A method of determining absolute dates for certain kinds of archaeological materials, particularly in situ hearths, kilns, and furnaces, based on the principle of thermo‐remnant magnetism. The method works because the materials used to construct these facilities contain fine particles of iron‐rich elements such as haematite. When these materials are heated above about 650 °C (the Curie point) all previous alignments in the magnetic particles are destroyed, and as the material cools the particles realign themselves on the Earth's natural magnetic field. Since the prevailing magnetic field moves over the course of time, and occasionally undergoes a complete geomagnetic reversal as the north and south poles change places, local and regional sequences in the pattern of magnetic declination, dip, and intensity can be established and in situ remains related to it to show when the last major episode of heating took place—the so‐called remnant magnetism. The technique is very sensitive and can sometimes be used to show the time lapse between the inner and outer walls of a long‐lived kiln or furnace. Compare paleomagnetic dating.