Recognition of ancient fires in archaeological sites containing gypsum rocks.

The presence in archaeological sites of architectural elements composed of gypsum rocks (selenite, gypsarenite and gypsrudite) is helpful to reveal traces of ancient fires because a characteristic white layer develop on gypsum surfaces directly exposed to temperatures in excess of 100 °C. The white surface is generally up to a few centimetres thick and is the consequence of gypsum dehydration to form a mosaic of microcrystalline bassanite, soluble anhydrite and/or anhydrite, depending on the temperature. With the exception of anhydrite, all phases rehydrate rapidly back to gypsum because of atmospheric moisture and/or groundwater capillarity rise along the walls. The shape of the original gypsum crystal outline is still recognizable, but the rock fabric is now composed of a mosaic of secondary, white microcrystalline gypsum. Anhydrite may also be present if the surface temperature during the fire was in excess of about 252 °C. The final result of gypsum heating appears indistinguishable from natural dehydration, which may occur because of several geological processes, such as diagenesis and burial. In these cases, however, the white areas are massive or have nodular shapes and obviously formed before the rocks were quarried. Criteria to distinguish between artificial and natural dehydration are thus easily applicable. White surfaces caused by fires develop only on exposed face of blocks which were not shielded from the heat, are thin, and correlate across adjacent blocks showing different gypsum facies and crystal orientation. The geological survey of gypsum facies in the source outcrops may reveal the presence of natural dehydration features, which may be misinterpreted as the effect of fires. The spatial extent of correlable white surfaces in the buildings, their variable thickness and composition, may give indications on the severity of damages caused by fire events, providing significant informations to unravel the evolution of archaelogical sites.