Timber-Laced and Vitrified Walls in Iron Age Forts: Causes of Vitrification

New field and geochemical evidence from vitrified forts in South Morar and Moidart, NW Scotland: further insight into melting and the process of vitrification

“…Presently, there appears to be some consensus that vitrification resulted from in situ melting caused by burning the timber interlacing (Childe, 1934;Gifford, 1992), as an act of destruction (e.g. MacKie, 1969b). There is also the view that vitrification occurred towards the end of occupation (e.g.…”

Local geology controlled the feasibility of vitrifying Iron Age buildings

“…If we assume 10 −12  <  k  < 10 −11 m 2 for the granular material infilling the spaces between the large boulders by using a model for the permeability of granular media2122, we obtain 22.4 < C < 707 m in the region of the solidus (by using 10 10  <  μ  < 10 12 Pa.s typical of high-silica partial melts at high temperature2324). We can see that this is large compared with the wall heights on the order of 1–10 m4725. From these considerations, we consider gravitational contributions to the sintering to be negligible and thus models for surface-tension-driven sintering sufficient.…”

“…However, in doing so they neglected both the material and the lengthscale considerations that we have explored above. Partially on the basis of the Childe & Thorneycroft28 reconstruction results, Mackie7 rejected a strengthening effect as a relevant consideration for interpreting vitrification. Above we have demonstrated that strengthening is a likely consequence of intense and thorough vitrification processes that result from heat above the material solidus.…”

“…Despite more than a century of academic investigation, the motives for and technology used in this anthropogenic high-temperature process remain a matter of debate. Currently, the leading theories are [1] fire-damage of the enclosure wall sustained during attack or as a result of warfare56, [2] post-occupancy deconstruction, abandonment, or site closure78, [3] a display of prestige or conspicuous consumption9, or [4] intentional strengthening of the enclosure by partial melting11011. Here we add experimental constraints to the debate by demonstrating that the rock types available to the builders were a first order control on the feasibility of vitrification, and by confirming that a strengthening effect can indeed ensue after wall burning.…”

Experimentally produced glass compared with that occurring at The Torr, NW Scotland, UK: vitrification through biotite melting

“…Childe and Thorneycroft, 1937a,b), are that vitrification was the result of destruction (e.g. MacKie, 1969bMacKie, , 1976Gifford, 1992;Ralston, 2006), particularly as in some cases vitrification is interpreted to have occurred towards the end of occupation (e.g. Sanderson et al, 1988;Ralston, 2006).…”