A Quantum Microscope Can Show Us How Neanderthals Tended Their Fires

Diamond sensors reveal processes from long ago.

The last flames kindled by Neanderthals died tens of thousands of years ago. But fire leaves traces behind — not only charcoal and ash, but also tiny magnetic changes locked inside minerals.

Researchers have now used a quantum diamond microscope to map that magnetic “afterimage” in a Neanderthal hearth from eastern Spain. The results suggest that its makers burned grass-based plant material and may have repeatedly added fuel to keep the fire alive. It’s the first known use of quantum diamond microscopy in archaeological science, according to the researchers.

An Ancient Hearth Under a Modern Lens

The hearth lies at El Salt, a Palaeolithic site close to a limestone wall near Alcoi, Spain. Its sediments preserve repeated Neanderthal visits spanning from roughly 80,000 to 45,200 years ago. But because the area was used for such a long time, fires overlap, forming what archaeologists call a palimpsest: many separate moments compressed into the same patch of ground.

Researchers led by Ada Dinçkal from the Universidad de La Laguna labeled the feature H89/90. Initially, Dinçkal and colleagues couldn’t even tell whether it was one hearth or two neighboring ones. It features a pale ash layer, about two centimeters thick in places, sitting above a thinner black zone of charred, heat-altered soil.

They first analyzed the features with an optical microscope, uncovering tiny bone fragments, plant remains, and lumps of reddish clay. Some ash particles still preserved the shapes of the plants that produced them.

But optical microscopes can only reveal a part of the story.

Burning changes iron-bearing minerals, leaving them magnetised according to their composition, temperature and position. Conventional instruments usually measure the combined magnetic signal of a sample. That can blur together grains that formed at different times or through different processes.

The quantum diamond microscope, or QDM, offers a way to separate the information. Its sensor contains defects in diamond known as nitrogen-vacancy centres. Under laser light, the quantum states of those defects respond to nearby magnetic fields. Researchers read that response to build a map of magnetic sources across the sample at micrometre scales.

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So, essentially, this microscope doesn’t offer a photograph but rather helps researchers map the magnetic effects of heating and cooling. Archaeologists can then compare that map with the visible structure of the hearth, deducing which microscopic components were altered together by the fire, where heating was most intense, and whether particular materials were incorporated during the hearth’s use (or entered later as contamination).

Clues in Red Soil

The clearest clue came from small red, clay-rich aggregates embedded high in the ash. They might have arrived long after the fire died, carried in by water, roots, or burrowing animals. But if they had, their magnetic signature should have stood apart from the surrounding hearth material. It didn’t.

The quantum microscope readily detected those contrasts elsewhere: limestone appeared magnetically weak, fossilised dung showed a distinct field pattern, and an iron-rich nodule blazed with an intense signal. The red aggregates blended into the magnetic map, suggesting their minerals changed alongside the ash before the hearth finished cooling — not centuries later as contamination.

Simply put, the red aggregates were already inside the hearth while it was still hot.

From this small clue, researchers can already reconstruct part of the fire’s history. Neanderthals appear to have lit it directly on an organic-rich surface, then gathered combustible vegetation nearby and fed it into the flames. This was probably not an elaborate fuel strategy, but it still reveals practical intelligence: noticing what would burn, collecting it and adding it at the right moment.

Neanderthals and Fire

Archaeologists have long known that Neanderthals used fire. The questions they’re now trying to answer are how often they returned, what they burned, how long they stayed and whether they cleaned or reused old hearths.

El Salt has become a key site for answering these questions. In 2024, researchers compared the six other Neanderthal hearts and found that the fires spanned at least 200 to 240 years. These were intergenerational hearths that likely were likely essential for the community. Another El Salt investigation identified an unusual pit hearth that reached roughly 500°C to 600°C and contained evidence of mixed plant fuels and discarded animal remains.

Of course, we don’t know that all Neanderthal populations used fire the same way. But this adds new evidence into how they managed things.

The evidence suggests they gathered local plant material, added fuel while the hearth was still active and adjusted the fire as it burned — small, deliberate choices that point to planning, attention and practical knowledge of different fuels. One hearth cannot define all Neanderthal behaviour, but it adds to a growing picture of people who organised living spaces, understood combustion and tended fires with far more control than the old image of opportunistic “cavemen” suggests.

The study was published in the Journal of Archaeological Science.