Vitrified Forts

By K. Kris Hirst, About.com Guide

Definition:

There are some 200 hillforts and other settlements in the world which evidence signs of being subjected to intense heat. Such burned forts range in age from Neolithic to Roman period. The heating was so extreme that all, some or part of the structures were vitrified or calcined. Vitrification is a chemical process by which silicate-based rocks are turned into a glass-like amorphous solid; calcination is the loss of moisture, reduction or oxidation in carbonate rocks.
Granite, basalt, gneiss or other silicate rocks begin to crystallize at temperatures about 650°C, and melt and vitrify when exposed to temperatures between 1050 and 1235°C. Biotite micas melt at 850°C. Carbonate rocks such as limestone and dolomite become calcined when exposed to temperatures of 800°C.
Why Vitrify?
In some cases, vitrification of timber-laced ramparts was done on purpose, to produce a more solid defensive feature. In others, vitrification was a result of an accidental or purposeful fire by people bent on destruction.
Vitrified forts (or vitrified structures) are difficult to date, because exposure to such intense heat destroys the organic materials, although recent research at Misericordia (Portugal) seems to suggest that archaeomagnetic dating may be a workable solution.

Sources
Catanzariti, Gianluca, et al. 2008 Archaeomagnetic dating of a vitrified wall at the Late Bronze Age settlement of Misericordia (Serpa, Portugal). Journal of Archaeological Science 35:1399-1407.
Friend, C. R. L., N. R. Charnley, H. Clyne, and J. Dye 2008 Experimentally produced glass compared with that occurring at The Torr, NW Scotland, UK: vitrification through biotite melting. Journal of Archaeological Science 35(12):3130-3143.

Examples:
Monte Novo and Misericordia (Portugal), Pico del Castillo (Spain), Tap o'Noth and Finavon Castle (Scotland)