What is Flotation?
Flotation is a recovery technique that archeologists and specialists use to separate out different types of materials from sediment or "matrix" samples. Basically, the sediment is poured into a frothy liquid (usually water) and some things float while others sink. The "light fraction," those things that are light enough to float (like charcoal and seeds), is skimmed off the top and set aside. The "heavy fraction," the stuff that falls to the bottom of the liquid, is put through a series of wire-mesh screens of different grades (fine, coarse, etc.) to sort the heavy stuff (stones, bone, potsherds, etc.) into size fractions. The various fractions (light fraction and several size fractions of the heavy stuff) are sorted using tweezers, good lighting, magnification (sometimes even under a microscope), and lots of patience. For most people, this is mind-numbing work, but amid the bits and pieces, some very informative things are found.
Two categories in particular have proven very useful to archeologists:
macrobotanical remains and microfauna. Macrobotanical remains (and some
microscopic ones) are small particles of charred (and sometimes uncharred)
plants like seeds and pieces of wood that can usually be seen by the naked
eye. Specialists known as paleobotanists study these tiny plant bits and
compare the archeological samples with modern comparative samples they
have collected and prepared.