Determining the karat of an alloy is a fairly simple process. The most common process used in jewelry stores is the touchstone process, which involves using different acids, a touchstone, and a testing star to establish the karat quality. The usual touchstone stars are 14, 18, and 22 karats, with each point of the testing star made from a different purity of the alloy. This process may seem like it requires expertise, but it can be done by those who do not know much about metals.
In order to determine the karat of an alloy, the metal is rubbed against the touchstone. Then, the touchstone is rubbed with a metal of known purity, such as 14, 18, or 22 karats. Acid is then poured onto the two rubbed marks and how the acids reacts is the purity of the unknown alloy. If the acid does not burn away the rubbed mark, then the unknown alloy has roughly the same, or greater, purity as the testing star.
An unknown alloy that is used against a 14 karat metal and has no reaction to the acid will have a value of 14 or greater. The same unknown metal is then used against an 18 karat metal, and if the same results are garnered, the metal is used against the 22 karat point of the testing star. No reaction means that the karat value of the unknown alloy is 22. This process will allow estimation of the number of karats in an alloy.