by Robin Ramey – CART Assistant Lab Director
Here at CART we utilize three primary screening methods to recover artifacts: (1) dry screening, (2) water screening, and (3) flotation. While dry screening and water screening are great methods for recovering most of the artifacts from cultural sediments, flotation is the best method for retaining the tiniest and most delicate artifacts and ecofacts. That’s why CART collects a flotation, aka “flot” or “float,” sample from each layer of sediment within every archaeological feature that we excavate.
In the field, a float sample is taken by collecting approximately 3 liters of sediment from the chosen context and depositing it into a sample bag. The bag is then labeled with the sample’s context information and two pieces of brightly colored flagging tape inscribed with the same context information are placed inside the bag. This way, if the label on the exterior of the bag rubs off or otherwise becomes illegible, the sample’s context information is not lost. The sample is then transported to the lab to be processed at a later date.
There are a variety of techniques used for processing float samples, ranging from set-ups that require little more than a 5-gallon bucket and a stirring stick to fully automated flotation machines. Thanks to our friendly neighbors at Mount Vernon who graciously allow us to use their facilities for flotation, CART’s float samples are processed in an apparatus called a flotation tank (for more information about Mount Vernon’s flotation set-up click here). However, regardless of technique or set-up, the principles behind flotation are the same.
Flotation tank – top view
Flotation works by exploiting the physical properties of different materials in a soil sample to sort them based upon their density. The flotation process entails adding water (and sometimes other agents) to soil samples and then somehow agitating them in order to separate out and collect artifacts. Agitation helps break up any clumps that may have formed between various components of the sample and the addition of water physically separates those components into two density categories: those that are less dense than water and those that are denser than water.
The contents of the soil that are less dense than water float to the top of the apparatus where they can be collected with very fine mesh or cheese cloth. This portion is called the light fraction. The light fraction usually consists of small (and mostly organic) artifacts such as seeds, charcoal, plant parts, and fish scales. Components that are denser than water—such as rocks, minerals, and some bone—sink below the water line and are collected in a separate layer of mesh. This portion is called the heavy fraction.
Burned maize kernel recovered from Old Colchester Park and Preserve (photo courtesy of Justine McKnight)
Once float samples have gone through the flotation process and been given adequate time to dry, the light fraction and heavy fraction are bagged separately, each retaining one of the pieces of flagging tape bearing the context information. CART staff and volunteers then pick through the heavy fraction to isolate just the cultural artifacts. The light fraction is sent off to be analyzed by faunal and archaeobotanical experts. Artifacts recovered from flotation samples such as the burned corn kernel pictured above, can help CART archaeologists learn more about the diets, seasonal patterns, gardening or agricultural practices, and environmental conditions of the occupants at sites we investigate. To learn more about the archaeobotanicals found at Old Colchester Park and Preserve, check out our previous blog post.