## 4 Discussion
### Vitrification Techniques in Ancient Anthropic Structure and Proposed Scenario at Serravuda  
Having stressed once again that vitrification of lithoid materials can also occur as a consequence of natural processes or as a result of specific human activities including the use of fire, only vitrification processes that affected ancient constructions erected for defensive purposes, i.e., fortified structures, will be discussed here. In this regard, it must be emphasized that in archaeological history, the term 'vitrification' denotes a construction technique in which man uses fire to raise temperatures to the point of partially melting lithoid materials. The aim is to generate liquids that, when cooled, undergo partial recrystallisation or turn into glass, thus binding the melted components together and strengthening the structure [38; 39]. As reported before, one of the best known examples of vitrification processes is provided by the so-called 'vitrified forts' that represent a hallmark of the Iron age civilization in northern continental and island Europe [4; 8; 9; 40]. Here, as occurred at the Serravuda site, the heat source capable of reaching the temperatures required for vitrification consist in the combustion of woody materials (timbers, logs, poles, and branches). Wooden materials were incorporated within loose rock fragments which normally were used to construct defensive structures. In dealing with the vitrification processes characterizing the vitrified forts of northern Europe, there have been some aspects that have been much discussed over the years, some of which are still somewhat controversial. One of these concerns is precisely about the presence and purpose of the inclusion of woody material in a structure fundamentally composed of fragmented lithoid material. After many studies and discussions on the typology of such materials, it is now generally agreed that in the case of the Nordic vitrified forts, timber was introduced by man not to facilitate the erection of a vertically developed structure, but solely to act as fuel in order to obtain a source of heat sufficient to melt the rocks, aiding their vitrification. The silicate liquids produced during melting, flowing through the loose lithoid materials, had the tendency to bond them, acting as cement. 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 \({}^{\circ}\)C) were attained during the heating event [4; 17; 41; 42; 43]. At this point, it should be noted that the SEM (scanning electron microscope) and X-ray diffraction (XRD) data of [PERSON] et al. [12] on the Serravuda rocks also provided similar thermometric indications. Similarities were also observed in the high-temperature mineral phase assemblages and in the quench, products present in the vitreous matrix. What distinguishes Serravuda from most North European vitrified forts is the overabundance of vitreous matter acting as a cement, as evidenced by the formation of a homogeneous vitreous crust of high thickness (up to 90 cm). This indicates that large volumes of liquid were generated during a single heating event, and this evidence does not support the hypothesis that only sporadic temperature peaks were reached. A possible interpretation of the overabundance of liquids at Serravuda is provided by the observations and results of some geochemical studies carried out on some vitrified forts in Scotland [7; 44].