Studying the remains of ancient fire is a tricky business, and over the years, researchers have used a variety of techniques to try to reveal them. They have examined individual objects (e.g., stone tools, bones, ceramics) to uncover such traces, and have carried out studies of the deposits in the field and in the laboratory using geophysical and chemical techniques, for example. One technique that has been employed recently is soil micromorphology, the study of intact blocks of soil or sediment using petrographic thin sections – slices of the sediment mounted on a slide and ground to a thickness of 30 µm, roughly that of a piece of paper. In the thin section, it is possible to observe the organic and inorganic components of the sediment preserved in their original orientation, along with minerals that might have been precipitated or modified after original deposition.
In the top image, Paul Goldberg is collecting an intact column of sediment from the Palaeolithic site of Pech de l'Azé IV in the Dordogne region of southwest France. The strategy used here to preserve the integrity of the deposits is to wrap them tightly with toilet paper and plastic packaging tape. They can then be transported to the laboratory where they are embedded with resin, sliced, and mounted on glass slides that typically measure 50x75 mm.
The bottom image shows a thin section of what appears to be a simple hearth from the Middle Palaeolithic site of Roc de Marsal, also in the Dordogne, which was scanned on a flatbed scanner. Visible here are the complex remains of multiple burning events represented by a fire-reddened base overlain by charcoal and ashes, respectively. Careful examination of the thin section, however, reveals in fact the vestiges of more than one fire, part of which had been eroded in antiquity.