Phytoliths on Fire – Experimental production of heated phytoliths for analysis of archaeological sediments

In thin sections from archaeological sediments, slag-like glassy components are very common. They can refer to differently heated materials, but most of them are considered to be phytoliths (amorphous, biogenic silicon dioxide (SiO₂)) of grasses. In order to gain more detailed knowledge about the combustion conditions of phytoliths, we carried out burning experiments with four typical cereals that are frequently found in archaeological contexts. The segmented plant parts leaf, husk and stem were burnt from 250 °C up to 800 °C. The main recognized heat-induced alterations are colouration, deformation, glassy phytolith slags and pseudo-crystallisation. Our results indicate that high temperatures are not necessarily required to produce heat-altered phytoliths from cereals, as they form at around 450 °C or, in some cases, even at lower temperatures. Glassy phytolith slags already occur at 600 °C. This temperature is typical for fireplaces, which are very common at archaeological sites. They could therefore be an important source for the frequent occurrence of glassy phytolith slags. At 800 °C pseudo-crystallisation occurs, but according to Raman spectroscopy silica remains amorphous. Furthermore, the plant parts stem, leaf and husk react differently to heat. The stem, particularly the parenchyma, shows first alterations at 250 °C and distinct deformations at 450 °C. In contrast, most phytoliths of leaves and husks are still identifiable at 600 °C. This implies that stems are more sensitive to heat and might be underrepresented in the archaeological record. As this part of the plant, commonly referred to as straw, was used extensively as building material, matting and fuel, their under-representation could be relevant to the reconstruction of archaeological sites.