Calculating Specific Gravity

Determining the specific gravity (SG) of a metal sample is good indicator of its likely alloy. It is not foolproof, but it is non-destructive, quick and easy. With reasonably inexpensive equipment, it is also quite accurate. SG is a measure of the alloy's weight when compared to water. Water's SG is the reference point, at 1.00 (see table at right). Here's how I determine the SG of my coins: The first step is to weigh the coin dry. The result here was 31.14g, which seems about right. (I'd like to think that bullion rounds are made ever-so-slightly oversize, rather than under. The nominal mass for this coin is 31.1g.) The resolution of this set of scales is 0.01g, though I'm not convinced that the accuracy is quite that good.   Here the coin is suspended by a single loop of microfine monofilament sewing thread. The entire length of thread (not just the loop) weighs less than 0.01g, so its effect will not be measurable. The scale has been zeroed. Ever so carefully the coin is dipped into the water until fully submerged and no, my finger isn't touching the water or the vessel! The result was 2.97g. SG is calculated by: Massdry / Masswet which in this case was 31.14 / 2.97 = 10.48 ...which is none too shabby, considering the published value of 10.49 for pure silver.

The sample coin here is an ordinary one troy ounce silver round. It is marked ".999 fine silver" and should prove to be a useful "calibration" exercise. Metal SG Water 1.00 Magnesium 1.74 Aluminum 2.70 Zinc 7.13 Tin 7.30 Iron (Pure) 7.87 Cartridge Brass ~8.50 Nickel 8.90 Copper 8.96 Coin Silver 10.31 Sterling Silver 10.36 Silver 10.49 10 K Green 11.03 10 K White 11.07 Lead 11.34 10 K Yellow Gold 11.57 10 K Red Gold 11.59 14 K White Gold 12.61 14 K Yellow Gold 13.07 14 K Red Gold 13.26 14 K Green Gold 14.20 18 K White Gold 14.64 18 K Red Gold 15.18 18 K Yellow Gold 15.58 18 K Green Gold 15.90 Gold 19.32 Platinum 21.45