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WHERE INFORMATION AND PROSPECTORS UNITE T
 
 

SPECIFIC GRAVITY

 

It’s a given that those of us who seek luxuriant rocks primarily rely upon sight and feel to determine possible/probable content. Furthermore, if a lump or chunk does not meet eye or heft approval it is promptly rejected. In general, I too follow this rudimentary course of action where my eyes and hands seek valuable clues.

Amazingly, the hand can sense minute differences good enough to discriminate between light, moderate or heavy minerals, allowing the prospector to discriminate between likely and unlikely keepers.

Most mineral books provide “Specific Gravity” (SG) designations to aid the reader about the measurement of density for all minerals and metals, which normally fall somewhere between 2 and 21. Unfortunately, the same books fail to show how to easily apply and utilize specific gravity. And, regrettably when a serious prospector decides to utilize this aspect of determination the mental gears all too often start to grind when trying to apply this formula and the various instruments.

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G = relative density,
W = weight
F = Force

Whether the prospector realizes it or not the effects of specific gravity are constantly employed by everyone who uses a gold pan to concentrate heavy metal from lighter minerals within the medium of water. Another way of looking at this reality is – although a 14.2” cube of gold weighs a ton it has not a trace more density than a 1/64th inch cube of gold.

A few years ago, while surfing the net I stumbled upon a calculation for determining SG, w/o all the associated expensive gizmos and mental gymnastics. After proving that this modus operandi worked I began using it, but, unfortunately due to carelessness I did not save the web page. Nonetheless, with the following improved instructions I’m passing it on, so that anyone who wants to know the SG of their rocks and metals can apply this test.

Presumably, metals stated as “pure” always have the same specific gravity. However, my experience indicates that many so-called pure metals, particularly silver bullion stamped as 999 do not always have the same specific gravity. There are a few reasons for this all too often irregularity: 1) I made incorrect measurements and calculations, 2) unrecognized impurities (contaminates) in the bullion, or 3) abuse of hallmarking purity. In addition, I constantly see that 1oz bullion bars vary in weight to the plus as well as the minus side of 31.1 grams.

I dislike relying upon SG conclusions for the majority of minerals due to impurities, as well as trapped air, which can cause a mineral to vary widely in SG. Nevertheless, utilizing SG can be a valuable tool in helping distinguish overall mineral composition.

Adhering to the following steps will make future SG determinations easy.

1. Tare the scale
2. Weigh the object (metal/rock) in a completely dry condition. This will be used in determining the SG of the object in question. Record this weight.
3. Make an apparatus that is heavy enough to hold the object to be weighed so that the object won’t cause the apparatus to fall off the scale. In other words make the apparatus heavier than the object.
4. Make another apparatus (wire basket), which will contain the object and this wire basket will attach to the main apparatus that rests upon the scale. These two wire apparatuses will not be weighed in the dry condition, but they can be weighed to better understand what occurs in the next step.
5. Weigh the complete apparatus (both sets of wires) while the wire basket is suspended in water and attached to the wire apparatus resting on scale. Record this weight.
6. Place the object in the basket and weigh both the basket and object in water. Record this weight.

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Position of scale is at the tarred position. In other words the scale is balanced.
The following exercise will be for discovering the specific gravity of a 1 troy ounce of 999 silver, which is supposed to have a SG of about 10.5
No photo provided showing the silver bullion being weighed.

A. Weight of silver bullion round = 31.1 grams

sg3

The above image illustrates a heavy copper wire resting on scale and a thinner and much lighter wire attached to heavier wire that has been formed into a basket to hold the silver bullion. The thin wire has a hook to be able to lift on and off the hook of the heavy wire, making it easy to remove or attach the wires.
The plastic cup contains the water.
Notice that there is a slight difference in the weights out and in the water. The out of water weight is not of concern, other than establishing a new tare, which is not mandatory.

sg4

This image shows the thin wire attached to larger wire suspended in the water.
Make sure there are no air bubbles trapped in the wire basket that will be holding the silver bar.

B. Weight of the wire in water and hanging from the heavy wire on scale = 73.8 grams

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This image merely shows the quickly crafted wire basket holding the 1oz silver bullion that will be suspended in the cup of water in next image.


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Weigh the wires and silver in the water.
Note: for those keen-eyed viewers – I took this image (with 35mm film) just prior to establishing the exact weight and it illustrates slightly less than what this images should show on the scale. So, although I made a slight goof in preliminary photography, at least you know how and why. Making slight errors is one of my better traits. The trick is to know an error has occurred. .

C. Weight of 2 wires & silver in water = 102 grams

To establish the SG of the silver:

  1. take the weight of the 2 wires in water (102gr) & subtract it from weight of the Ag & wires in water (73.8gr).
  2. subtract the result (28.2gr) from the weight of the object (31.1gr),
  3. divide the result (2.9) into the weight of the object (31.1) to yield a SG of 10.7

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The SG of theoretically 100% pure silver, under ideal laboratory conditions is 10.49. So why did I get a higher SG for this 999 bullion round? The answer is that either I miscalculated, unintentional introduced scale errors or the silver is harboring one or more heavier metals. The stamped purity of this silver bullion is 999 or 99.9%. Therefore, there is at least a .1% amount of impurity. So, what are the likely contaminates in this 1oz silver bullion? Based upon multiple previous filtrations of digested silver bullion I’ve found gold, PGMs, as well as Lead and Bismuth, which could account for the discrepancies.

Keep in mind that this methodology of achieving SG is not 100% accurate, but close enough to recognize high probabilities.

After a little practice try some alternative metals that are supposed to be pure.

Here are a few anomalies I found interesting: I happened to have a few old non-clad copper pennies and decided to see what the SG of these were.

The Specific Gravity of pure Copper = 8.96

Wt of 1958 penny = 3 grams
Wt of wires in water = 73.8 grams
Wt of wires & penny in water = 76.45 grams
SG = 8.57

Wt of 1954 penny = 3.1 gr
Wt of wires in water = 73.8 gr
Wt of wires & penny in water = 76.6 gr
SG = 10.3

Wt of 1956 penny = 2.95 gr
Wt of wires in water = 73.8 gr
Wt of wires & penny in water = 76.5 gr
SG = 11.8

Even though all 3 pennies were of different weights, probably due to minor wear it makes no difference what the object weighs to obtain the specific gravity. But, why would there be such major inconsistencies? I can only speculate, but it seems that serious contamination is a willing partner in two of the instances.

Just prior to deciding to post this specific gravity web page I ran a SG test on a Lead bar because I purchased it as being of high purity.

The SG of pure Pb = 11.35
Weight of 99.95% Lead bar = 84.2 grams
Weight of wires in water = 240.3 grams
(Note: I added weight to the scale to hold this bar)
Wt of lead and wires in water = 317.3 grams
SG = 11.69

Obviously, there is a discrepancy, but it is not enough for me to be concerned with, but does indicate a high probability that some gold or pgm is present. Furthermore, even though I re-ran this test 3 times and came up with almost exact same results it is possible that my scale is introducing some amount of error. If I had come up light instead of being heavy I would immediately suspect that an unseen air bubble was attached to the basket or lead bar, or, that considerable contaminates were present.

Hope this information becomes useful.

Created: 1-30-07
Revised:

Joseph Cummins