An ancient clay artifact presented as thousands of years old could turn out to be a forgery simply by studying the magnetism accumulated in its material.
Researchers from the University of California, San Diego, tested a new method on ancient objects, modern souvenirs, and known forgeries. Their principle relies on a subtle property of fired clay: its tiny magnetic grains record different signals over time.
A collection of ancient clay amphorae
BI ravencrow · Pexels
During the initial firing, the intense heat erases previous magnetic signals. As it cools, some grains align with Earth’s magnetic field and retain this direction for a long time. But other, smaller grains subsequently continue to slowly acquire new magnetization over the years.
This second trace is what interests the researchers. The longer the object remains exposed to Earth’s magnetic field, the more time this slow magnetization has to build up. Its behavior when a small sample is gradually reheated therefore provides an indication of the time elapsed since the pottery was fired.
Specifically, the team heated samples starting at 50 °C, then increased the temperature in 10 °C steps. For the modern objects studied, the magnetic trace acquired over time disappeared at a relatively low temperature. In samples more than 1,000 years old, it resisted temperatures above 112 °C.
In the published tests, the method distinguishes ancient artifacts from modern objects. It relies solely on laboratory magnetic measurements and on analyzing the signal during heating, not on the object’s appearance after being placed in the oven.
To establish this threshold, the experiments were supplemented by computer models reproducing the behavior of very small magnetic grains over long periods. The physical properties used had been known for several decades, but the researchers indicate that the calculations needed to finely simulate their evolution had until now been inaccessible.
This approach could interest museums and archaeologists when a piece of pottery has an uncertain provenance. It could also provide physical evidence in investigations into antiquities suspected of being fake. The authors notably cite the difficulties encountered when an object’s authenticity cannot be established using conventional methods.
The team now plans to apply this measurement to objects held in museums whose authenticity is disputed. And, it is feared, scandals may break out...