There are some 70 currently known prehistoric fortified sites in scotland including structures where the rock has been melted and fused together. The possible technology involved in this vitrification has been much discussed. Several carbon-14 dates have been obtained but covering a wide span - c. 1200 BC to 650 AD, that is, the period when correlation between tree ring dates and carbon-14 dates is less precise. It was thought that thermoluminescence might provide information on when the last vitrification took place. The background to the technique is explained, especially in relation to vitrified fort dating and some very preliminary experimental work described. References.