An obsession by ancient rulers with placing their mark on pottery could help us work out whether the Earth's magnetic field is likely to change any time soon.

The Earth's magnetic field is always changing, and there are reasons to think we might see a reversal in direction some time soon. To predict the future, we need to understand the past. However, our direct measurements of the strength of the field are recent, and paleomagnetic studies of past changes are hindered by imprecision in dating. Now, a study of pottery from the Jerusalem area has given us a precise record over a 600-year period.

In the ancient kingdom of Judea, clay jars were stamped with royal seals on their handles. The stamps changed according to who was in power, giving us a detailed record of the jars' ages. Professor Lisa Tauxe of the Scripps Institute of Oceanography took advantage of this dating and the fact that ceramics hold a record of the magnetic field in which they were fired.

In Proceedings of the National Academy of Sciences, Tauxe reported measurements of magnetic field strengths around Jerusalem between 2,800 and 2,200 years ago. Judea was small enough that, even if the pots were not fired right where they were buried, the difference in location should barely matter.

During this period, the Earth's magnetic field in the region weakened. Some of this may have been related to movement of the magnetic poles, but the change was so large, it almost certainly indicates a worldwide decline.

Most significantly, the field weakened by 27 percent in the space of 31 years during the 8th century BCE, from a level around twice that of today. Evidence for such a dramatic change has been observed before from Middle Eastern pottery, but had been disputed on the basis that the findings depended on unreliable dating methods. Geologists are surprised the magnetic field can change this fast.

Pottery is a great tool for measuring the Earth's ancient field. Magnetite particles in hot clay line up with any magnetic fields they experience at the time, and the stronger the field, the more they fall in line. As the clay cools, the particles are locked in place, keeping a record of the strength and direction of the field at the time.

However, most archeological discoveries have trouble dating pots exactly. Even if an event such as a volcanic eruption tells us exactly when an item was buried, this is of limited value when the pot records the field at the time and place it was fired. The administrative obsessiveness of the ancient Judeans was probably very annoying to the potters of the day, but has been invaluable to modern students of geomagentism.