trickyt57

Finding Specific Gravity for different metals I find it difficult to find the specific gravity of certain metals. There are plenty of tables on the internet, but the results often don't agree and are slightly different. As far as I can tell the below specific gravity of metals in g/cm2: Gold: 19.32 (Some sites say 19.3) Silver: 10.49 (Some sites say 10.50) Copper: 8.96 (Some sites say between 8.80 and 8.95) Specific Gravity of an alloy There appears to be more than one opinion as to what the correct formula is. As far as I can tell the correct formula is: 1. Find the reciprocal of the specific gravity of each metal in the alloy. This is done by dividing by the specific gravity. For example, the specific gravity of silver is 10.49 and the reciprocal is 1 divided by 10.49 or 0.094966. 2. Multiply each reciprocal by the number of parts per thousand of that metal to be used. 3. Add the results of the multiplication together. 4. Divide 1000 by this total - the answer is the specific gravity of the alloy. So in Excel the formula would be: =1000/(1/SG1*P1+1/SG2*P2) Where: SG1 = specific gravity of metal 1 SH2 = specific gravity of metal 2 p1 =purity of Metal 1 in parts per 1000 of the coin p2= purity of metal 2 in parts per 1000 of the coin So, for a Krugerrand which is 91.67% (22ct) gold and 8.33% Copper the formula would be: =1000/(1/19.32*916.7+1/8.96*83.3) =17.62266 (However I found the specific gravity quoted in one place as 17.77. Close enough. Testing for specific gravity Most digital scales won't do the job well enough. Some only measure weight to 1 decimal place (you need at least 2. Three decimal places is better.) Some can't weigh more than 125g or 150g. That's fine for weighing a coin (1oz = 31.1035g), but with specific gravity test you need a glass of water. That will likely weigh at least 150g. Some scales don't give very accurate readings, particularly for heavier weights. Accurate scales are more expensive. Be prepared to pay up if you want accuracy. Before doing the test you need to calibrate the scale. This doesn't mean just pressing the Tare button and seeing 0.000 on the scale. Calibrating means that you have a metal calibration weight (e.g. 100g). The calibration process will vary according to the manufacturer, but generally the calibration process goes something like this: 1) Turn on the scale. If necessary press the Tare button so that the scale shows 0.000g. 2) Press and hold the "M" button (could also be the button used to choose between grams and ounces). After a second or two it will show a weight (e.g. 200.000g). Press the Tare button as many time as necessary until the weight showing corresponds with the weight shown on metal calibration weight. 3) When the display shows the same number of grams as embossed on your metal calibration weight, press the "M" button again. You should see the word "CAL" flash on the display. 4) Place the metal calibration weight on the scale. It should say "Pass". 5) You can now remove the metal calibration weight and commence weighing (you might need to turn off and restart, or just wait a minute depending on the manufacturer of the scale.) Testing equipment Apart from the scales you don't need any special equipment. For suspending the coin in the water I use dental floss with a slip-knot tied across the diameter of the coin. Dental floss is almost weightless, so it won't affect the results much. String would be too heavy and thick and would absorb some of the water. Other ways of testing your gold or silver Make sure the weight and size of your coin corresponds to the official specification. For size the best way is to compare it side-by-side with a coin which is known to be genuine. In particular look at how fat it is. Use a "ping" tester that you can buy for around $3 per month on your phone. You enter the coin name and year, (or specifications if it's not in their database). Use a surface purity tester - often called a "diamond tester". It only checks the surface, not the interior. But it will help you quickly distinguish between gold or silver-looking and genuine gold or silver, at the surface level. Gold or silver plated coins would pass this test. Magnet test. Buy a large circular magnet ("fishing magnet"). Tilt it at 45 degrees. A silver coin should slide slowly, maybe in a semi-circular pattern, whereas a cupro-nickel coin will slide fast. Gold is non-magnetic, so it should slide fast. Anything which sticks to the magnet is not a genuine precious metal. Visual test: Buy a coin magnifying loup and examine your coin carefully comparing every detail with pictures of the real thing. Often modern genuine coins will have features that are hard to replicate, such as very fine hair-like lines, or a hologram. If the lettering seems less distinct, or if the fine lines seem less fine than on the image, you could be handling a fake