Potassium is a common element found in many materials, such as micas, clay, tephra, and evaporites. In these materials, the decay product Ar is able to escape the liquid (molten) rock, but starts to build up when the rock solidifies (recrystallises). Time since recrystallization is calculated by measuring the ratio of the amount of Ar to the amount of K remaining. The long half-life of K is more than a billion years, so the method is used to calculate the absolute age of samples older than a few thousand years.