To assemble a set of guaranteed fakes, researchers walked into Jerusalem souvenir shops and bought tourist pottery off the shelf, added known forgeries to the pile, and tested the lot against pottery excavated from Iron Age Judah — a bake at 112°C now tells them apart

Every forgery-detection method has the same awkward requirement: to teach a test the difference between real and fake, you need samples you are absolutely sure about, on both sides.

The genuine side is easy if you’re an archaeologist. Pottery excavated in context from Iron Age sites in Israel — vessels and figurines of Kingdom-of-Judah vintage, dug up under scientific supervision — is as certified as antiquity gets.

The guaranteed-fake side required a shopping trip. For a study published August 31 in PNAS, a team at UC San Diego’s Scripps Institution of Oceanography assembled its counterfeits the direct way: pottery bought from souvenir shops around Jerusalem — mass-produced tourist ware, ancient in style, weeks old in fact — plus pieces already known to be forgeries.

Then they baked everything, and the clay confessed.

The two magnetisms

The method rests on a property every fired clay object carries: a magnetic time stamp.

When pottery is fired in a kiln, the magnetic particles in the clay align with Earth’s magnetic field and, as the piece cools, the larger particles lock that direction in permanently — thermal remanent magnetism, frozen at the moment of creation.

But the smallest particles never fully settle. Their signals keep drifting with the wobbles of Earth’s field across the years — a second, restless signal called viscous remanent magnetism, which accumulates for as long as the object exists.

That second signal is the age detector, because of how it dies. Reheat the pottery and the viscous magnetism erases — and the newer the pottery, the lower the temperature needed to wipe it. An object that has been accumulating its drift for three thousand years holds it far more stubbornly than one that left a Jerusalem gift shop in 2024.

Finding the number

The team — led by postdoctoral scholar Yoav Vaknin, with Scripps geoscientists Lisa Tauxe, Brendan Cych and Jeff Gee — ran the whole collection through the oven, starting at 50°C and stepping up ten degrees at a time, measuring what each bake erased.

A clean threshold emerged. In samples more than 1,000 years old, the viscous magnetization could only be erased above 112°C. The modern pieces — the souvenirs, the known fakes — gave up their signal below it. Micromagnetic computer models, simulating hypothetical samples aging over varying spans, confirmed the cutoff; the modeling only recently became possible, the authors note, because the computing power hadn’t existed before.

The result is a lie detector with a dial: heat a disputed object past 112°C, and if its accumulated magnetism survives, the object has genuinely been sitting around for centuries. If the signal wipes easily, somebody’s ancestor was a potter’s wheel in a workshop last spring.

Why the courts wanted this

The study’s motivation is spelled out in its own pages: a recent high-profile forgery prosecution in Israel collapsed for lack of conclusive evidence — the notorious antiquities trial in which spectacular biblical-era artifacts could be neither proven fake nor proven real, and the defendant walked.

That is the gap the method aims at. Forgery plagues every archaeological hotspot — Israel, China, Mexico, “particularly in places where treachery abounds,” as Tauxe put it — and the existing scientific tests each have blind spots a good forger can work around. A physical threshold written into the clay’s magnetic grains is a harder thing to fake, because the forger would need to counterfeit three thousand years of geomagnetic drift.

Vaknin’s stated hope covers the whole chain: museum curators who want to display only authentic pieces, historians who need to know which objects can bear the weight of a conclusion, and law enforcement trying to choke the illicit antiquities trade that funds the looting of real sites.

The souvenir’s second career

There is a pleasing symmetry in the study’s design. The tourist pottery of Jerusalem exists to impersonate antiquity — cheap, honest fakery, sold as such, the bottom rung of a ladder whose top rung is the master forgery that fools a museum.

The researchers drafted the bottom rung to catch the top. The gift-shop jars served as certified specimens of newness, the one credential no forger’s product can escape, and their eagerness to shed their magnetism at low heat is now part of the calibration that will be aimed, the team says, at disputed objects in museum collections worldwide.

The forger’s whole craft is making the new look old. The clay, it turns out, has been keeping its own records of the difference all along — in particles too small to settle, faithfully logging every year since the kiln. All anyone had to do was warm it up and ask.