Figure 1: Weight of the displaced water.
Club member Vince Simkins asked "How is a specific gravity test done with a nugget containing quartz?". That is, how to determine the amount of gold in a specimen containing gold and quartz. I sent him a link to another site but in looking closely at that site and other sites, I was either confused about their discussion or their formulas. Thus I have written the following notes which give the formula in general terms for any two known minerals, using two different methods, with examples, followed by a complete derivation of the formulas for those who want to know how the results were obtained. I have also added an online calculator to simplify the calculations. Thanks to Vince for bringing this omission to my attention.
These tests will only give a general idea since it is extremely unlikely that the specimen only contains two pure minerals. However it will at least give a ballpark estimation. If possible, for best results try to obtain a sample of each mineral from the same area that appears to only contain that mineral and measure the specific gravity of each for the values in the following formulas. One can look up the average specific gravity of gold for many areas since gold is almost never found in a pure state. Many minerals have a range of values for specific gravity as shown in the list of minerals on the Gold Facts page. For more information about sources of error see the discussion below. This discussion assumes gold in a matrix of another mineral such as gold in quartz. However this test can also be used to determine the amount of gold in a nugget that also contains silver or copper.
For both methods discussed below, the weight of the sample should be first measured dry and noted. Call this Wsa Weight of the sample in air.
For both methods, you need a container of water large enough that the sample can be completely submerged without water going over the top. Figure 2 below is adapted from a graphic at Gem Identification.
Method 1
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Figure 1: Weight of the displaced water. |
For the first method, place the container with water on a scale and note the weight or zero the scale for a direct measurement. Suspend the sample in the container of water by a very thin thread or thin fishing line or similar line so that the volume of the line is negligible compared to the volume of the sample. You can hold the string or tie it to a brace such as a simple banana holder. The sample must be completely submerged but not touch the bottom or sides of the container. Then note the weight of the container with the sample suspended in the water. The increased weight of the water should be noted. Call this Ww Weight of the displaced water. (See Figure 1) The advantage of this method is that a variety of scales can be used such as the triple beam balance that is shown.
Method 2
 Figure 2: Weight of the sample in water. |
The second method requires a balance or similar type scale as the weight of the sample itself must be measured while it is completely suspended in water. Again the sample must be completely submerged but not touch the bottom or sides of the container. Note the weight, which we will call Wsw Weight of the sample in water. (See Figure 2)
Two formulas for the weight of gold (or first mineral) are given below corresponding to the two methods above. All the weight measurements must be in the same units, i.e. all in grams or all in ounces or all in some other unit.
Both formulas above are related by the fact that the weight of the sample in air is equal to the weight of the sample in water plus the weight of the displaced water, Wsa = Wsw + Ww. The specific gravity of a single mineral is Wsa / Ww which is the same as Wsa / (Wsa - Wsw) as shown at Gem Identification.
For the specific gravities of several common minerals, see our Gold Facts page.
To use this calculator, enter the proper values in the Common Variables form, then enter the first value in either the Method 1 or Method 2 form and click the Calculate button. The computed values will appear below the Calculate button.
The default values below are for gold, using an average specific gravity of 18.6, and quartz, using a specific gravity of 2.65. These values assume a specimen of gold in a matrix such as quartz. However for a gold nugget, containing another mineral such as silver, use the specific gravity of pure gold, 19.3, and pure silver, 10.5.
I thought that modern U.S. coins might make good examples but most are composed of Ni and Cu which have almost the same specific gravity or are composed of 3 elements. If anyone knows of objects with a known composition of 2 substances with each substance having a known specific gravity, please let me know. I am looking for a good test of the above methods with known quantities.
Two examples are shown and compared to results from other web sites. All weight measurements must be in the same units and the resulting weight of gold will be in the same units.
The value of 19.3 used below for the specific gravity of gold is too high for placer gold. This was only used for comparison to other web sites.
According to the "Dictionary of Geological Terms" the definition of specific gravity is as follows. "The ratio of the weight of a given volume of a substance to the weight of an equal volume of water." More accurately, "Specific Gravity is the ratio of the weight of a substance and the weight of an equal volume of pure water at 4 degrees C."
For best accuracy on our measurements the above definition has restrictions on water purity and temperature. There are charts if one wants to correct for temperature. However neither of these compare to the inaccuracy due to the unknown composition of the sample as we are just making guesses about composition and specific gravities. One source of error can be caused by air bubbles clinging to the sample. It is best to use distilled water. If air bubbles form, a slight movement of the sample in the water will help to remove them.
There are special scales for calculating specific gravity but these are not needed for the tests on this page. Ore samples found in the field have too many unknowns for any exact calculations. This section will mainly discuss the types of scales that can be used for the specific gravity tests on this page and for weighing your gold.
An Equal Arm Balance/Trip Balance scale can be
used for either of the methods discussed above. This consists of
two pans, each equidistant from the center or fulcrum. The material
to be weighed is placed on one pan and standard weights are placed on
the other pan until the scale is balanced. Any Balance scale measures mass,
not weight. The scale shown is from
Scales & Balances at
Schoolmasters Science. The scale is only $42.50 but weights, total weight
of 100 grams with a resolution of 1 gram, cost another $16.25.
Most of these types of scales are fairly inexpensive but are designed for high
school projects. They have minimum range and low resolution.
Analytical Balances are highly accurate but also much more expensive.
Gun reloading Grain balance beam scales are great for weighing small
pieces of gold.
These usually have a 0.1 grain
(0.0065 gram) accuracy but are limited in maximum capacity, usually
around 500 grains (33 grams) and cost around $45 to $50. They can usually be
found at a local gun shop or online. Bob Boor has
one that he bought many years ago from a local gun shop, manufactured by
Dillon Precision Products,
that has a maximum capacity of 511 grains and is accurate to 0.1 grain. The present
model, shown here, is Dillon's 'Eliminator' Scale with a price of $50.
There is also an electronic version with a 900 grain capacity and
accurate to 0.1 grain but it costs $140. In general, these have too small a
capacity to be used for the specific gravity tests but are great for weighing
small amounts of gold.
For weighing samples of gold in quartz, meteorites, or large nuggets (lucky you), a larger capacity may be needed. There are digital and analog pocket scales ranging from a 50 gram capacity with 0.01 gram accuracy to 500 grams with 0.1 gram accuracy; though some go up to 2000 grams with 0.1 gram accuracy. Shown here is a ProScale XX 500g x 0.1g digital scale. Most of these are less than $50. Depending on the scale capacity and sample size, these scales could be used with Method 1 above.
For very large items, Triple Beam Balance Scales have capacities of
up to 2610 grams (2.6 kilograms or 5.75 pounds) with an accuracy
of 0.1 grams. A MyWeigh brand balance can be purchased for around $75. These
scales can definitely be used with Method 1 above for the specific gravity test.
Club member Vince Simkins mentioned that www.saveonscales.com has quality stuff and fair prices. Some of the above prices came from this web site.
Both Bob and Vince have scales made by a variety of manufacturers and both would be a good source of information about purchasing a scale. You can buy a calibrated weight to check your scale occasionally or even use a new coin. Coins have a known weight when minted and this weight is usually easy to find in coin books or on the Web. For instance, from the U.S. Mint web site, the dime weighs 2.268g, the nickel 5.000g and the cent 2.500g. Of course these weights only apply to newly minted coins.
The above formulas are derived from first principles.
The volume of the sample is the sum of the volume of gold and the volume of the other mineral.Please send any comments and suggestions on these pages to the DGD Webmaster
This page was last updated on 9 Feb 2008.