CHARRED SEEDS : INTRODUCTION
About charred plant remains
Charred seeds are the ‘bread and butter’ of archaeobotany in the sense that, together with wood charcoal , they are the most commonly found type of archaeobotanical remains on most dry sites. They have been a subject of study since 1880s (Wittmack 1880). Charred plant remains are found only when they have been exposed to heat sufficient to convert their organic content, at least partly, to carbon but insufficient to reduce them to ash. For this reason they are also referred to as carbonised. The plant materials most likely to be preserved through charring are the heavier, denser, more robust parts of the plant such as the seeds of cultivated and wild species, the pips or stones of soft fruits, and nutshell. More fragile parts of the plant, such as cereal chaff, are represented by the relatively robust bases of the glumes (which enclose the grains of wheat and rye), the nodes (and internodes) of the rachis (the central axis of the cereal ear) and, less commonly, culm nodes (the points on the straw from which the leaves arise).
Conditions of preservation
Modern charring experiments have shown that cereal grain survives better than glume bases and that both survive better than culm nodes (Boardman and Jones 1990), More recent experiments have established that the conditions most likely to result in preservation by charring are heating to temperatures between 220 and 240oC with a limited supply of oxygen. Below this temperature range, the material does not became charred even after several days exposure to heat while, above this range the material is in a very fragile state unlikely to survive archaeological deposition and recovery.
Once charred, the remains are resistant to chemical damage and microbial decay, and the major barriers to their archaeological survival are mechanical damage and alternate wetting and drying of the deposits in which they are contained.
Archaeological significance
Charred archaeobotanical remains provide information on both cultivated plants and plants collected from the wild, and their exposure to heat usually implies human activity. As such, they are ideally suited for providing information on the plant component of human diet, and other plant uses. The relative proportions of different species and plant parts can also provide information on plant processing, and the weed seeds associated with crop remains can provide information on cultivation practices (see interpretation section). However, as their preservation is dependent on exposure to heat, they provide only a very partial record of what was eaten or used (compared for example to animal bones, which are themselves subject to differential loss and destruction). Nevertheless, their relatively distinct morphologies provide the best evidence available for the species of non-woody plants used.
References
- Boardman, S. and Jones. G. 1990. Experimants on the effects of charring on cereal plant components. Journal of Archaeological Science 17: 1-11.
- Wittmack, L. 1880 Antikc Sarnen aus Troja und Peru. Monatsschrift des Vereines zur Beforderung des Gartenbaues in den Koniglich Preussischen Staaten und der Gesellschaft der Gartenfreunde Berlins 23: 120-121.