Bulk matrix samples of about 5 liters of soil are collected in the field from each feature, particularly from special zones such as ash lenses in hearths. The soil is brought back to the lab and dried. The soil volume is measured, and then the soil is poured, a bit at a time, into a bucket of water. Charcoal and other charred plant materials are lighter than the water and float to the surface. They are poured off into brass screens with openings of 0.425 mm, smaller than the holes in window screens.
More water is added and the process continues until no more floats.
This produces the light fraction. Then the residue of soil and artifacts in the bucket is washed through another screen with holes 0.85 mm diameter, or about the same as window screening. This produces the heavy fraction. Here are the tiny flakes, pieces of pottery, and animal bones. This is the only way to recover the tiniest bones, such as those of crawfish, or small fish or small mammals or birds.