One of the principle procedures within environmental archaeology is that of flotation. While many archaeologists during hot summers relish an opportunity to submerge their arms in cool water, the process is carried out year round as an important first step in the majority of post-excavation environmental analyses.

The premise behind the use of water filled tanks is that charred environmental remains such as cereal grains, seeds and charcoal will float in water due to their air filled voids. With a constant supply of running water the floating remains can be directed to flow out of the tank and are caught in sieves, often of 0.25mm size. Using the flow of the water from beneath the sample, and with the encouragement of the operator, the soil is broken up to release the charred remains. Silty elements to the soil will frequently float, flow out of the tank and pass through the sieves into outflow tanks where the particles will eventually sink as the water flows through the settling tanks, this prevents silt pollution and enables the cycling of the water. Sandy, gritty and stony elements to the soil will sink within the tank and be retained by a mesh that the soil sample is processed in. The mesh is often of 1mm size and needs to be lifted out of the tank once the soil sample has completely broken up. Sand particles smaller than the mesh size will sink through it and settle in the bottom of the tank, depending on the soil being processed and the size of the tank this means the sludge must be emptied at intervals before it builds up to the position of the mesh. Clay elements are more problematic as they are slower to break up and clay soils will need longer to process or may need pre-flotation treatment.

Environmental remains that have not floated, been caught in the sieves and classified as 'flot' will sink and be caught by the mesh, this is then classified as 'retent'. Environmental remains within retents include bones; artefacts and slag; heavier or waterlogged cereals, seeds or charcoal; shells; hazelnut shell fragments and many other less frequently encountered materials. The processing of the retent will be discussed next week.

Waterlogged deposits can cause problems in flotation, this is primarily due to the voids within environmental remains being filled with water rather than air and therefore not providing buoyancy. Often the solution of drying the sample first is not feasible as waterlogging can preserve environmental remains that would not survive otherwise.

Flotation procedures are also described in the IAI Environmental Archaeology Guidelines.