Friendly fire: Engineering a fort wall in the Iron Age

There is widespread evidence that the walls of Iron Age forts across Europe were set on fire, causing partial melting of the stonework followed by either recrystallization or glass formation on cooling - a process termed "vitrification". The motivation for fort wall firing has remained speculative since its first description in 1777. Since the suggestion of MacKie (1969) that fort vitrification might destabilize fort walls, the debate as to motives has focused on combative or destructive intentions. Here, a multidisciplinary analysis of experimental fort wall samples shows that in fact vitrification results in strengthening, not weakening. The strengthening involves diffusive and viscous sintering of material aggregates and size-dependent heat transfer. These new results support a long-since-dismissed idea that Iron Age fort walls were intentionally set ablaze in order to fortify the walls.