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Learn How to test Karat gold with touchstone and acid

Are you interested in learning how to authenticate and determine the purity of gold and silver jewelry? Look no further! In this tutorial, we'll show you how to test gold using a touchstone and acid solutions.

Testing gold and silver jewelry using a touchstone and acids is a common method used by jewelers to determine the karat of gold jewelry. It's a simple yet effective method commonly used by jewelry buyers, sellers, and collectors. By the end of this tutorial, you'll have the knowledge and skills to confidently test gold and make informed decisions when buying or selling scrap gold jewelry. So let's get started!

This method involves making a scratch on a touchstone, a black stone with a smooth surface, with the jewelry being tested and then applying a drop of a gold testing acid to the scratch.

The reaction of the acid with the gold will depend on the karat of the gold, and by comparing the reaction to known standards, the karat of the gold can be determined.

To begin, you will need a touchstone, a set of gold testing acids, and a set of gold testing needles for comparative testing.

The gold testing acids should include solutions for 10K, 14K, and 18K gold, as well as a solution for testing silver. It is also helpful to have a solution for 22K gold, but this is not necessary.

You can also make the gold and silver acid test solutions yourself quite easily, by mixing the following amounts of reagents:

  • 10K acid: 22.2 mL nitric acid and 7.4 mL distilled water

  • 14K acid: 23.7 mL nitric acid, 5.9 mL distilled water, and 10 drops hydrochloric acid

  • 18K acid: 5.9 mL nitric acid, 5.9 mL distilled water, and 17.8 mL hydrochloric acid

  • Silver acid: 20 grams potassium dichromate, 22.2 mL nitric acid, and 7.4 mL distilled water

To test the karat of gold jewelry using a touchstone and acids, follow these steps:

  1. Scratch the gold jewelry being tested over the surface of the touchstone, pressing hard to leave a visible deposit. It is best to leave a line of about one inch long.

  2. For the highest sensitivity, place a scratch line with a gold test needle next to the scratch line of the jewelry being tested. Compare the speed at which the scratches dissolve. If the test scratch dissolves faster than the needle scratch, it is a lower karat than the needle.

  3. Place a drop of the 10K acid on the scratch made by the jewelry being tested. If the acid dissolves the scratch, the jewelry is less than 10K gold or not gold. If the acid leaves the scratch intact, the jewelry is 10K or greater than 10K.

  4. Repeat the process with the 14K acid. If the acid dissolves the scratch, the jewelry is less than 14K gold. If the acid leaves the scratch intact, the jewelry is 14K or greater than 14K.

  5. If available, repeat the process with the 18K and 22K acid.

Few things to note:

  • If the 14K solution dissolves the scratch slowly and leaves rusty color particles, it is probably 12K gold. Also, be aware that some items marked as 14K were fabricated prior to 1982 when it was legal to mark items as 14K, even if the gold was only 13.5K. In this case, the 14K solution will not dissolve the scratch but will make it lose its brightness and turn it into a yellow-rusty color.

  • When testing gold jewelry using a touchstone and acids, it is recommended to use reagent grade or technical grade nitric acid. The strength of the acid may vary slightly, but this should not make a significant difference in the test results. It's important to remember that this test is a comparison test vs. our known gold piece, so the exact formulation of the acid is less important.

  • It is also important to remember that the acids containing hydrochloric acid will weaken over time, so it is best to use fresh acids for the most accurate results.

To test platinum or white gold, repeat the acid test above with the 18K or 22K acid.

For testing silver jewelry, follow these steps using a touchstone and silver testing solution:

  1. Begin by scratching the jewelry you want to test over the surface of the touchstone. Make sure to press down firmly to create a large and thick visible deposit, ideally a line about one to one-half inches long.

  2. Next, apply a drop of the silver testing solution to the scratch you just made. The color reaction of the solution with the metal scratch will help determine the purity of the silver.

  3. Observe the color of the solution on the touchstone. Keep in mind that the background of the touchstone is black, so the colors you see will be in contrast to this. Here's what to expect:

  • Fine Silver (.999): Bright Red

  • Silver 925: Dark Red

  • Silver 800: Brown

  • Silver 500: Green

It's worth noting that you can also test the silver directly on the piece of jewelry you're testing, although this will likely dull the piece's polish and leave a mark where the acid was applied.

Finally, keep in mind that when testing items of heavy weight and volume, such as chains and coins, where plating could hide the true metal, it is recommended to make a deep notch in the jewelry being tested and test the metal inside the piece. This will ensure that the true karat of the gold is being determined and not the karat of any plating on the surface.

That's it! With a touchstone and acid solutions, you can accurately test gold jewelry to determine its karat and authenticity. Just be sure to handle the acids safely and clean the touchstone well between uses - use an 18K or 22K acid for 10 minutes to dissolve the scratch and wash well with plenty of water.


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