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From: "Buell Ish" <buell@vectrafitness.com>
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Subject: Re: Specific Gravity Test set-up
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Ray,
Great photos! See below for a pasted in explanation of specific
gravity and how to measure/calculate it. In Ray's first photo, the
scale is zeroed (the glare makes it not appear so, but Ray assured me
it was). In the second photo, we see a dry weight of 178.9 grains. In
the third photo we see a wet weight of 159.0. The formula is below.
First we take the differnce, which is 19.9 grains. This is how much
the coins volume in water weighs. Now we divide the dry weight by
this number. 178.9/19.9 = 8.99 In other words, the specific gravity
is 8.99 in this example. Look at the chart below. Copper is 8.92. So
this is very close, slightly heavier. Notice that Nickel is close
(8.90), but Lead is much heavier (11.34). Specific gravity won't tell
you what material a coin is made of, but it provides a piece of
information that can make a determination possible. As we are dealing
with alloys of copper (often), colonial coins come out with various
specific gravities. The coin in Ray's test is a Gallery Mint Museum
Fugio. And Ray, I'll bet it is not an electro.
Regards, Buell
***********************************************************
Specific Gravity, An Introduction.
Specific Gravity is the weight of a substance compared to that of an
equal amount of water. Because it is expressed without units, the
numbers are the same whether one uses the Metric System, the English
System, or any other measurement system for that matter. Because
water is the basis, it has an assigned specific gravity of 1.
Something with a specific gravity of 2 weighs twice as much as an
equal amount of water.
This method of determining specific gravity involves weighing an
object in air and then under water. The difference between these two
weights is the weight of the water displaced. The weight of the
object in air divided by the weight of the water it displaces equals
the object's specific gravity.
Specific Gravity = Dry Weight / (Dry Weight - Wet Weight)
Set Up.
Set up according the photograph above. The large shelf holds the
scale, and the small shelf holds a cup of water. Using distilled
water will increase the accuracy of the results. The wire frame is
placed on the scale such that the coin holder is submerged in the
water. A cotton protector may also be placed on the scale. Ensure
that the wire does not touch the upper shelf or the cup itself. Also
make sure that there is enough water in the cup to cover the coin
when it is resting in the coin holder section of the wire frame.
Basic Method.
First zero the scale. Place the coin to be weighed on the cotton
protector on the scale. Note the "Dry Weight". Remove the coin and
verify that the scale again reads zero. Now place the coin in the
coin holder portion of the wire frame. The best way to do this is to
lift the wire frame a few inches such that the coin can be gently
placed in the coin holder without getting wet fingers. Now lower the
coin into the water returning the wire part to the scale. Make sure
that the coin is fully submerged and that the coin and wire are not
touching anything that they should not be touching. If bubbles cling
to the coin, dislodge them by tapping the wire frame with a pencil.
Now note the "Wet Weight". Remove the coin. Again, the best way to do
this is to lift the wire frame such that you can remove coin without
sticking your fingers into the water. Replace wire frame and verify
that the scale again reads zero. If it does not, redo measurements.
Calculate the Specific Gravity.
Using the above formula, calculate the specific gravity. The result
will be the same no matter which unit of measurement (e.g. grams,
grains, etc.) you have been using.
Compare result to chart.
In general, most coins are alloys. Remember that specific gravity is
just one piece of information. It does not conclusively prove what a
coin is made of, because many alloys have similar specific gravity
numbers. If the number obtained is far from the anticipated result,
repeat the test several times to confirm your readings and
calculations. Many reference books contain specific gravity numbers
for various coinages.
Specific Gravity , Elements.
Aluminum (AL) 2.700
Antimony (Sb) 6.684
Beryllium (Be) 1.850
Bismuth (Bi) 9.800
Cadmium (Cd) 8.640
Chromium (Cr) 7.190
Copper (Cu) 8.920
Gold (Au) 19.320
Iridium (Ir) 22.420
Iron (Fe) 7.860
Lead (Pb) 11.340
Magnesium (Mg) 1.740
Manganese (Mn) 7.300
Mercury (Hg) 13.550
Molybdenum (Mo) 10.200
Nickel (Ni) 8.900
Osmium (Os) 22.480
Palladium (Pd) 12.000
Platinum (Pt) 21.450
Rhenium (Re) 21.200
Rhodium (Rh) 12.440
Ruthenium (Ru) 12.300
Silicon (Si) 2.330
Silver (Ag) 10.490
Tantalum (Ta) 16.600
Tin (Sn) 7.298
Titanium (Ti) 4.540
Tungsten (W) 19.300
Zinc (Zn) 7.130
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