Celtic vitrified forts: Implications of a chemical-petrological study of glasses and source rocks

The term “vitrified forts” refers to remnants of stony fortifications of early Iron Age in which constituent rock fragments and boulders have been melted and/or welded together by heat, apparently in situ. The methods used are a matter of controversy, and even the question whether the firing was constructive or destructive is disputed. In this study we present bulk analyses of some twenty source rocks and of fifty glasses (in petrographic thin sections) from eleven sites and attempt to relate these data to the processes (and purposes) producing the observed sintering, welding and melting. The results are consistent with progressive partial, sometimes total, melting of the rocks used. In agreement with most previous investigators we conclude that no exotic rocks (although limited amounts of clays or organic debris cannot be excluded) were used as flux nor were easily fusible materials selected. On the other hand, simply burning an ordinary (of the later murus gallicus type) timber-laced wall cannot account for the evidence of strongly reducing conditions and sustained high temperatures, probably for days and in some cases well above 1000 °C. Rather, the fires seem to have been contained, perhaps by filling the space between the rocks in the timber framework with soil, clay and combustible materials, e.g. peat, brushwood and domestic debris. We cannot conclude whether the firing was accidental, or set by enemies or by the builders. In any case, the builders appear to have used more sophisticated techniques than previously believed to ensure the durability of their constructions.