A technique developed to assist in the recovery of plant, insect, and molluscan remains from archaeological deposits; a method of screening in which minute pieces of flora are separated from the soil by agitation with water. The technique works on the principle that organic material such as carbonized seeds, snail-shells, and beetle wing-cases have a lower specific gravity than inorganic materials such as soil and stone, and will thus float on the top of a suitable liquid medium while the rest will sink. Water is commonly used for flotation, though there are disadvantages since it has a fairly low specific gravity and heavier material such as fruit stones will sink. Other media have been used, such as carbon tetrachloride solution or zinc chloride solution. Flotation of samples by hand is called wet sieving. Samples of material are slowly poured into water, any lumps are broken up, and the flot is drawn off with a sieve. The method is more controlled than flotation by machine, and the recovery rate is better. For large-scale excavations, machines are used. Operating principles vary: samples are poured into a large container of water, or water and paraffin, which is agitated by air injection or by currents of inflowing water. The addition of a floculating agent increases surface tension, though not all machines are 'froth flotation' machines. The flot is carried off the surface through a mesh, or series of meshes to allow preliminary sorting. Samples retrieved are sent away for specialist identification and analysis by an archaeobotanist.