Quantum Microscope Reveals How Neanderthals Managed Fires at El Salt

Unlocking Neanderthal Fire History at El Salt

The last flames kindled by Neanderthals died out tens of thousands of years ago, but fire leaves behind a silent witness. Beyond charcoal and ash, intense heat locks microscopic magnetic changes deep inside minerals. Researchers working at the Palaeolithic site of El Salt in eastern Spain have turned to advanced physics to read those magnetic afterimages, shedding light on how ancient groups managed their campfires.

The site, situated near a limestone wall close to Alcoi, preserves repeated Neanderthal visits spanning from roughly 80,000 to 45,200 years ago. Because generations of people used the exact same spot, the resulting deposits formed what archaeologists call a palimpsest—many separate moments compressed into a single patch of ground. This overlapping accumulation made it difficult to determine whether a specific burned layer represented a continuous, long-term fire or a series of shorter, distinct events.

To untangle this history, a team of researchers led by Ada Dinçkal from the Universidad de La Laguna focused their attention on a specific combustion feature labeled H89/90. Initially, the team could not even determine whether the feature represented a single hearth or two neighboring ones. It features a pale ash layer about two centimeters thick resting above a thinner, black zone of charred, heat-altered soil.

How Quantum Diamond Microscopy Reveals Hidden Magnetic Patterns

Conventional instruments typically measure the combined magnetic signal of an entire archaeological sample. That broad measurement tends to blur together mineral grains that formed at different times or through separate heating processes. To overcome this limitation, the research team applied a quantum diamond microscope, or QDM, marking what the investigators report as the first known use of the technology in archaeological science.

The sensor inside a quantum diamond microscope contains defects in diamond known as nitrogen-vacancy centers. When exposed to laser light, the quantum states of those defects respond directly to nearby magnetic fields. By reading that response, scientists can build a high-resolution map of magnetic sources across a sample down to the micrometer scale. Instead of offering a simple photograph, the instrument maps the magnetic effects of heating and cooling.

With the QDM data in hand, researchers compared different spots within the hearth slice. The magnetic signatures in the ash layer were almost identical across samples and aligned uniformly in the same direction as Earth’s magnetic field at the time. This consistency indicated that the ash cooled in place following a single major burning episode.

Distinguishing In Situ Campfires From Later Contamination

Proving that a burned patch of soil still lies in situ—meaning right where the fire was originally made—is essential for interpreting ancient behavior. Earlier excavations at El Salt revealed that many hearths possessed a blackened organic layer beneath a white ash layer. In contrast, feature H89/90 lacked that dark layer, suggesting the fire burned the ground clean of organic material.

Additional clues emerged from small, red, clay-rich aggregates embedded high in the ash. Researchers needed to know whether those lumps arrived long after the fire died, carried in by water, roots, or burrowing animals, or if they were present while the fire burned. If they were later contamination, their magnetic signature would have stood apart from the surrounding hearth material.

The quantum microscope detected clear contrasts elsewhere: limestone appeared magnetically weak, fossilized dung showed a distinct field pattern, and an iron-rich nodule blazed with an intense signal. Yet the red aggregates blended seamlessly into the magnetic map. Their minerals had changed alongside the ash before the hearth finished cooling, proving they were already inside the structure while it remained hot.

Deliberate Fuel Management and Practical Intelligence

Combining geophysical analysis, chemical testing, and quantum microscopy allowed the team to reconstruct portions of the fire’s operational history. The consistent mineralogy, undisturbed layers, and uniform magnetism point to it being a single hearth formed with deliberate human fuel management, according to findings published in the Journal of Archaeological Science.

Optical microscope analysis previously uncovered tiny bone fragments, plant remains, and lumps of reddish clay, with some ash particles preserving the shapes of the plants that produced them. Combined with the quantum mapping, the evidence indicates that Neanderthals lit the fire directly on an organic-rich surface, gathered combustible grass-based plant material nearby, and repeatedly added fuel to keep the flames alive.

While the strategy was likely straightforward rather than elaborate, it demonstrates practical intelligence: recognizing what would burn, collecting it from the surrounding landscape, and feeding it into the fire at the right moment. El Salt continues to yield insights into how Neanderthals managed their living spaces, building on 2024 research across six other hearths at the site that showed fires spanning timelines of at least 200 to 240 years.