How to Check the Purity of Gold

by Kristen May

Most gold sold in the U.S. is stamped with a number indicating its purity. While 24-karat gold is 100 percent pure, you can still buy gold at levels as low as 10 karats, which is only 42 percent pure, with other metals mixed in. Several methods are available to test the purity of gold, and the best one to use depends on how much you are willing to spend and how accurate you need it to be.

Acid Testing

Clean and dry the black acid testing stone to ensure there is no residue on it from previous tests.

Select the gold test needle with the lowest karat amount, usually 10 karats.

Scrape the needle across the testing stone, leaving a light trail of gold.

Scrape an inconspicuous part of the gold item being tested across the stone to leave a light trail. The trail should be approximately the same thickness as the trail your needle left, parallel to the needle trail for comparison.

Apply a drop of the acid testing solution that matches the karat level of the gold needle to each scratch. If the gold scratch dissolves, it is equal to or less than the purity listed on that test solution. If it dissolves faster than the scratch from the needle, it is a lower purity than the needle. If the scratch remains, it is a higher purity than the testing solution.

Wash and dry the stone and repeat the process with the test needles and acid solutions for higher karat levels until you find the level at which the scratch from the gold dissolves. If the scratch mostly dissolves but leaves some rust-colored residue, the gold is likely to be about 2 karats lower than the level indicated on the test solution.

Other Methods

Put the gold in an electronic gold-testing instrument, which measures purity based on how much electricity the gold conducts.

Hold a magnet next to gold marked as 24 karats. If the gold is attracted to the magnet, it is not actually 24-karat gold because gold is not magnetic.

Take a sample of the gold to a jeweler to have it assayed. This tests the gold for purity by melting it and gets the most accurate measurement.

Warning

  • Acids in a test kit can burn skin and cause severe damage to eyes and organs if it comes in direct contact. Store them out of reach of children. Wear gloves and goggles to protect yourself while using the acid.

Items you will need

  • Black acid testing stone
  • Gold needles at several known karat levels
  • Acid test solutions for gold purity
  • Electronic gold testing instrument
  • Magnet

Photo Credits

  • Ryan McVay/Photodisc/Getty Images