Flotation
Flotation is a method for recovering small and fragile remains from archaeological sites. Dirt from an archaeological context is dumped into a screen-bottomed container that is immersed in a liquid (usually water): materials less than the specific gravity of the liquid are suspended or float to the surface (the light fraction) where they can be scooped or decanted off; materials heavier than the specific gravity of the liquid sink (the heavy fraction) and are captured in a fine-mesh screen.
Light fractions typically consist of old plant remains, as well as other objects such as modern rootlets, gastropods, and so forth. These go straight to the paleoethnobotanist for sorting and identification. Heavy fractions generally contain a mixture of things such as sand, artifacts (e.g., ceramic sherds, bone), and plant remains. Heavy fractions need sorting into analytical categories before the plant remains end up with the paleoethnobotanist.
Light fractions typically consist of old plant remains, as well as other objects such as modern rootlets, gastropods, and so forth. These go straight to the paleoethnobotanist for sorting and identification. Heavy fractions generally contain a mixture of things such as sand, artifacts (e.g., ceramic sherds, bone), and plant remains. Heavy fractions need sorting into analytical categories before the plant remains end up with the paleoethnobotanist.
This heavy fraction contains a lot of charred plant material, but also a lot of other kinds of artifacts that will go to other analysts.
See Processing Heavy Fractions on how I separate out the charred plant material from the rest of the material. |
Types of flotation systems
Flotation systems can be categorized into two basic types: hand-flotation systems, and machine-assisted flotation systems. The former systems are inexpensive to make, but result in less consistent recovery that is dependent on the skill of the individual operator. The latter systems cost more to make, but may help even out recovery rates among different operators. However, in both systems, the operator needs to take meticulous care to achieve the best recovery rates.
Both systems use a fine-mesh screen in the bottom of a container to capture the heavy fraction. However, in a machine-assisted system a spray of water shoots up against the bottom of the container. Additionally, instead of scooping out the light fraction (which takes a great deal of skill), the light fraction decants into a very fine mesh of some sort (preferably, a polyester cloth with sizing).
Waterscreening is NOT flotation!
Waterscreening is not at all the same as flotation. In flotation, the screen-bottomed container starts out and remains under water so that fragile remains suspend or float up gently. In waterscreening, water is used to screen material through a series of nested screens ranging from 1/2-inch to 1/4-inch to window screen (1.0-1.5 mm openings). Not only is the smallest screen size larger in waterscreening than it should be in flotation, but the process is far from gentle.
Still under construction!
© Gail E. Wagner, 2014. The views and opinions expressed in this page are strictly those of the page author. The contents of the page have not been reviewed or approved by the University of South Carolina. Page last updated 15 Sept. 2014.