Testing for the Purity of Gold

Posted on October 23, 2014 by Mary Hood | 0 Comments

Often, the karat stamp on antique rings is worn down and illegible. A few testing methods can help you determine the purity your gold piece. 

The purity of gold is described in karats, the highest possible level of purity being 24 karats (kt.) containing at least 99.95% gold by mass. 18kt gold is 18 parts gold and 6 parts another metal. Most gold jewelry is 14kt (14 parts gold, 10 parts another metal).

Usually gold jewelry is stamped with its karat amount, but polishing, repair, and wear can efface the stamp. Fortunately, there are a few reliable ways to determine the karats or your piece.

1. The Chemical or “Touchstone” Method

Although results from this method tend to be subjective, they are generally reliable. This is a handy method for testing gold of any shape or size.

Using a gold purity testing kits, you can determine whether your gold is 10, 14, 18, or 22 kt. Each kit comes with a testing stone, testing needles with tips made of 10, 14, 18, and 22 kt. gold, and corresponding testing acids. (You will need yellow gold needles to test yellow gold and white gold needles to test white gold.)

Touchstone Testing Kit

You will also need two glass beakers, water, baking soda, protective gloves, 320 grit sandpaper, and a well-ventilated area.

If you suspect that your piece is not gold, but rather gold-plated or gold-filled, use a graver to make a small indentation on the inside (or on a less visible area of the piece). Place a drop of acid from 18k testing bottle onto the exposed area. If there is a reaction of any kind, the inner metal is not gold. No reaction means that it’s possibly gold.

To test the karat amount of gold, swipe the piece across the testing stone to deposit a thin film of gold. Repeat this step with each gold testing needle and indicate which line corresponds to which karat amount. 

 Beginning with the 10 kt. testing acid, apply a thin strip of acid across the gold lines. After about 30 seconds, the part of the 10 kt. that came in contact with the acid will dissolve. If part of the test line line dissolves as well, that line was also 10 kt. 

Apply a thin line of testing acid across each line of gold. 

In this instance, 10 kt. can be ruled out since the 10 kt. testing acid did not dissolve the test line.

If the test line does not dissolve, dip the testing stone into a solution of water and baking soda and then into pure water. Dry and resume testing, this time with the 14 kt. testing acid. Continue in this way until part of the test line dissolves. To completely clean the testing stone, buff with 320 grit sandpaper.

To wash touchstone of testing acid, dip into a solution of baking soda and water, then dip in pure water.

In this example, the 14 kt testing acid dissolved the test line, indicating that the mystery gold is approximately 14 kt. 

Watch this video of the chemical testing process for further information.

2. The X-Ray Fluorescence (XRF) Method

When exposed to high-energy radiation, atoms usually emit X-rays which correlate with the source element’s atomic number. Upon analysis of the emitted X-rays, one can determine the presence of gold and alloys. Devices analyzing metal content, like this one (pictured below) by Thermo Scientific, can perform this analysis with the mere press of a button.

The Niton DXL Precious Metal Analyzer by Thermo Science

While fast and convenient to use on some pieces, the XRF method may not be appropriate for any test subject. X-rays cannot travel beyond the surface of metal, meaning that only a small area can be tested. Developing technologies, however, can help determine the probability that a piece is gold-plated. Finally, pieces with a unique surface, such as relief work, may prove difficult to accurately test. 

If you're uncertain about the purity of a gold piece, bring it to your local jeweler. He or she may recommend a particular purity testing method based on the size and shape of your piece. 

Photos: Marbara Michelle Jacobs, The Ganoksin Project, Thermo Science

Posted in gold, gold purity, informational, jewelry, karat, quality stamp

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