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April-November 2003InteractiveDig Mount Vernon
An overhead view of the flotation tank
Water flowing through the screen in the flotation machine
Artifacts recovered using flotation

Photos courtesy Historic Mount Vernon. Click on images for larger versions.
by Laura Shick

Flotation 101

I'm in charge of the flotation laboratory at Mount Vernon this summer. Flotation provides archaeologists with insight into people's diet and agricultural practices, as well as information on past environmental conditions. We hope soil from the distillery will yield some of the grain that was used to produce whiskey. Such remains can only be recovered through flotation, because they pass through the larger screens we use in the field.

At the beginning of this season, I spent several days in the lab teaching myself how to use the cumbersome flotation tank that looks like a hot dog cart! The first obstacle was that the tires on the 220-pound machine were flat, so I was unable to move it out of storage. Thankfully, the helpful staff in Mount Vernon's maintenance department provided me with a portable air pump.

Next, I set up shop right outside the archaeology building, so I would have easy access to water, electricity, and laboratory supplies like screens and bags. The flotation tank holds 100 gallons of water, and requires electricity to run a pump to keep water flowing through the soil so that the sediment breaks into fine particles and artifacts float to the surface.

The flotation tank was soon working beautifully. However, it can only store 113 liters of sediment, so I had to figure out how to empty and clean it. Emptying tons of mud onto the front steps of the archaeology building or the parking lot wasn't an option, so I bought 80 feet of hose at Home Depot to connect to the tank and direct the sediment into the woods where it isn't a hazard or eyesore.

Once I solved these logistical problems, I began processing soil samples. I collect the light and heavy fractions separately in bags made of fine nylon mesh, hang the bags on a clothesline to dry, and then discard the sediment. I store the light and heavy fractions in plastic bags after they dry until it's time to sort and analyze the artifacts. It is particularly important to note from which features of the distillery the soil samples were collected--drains, troughs, or floors where malt kilns were once located--because the artifacts recovered from these features will provide further insight into the distilling process.

In future posts, I plan to share my findings as I sort artifacts from the distillery recovered by flotation.

Update July 31, 2003

Before I could begin performing flotation on soil samples from the distillery, I had to make certain that our flotation tank is working properly. To do this, archaeologists often use the "poppy seed" test. I purchased a jar of poppy seeds at the grocery store, counted out 100 seeds, and charred them on a baking sheet in an oven. The reason for charring the seeds is to simulate archaeological conditions: only carbonized, desiccated, or waterlogged botanical remains are preserved in archaeological sites. I buried the seeds in a bucket of soil, and ran the sample through the tank. Next came the difficult part--sorting through the light fraction with a pair of tweezers to separate the tiny poppy seeds from other organic material in the soil, such as twigs and charcoal. Thankfully, the test was a success: I recovered 89 of the 100 poppy seeds in the test sample.

This season at the distillery, we are collecting flotation samples (two bags of soil) from every eighteenth-century layer we open. Sandy soil is processed quickly in the flotation tank, but two bags of clay can take an hour or longer to filter through the screen. I certainly have enough samples to keep me busy for the rest of the summer and into the fall! My task as lab manager is to perform flotation on all the soil samples we collect, before we run out of space to store them in the lab. The light fraction material from the distillery will then be available for examination by an archaeologist who specializes in identification and analysis of botanical remains.

Questions about flotation? Stop by the bulletin board!

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