Flotation

Jesse processing soil samples

Flotation is a frequently used method in archaeology which separates small organic material such as seeds and shell from soil by using water. The goal of flotation is to collect small artifacts, plant materials, and small animal bones that would normally be lost during excavations and screening. Samples of soil (from 5-10 liters) are taken from each level of each excavation unit and then processed later using flotation. Flotation works on the principle that organic material such as burnt or carbonized seeds, charcoal, and some animal remains are lighter than inorganic materials such as soil and stone, and will thus float on the top of water while the rest will sink. Although there are many ways of floating soil samples, the most low tech and affordable involves using buckets to agitate the soil and separate the heavy material (heavy fraction) from the light material (light fraction).

At Cane Garden, we used a modified bucket flotation method. We lined a bucket with window screen and slowly poured the soil sample into the bucket once it was full of water. A hose was placed between the bucket and the screen and ran constantly at very low pressure. As we agitated the soil carefully with our hands, the silt fell through the screen and the gravels and small artifacts collected in the screen. At the same time, charcoal and other lighter bits such as seeds floated to the top, ran through the bucket spout, and into a second bucket lined with a chiffon fabric. Afterwards, both the light screen and window screen were tied up, dried and bagged for processing and analysis.  18

Our enthusiastic helper

 

 

 

 

 

 

 

Why go through all the trouble? For one, microartifacts (those too small to collect during excavation) are just as important for learning about the past. Small fish bones, bits of lithic flakes, and plant remains are some of the things recovered during flotation.

Most important for my own project is the recovery of plant remains. Plant remains can tell us about changes in the natural environment over time, changes in agricultural strategies, tell us what crops people are farming, and importantly, what domesticated and wild species people are using and consuming. Companionate species, or those species of ‘weeds’ that grow only in tandem with certain domesticate crops and farming practices, can be used as a proxy for determining what kinds of crops were grown and how they were grown. Wild plants were often collected and used as a basic foodstuff in addition to medicine and for spiritual purposes. Small bits of charcoal can be used to identify which species of wood were used for cooking fires and as architectural materials. Changes in charcoal species and frequency over time can tell us if available wood species changed through time and how people adapted to these changes. In sum, hours of mind-numbing flotation and sorting tiny bits of gravel and seed under a microscope are totally worth it given the plethora of information we can learn!

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