The most reliable ways to test gold content are with an electronic gold-testing machine or with acid testing. Electronic machines test the metal's conductivity and can be accurate up to 24 karats, and acid mixes test the metal's chemical reactions to indicate purity to an accuracy of over 18 karats. The acids used to test gold quality are very corrosive and are not as exact as electronic testers.
To use an electronic gold tester, set the machine up by plugging in all necessary wires and applying a conductivity agent to the tester's probe. Next, place the gold sample in the machine's testing tray and touch the probe to it. The karat content displays on the machine's screen instantly.
To test gold using acid, mix three solutions individually: one containing 1 gram of potassium dichromate, 22 milliliters concentrated nitric acid and 8 milliliters distilled water; one containing 1 milliliter hydrochloric acid, 50 milliliters nitric acid and 12 milliliters distilled water; and one made of 1 milliliter nitric acid and 3 milliliters hydrochloric acid. File a notch in the gold for each of the three solutions.
A strong green reaction to the first solution indicates copper or brass and a strong pink reaction indicates silver. A slight reaction indicates a gold content of 10 karats and little to no reaction indicates greater than 10 karats. When the second solution is applied, a slight reaction indicates a quality of 14 to 18 karats and little to no reaction indicates more than 18 karats. With the third solution, a slight reaction indicates gold quality of more than 18 karats.