On the formation of charred millet aggregates in archaeological assemblages

Charred aggregates are one of the most common forms in which millets are preserved on archaeological sites. Despite the lack of consensus on their origin, few studies have attempted to determine how aggregates are formed. Knowing how aggregates are produced allows us to understand the diversity of processes operating in the formation of charred archaeobotanical assemblages. As a contribution to filling this gap, we investigated the charring conditions of archaeological millet assemblages by comparing them to experimentally charred millets grains exposed to different temperatures , and reducing and oxidizing atmospheres, using pyrolysis-gas chromatography-mass spectrometry (Py-GC-MS) and thermally assisted hydrolysis and methylation-GC-MS (THM-GC-MS). The results show that the formation of aggregates in the millet species studied is due to the transformation of organic material into char and the emission of volatiles as a consequence of the high thermal impact that is produced in some areas of grain clusters. Substances derived from the charring act as a 'glue' that holds a grain cluster together, in which some grains are exposed to a temperature range allowing preservation of recognisable grain morphologies.

Charring conditions. Experimental archaeology. Millet preservation. Pyrolysis analysis.