Archaeobotanical Flotation

In most archaeological settings, organic plant tissue decays unless it is altered into a more durable form, and on dry sites the dominant mode of preservation is carbonization. When plant material is burned under reducing conditions it turns to charcoal, which resists decay for thousands of years and, crucially for flotation, is light and water-repellent so it floats. Uncharred seeds normally rot away, so a typical flotation assemblage is overwhelmingly carbonized. Waterlogged, desiccated, or mineralized deposits can preserve uncharred remains, but those require different sampling and may not respond to standard water flotation, as Pearsall emphasizes.

The light fraction is the buoyant material that floats to the surface during processing and overflows into a fine mesh; it consists mostly of charred seeds, nutshell, and wood charcoal and is the primary target of paleoethnobotanical analysis. The heavy fraction is the denser residue that stays in the tank and is recovered over a coarser screen; it contains bone, lithics, beads, and any seeds too dense or waterlogged to float. Best practice scans both, because some plant remains and most small artifacts are only found in the heavy fraction, and reporting volume-standardized counts from each keeps the recovery transparent.

Identification is morphological and comparative. Under a low-power stereomicroscope the analyst examines each specimen's size, shape, surface features, and, for charcoal, its cellular anatomy in three planes, then matches it to authenticated specimens in a modern reference collection of seeds and woods. Diagnostic traits — hilum form, embryo position, testa pattern, vessel arrangement — distinguish taxa, and confidence is recorded honestly, with ambiguous specimens left as indeterminate. Because identifications rest on direct comparison with known material, the quality and breadth of the reference collection are as important as the recovered remains themselves.