
Wa = 26.965 g
Ws = 24.306 g
Sa = 0.043 g
Sw = 0.023 g
_________________ = _______________________________ = 10.2
(Wa-Sa) - (Ww-Sw) (26.965 - 0.043) - (24.306 - 0.023)
The math works out to 10.20159 but the result is best expressed to 1/10 - this coin was a real silver 8R within a normal range.
If you do the same math and leave out the weight of the wire harness you get 10.4 (10.3586) which is too high so accuracy is super critical when you are looking for small changes.
I usually repeat all the steps at least 3 times and average the results because a change of 1/100th gram will modify the results.
This process will take about 15 to 20 minutes per coin at first, but with practice, I have gotten production to about 6 coins per hour.
You can do less precise tests of course and do one coin per minute by taking only one weight and using a 1/100th gram accuracy. This will distinguish between a copper fake and a silver coin, but the accuracy level to 1/1000 is needed to distinguish between different silver alloys.
This is where paying attention in Chem Lab (and liking science in general) comes in handy.