If you hung out in Western Europe during the Iron Age, you didn’t want to get on the wrong side of the Gauls. Ancient texts often talk about the bad behavior of this blonde-headed band of Celtic peoples, most notably claiming that they used to decapitate their slain rivals, embalm their heads, then stick them on spikes – the ultimate “don’t mess with us” message for any would-be foe. However, many of these accounts were written by their enemies, and some historians argue that they were made up to make the Gauls look bad.

For the first time, scientists have used chemical analysis to prove that this isn’t the stuff of distorted historical memory, reporting that the Gauls really did perform this grisly ritual on their enemies in the Journal of Archaeological Science.

At an excavation site in Le Cailar in the picturesque portion of southern France, archaeologists came across a number of skulls from the third century BCE that appeared to have been purposefully severed from the spine. They took them back to the lab to see whether they showed any traces of chemicals that would suggest the heads had been preserved.

They discovered that six of the 11 human skulls contained diterpenoid, a key biomarker of conifer resin, the gooey stuff that oozes out of trees. Remarkably, we know from historical written sources that the Gauls used oil derived from various types of conifers, namely cedar trees, to preserve body parts.

The ancient Greek historian Strabo (64/63 BCE to 24 CE) wrote in his seminal book Geography: “When they depart from the battle they hang the heads of their enemies from the necks of their horses, and, when they have brought them home, nail the spectacle to the entrances of their homes.

“The heads of enemies of high repute, however, they used to embalm in cedar-oil and exhibit to strangers, and they would not deign to give them back even for a ransom of an equal weight of gold.”

If you’re having lots of severed human heads planted around your settlement, conifer oils are probably a good idea. Many of these oils smell nice – pine fresh! – and are known to hold anti-bacterial properties. It’s likely that they used the preserving oil to keep the heads intact, meaning more heads could be on display before they turned into a nasty sludgy mess.

However, the researchers have another interesting theory. Réjane Roure, co-author of the study from Paul Valéry University of Montpellier, told The Guardian that they suspect the Gauls might have also been preserving the heads to sell them.

Perhaps the severed head of a rival was a must-have accessory for any self-respecting Iron Age settlement in Northern Europe.