Having the ability to test the purity of gold is important for anyone who is involved with buying or selling gold. Just because an item is stamped as 10 carat, 14 carat or 18 carat does not mean that it actually contains that type of gold. It is a sad reality that fakes exist in the gold market, particularly because gold jewellery has virtually no regulation or oversight. Sometimes a lower quality of gold will be stamped with a higher-grade marking, or other times it may not be gold at all, but rather layered with gold in such a way that it is not immediately apparent that it is a plated item. To protect your investment against forgeries and to make sure you are getting the gold you pay for, it is important to test the purity of the gold. There are simple methods you can do at home without sending your gold away to an assayer's office.
Examine the gold item that you would like to test. Look it over carefully for any identifying marks, such as 10k, 14k, 18k and so forth. This is because the test needle and acidic solution will vary according to how many carats (k) the gold is, so if you can identify what it is marked as, you will have the most logical place to start.
Pick up the touchstone and the test needle that corresponds to the marking on the gold. For example, if you want to test the purity of an item that is marked as 14 carat, you will want to use a 14 carat test needle. Because the test needle is from a known source that is proven to be 14 carat, it can be used for comparative analysis.
Scratch the test needle against the touchstone. It should be a firm scratch, but at the same time, you should not scratch like mad across the entire surface of the touchstone. That will only result in needing to buy a new touchstone every time you test the purity of gold. Just a small, firm scratch with the test needle is all that is required.
Use the gold item you are testing to make a second scratch, next to the one that was made with the test needle. The result is that your test stone now has two scratches. Ideally, these scratches will be located side-by-side for easier analysis.
Use the acid solution that corresponds to the test needle. For example, if you are testing with a 14-karat test needle, then use the 14-karat gold solution, or use an 18-karat solution if you used the 18-karat test needle, etc. Because each acid solution is developed to work with a specific gold purity level, it is important to use the right acid. Simply drip one or two drops on the scratches you made with your needle and the test item.
Analyse the results. The scratches should appear to change colour in reaction to the acid that has been dripped on them. If the colour of the scratch from the test needle is the same as the colour generated by the scratch from the test item, then you have a positive identification. If the colours do not match, then repeat the process using a different test needle and acidic solution.
You can also purchase an electronic test kit, which measures the purity of gold without scratching or using acid. These testers, however, are more expensive, and results may vary when the battery begins to wear.
Although the acid test method will verify the purity of gold with a degree of certainty, the only way to conclusively guarantee the purity of gold is to have a professional assayer conduct a fire test.