Gold is rare, easy to work, resistant to corrosion, and above all, beautiful. These are all reasons why the precious metal has served, not only as the basis of monetary systems throughout history, but has long been a highly favored medium for jewelry and art. The beauty of gold aside, its intrinsic value also attracts dishonest people who attempt to pass off alloys and other impure forms of gold as pure. Fortunately, there are simple tests to determine an object's gold content.
Concept of Karat
In its pure form, gold is so soft that you can scratch it with a fingernail, a property that makes pure gold too soft to use for coins or jewelry. To make gold more durable, it is mixed with other metals, such as copper, zinc and nickel. Alloys of gold are characterized by a karat designation, where each karat means that the alloy contains 1/24 gold by weight. In the karat system, pure gold is designated 24-karat, 75 percent gold is 18-karat, and 50 percent gold is 12-karat. Gold's karat value is unrelated to the type of metal or metals used in the alloy.
The most accurate testing methods for determining the quantity of gold in an alloy require powerful and expensive laboratory equipment. These methods include spectroscopic analysis and X-ray fluorescence. Another accurate test method is fire assay, but employing this method requires melting the gold object, so it is unsuitable for coins, jewelry and works of art.
A touchstone kit for testing gold alloys is simple to use, inexpensive and non-destructive. The kit includes a block of fine-grained black stone called the touchstone, a set of probes tipped with small amounts of standard alloy compositions and small quantities of test chemicals. The chemicals include nitric acid, hydrochloric acid and aqua regia, a mixture of the two acids. The acids in these kits can cause chemical burns, so they must be handled with care.
Using a Touchstone
To test composition of a gold object, draw a clean sample of the metal lightly across the touchstone to create a faint "streak" of powdered metal. Draw lines beside the streak with the kit's needles of known composition, bracketing the assumed karat value of the unknown sample. For example, if a ring is believed to be 14-karat - or 58.33 percent pure -- gold, draw a line with the 12-karat probe on one side, and with the 14-karat probe on the other. Apply a small drop of a test acid to the surface of the stone and drag the drop across all three lines. The test acids react with alloys within a defined range of purity, so a streak that does not react with the acid is more pure than the test range.
A substance's density is its mass per unit of volume. Pure gold has a density of 19.3 grams per cubic centimeter. By comparison, the density of water is 1 gram per cubic centimeter. There are several ways to calculate density. The simplest method is to weigh the sample and measure its volume. To measure volume, drop the object in a graduated cylinder partially filled with water and observe the amount of water it displaces. Divide the weight in grams by the volume in cubic centimeters to determine density. If the answer is not 19.3, the sample is not pure gold. Density decreases as the percentage of gold in an alloy falls, and even gold objects with the same karat value vary in density, depending on the percentages of different metals present in the alloy. Therefore, density is useful for estimating gold content, but will not tell you much about the components.
- Gems & Gemology, "Methods for Determining Gold Content of Jewelry Metals," Meredith E. Mercer
- Argonne National Laboratory-Ask a Scientist: Test for Gold
- Elmhurst College: Do the Movies Depict the Weight of Gold Bars Accurately?
- Spike Mafford/Photodisc/Getty Images