Once the clay cools, the iron particles maintain that magnetism until the clay is reheated.
The term that refers to changes in the Earth's magnetic field in the past is paleomagnetism.
Any changes that occur in the magnetic field will occur all over the world; they can be used to correlate stratigraphic columns in different locations.
In addition to changing in orientation, the magnetic north pole also wanders around the geographic north pole.
Archaeomagnetic dating measures the magnetic polar wander.
On the earth's surface, when you hold a compass and the needle points to north, it is actually pointing to magnetic north, not geographic (true) north.
The Earth's magnetic north pole can change in orientation (from north to south and south to north), and has many times over the millions of years that this planet has existed.In general, when clay is heated, the microscopic iron particles within it acquire a remnant magnetism parallel to the earth's magnetic field.They also point toward the location around the geographic north pole where the magnetic north pole was at that moment in its wandering.This heating, or firing, process resets the iron particles in the clay.They now point to the location of magnetic north at the time the firepit is being heated.This correlation process is called magnetostratigraphy. Lava, clay, lake and ocean sediments all contain microscopic iron particles.