WOOD CHARCOAL : RECOVERY
Wood charcoals concentrated in the sediment
For wood charcoals concentrated in the sediment (mainly timber and remnants of a last fire in situ), the find should be photographed in the field. Timber wood charcoals can be used for dendrology-related studies, i.e. dendrochronology, dendroecology and dendroethnobotany. For this purpose, the whole piece should be extracted as it is in situ so that a proper laboratory study can be undertaken (notably measuring growth-ring width). The piece should be wrapped in tinfoil and secured with tape, or put securely into a box which fits as far as possible the shape of the find. Concentrated wood charcoals can be used for radiocarbon dating. For that purpose they should be collected using tweezers. Wood charcoal can be identified prior to the dating process without affecting the quality of the date obtained (Vernet et al. 1979).
Charred fruits and seeds concentrated in the sediment
Concentrated deposits of charred seeds (as sometimes found for example in storage contexts destroyed by fire, or resulting from a large accident during grain drying) are most efficiently processed by hand, either on-site or in the laboratory. For this purpose the deposit is collected in a clean plastic bag and processed as soon as possibly by pouring the sediment into a bucket or bowl of water and stirring the deposit gently by hand. Floating material is then poured off into a pair of sieves, usually of mesh sizes 1mm and 300 microns. Further water is added and the process repeated until all the floating material has been collected in the sieves. The heavy residue remaining in the bucket/bowl is then poured into a 1mm mesh sieve. The contents of all the sieves are washed with clean water (keeping the 300 micron sieve below the 1mm flot sieves to retrieve any plant material that is washed through). This process replicates, on a smaller scale, the mechanical methods for processing bulk samples described below. Samples can be packaged in strong kitchen paper or muslin bags as described below for bulk processing.
Wood charcoals and charred fruits/seeds dispersed in the sediment
More commonly, wood charcoals, roots and tubers, and charred seeds and fruits are dispersed in the sediment. To recover such dispersed plant material, the sediment is processed using a mechanical ’water-separation’ machine. This type of machine can be adapted for use in the field or laboratory. The field version is often constructed from a 40-50 gallon barrel, as shown here. Essentially, it consists of a water supply under pressure which is projected upwards through the descending sediment, thus lifting light material (such as archaeobotanical macroremains) to the surface and into flot sieves positioned on the outside of the machine, Mesh sizes of these outer flot sieves range from 250 microns upwards (often a pair of sieves – 1mm and 300 microns). An inner mesh retains the heavy portion of the sediment, and usually ranges from 0.5 to 1 mm (smaller mesh sizes tend to clog for most deposits). The aim of this method (and the hand processing method described above), is to recovery as much as possible plant material through the relatively gentle process of flotation while retaining to ability to recover plant material that will not float. Other methods of recovery, such as water-sieving and dry-sieving, are not recommended because of the potential damage to fragile plant material. A possible exception to this is the processing of very dry sand, for which dry-sieving may be appropriate.
The field version of a water-separation machine requires an on-site source of water, which may be a natural source such as a pond or stream, or an artificial tank or series of other containers such as water barrels. It can be helpful to dry-sieve deposits through a 1cm sieve prior to water separation, to remove large stones and break the sample up. Water is pumped from the water source into the machine and projected upwards through the sediment which is slowly poured into the internal mesh. Water carries the floated material over a spout into the flot sieves positioned on the outside of the machine. It is essential to be able to regulate the flow of water into (or out of) the machine to increase or decrease the pressure under the sediment and to regulate the flow of water into the flot sieves (to prevent overflowing). An equipment list for on-site machine flotation...
On-site equipment checklist
- wet-sieving machine with valves, support mesh and weir;
- water containers for reservoir;
- pipes for connecting reservoir to pump, pump to machine, with connectors/clips;
- pipes for bringing water to sieving area with Y connection and hand spray;
- roll of plastic sheet for stockpiling samples and drying residues;
- gutter for drainage channel and plastic pipe;
- at least 2 pieces of 1mm mosquito mesh 120x120 cm;
- clothes pegs;
- 2 pairs of granulometry sieves, 1mm and 300 micron mesh;
- small plastic sieve (for cleaning machine between samples);
- 4 12.5 litre plastic buckets (for soil), 1 large washing up bowl (for water);
- 2 metal shovels (for filling buckets);
- long handled brush (for sweeping plastic sheet – soil and residue);
- table (for bagging flots) and box for equipment;
- 1 large bowl (for hand sieving);
- strong blue lab paper for bagging flots;
- large (A4+) strong bags for bagging residues;
- garden tags with green wire;
- clipboard (for current sample sheets) and ring binder (for finished sample sheets);
- pens for writing on paper and plastic;
- stapler (for attaching labels to drying residues);
- string for washing line;
- dustpan and brush (for collecting dried residues);
- large sample splitters (riffle box);
- rubbish bags.
A typical field machine can process two small buckets (c. 25 litres) of deposit at a time, so that a 50 litre sample can be processed in two ‘runs’ of the machine. It is advisable to record the volume of soil before processing so that the ‘density’ of plant material recovered can be calculated later. This type of machine is designed efficiently to process large quantities of deposit, and it is desirable to keep pace with the excavation so that the largest mesh flot sieve (and the heavy residue) can be quickly scanned for plant material, and more samples of the same or adjacent deposits requested form the excavation before it is discarded. However, if a backlog of samples develops, these can be stockpiled on plastic sheeting near to the machine.
Laboratory versions of the water separation machine follow the same principles but can usually be left to run slowly with minimal supervision until the deposit is fully processed. However a laboratory machine usually imposes severe limits on sample size, which restricts its usefulness for dispersed charred remains. It is more often used for waterlogged deposits where the plant remains are more concentrated.
Regardless of the method of processing, whether by hand, field or laboratory water-separation, the flot is usually packed in strong kitchen paper, in a way which prevents material spilling out of the packaging. Alternatively, muslin bags may be used within the flot sieves during processing, and removed intact at the end. It is useful to have some means of suspending both paper and muslin packages in the shade to dry. Heavy residues from the inside of the machine can be spread on plastic sheeting to dry. Once dry, both flots and residue can be packed in plastic bags for storage – seal-easy bags work well for the flots, and the bags used for the residue need to be made of heavy duty plastic.
Video tutorials on operating a water separation machine
This tutorial demonstrates the operation of an on-site water-separation machine for the recovery of charred plant remains, small bones and artefacts etc. If you are viewing it for the first time, it is recommended you do so in the order below.
- How to set up a flotation system
- Set up animation (FLASH animation download)
- Water separation machine
- Water separation process (part 1)
- Water separation process (part 2)
- Water separation animation (FLASH animation download)
- Sorting and packaging
Video filmed at Assiros Toumba. Created by Graham MacElearney. Narrated by Mark Edmonds.