We know that Neanderthals used fire, but whether they started the fires or they took advantage of naturally occurring ones was a mystery. Now, researchers studying 50,000-year-old "black blocks" and conducting modern combustion experiments suggest that Neanderthals made fires on demand with the help of manganese dioxide. The findings are published in Scientific Reports this week.
Several Neanderthal sites in France have yielded lots of small black blocks (pictured above), which have often been interpreted as manganese oxides collected for their coloring properties and used in body decoration and cave art, for example. However, any black material that’s soft enough to mark with yet resilient enough to last could have been used for decorative purposes. And since Neanderthals habitually used fire, carbon-rich materials like soot or charcoal would have been readily available. Manganese oxides would had to have been transported into the cave from elsewhere.
To explore other potential uses of manganese oxide, Leiden University’s Marie Soressi and colleagues studied multiple small, blackish colored blocks previously excavated from the Neanderthal site of Pech-de-l’Azé I in southwestern France using optical and scanning electron microscopy, X-ray diffraction, and X-ray fluorescence spectroscopy. These compositional analyses suggest that the Neanderthals were deliberately selecting blocks that are predominantly manganese dioxide from a range of manganese ores in the area. Indeed, manganese dioxide can be found in the limestone karst close to Pech-de-l’Azé I.
The team then conducted a series of combustion experiments (some of which you can see in the video below). Many of the blocks they examined showed evidence of abrasion, likely from a grindstone. So they converted some of the Pech-de-l’Azé I manganese dioxide material into powder, placed it along with wood onto steel gauze on a stand, and heated it from below using a flame for at least 15 seconds. They did the same to small amounts of untreated beech wood as well as a mixture of beech and commercially available manganese dioxide.
Manganese dioxide reduces the wood’s auto-ignition temperature: While untreated wood didn’t ignite at 350°C (660°F), the mixtures of wood with manganese dioxide could ignite at around 250°C (482°F). Additionally, manganese dioxide substantially increases the rate of char combustion: The peak rate occurs at about 370°C (698°F), which is well below the 460°C (860°F) for wood alone. The team concludes that, for Neanderthals at Pech-de-l’Azé I, the most beneficial use of manganese dioxide was for fire-making and producing fire on demand.