Now a group of archeologists and geophysicists have come up with a novel way to produce decade-scale temporal measurements of the Earth's magnetic field strength from before the invention of the magnetometer. When iron-age potters fired their pottery in a kiln to harden it, it loosened tiny ferromagnetic particles in the clay. As the pottery cooled and these particles hardened, it captured a snapshot of the Earth's magnetic field. Crucially, the governments of that time required pottery used to collect taxed goods (e.g. a portion of olive oil sold) to be stamped with a royal seal. These seals changed over time as new kings ascended, or governments were completely replaced after invasion. Thus by cross-referencing the magnetic particles in the pottery with the seals, researchers were able to piece together a history of the Earth's magnetic field strength spanning from the 8th century BCE to the 2nd century BCE. Their findings show that large fluctuations in the magnetic field strength over a span of decades are normal.