Many of us in the art jewelry world work with multiple metals like silver, copper, various karats of gold, and brass and sometimes you might get these metals mixed up. You also might have a client that wants you to make something special from some old jewelry they no longer want, you need some way to test these metals so you will know what you are working with.
Image #1 shows two pieces of metal one is gold the other is red brass and it is virtually impossible to tell the difference between the two, however there are two methods available to the studio jeweler to test these metals. One is an electronic tester which is shown in image #2. This devise uses electric current in conjunction with chemical agents to decipher the metals content. As you see in image #3 sample B is tested and is not gold this is indicated by the glowing red light to the left. Sample A is tested and in image #4 all four of the lights have illuminated; indicating that sample A is at least 18k gold. This is a very quick and easy testing method but the equipment is quite expensive and limits you to the testing of gold only.
Another way of testing metals, that has been around for centuries, is called touchstone testing (see image #5). This system uses a piece of slate and various concentration of acids to accurately decipher the metals purity. There are acid solutions for 10k, 14k, 18k and 22k gold as well as platinum and silver . Testing with this system begins by taking the metal sample and scratching it across the piece of slate which will leave a small deposit of metal on the surface of the stone (see image #6). Then using the various concentrations of acid one at a time, starting with the lowest concentration, apply one drop of the solution to the sample (image #7). If the the sample is not gold the metal beneath the drop of solution will disappear almost instantly as the solution dissolve the metal as shown in image #8. If it is gold, say 18k, then any solution less than the 18k test solution will have no visible affect on the sample. The 18k solution will cause the sample to lighten but not disappear and the 22k solution will dissolve the sample completely this is seen in image #9 from left to right. This system also comes with test needles, small samples of karat gold that you can use as a control, which will allow you to compare the reaction of the acid solutions on unknown samples.