In a metals shop the most common method for determining the karat of an alloy is with the use of a touchstone. It's an easy method and frequently used to determine the karat of gold.

The basic equipment needed for touchstone testing includes different acids, a touchstone, and a testing star. The most common touchstone acids are of 14, 18, and 22 karats. Each point of the star is made of a different purity of gold.

However, touchstone testing is not as precise as a chemical analysis done by an expert, which would indicate with greater precision the standard of the metal in parts per thousand.

Testing
First, rub the touchstone with the metal of unknown purity. Alongside this mark, rub the touchstone with an alloy of a known standard. Apply touchstone acid to the two marks and observe how they burn. If neither mark burns away then the standard of the metal is close to that of the known alloy.

For example, to determine whether a gold ring is 18 karats, first file down a place on the ring to eliminate the possibility that it is plated. Next, rub the touchstone with the filed edge. Right next to this mark make another mark with a metal that you know is 18-karat gold. Then put a drop of 18-karat touchstone acid on the two marks and watch to see if the first mark burns off. Burning indicates that the gold is inferior to 18 karats; if it doesn't burn, then the gold is equal or superior to 18 karats, but the exact standard is still unknown.

The touchstone acid method indicates with a certain imprecision that the metal is superior to the karat of the acid used because the gold didn't burn away. To know if he gold is of 22 karats repeat the above operation with an acid of 22 karats. If instead it burns away, repeat the test with an acid for 14 karats, and so on, until no burning occurs.

There are specific touchstone acids to test of silver content as well, but if we were to analyze them with 18-karat gold acid, the mark would turn light blue, reacting to the chloride in the silver.