The Organic Jacuzzi

Hey Fort Followers,

We had a bit of an exciting treat in the lab this past month, busting out one of our biggest tools on campus. This beautiful work of stainless steel and PVC is a flotation machine; it’s a large, divided basin with an attached water pump that helps us turn a bag of soil sample from the site into manageable, sorted collections of sediments. The first step in the process is collecting samples.

The soils are collected from particular units with specific research questions in mind, often relating to unique qualities of the units they come from. The soil sample I processed came from N6 E2, and it was collected because evidence of burning, apparent from the ashy, oxidized nature of the soil matrix, was noticed at the 30-35cm depth. One might expect abundant charcoal from a fire-related feature which is an excellent candidate for gathering by flotation.

The next stage involved filling the flotation machine with water and depositing the sample. The soil enters the main basin of the machine into a fine screen, where water pumping up from the bottom helps to loosen and separate the fine particles of clay, silt, and sand from a collection of pebbles, organic matter, and artifacts. As the sediment loosens up, the less dense organic material like seeds and charcoal float to the surface of the moving water. This portion of the sample will exit the spillway into the second basin, landing on another separate fine screen, where it can be dried for later analysis. This portion is called the “light fraction.” What solid matter remains in the screen in the main basin is also collected as the “heavy fraction.”

        After all of the soil is freed from the larger sediments the two screens are collected and are left to dry so that the many discrete pieces can be sorted and cataloged. So far, we have been working with the heavy fraction which involves filtering the material through a series of mesh sieves. The dry heavy fraction is poured into a tower of sieves arranged from largest to smallest mesh size, so that the screen of each sieve can be separated with finer and finer grain size.

        The sorting of all of this material is a meticulous and tedious process that can come down to sorting with tweezers under a microscope, to get a higher resolution picture of the context  from which these samples were drawn. I have worked through just two layers of the dried, sieved heavy fraction samples, but already I am seeing a very detailed picture of this little corner of N6 E2. Even the heavy fraction included lots of charcoal–much of this was stuck in soil concretions that remain stubbornly fused even after their time in the flotation machine. Also prominent are the many fragments of bone which we find regularly across the site. A couple of small favorites of mine are a particularly pleasant little prism of container glass, and a sturdy but diminutive iron tack.

        Sorting through all of the material from just a small sample of soil will continue throughout the semester. Hopefully a clearer picture will develop by the end of this year and we’ll have some valuable insights to share!

-Jacob