As a gold buyer this is important to remember gold metal is expensive relative to most other metals therefore there is always an economic incentive to pass lower content gold as higher content gold. Hence, remember to test for gold purity.
This test involves the use of strong acids so before you begin this test, please ensure you have the personal protective equipment (PPE).
- Safety goggles - to protect your eyes
- Protective gloves - to protect your hands
- Water, baking soda, paper towels - to dilute, neutralize, and clean acid spills respectively.
Also ensure you are in a well-ventilated area as the acids are strong and noxious.
This test involves rubbing the jewelry on a touchstone, an abrasive stone similar to fine sand paper, leaving behind some metal from the jewelry. The gold testing kit will have different strengths of nitric acid, the higher the karat rating the stronger the acid. Start with the lowest strength acid (lowest karat rating) and work upwards. If the acid does not dissolve the metal left of the touchstone after after 20 to 40 seconds, then it means that it is a higher karat then the test. Continue testing until it does dissolve, then you can confirm that the karat rating is between the previous rating tested and the test that dissolved the metal. Not a definitive test for the exact karat rating, but simple and cost effective.
A couple of things that may help you do this test is to use a gold test needle (see section on miscellaneous equipment below) to use as a reference. Draw lines with the reference needle along with a line with the jewelry being tested, and test the acids perpendicular to the lines. This will make the test result more obvious seeing the result side by side a known gold content.
If you notice a highly effervescent green reaction then this indicates base metals and not solid gold jewelry.
This method of testing is non-destructive and is suitable for normal assaying requirements. A computer, called a x-ray fluorescent (XRF) analyzer, will use x-rays to determine the composition the the jewelry. It takes less than a few minutes to complete and the results may be printed out. Popular brands include Thermo Scientific Niton range or Olympus GoldXpert or DELTA Precious Metals XRF analyzers.
There is also a method called x-ray fluorescence assay which involves melting a sample into a uniform composition before testing the sample on a XRF analyzer. This method is more accurate then a XRF analyzer but less accurate then a fire assay.
This method of testing is totally destructive however provides the greatest accuracy. The first part of the process is called fusion which involves mixing the sample with a fluxing agent (which helps melt, fuse, and separate the precious metal) and heating it to a high temperature. The second part of the process is called cupellation which involves using a collector, such a nickel or lead, to extract the precious metals.
The precious metals extracted from fire assay is then analyzed by one of the following methods:
- Flame Atomic Absorption (AA)
- Inductively Coupled Plasma-Atomic Emission Spectrometry (ICP-AES)
- Inductively Coupled Plasma-Mass Spectrometry (ICP-AES)
- Instrumental Neutron Activation Analysis (INAA)
Other equipment that is common in gold testing kits include:
- Jewelers loupe provides magnification to see small details more closely. Handy for inspecting hallmarks.
- Digital scale for precisely measuring the weight of jewelry.
- Magnet for testing if there are magnetic metals in the jewelry.
- Gold test needles for using as a reference in conjunction with the jewelry on the touchstone test.
- Diamond tester for confirming a precious stone is a genuine diamond (or moissanite).