Some historians have made the assumption that Caesar was referring to the Picts being dyed with the woad plant, Isatris tinctoria, but this is by no means a proven fact. It could have been another dye altogether.

Vitrum is most commonly translated as glass, but Caesar could have been referring to the use of flint or other glassy material for scarification.

Vitrum also is in Latin dictionaries as woad, or dyestuff. When did vitrum adopt this meaning? Was this post Caesar's time?