Archaeomagnetism dates the last time a hearth or kiln was used. Heating a material to a temperature greater than 600 degrees Celsius and then cooling it causes magnetic grains to align themselves along the direction of the earths magnetic field during cooling. The grains will remain orientated in this direction unless they are reheated.
In order to obtain a date by this method it is necessary to establish true north and the horizontal level on site and it is essential that the material to be sampled has been heated and remained undisturbed since it was last heated. The objects orientation to north and the horizontal level is recorded on the object on site before removal to the laboratory. The recorded magnetic direction of the sample is then measured in the laboratory and compared to the current geomagnetic field; the results are calibrated to give dates. Because magnetic north has moved considerably over time it is possible to date sites with varying degrees of accuracy, for example during the Bronze Age magnetic north was close to the Baltic Sea so hearths last lit in this period will be magnetically aligned in that direction.
The technique can be applied in the UK to objects fired within the last 3000 years. This is because a map of the movement of magnetic north has been created by plotting magnetic alignments from features that have be closely dated by other means. Archaeomagnetic dating is not an independent method of dating and requires a reference curve to convert the magnetic direction measured into a date. (Information from Bradford University's Archaeomagnetic Research Group http://www.brad.ac.uk/acad/archsci/depart/resgrp/archmag/menu.php?2)