In the third century B.C.E., the Celtic warriors of what is now southern France cut off the heads of their defeated enemies and put them on display. Ancient Greek and Roman authors documented this practice and referred to the heads being “embalmed.” But archaeologists weren’t sure how—or whether—the heads were actually preserved. Now, a team of researchers has analyzed the chemical residues left behind on 2300-year-old skull bones excavated from the town of Le Cailar in southern France. They found traces of conifer resin on the bones, a sure sign the heads had been embalmed, The Guardian reports. The researchers say the heads could have been dipped in the resin, or it could have been poured on, perhaps many times over the course of a head’s afterlife.