To recover very small artifacts and charred seeds and nuts, soil is "floated." In this process, the soil is dried, then poured into a bucket of water. Charcoal and other plant materials are lighter than the soil, and float to the surface. They are poured off into a screen with holes of 0.5 mm, which is finer than window screening. Then the residue in the bucket is washed through another fine screen to recover small artifacts such as tiny beads, and animal bones.
The light fraction of a float sample is that material that floated, usually roots, charcoal, charred seeds and nuts. The heavy fraction is all the small artifacts, bones, beads, and so forth that are caught by a very fine screen. Each sample is sorted carefully, often under a microscope.