Saturday, January 26, 2013

Winter update: flotation samples from Mission Escambe

Thanks to extensive work this past fall by UWF grad student Lauren Walls (whose thesis project focuses on prehistoric subsistence at the Thompsons Landing site on the UWF campus), our new flotation system is in full operation, and soil samples from Mission Escambe are now beginning to be processed. With the help of student Morgan Smith, this past Wednesday we used the system to process four soil samples in order to extract lighter-than-water charred plant remains from excavated samples taken at the site in 2012 (and many more remain to be done from previous years as well).

The photo-essay below will provide an overview of how flotation is done, and shows that we are continuing to work on different aspects of the Mission Escambe project year-round. Artifact processing is nearly done from the 2012 season after a fall full of labwork, but until now, our many flotation samples have been awaiting attention. We hope to learn many new things from this process, most notably to include detailed evidence about crops and other plant foods used at Escambe, as well as the range of woods used for architecture and heating at the site.

The first and last step in flotation is proper record-keeping, which involves creating a record of precisely which samples have been processed, and attaching waterproof labels to each portion of the processed sample (both the light and heavy fractions, noted below).

Since the soils at Mission Escambe are so full of clay (they are alluvial soils alongside the Escambia River), each soil sample has to be deflocculated by soaking the soil in water and adding baking soda in order to help separate the tiny clay particles from one another and thereby release charred wood samples so they can float to the surface of the water.  Below are Lauren and Morgan gently stirring the sample.

Some charcoal particles immediately float to the surface and are poured off into a strainer to be incorporated into the light fraction of the processed sample.

Once the rest of the deflocculated sample is poured into the flotation tank, the sieve just below the water surface captures everything, including pebbles and artifacts and charcoal fragments that are heavier than water, as well as any additional particles of charcoal that will be released upon further soaking and agitation by tiny jets of water projecting upward from just below the sieve.  Gentle agitation by hand helps free these particles, which then flow on the surface of the water into another sieve which captures them as the light fraction, as the water falls through and below.

Once the sample has been thoroughly processed, the larger sieve is lifted out from the water in the tank, revealing all the artifacts, pebbles, and small rocks that may be present, including charred plant remains that were too heavy to float away.  This constitutes the heavy fraction of the flotation sample.

Both the heavy and light fractions are then placed in rectangles of curtain shear or other light cloth in order to be able to dry them for later processing and/or storage.  Below is the heavy fraction.

The light fraction is substantially smaller and lighter, consisting only of those particles (and sometimes tiny modern rootlets) that floated to the surface of the water and were caught in the sieve on the side of the tank.

The two fractions are then tied up with their appropriate labels...

...and hung up to dry.

This entire process results in a series of paired light and heavy fractions that can be examined with a microscope to sort out the assorted plant remains into their various types and individual species, allowing a remarkable glimpse into the daily lives of Mission Escambe's inhabitants.