If you wish to find out if a piece of jewelry is made of gold, touch it with a glass stopper which has been wetted with nitric acid. If it is pure gold it will remain untouched and unchanged; if not, the solution will become blue from the formation of nitrate of copper. (False “gold” always contains copper.)
To find the number of karats in a gold object is not as easy as testing for gold content. First, weigh the gold object, then break it up and mix it with seven times its weight in silver. Beat this mixture into thin leaves and then add nitric acid; the silver and copper will dissolve. Now fuse the remaining gold and weigh it. Compare the first and last weights: the difference will be equal to the number of karats of pure gold.
Repeat several times to check.