New research suggests ancient pottery may contain a magnetic “clock,” providing researchers with a lasting record of how long ago ceramic artifacts were created.
According to scientists at the University of California, San Diego, the newly-developed method allows this lingering magnetic signal to help distinguish genuinely ancient clay artifacts from modern forgeries.
In experiments, clay objects more than 1,000 years old retained a magnetic signature that could only be erased by heating them above 234°F. (112°C). Modern objects lost the same type of magnetism at lower temperatures.
Published in the Proceedings of the National Academy of Sciences (PNAS), the research could give archaeologists and museums a new forensic tool for investigating artifacts whose origins are uncertain, while potentially giving law enforcement another way to build cases involving antiquities fraud.
“I hope that our new method will be useful for museum curators, who wish to display only authentic artifacts,” lead author and postdoctoral scholar at UC San Diego, Dr. Yoav Vaknin, said in a press release. “For archaeologists and historians, studying the societies that created them and for law enforcement authorities in their effort to eliminate the illicit antiquities trade, which involves looting archaeological sites and forgery.”
The technique takes advantage of a property baked into clay when it is fired.
As pottery heats in a kiln and then cools, magnetic particles inside the clay correspond to Earth’s magnetic field. Larger particles can preserve that orientation for extremely long periods, producing what geophysicists call thermal remnant magnetization.
But another signature slowly develops afterward. Smaller magnetic particles remain less stable and gradually respond to Earth’s magnetic field, while an artifact sits buried, stored, or displayed. This secondary signal, known as viscous remanent magnetization, effectively accumulates with time.
Instead of trying to determine an artifact’s age simply by measuring which direction its magnetism points, researchers wondered if they could measure how difficult its accumulated magnetic signal is to erase.
Researchers examined ancient and modern clay objects, including known archaeological specimens, objects purchased from souvenir markets around Jerusalem, and known fakes. They progressively reheated samples, starting at 112°F (50°C) and increasing the temperature in 10-degree increments while measuring changes in their magnetic signatures.
All 26 modern objects examined in the study lost their viscous magnetization below around 234°F. (112°C). By contrast, the 19 ancient specimens, each over 1,000 years old, retained the signal until temperatures exceeded that threshold. Computer simulations modeling hundreds to thousands of years of magnetic exposure independently supported the same result.
Researchers also tested the method on three disputed artifacts that the Israel Antiquities Authority obtained in connection with illicit antiquities trafficking. The evidence was consistent with all three being genuinely ancient.
The technique does have limits. It does not reveal an artifact’s exact date of manufacture, and the 234°F threshold cannot simply be applied to every object. Genuine artifacts younger than roughly 1,000 years, and sophisticated forgeries that have aged for decades or centuries, could be harder to distinguish. The researchers, therefore, envision magnetic testing as another line of evidence, not a replacement for existing archaeological authentication methods.
Nevertheless, the breakthrough could still be significant. Forged antiquities have plagued museums, private collections, and archaeological markets for generations.
Researchers note that a recent Israeli prosecution involving allegedly fake artifacts failed in part because investigators lacked sufficiently conclusive evidence. This new approach could eventually provide measurable physical evidence that is harder for a skilled forger to imitate.
Researchers now plan to examine disputed artifacts held by museums worldwide, possibly turning a phenomenon geophysicists have understood for decades into a new form of archaeological forensics.
The study, “Viscous Remanent Magnetization (VRM) Authentication Method for Archaeological Artifacts” appeared in Proceedings of the National Academy of Sciences.
Tim McMillan is a retired law enforcement executive, investigative reporter and co-founder of The Debrief. His writing typically focuses on defense, national security, the Intelligence Community and topics related to psychology. You can follow Tim on Twitter: @LtTimMcMillan. Tim can be reached by email: tim@thedebrief.org or through encrypted email: LtTimMcMillan@protonmail.com