Friday, August 20, 2010

The Recovery of Carbonized Plant Remains through the Flotation Method

Composite photograph showing clockwise 1) sample selection, 2) stream flotation, 3) carbonized beans, 4)beans, corn kernels, and cob fragments

Dr. Irvin Uhler who volunteered at the North Museum of Natural History’s Archaeology Section during the 1990’s once stated that, “archaeologists prefer to revel in minutia – you guys [meaning we archaeologists] strive to learn more and more about less and less”. The subject, which also happens to be the topic of this week’s TWIPA blog he was commenting on, just happened to come up during one of our many lunch hours which usually took place in the archaeology lab located in a crowded, dimly lit room of the museum’s basement. The topic Dr. Uhler was referring to is the water separation technique or flotation method whereby small plant and other tiny non-plant remains are recovered from archaeological sediments.

The short history shows that the technique immediately gained widespread use by the archaeological community during the 1960’s when Stuart Struever successfully employed the flotation method to recover small fragile remains from feature soils at the Koster Site, a deeply stratified prehistoric site located in the Lower Illinois River Valley. Since then the technique has become the principal method used by paleobotanists to recover small scale carbonized plant remains.

In form, the process simply involves immersing a soil sample of a standard volume in a tub of water, often determined by the total size of the feature from which it was taken. A bracket-supported window screen size hardware cloth, serving as the tub’s bottom, facilitates the discharge of soil as the carbonized plant particles are skimmed off the water’s surface. The larger, more dense material such as lithics, bone, shell, certain nutshells, and not infrequently water logged charcoal, are some of the kinds of objects that settle to the bottom, trapped by the hardware cloth as the sediment particles fall through the tub. Such trappings are called the sample’s heavy fraction. Both the light and heavy material fractions are then dried, sorted, and later identified by trained personnel.

Other more environmentally friendly flotation devices have been designed for use in the laboratory where the sediments are self contained in such a way that they are returned to the archaeological site and used as backfill. Some of the more sophisticated devices have “streamlined” the process into one system where the water is recycled and used again and again.


Hunter, Andrea A. and Brian R.Gassner
1998 Evaluation of the Flote-Tech Machine – Assisted Flotation System. American Antiquity 63(1):143-156.

Minnis, Paul and Steven LeBlanc
1976 An Efficient, Inexpensive Arid Land Flotation System. American Antiquity 41(4):491-493.

Struever, Stuart
1968 Flotation Techniques for the Recovery of Small Scale Archaeological Remains. American Antiquity 33(3):353-362.

Struever, Stuart with Felicia Antonelli Holton
1979 Koster: Americans in Search of their Prehistoric Past. Anchor Press/Doubleday.

For more information, visit or the Hall of Anthropology and Archaeology at The State Museum of Pennsylvania .

No comments:

Post a Comment