A Specific Gravity Test is used to calculate the amount of gold contained in a gold nugget that contains a mixture of gold and quartz.
You need two pieces of information to do the calculation. You need the weight of the specimen in air and you need the weight of the specimen when it is suspended in water. Weighing the specimen in air is easy, just put the specimen on the scale and write down the weight (dry weight). Weighing the specimen in water is a little more complex and you will need a few things to do it:
- Electronic or Digital Scale
- A piece of wood 36" long x 3.5" wide x ¾" thick
- A piece of wood 7.5" long x ¾" wide x ½" thick
- A small metal picture hanger or piece of bent wire
- A small ½ diameter simple key ring
- Container of Water
- Bottle of 91% Isopropyl Alcohol. To soak the specimen in for 15 minutes after you remove it from the water. This keeps any iron in the specimen from rusting.
To set up for weighing the specimen in water, first place the 36" long piece of wood on a table and hang about 8" of the wood over the edge of the table. Put something of weight on the other end of the piece of wood to hold in firmly in place.
Then place the scale on the part of the wood that is hanging over the edge of the table. Tie a piece of string about ½" back from one end of the 7.5" piece of wood, then thread the string through the key ring and tie the other loose end of the string about ½" back from the other end of the 7.5" piece of wood. Leave approximately 24" of string hanging loose between the two places where it is tied onto the wood.
Tie a piece of string to one end of a metal picture hanger or piece of bent wire.
Place a container of water on the floor below the scale.
Now turn on your scale. Place the stick on the center of the scale with the string hanging down. Hook the picture hanger or bent wire onto the key ring so the end of the string hangs down into the container of water. Let the string that is attached to the picture hanger soak up water until it is saturated.
Lift up the stick slightly off the scale and turn on the scale. Set the stick back down on the scale and press "Tare" to zero out the scale. Tie your specimen to the string hanging from the picture hanger and re-hang the picture hanger on the key ring. Make sure the specimen is totally submerged in the water and that neither the specimen or the string is touching the sides or the bottom of the water container.
Note the weight of the specimen (wet weight).
Now you have the dry weight and the wet weight of the specimen. Use the following calculation to calculate the amount of gold in your specimen.
- Subtract the wet weight from the dry weight and note the difference (D).
- Divide the dry weight by (D) and write down the result (SG) which is the specific gravity of the specimen.
- Subtract the specific gravity of quartz, (I am using 2.60 specific gravity for the quartz in this calculation), from (SG) to get the result (R). The specific gravity of quartz typically varies from 2.60 to 2.63 because some quartz is more dense than others. This variation does not affect the result greatly, but it does show that the calculation, though close, will never be 100% accurate.
- Multiply the result (R) by (D) to get a result (R2).
- Divide (R2) by 25.97, which will give you the contained amount of gold in the specimen in ounces.
- Multiply the contained amount of gold in ounces by 31.103 to get the contained amount of gold in grams.
Below is an example using the formula above:
Specimen dry weight: 35.8 Grams
Specimen wet weight: 27.9 Grams
35.8 minus 27.9 equals 7.9 Difference (D)
35.8 divided by 7.9 equals 4.53, the specific gravity of the specimen (SG)
4.53 minus 2.60, the specific gravity of white quartz, equals 1.93 (R)
1.9 (R) multiplied by 7.9 (D) equals 15.25 (R2)
15.25 divided by the constant 25.97 equals 0.587 ounces of contained gold
0.587 ounces multiplied by 31.103 (the number of grams in a troy ounce) equals
18.26 grams of contained gold in the specimen.