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

Abstract

European Bronze and Iron Age vitrified hillforts have been known since the 1700s, but archaeological interpretations regarding their function and use are still debated. We carried out a series of experiments to constrain conditions that led to the vitrification of the inner wall rocks in the hillfort at Broborg, Sweden. Potential source rocks were collected locally and heat treated in the laboratory, varying maximum temperature, cooling rate, and starting particle size. Crystalline and amorphous phases were quantified using X-ray diffraction both in situ, during heating and cooling, and ex situ, after heating and quenching. Textures, phases, and glass compositions obtained were compared with those for rock samples from the vitrified part of the wall, as well as with equilibrium crystallization calculations. ‘Dark glass’ and its associated minerals formed from amphibolite or dolerite rocks melted at 1000–1200 °C under reducing atmosphere then slow cooled. ‘Clear glass’ formed from non-equilibrium partial melting of feldspar in granitoid rocks. This study aids archaeological forensic investigation of vitrified hillforts and interpretation of source rock material by mapping mineralogical changes and glass production under various heating conditions.

Effect of network connectivity on behavior of synthetic Broborg hillfort glasses

“…The first material has been identified as felsic-like and is rich in quartz, plagioclase, and minor microcline, with higher and lower microcline content resulting in a red and a white granite, respectively. 4,15,36 The second material is maficlike and is an amphibolite comprising amphibole and clinochlore with minor amounts of quartz, plagioclase, microcline, and mica. 4,15,36 The Broborg archeological glasses are as heterogeneous as their starting materials.…”

The Forerunners on Heritage Stones Investigation: Historical Synthesis and Evolution

“…The first walled settlements of the Old Iron Age were areas surrounded by stone barriers driven into the ground and wide walls of adobe masonry. Vitrified walls were made with masonry and fired adobes [71][72][73] composed mainly of bones, rubble and earth that served to compact the dry-laid masonry from the late Bronze Age (7th-6th centuries BC) to that of the late Iron Age (2nd century BC).…”

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

“…Various vitrified forts, dated from the Late Bronze Age to the Early Medieval period, have been documented throughout western Europe [31,32]. Interpretations related to the building technique and their function through Bronze and Iron Ages suggest a certain degree of craft specialization and work planning.…”

“…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].…”

“…Stagnant air due to the poor ventilation inside the inner walls can be saturated with carbon and other products of decomposition from wood burning, which can consume oxygen, generating a low-oxygen environment. When the wood dries and burns, temperatures gradually rise to around 900-1000 • C in the wall core [32]. When the fuel is all consumed, the timber-laced wall is susceptible to the inward diffusion of air (i.e., H 2 O and O 2 ) during the early phases of cooling, while the burning temperature is still high (900-1000 • C).…”