Local geology controlled the feasibility of vitrifying Iron Age buildings

Abstract

During European prehistory, hilltop enclosures made from polydisperse particle-and-block stone walling were exposed to temperatures sufficient to partially melt the constituent stonework, leading to the preservation of glassy walls called ‘vitrified forts’. During vitrification, the granular wall rocks partially melt, sinter viscously and densify, reducing inter-particle porosity. This process is strongly dependent on the solidus temperature, the particle sizes, the temperature-dependence of the viscosity of the evolving liquid phase, as well as the distribution and longevity of heat. Examination of the sintering behaviour of 45 European examples reveals that it is the raw building material that governs the vitrification efficiency. As Iron Age forts were commonly constructed from local stone, we conclude that local geology directly influenced the degree to which buildings were vitrified in the Iron Age. Additionally, we find that vitrification is accompanied by a bulk material strengthening of the aggregates of small sizes, and a partial weakening of larger blocks. We discuss these findings in the context of the debate surrounding the motive of the wall-builders. We conclude that if wall stability by bulk strengthening was the desired effect, then vitrification represents an Iron Age technology that failed to be effective in regions of refractory local geology.

Reproduction of melting behavior for vitrified hillforts based on amphibolite, granite, and basalt lithologies

“…We know that a reducing atmosphere and a slow cooling rate are required to produce the olivine seen in the archaeological samples. These melting and cooling reactions in the amphibolite rocks are much more complicated than those reported for hillforts that involve only sandstones or granites 20 .…”

“…Heating times were likely much greater than the 15 min studied here, so it is quite likely that extensive melting (i.e., above the solidus) would have taken place regardless of lithology. Thus, while lithology may have affected the feasibility of extensive vitrification, as suggested by Wadsworth et al 20 , the evidence that humans selected rocks for their lower temperature melting characteristics, as suggested by Kresten et al 21 , is harder to corroborate here from just the melting temperatures. Rather, as noted by Nisbet 26 from empirical observation, vitrification in hillforts has been observed for many different source rocks and many lithologies throughout the world.…”

Tracing the Origins of an Anthropic Vitrified Structure with “Pre-Bleached with Blue LED” Thermoluminescence Dating: The Enigmatic Serravuda Hilltop Fortification in Calabria, Italy

“…Concerning this hypothesized process, scientists of the northern forts have long wondered whether the combustion of wood, the only source of heat available in those ancient times, was sufficient to induce significant vitrification of the lithoid material used for construction. This is because the mineral paragenesis observed in pyrometamorphosed materials and, in particular, the quench crystalline phases present in the glass matrix, suggest that frequently, very high temperatures (of the order of 1000-1100 °C) were a ained during the heating event [4,17,[41][42][43].…”

Insights on the Origin of Vitrified Rocks from Serravuda, Acri (Italy): Rock Fulgurite or Anthropogenic Activity?

“…Interpretations related to the building technique and their function through Bronze and Iron Ages suggest a certain degree of craft specialization and work planning. Many authors claimed that the vitrification was obtained through the combustion of timber interlacing within the walls and the fire was a deliberate act at the end of the fortification's life [31][32][33][34][35][36][37][38]. Further interpretations of the cause of the vitrification at these sites include ritual destruction (e.g., decommissioning of a site), destructive burning by enemies, incidental melting due to signal fires or lightning strikes [31,32,34,[36][37][38].…”