CHARRED TUBERS : REFERENCE COLLECTION
How to make a root and tuber reference collection
The best way to create a root and tuber reference collection is to collect modern plant material from the area in which your archaeological material comes from. One method for starting a reference collection is to target all the known economic root and tuber plants used in the region you are studying, a list of which can be derived from archaeobotanical and ethnobotanical literature. By targeting known economic plants you are narrowing your field of focus to those plants that may potentially be found archaeologically. This reduces the amount of taxa collected and minimises the work involved in processing the reference specimens as well as providing a clear collection strategy. By limiting a collection to only known economic plants, however, there is the potential that your reference collection may not include all the possible taxa that could be present in your samples. Often when out collecting it is useful to gather other root and tuber bearing plants from the same habitats as the economic plants being targeted. This will broaden the reference collection to include other plants that could have been collected and used in the past but are not known in the literature. Having a too broad collection strategy, however, can create an unnecessarily large workload. When creating a reference collection it is best to collect plant material from the same climatic zone as you are working archaeologically.
If it is not possible to collect plant material from the same area as your site then common plant species can be found in multiple regions. It is important to record the location and describe the environment your specimens are collected in since plant anatomy and morphology can vary according to environmental conditions. For a comprehensive reference collection it is necessary to include specimens of the same species from different habitats. Cyperus rotundus tubers, for example, differ when growing in dry or waterlogged conditions (see Hather 2000). When creating a reference collection it is important to be aware of seasonality. This is important since vegetative storage organs change in morphology as the seasons progress and certain roots and tubers are more palatable at different stages in their growth cycle. Inherent in any reference collection is a compromise between time efficiency, plant availability and the need to create a comprehensive reference collection.
For a reference collection it is essential that correct identifications are made. If possible enlist the help of a field botanist when collecting plants or take photographs of the whole plant to aid later identifications. Be aware that root and tuber collection is destructive; it involves uprooting the whole plant and can adversely impact on plant communities. Avoid over collection of rare species. Before collecting specimens, check and gain the appropriate government permission for plant collecting in that region. If collecting overseas you may need an import permit or quarantine certificate for bring material into your home country. Plant specimens that cannot be processed immediately should be placed in a cold environment (refrigerator) inside open plastic bags, although do not leave them in storage for too long as they will rot. Do not freeze plant specimens as this can destroy the internal anatomy by rupturing cells. Another option is to preserve plant material in ethanol (70% alcohol) in glass jars. See Hather 2000 for more information.
Charring modern material
For archaeobotanical comparisons, root and tuber samples should be charred from both a fresh and dry state. Moisture content affects the level of distortion and vesicle formation during charring as the water and oils volatilise. Vegetative storage organs can be dried in a drying oven at moderate temperatures 40-70oC for two to three days depending on the size of the specimen being dried, before being charred. Experimental charring can be conducted in a muffle furnace or oven at temperatures between 220 and 300oC for anything between 2 and 6 hours depending on the plant material. The smaller and drier the organ the shorter the charring of time needed to recreate the same characteristics as found in archaeological samples. Material should be charred in a reducing rather than oxidising atmosphere. Archaeological remains are thought to form under reducing conditions lodged in the ash at the base of the fire or shielded from the direct flame. To simulate archaeological conditions the sample can be buried in a substrate like sand or ash or wrapped in tinfoil rather than be exposed to air when charring. If buried in sand or ash the charred specimen can be retrieved by sieving. The reference collection can be stored in small glass vials. For work on experimental charring see Hillman et al 1985, Boardman and Jones 1990, Hather 1991, Wright 2003, Braadbaart and van Bergen (2005). If you have time it is also useful to make thin sections of fresh material to understand the internal anatomy of the organ. For a detailed description on how to make a reference collection see Hather 2000.
Tips for Charring
- Char fresh and dried.
- Char in sand or ash or wrapped in tinfoil.
- Char between 220 - 300oC for 2 – 6 hours