The long half-life of 40K allows the method to be used to calculate the absolute age of samples older than a few thousand years The quickly cooled lavas that make nearly ideal samples for Kâ `` Ar dating also preserve a record of the direction and intensity of the local magnetic field as the sample cooled past the Curie point of iron . The quickly cooled lavas that make nearly ideal samples for Kâ `` Ar dating also preserve a record of the direction and intensity of the local magnetic field as the sample cooled past the Curie temperature of iron .