This set of test needles has 6 different gold & silver karats represented: 8K, 10K, 12K, 14K, 18K, 999S
Use these test needles to compare acid reactions with gold & silver items to be tested. Choose a karat needle, rub or scratch the needle against a test stone, and make a rubbing or scratch with the item to be tested next to it. Place a drop of testing acid across both marks, and carefully observe the reaction to the acid. If the scratch or rubbing made by the item being tested reacts the same as the test needle scratch or rubbing, you could infer that the karat of the item being tested is the same as the test needle. If the item being tested dissolves quicker than the comparison test needle mark, the gold content of the item being tested may be a lower karat than the test needle used. If the item being tested dissolves slower than the comparison test needle mark, the item being tested may have a higher karat than the test needle used. Please note-this kind of gold testing is subjective, and requires careful observation and practice to master.