FLOTATION: an Idiot's Guide!

So what's it all about?

Basically flotation is considered the most efficient way to retrieve charred grain, seeds and charcoal from bulk soil samples. Flotation is also useful for retrieving fish bone. The 'science' behind flotation is that charcoal (and charred material) float. You therefore end up with three products:

the 'flot' (light fraction)
the residue (heavy fraction)
soil

What equipment do you need?

Flotation tank: these can be ordered from people such as Palaeoecology Research Services or get your own made with the help of an old oil drum and a welder
2 'settling' tanks, i.e. 50 gallon watertank / adapted oil drum / old bath
300 micron and 1mm mesh sieves
1mm nylon mesh - c.1m sq. but make sure plenty of overlap on tank
Bulldog clips or similar
10 litre bucket (for measuring sample volume)
3-ply tissue or similar
Recording forms
Labels and marker pen
String or garden wire

Methodology

The essence of flotation is simple. The soil sample is emptied into a tank full of water (flot tank); the sediment sinks to the bottom; the charred material floats; and the artefacts are captured in the residue mesh. In the examples used here a system for recycling the water has been used. This is not only a practical solution in arid and semi-arid countries, it keeps things 'greener' and makes sense for those who are on metered water!

Recycling water:

water in the flotation tank flows into 'settling' tank no. 1.
An attachment near the top of the barrels connects 'settling' tank no. 1 and no. 2. This allows water to flow from the first into the second.
A pump is placed between the second settling tank and the flotation tank. The uptake pipe travels from 'settling' tank no. 2 to the pump. The output pipe travels from the pump to the bottom of the flotation tank, and comes out as jets through the bars or altnative (i.e. shower head).
A filter (foot valve or 1mm nylon mesh) is placed over the end of the uptake pipe to prevent contamination by larger particles such as leaves.

The process...

Fill the tanks with water. Note: only partially fill the flotation tank itself at this point (see 'Little Extras' below).
Pick a sample; gather together all of the bags / buckets taken for that one sample (this should be recorded on the sampling sheets and on the bags themselves, i.e. 1 of 4, 2 of 4 etc); note down its sample no., context no., and the sample volume.
Secure the 1mm nylon mesh in the flotation tank using bulldog clips. It needs to line the sides as well as the support bars which sit just above the water jets). If you don't do this the mesh will probably collapse into the tank with the weight of the soil, resulting in the loss of the sample. If you have a spout, secure this over the mesh as well. If you don't, use a large stone or brick to help keep things in place).
Stack the sieves under the spout.

Empty in the first 10 litres of soil; turn the pump on; start gently breaking up the soil. If there is charcoal or charred material in the sample, you should quickly notice it floating to the surface.
When all the bags have been processed and you are happy that the residue is relatively clean, increase the amount of water in the flotation tank, and let it wash over into the two sieves. You may need to pat the mesh from BEHIND as charred material can stick to the mesh. When you are happy that you have collected all of the flot, remove the spout / rock and dunk it in the flotation tank - again the flot often gathers underneath this. A tea-strainer is an excellent way to gather up any strays in the tank! It can be easier to complete these last few tasks with the water pump turned off.
Gather up the residue mesh and empty onto a clearly labelled tray / large plastic bag / other.

Fold-up the 3-ply tissue (kitchen towel is a weaker alternative that will collapse with large flots - support it a in foil tray) avoiding the weaker perforations. Take the coarse (1mm) sieve and empty it onto the paper by giving it a sharp tap on the ground / table by flicking your wrist. Do not bang it with the force of your whole

Fold the paper up so the flot is secure but not crushed and secure the two ends with string / wire and a label (sample no., context no.). Mark with 'C' for coarse (1mm sieve) or 'F' for fine (300 micron sieve). Repeat for other sieve. These can then be hung up to dry.
Rinse off the sieves and mesh and repeat the process with next sample.
At some point you will find that the flotation tank and the settling tanks fill up with sediment. In particular, if you find samples are not cleaning well, it is probably due to the settling tanks containing too much sediment. This signals the time to get muddy and clean out the tanks before starting again. If in a built-up area, you will need a silt trap so you don't block the drains, and a skip to empty the sediment into. In the field you may just be able to tip the tanks over and shovel the sediment out of the way. It is recommended that you clean the tanks out between sites to reduce contamination.

Little Extras...

Make sure you have all of the bags / buckets for one sample together before processing, otherwise you'll end up with two or even three sets of residues and flots, and life just gets confusing.
Before emptying a sample into the flotation tank, its worth reducing the water level to a few centrimetres below the overflow level. This prevents soil splashing over into the sieves.
Never scrape the sides of the residue mesh as this will crush any seeds and charcoal - this is with either your hands or the tea-strainer. Tap or flick the mesh from behind instead.
Label all flots and residues clearly, securely and using waterproof pens and labels. There's nothing worse (in the world of processing at least) than finding half of your hard work is wasted as it is unmarked.
Singing makes the work go a lot faster!

This is by no means the be all and end all of Flotation. It's just what has worked for me, both as the floater; instructor; and as the analyst.