Formation, morphology and interpretation of darkened faecal spherulites

Faecal spherulites are a common indicator of dung in archaeological deposits and most of the basic processes of their formation and taphonomy have been explained. However, a darkened form is also regularly found, ranging from slightly transparent through to completely opaque. These have been less well studied, so we set out here to understand what actually causes the darkening and to determine the range of conditions required to produce the changes. Darkened spherulites were successfully created by heating dung to between 500 °C and 700 °C with the gaseous products constrained. The maximum production in our experiments was at 600 °C. The darkened spherulites often expanded during the alteration process and some of the expanded ones become distorted. SEM examination was only possible through destructive preparation processes, but examples were found showing expansion beyond the normal size range. These had a distinctive internal structure characterised by very fine crystallinity and larger scale fracturing, perhaps resulting from organic matter loss and/or CaCO3 alteration. Prolonged oxidative heating failed to remove the darkening, leading to the possibility that it is partly a structural phenomenon, with opacification arising from compound relief. Based on these findings, darkened spherulites can now be confidently interpreted as; resulting from dung being heated in conditions of limited gaseous exchange to between 500 and 700 °C, then not heated again beyond ca. 700 °C. These sorts of conditions could occur, around the edge of, or beneath, any fire where fresh dung is; being burned or where the existing stratigraphy has a dung component.