The soil samples collected during the course of excavation – mainly from the material filling pits, post-holes and ditches – will initially be processed by a method known as flotation. The bulk of these samples will comprise soil (and, in the case of the Undifferentiated Head, a lot of gravel), but hidden among this soil can be tiny pieces of potentially invaluable environmental remains, much of it barely visible to the naked eye – flecks of charcoal, seeds, pollen grains, fragments of hazelnut shells, tiny pieces of bone and so on. As well as aiding our understanding of the past environment, these organic remains can be invaluable for obtaining radiocarbon dates.
Flotation is a technique that takes advantage of the relative buoyancy of different materials in order to extract these organic remains from the soil and stones that make up most of the sample. The sample is placed on a screen or mesh within a tank and a constant water source introduced from below. The soil breaks down in the water and disappears washes away through the mesh. Larger items (the heavy fraction) are caught in the mesh, while the lighter organic remains float to the surface, are carried out through a run-off and a caught in a finer mesh or screen. The heavy fraction can also contain items of value – pieces of ceramic or of worked flint, for example, that can be extracted via sieving.
Flotation of our soil samples is being carried out by some of our volunteers, using the facilities at Fort Cumberland, Southsea, which is home to English Heritage’s Environmental Studies team. We are extremely grateful for all the assistance they have provided.